ABSTRACT

Flexible and stretchable organic materials and devices for application in emerging optoelectronics

Emilie Dauzon

New technologies will require more and more compliant materials capable of conforming to curved surfaces, i.e., able to stretch and mechanically resist body motions for wearable and on-skin applications. In this regard, this work discusses strategies to induce stretchability in materials. We focused our attention on improving the elasticity of transparent conducting electrodes (TCE) based on PEDOT:PSS and semiconductors (active layer) for organic solar cells.

Firstly, the introduction of DMSO and Zonyl as additives into PEDOT:PSS was shown to produce highly transparent conducting electrodes (FoM > 35) with low Young's modulus and high carrier density. We investigated the relationship between the transport properties of PEDOT:PSS and the morphology and microstructure of its films.

The combination of the two additives enhances the fibrillary nature and the aggregations of both PEDOT and PSS components of the films.

Secondly, stretchable TCEs based on PEDOT:PSS were fabricated using an innovative approach that combines an interpenetrated polymer network-based on polyethylene oxide and Zonyl. The presence of three-dimensional matrix provided high electrical conductivity, elasticity, and mechanical recoverability. The potential of this electrode was demonstrated with indium-tin-oxide (ITO)-free solar cells with a power conversion efficiency similar to ITO.

Finally, the research was completed by integrating a cross-linker or an elastomer into the active layer to enhance its stretchability while maintaining excellent photovoltaic performance. In particular, SEBS elastomer exhibited a tailored elasticity with various fullerene and non-fullerene blends: P3HT:PC61BM, PCE10:PC71BM and PCE13:IT-4F. This versatile approach highlights the ease of manufacturing and scalability achieved by the solution casting processes along with a high compatibility of acceptor and donor blends. My husband Zain Ali has supported me more than anyone else has. He has lived and shared all the way this Ph.D. and its emotions. From obstacles and challenges to achievements, Ph.D. is full of ups and downs. My husband helped me a lot in those moments with his encouragement, comforting and understanding. He has helped me relativize whenever I get too involved physically or emotionally in my work.

I thank my parents and siblings who have supported me throughout my career, and without their efforts, I would not be here. I thank my sister, Marine, for her support, of course, but especially for making me laugh so much, even when I was not in the mood.

There is nothing better than a good laugh. I hope you will keep this positivity and hilarity through life. To Kathleen who understands every aspect of my life, supporting me on the phone whenever I needed, organizing holidays to forget about my Ph.D. for a weekend.

As a part of my Ph.D., I had to move out often and I would like to thank all the roommates I had. Some were ghost-like roommates, some were kind, some did not know anything about cleanliness, and some others became great friends. Thanks to Malou and Diego, you were the best "colox".

I would like to thanks the people and my colleagues from KAUST and from the LPPI.

Thanks to all professors, scientists, engineers and technicians who always provided Figure 2-3: Noticeable applications for stretchable electronics such as: A) paper-like display, [10] B) braille display, [76] C) stretchable photovoltaic, [20] D) stretchable LED, [56] E) tattoo-like sensors, [65] F) eye camera, [START_REF] Ko | [END_REF] G) bio-medical smart bandage, [5] H) haptic gloves from HaptX Inc., [START_REF]HaptX | Haptic gloves for VR training, simulation, and design[END_REF] I) Samsung Galaxy Fold, [57] and J) ease and cheap roll-to-roll processing. [54] [START_REF] Connor | Stretchable Bioelectronics for Medical Devices and Systems[END_REF] B) Range of Young's modulus regarding the materials. The numbers in brackets are in Pa for soft, MPa for tough, and GPa for hard materials, respectively. [42] .. principle by deposition on thin material on the soft elastomer. AFM images of a buckling-induced structure of silicon membrane on B) uniaxially and C) biaxially prestrained substrate. [120] D) SEM image of single-crystal Si ribbons on elastomeric substrate. [119] .. Patterned structure for stretchable electronics. A) Spring-like structure for dye-sensitized solar cells. [129] B-C) Stretching an open mesh geometry. [6] D) Photographic, (E) optical and F) scanning electron microscopy image of metal fractal design mesh. [26] G) Designs of interconnecting meshes. [138] H) Silicon S-shaped patterned structure. [128] I-J) stretchable electrochromic E-textile. [148] K-L) Stretchable battery made of E-textile. [153] .. Resistance response under stretching of A) Metal ion implantation of a gold cluster into PDMS, [154] B) Ionic polymer-metal composites (IPMCs) made of Pt salt. [100] C) Energy response under stretching of intrinsically stretchable light-emitting electrochemical cells (LEECs) fabricated from Ru/PDMS. [166] D) Graphene film on PDMS. [117] .. Figure 2-9: All-polymer stretchable electronic composite. A) PDMS filling with liquid metal and evolution of its resistance under tensile strain. [187] B) Electroluminescent device based on PEDOT:PSS/EMIM TCB Composite Electrodes. [23] C) Enhancement of conductivity and mechanical properties of PEDOT by incorporation of ionic liquid (STEC). [207] D) DPPT-TT semiconductor confined into SEBS matrix with mobility behavior under stretching at 100%. [32] ..................... 55 Effect of the regioregularity rate on the optoelectronic and mechanical properties in conjugated P3HT polymer. [233] .. [241] .......... 61

Figure 2-14: Copolymerization of A) poly(3-alkylthiophene), [111] and B) PDPP2FTseg-2T [113] for tunable tensile modulus and photovoltaic performance. ...................... 62

Figure 2-15: Effect of the spacer incorporation in DPP-x on mobility and elasticity. [244,247] Figure 2-16: Effect of (A) length and (B) bulkiness of side chain on the mechanical properties. [112,267] Figure 2-18: Stress-alignment of P3HT and effect on mobility. On the left, a schema of P3HT film: A) at rest, P3HT is randomly oriented with a preferential edge-on arrangement. B) Under stress, there is a reorientation of P3HT nanocrystals with the alignment of the backbone with the strain direction. Characterization of carrier mobility with the applied strain: C) Effect of the strain on the mobility and the stretching direction with the transistor channel (parallel and perpendicular to the channel). D) Evolution of anisotropic mobility with the strain. [277] ............................ 69

Figure 2-19: Stretchable inorganic: A) Silicon based [282] and B) GaAs based solar cells. [34] C) Stretchable methylammonium lead trihalide based perovskite solar cell. [285] D) Stretchable dye-sensitized solar cell. [129] .. Figure 2-20: Stretchable organic solar cell based on A) P3HT:PCBM using buckling method, [288] B) PNTz4T:PCBM using buckling method, [20] C) P3HT:PCBM using network of thiolene, [33] D) PTB7:PC71BM:DIO [35] and E) molecularly change of alkyl chain in P3AT:PCBM. [112] .. (FoM) with respect to the additive composition for Zonyl (blue), DMSO (red) and the mixture of Zonyl with 5 wt% DMSO (yellow). (d) Sheet resistance-transmittance (550 nm) plot for highly conducting and stretchable PEDOT:PSS films (1 wt% Zonyl and wt% DMSO) and its comparison with commercial ITO and previous reports using the same additives. [3,13] The first additive corresponds to mPEG addition. In case of two additives, we change the additive content of only one additive (mPEG). We suppose that Zonyl was kept at 1 wt%. Without any additive, PEDOT represents almost only a third of the annealed film. The addition of an additive decrease in its proportion. With two additives, the PEDOT content is only 16%. .... Note that 5 wt% Zonyl gives Young's modulus as low as 32 MPa, and this concentration is the limit where self-supported films could be manipulated. At higher concentration, Zonyl/PEDOT:PSS films were extremely viscous and not tractable.

LIST OF TABLES

CHAPTER Introduction

In the actual economic context, energy demand is increasing annually due to world population growth. The International Energy Agency (IEA) has proposed in its World Energy Outlook 2018, a scenario in which an increase of 40% in energy demand will occur by 2040 if current governmental policies remain the same. [1] Today, fossil fuel (coal, oil, and gas) is the primary source of energy production in the world and represents 60% in total. [1] The majority of political and scientific opinions agree that human activities contribute to the greenhouse effect (confinement of solar radiation in the Earth's atmosphere) inducing global warming. A considerable increase in carbon dioxide (CO2) concentration in the atmosphere, contributing to the greenhouse effect, is observed since 1950, the beginning of the industrial revolution. [2] To fight against climate change by reducing CO2 emissions, there are other alternative energy resources.

Nuclear energy does not release greenhouse gases other than water vapor into the atmosphere. However, the extensively long half-life of uranium 238 (4.5 billion years) requires to bury radioactive waste for now and into the indefinite future. [3] Hence, the geologic impacts, high costs, and political controversies divide the public opinion for atomic energy. Besides the fact that fossil fuel and nuclear have significant environmental liabilities, these energies use limited resources. The IEA estimates that natural reserves can last 40 years for oil, 60 years for natural gas, and 130 years for coal of production at current output rates. [1] Predicted nuclear resources are present sufficiently for a long-term period (over 135 years) to supply electricity and other applications. [4] Faced with this increasing energy demand, the environmental impacts, and the impoverishment of fossil fuels, the use of clean and renewable energies has become a priority for sustainable development by achieving the objectives of the Paris Agreement. In the Kingdom of Saudi Arabia, where this dissertation partly took place, the environmental concerns and the increase of energy demand has lead Mohammed Bin Salman's vision for 2030 to exploit sunlight with the wind in complementary progressively. [5] Earth receives permanently 1361 W.m -2 in solar energy, i.e., 170 000 TW, while the world energy consumption reached about 162 000 TW.h in 2018. [1] Overall, every hour the Earth receives over 9100 times the need for energy (170 000/ (162 000/365days /24h)). The ability to transform the sunlight into electricity defines the photovoltaic (PV) effect, and Antoine Becquerel discovered it at the end of the 19th century.

Unfortunately, the phenomenon was considered anecdotal until the '70s. The conversion of solar energy into electricity and the development of the PV sector represent significant scientific economic and environmental challenges. Today, the most technologically advanced and readily worldwide-traded photovoltaic cells are silicon-based devices. Silicon cells have recently yielded to 26.1% record efficiency in the laboratory [5] and 27.6% efficiency using a silicon concentrator cell. [6] However, the manufacturing cost of these cells is high, and it is not an "on-demand" energy since daylight is required, compared to fossil energy production. Among the alternatives, there are GaAs cells based on gallium (Ga) and arsenide (As). Its structure is comparable to silicon, but its efficiency surpasses silicon technologies. Multi-junction GaAs thin-film device shows a world record of 47.1% efficiency -nowadays the highest power conversion efficiency. [7] Thin film technology regroups copper indium gallium selenide/copper indium selenide (CIGS/CIS), cadmium telluride (CdTe), and amorphous silicon. CIGS is still in the development phase and expected to become the future of PV technology with a long lifetime. However, an environmental issue can be pointed out during the recycling phase due to the toxicity of the materials.

New PV technologies such as dye-sensitized, perovskite, quantum dots, and organic cells have recently emerged, and represent promising solutions. Although their efficiencies remain lower than inorganic cells, it offers different advantages like mechanical compliance, easy processing, and low cost.

Organic photovoltaic (OPV) cells are used to convert light energy into electrical energy through the absorption of incident light by a photoactive layer between two electrodes.

The first organic photovoltaic cell was published in 1982 by Weinberger et al. [8] It was made from a polyacetylene layer inserted between two metal electrodes. This configuration is called the Schottky junction. The photocurrent is due to the dissociation of the excitons at the organic/metal-semiconductor interfaces. The nature of the electrodes strongly influences photovoltaic properties. Different polymers have been tested since Weinberger's work, but the power conversion efficiency remains low (< 1%). [9][10][11][12][13][14] This monolayer structure requires high mobility materials with high optical absorption and really thin film to achieve high efficiency.

Tang contributed to a significant advance in the organic photovoltaic field by publishing the first solar cell composed of two superimposed organic materials: copper phthalocyanine and a tetracarboxylic perylene derivative. [15] This breakthrough led to an energy conversion efficiency record of 1%. A bilayer of pure donor/acceptor materials, also called PN heterojunction, was used as the active layer (Figure 1-1a). The dissociation of the exciton is done at the heterojunction between the two semiconductors thanks to the local electric field. This type of structure was intensively studied from the early 90s. A next breakthrough was the incorporation of C60 into organic solar cells. In addition to having a high electronic affinity, the C60 has high electron mobility, making it an ideal candidate as an electron-accepting material. The Nobel team of Heeger and Sariciftci worked on C60 and poly[2-methoxy-5-(2'ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and showed an increase of photocurrent and the existence of a high-speed charge transfer. [16,17] Ideally, the active layer of these cells should be as thin as the exciton diffusion length to avoid recombination losses. In return, these cells will suffer because of the low absorption of the incident light. Hence, excitons can be generated far from the interface, and many recombinations can take place limiting considerably, charges extraction at bilayer configuration. [18,19] To overcome this recombination problem, a new solar cell configuration has been introduced by Yu et al. by mixing MEH-PPV and [6,6]-phenyl-C61-butyric acid methyl ester commonly named PC61BM or PCBM, a fullerene derivative. [20] This initiative was the first bulk heterojunction (BHJ) solar cell with a record photovoltaic conversion efficiency of 2.9%. Unlike a bilayer-type cell (Figure 1-1a), BHJ solar cell has a more extensive interface between the two materials of the photoactive layer (Figure 1-1b).

Consequently, a typical diffusion length of excitons becomes similar to the domain size of the donor and acceptor. [21] This configuration remains today the most used, and fullerene derivatives are now a standard in organic solar cells. [22] During the last 30 years, this type of solar cell has become a hot subject and can exhibit a high efficiency of over 17.3%. [23] Compared to inorganic semiconductors, organic offers an infinite potential of chemical structure, leading to low cost, large scale, and mechanically flexible materials with tunable properties. Unfortunately, organic semiconductors have a relatively low dielectric constant and a small average exciton diffusion length, which is a primordial factor involved in the photovoltaic mechanisms. [24] Organic solar cells have theoretically, a maximum power conversion efficiency (PCE) up to 20%, [25] leaving them behind the conventional inorganic photovoltaic device.

Nevertheless, they certainly offer many advantages over silicon devices. Organic films are lightweight, soft, have excellent compatibility with plastics substrates, flexible, lowcost, ease of processing, and structurally tunable. They are typically solution based which allow processes that offer benefits like printing large area and ease of upscaling lab-process, e.g., spin-coating, to industrially fabrication (printing, spray coating, evaporation, roll-to-roll). [26] In comparison, silicon devices are problematic to apply to large-area electronics. Moreover, future applications will required high compliance to cover unconventional surfaces such as building, car or even biological tissues. These materials could be stretchable and that will open many new application perspectives.

Stretchable solar cells will be the focus on this Ph.D. work. They offer many advantages, including ease processing, low cost, and elasticity due to their low Young's moduli and their organic character. Our goals of this Ph.D. thesis are to understand and to design stretchable organic solar cells. The conventional organic solar cell structure combines typically inorganic and organic materials. The substrate is usually glass, on which one of the electrodes is deposited. The cathode is often indium tin oxide (ITO), while the anode is aluminum. The organic photoactive layer achieves the carrier generation and recombination, whereas the hole transporting layer (HTL) and electron transporting layer (ETL) are implanted to facilitate the carrier's extraction and improve the wettability between the electrodes and bulk heterojunction. The conventional structure has to be rethought entirely to design an inherent stretchable solar cell. Every layer will bring new challenges, which lead us to divide this Ph.D. dissertation into several chapters.

Chapter 2 demonstrates the benefits of stretchable materials for future applications. We Chapter 7 gives general conclusions and future recommendations.

CHAPTER Stretchable bulk heterojunction organic photovoltaics:

A literature review

Introduction

Research in electronics has been dominated by a focus on device performance. [1,2] Other enabling features and properties such as mechanical compliance, and the ability of materials to be printed at low-cost and at large-scale, are considered increasingly important. They are more and more crucial to enabling penetration of electronics in new sectors of the market such as bioelectronics, [3][4][5][6] wearable and textile, [7][8][9] e-paper, [10][11][12] among others. Undisputedly, silicon-based technologies are still the leader in electronics, but despite their high electronic performances, they appear -as they areincompatible with soft or stretchable applications such as skin-like devices. In this regard, structuration tricks such as origami [13] could be used to create a "so-called stretchability."

On the other hand, using composite and conjugated polymers may never compete headto-head with silicon-based technologies on their home turf, but they offer several unique and intriguing advantages that cannot be easily met by Si. The inherent mechanical softness and stretchability attainable in these electronic materials is a case in point.

Additional benefits include the ability to be manufactured in ambient conditions, at low-cost, and ease of upscaling lab-process, e.g., spin-coating, to industrially large area fabrication (printing, spray coating, evaporation, roll-to-roll). [14,15] In comparison, silicon devices are problematic to apply to large-area electronics.

In this chapter, we give a detailed introduction to stretchable electronics, discuss the opportunities of wearable and stretchable photovoltaic devices as a green energy source, then discuss the evolution of the field in terms of applications [3,[16][17][18][19][20] and strategies to achieve stretchability. [6,[21][22][23] We report the most effective and up-to-date ways to induce elastic behavior into organic, but also inorganic materials. Structuration of materials via ingenious designs such as buckling, [21,24,25] mesh, [26] fish-net, [6,27] among others, represents one prospective way to create extrinsic stretchability up to 200%. [9] Intrinsically stretchable materials also respond to the stretchability desire and answer the call for ease of processing. Conductor/polymer composites and inherently stretchable conjugated polymers could be deposited in one-step for applications requiring stretching. In this regard, we present many intrinsically stretchable electronics materials such as semiconductors, conductors including transparent conducting electrodes (TCE) pertaining to organic photovoltaics (OPV) devices and beyond. The insertion of metal nanowire [22,28,29] or semiconducting polymer [30][31][32] into an elastomeric matrix shows competing properties between conduction transport and elasticity. We explore the requirements for maintaining the percolation pathway under strain in conductor/polymer composites. We present the limiting factor of stretchability for elastic conjugated polymers, the potential ways, and the importance of selectively designing conjugated polymers to tailor and further improve device performance and stretchability. Then, we will focus our attention upon stretchable solar cells that have been reported [33][34][35][36] and discuss the various design approaches. We will discuss how these breakthroughs in designs and implementation of stretchable electronic materials and devices will bring a new perspective of efficient design and guidance toward the development of stretchable solar cells, which is the objective of this PhD work.

Definitions and motivations

What are flexibility and stretchability?

Flexible systems can undergo a small bending motion, the extent of which is determined by the Young's modulus of the materials and their thickness. [37][38][39] All materials, from metals to wood, from inorganic to organic materials, are bendable. Their ability to be bent depends on their mechanical properties but, also on the geometry of the system.

Bendability and flexibility increase significantly with the reduction of the thickness of the film and the substrate. This implies that even poorly deformable materials can be bendable and rollable if their thickness is sufficiently low. The strain experienced by a material in the bending direction ε can be calculated by the ratio of the stack thickness, including substrate ts and film tf, to twice the radius of curvature, 2r, or 𝜀 = (𝑡 𝑠 +𝑡 𝑓 )

2𝑟

. [37] Figure 2-1 A shows the radius of curvature. Any brittle material will bend to a radius of curvature approximately a thousand times its thickness. [40][41][42] The first work on an unconventional form of electronics was described in 1994 with an all-polymer field-effect transistor (FET) made with a polyester (polyethylene terephthalate) insulating layer. [43] This effort highlighted a promising perspective for flexible devices, including low-cost fabrication methods over large-area substrates.

Several attempts in the '90s led to the development of the plastic transistor. [44,45] In particular, Bao pushed the progress of this new field by incorporating the polymer semiconductor P3HT to demonstrate a high-performance transistor. [45] The interest in flexible electronics has increased exponentially, as demonstrated by the number of publications per year (Figure 2-1 B). Stretchable electronics is a more recent emerging field that is concerned with the manufacture of electronic devices that can operate under or conform to complex mechanical deformation. The strain or deformation that stretchable electronic devices can experience includes a wide range of deformation (Figure 2-2), such as bending, [40,41] shaping, [3,46] stretching, [47] wrappings, [3] pokings, [48] crumplings, [49] and twisting. [48,50]

Benefits of stretchability and its applications in electronics

The intrinsic mechanical properties of the stretchable system directly allow numerous advantages that rigid bulk materials like silicon or rigid inorganic thin-film semiconductors like CIGS, CdTe, and hybrid perovskites, cannot offer intrinsically.

While the stretchable electronic devices would potentially recover from stress, most electronic elements, such as metals and semiconductors, fracture at less than about one percent strain. [41] The compliance of stretchable materials offers benefits such as foldability, [51] conformability, [6] covering a nonplanar surface, [52] among others. Such properties would allow a broad range of new applications. In the following, we list potential applications enabled by stretchable electronic devices that are summarized in Figure 2-3. The capacity of data storage would increase in a highly packed folded system. [45] Bendability and rollability could yield to a large area by printing technique [45] and roll-to-roll process, [53,54] and thereby a great potential to produce lowcost electronics. [14] The use of such materials can simplify processing and installation but also reduce weight and improve portability for mobile and wireless applications.

Stretchable electronics have received attention for as well flexible displays, [55] cover arbitrary surfaces, [56] textile and clothing, [7][8][9] and paper-like displays or e-paper. [10][11][12] The ability to be foldable and bendable, characteristic to stretchable, represents the most prominent applications in the electronics field at the moment. Most of the applications cited above have reached commercial reality. In 2019, the commercialization of a new generation of foldable smartphones had begun. Several companies have launched this kind of phone, such as Samsung Galaxy Fold, [57] Huawei Mate X, Royole FlexPai, among others. Entirely new applications have been results of intense research in stretchable electronics. Impressive applications have been developed ranging from electronic paper-like display devices, [10][11][12] to sensor skins [6] and sheet scanners [58] to radio frequency identification tags and smart labels, [59,60] from non-volatile memories [18] to large-area wireless power-transmission [61] wearable technologies for harvesting energy [62] and photovoltaics. [19,20] Nevertheless, the conformability offers such advanced applications that it could be from a sci-fi book. Indeed, stretchable electronics have gained attention in recent years due to its high potential in wearable electronics such as skin-like and bio-implanted electronics. [16] It represents a challenging class of applications where a system can be wrapped conformably around complex curvilinear shapes or integrated with biological tissues in ways that only simple bendability is not enough. Stretchability allows integration and compatibility properties for bioelectronics. Just like our skin and our organs, conformable materials are soft, capable of deep, inward, and non-invasive integration with soft complex biological tissues. [4] Incredible bioelectronics examples can be developed here such as health sensor, [3] wearable devices, [4] drug delivery, [5] pressure skin-like sensors, [6] rubber-like artificial skin, [63][64][65] conformable sensors and displays, [10][11][12]66,67] electronic bio-interfaces, [68][69][70] or artificial muscles. [71][72][73] Most of these devices are for personal monitoring healthcare and medical issues with more or less invasive methods like a stretchable microelectrode array (SMEA) with the capability of monitoring neural activity. [74] In the near future, bio-medical integrated electronic skin will allow the use of different healthcare sensors (heartbeat, temperature, blood pressure, etc.) to monitor and detect health issues in real-time. [5] Illness and health conditions could be taken care of pro-actively or reactively with very short delay as soon as the device notices change in average level, by avoiding a long and potentially painful incubation period and thus enhancing health and quality of life, as well as reducing morbidity and mortality. Honda et al. show the possibility to control and deliver a drug to improve health with a "smart band," making such devices both diagnose and treat ailments. [75] To facilitate access for people with disabilities, new bio-applications have as well attracted attention like braille displays, [71,76] actuators that can electrically create a sensation of touch, [76] and eye cameras that allow communication via the eye motion. [4,52,[77][START_REF] Jung | Proc. Natl. Acad. Sci[END_REF][START_REF] Ko | [END_REF] Textile and wearable electronics gained notable successes in the entertainment sector and virtual reality in particular. More than visual support, haptics

give a new dimension of reality for extensive virtual experience and full immersion.

Several products are now available such as haptics gloves. [START_REF]HaptX | Haptic gloves for VR training, simulation, and design[END_REF] Kuchenbecker et al.

worked intensively on haptics devices for robotic. [17] Figure 2-3: Noticeable applications for stretchable electronics such as: A) paperlike display, [10] B) braille display, [76] C) stretchable photovoltaic, [20] D) stretchable LED, [56] E) tattoo-like sensors, [65] F) eye camera, [START_REF] Ko | [END_REF] G) bio-medical smart bandage, [5] H) haptic gloves from HaptX Inc., [START_REF]HaptX | Haptic gloves for VR training, simulation, and design[END_REF] I) Samsung Galaxy Fold, [57] and J) ease and cheap roll-to-roll processing. [54]

Importance of stretchable solar cells

The photovoltaic (PV) concept offers many economic and environmental benefits, especially for electricity production on the site and the sustainability of the primary source. Photovoltaic technology has emerged with now, the ability to power cities by megawatt power plants. [START_REF] Parida | [END_REF] In addition to producing electricity for homes and out-ofthe-way locations, this source is suitable for smaller technologies available and accessible on the market, such as solar refrigerators, watches, power banks, and phone chargers, outdoor lights, solar power toy (car racer) and calculators, among others. The embedding of deformable solar panels makes possible PV applications in self-powered backpack, tents, and greenhouses.

In particular, solar cells with significant mechanical compliance will bring the next level of smart solar cells, making opportunities to create innovative applications such as smart windows, cars, foldable smartphones, human skin-like, and wearable electronic textile (e-textile). [6][7][8][9]16,[82][83][84] We described the benefits and applications of stretchable electronic devices previously. Those new independent electronic technologies will require independent power generation. Although stretchable electronic devices have progressed, soft and stretchable power sources have been studied far less. Power energy conversion systems to supply these stretchable electronic devices are critical to the development of this technology. O'Connor et al. studied the viability of each technology for stretchable bioelectronics devices, as depicted in Figure 234A. [START_REF] Connor | Stretchable Bioelectronics for Medical Devices and Systems[END_REF] Few electricity sources could potentially have the desired profile, such as thermoelectric, piezoelectric, triboelectric, and photovoltaic. Piezoelectric and triboelectric generators need a motion or external strength to work and are mainly made of hard materials. [START_REF] Roundy | [END_REF] Thermoelectric convert heat to electricity. [87] Thermoelectric generators provide energy under a gradient of temperature, but it is inefficient for lowtemperature change. Photovoltaic (PV) devices offer the highest power density compared to the other previous technologies. [START_REF] Raghunathan | IPSN 2005. Fourth International Symposium on Information Processing in Sensor Networks[END_REF] Outdoor sunlight at noon can produce up to 15 mW/cm 2 , while the other generator could provide a power density lower by a few orders of magnitude (piezoelectric 330 µW/cm 2 and thermoelectric 40 µW/cm 2 ). [START_REF] Raghunathan | IPSN 2005. Fourth International Symposium on Information Processing in Sensor Networks[END_REF] Besides piezoelectric and thermoelectric harvesting technologies provide to some extent power density, some applications do not require much more like a pacemaker and implantable transmitter. [START_REF] Connor | Stretchable Bioelectronics for Medical Devices and Systems[END_REF] However, implantable devices like pacemakers are unsuitable with solar energy sources due to the opacity to light of the human body. On the other hand, the photovoltaic system generates a large amount of energy, suitable for most of the mobile applications. [START_REF] Deterre | Toward an energy harvester for leadless pacemakers[END_REF] Consequently, stretchable solar cells have increased attention in the past few years due to their importance in self-generator for emerging wearable and/or bioelectronics applications.

Most bioelectronics applications require extreme softness and conformability with the bio-matter. [START_REF] Rivnay | [END_REF] Using photovoltaic devices for bio-applications yields to several challenges. First, the periodicity (day/night) and the body area influence the intensity of light received. For prolonged use, a micro-battery or micro-supercapacitor could potentially be implanted for complementary energy when no light is available, and it could be recharged by the photovoltaic cell. [91] Second, the rigidity of the photovoltaic system is not suitable for biological materials. Conventional solar cells are planar and rigid, while in vivo materials are elastic, soft, and curved. The capability of the material to stretch while keeping photovoltaic performances is required for compatibility with soft material such as skin, organs, textiles, etc. To fulfill such deformations, the selfpower solar cell would need to be stretchable. Several strategies can be used to address this challenge.

Characteristics of stretchable materials

The Young's modulus, also called the elastic modulus, characterizes intrinsic stiffness or resistance to deformation and then can be related to the ability of a material to be deformed reversibly. It is independent of the geometrical shape. Various methods have been used to determine the elastic modulus such as stress-strain, [92] buckling, [93] nanoindentation, [94] Peak-force Quantitative Nanomechanical (QNM) mapping, [95] and Brillouin spectroscopy. [96] By definition, the way to determine Young's modulus is by measuring stress under tensile strain. New techniques have also appeared which allow its characterization on thin films coated on a substrate or even at the nanoscale, including nanoindentation and Peak force mapping. [94] These are primarily surface mechanical characterization techniques based on a scanning probe, including by using atomic force microscopy (AFM). Rayleigh-Brillouin microscopy measures the response of light scattering through a viscoelastic medium. [96] This technique enables in-depth three-dimensional mapping of a material based on mechanical information, i.e. longitudinal modulus. It is a non-invasive characterization tool for samples that can be damaged, such as biological, or present in an inaccessible environment. In the same way as nanoindentation and peak force measure local properties, Brillouin spectroscopy measures the local properties at the microscale and can provide different results than the mean-field bulk properties. The buckling-based method is a well-known technique to define Young's modulus of thin layers and has been described by Stafford et al. [93] The process is based on the formation of wrinkles in a relatively rigid film on a relatively compliant substrate under compressive strain. [97] The buckling method is known to be tedious and approximate, but suitable for thin films. Tensile strength experiments provide an absolute value of Young's modulus and additional information about the mechanical characteristics of materials such as ultimate stress and strain at break, but this procedure requires thick self-standing films.

The higher the modulus, the more stress is required to stretch the material. Figure 234B shows the range of Young's moduli of common materials. One critical challenge is the mismatch between Young's modulus of bio-materials and semiconductors in general, which leads to unsuitability and non-conformability. [42] Generally, conventional electronic materials fail for a tensile strain between 1 and 3%, [47] and less than a percent in the case of metal-based materials. [98] Depending on the intended application, the strain range would vary from a few percent to more than a hundred percent. For example, hemispherical retinal electronics require less than 5% strain, [99,100] while artificial muscles are deformable up to 60%. [98] Epidermal technologies need over 30% strain to accommodate the electronics skin-like behavior, [65,98] and over 100 % strain at the knee, hip, and ankle joints. [START_REF] Raghunathan | IPSN 2005. Fourth International Symposium on Information Processing in Sensor Networks[END_REF] A low Young's modulus is preferable for an excellent conformability in bio-electronic applications.

Usually, a soft polymer such as elastomer has a low Young's modulus (<1 MPa) and hard metal have a high elastic modulus (~100 GPa). Without a doubt, the significant advantages of organic conducting polymers over inorganic materials are the promise of elastic deformation without fracture.

Crack-onset (CoS) strain defines the strain at which the first crack appeared under stress. Above this point, the material is irreversibly degraded. The crack-onset is commonly observed for a film and determine optically during gradual elongation. For bulk materials, the limit of reversible elastic behavior is associated with yield strength.

Usually, a low elastic modulus generates a high ability resisting to crack and so, a high crack-onset. Even if mechanical characteristics such as Young's modulus and crackonset are essential, we will see in the next section that they are not necessarily a prerequisite, and it is possible to get a stretchable structure from hard and brittle materials.

Strategies to make stretchable electronics

How are devices made to be stretchable? There are a limited number of strategies, which have been shown to work. In the early days, one approach was favored to create compliant and flexible systems, as described in the review by Sun and Rogers. [101] This method was based on using a small loading of one-dimensional functional materials, such as nanowires, [102] ribbons [103][104][105] or carbon nanotubes (CNT), [106] and relying on their distribution and the creation of percolation pathways in a film or a bulk material.

This process uses the percolation of materials that are not intrinsically stretchable to achieve the desired outcome. As the field progressed, new techniques emerged, and two strategies have been adopted to induce stretchability: either extrinsically or either intrinsically. Extrinsically, the design of a device structure is changed to become stretchable. [107] On the other hand, intrinsically, the material is either blend with elastomers [32,108] or surfactants [109] or either altered at the molecular level. [110][111][112][113] In the next sections, we will intensively review the techniques of making stretchable electronic devices by extrinsic and intrinsic strategies. A few examples will be given to illustrate the different strategies, but the properties of the materials will be discussed in a separate section.

Extrinsically stretchable electronic devices

Extrinsic stretchability relies upon implementing architectural modifications to a device in order to influence the stretchability of the system. Different approaches and tricks can be used to make a system more stretchable.

Buckling structures

Taking advantage of a material's ability to bend, even to a small extent, a strategy has been developed to convert local bending/unbending of stiff and non-stretchable material to macroscopic stretchable devices. This strategy, called the buckling method consists of the deposition of a very thin layer of rigid material on a pre-strained elastomer. After releasing the strain, the stack reaches an internal equilibrium of stresses to form a periodic "wavy structure", as illustrated in Figure 2345A. [21,114] In this case, stretching folds the film and the substrate surface in the manner of an accordion without breaking the fragile film. The difference between elastic modulus and the Poisson coefficients of the two materials causes buckling because the flexible substrate can and would like to return to its initial state, but the rigid film cannot. The wrinkled structure also appears, for example, after evaporation of a thin metal layer on top of the elastomeric substrate. [21] The self-assembled ordered wavy structure has been intensively studied by Yoo et al., giving a fundamental understanding of the buckling technique. [24,25,115,116] The sine waves are characterized by an amplitude and period, itself characterized by the thickness of the deposition and Young's moduli of the substrate and the film. The use of graphene can be illustrated by the work of Kim et al., [117] which showed that a graphene film directly deposited on PDMS maintained under isotropic stretching could, after relaxation, be further stretched up to 11% and this, without changing the resistance of the material. A stretch of 25% leads to an increase of resistance about an order of magnitude. The formation of wrinkles based on stiff inorganic materials on top of a soft substrate has shown its suitability for highperformance stretchable electronics, even applied to silicon. Naturally, making stretchable silicon-based technology became one of the most studied in the field. [51,118,119] For example, wavy silicon based on a continuous monolayer [51,107,120] or nanoribbons [51,118,119] presents high mechanical behavior with uniaxial or biaxial deformations. Figure 2-5 B-D gives examples of wrinkled silicon structures. [119,120] More generally, wavelike structures have been extensively examined for inorganic materials. [6,51,107,114,[121][122][123][124][125] Uniaxial pre-stretching makes a uniaxially stretchable system, [107] while biaxial [120,126] or radial [127] pre-stretching supports lead to a biaxially stretchable device. C) biaxially pre-strained substrate. [120] D) SEM image of single-crystal Si ribbons on elastomeric substrate. [119] Different strategies have been used to improve the extrinsic stretchability of systems.

The controlled delamination buckling method is based on the previous technique, but the surface of the elastomer has been modified to get some designated delamination on certain parts of the wavy structure. [107,114,122,123] An elastomeric substrate such as polydimethylsiloxane (PDMS) can be patterned by ultraviolet (UV)-ozone passivation of its surface, leading to a change of adhesion sites. [101,123] After deposition of metal nanoribbons on pre-strained PDMS, the relaxation creates a buckling structure. The only difference is defined by the delamination of ribbons on the non-oxidized sites, as shown in Figure 23456. The wavy structure leads to system-level stretchability. In this case, the microstructured metal ribbons increase the stretchability of up to 100% of the overall structure. [123]

Spring-like structures

Patterning and shaping of inorganic materials have a well-proven track record of inducing extrinsic stretchability in materials. [128][129][130] Figure 234567A shows an example of a spring-like structure. [129] The helical structure can elongate considerably, even though the material itself is intrinsically stiff. [50,129,131] Such structures behave like a spring, which behaves as a model elastic architecture. An ideal spring has elastic behavior, i.e., after deformation, it returns promptly to its initial position.

In-plane meanders or serpentine shapes enhance the ability to be stretched in-plane as well. [4,50,64,123,132] In this approach, the principle of the spring is transposed to two dimensions to lead to what could be called a "2D spring". The metal deposition is performed on the substrate in a pattern designed to resist elongation mechanically.

Nano-springs in thin films with helical or zigzag columnar microstructures can be fabricated via glancing angle deposition (GLAD) or dynamic oblique angle deposition (DOD), leading to a drastic decrease of the stiffness compared to dense films. [START_REF] Hawkeye | Glancing Angle Deposition of Thin Films[END_REF][START_REF] Dice | [END_REF][135][136]

Mesh structure

In open mesh geometry such as fishnet [6] or more complex meshes with fractal motifs, [26] the framework is a repetition of motifs forming a network that gives the architecture anisotropic deformation properties. Contrary to the wavy structure, a mesh offers uniaxial, biaxial, and radial strains. It can accommodate other out-of-plane deformation modes such as scissors, twisting, and shaping, achieving a deformity for which the behavior is similar to an elastomer. This structure can be easily stretched up to 25%. [27] In Figure 2-7 B-F, these metal-coated patterned structures are reported.

Origami and kirigami structure

3D structures derived from mesh design and origami/kirigami-inspired folding techniques, using flexible interconnection, [128,[137][138][START_REF] Lacour | Proc. IEEE[END_REF] are also found in stretchable electronics. Usually, this approach is made of rigid islands with soft metal interconnects, as shown in Figure 2-7 G-H. [4,[START_REF] Ko | [END_REF][START_REF] Lee | [END_REF]141] The presence of these interconnections between components allows the full system to stretch. Several reviews show how the arts of origami and kirigami have inspired various highly deformable structures in electronics and other applications. [13,142,143] 2.3.1.5 Textile structure

Inspired by traditional weaving techniques, the concept of the textile structures appeared a solution for stretchable electronics. Electronic textile (e-textile) due to the weaving of fibers have a high potential for flexibility [144][145][146][147] and stretchability. [148] For example, Lycra Spandex is a textile that can be stretched up to 200%. When this fabric is soaked with PEDOT:PSS solution and exposed to the electrochromic precursor, it becomes a stretchable conductive electrochromic e-textile as shown in Figure 2-7 I-J. [148] Due to the complexity and the different levels of textile structure and manufacture, the optimization of the e-textile configuration could be complicated.

Indeed, a fabric consists of intertwined threads, themselves made of micro-fibers. The nature of the fibers and their weaving to form a yarn affect the mechanical properties. Some theoretical models have been employed to predict the mechanical properties of such structures. [149][150][151][152] A textile structure that can be stretched without fracture up to 200% has been successfully used for stretchable batteries and supercapacitors, as illustrated in Figure 2-7 K-L. [9,153] Figure 2-7: Patterned structure for stretchable electronics. A) Spring-like structure for dye-sensitized solar cells. [129] B-C) Stretching an open mesh geometry. [6] D) Photographic, (E) optical and F) scanning electron microscopy image of metal fractal design mesh. [26] G) Designs of interconnecting meshes. [138] H) Silicon S-shaped patterned structure. [128] I-J) stretchable electrochromic Etextile. [148] K-L) Stretchable battery made of E-textile. [153]

Intrinsically stretchable electronic materials

Intrinsically stretchable devices can withstand mechanical deformation within the constituent materials as opposed to extrinsic and architected systems. Intrinsically stretchable materials include composite materials and organic semiconductors.

Stretchable composites

An approach is to composite soft elastomers with electronic materials. The embedment of materials in a stretchable matrix allows withstanding extreme stretching motions.

This strategy is often used for making stretchable conducting materials where the conduction is ensured by a percolation network of conducting inclusions. [28,154] The density of conducting material must, therefore, be large enough to create a percolated network within the entire composite matrix and yet remain sufficiently low not to hinder the stretchability of the matrix. In the next section, we will detail the electronic and mechanical characteristics associated with stretchable conductor composite.

Inherently stretchable semiconducting and conducting polymers

The design of elastomers has inspired chemists to replicate their benefits using conjugated polymers, whereby the molecular structure can be tailored to give elasticity to the polymers. The origins of elastic properties of non-conjugated materials are dependent on the chemical nature of the monomer, the branching, the conformation, the packing structure, the extent of crosslinking, and the chain length of macromolecules.

The translation of those facts to conjugated systems is possible to achieve similar enhancements of mechanical deformation. An intensive section correlating the conjugated polymer structure to the mechanical and electronic properties will be described later in this chapter.

Materials for intrinsically stretchable solar cell

The intrinsic mechanical properties of the stretchable system directly allow numerous advantages such as direct deformability in any direction, low-cost and printability, that rigid bulk materials like silicon or rigid inorganic thin-film semiconductors like CIGS, CdTe and hybrid perovskites, cannot offer intrinsically. Here, we focus our attention on intrinsically stretchable materials, including composites and organic electronic materials for photovoltaic applications. Organic solar cell (OSC) refers to the stacking of an active layer sandwiched by two electrodes. In brief, electrodes require conducting materials such as metal, metal oxide, or conducting polymer where low electrical resistance is needed. In this case, the properties of conduction have to be considered while making an intrinsically stretchable electrode. Oppositely, the active layer is a mixture of semiconducting polymers named bulk heterojunction. In this instance, charge transport and light-harvesting are essential and should not be disrupted through making the material intrinsically stretchable. We will discuss conductors and semiconductors materials separately, consider their properties, and how to maintain them in light of different strategies.

Stretchable Conductor

Stretchable conductors associate good conductivity and elasticity. Some reviews discussed tricks to manipulate the structure and make extrinsically stretchable conductors. [66,155,156] Conductive materials that intrinsically stretch, such as elastomerbased composite and conducting polymer, could represent a successful pathway for electronics. We will discuss stretchability along with conductivity. The addition of other properties such as transparency for transparent conducting electrodes (TCE) reduces the number of materials candidates exponentially. For the comfort of the reader, we will not distinguish the absorption properties, but we will point out the case of transparent electrodes (nanowire, CNT, graphene, and polymers).

Metallic nano-object

Overall conduction is limited by the transport of carriers within and between the conducting inclusions of the composite. As a result, the dispersion of the inclusion, its aspect ratio (between the two characteristic lengths of an object), and its wettability on the elastomer and vice versa are key morphological parameters that have been shown

to impact the conductivity of the composite. [157] The percolation can be assured through conducting particles embedded in a stretchable matrix or substrate. [70,154] The properties of these composites are not sufficient for the field of stretchable electronics: the conductivities are generally too low (~10 -12 to 10 S/cm) compared to ITO or bulk metals, and the stretching range is also rather limited. Percolation can only be achieved with high loading factors (> 20%), which constrains the deformability of the material. [157,158] Rosset et al. have shown that a sample of PDMS treated by a metal vapor deposition technique (metal vapor arcs) can stretch up to 175%. [154] Goldimplanted PDMS strips present high cyclic durability at 10% strain keeping its electrical resistance intact (398 Ohm.sq -1 ), as shown in Figure 2345678A. [154] A composite material has been proposed in which PDMS is infused into a nanoporous gold film, obtained by etching an alloy. [159] This material stretches to 25%, with an electrical resistivity only 1400 times greater than that of pure gold, which remains an impressive performance. Ionic polymer-metal composites (IPMCs) [160,161] and polymer-metal salt composites [100] have shown a good mechanical response for actuators. Insertion of platinum into an elastomer shows a sheet resistance of 2 Ω.sq -1 when it is not stretched and increase by order of magnitude when it is stretched up to 140 % (Figure 2345678). [100] Nanoparticles (NP) and nanowires (NW) of metal elements such as copper, [28,162] silver [29,[163][164][165] or ruthenium [166] have also been composited with soft polymers, offering a promising solution for transparent, stretchable electronics (see Figure 2-8 C). For example, CuNW/polyurethane composite conductors exhibit low sheet resistance (<10 2

Ohm.sq -1 for T ~ 80%) even under 60% strain. [28] Modified silver nanowires (AgNW)

with graphene oxide (GO) into polyurethane acrylate matrix have been used to make fully transparent stretchable electrodes (14 Ohm.sq -1 with T = 82%) for polymer lightemitting diodes (PLEDs). [29] 2.4.1.

Carbonaceous materials

An improvement has been obtained by the use of carbonaceous materials, namely carbon black, [167][168][169] carbon allotropes such as nanotubes [157,170,171] and more recently, graphene (Figure 2-8 D). [22,117] Indeed, in this case, the percolation of the conductive network is more efficient, thanks to more favorable contacts between carbon nanomaterials, and, in the case of carbon nanotubes, for low charge rates (0 to 2%). [157] Single-wall carbon nanotubes (SWCNT) dispersed into an elastomeric matrix show extremely elastic conductive materials. [56,[172][173][174][175] For example, Sekitani et al.

demonstrated several elastic conductor composites based on SWNTs, ionic liquid, and fluorinated rubber, reaching a conductivity of 9.7 S.cm -1 at 118% strain. [56] Metal-free SWCNTs/polymer composites have been manufactured by vacuum impregnating SWCNT aerogels with PDMS. [22] These samples are conductive up to a strain of 250%.

For a strain between 0 and 100%, the resistance varies only about 15%, and the robustness of the material during 20 cycles of stretching has been shown. CNTs could offer the possibility to associate elasticity, conductivity, and transparency suitable for optoelectronic devices requiring transparent conducting electrodes (TCE). [22,176] Figure 2-8: Intrinsically stretchable composite. Resistance response under stretching of A) Metal ion implantation of a gold cluster into PDMS, [154] B) Ionic polymer-metal composites (IPMCs) made of Pt salt. [100] C) Energy response under stretching of intrinsically stretchable light-emitting electrochemical cells (LEECs) fabricated from Ru/PDMS. [166] D) Graphene film on PDMS. [117]

Eutectic gallium-indium

Eutectic gallium-indium (EGaIn) is a stable metal liquid at room temperature that can be mechanically deformable. [177] Due to its compliance, it has also been using as a nontransparent top electrode for stretchable solar cells. [110,[178][179][180][181][182][183] However, these systems can be more vulnerable to leakage as the metal remains permanently in the liquid state.

Intrinsically stretchable EGaIn can be incorporated in an elastomeric matrix. [184][185][186][187][188] These composites, such as liquid-alloy-filled elastomeric microchannels, can maintain metallic electrical conductivity at extreme strains, as seen in Figure 2-9 A. Liquid

EGaIn in styrene ethylene butylene styrene block copolymer (SEBS) fibers can be stretched up to 700%, maintaining the metal's high conductivity. [189]

Conducting polymers

Conjugated polymers in their doped state, such as polyaniline (PANI), [190] and polypyrrole, [191,192] are electronic conductors, can also be composited with elastomers and have been exploited for electronic muscle, i.e., actuators. Poly(3,4ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) represents a solutionprocessed alternative to ITO for stretchable TCE. [109] Use of inherently stretchable materials may offer stretchability, but in the case of PEDOT:PSS, Young's modulus is found to be between 0.9-2.8 GPa, depending upon testing conditions (relative humidity, characterization technique…). [193,194] To improve the elasticity of PEDOT:PSS, numerous studies point out the benefits of the incorporation of elastomeric materials such as polydimethylsiloxane (PDMS) [195,196] or polyurethane, [194,[197][198][199] surfactants such as Zonyl, [109,183,[200][201][202] Triton [203,204] or Xylitol, [205] and soft polymers, including poly(ethylene glycol), or poly(vinyl alcohol) [206] or ionic liquids. [207] The mechanical properties of PEDOT:PSS were also enhanced by buckling method or forming into micro/nanostructures. [180,208,209] Recently, ionic liquids appeared to be a new synergistic solution to improve at the same time conductivity and softness. For example, when 1-ethyl-3-methylimidazolium tetracyanoborate (EMImTCB) was incorporated into PEDOT:PSS solution, a conductivity over 1000 S.cm -1 was found attesting the role of EMImTCB as a secondary dopant. [23] However, it was established to act as a plasticizer for PEDOT:PSS by decreasing the elastic modulus down to 38 MPa, giving stretchability up to 50%. recent example: different ionic liquids were tested and found a similar effect on conductivity and mechanical properties. [207] In particular, bis(trifluoromethane) sulfonimide lithium salt, also named stretchability and electrical conductivity enhancer

Stretchable Semi-conductor

Intrinsically stretchable semiconducting polymer composites

In their neutral state, conjugated polymers presents electronic structure similar to semiconductors, which allows them to be used as active layer in electronic devices.

Semiconducting polymers play a crucial role in electronic devices such OPVs, OLEDs, and OFETs, but many of the conjugated materials involved are not intrinsically stretchable due to their rigid structure of the polymer. [210,211] The blending of conjugated polymers with soft elastomers has been shown to offer great stretchability improvements. Polymer/elastomer composites with poly(3-hexylthiophene) (P3HT)

have led to high materials compliance without suffering a significant charge mobility penalty. For example, the mobility remains constant under 100% strain with content as low as 10 wt% P3HT into PDMS matrix for over 1000 stretched-and-release cycles [108] or even for lower content (0.49 wt% P3HT). [31] The ability of these composites to maintain a percolation pathway even for very small loading of the semiconducting polymer is remarkable. Highly stretchable poly(2,5-bis

(2-octyldodecyl)-3,6- di(thiophen-2-yl)diketopyrrolo[3,4-c]pyrrole-1,4-dione-alt-thieno[3,2-b]thiophen)
(DPPT-TT) mixed into SEBS matrix has also been achieved and shows a slight decrease carrier mobility under 200% strain (Figure 23456789). [32] Figure 2-9: All-polymer stretchable electronic composite. A) PDMS filling with liquid metal and evolution of its resistance under tensile strain. [187] B) Electroluminescent device based on PEDOT:PSS/EMIM TCB Composite Electrodes. [23] C) Enhancement of conductivity and mechanical properties of PEDOT by incorporation of ionic liquid (STEC). [207] D) DPPT-TT semiconductor confined into SEBS matrix with mobility behavior under stretching at 100%. [32]

Inherently stretchable semiconducting polymers

The rigidity of silicon is due to its covalent bonds, where each atom is linked with four neighbors. In contrast, a pi-conjugated polymer consisting of molecular stacking maintained together via electrostatic interactions (π-effects) and van der Waals bonds can be much softer. [START_REF] Rivnay | [END_REF] The hybridization of the carbon atoms and the molecular arrangement control the conduction properties of organic semiconductors, while the intrinsic properties of the polymer chain are essential, its hierarchical arrangement can make it either much more or much less stretchable. The π-π stacking between the molecular chains is responsible for an increase of crystallinity. In the next sections, we will see that the molecular packing and crystalline order significantly change the transport properties and the mechanical properties of materials. [212][213][214][215] Several examples will be presented that deal with the design of conjugated polymers that can comply with mechanical stress and be stretched. We point out the effect of molecular structure such as rigidity of backbone, randomness induced by segmentation, the bulkiness of side chain, and molecular weight of the chain on the electronic and mechanical properties, as illustrated in Figure 2-10. As such, materials may exhibit an extensive and tunable range of mechanical properties. Backbone -Fused ring:

The fusion between aromatic rings on the conjugated backbone is well known to enhance electron delocalization. [216][217][218] Fused rings lead to a rigid planar geometry avoiding rotational disorder and improving the carrier mobility. [START_REF] Bredas | Proc. Natl. Acad. Sci[END_REF][START_REF] Coropceanu | [END_REF][221] Moreover, the co-planarity increases the π-π stacking order favoring the inter-chain transport, but it could render the polymer insoluble. [222] In non-conjugated polymers, the high order and the crystallinity increase Young's modulus. [START_REF] Cheremisinoff | Handbook of polymer science and technology[END_REF] As an extreme case, graphene can be considered as a highly fused system extended in two-dimensions, and its elastic modulus is at 1 TPa. [START_REF] Sakhaee-Pour | [END_REF] For conjugated polymers, similar behavior is expected. Indeed, fully conjugated fused polymers -also named conjugated ladder polymers (cLPs) -are a class of polymers in which the backbone is a succession of the π-conjugated fused ring. [225] cLPs show unique properties such as chemical and thermal stability, high π-π stacking interactions, and ultra-fast charge transport, among others. [225] Polyacene is an example of cLP. Although polyacene has not been successfully synthesized, its Young's modulus was found at 745 GPa, by the density functional theory (DFT)

approach. [226] Poly(benzimidazole benzophenanthroline) (BBL) is a polymer containing six fused rings and is extensively investigated in electronic applications. [225] BBL shows charge mobility of up to 0.1 cm 2 V -1 s -1 due to its conjugation and configuration [227] and a Young's modulus of about 120 GPa. [228] Although cLPs may present benefits for charge transport due to the high planarity of their backbone, they have several disadvantages such as low solubility and high elastic moduli, making them unsuitable for stretchable electronics.

A lower fusion of rings leads to higher ductility compared to cPLs. Roth et al.

demonstrated that fused rings are stiffer than a single isolated ring. [229] Another study observed a proportional trend between electrical and mechanical properties, i.e., ring. [213] Highly crystalline pBTTT shows more substantial charge mobility than P3HT due to the stability and order of the backbone and sidechains. [230] O'Connor confirmed the charge transport enhancement, and their investigation of mechanical behavior, which disclosed a correlation between crystallinity and stiffness. [213] The compliance of pBTTT film decreases due to the fixation of interdigitated sidechains and densely packed fused pBTTT structure that leads to the reduction of chain flow and movement.

Young
Consequently, Young's modulus is found higher for pBTTT than P3HT. Brittleness is related to crystallinity, which is also connected to an increase in the charge carrier mobility. [213] In this example, the molecular organization and packing due to the rigidity of the fused ring highlight the consequences of intrinsic mechanical properties.

Figure 2-11: Effect of the introduction of the fused rings on mobility and Young's modulus. Comparison of fused pBTTT as-cast (AC) and annealed at 180

°C (AN) with non-fused P3HT. [213] Regioregularity:

Similar to elastomers, the spatial configuration and the regioregularity of semiconducting polymers can play even more of a crucial role in the electronic behavior because of π-interactions. For example, regioregular poly(3-alkylthiophene) (P3AT)

shows better conductivity and mobility than randomly arranged P3AT. [231,232] Charge transport and crystallization are improved in this regioregular component, while the randomness of the non-regioregular chains makes P3AT amorphous and reduces its mobility. Kim et al. demonstrated a strong relationship between regioregularity, selfassembly, crystallinity, and electronic performances of P3HT. [232] In conclusion, the polymerization conditions are responsible for the structural organization, i.e., the packing and crystallinity, which alter conducting behavior. Controlling the regioregular portions in P3HT confirmed its beneficial effects for tailored mechanical, optical, and transport characteristics. [233] The degree of crystallinity increases linearly with the regioregularity ratio in a conjugated polymer, and so does the charge carrier mobility and the stiffness (i.e., Young's modulus). [233] Highly crystalline and regioregular P3HT

shows significantly higher stiffness and brittleness than low regioregular content of P3HT, as shown in Figure 2-12. Again, the regioregularity results in enhancement of molecular packing due to an improved chain bonding and the higher interactions between the chains. Regioregular P3HT will, hence, possess a higher chance of failure under strain. [233] Figure 2-12: Effect of the regioregularity rate on the optoelectronic and mechanical properties in conjugated P3HT polymer. [233] Molecular weight:

The molecular weight (MW) of a conjugated polymer influences its electrical transport and mechanical properties. The optimization of the MW has been the subject of extensive research for enhancing carrier mobility and morphology. [234][235][236] For example, in the case of P3HT, the MW has a beneficial impact on charge mobility. [236] For longer chains, the charge carrier can move further down the length of the chain. On the opposite, the π-stacking and crystallinity decrease significantly with the chain length, making it difficult for the carrier to undergo interchain hopping. This behavior was confirmed by Li et al., who observed that an optimal MW (~40 000 g/mol) was required to adjust the competition between electronic and morphological properties in PBnDT-FTAZ acceptor combining benzodithiophene (BnDT) and fluorinated benzotriazole (FTAZ). [235] They explained that beyond a threshold, the MW has a deleterious effect on the carrier mobility. In other words, there has to be a compromise between crystalline domains influence and MW. The Heeger group also observed that an average MW yields optimal photovoltaic performance superior to low and high MW versions of the polymer, that is to say between a densely packed and disordered microstructure. [237] They found the optimal MW to be 55 000 g/mol -1 for P3HT. [237] Getting an optimal OPV device based on a high MW polymer could be technically challenging. Several factors can intervene in the polymerization process and need to be optimized. Han et al. demonstrated that the steric hindrance of the side chain could prevent the formation of a high MW polymer with a planar conjugated backbone. [238] However, for high carrier mobility a high MW is preferred to promote tie-chains along with enhanced backbone planarity.

Koch et al. established that the side-chain interdigitation is thermodynamically favored

for short regioregular oligo(3-hexylthiophene)s (3HT)n while kinetically, long polymers organize end-to-end and do not interdigitate. [239] The interdigitation brings a high crystalline order with a close lamellar packing. From their observation, they described the interdigitated material to be "somewhat brittle," and the non-interposed one was "plastic." [239] In general, increasing the MW leads to higher elastic deformation due to higher chain entanglement. Koch et al. also confirmed the softening effect of MW in P3HT polymer and related it to the structural organization. [240] High MW P3HT (> 90 000 g/mol) can be stretched almost to 300% while highly crystalline and low MW chains break after a few percent. [240] Rodriquez et al. reaffirmed that the elasticity of P3HT increased with the MW. Figure 2-13 represents the dependence of the MW of P3HT on its mechanical properties. [241] [241] Copolymerization:

Block copolymerization is one of the solutions proposed to engineer the mechanical and electronic properties of polymers. Lipomi's work brought a new, more profound understanding of the impact of the alkyl side-chains on the electronic and mechanical properties. They copolymerized two thiophene monomers with an average length of the side chain of n = 7. [111] A systematic comparison of P3HpT, a P3HT:P3OT blend, a block copolymer (P3HT-b-P3OT), and a random copolymer (P3HT-co-P3OT) were investigated, as seen in Figure 2-14 A. P3HT-b-P3OT offers an expected tensile modulus between the two homopolymers. However, the random copolymer and the blend display a synergetic mechanical effect, giving a lower modulus than anticipated, emphasizing that the benefit of the copolymerization compared to the individual unit.

The exploration of the structural polymer rearrangement continued with Müller et al. [242] The copolymerization of semiconducting P3HT with insulator polyethylene (PE) resulted in a highly crystalline structure capable of being elongated over 600%. The carrier mobility of the block copolymers varies with the proportions of P3HT and PE but remains extraordinarily high for P3HT content greater than 10 wt%. Printz et al. examined the plasticizing effect of the copolymerization of two semiconducting polymers. [113] Randomly associating PDPP2FT and 2T blocks to provide PDPP2FT-seg-2T (see Figure 2-14 B for structure), offered a softening effect of 2T-formed polymer (PT2T) with similar electronic performance to PDPP2FT. The segmentation of the block copolymers drives a structural disorder which primarily affects the mechanical performance. [113] This study emphasized the possibility of creating fundamental randomization of all-conjugated systems with high elasticity while keeping the electronic fulfillment consistent. [111] and B) PDPP2FT-seg-2T [113] for tunable tensile modulus and photovoltaic performance.

Molecular spacer:

The introduction of spacers in the polymer main chains has shown a net improvement in the processability, self-assembly, and morphology of conjugated polymers. [243][244][245][246] The flexibility of the conjugation linker appears to be a unique way to design materials combining desirable electrical and mechanical properties. Savagatrup et al. used this strategy to improve the elasticity of diketopyrrolopyrrole (DPP) polymers with an alkyl chain. [247] They revealed that increasing the ratio of flexible conjugation spacer, improved the crack-onset and the elastic modulus. On the other hand, the carrier mobility drops significantly by the addition of spacers as previously measured and shown in Figure 2-15. [244] The fabrication of microfibers by the melt-drawing process exhibited a rise of carrier mobility and a fracture strain about 130% for these same compounds. [248] Lu et al. found that the addition of thiophene spacer in the main chain affects the structural organization and mechanical properties. [249] The polymer can be stretched up to 40% without the crack formation and can maintain hole mobility of 0.1 cm 2 V -1 s -1 . They attributed the ability to retain mobility under stretch to the edge-on packing configuration. elasticity. [244,247] Side-chain:

The side-chain bonded to the conjugated main chain affects several polymer properties such as solubility, crystallinity and, MW, among others. [250][251][252][253] These factors directly influence the electronic transport, morphology, and ductility. The distribution of the side chain can improve or, in contrary, disturb the intermolecular π-packing. In a ringlike polymer (for example, polythiophene), the position, the length, and the spatial size of the side chain affect the solubility as well as the optical and structural organization. [216] It is well-known that the substitution may generate a steric hindrance.

For example, Cho et al. show the effect of the side-chain substitution in poly(2,7-

dihydroindeno[2,1-a]indene-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole) (PININE- DHTBT)
. [254] They substituted hydrogen atoms by alkyl chains in the structure. The steric hindrance generated by the substitution displayed a damaging effect on the carrier mobility and the photovoltaic characteristics. The position of the substitution is crucial for the solubility but also disturbs planarity, conjugation, and energy levels of the polymer. [252] In one copolymer made from a bithiophene fused with a benzene moiety (benzo[2,1-b:3,4-b′]dithiophene, (BDT) with 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT), named PBDT-DTBT, the position of the side chain also influenced electronic properties. [253] Ocheje et al. indicated that the introduction of amide-containing sidechains into diketopyrrolopyrrole-based polymer affected the softness significantly. [255] In the absence of a side chain, the polymer is brittle, while in its presence, the elasticity increases. Unfortunately, few data show the mechanical impacts of the side-chain positions. We can speculate that the position of the pendant group might be beneficial as long as it generates steric hindrance due to an increase in packing disorder.

More investigations have been done on the effect of the side chain geometry and the length on properties of conjugated materials. Lipomi's group has intensively investigated "molecularly stretchable" electronics by comparing and manipulating polymer chains to gain mechanical softness. [111,112,178,256,257] For example, a modification of the alkyl-side chain length of the poly(3-alkylthiophene)s (P3ATs) has been explored. [112,179] Poly(3-octylthiophene) (P3OT) was studied, which displayed higher mechanical compliance than the reference P3HT. [179] The crack-onset was pushed to 11% strain for P3OT while P3HT film starts cracking after only 4%. For proof of the stretchable concept, they transferred the P3AT on a hemispherical surface, and the photovoltaic performance was successfully achieved and measured for P3OT

but not for P3HT. [179] Although the elasticity increases, the electronic properties are not sufficiently interesting in this case. Later, the alkyl-chain length impact was investigated more closely. [112] The number of carbon atoms in the side-chain was increased from 4 to 12, specifically a series of P3ATs with A = butyl, hexyl, octyl, and dodecyl. [112] The backbone chain was kept the same, a comb-like thiophene polymer.

Figure 2-16 A highlights the effect of the elasticity on the side-chain in this study. The chain folding decreases with the length of alkyl-chain, generating a large decrease in the elastic modulus and, consequently, an increase in ductility. On the electronic side, several papers displayed that the main-chain and side-chain crystallizations are a function of alkyl side chains, which influence the charge-transport. [258][259][260] Park et al.

investigated the consequence of the alkyl length on the mobility and deduced that the hole mobility decreased with increasing alkyl chain. [260] The alkyl length also affects the absorption and energy levels. [251] Several publications followed Lipomi's investigation to show P3AT commercial viability, [181] and improving the elasticity by using random segmentation of P3ATs copolymers. [111,113,261] Complementary to these studies, the mechanical compliance of poly(3-heptylthiophene) (P3HpT) showed an improvement of the ductility and a decrease of Young's modulus compared to shorter carbon side-chain. [111] The photovoltaic characteristic of poly(3-alkylthiophene)

suggested that although P3HT is much stiffer than P3HpT, however, its electronic properties remain similar. [111] For the first time, it was shown that the mechanical deformability and the electronic properties were not completely incompatible, contrary to several reports describing the damaging effect of the side-chain length on the carrier mobility. [213,[START_REF] Moulton | [END_REF] Nevertheless, Lipomi et al. also showed a net decrease of carrier mobility in P3HpT films, indicating a deleterious effect on the electronic transport properties. [181] That is more in line to the generally understood concept that electronic properties of conjugated polymers suffer from high mechanical compliance.

P3HpT:PCBM allowed to power small electronic devices such a watch undergoing wrist bending. [257] The side-chain polymer seems to have an important benefit for the ductility to the detriment of semiconducting properties. Regarding the charge transport, several studies confirmed that the larger side chain leads to an increase of π-π stacking spacing and the decrease of mobility. [263][264][265][266] Bulky chains increase the steric hindrance and prevent interchain interdigitation, but in some cases, the latter can be optimized by the side chain length and the branching, improving the self-assembly. [264] Figure 2-16: Effect of (A) length and (B) bulkiness of side chain on the mechanical properties. [112,267] The increase of side-chain bulkiness by branching translates to a decrease of Young's modulus because the space occupied by a branched-chain is larger than the linear chain.

This fact was confirmed in Roth's library, where Young's modulus of fifty conjugated polymers was measured and listed according to the backbone chain, side-chain length, and branching. [229] They concluded that branching side chains showed a lower tensile modulus and higher elasticity compared to the linear side chain. [229] Yiu et al. examined the length of the side chain and the branching effect on the optical, structural, and electronic performances in a furan-containing polymer (PDPP2FT). [265,268] They confirmed that the length of the side chain mainly affects the lamellar spacing over the π-π stacking. [265] Decorating branched chains in PDPP2FT polymer revealed a smaller correlation length of π-π stacking than the linear chain, even if the number of carbon is lower in the branch. [265] Lipomi's group found interest in the previous study and resumed it by looking at the influence of the side chain bulkiness and flexibility on the mechanical and electronic properties. [267] They incorporated linear alkyl, branched alkyl, and linear oligo(ethylene oxide) side chains on PDPP2FT as depicted in Figure 2-16 B. As expected, the length of the oligomers exposed a negative linear regression with the elastic modulus, whereas both linear and branched carbon sidechains exhibited better electronic properties such as higher mobility than oligomer. In this work, branching with the same number of carbon atoms compared to linear chains also increased the crack-onset, in accordance to the Roth's library. They concluded that compared to the bulkiness, the flexibility of the chain is as crucial to look at, meaning that the chemical structure of the side chain is as important as the size. [267] Crosslinking:

Crosslinking strategies have recently attracted much attention in organic photovoltaics as a viable solution for long-term stability. [269][270][271] Rumer and McCulloch have reviewed different reported strategies to lock the active layer morphology by crosslinking. [271] Furthermore, the molecular network generated by crosslinking could provide opportunities to prevent mechanical instability and increase elasticity and strength. Few studies confirm this theory. To prevent chain sliding and increase the elastic deformation, Wang et al. introduced PDMS as a crosslinker into 3,6-di-2-thienylpyrrolo [3,4-c]pyrrole-1,4-dione (DPP)-based conjugated polymer. [272] They demonstrated a decrease of the elastic modulus and a significant increase of crack-onset with the decay of the relative crystallinity, as illustrated in Figure 2-17. The same group disclosed a strategy involving non-covalent crosslinking of the conducting polymers via hydrogen bonding for a stretchable and healable organic transistor. [273] In conclusion, a three-dimensional network is a promising way to increase the elastic deformation of conjugated polymers.

Up to here, we reviewed the electronic and mechanical properties of the polymeric semiconductors. In the optoelectronic device, substituents are also used to tune the electronic properties (energy level, bandgap, etc.). For instance, fluorination of the backbone affects photovoltaic efficiency. [274,275] Here, we did not focus our attention on the substitution due to a lack of reports on their mechanical properties; nevertheless, it could open up study prospects. [272] Behavior under stress:

Conducting polymer chains interact to some extent with each other, naturally creating a stacking order due to several interactions, enhancing the mobility of the charge but causes fragility of the polymer during stretching. Heeger et al. have studied the effect of chain alignment on the conductivity and the mechanical behavior in conducting polymers. [214,215,276] In polyacetylene films, Young's modulus increased linearly with the conductivity due to the improvement of chain alignment under stress. [214,215] They highlighted the strong correlation between transport, morphology, and mechanical properties. Electrical and elastic properties are often inversely beneficial. [212][213][214][215]276] P3HT has also attracted some attention regarding its mechanical and electrical properties under applied stress. O'Connor et al. showed a strong correlation between strain-alignment and charge mobility in P3HT polymer. [277,278] The strain-dependence of P3HT packing orientation revealed an anisotropic structure with an improvement of the mobility in the direction of the polymer backbone, through their investigation carried out under stress by grazing-incidence X-ray diffraction, as depicted in Figure 2-18. [277] The enhancement of charge transport and chain orientation in the stretching direction of regioregular P3HT was also investigated by Yasudal et al. [279] The stretchalignment of P3HT shows an increase of the field-effect hole mobility in the parallel direction of the stretching.

Characterization of carrier mobility with the applied strain: C) Effect of the strain on the mobility and the stretching direction with the transistor channel (parallel and perpendicular to the channel). D) Evolution of anisotropic mobility

with the strain. [277]

Stretchable photovoltaic system

Stretchable electronics materials have been highly studied, as we described in the previous section. The photovoltaic system comprises several layers that require mechanical improvement in order to induce stretchability. Extrinsically and intrinsically, methods can help to create a highly deformable structure, as described in section 2.3. We believe that efficient solar cells will most likely combine strategies. In this section, we take the opportunities that stretchable photovoltaic is a new field to look at all kinds of strategies and devices tested under stress. In the next part, we will specify the state-of-art of the stretchable solar cell by separating different kinds of solar cells (inorganic, perovskite or dye-sensitized, and organic).

Stretchable inorganic solar cell

The most common solar cells are usually made from inorganic elements such as silicon.

Although inorganic is considered as brittle, its structuration can bring an extrinsic stretchability as described in Section 2.3.1.

A few examples in the literature show stretchable inorganic solar cells using structuration. The buckling method may be used to pattern silicon nanoribbons on PDMS. [281] As a result, a wavy structure is obtained and is expected to reach at least 10 % stretching without delamination. Indeed, such nanoribbons layout has been used to create stretchable p-n Si diodes, which could undergo a strain of 15.5% without performance alteration. [119] Moreover, highly flexible amorphous silicon solar cell has been successfully obtained using a transfer printing technique and has been tested under 50% strain without degradation, as shown in Figure 2-19 A. [282] Yoon et al. give another example of a silicon solar cell using a new approach based on an array of Si micropillars embedded into the elastomeric matrix. [283] They showed the device could be stretched up to 40% while keeping a power conversion efficiency of about 3.3%.

Besides silicon, other inorganic elements demonstrate extrinsically stretchable photovoltaic devices. With interconnect-island geometry, GaInP/GaAs cells could be biaxially stretched by 60%, due to the serpentine interconnections. [284] Efficiency reached 19% when unstretched, but the characteristics under strain have not been tested.

Copper indium gallium selenide (CIGS) photovoltaic cell on glass-fiber textiles demonstrates the conversion of more than 8% and potential stretchability due to its weaving textile structure. [144] Lee et al. have shown another example of a stretchable inorganic device where single-junction GaAs interconnected microcells design achieves high stretchability without affecting the device functionality showing an efficiency as high as 12.5% staying constant over more than 500 cycles. [34] The design and performances can be depicted in Figure 2 [282] and B) GaAs based solar cells. [34] C) Stretchable methylammonium lead trihalide based perovskite solar cell. [285] D) Stretchable dye-sensitized solar cell. [129]

Stretchable hybrid metal halide perovskite solar cells

Recently, perovskite solar cells have emerged as a solution for mechanically stable solar cells. Based on wavy layouts, stretchable methylammonium lead trihalide (MAPbX3) perovskite solar cells can be prepared with power conversion efficiency (PCE) of 12%. [285] Such a device has been tested under compression up to 50% that damages the PCE by 65%, as illustrated in Figure 2-19 C. On the other hand, a significant mechanical endurance is observed under 100 compression-and-relaxed cycles at 25% with only a 30% reduction in PCE. Perovskite fiber for stretchable solar cells, which offers a potential application in E-textile, displays an energy conversion efficiency of 3.3 %, which remained stable during bending. [286] A hierarchical multilevel fiber design of perovskite covered between TiO2 on CNT fibers and P3HT/SWCNT/Ag nanowires also demonstrates 3.03% PCE and decent mechanical consistency (>1000 bending cycles). [36]

Stretchable dye-sensitized solar cell

Dye-sensitized solar cells (DSSC) in fiber-shaped, which could be used as a textile, has been described for stretchable applications. [129,145,287] Based on transparent conductive oxide, a single fiber demonstrates an energy conversion efficiency of 1.3%. Although weaving fibers fabric has been achieved, no stretching analysis has been carried out. [287] Clothes were made from fiber-like dye-sensitized solar cells leading 4.8%

efficiency. [145] Spring-like DSSC with two fiber electrodes establishes high energy conversion efficiencies over 7% that could be preserved under strain (Figure 2-19 D). [129] This type of stretchable photovoltaic textile maintains ∼90% of its initial yield after 50 stretching cycles at 20% strain.

Stretchable organic solar cell

Stretchable organic photovoltaic (OPV) cells have attracted more attention than any other kind of stretchable cells. Polymer-based semiconductors are an excellent alternative to inorganics semiconductors for this application. Here, we study mainly organic blend systems that have been tested in organic solar cells (OSCs). Several stretchable OSCs have reached a proof of concept. To improve the ductility of P3HT:PCBM, which has an elastic modulus about 2 GPa, [112] a strategy to make the structure extrinsically stretchable is to pattern it by buckling method, spring-like structure or other techniques as previously described. This approach is particularly efficient since the standard electrodes are commonly inorganic elements such as Indium Tin Oxide (ITO), Aluminum (Al), Titanium dioxide (TiO2), etc. In 2011, Lipomi et al.

demonstrated the first stretchable P3HT:PCBM based OPVs using the well-known buckling method with PEDOT:PSS as the anode and liquid inorganic eutectic galliumindium (EGaIn) as the cathode. [182] Rivaling with Lipomi's solar cell, Kaltenbrunner et al. showed a wavy structure of PEDOT:PSS/P3HT:PCBM/Ca/Al with a 3.9% PCE capable of being uniaxially compressed by up to 80%. [288] Zhang et al. applied another extrinsically stretchable technique based on spring-like fibers-shaped OPV. [130] They used P3HT:PCBM as a photoactive layer and both, TiO2/Ti and PEDOT:PSS/MWCNT, as electrodes. A textile-like structure may be complex to process, but here, stretchable fibers were integrated to make a fabric, and power conversion efficiency deviation is below 15% of initial PCE (1.23%) under a strain of 50% strain. [130] Figure 2-20 A describes the highly deformable behavior of this cell.

Switching from P3HT:PCBM to the high-efficiency donor:acceptor such as PTB7:PC71BM or PTB7-Th:PC71BM, among others, have helped go beyond 3% PCE.

With the buckling method, a PCE of 5.2% can be achieved with PEDOT:PSS/PTB7:PC71BM/EGaIn configuration. [289] OPV device displayed 80% of its original PCE after 50 cycles of stretching at 20% strain. [289] PTB7-Th:PC71BM conversion efficiency of 2.82% after 10% strain without pre-strained the elastomeric substrate. [290] Wen-Chang Chen's group used the same donor:acceptor achieving by buckling method a 5.32% PCE, which is retained after 50 cycles of stretching under a 20% strain. [291] Similarly, the same group showed a photovoltaic efficiency of 5.6%, capable of 30% compression with a 25% loss of initial PCE. [292] By controlled delamination buckling method, PCDTBT:PC71BM achieved 5.1% PCE and was compressed-and-released for 12000 times of at 20%, which lowered by 35% its efficiency. [293] With a silver mesh and buckling method, PTzNTz:PC71BM based OPV achieved a power conversion efficiency of 9.7% and robustness with 74% efficiency retention after 500 compression cycles at 37% strain. [294] Naphtho[1,2-c:5,6-c′]bis [1,2,5]thiadiazole (NTz) (PNTz4T) and PC71BM blend demonstrated a stretchability over 52% compression while the initial power conversion efficiency (7.9%) decrease by the same geometrical factor, i.e., 50%. [20] This can be seen in radiation. [33] For a small quantity of small reactive molecules (< 20%), photovoltaic performances are not changed compared to P3HT:PCBM while the ductility is largely improved, seeing in the increase of crack-onset strain and decrease of elastic moduli.

They studied the photovoltaic behavior under stress with a variation of thiol-ene concentration. It revealed highly stretchable solar cells keeping 90% of its original PCE (1%) under 20% strain, as demonstrated in Figure 2 ) matrix. [295] Flexible TQ-F:N2200 all-PSC matrix achieved a high PCE of 5.60 % and preserved up to 90 % after a few hundred bending cycles at a low curvature radius (3 mm).

While diiodooctane (DIO) is often used to improve the morphology and photovoltaic performances, [296] Li et al. showed that PTB7:PC71BM becomes softer in the presence of DIO as seen in Figure 2-20 D. [35] They also demonstrated a solar cell with 3.48% efficiency, which only dropped to 2.99% after 100 stretching cycles at 50% strain.

Figure 2-20:

Stretchable organic solar cell based on A) P3HT:PCBM using buckling method, [288] B) PNTz4T:PCBM using buckling method, [20] C) P3HT:PCBM using network of thiolene, [33] D) PTB7:PC71BM:DIO [35] and E) molecularly change of alkyl chain in P3AT:PCBM. [112] Lipomi's group investigated polymers' mechanical properties to understand its intrinsic origin. First, they examined the stiffening effect of P3HT:PCBM and DPPT-TT:PCBM, not pre-strained. [178] Intrinsically stretchable organic solar cells using EGaIn as a cathode were obtained. DPPT-TT showed better compliance under strain than P3HT, which highlights the importance of the identity of the conjugated polymer in mechanical properties. This investigation opened a new route of molecularly modified soft polymers for photovoltaic applications. As described in section 2.3.2.2, several factors affect the mechanical properties of organic semiconductors. They showed the brittleness of poly(3-alkylthiophenes) (P3AT) is correlated to alkyl side chain by improving Young's modulus with the length of the polymer. [112] More than mechanical properties, they investigated the photovoltaic characteristics and found a good deal between photovoltaic behavior and mechanical property. In Figure 2-20 E, the difference between P3HT and poly(3-dodecylthiophene) (P3DDT) under strain is showed: photovoltaic performance of P3HT suffers, a contrario P3DDT maintain its performance under 20% stretching. Based on P3AT study, they also reported P3HpT:PCBM enduring 1000 cycles at a compressive strain of 75% and maintaining more than 80% of its original PCE. [257] P3OT also seems to improve stretchability due to its high crack-onset (47%). [112] This polymer demonstrates conforming to curved surfaces, which are equivalent biaxial tensile strains of 24%. [179] Despite fair mechanical compliance, P3OT:PCBM based solar cells are converting only about 0.36% of the light into electricity. Molecularly conjugated polymers modification improves the mechanical properties by incorporating spacers into polymers. [297] Lipomi published a review recently addressing more details about the mechanical behavior of conjugated polymer for photovoltaic by manipulation of the chemical structure. [298] Replacing fullerene (PCBM) to a conjugated polymer brings mechanical ductility and power efficiency, as supported by Kim et al. [299] A systematic comparison between PBDTTTPD:PCBM and all-polymer PBDTTTPD:P(NDI2HD-T) has demonstrated the benefit of all-polymer based solar cell improving the PCE (up to 6.64%) and the crackonset (7.16%). They also verified the flexibility by bending cycling (r = 1.5 mm, 150 cycles). In the case of the extrinsic method, the stretchability could be tested under tensile or compression strain due to its initial configuration. For example, the stretchability of the wavy structure obtained by buckling method is a compression. It is commonly considered that the initial length (L0) is the length of the flat surface obtained after the deposition of the materials. The wrinkles are then created by relaxing the pre-strained elastomer. Extrinsically stretchable configuration can be tested under tensile strain in the case of mesh, textile, or spring-like structures.

Comparison of stretchable solar cell

On the contrary, the intrinsically stretchable devices are more difficult to be tested under compression and are mainly tested under tensile strain. Internal stretching could change the molecular assembly and morphology of the film, leading to a potential decrease in the carrier mobility and the PCE. In the reported data set, extrinsically stretchable OPVs are mostly obtained by the buckling method. They have generally higher efficiency than intrinsically deformable OPV for the same strain, as the applied strain does not alter the internal structure. The pre-mechanical characterizations, such as crack-onset, allow determining the maximum strain to apply. Intrinsically stretchable solar cells show excellent PCE retention under stress in Moreover, with the recent advance OPV, the invention of new organic donor/acceptor such as PTB7-Th:PC71BM (~10% PCE) [300,[START_REF] Wan | dithiophene-4,8-dione)[END_REF] allowed the growth of the power conversion efficiency for stretchable OPVs, as labeled in Figure 2-21 B. New conjugated polymers are also found softer. The Young's modulus of PTB7-Th:PC71BM is established between 0.6 and 1 GPa, [290] while it is shown at 2 GPa [112] for the standard P3HT:PCBM. Usually, a low elastic modulus generates a high ability resisting to crack and so, a high crack-onset. For example, P3OT:PCBM has a 0.5 GPa modulus and crack-onset of 47%, [111] while P3HT:PCBM cracks at 3% and Young's modulus is about 2 GPa. [112] Exceptions still exist. For example, besides PTB7-Th:PC71BM has a low elastic modulus, it was found that its crack-onset was relatively small (~5). [290] On the contrary, PTB7:PC71BM blend has a high Young's modulus (>5.5 GPa) and unexpected high crack-onset (100%). Those differences are mainly due to the mechanical testing tool used to determine Young's modulus. Indeed four techniques were used: stressstrain, buckling based method, nanoindentation, and PeakForce mapping, which can diverge due to the characterization difference (surface or bulk) as previously discussed.

In Table 2-1, only one donor/acceptor couple contains a non-fullerene acceptor. The benefit of the all-polymer acceptor is challenging to be discussed. Fullerene replacement would potentially combine high power conversion efficiency with high strain. In general, the differences between P3HT:PCBM and new conjugated polymers in terms of mechanical properties and electronic properties have allowed an improvement of the PCE and, on average double the applied strain in the last past five years. P3HT: :PC61BM: 20wt% thiolene 20 0.90 1.00 0.9 Intrinsic -Tensile strain 2.9 3 24 [33] P3HT:PC61BM: 30wt% thiolene 43 0.78 0.93 0.84 Intrinsic -Tensile strain 2.0 3 >50 [33] P3HpT:PC61BM 75 0.96 1.20 0.8 Extrinsic -Compression 0.07 2 4 1000 [257] P3OT:PC61BM 24 0.36 0.36 1.0 Intrinsic -Tensile strain 0.5 2 47 [179] P3DDT:PC61BM PTB7:PC71BM:DIO 50 3.20 3.50 0.9 Intrinsic -Tensile strain 5.5-9.5 4 100 100 [35] PTB7:PC71BM:DIO 100 2.14 2.90 0.7 Intrinsic -Tensile strain 5.5-9.5 4 100 [35] PCDTBT:PC71BM 20 3.32 5.10 0.65 Extrinsic -Compression 12000 [293] (PNTz4T): PC71BM 52 3.92 7.40 0.53 Extrinsic -Compression 20 [20] PNTz4T:PC71BM 37 5.14 9.70 0.53 Extrinsic -Compression 500 [294] PTB7-Th:P(NDI2HD-T) 10 2.82 3.00 0.9 Intrinsic -Tensile strain 0.8 4 >10

[290]

Conclusions and outlooks

As described previously, enhancing the stretchability can be done extrinsically by patterning the structure or intrinsically by chemical modification of conjugated polymer or addition of soft material. Intrinsically stretchable organic materials generally offer more advantages than extrinsically stretchable cells. In principle, they do not require structuration and can be directly stretched. Besides high deformability in any direction, inherent stretchable electronic materials -whether it is a conductor or a semiconductor -seem to be less complicated than the extrinsically deformable one, avoiding potential difficult multi-step fabrication. They could grant fast and readily solution-processable for large-scale and industrial applications.

Wagner et al. have indicated that "Stretching pre-formed waves, meanders, and spirals develop smaller strains in the conductor than stretching inherently stretchable materials." [42] We agree with that statement. Intrinsic elasticity could maintain its structural integrity under strain, contrary to a patterned structure. Also, a mechanical accumulative effect of structuration and natural stretchable materials might be possible.

A soft intrinsically deformable material structured, for example, by buckling-method, will benefit from its low elastic modulus and its patterned structure. Intrinsically stretchable electronic materials offer many advantages, including ease processing, low cost, and elasticity. In this regard, we focused our attention on intrinsically stretchable composites and soft polymer.

In the conductor/elastomer composite materials, the conducting material such as nanowires, CNTs, PEDOT:PSS, among others, is directly dispersed into an elastomeric matrix, which offers the benefits of ease processing and ascribing stretchability to the composite as a whole. Nonetheless, these materials may have limited application due to the competition between conductivity and stretchability. Conjugated polymer like PEDOT:PSS may be a better option with a higher conductivity at the same strain than inorganic/elastomer composite. The percolation pathway could readily be maintained under stress by long-chain polymers compared to nanoparticles or nanowires composite.

Stretchable semiconductor composites and inherent soft conjugated polymers cases are more complex and astonishing. A robust and intrinsically stretchable transistor can be obtained with P3HT/PDMS blend, which formed nano-fibrillar structure and maintained mobility under considerable strain (>100%) even at ultralow P3HT concentration (0.49%). [31] The mechanisms of this extraordinary charge transport preservation under stress remain uncertain and need further investigations.

Inherent stretchable conjugated polymers could be designed by following some rules.

We reported significant understanding points for improving the mechanical properties of a conjugated polymer. The rigidity of the backbone, the regioregularity, the side chain, and the molecular weight plays a critical role in the material softness. Designing new materials requires researchers to take into account all of these factors, based on the aimed application. In particular, the packing arrangement and degree of crystallinity are deleterious for stretchable material but vital for the charge transport. In general, the rigidity of the backbone is preferable for inter-chain interactions and electrical behavior but competes with other benefits such as the ductility and the process-ability. The presence of large side-chain induces steric hindrance and decreases the charge mobility,

CHAPTER Materials and Methods

In this chapter, we report the experimental materials and methods used through this thesis. We present materials, thin-film formation, and device fabrications succinctly, before drawing our attention to the characterization. These include ex-situ and in situ methodology.

Materials

PEDOT:PSS aqueous solution (Clevios PH1000) was purchased from Heraeus containing 1-1.3% of solid with a PEDOT to PSS weight ratio of 1:2.5. Zonyl FS-300

(

Preparation of PDMS

PDMS Sylgard 186 substrate was prepared, as described in the literature. [1] We mixed 3 g of silicone base with 0.3 g of cross-linker and 2 g of silicone solvent (commercial mixture of 65% hexamethyldisiloxane and 35% octamethyltrisiloxane from Dow Corning) to decrease the viscosity. A vacuum pump was used to remove the bubbles.

The silicon mixture was deposited on glass by bar coater at the speed of 3 mm/sec. Then it was cured at 80 °C for 30 min in the oven to produce PDMS substrates with thicknesses of ∼100 μm. For self-standing PEDOT:PSS films, we used drop-casting deposition with a rectangular mold made from a glass slide and standard tape. The mold was cleaned with isopropanol. We covered the substrate with mixed PEDOT:PSS solution and dried slowly at 70 °C for 15 min, then 110 °C for 30 min before the tensile test.

Preparation of PEDOT:PSS electrode

Synthesis of C12N3

1,12-diazidododecane (C12N3) was obtained by a reaction of 1,12-dibromododecane (DBDD). This synthesis (Figure 3-1) was realized by the method described by

Thomas. [9] Figure 3-2: Synthesis of 1,12-diazidododecane from 1,12-dibromododecane.

In a solution of 1,12-dibromododecane (2 g, 9 mmol) in DMF (15.0 mL) was added NaN3 (1.18 g, 27.3 mmol). The mixture was stirred for 16 h at 60 °C. Then, 100 mL of water and 25 mL of diethyl ether (Et2O) were added. After phase separation, the aqueous solution was extracted twice with Et2O (2x25 mL). The organic phase was washed with water (3x10 mL) and dried with magnesium sulfate. The solvent was evaporated, and the product was purified by column chromatography on silica (petroleum ether as eluent).

Acceptor:donor solution

Fabrication of OPV devices

Glass patterned with 140 nm of Indium Tin Oxide (ITO) substrates were cleaned by an ultrasonic bath with sodium dodecyl sulfate solution (SDS), deionized water, acetone, and isopropanol. UV-Ozone cleaner was used for 15 min to remove all organic residues and to improve the wettability of PEDOT:PSS. We used PEDOT:PSS AI 4083 from Heraeus Clevios as a hole transport layer (HTL). After a pre-filtration (0.25 µm disc), the PEDOT:PSS mixture was deposited by spin coating (4000 rpm, 50 s, 2000 rpm/s).

Thermal treatment was performed at 140 °C for 10 min.

Characterization

Profilometry

A KLA Tencor profilometer was used to measure the thickness of thin films. The film is etched to make a step, which is measured by a diamond stylus in contact with the surface and moving across the sample.

Resistance and conductivity

The four-points probe or the Van der Pauw method was used to determine the sheet resistance Rs. Ohm's law defines the resistance as the ratio of the applied voltage to the current flowing. It is linked to the ability of a material to oppose the passage of an electric current. Rs is the resistance of one square and expressed in Ω/□. However, the resistance does not take account of the dimension of the material. The conductivity and the resistivity are more appropriate to define the intrinsic properties relative to the charge movement and the ability to conduct electric current. The conductivity (σ) and its inverse, the resistivity (ρ) are defined by:

𝜎 = 1 𝜌 = 𝑙 𝐴 × 𝑅 (Eq. 3-1)
where l is the material length, A the cross-section, and R the resistance. The crosssectional area of a thin film is the product of the thickness (t) and the width (w).

The resistance measurement could be done by a two-points probe method. It requires only two probes that serve for both applying voltage and measuring the current, as seen in Figure 3-3A. The measure obtained is the sum of the resistances (probe resistance, contact resistance, and resistance of the sample). The measurement with the in-line four-points probe has the advantages to minimize the contact resistance compared to a standard two-points probe. [10] In this case, four probes are equidistant with a spacing of s from each other, as depicted in Figure 3-3B. The resistivity is defined in this case by:

𝜌 = 2𝜋𝑠 𝑉 𝐼 (Eq. 3-2)
Here s is the section, V the voltage and I the current. The Van der Pauw method can be used as an alternative to determining the sheet resistance and, by extension, the resistivity. [11,12] This technique requires a flat and uniform sample of any shape. Van der Pauw method requires four ohmic contacts which need to be as small as possible and placed on the boundaries of the samples as illustrated in Figure 34 ; where V34 is the voltage difference between the contacts 3 and 4, and I12 is the current injected from 1 to 2. R23,41 is defined analogously. The Van der Pauw method is defined by the relationship between R12,34 and R23,41: [12] 𝑒 (-𝜋𝑡 𝜌 𝑅 12,34 ) + 𝑒

(- 𝜋𝑡 𝜌 𝑅 23,41 ) = 1 (Eq. 3-3)
This results in the appearance of an electromagnetic force 𝐹 𝐵 = -𝑒𝐵 𝑧 𝜐 𝑥 (Lorentz force) where e is the elementary charge (1.602×10 -19 C) and υx the carrier velocity. This force generates a displacement of electrons; it appears a concentration of negative charges on one of the sides of the sample as well as a deficit of negative charges of the opposite side. This charge distribution gives rise to an electric field Ey. The latter is itself responsible for an electric force that acts on the electrons: Fy = eEy (Coulomb force). This produces a voltage called Hall voltage in the transverse y-direction can be defined as:

𝑉 𝐻 = - 𝐼 𝑥 𝐵 𝑧 𝑒𝑁𝑡 (Eq. 3-6)
Where N is the carrier density and t the thickness. The Hall voltage VH is proportional to both the electric current I, magnetic field B, and the thickness of the sample. We could introduce a coefficient RH called Hall coefficient such that:

𝑅 𝐻 = - 1 𝑒𝑁 (Eq. 3-7)
Depending on the type of charge carrier, the measured Hall voltage will be positive or negative, which makes it possible to determine the type of doping of the material. If The Hall mobility can be determined from sheet resistance measurement by: 𝜇 = -𝑉 𝐻 𝑅 𝑠 𝐼 𝑥 𝐵 𝑧 (Eq. 3-8)

The transport properties have been studied for the PEDOT:PSS electrode layers. Halleffect measurements were performed using a Lakeshore 7700 system on film using the Van der Pauw configuration. Measurements were conducted by applying a reversible sweep of 5 kG magnetic fields.

Mechanical properties

Young's modulus

Tensile test:

Young's modulus, also called elastic modulus or tensile modulus, describes the stiffness of a material. The Young modulus is deduced as:

𝐸 = 𝜎 𝜀 (Eq. 3-9)
Where σ is the stress and ε the strain. The elastic limit or yield strength is shown as point A. Further stress induces an irreversible deformation named plastic deformation when the material can no longer return to its original form. Beyond the ultimate tensile strength (point B), the material starts breaking in a small cross-sectional area, commonly named "necking." The material strain hardens under stress, but after this point, the material appears to soften, and the stress decreases. In reality, the actual stress keeps increasing, and the diminution of the cross-section area by necking is not adjusted in real-time. Additional stress leads rapidly to fracture (point C). If a material breaks in its elastic domain, it is considered fragile. On the other hand, it will be considered ductile if it can undergo large plastic deformations before its rupture.

We measured by traction on Q800 Dynamic Mechanical Analysis (DMA) from TA Instruments. The self-standing thin film (≈ 10µm thick) was clamped, and a preload force of 0.05 N was applied until 0.05% strain. We used the mode strain rate at 20%/min to failure. The tensile test has its limitations. Mainly, this method requires a thick uniform film.

Buckling method:

Young's modulus can be determined by buckling technique initially described by Stafford et al. [14] The method is based on the formation of wrinkles in a relatively rigid film on a relatively compliant substrate under compressive strain. [15] Figure 3-7 shows the wrinkles induced by the buckling technique.

Figure 3-7: Schema of buckling principle

The tensile modulus of the film (Ef) is correlated to the modulus of the substrate (Es), the thickness of the coated film (tf), the wavelength of the buckles (λb), and the Poisson's ratios of the film and substrate (νf and νs): [14] 𝐸

𝑓 = 3𝐸 𝑠 ( 1 -𝜈 𝑓 2 1 -𝜈 𝑠 2 ) ( 𝜆 𝑏 2𝜋𝑡 𝑓 ) 3 (Eq. 3-10)
This equation is valid only for: (1) low strain (<< 10%), (2) a Young's Modulus much greater for the film compared to the substrate (Ef >> Es), (3) a film significantly thinner than the substrate (tf << ts), and (4) the wavelength are higher than the amplitude of the wrinkles. This method is also limited by surface and interface adhesion, delamination, film, and substrate defects.

Incompressible elastomeric material such as polydimethylsiloxane (PDMS) has a

Poisson's ratio at 0.5. [16,17] In this study, several chemical components have been used. Their Poisson's coefficient was taken at 0.35 for polymers and blend such as PEDOT:PSS, [18] P3HT:PCBM, [16] or PCE10:PCBM in accordance with the literature.

One may note that a variation of the Poisson's ratio from 0.3 to 0.4 changes the tensile modulus less than 10%. It is considered than a Poisson's ratio 0.35 ± 5 gives an accurate tensile modulus within 5%.

Strain

The strain defines the deformation under stress (Eq(Eq. 3-11). The longitudinal strain (εl) defined by Cauchy in 1827 is:

𝜀 𝑙 = Δ𝑙 𝑙 0 = 𝑙 -𝑙 0 𝑙 0 = 𝜆 -1 (Eq. 3-11)
Where l0 and l are the lengths before and after stretching respectively, Δl is the length difference between l and l0, and λ is the stretch ratio. This definition is the most commonly used. It is also called the engineering strain. In the same way, the transversal strain (εw and εt) can be expressed as a function of width w and thickness t:

𝜀 𝑤 = Δw 𝑤 0 = 𝑤 -𝑤 0 𝑤 0 (Eq. 3-12) 𝜀 𝑡 = Δt 𝑡 0 = 𝑡 -𝑡 0 𝑡 0 (Eq. 3-13)
𝜀 𝑡 = 𝜀 𝑤 = 𝜀 𝑇 (Eq. 3-14)

The Poisson's ratio (υ) is related to the compliance of the material under an axial strain.

It is defined by the rate of relative transversal contraction (w) to relative longitudinal expansion (l):

𝜈 = - 𝜀 𝑤 𝜀 𝑙 = - 𝜀 𝑡 𝜀 𝑙 = - Δw 𝑤 0 ⁄ 𝜀 𝑙 (Eq. 3-15)
We obtain the following expression:

𝜀 𝑇 = Δw 𝑤 0 = -𝜈 × 𝜀 𝑙 (Eq. 3-16)

Crack-onset strain

We assayed the ductility of the films by measuring the crack-onset strain. A thin film is deposited on top of soft elastomer pre-treated with UV-Ozone for 15 min. The uniaxial elongation of the system deforms the material. Within the elasticity limit, the material elongates under an applied strain and recovers its initial shape and dimensions when the stress is interrupted. The stress becomes more important with higher strain, and the apparition of cracks dissipates the excess of mechanical energy. This critical strain at which the first crack appears spontaneously is called crack-onset strain (CoS).

Figure 3-8 shows the propagation of cracks observed when a material is submitted to a strain superior to the crack-onset.

Figure 3-8: Schematic of crack-onset determination

In practice, the elongation was performed using a linear stage and a DC controller from Thorlabs with a speed of 2.5 mm.s -1 . Optical microscope (Nikon eclipse LV100POL) mounted with a camera was used to observe the generation of cracks. This technique determines the toughness of material quickly. However, it remains qualitative with a certain number of limitation such as: (1) it requires a good adhesion between film and substrate, (2) it is limited by the resolution of the microscope (cracks can be submicroscopic), and (3) it is a destructive technique.

Relative resistance under strain

As the percolation pathway increases with the applied strain, the electrical features are expected to be driven by the stress. In situ characterization of resistance allows us to examined the change of the conduction properties under strain. We used the same system for CoS. Copper double-side tape maintains the position of the film-coated on PDMS and allows the contacts and interface with a Keithley source meter, as depicted in Figure 3456789.

AFM Imaging

Atomic force microscopy (AFM) from Bruker® was used to image the surface topography of a sample by analyzing point by point with a local probe scan. The images were taken in ScanAsyst mode (constant force, frequency of 2 kHz).

Ex situ and In Situ GIWAXS Characterization

Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) is a versatile technique for characterizing an object at the nanoscale structure. GIWAXS is typically used to probe the morphology and crystallinity of polymer-based thin film. [19,20] Figure 3-10 shows a schematic representation of the GIWAXS experiments and the typical scattering image. An X-ray beam with wavelength produced by synchrotron source is sent to a horizontally placed sample, at a given incident angle (αi). A total reflection of the beam occurs below a given angle named critical angle (αc), which is relative to the considered material. Depending on the incident angle and the critical angle, three probing configurations are distinguishable as:

-αi< αc(film), the penetration depth of the beam is only a few nanometers allowing the probing of near-surface structures αc(film) < αi < αc(substrate), the scattering is only caused by the entire film (surface and inner structure). This is the most common regime for organic thin film.

αi > αc(substrate), the beam penetrates deeply and scatters the overall sample from the surface of the film to the substrate. A large background scattering from the substrate is detected.

Figure 3-10: Schematic of GIWAXS experiments

The collision of the beam with the sample scatters the beam at different angles. θf and αf are the out-of-plane scattering angle and horizontal in-plane scattering angle, respectively. The two-dimensional detector collects the scattering pattern. The detector is placed typically 10 to 40 cm away from the sample for GIWAXS measurements, while 100 to 500 cm will be performed for grazing-incidence small-angle X-ray scattering (GISAXS). Figure 3-10 shows characteristic collected 2D image. From this pattern, we can determine the peak position (atomic plane distance, i.e., pi-pi and lamellar spacing for conjugated polymer), the intensity of the peak (crystallinity), and as well the molecular orientation can be determined by the shape of the pattern. In the image, qz and qxy define the scattering vectors in the out-of-plane and in-plane directions, respectively. The scattering vectors are relative to the wavelength (λ) and the scattering angle by the relationships:

𝑞 𝑧 = 4𝜋 sin ( 𝛼 𝑓 2 ) ÷ λ (Eq. 3-17) 𝑞 𝑥𝑦 = 4𝜋 sin ( 𝜃 𝑓 2 ) ÷ λ (Eq. 3-18)
Bragg's law is:

2𝑑 sin(𝜃) = nλ (Eq. 3-19)
where d is the real-space distance between atomic planes and n is an integer. Bragg's law can also be expressed as:

𝑞 = 𝑛 2𝜋 𝑑 (Eq. 3-20)
Figure 3-11 illustrates the fundamental transformations of the GIWAXS data. 2D

GIWAXS image is reshaped with GIXSGUI software. [21] The reshaping is a simple two-dimensional image transformation taking account of some geometric and intensity correction due to the flatness of the detector, the medium attenuation, and the polarization of synchrotron radiation. In the raw scattering 2D plot (Figure 3 The correlation length (CCL) can be defined by using the Scherrer equation:

𝐶𝐶𝐿 ≈ 2𝜋 𝐹𝑊𝐻𝑀 (Eq. 3-21)
Where FWHM is the full width at half maximum of pattern peak in I(q) vs. q plot.

Figure 3-12: Correlation between GIWAXS pattern and crystal orientation

In practice, in situ and ex situ 2D GIWAXS experiments were performed at D1 beamline at the Cornell High Energy Synchrotron Source. The beamline energy was 1.17 Å, and the sample detector distance was 167.5 mm. An exposure time of 5 s was used for ex situ experiments. Ex situ GIWAXS experiments were performed at variable angles ranging from angles below the critical angle to above the critical angle.

Practically, in situ experiments during spin coating were performed on a miniature spin coater with a glass substrate that was placed on the X-ray beam path. A laser was used to verify proper sample alignment and low wobble during rotation. The spin-coater and measurement depends on the photoelectric effect. UV photons emitted from a deuterium lamp are absorbed by the material resulting in the electron excitation and its ejection as a photoelectron. Figure 3

Absorption and transmission

The UV-Visible absorption and the transmission spectra were measured using a UVvis-NIR spectrophotometer (JASCO V-570). Note that Beer-Lambert law could be used to correlate the absorbance A and T at a specific wavelength (λ) by (𝜆)= -𝑙𝑜𝑔10𝑇. The values of transmittance were measured at the wavelength of 550 nm for the transparency measurement. Glass substrate was used as a reference.

Solar cell characterization

Organic solar cells were tested under AM 1.5G solar irradiation, in the nitrogen glove box, by solar simulator from Newport. Before every measurement, silicon solar cell certificated by NREL was used as a calibration to ensure the correct measurement. J-V characteristics curves were measured by a source meter (Keithley 2420) under 100 mW.cm -2 . The open-circuit potential (Voc), the photocurrent density (Jsc), the maximum power (Pmax) delivered, and the fill factor (FF) were extracted from the current-voltage curve (Figure 3-14).

Figure 3-14: typical J-V characteristics

The following relationship calculates the power conversion efficiency of the solar cell:

𝑃𝐶𝐸 = 𝐽 𝑠𝑐 𝑉 𝑜𝑐 𝐹𝐹 𝑃 𝑠 (Eq. 3-23)
With Ps, the power of the light excitation (1000 W/m²).

CHAPTER Conducting and Stretchable PEDOT:PSS Electrodes:

Role of Additives on Self-Assembly, Morphology, and Transport

Introduction

Transparent conducting electrodes (TCEs) are a critical component in many optoelectronic devices, ranging from touch screens and displays, [1] to photodetectors [2] and solar cells. [3,4] Increasingly, such devices are also more and more designed and integrated onto flexible and stretchable platforms, for instance, in the context of wearable electronic applications. The need for mechanical properties favoring flexible and stretchable devices challenges rigid TCE materials, such as indium tin oxide (ITO), currently the most commonly used TCE. In recent years, alternative TCE materials exhibiting mechanical properties more compatible with the needs of stretchable electronics have emerged. For instance, silver nanowires, [5] carbon nanotubes, [6] conjugated polymers [7] have been developed for wearable, biological, medical, and skin-like technologies. [8,9] Among these, conducting and stretchable polymers, such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), are particularly interesting because their mechanical, electrical and optical properties can be tuned with the help of additives in the ink formulation, making them compatible with low-cost solution coating and printing. Moreover, they exhibit unique emergent properties, such as mixed transport of electrons and ions, making them also suitable for bioelectronic applications. [10,11] A variety of methods have been reported to successfully increase the electrical conductivity of PEDOT:PSS, [12] including by addition to the ink of polar solvents (dimethylsulfoxide (DMSO), [13][14][15] dimethylformamide (DMF), [16] ethylene glycol (EG), [17] xylitol, [18] waterborne polyurethane (WPU) [19] and surfactants, such as Zonyl-FS 300. [13,14,20] Post-treatment of PEDOT:PSS films with salts, [21] acids, [22] or solvents [3] have also been shown to improve conductivity. Moreover, the processing conditions have also been suggested

to influence the conductivity of PEDOT:PSS. [23] Yet, despite the popularity of PEDOT:PSS and the development of myriad recipes and formulations for tuning its properties, the conductivity of this material remains a bit below that of ITO, with the highest reproducible conductivity on the order of 4380 S/cm [24] compared to ITO's 800-5000 S/cm. [25] This deficiency is not easy to address, given that very little is actually understood about the mechanism of transport in PEDOT:PSS. The role of additives on its structuration which ultimately leads to high conductivity and, in some instances, simultaneous mechanical stretchability, remains vague. The basic question of whether the additives enhance the conductivity by inducing morphological changes, by doping or due in some way to the intrinsic properties of PEDOT:PSS has not been addressed to this day.

Numerous studies have attempted to explain the enhancement of conductivity, often presenting differing and sometimes competing conclusions. [3,24,[26][27][28][29][30][31][32][33][34] Hence, a complete and coherent picture remains elusive. The proposed mechanisms include PEDOT chemical conformation changes, [27] removal of PSS, [3,30] phase separation of PEDOT and PSS, [31,32] screening effects, [16] as well as morphology and microstructure changes, such as crystallinity and crystal orientation. [26,35] In spite of all these efforts, the field lacks consensus regarding the origin of conductivity improvements. Some claim the improvement of conductivity is due to a change of the PEDOT conformation from benzoid to quinoid during EG treatment. [27] Others seem to agree that the addition of a polar solvent increases the conductivity because of the removal of PSS [3,[28][29][30] and a better phase separation between PEDOT and PSS. [31,32] A screening effect has been as well evoked by Kim et al. [16] Also, a change of morphology, [24,33] the improvement of the crystallinity [34] , and orientation of the crystalline domains [26,35] have been put forth as explanations for the enhancement of conductivity. A number of reports in the literature have also recently speculated that a core-shell structure forms in highly conducting PEDOT:PSS, wherein PEDOT constitutes the core and PSS enriches the shell. [28,36,37] Clearly, understanding the mechanisms behind the formation of highly conducting PEDOT:PSS would lead to improvements in the electrical properties by optimization of the structuration of PEDOT:PSS through the development of suitable formulations and fabrication processes.

Here, we investigate the addition of two very different additives on the self-assembly, structuration, transport, and mechanical properties of PEDOT:PSS. The polar solvent DMSO is proven to improve the conductivity, while Zonyl FS 300 is a surfactant that remains in the film, improving both the conductivity, the wettability on elastomer-type material and the stretchability of PEDOT:PSS. [13,14] Our combined in situ and ex situ analyses shed light on the distinctive impact of the two additives on the aggregation of PEDOT and PSS components in solid-state PEDOT:PSS film. The DMSO preferentially solubilizes PSS and thus enhances the aggregation of PEDOT and its phase separation from PSS. Meanwhile, Zonyl preferentially interacts with PSS, providing a pathway for phase separation and introduces order into PSS domains. Both additives induce fibril formation in the film, but their combination dramatically enhances the fibrillar characteristics, such as the length, width, and mesoscale arrangement, and promotes the formation of core-shell fibrils which form a percolated conducting network. These results show that both additives impactin their own way the microstructure, phase separation and ultimately the electrical and mechanical properties of PEDOT:PSS film. DMSO promotes phase separation by initiating PEDOT fibrils growth, while Zonyl promotes PSS phase segregation and the selfassembly of core-shell PEDOT:PSS fibrils within a plasticizer matrix, increasing the stretchability of the material. The addition of both additives causes a synergistic effect, which further improves the electrical transport and the stretchability through a selfassembly mechanism of highly percolated core-shell fibrils. Special care was taken to minimize artifacts, [38][39][40][41] , and resistances from Hall Effect and four-point probe techniques were found to be in good quantitative agreement (Figure 4-1d), indicating the reliability of the Hall Effect data. Nevertheless, an anomalous negative Hall coefficient was obtained for some of the samples known to exhibit very low conductivity (i.e., pristine and 0.1% Zonyl), despite PEDOT:PSS is a p-type material. This highlights the limitations of the Hall Effect measurement for the low conductivity samples. However, as the conductivity of samples prepared with additives increased by orders of magnitude, the data was reproducible, and its accuracy is expected to be within 5%. We find that the addition of DMSO co-solvent increases the conductivity by 4 orders of magnitude from 0.06 to 700 S.cm -1 , with most of the variation occurring for 0-5 wt% DMSO, in agreement with previous reports. [16,31] Zonyl also enhances conductivity, but its effect is more modest in comparison to DMSO, as the conductivity peaks at 67 S.cm -1 for 5 wt% Zonyl addition. [13,20] The combination of the two additives in the high conductivity regime can be seen by varying the amount of Zonyl while the concentration of DMSO is kept constant at 5 wt%. The conductivity is very slightly decreased with the addition of Zonyl to DMSO, but it remains far better than films prepared with Zonyl alone. It is important here to note that while DMSO is a co-solvent which evaporates from the film upon annealing, as will be shown later, Zonyl remains embedded in the film and represents a significant volume of the film, in some cases equal to or surpassing the amount of PEDOT:PSS. Indeed, adding 5 wt% Zonyl in a 1 wt% PEDOT:PSS solution represents ca. 80 wt% of Zonyl in the solute and, eventually, the dried film. Given the PEDOT:PSS ratio is 1:2.5, the final film is expected to be made up of a whopping 94 wt% insulators (Zonyl and PSS), assuming that solute composition in the solution does not change in the solid-state film. It is quite remarkable then that the conductivity of the films is maintained essentially unchanged with the addition of so much Zonyl and suggests that the PEDOT:PSS content must be very well percolated and possess excellent intrinsic conductivity for the conductivity of the macroscopic film to remain so high. The conductivity (𝜎) is directly related to the free carrier density (n) and the mobility () of the latter through the relationship 𝜎 = 𝑛𝑒𝜇, where e is the elementary charge. To distinguish if the enhancement of the conductivity is due to an increased availability of charge carriers or changes in their mobility, both carrier density and mobility have been extracted from Hall Effect measurements. The carrier density and carrier mobility are summarized in Figure 4-1b and c, respectively. We observe that free carrier density increased similarly by 6 orders of magnitude, from 10 16 to 10 22 cm -3, with the addition of either DMSO or Zonyl. However, the increase of carrier density is steeper with the addition of DMSO co-solvent than with Zonyl, and while it remains high for increasing concentrations of DMSO, it peaks at 5 wt% of Zonyl and decreases. The Hall mobility suffers in all cases by the addition of either additive into the PEDOT:PSS ink, decreasing by 2 to 3 orders of magnitude. With the exception of data points at the lowest conductivity, the measured mobilities and carrier densities are within the norm of what has been reported in the literature for PEDOT:PSS, including mobility in the range of 0.04 to 4 cm 2 .V -1 .s -1 for different treatments, [24,42,43] and carrier density between 10 18 and 10 22 cm -3 . [24,42] The decrease of mobility is attributed in part to the increase in the number of charge carriers which produce more scattering centers. [44] This is probably not the only factor influencing the Hall mobility, which has a steeper decreasing trend with Zonyl addition and is typically an order of magnitude lower than with DMSO addition alone, despite yielding a lower carrier density. We can attribute this to the presence of a large fraction of the insulating Zonyl in the PEDOT:PSS film, which is likely to disrupt some of the conducting pathways in the film and could additionally explain this decreasing trend.

Results

Electrical, optical and mechanical properties of PEDOT:PSS

On the whole, the drastic increase in carrier density easily overcomes the decreases in mobility, explaining the net increase of the conductivity whether DMSO, Zonyl or a mixture of the two is added. Nevertheless, it is clear that the one order of magnitude reduction of Hall mobility for Zonyl vs. DMSO, and slower increase of carrier density with Zonyl addition are the culprits for the lower conductivity achieved in Zonyl-based films. Although such a large volume fraction of insulating material is used, the ability of PEDOT:PSS to maintain its conductivity is impressive. Mixing Zonyl with DMSO leads to much higher carrier density, closer to that of DMSO only, and the mobility values remain higher than Zonyl alone for an amount superior at 3 wt%, which is also impressive. This suggests that the large amount of Zonyl present in the film does not disrupt the charge transport pathways as effectively when the DMSO co-solvent is present or that the presence of DMSO also helps enhance the percolating conduction pathway in the overall film.

We turn our attention to the study of mechanical properties for the different ink formulations described above (Figure 4-2a). We have measured the Young's modulus (tensile modulus) from the slope at origin of strain-stress curves on thick films (Figure 4-2b). The Young's modulus is found to be ca. The Young's modulus of pristine PEDOT:PSS is consistent with previous reports, which showed it to range from 900 to 2800 MPa, [45] depending upon the relative humidity during the tensile test. On the other hand, we note that our values of Young's modulus with addition of Zonyl are approximately an order of magnitude lower than those published by Lipomi. [20] We ascribe this to the measurement method. We measured the Young's modulus directly under tensile strain on thick film around 15 µm, while Lipomi deduced the values from the buckling method on thin films (150 nm).

Both methods have their advantages and limitations. The buckling method is known to be tedious and approximate, but suitable for thinner films. While stress-strain measurements provide an absolute value of Young's modulus and additional information about the mechanical characteristics of materials, but requires thicker films to be made using alternative processing approaches for the sake of measurement accuracy. The conductivity of free standing films tracks the conductivity trend of the thin films very closely, exhibiting good fidelity, albeit slightly lower absolute conductivity (Figure 4-3). The free-standing samples are therefore realistically representative of the properties of the thin films, including their mechanical properties and stiffness, as defined by the Young modulus. of an electrode to be both conductive and transparent. [46] The FoM combines both optical and electrical transport properties and can be estimated by the ratio of direct current conductivity (σdc) to optical conductivity (σop):

𝐹𝑜𝑀 = 𝜎 𝑑𝑐 𝜎 𝑜𝑝 = 𝑍 0 2𝑅 𝑠ℎ𝑒𝑒𝑡 (𝑇 -1 2 -1) (Eq. 4-1)
where Z0 is the impedance of free space (= 377 Ω), Rsheet is the sheet resistance (Ω.sq - 1 ) and T is the transmittance conventionally at λ = 550 nm. A minimum FoM of 35 [23] has been proposed as the lower limit for commercial viability of any transparent conducting electrode for various optoelectronic applications such as liquid crystal display (LCD), [47] touch screen [48] smart windows, electrochromic devices, [49] organic photovoltaic (OPV) devices, [13] light emitting diodes, [50] etc... In Figure 4-4c, we present the FoM of PEDOT:PSS films with different additives: DMSO, Zonyl and their mixture. Adding Zonyl (blue) increases the FoM from 0.01 to 13 thanks to improved conductivity and transparency of the film, but remains below the commercial viability limit. The FoM decreases at higher concentration after peaking at 5 wt%, as the conductivity drops with the likely loss of a percolating network (Figure 4 The results demonstrate that it is possible to produce high performance electrodes (FoM > 35) with broadly tailored mechanical properties and plasticity through a mixture of DMSO and Zonyl. Indeed, the implied ability to decouple optoelectronic and mechanical properties, the latter being highly tunable by an order of magnitude for 0 < [Zonyl] < 2 wt%, while the former is maintained essentially constant, is remarkable and likely beneficial technologically. It opens up tremendous opportunities to produce complex designs achieving uniform optical and electrical properties, which are crucial for device operation and aesthetics, but with different mechanical properties, which makes it possible to achieve complex mechanical functionalities by adjusting, for instance, to different mechanical properties and stretchabilities in different parts of a wearable and/or stretchable device. The plasticizing effect of Zonyl is probably due to a hidden microstructural features which increase the integrity of the films under stress, as will be revealed later in this study.

yellow). (d) Sheet resistancetransmittance (550 nm) plot for highly conducting and stretchable PEDOT:PSS films (1 wt% Zonyl and 5 wt% DMSO) and its comparison with commercial ITO

and previous reports using the same additives. [3,13] Even as stretchability is a sought after property, the same electrodes must achieve excellent TCE properties, including low sheet resistance and high transparency in the visible, which have thus far made ITO a particularly interesting material. We compare the sheet resistance-transmittance of our best stretchable TCE to commercial ITO in Figure 4-4d. We have prepared dozens of PEDOT:PSS films with different thicknesses using the optimal formulation (5 wt%/1 wt% DMSO/Zonyl) for high conductivity and low Young's modulus. The scatter plot of sheet resistance vs. visible transmittance (550 nm) in Figure 4-4d combine our data with those of commercial ITO [51] and other reports on PEDOT:PSS. [3,13] The plot shows that increasing the layer thickness lowers the transmission but also decreases the sheet resistance. The data reveal that sheet resistance < 50 /square can be achieved, but with transmission < 85%, whereas several data points show the possibility to achieve < 100 /square with transmittance between 85 and 95%.

While a lot of literature exists on the conductivity improvement of PEDOT:PSS, [12] few researchers have investigated the enhancement of ductility and transparency in parallel with the conductivity. The Young's modulus characterizes the deformation behavior of a material and by definition, is a parameter directly linked to the intrinsic mechanical properties of a material, independent of thickness. Free standing PEDOT:PSS electrodes have shown improved stretchability by addition of WPU or PDMS, yet they exhibit poor transparency. [19,52] Deformability of PEDOT:PSS is often associated to plasticizers (such as surfactants) added during its fabrication. Bao's group initialized the work with Zonyl fluorosurfactant and intensively studied flexible and stretchable TCEs. [13] In Table 4-1, we summarized the data available in the literature for soft transparent conducting electrodes and compared it to the data from this study. In particular, we focused on PH1000-based electrode studies which study transparency, resistance and Young's modulus. Our results are in accordance with published work based on Zonyl and DMSO. [13,14,20] We also found more prominent effects of Zonyl at 1 wt% than in previous work, especially in the presence of DMSO. [20] It is important to notice that although Zonyl has been described as a plasticizer, [13,14,20] it was found that a mixture with 1 wt% Zonyl and 5 wt% DMSO led to a more rigid film than pristine PEDOT:PSS. [20] Again, the main difference can be attributed to the characterization methods, as previously discussed. Recently, studies on addition of ionic liquids, such as 1-ethyl-3-methylimidazolium tetracyanoborate (EMIMTCB) [53] and bis(trifluoromethane) sulfonimine lithium salt, [54] into PH1000 solutions have emerged.

The plasticizing effect of ionic liquids induces a low Young's modulus and enhances the conductivity of PEDOT:PSS. However, their toxicity could pose a significant issue in applications such as bioelectronics or wearable electronics. N/A N/A 2.2 [56] Buckling [55,56] 0.9-2.8 [45,57] Tensile test [45,57] 300 x10 3 87 0.01 1.2 Tensile test this work PH1000 + 5 wt% DMSO 190-370 [13,55] 97 [13] 33 [13] N/A Buckling [ [13,14,20] 95-97 [13,14] 27-43 [13,14] 3.14 [20] Buckling [13,14,20 [54] roughness is associated to the large number of prominent and well-defined fibrils apparent in the AFM image of the mixed additive film in contrast to DMSO-only and Zonyl-only films and points to a synergistic self-assembly effect of the two additives.

In fact, the additives mixture appears to convert the entire film into well-defined and similarly formed fibrils, implying that all three solid components are incorporated into the fibrils, not just PEDOT and PSS. These wider three-component fibrils are presumably core-shell structured fibrils, as will be shown later, that form a highly conducting network, which is also mechanically soft and stretchable. By contrast, pure DMSO yields core-shell fibers, which are conducting but result in a rigid film, and pure Zonyl yields a soft but less conducting electrode. These observations point to a rather complex self-assembly process in the presence of additives that deserves to be investigated further. To gain more insight into the role of additives and gain important clues about the formation of the fibrils, we have performed in situ x-ray scattering measurements during the spin-coating, drying and annealing stages commonly used for the preparation of PEDOT:PSS films. To do so, we have used an experimental platform which has successfully been used to investigate the solution-to-solid phase transformation of organic semiconductors [59] and hybrid perovskite semiconductors. [60] In situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed during spin coating and thermal annealing to monitor the aggregation of PEDOT and PSS throughout the fabrication process. GIWAXS snapshots taken in situ at different stages of the coating step of the different formulations, namely pristine, DMSO, Zonyl and mixed DMSO/Zonyl additives are shown in Figure 4567. The GIWAXS snapshots reveal liquid water and liquid DMSO scattering halos centered around q = 19 and 14 nm -1 , respectively. These features weaken during spin-coating and entirely disappear after annealing, giving way instead to solid-state aggregation peaks for PEDOT and PSS. Performing a cake integration of each GIWAXS snapshot (Figure 4-7 and Figure 4-8), we obtain a plot of intensity vs q for each time stamp. Combining these together, as we have done in Figure 4-9, allows us to summarize the time-evolution of scattering features during spin-coating (left-most column) and thermal annealing (central column). Importantly, we note from in situ GIWAXS measurements performed during spin-coating that PEDOT and PSS do not aggregate in the as-cast wet film even after 300 s of spin-coating. We notice for the DMSO-containing formulation, a weak scattering feature at 18 nm -1 , which is distinct from water scattering. This may be attributed to PEDOT pre-aggregation in the presence of DMSO in the solution, as will be seen below. This provides a means of extending the duration of solvent evaporation and therefore the onset of solidification without changing the formulation or annealing conditions.

To do so, we drop-cast the PEDOT:PSS ink, let it spread over the substrate prior to placing it on the hotplate, thus preventing the excessive loss of solution by ejection and outflow [61] during spin-coating. ) from the surface of the solution corresponding to water, as the bulk solution forms a wet film that is sufficiently thin and flat. In the pristine PEDOT:PSS film case, we observe a shift of the dominant scattering feature of water towards lower q values, namely the scattering of PEDOT (q = 18 nm -1 ), indicating loss of water and aggregation/crystallization of PEDOT. After 150 s, the water appears to be fully evaporated and both PEDOT and PSS scattering features become apparent. A similar behavior is observed for Zonyl, but the PSS scattering (q = 13.1 nm -1 ) is now more strongly developed, confirmed by the ex situ observation (see below). In the presence of DMSO, liquid scattering evolves from water, which evaporates first given its lower boiling point (100 o C) and is replaced by scattering of the higher boiling point liquid DMSO (189°C) co-solvent at 14 nm -1 . Upon DMSO removal we observe a strong scattering signal associated to PEDOT while the PSS scattering is weakly developed. In Figure 4-11, we plot the dominant q value versus time showing that after 150 s, the solvents (water and DMSO) are evaporated and the scattering comes primarily from PEDOT and PSS aggregates. The kinetics of drying are affected by the amount of DMSO loading, as the latter has a much higher boiling point than water. For instance, with 10 wt% DMSO, annealing for 200 s is required to observe the solid state features of PEDOT and PSS. Indeed, we found that the time to evaporation depends on the concentration of the co-solvent, DMSO in this case. At 1 wt% DMSO, 15 s is necessary to fully evaporate the DMSO after the removal of water, while 25 s of annealing is required for 5 wt% DMSO and 50 s for 10 wt%. DMSO therefore delays the onset of crystallization of PEDOT:PSS compared to the pure water case. This is not surprising as it has been established that annealing at 50°C is required to last 450 s in order to crystalize PEDOT: PSS with EG, [26] which has a slightly higher boiling point (197°C) than DMSO. We note that PEDOT aggregation starts before PSS aggregation in the presence of DMSO, which offers an important clue about the phase separation and probable core-shell arrangements of the two components. In Figure 4-11, the kinetics of the peak position shows that PEDOT pre-aggregates 5 s earlier than PSS at 5 wt% DMSO and up to 30 s for 10 wt% DMSO. This confirms that PEDOT aggregates and crystallizes first into the fibrils seen in AFM images before PSS assembles and aggregates around it. v/v% of water with respect to the solid state film thickness. We note a change of the slope of drying after 30 s due to the temperature increase. Addition of DMSO co-solvent increased the wet thickness and changed the solvent composition drastically. We measured a wet film thickness of 9.75 m for 5 wt%, 9.91 m for 10 wt% and 9.93 m 15 wt% DMSO. These results confirm that swelling is dominated by the DMSO co-solvent and that as-cast films contain a minority of water compared to the co-solvent.

The presence of 1 wt% Zonyl also increases the initial thickness of the wet film as a result of its presence.

Upon ramping the temperature, we observe the thinning of the wet film because of drying of the solvent and co-solvent. In the pristine case, the drying process is completed within 85 s, as indicated by a transition to a steady-state thickness of the film. The rate of thinning in the presence of water is estimated to be 2 m/min after thermal annealing started. In the presence of DMSO, we observe a two-step thinning behavior with a steeper slope for short times (10 m/min) and a shallower slope (2.8 m/min for 5 wt%, 2.5 m/min for 10 wt% and 1.3 m/min for 15 wt%) for long times, indicative of water evaporation followed by DMSO evaporation, in agreement with in situ GIWAXS observations of annealing experiments on thicker films. This is also responsible for delaying the onset of solid film formation. In Figure 4-12, in situ thickness was measured at higher temperature (110°C) and for a longer period of time.

It appears that these two steps are not observed because of the fast process (less than a few seconds after annealing). The thinning behavior for the case of Zonyl differs from that of DMSO. The thinning process is complete in the same timeframe as pristine films, as water is the only solvent present. However, the film appears to be effectively thinning faster as the wet film thickness of as-cast film is greater, likely due to loading with Zonyl. The increased thickness is attributed to the non-evaporating Zonyl. When both types of additives are mixed, we observe a behavior consistent with the evaporation of two solvents of different boiling points (water and DMSO), consistent with Raoult's law of solvents. [62,63] Line-cuts of the integrated scattering intensity versus q taken from GIWAXS snapshots has been plotted in Figure 4-13 e-f to investigate, respectively, the DMSO and Zonyl effects. Both plots also include the mixed additive case for comparison. Increasing the amount of DMSO clearly results in increasing scattering intensity for the PEDOT intermolecular -stacking with no effect on PSS scattering. Similarly, the effect of Zonyl addition is to increase the intensity of the PSS diffraction feature, but also has a non-negligible effect on the PEDOT scattering, which increases slightly as well. The mixture of Zonyl and DMSO influences the scattering features of both PEDOT and PSS at the same time. Whereas Zonyl-only GIWAXS data contributes to strengthening the scattering signals of PEDOT and PSS, with the latter more prominently than the former.

We have performed further quantitative analysis by fitting a Gaussian functional to the scattering features of PEDOT and PSS. This allows extraction of the orientation, crystalline correlation length (CCL) and stacking distance, which are summarized in . From the full width at half maximum (FWHM) of the Gaussian fit, we deduce the CCL. The crystalline correlation length is given by 𝐶𝐶𝐿 = 2 𝐹𝑊𝐻𝑀 . It corresponds to the average size of crystallographically coherent regions. In addition, the relative degree of crystallinity of the PEDOT and PSS domains are deduced from I(χ) polar plots, χ being the polar angle and the area bellow the I(χ) × sin(χ) vs χ plots was obtained by fullintegration between 0 to 90°. To compare the orientation of crystalline domains, we integrate the areas of I(χ) × sin(χ) vs χ with polar angle χ ranges of 90-135° and 135-180° corresponding to the face-on and edge-on orientations, respectively. The ratio between the I(χ)×sin(χ) vs. χ integral from 90° to 135° and the one from 90° to 180° estimates the fraction of face-on oriented crystallites.

We show in Figure 4-13g that addition of 5 wt% DMSO shifts the q value of PEDOT The CCL of PEDOT domains in the solid state increased with all treatments. The CCL for PEDOT increased from 1.2 nm to 1.4 nm upon adding 5 wt% DMSO, to 1.5 nm upon adding 1 wt% Zonyl and up to 1.7 nm with the mixture of 1 wt% Zonyl and 5 wt% DMSO, highlighting the benefits of the mixture. By contrast, the CCL of PSS decreased slightly from 1.8 nm to 1.4 nm with DMSO addition, to 1.63 nm with Zonyl addition and remained constant at 1.4 nm for mixtures. The increase in the CCL and contraction of d-spacing of PEDOT upon polar solvent addition are consistent with previous reports. [26,35] The face-on stacking amount for PEDOT was found to be constant in the presence of different additives and increased slightly for PSS with the addition of Zonyl alone (from 0.38 to 0.43) or with DMSO (0.39 to 0.44). DMSO addition promotes more face-on lamellar stacking, in agreement with previous studies. [26,64] Overall, it appears that the crystalline quality of the PEDOT and PSS domains improve substantially upon mixing of the additives, indicating they become more clearly segregated from each other.

Discussion

The aim of this discussion is to propose a holistic model that captures the self-assembly of fibrillar morphology in the presence of polar solvent and surfactant additives and explains the remarkable electrical and mechanical properties simultaneously achieved in PEDOT:PSS films. Let's recall that the addition of 1 wt% Zonyl and its retention in the dry film imply that Zonyl represents nearly half of the solid-state material, and considering PSS presence, the film is nearly 85% insulating, with only 15% of conjugated PEDOT, by weight. Using the analogy of carbon nanotubes or silver nanowires, the high conductivity in such electrodes is closely related to the density and percolation pathway, as well as to the contact junction between the wires/fibers. [65,66] We note that a much higher concentration (>5 %wt) of the surfactant appears to compromise the percolation pathway. Remembering these facts, it must then be true, given the remarkable conductivity of the fibrillary PEDOT:PSS films and their simultaneous mechanical softening, that sophisticated, multiscale self-assembly is at play, resulting in a morphology which maintains long-range conducting pathways with efficient localized doping and mechanical softness.

It is important to recall that PEDOT and PSS are ionic moleculesespecially PSS, while at the same time being hydrophobic and hydrophilic, respectively. Hence, there are electrostatic and hydrophobic forces in the system which are likely to govern the local self-assembly and phase separation of this material. [67] We also recall that PEDOT is the conjugated component, while PSS is an insulating dopant. Hence, the requirement for good macroscopic electrical conductivity undoubtedly means the PEDOT:PSS blend system must form a percolating pathway of the PEDOT conjugated material with sufficient pi-stacking, while maintaining sufficient local proximity to PSS anions to achieve doping. It helps to compare and contrast this situation to other common blend systems in organic electronics and photovoltaics where van der Waals interactions are the dominant forces. [59] In such systems, phase segregation is also an important requirement in order to achieve a percolation pathway for energy and/or charge transport. For instance, blends used in an organic field-effect transistor (FET) applications are required to undergo layer stratification into bilayer or trilayer films to promote in-plane transport, as is the case for small-molecule:insulating polymer and small-molecule:semiconducting polymer blends. [68] In the case of bulk heterojunction (BHJ) solar cells, a percolating bicontinuous network is instead formed between donor (D) and acceptor (A) components with the purposes of carrying excitons to the nearest D/A interface and letting complementary charges move through complementary percolating networks in the out-of-plane direction to reach their respective electrode. [61] In BHJ systems, the requirement for intimate contact between D and A component is to form charge-transfer states and an energy landscape that provides a driving force for charge separation. [69] In this regard, the presence of additional interactions as compared to conjugated organic blends, including hydrophobic and ionic interactions between PEDOT and PSS, are likely to promote their self-assembly into hierarchical structures that allow their segregation yet locally maintain charge neutrality. A very intimate intermixing of PEDOT and PSS limits charge transport although it satisfies charge neutrality requirements, similar to the poor charge generation in D-A blends, which are quenched and unable to phase segregate. [61] This makes the thesis of core-shell particles with a PEDOT-rich core and a PSS-rich shell a very plausible one. [28] It also explains, in our view, why previous reports have associated high conductivity to the segregation of PEDOT and PSS. [3,[28][29][30] The presence of ionic and hydrophobic interactions makes a strong case for additives, which provide a means of overcoming these local constraints, either thermodynamically or kinetically. Consequently, PEDOT and PSS can self-assemble into hierarchical structures that promote both extended percolation and local doping. At the molecular scale, PEDOT must be allowed to aggregate freely and for PSS to be pushed out of its way, but remain in its immediate surrounding to allow its doping.

Achieving macroscopic conduction further requires a hierarchical network of charge transport highways to be connected, as otherwise unconnected fibrils would not participate in providing a conduction path. Moreover, to be also stretchable, the film requires a connection between each fibril through a soft matrix that does not disrupt the fibrils or block their percolating network. It appears that the combination of DMSO and Zonyl meets all these criteria and assists in producing this type of resilient and highly percolated network of fibrils. Indeed, the AFM images showed the pristine formulation did not promote fibrils formation on its own, clearly requiring the assistance of DMSO and/or Zonyl to do so. The fibrils exhibit a mesoscale order in that they are locally aligned and oriented with respect to each other, almost like a liquid crystalline assembly behavior. They are also connected to each other by some linkage points to promote interfiber charge transport and maintain the mechanical properties of the film when Zonyl is also present, although AFM does not readily reveal these. We note that addition of excessive amounts of surfactant (>5 wt%) compromises the percolation, but for smaller amounts the fibrils become more prominent in numbers, size (length and diameter) and closely connected, indicating that the Zonyl presence participates directly in fibril formation and interconnectivity.

Fibril formation and their interconnection have been seen in polymer-based solar cells, where liquid-liquid phase separation helps the formation of a polymer scaffold. [70] Additives also induce the selective aggregation of the polymer and thus enhance the phase separation by selectively solubilizing the acceptor molecule. [71] For instance, the high boiling point 1,8-octanedithiol for P3HT:PCBM was first chosen to preferably solubilize the fullerene acceptor and causes the P3HT polymer to aggregate when the main solvent (chlorobenzene) evaporates to form a network in which the fullerene is solvated. When the additive eventually evaporates, the acceptor aggregates and solidifies in the surrounding of pre-formed P3HT domains. [67] Our in situ experimental results in this study have clearly shown the selective solubility of PSS [31] and the earlier aggregation of PEDOT in the presence of DMSO. They also show that in the absence of DMSO, water surrounds PSS and PEDOT in the same way and provides the insufficient driving force for PEDOT to separate from PSS, thus hindering the formation of extended and hierarchically connected fibrillary networks. The system is thus quenched. In this way, the solubility of the components in the solvent additive plays an important role in aggregation, and whether fibrils can form and interconnect.

When DMSO is added to the system, in situ experimental data suggest that while DMSO is in the presence of a lot of water it is a co-solvent. However, by the time the film is on the hot plate and the solvent composition is in favor of DMSO due to its higher boiling point, the solution and its self-assembly become entirely dominated by DMSO. Hence, PEDOT and PSS are self-assembled in an environment where there is preferential solubility toward PSS by DMSO and less toward PEDOT. The polar cosolvent causes chain unfolding and promotes unhindered growth of PEDOT-rich grains by promoting its phase separation from PSS and providing a driving force for aggregation via selective reduced solubility. [26,31] This is a classical way of getting PEDOT to aggregate and form fibrils, while PSS will subsequently deposit around them, perhaps even forming a shell due to electrostatic interactions.

In contrast to DMSO, Zonyl is a surfactant which behaves like a self-assembly agent.

Zonyl is an amphiphilic oligomer that can interact with PEDOT and PSS by bonding with the electronegative fluorine and through hydrogen bonding. The OH groups on Zonyl and the PSS anion may also exhibit a preferential interaction. Zonyl does not preferentially solubilize in either DMSO or water, but it is solubilized by DMSO, as is PSS, while PEDOT is not a priori. This means that as water evaporates faster than DMSO and the DMSO concentration increases, DMSO solubilizes PSS and the Zonyl with which PSS interacts while PEDOT is free to aggregate. We propose a liquid-liquid phase separation between PEDOT-rich and PSS/Zonyl-rich regions, allowing PEDOT to nucleate and grow fibrils without disruption, surrounded by the solvated anionic PSS.

In situ/ex situ GIWAXS confirmed a significant PSS aggregation in the presence of Zonyl only, along with a small increase in PEDOT aggregation. The aggregation of PEDOT occurs earlier and is further enhanced when DMSO is also present, allowing PSS and Zonyl to remain soluble while PEDOT aggregates. Given the amphiphilic nature of Zonyl, it is indeed likely that hydrophobic PEDOT and hydrophilic PSS selfassemble around it, thus forming core-shell fibrils with Zonyl present at the interface.

Other amphiphilic surfactants such as Triton X have previously shown to promote nanofibrils with a PEDOT core and at the interface with a PSS shell. [72] The mechanism proposed behind the fibrous structure formation was attributed to the miscibility of the surfactant preferentially with the PSS at low concentration and PEDOT at higher concentrations. Thus, adding a surfactant promoted phase separation and larger grains in most of the case. [13,21] Despite the fact Zonyl has a low surface tension (~23 mN.m - 1 ) [73] and is used to help PEDOT:PSS wet hydrophobic surfaces, we note that the contact angles of films decreased with small amount of Zonyl (Figure 4

Conclusion

In this chapter, the mechanisms of structuration of PEDOT:PSS film with different additives have been studied. A precise correlation was highlighted between the morphology and Hall Effect measurement giving new perspectives for futures applications in the stretchable transparent conducting electrode. In situ experiments show a valuable approach towards understanding mechanisms of film formation.

PEDOT:PSS phase separation is improved by the synergetic effect of the two DMSO and Zonyl. DMSO improves the structuration and the crystallinity of PEDOT, creating a pathway for carrier transport. Zonyl additive involves twisted fibrils with thicker PEDOT core. The addition of both additives generates a synergetic effect producing large fibrils well oriented. This enhanced understanding of the crystallization dynamics should help to pull up the challenges in achieving highly stretchable TCE.

CHAPTER Stretchable transparent PEDOT:PSS-based electrodes

for efficient strain sensors and organic photovoltaics

Introduction

The physical properties of current stretchable semiconductor technologies are getting closer to meet the requirements for many electronics applications. [1][2][3][4][5][6][7][8][9] In addition to being bendable, the ability to stretch out over considerable distances promises a high conformability with soft materials, including biological ones. [10] Considering that our body is mechanically heterogeneous with soft cells and rigid bones, the strain-range varies from a few percents, e.g., for a retinal device, [11,12] to over 100 percent for joints. [13] However, most of the skin-based technologies require more than 30%

strain. [14,15] Thus, the future generation of wearable bio/electronics should combine mechanical compliance under stress with high electrical conductivity. [16][17][18][19][20] To this end, previous research has reported compliant optoelectronic devices using structuration as a promising route to build up extrinsic stretchability from patterned, [21] buckled, [22] spring-like, [23] or fishnet structures, [2] among others. On the other hand, certain materials show intrinsic elastic behavior either by a change at the molecular level [24][25][26][27] or by the addition of elastomers [28,29] or surfactants. [30][31][32] Intrinsically stretchable material offers advantages like the ease of processing over large-areas, whereas, in comparison, the application of complex structures to achieve extrinsic stretchability is more problematic to apply.

All optoelectronic devices, whether they are photodetectors, [33][34][35] displays, [36] or solar cells, [37,38] utilize a common component, a transparent conducting electrode (TCE).

Poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising TCE candidate. However, reliably enhancing its conductivity further remains despite the decades-long research effort. [16,22,[45][46][47]31,37,[39][40][41][42][43][44] Most of the studies focused on the improvement of the conductivity while the stretchability was less investigated. [48][49][50][51] Our group has recently shown the benefits of a poly(oligo ethylene glycol methacrylate) (mPEG) network within PEDOT:PSS for microactuators with an excellent electromechanical response. [52] We have also shown that the addition of Zonyl to PEDOT:PSS demonstrates the formation of fibrils improving the transport properties and the softness, previously seen in Chapter 4 .

In this work, we propose for the first time to use PEDOT:PSS and poly(oligo ethylene glycol methacrylate) network to make stretchable TCEs. We demonstrate the use of alternative additives to increase the conductivity of PEDOT:PSS while increasing its intrinsic mechanical stretchability. We achieved this through the use of: i) reactive oligoethylene glycol methacrylate (mPEG) that can be polymerized and cross-linked in situ, and ii) Zonyl FS300 (Figure 5-1). Zonyl is necessary to promote adhesion of PEDOT:PSS on the hydrophobic substrate surfaces such as polydimethylsiloxane (PDMS). [31,40] This has been associated with the amphiphilic nature of the Zonyl molecule consisting of a poly(ethylene oxide) head and a poly(tetrafluoroethylene)

tail. [39] It also has a plasticization effect on the mechanical properties of the host material(s). [31,32] On the other hand, a reactive mixture of poly(ethylene glycol) methyl ether methacrylate (PEGM) and poly(ethylene glycol) dimethacrylate (PEGDM), from here on referred as mPEG, can be introduced directly in commercially available versions of PEDOT:PSS dispersions. Introducing the water-soluble free radical initiator (Potassium persulfate, KPS), the mPEG additives yield a cross-linked network after annealing (Figure 5-1). Here, we exploit a similar PEDOT:PSS/mPEG system for application in mechanically robust and intrinsically stretchable transparent electrodes and their practical application. Analysis of the Young's modulus, crack-onset, stretching behavior, strain sensing ability, and mechanical durability provide important information on the key role of the polymer network within the PEDOT:PSS and the resulting physical properties. Lastly, the use of transparent electrode technology in stretchable organic solar cells and on-skin sensors is demonstrated.

Results and discussion

Prerequisites of the precursors' solutions

Improving the conductivity of commercial PEDOT:PSS further is necessary for application as a standalone electrode. It was previously shown that the addition of 5 percent weight (wt%) dimethyl sulfoxide (DMSO) into PEDOT:PSS dispersion improves the electrodes' electrical conductivity. [24,32,39,41,42,47,53] Motivated by this work, (𝑊 𝑎𝑑𝑑𝑖𝑡𝑖𝑣𝑒 + 𝑊 𝑃𝐸𝐷𝑂𝑇:𝑃𝑆𝑆 + 𝑊 𝐷𝑀𝑆𝑂 ) ). mPEG was prepared by combining PEGM and PEGDM (Figure 5-1), as water-soluble monomer and cross-linker, respectively, and potassium persulfate (KPS) as a free radical initiator (10:1:0.1, w/w). mPEG precursors can be polymerized via thermal curing for 30 min at 110 °C; a process step identical to that often applied for thermal annealing of pristine PEDOT:PSS layers.

We monitored the polymerization of mPEG solution in water during thermal annealing by Fourier-transform infrared spectroscopy (ATR-FTIR). Figure 5-2 shows spectra before and after annealing. The mPEG polymerization also takes place in the absence or presence of Zonyl (Figure 5-2b). Analysis of the data for mPEG in solution and after thermal annealing of the deposited layer reveals an 85% polymerization with the 15% of the monomers remaining unreacted. To overcome this problem, each film was rinsed three times with methanol, thereby purging the non-reacted monomers. The transparent electrodes were therefore grown by spin coating of the formulations on top of glass substrates and annealed at 110°C for 30 min. Resulting layers were then rinsed with methanol and annealed for 10 min at the same temperature. We find the last step to benefits the electrical conductivity of the PEDOT:PSS, as seen in Figure 5-3, in agreement with previous reports. [54]

Electrical and transport properties

Next, we studied the role of Zonyl and mPEG on the physical properties of the electrodes after rinsing and the second thermal annealing step. In the case of mPEG, the conductivity increases up to 1230 S/cm for 5 wt% mPEG before it starts reducing at higher concentrations (Figure 5-4a). Charge transport measurements show that the carrier density remains relatively constant for all Zonyl concentrations studied (Figure 5-4c), while the Hall mobility increases with the addition of mPEG up to 5 wt% (0.44 cm 2 /Vs) (Figure 5-4d). At 10 wt% mPEG, the Hall mobility falls to 0.08 cm 2 /Vs. This may be due to a positive effect on the PEDOT:PSS morphology up to a certain point where the mPEG content is non-negligible and disrupts the conduction path of PEDOT:PSS chains. We also studied the electrical properties of mixtures of mPEG and Zonyl, where the concentration of the latter was kept fixed at 1 wt%. Here one should expect that the interpenetration of PEDOT:PSS with mPEG and 1 wt% Zonyl should reduce the conductivity of the layers when compared to the PEDOT:PSS:mPEG composition; as a result of the aforementioned adverse effects of Zonyl on charge transport. This expected trend is indeed observed, as the electrical conductivity of the PEDOT:PSS:mPEG:Zonyl layer is lower than the PEDOT:PSS:mPEG formulation. However, the mixture with both additives still remains more conductive as is the case when only Zonyl is used (Figure 5-4a).

At this point, it is essential to recall that the additive weight over the total weight gives the additive content in wt%. This definition implies that the addition of two additives into PEDOT:PSS solution results in a reduction in the proportion of the conducting part (i.e., PEDOT) that is more than when only one additive is used. In

Transparency and Figure of Merit

Another important characteristic of high-performance TCEs is their optical transparency. To better understand the relationship between electrical conductivity and optical properties of the conductive layers, we calculated the Figure of Merit (FoM), a parameter describing the ability of a TCE to be both optically transparent and simultaneously electrically conductive, as mentioned in chapter 3.

Figure 5-7a shows the dependence of sheet resistance on additive content (wt%) together with its relation to the optical transmission measured at 550 nm for all films investigated. We found the resistance of 100 Ω•sq -1 at 94% T for PEDOT:PSS without any additive. The sheet resistance reduces to 72 Ω•sq -1 with 5 wt% mPEG (93.5% T), and increase to 130 Ω•sq -1 (93.2% T) for 1 wt% Zonyl. While the impact of mPEG on the sheet resistance of PEDOT:PSS has not been previously reported, Zonyl has been reported to yield results comparable to those in the present work in terms of sheet resistance and transparency measured at 550 nm. [22,32,37,39] The calculated FoM values as a function of Zonyl and mPEG concentrations are summarized in Figure 5-7b. Zonyl decreases the conductivity drastically, and the slight improvement observed in the optical transparency does not counterbalance the deleterious effect on the FoM. At 2 wt% Zonyl, the FoM reduces below 35, which is the limit of commercial viability. [55] The addition of mPEG at concentrations between 0 and 3 wt% increases the conductivity and the FoM. Interestingly, the addition of mPEG leads to a noticeable increase in the layer thickness (from about 100 nm at 0 wt% to 150 nm at 15 wt%), even when processed using the same spin-coating parameters. The latter effects lead to a reduction in the optical transmittance measured at 550 nm and a drop in the FoM for mPEG concentrations > 3 wt% despite the higher electrical conductivity of the layers.

When both mPEG and Zonyl are present in the PEDOT:PSS layer the FoM increases for mPEG content in the range of 0 to 3 wt%, due to the increase in the layers transparency, followed by a noticeable reduction at mPEG >5 wt%, due to the deterioration in the layer's electrical conductivity. Evidently, the addition of mPEG leads to high FoM when wt% is maintained in the range 0 to 12 wt%, while Zonyl results to a FoM of over 35 when its content is <1.5 wt%. For PEDOT:PSS containing both additives, the wt% of mPEG can be increased up to 8 wt% without crossing the commercial viability limit required for ITO-free devices. The aforementioned analysis helps to define the parameters window of transparent conductive PEDOT:PSS electrodes containing Zonyl and mPEG additives.

Mechanical properties

Crack-onset strain

The mechanical properties of each electrode composition has also been characterized to assess the material softness and elasticity. The ductility of a system containing of PEDOT:PSS and stretchable substrate (PDMS) was first investigated using the crackonset strain (CoS) method. [56] The latter represents the strain at which the first crack appears and reflects the ability of a material spin-coated on an elastomer to undergo stress without crack formation on its surface. The dissipation of mechanical energy caused by the stress induces cracks beyond a specific elongation. As mentioned previously, the addition of Zonyl into PEDOT:PSS is essential for the deposition on PDMS, an effect most likely attributed to the amphiphilic micellar character of Zonyl, which promotes adhesion to the elastomeric substrate. On the other hand, the unfavorable wetting properties of the PEDOT:PSS/mPEG formulations do not allow spin coating of the solution onto the hydrophobic PDMS. Even though PEDOT:PSS formulations containing mPEG seem to offer an excellent compromise between conductivity, transparency, and elasticity, their deposition on the particular elastomer is extremely challenging. Treating the PDMS surface with oxygen plasma [40] , or ultraviolet/ozone (UV/O3), does not improve the processability of the formulations.

Addition of Zonyl into PEDOT:PSS and PEDOT:PSS:mPEG formulation allows for direct solution processing onto PDMS via spin coating. The resulting conducting films were studied, with and without mPEG, before CoS measurements. In situ monitoring was performed using an optical microscope while gradually increasing the strain to determine the crack-onset.

Elastic modulus

The Young's modulus (elastic modulus) was also determined from the slope at the origin of respective stress-strain experiments on thick, freestanding films obtained by drop-casting (Figure 5-9a-c). Addition of Zonyl and mPEG into PEDOT:PSS results in an exponential decrease in the elastic modulus with increasing additive content (Figure 5-9d). Pristine PEDOT:PSS layers yield an elastic modulus value of 950 MPa, which is consistent with previous reports. [32,57] The addition of mPEG yields conducting electrodes with the lowest Young's modulus of ≈6.8 MPa (10 wt% mPEG), making this particular composition extremely elastic. As the Young's modulus decreases, the elasticity increases as does the crack-onset. The availability of the FoM (Figure 5-7b) and CoS (Figure 5-8) data allows one to select the optimal material composition for application as stretchable, transparent and conducting electrode.

Broadly speaking, a FoM value of over 35 and the highest possible CoS value are generally desirable. From this data, it becomes apparent that conductive layers containing a high wt% of additives exhibit improved stretchability but non-optimal optoelectronic properties. PEDOT:PSS electrodes containing only Zonyl can be used as ITO replacement when the additive content is maintained <1 wt%, but the stretchability of those electrodes remains limited since the CoS is only 33%. Electrodes

Electrical behavior

Under uniaxial strain

In addition, we also investigated the electrical behavior of the conductive electrodes under mechanical deformation. Below the crack-onset, the stage (i) is characterized by the slow increase of R/R0.

Uniaxial stretching of an object will affect the initial geometrical shape in all directions.

The length (l) increases by a strain (𝜀 = 𝑙-𝑙 0 𝑙 0

) while both the thickness (t) and width (w) shrink. By assuming uniaxial stretching, R/R0 change due to geometrical variation is calculated (Figure 5-10a-b) using a Poisson's ratio of 0.35 in accordance with the literature. [58] The increase of R/R0 in the presence of Zonyl or mPEG is less dramatic as compared to the expected values due to geometric change under strain below CoS.

As a result, the conductivity (Figure 5-10c-d) exhibits a net rise below CoS for both Zonyl and mPEG-containing PEDOT:PSS electrodes. Similar behaviour has been previously reported. [59][60][61] In particular, the presence of Zonyl increases the conductivity by 2.5-fold at 55% strain with an additive content of 5 wt%. A rise of conductivity to ≈500 S/cm is measured for 5 %wt mPEG at 60% strain. The contradiction with the geometrical shape deformation prediction suggests that the electrode deformation is not isotropic. Strain-induced morphological rearrangement or alignment may explain the electrical comportment below the crack-onset.

Above CoS, the evolution of R/R0 appears to depend on the additive content. For example, at 0.5 wt% Zonyl, the slope of the curve is extremely steep, while at 5 wt% Zonyl, it becomes less pronounced. In the presence of mPEG and Zonyl (Figure 5-10b), the slope get less steep with increasing mPEG content, except for 5 and 10 wt%, for which an abrupt response above the CoS is observed. To determine the cause of the resistance change, the evolution of the electrode morphology was monitored using optical microscopy (Figure 5-11 and 5-12). As already discussed, the presence of additives improves the stretchability and impedes the formation of cracks. For PEDOT:PSS electrodes with 0.5 wt% Zonyl cracks appear for ≥20% strain. As the strain is further increased, cracks grow denser and expand quickly along the perpendicular direction of the strain. The fissures are a few millimetres long at ≥40% strain and for a lowest Zonyl content of 0.5 wt% (Figure 5-11). The crack-onset is pushed to a higher strain of around 55% for 5 wt% Zonyl (lower row Figure 5-11), and the crack length is considerably reduced to <50 μm even at 120% strain. Based on these results, we conclude that Zonyl has a dual effect on the mechanical, and consequently electrical, behavior under mechanical strain. Firstly, and in accordance with previous results, increasing the Zonyl concentration delays the appearance of cracks upon increasing strain. [31] Secondly, the cracks that do eventually form are smaller at higher Furthermore, the evolution of the electrode's electrical resistance upon increasing and decreasing strain applied on the films from 0 to 120% and back to 0% (Figure 5-13), was also investigated. A large hysteresis for all electrode compositions is evident.

Despite this and the appearance of dense, large cracks, the electrical characteristics appear to recover to some extent highlighting the ability of electrical conductivity to self-heal by possible cracks colliding.

Under stretched-and-released cycling

To access the durability, conductive layers deposited on PDMS were cycled at different tensile strains using a motorized linear stage (Figure 5-14a). Based on the aforementioned results (Figure 567), Zonyl content should be limited to ≤1 wt%, whereas the mPEG should not exceed 5 wt%. We chose these two extreme compositions to get an optimum elasticity while maintaining good electrical transport and optical transparency. Waves-like structure with an amplitude of 0.3 μm are seen in the AFM images.

Moreover, Figure 5-15 compares the relative resistance parallel and perpendicular to the stretching direction, under 30% strain. The parallel resistance (as previously measured) is relatively constant over more than 150 cycles. On the other hand, the resistance rises gradually by 2-fold in the perpendicular direction during the same number of 0%-30%-0% cycles. An irreversible plastic deformation along the stretching direction is probably at the origin of the presence of waves and the resistance rises in one direction, which could be based on strain-induced chain rearrangements or sliding. The relative conductivity (σ/σ0) oscillates between 1 and 3 during each stretching cycle caused by the geometrical change. This particular composition exhibits a remarkable elastic behavior up to 60% strain with fast response, high durability, and high recoverability. Further increase of the strain to 80% leads to an irreversible increase of the resistance (Figure 5-16a). However, after 250 cycles, the relative resistance rises by only 1.7-fold due to the formation of small cracks (Figure 5-16d). This is a typical characteristic of mechanical stress-induced fatigue.

Morphology under uniaxial deformation

We also examined the morphology of the PEDOT:PSS films coated on PDMS with different compositions before and after elongation, using an AFM. shows an intimate fibrillary network at 0% strain. Previously, we have shown the mechanisms of self-assembly PEDOT:PSS:Zonyl fibrils, while a core-shell structure is present in the absence of additives. [32] Elongation at 20% strain broadens the network and orientates slightly the fibrils along the stretching direction (Figure 5-17b). Figure 5-17e shows that the height distribution decreases slightly at low strain, probably due to the special expansion of the fibril network. As the strain increases further over the crack-onset, the apparition of cracks may broaden and rise the statistical height. The nanoconfinement has shown to improve mechanical compliance of conjugated polymer in elastomeric matrix. [29,62,63] It also well-known that interface effects play a

For both PEDOT:PSS-based electrode compositions, the drop in PCE is the consequence of the reduced Fill Factor (FF) and open-circuit voltage (VOC) values.

Despite this, however, the short circuit current (JSC) remains high (>20 mA/cm 2 ) regardless of the electrodes, which reflects the good quality of the electrode/BHJ interface. When a PEDOT:PSS electrode with a relatively higher resistance is used, the The bending strain ε can be related to the ratio of the thickness of the substrate ts and the film tf, and the radius of curvature as: [65] 𝜀 = (𝑡 𝑠 + 𝑡 𝑓 ) 2𝑟

(Eq. 5-1)

Experimentally, the thickness of the substrate was about 150 μm, while the film was 150 ± 20 nm. As ts >> tf , the strain could be simplified, such as:

𝜀 ≈ 𝑡 𝑠 2𝑟 (Eq. 5-2)
The radius of curvature on the user's finger was estimated at 4 ± 1 mm, as described in Figure 5-23a, corresponding to 18.75 % ± 2% strain.

The strain can be accessed as well, by the relationship:

𝜀 = 𝐿 -𝐿 0 𝐿 0 (Eq. 5-3)
Where L0, the initial length, and L, the length after bending. Accordingly, the bending strain was calculated at 23% strain after bending. Considering the different measurement uncertainties (radius of curvature, length, thickness) due to the manual technique, the rough estimation of the strain during bending is between 15 to 25% strain. The strain could locally be higher.

Bending the finger between 15 to 25% strain (Figure 5-23b), induces an instantaneous rise of the relative resistance. The resistance returns to its initial value when the finger is straightened. The sensor with electrodes containing Zonyl shows a fast 100-fold increase of the specific resistance on the first bending and returns to its initial state when the finger straightens. A second bending doubles the electrical response and continues to double every cycle. After four bending-and-release cycles, the increase of resistance due to the bending is relatively constant about 600-fold (80). We note that the gradual increase in the first bending-and-flattening cycles may be due to either the poor human reliability to apply constant strains but more problably either to some irreversible behavior. For sensors made with electrodes containing 0.5 wt% mPEG and 1 wt% Zonyl, the electrical response upon finger bending is significantly smaller (Figure 5-23b). The relative resistance rises only by 2.5-fold after each bending, before returning to its initial value in the horizontal position of the finger. During the course of the experiment, the level of resistance variations has been recorded for twenty cycles.

Conclusion

Transparent and highly stretchable conducting PEDOT:PSS electrodes were developed using two additives, namely Zonyl and mPEG. Addition of Zonyl was found to improve the mechanical properties of the electrodes as well as their processability on elastomeric substrates but adversely affecting the electrical conductivity. On the other hand, the incorporation of mPEG precursors improves both the conductivity and elasticity of the PEDOT:PSS electrodes. Electrical measurements as a function of mechanical stress, demonstrate the ability of the electrodes to stretch up to 80% strain. To demonstrate their potential for practical applications, we fabricated ITO-free organic solar cells.

Optimized cells exhibit excellent operating characteristics, including high short circuit current (JSC = 20 mA/cm 2 ), open-circuit voltage (VOC) of 0.81 V, fill factor (FF) of 65.9

and a maximum PCE of 11.9%. Furthermore, the dependence of electrical resistance on the strain-induced morphological changes of PEDOT:PSS-based electrodes was explored to construct prototypical strain sensors that were characterized using the repeated manual as well as mechanically assisted motion. The microstructural changes were examined using AFM under different levels of strain. Based on these results, we proposed a mechanism that correlates the mechanical stretchability of these electrodes to their electrical conductivity. The PEDOT:PSS-based electrodes reported in this study could be used in a wide range of applications in the emerging field of stretchable electronics.

demonstrates to be a viable solution in organic photovoltaics for thermal and long-term stability. [11][12][13] Furthermore, the three-dimensional network caused by crosslinking could provide opportunities to prevent mechanical failure and improve the overall elasticity. The benefit of crosslinking under strain has been demonstrated as an effective method. [14][15][16] The decrease of the elastic modulus of 3,6-di-2-thienyl-pyrrolo [3,4c]pyrrole-1,4-dione (DPP)-based conjugated polymer using siloxane as a cross-linker was confirmed by Wang et al. [14] While there are many studies on the modification of the BHJ polymers chemistry to enhance the deformation, [7,[17][18][19] less work has explored the effect of the cross-linkable molecule on the elasticity of organic blend. Derue et al.

developed crosslinking molecules having two functional azides, 4,4'-bis(azidomethyl)-1,1'-biphenyl (BABP), to limit the diffusion of PC61BM and hinder its crystalization. [11,12,20] We, therefore, explore two clever strategies that allow the mechanical compliance of the active organic layer: crosslinking or blending pathways.

Here, as a first strategy, we investigate the effect of cross-linker on the elastic deformation of conjugated polymers. and PEG3N3 reagents for commercial use. [21,22] The PEGn bis-azide is particularly interesting in order to make the active layer elastic and is not patented. Indeed, PEGnN3 has a molecular weight (MW) of 1100 g/mol, compared to 244 g/mol for PEG3N3. High MW can increase the elasticity as organic polymers show a correlation between molecular weight and mechanical properties. [23][24][25] The chain entanglement enhances the elastic deformations. Nevertheless, we may note that beyond a threshold, MW shows a deleterious effect on the carrier mobility in a conjugated polymer. [26,27] Therefore, PEGnN3 seems to be an excellent candidate as its chain is reasonably long enough to improve the stretchability.

Beside crosslinking, a physical blending of elastomers has been proven to be an effective way to make a soft composite conjugated polymer. [28][29][30][31][32] Materials embedded homogeneously in a rubber matrix show sufficient mechanical strength to stretch semiconductor polymers intrinsically. For example, poly(3-hexylthiophene) (P3HT) blended with poly(dimethylsiloxane) (PDMS) leads to a stretchable material maintaining good charge mobility. [29,32,33] Similarly, P3HT:SEBS composites show high deformation. [31] However, few reports demonstrate the mechanical benefits of the blending in OPV devices. [34,35] For the first time up to date, we propose, as a second strategy in this chapter, to investigate two elastomers as a plasticizer component in active blends for OPV. Unlike bis-azide molecules, which connect two molecules of PCBM, the elastomer will not react with P3HT either PCBM. Hence, this system could perfectly be adaptable for non-fullerene organic blends. We propose to study SEBS (Styrene Ethylene Butylene Styrene Block Copolymer) and PDMS (Polydimethylsiloxane). The chemical structures are given in Figure 6-2. These strategies will be first evaluated on "classical" active layer, i.e; P3HT:PC61BM, then other acceptor:donor blends will be explore. In a first part, the reactivity of the cross-linker additives with the semiconductors blend was studied to ensure the crosslinking occurs. In a second part, we will investigate the impact of the additives on the film morphology, before to explore the mechanical properties and the photovoltaic characteristics. Finally, other bulk heteronjunctions will be tested.

Results and discussion

Preamble -Crosslinking characterization

Due to the thermodynamic instability of P3HT:PCBM active layer, the diazidoadditives were used to stabilize the structure over time by decreasing the diffusion of the PCBM [11,12,20] and here, we explore the possible improvement of the stretchability by cross-linking. The reaction of fullerene with organic diazides produces "azafulleroids." The mechanism of the reaction between diazide with PC61BM is shown in Figure 6-3. Under heat treatment, bis-azide molecules react selectively on the derivatives of fullerene by a 1,3-cycloaddition and release two molecules of dinitrogen. [36] Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and infrared spectroscopy have been carried out to prove the selective reaction of cross-linker molecules on the fullerene. presents two endothermic peaks at 247 and 280°C, corresponding to the crystallization and the melting transition, respectively. [37] P3HT exhibits a Tg between 100 and 140°C and a melting point at 220°C.

For PEG3N3 cross-linker, the TGA exhibits a rapid weight loss after 150°C (Figure 6-4b). The decomposition of azide function into dinitrogen is not seen; the decomposition of the entire molecule hide it. For PEGnN3, the thermal analysis reveals a weight loss of 6.5% between 140 and 180°C, revealing the azide decomposition.

Firstly, the molecule decomposes theoretically, by liberating four nitrogen atoms from the two azide groups, corresponding to an expected 5% weight loss (4*MN/Mn = 56/1100). Then a second weight loss is observed from 250 to 330°C, indicating the decomposition of the molecules. DSC was performed at several thermal ranges: from -90°C to 250°C (Figure 6-4c), and from room temperature to 160°C (Figure 6-4d) to determine the thermal properties accurately. Below 160°C, PEG3N3 exhibits an exothermic peak onset at 132°C, corresponding to the decomposition of the azide function. PEGnN3 displays a transition peak that starts at 29°C, which the melting of the molecule, while the decomposition of the bis-azide is found at the onset of 143°C.

Thermal annealing up to 200°C reveals the decomposition of both components above 170°C, in agreement with TGA. Kinetics of reaction the bis-azide function can be detected by attenuated totalreflectance Fourier transform infrared (ATR-FTIR) spectroscopy (Figure 6-6). The infrared spectra of each precursor are presented in Figure 6-6a. The peak at 2090 cm -1 is attributed to azide functions, for both cross-linkers: C12N3, PEG3N3, and PEGnN3.

PCBM shows several FTIR C=C characteristics peaks at 1464 and 526 cm -1 , due to the C60 cage, and 1730 cm -1 corresponding to the C=O bond of the butyric acid methyl ester, in agreement with the literature. [38] IR spectra of P3HT:PC61BM:PEG3N3 with weight ratio (1:0.8:0.4) after thermal annealing at 150°C during 0, 1, and 5 min is shown in

Preparation of active layers with additives

In the previous section, the cross-linking process was studied. The diazide cross-linkers react selectively with PC61BM molecules. From now, the preparation of P3HT:PC61BM

(1:0.8 ratio) film involves blending fixed amounts of reactive cross-linkers or elastomers with the organic semiconductors. Solutions were mixed to achieve a target additive content in weight percent (wt%) compared to the acceptor weight. The blend solution was spin-coated and annealed at 150 °C for 15 min before testing. Chapter 3 provides more details on the method. In the next sections, the impact of the addition of cross-linkers and rubbers into the active layer on the morphology, mechanical properties along with the photovoltaic characteristics is studied.

Film Morphology in the presence of additives

Optical image

Optical microscopy allows us to understand, to some extent, the phase separation. Thus, P3HT:PCBM active layer is thermodynamically unstable. [39] The morphology changes with temperature. After 60h thermal annealing at 150 °C, PCBM molecules diffuse into the active layer and form microcrystals, as shown in Figure 6-9 (left column). This problem is well known in the literature, and ascribed to the dissemination and crystallization molecules PCBM under thermal annealing. [40,41] Crystallization could demonstrate dramatic effects on photovoltaic performances, [39] but also on mechanical properties, [14,42,43] which is a main concern of this dissertation. While the mechanical stability is the main concern of this dissertation, the prevention of PCBM crystallization remains essential to ensure elasticity. The thermal stability of films with different concentrations of additives was followed by optical microscopy under the same conditons. Without additive, after heat treatment, PCBM molecules form microscopic size crystals (about 20 μm 2 ). When C12N3 is added, the crosslinking of the PC61BM molecules by -N3 group prevents the growth of PCBM microcrystals in the photoactive layer after 60 hours at 150°C. The same effect is observed with PEG3N3. The addition of PEG3N3 stabilizes the morphology even after 60 h at 150°C (Figure 6789). In the presence of 10 wt% poly(ethylene glycol) bis-azide (PEGnN3), the morphology after aging is not stabilized. This shows that 10 wt% PEGnN3 cross-linkers does not have the desired effect. On the contrary, we observe an increase in the crystal size. Indeed, with this weight ratio of PEGnN3, there is only one azide function for about 24 molecules of PC61BM, while two other cross-linkers are in a 1:1.4 ratio of azide compared to PC61BM. Consequently, only a few of the fullerene derivatives are cross-linked with PEGnN3 and may explain the difference in cross-linker stability behavior compared to C12N3 and PEG3N3. This crosslinking prevents the crystallization of PC61BM during heat treatment at 150°C. Several reports confirm the favorable effect of crosslinking on the thermal aging. [11,31,44] The embedding of P3HT:PCBM with elastomeric SEBS slows down the diffusion of PCBM molecules as a net reduction of the crystal is seen compared to the neat P3HT:PCBM sample. This behavior can be beneficial for longterm stability.

Absorption Spectra

UV-Visible absorption has been used for measuring the optical properties of semiconductors in films. The UV-vis spectra for as-casted films of PCBM, P3HT, and P3HT:PCBM blend is shown in Figure 6-10a. PCBM shows a low optical absorbance between 400 to 800 nm and a maximum at 335 nm. P3HT exhibits a broad absorption peak centered at 550 nm with the presence of three significant vibronic polymer bands at 520, 553, and 602 nm, due to the relative good organization of the polymer chains in the film. [45,46] The effect of the additive on P3HT:PCBM film morphology was investigated on the annealed thin films. Figure 6-10b shows the absorption spectra normalized to the PC61BM maximum (335 nm). An increase of the absorbance intensity at 520 nm is observed for the cross-linked film with azide based molecules, indicating an increase in the degree of P3HT aggregation. On the contrary, the elastomer addition decreases the absorption intensity with a blue-shift of the peak at 510 nm. This indicates that the degree of P3HT aggregation decreases in the presence of elastomeric additives.

GIWAXS

Understanding the effect of the additives on the morphology is critical to ensure having good charge transport in the OPV device. X-ray incidence angle was fixed at an optimal angle of 0.12° probing P3HT:PCBM film and minimizing the signal from the substrate.

For more details, please refers to chapter 3. lamellae layer plans (100), (200), and (300), respectively. The (010) peak at q ~ 16.5 nm -1 , along qxy, is typical for P3HT π-π stacking, which has a spacing of 0.37 nm. The ring positioned at q = 14.2 nm -1 is due to scattering from disordered PCBM molecules. This is in agreement with previous studies. [47][48][49][50] The previous result revealed no change in the correlation length with cross-linkers. This effect does not take into account the volume distribution of the material. The quantification of the material blend could play a crucial role in understanding the morphology of film formation. The relative degree of crystalline aggregation (rDoA), also called the relative degree of crystallinity, is giving in Equation 6-1. [51] It is proportional to the azimuthal integration of a Bragg peak (in our case, it will concern (100) P3HT lamella peak), as seen in Figure 6-14 inset, taking account of a geometrical correction factor sin(χ).

𝑟𝐷𝑜𝐴 ∝ ∫ sin(𝜒) 𝐼(𝜒) d𝜒 𝜋 2 ⁄ 0 (Eq. 6-1)

The integration result was extracted for all compositions and normalized to material volume (density and thickness). rDoA was determined as relative to pristine P3HT:PCBM, as plotted in Figure 6-14b. The addition of cross-linkers increases the aggregation of P3HT lamella. On the contrary, the addition of elastomer decreases the additives are distinguishable here: 1) the tensile modulus of a P3HT:PCBM is drastically reduced with the concentration of additive, and 2) the increase of the chain length of the cross-linker increase the softening effect in the semiconductors blend. The addition of small molecules has a plasticizing effect, which softens rigid polymers. The Young's moduli of films decrease with elastomer blending, which agrees the incorporation of soft rubber in the film concommitantly with the decrease of relative crystallinity of P3HT aggregates, deduced by UV-visible spectra and GIWAXS. Furthermore, the Young's modulus determines the ability to resist stress, i.e., the stiffness of a material. The material softness is not automatically related to the ability to be stretched. The ductility of the material needs to be determined. For that reason, the fracture strain was extracted from a tensile test for an average of three samples. Table 6-2 shows a summary of the mechanical properties extracted. While the elastic modulus decreases exponentially in the presence of additive due to a plasticizer effect, the fracture strain does not increase as much. The fracture strain increases by ~50% for all cross-linked films. Surprisingly, PDMS does not contribute to the elasticity more Diazide components have demonstrated their ability to improve the softness and to maintain good electrical properties for OPVs. While these cross-linkers are suitable for fullerene acceptors, they require thermal curing in agreement with the TGA and DSC.

The recent progress of our group in the OPV devices shows efficiency up to 17% without thermal annealing. [1] This solar cell was made from a ternary PBDB-T-2F:Y6:PC71BM bulk-heterojunction. Recently, the chlorination or florination of conjugated polymer have demonstrated an annealing-free OPV with high crystallization and aggregation due to the intermolecular interactions between the halogens and carbon/hydrogen. [58] The morphology is optimized and ordered without thermal annealing, which simplifies the fabrication process. While some of new systems do not require thermal treatment, any strategy involving thermal annealing like diazide cross-linkers is de facto forbidden. However, the second strategy, i.e. the rubber blending, does not require any additional step for curing and appear as the ideal pathway to be explored for high efficiency molecules. This method reveals to be an excellent simple and versatile technique to create stretchability and may be suitable for other acceptor/donor blends. Our study reveals that SEBS is a great candidate to improve the mechanical properties and keeping excellent photovoltaic performance when it is blended with P3HT:PC61BM at 25 wt% or less. In the next section, we investigated high-efficiency bulk-heterojunction mixed with SEBS. weight. DIO is used to improve morphology. [60] Thick films were made by the solution drop-casting of PCE10:PC71BM:SEBS with 3% DIO on glass, annealed at 50°C for 2 h, and washed for 30 min in methanol bath to remove any residue of DIO co-solvent. While DIO induces a better morphology, [60,62] it may act as a lubricant and decreases the Young's modulus in some cases [18,63] and increase the stiffness of polymers blend in others. [64] The drying time is critical to avoid any contribution of the solvent and co-solvent on the mechanical properties. We measured the loss of mass under vacuum at 50°C, at a fixed time to make sure that the elasticity is not due to an excess of solvent in PCE10:PC71BM films.

Other bulk heterojunction blends for high power conversion

PCE13:IT-4F

With the recent development of the non-fullerene acceptors, the PCE has progressively increased by over 13%. [1,66] cell demonstrates a power conversion efficiency of over 11%, in accordance with the literature. [1] In the next section, we investigate the mechanical and electrical properties of PCE13:IT-4F bulk heterojunction in the presence of SEBS. depending on the studied BHJ. For all the acceptor:donor blends, the addition of SEBS as a ternary blend component improve the ductility of the film as suggested by a decrease of the elastic modulus over 70% at 50 wt% SEBS, while PCE remains within 60%. The introduction of SEBS is a versatile tool that can be adaptable to any BHJ system (providing that common solvent can be found) assuring good mechanical properties and a minimal loss of PCE. These results demonstrate the possibility of tailoring mechanical properties and photovoltaic performances. A concentration up to 25 wt% SEBS could be introduced into acceptor:donor blend with largely tailored softness i.e., Young's modulus. Moreover, it seems that is chemical nature of SEBS is well adapted whatever the used acceptor (fullerene or non-fullerene) and donor. Until 25 %wt SEBS, miscibility have a minor importance in PCE results. Certainly, the GIWAX analysis of PCE10:PC71BM and PCE13:IT-4F in the presence of SEBS would reveal similar results than P3HT:PC61BM. Furthermore, the crack-onset has to be studied to attest the ability to be stretched on PDMS. Beside the efficiency and the elasticity, nowdays the thermal stability remains an issue and the addition of SEBS in acceptor:donor blend could be explored as a solution to limit the losses over time.

Toward stretchable solar cells

The fabrication of P3HT:PC61BM semi-stretchable solar cell is demonstrated. The aim is to confirm the potential replacement of indium tin oxide (ITO) by a stretchable electrode coated on the elastomer. First, ITO was replaced by a highly stretchable and PCE is achieved at 1000 rpm. Increasing the velocity reveals a proportional decay of current density. For the modified electrode directly coated on glass, the best performance is recorded at 7.8% PCE for 750 rpm. ITO and modified PEDOT:PSS with 1 wt% Zonyl and 5 wt% DMSO electrode has the same velocity dependence. At least 20 % loss of efficiency is observed in PCE10:PC71BM when the electrode is switched from inorganic to an organic system. This is more than the detected loss with P3HT:PC61BM bulk heterojunction. Based on those results, the optimized thickness should be obtained below 1500 rpm, and so, only four velocities were tested on the PDMS substrate. An increase of the velocity shows an increase of the PCE with a maximum detected for 1500rpm at 4.3% PCE, leading to a 55% decay compared to the best reference cell.

Conclusion

Different strategies have been used to improve the mechanical properties of semiconductors. Both elastomer blending and crosslinking strategies demonstrate a 7. CHAPTER 7. Conclusions and recommendations

Conclusions

Stretchability is a new field where materials and devices are mechanically deformed.

The benefits of stretchable materials are numerous for future applications in e-textile, bioelectronics, electronic skin, health sensor, among others. Stretchable applications have great potential in the near-and long-term future, and in particular, the mobile and wearable applications will require a wireless power supply. In this regard, organic photovoltaic (OPV) seems to be an excellent candidate. Excellent research progress in this domain is seen by the fast improvement in power conversion efficiency from 12% to over 17% during the duration of this Ph.D. For this reason, the improvement of the stretchability of such devices needs to be easily adaptable for organic solar cells replacement. This research aimed at developing versatile methods to fabricate stretchable electrode and stretchable active layer for mobile applications such as wearable electronics and photovoltaics devices. Chapter 2 reviews the recent progress in materials for stretchable electronics and their application in photovoltaics. Given the current bibliography, the advancement in stretchable organic solar cells demonstrates that the fabrication of composite based on the addition of surfactants or elastomers represents a great strategy to enhance intrinsically the stretchability, which offers lowcost, ease of process by solution coating, and high compatibility and tunability. We Finally, the insertion of a third component into the active layer and its impacts on the optoelectronics and mechanical characteristics are characterized. The modification of the active layer based on P3HT and PCBM was achieved in two ways: a crosslinking process and an insertion of an elastomer. We tested three bis-azide cross-linkers with an adjustable carbon chain and two elastomers. This addition increases the phaseseparation between the blend and the additives, especially for high content and high molecular weight, as seen via the optical microscope images and AFM images. The lamellae stacking of P3HT is essential for good carrier transport. The stacking is reinforced after the addition of a cross-linker, but decreases with the additions of elastomers due to its poor miscibility with the blend at high content. On the other hand, the improvement of mechanical properties make these methods attractive for stretchable solar cell. Indeed, the Young's modulus decreases by 7-fold with 50 wt% SEBS (relative to the PCBM weight) and the associated fracture strain is largely increased creating a stretchable layer up to 54% strain. The photovoltaic performance of P3HT:PCBM solar cell demonstrates a slight enhancement of the PCE below 10 wt% of cross-linkers and decreases above and with elastomers. This is in agreement with GIWAXS analysis where the ordering of the semiconductor polymer decreases in presence of elastomers but tends to increase in presence of cross-linkers. Ideally, we shows excellent mechanical properties in a low content range with good photovoltaic characteristics. However, their potential applications are restricted by certain factors such as stretchability and mechanical durability, the development and the integration of other layers for the creation of a fully stretchable solar cell, the thermal and chemical stabilities over time, among other.

Indeed, several tests are envisaged to attest to the real stretchability and durability: the crack-onset strain, the yield strain, and fatigue stress testing monitored by stretchedand-released cycling at a fixed strain. Those testing can provide better insight into the applicability of such materials and their robustness.

We showed the proof of concept of the incorporation of a ternary additive. However, the hole and electron transport layers and electrodes have to be developed and optimized extensively in order to create a fully stretchable solar cell. It is possible to make a solar cell with an active layer sandwiched between two electrodes, the introduction of a hole transport layer (HTL) allows the matching of energetic levels between active material and electrode, and avoids charge recombination and leak current. We believe that the HTL could be one of the keys to a functional, efficient, and stretchable solar cell. The insertion of Zonyl into a low conducting PEDOT:PSS solution (AI 4083) could be a viable solution for stretchable HTL. Additionally, Zhou et al. have introduced as a method for altering the work function of electrode materials by using polyethyleneimine (PEI). [1] Lipomi's group has proven that PEI can be used to allow PEDOT:PSS to behave as the cathode. The change of aluminum as the cathode could be allowed by PEDOT:PSS on top of PEI or using silver nanowires. As PEDOT:PSS has been highly studied in this work, this option seems to be a great alternative. On the other hand, Ag nanowires (NW) have been studied and published in our lab. [2] They form a highly conducting, transparent top electrode. We propose that

FOREWORD

  This work was done in collaboration between Université de Cergy-Pontoise (a member of Université Paris Seine) and King Abdullah University of Science and Technology. The Laboratoire de Physico-chimie des Polymères et des Interfaces (LPPI) provided valuable knowledge and expertise on the mechanical properties of the materials used while Laboratory of Advanced Materials & Applications (LAMA) provided support with optoelectronics properties and devices fabrication. Sometimes, all you need it some acknowledgment and gratitude. Aram for his support and his guidance. I am grateful to him for the outstanding scientific contribution and input he has added and his encouragement to think out of the box during the course of my Ph.D. Thomas Anthopoulos for taking me in, while my research was already mature, the support he provided, as well as his time and his scientific involvement that improves my work. I would like to acknowledge Xavier Sallenave. He has accompanied me significantly from the start and always gave me advice, always had his doors open, read all my work and gave me his advice in a professional and friendly way. I am grateful to professor Detlef Smilgies for the training on X-ray scattering techniques and his guidance at the D1 beam-line at Cornell High Energy Synchrotron Source (CHESS).

Figure 1 - 1 : 26 Figure 2 - 1 : 33 Figure 2 - 2 :

 1126213322 Figure 1-1: Structure of an organic solar cell (a) bi-layer heterojunction type and (b) bulk heterojunction type. The donor is represented in pink and acceptor in yellow. .. 26 Figure 2-1: A) Schema of bending material and definition of the curvature radius. B) Evolution of interest in flexible and stretchable electronics per year. ......................... 33

39 Figure 2 - 5 :

 3925 Figure2-4: A) Viability of bioelectronics systems with harvesting technologies determined by power density consumption and generation.[START_REF] Connor | Stretchable Bioelectronics for Medical Devices and Systems[END_REF] B) Range of Young's modulus regarding the materials. The numbers in brackets are in Pa for soft, MPa for tough, and GPa for hard materials, respectively.[42] ..................................................... 39

44 Figure 2 - 6 : 45 Figure 2 - 7 :

 44264527 Figure2-5: Structures based on the buckling process. A) Drawing of buckling strategy principle by deposition on thin material on the soft elastomer. AFM images of a buckling-induced structure of silicon membrane on B) uniaxially and C) biaxially prestrained substrate.[120] D) SEM image of single-crystal Si ribbons on elastomeric substrate.[119] ................................................................................................................. 44

47 Figure 2 - 8 :

 4728 Figure 2-7: Patterned structure for stretchable electronics. A) Spring-like structure for dye-sensitized solar cells.[129] B-C) Stretching an open mesh geometry.[6] D) Photographic, (E) optical and F) scanning electron microscopy image of metal fractal design mesh.[26] G) Designs of interconnecting meshes.[138] H) Silicon S-shaped patterned structure.[128] I-J) stretchable electrochromic E-textile.[148] K-L) Stretchable battery made of E-textile.[153] ....................................................................................... 47

  Figure 2-8: Intrinsically stretchable composite.Resistance response under stretching of A) Metal ion implantation of a gold cluster into PDMS,[154] B) Ionic polymer-metal composites (IPMCs) made of Pt salt.[100] C) Energy response under stretching of intrinsically stretchable light-emitting electrochemical cells (LEECs) fabricated from Ru/PDMS.[166] D) Graphene film on PDMS.[117] ......................................................... 52

Figure 2 - 10 : 56 Figure 2 - 11 : 58 Figure 2 -

 21056211582 Figure 2-10: Schematic example of conjugated polymer engineering for improving the stretchability intrinsically............................................................................................. 56 Figure 2-11: Effect of the introduction of the fused rings on mobility and Young's modulus. Comparison of fused pBTTT as-cast (AC) and annealed at 180 °C (AN) with non-fused P3HT. [213] ............................................................................................ 58 Figure 2-12: Effect of the regioregularity rate on the optoelectronic and mechanical properties in conjugated P3HT polymer. [233] ............................................................... 59

59 Figure 2 - 13 :

 59213 Figure 2-12: Effect of the regioregularity rate on the optoelectronic and mechanical properties in conjugated P3HT polymer. [233] ............................................................... 59

66 Figure 2 -

 662 Figure2-16: Effect of (A) length and (B) bulkiness of side chain on the mechanical properties.[112,267] .......................................................................................................... 66

  Figure 2-19: Stretchable inorganic: A) Silicon based[282] and B) GaAs based solar cells.[34] C) Stretchable methylammonium lead trihalide based perovskite solar cell.[285] D) Stretchable dye-sensitized solar cell.[129] ................................................... 71

75 Figure 2 -

 752 Figure 2-20: Stretchable organic solar cell based on A) P3HT:PCBM using buckling method,[288] B) PNTz4T:PCBM using buckling method,[20] C) P3HT:PCBM using network of thiolene,[33] D) PTB7:PC71BM:DIO[35] and E) molecularly change of alkyl chain in P3AT:PCBM.[112] .................................................................................. 75

Figure 3 - 1 :

 31 Figure 3-1: Chemical structure of the component used in this work ...........................

Figure 3 - 2 :

 32 Figure 3-2: Synthesis of 1,12-diazidododecane from 1,12-dibromododecane. ...........

Figure 3 - 3 :

 33 Figure 3-3: Schematics of A) two-points probe and B) four-points probe for sheet resistance measurement. ............................................................................................

Figure 3 - 4 :

 34 Figure 3-4: Schematic of Van der Pauw method on A) Arbitrary shape, B) symmetrical shapes such as square or round .............................................................

Figure 3 - 5 :

 35 Figure 3-5: Schematic of the Hall effect principle .....................................................

Figure 3 - 6 :

 36 Figure 3-6: Typical stress-strain curve ......................................................................

Figure 3 - 7 :

 37 Figure 3-7: Schema of buckling principle .................................................................

Figure 3 - 8 :

 38 Figure 3-8: Schematic of crack-onset determination .................................................

Figure 3 - 9 :

 39 Figure 3-9: Apparatus used for resistance measurement under strain. ......................

Figure 3 - 10 :

 310 Figure 3-10: Schematic of GIWAXS experiments ....................................................

Figure 3 - 11 :

 311 Figure 3-11: GIWAXS data collection of (a) raw 2D GIWAXS image, reshaped as (b), and converted into (c) 1D intensity plot. The accumulation of hundreds of graphs over time gives a (d) time-resolved GIWAXS image ................................................

Figure 3 - 12 :

 312 Figure 3-12: Correlation between GIWAXS pattern and crystal orientation ............

Figure 3 - 13 :

 313 Figure 3-13: PESA technique: A) Photoelectric effect principle exposed to the band diagram of metal or semiconductor and B) typical photoelectron spectra for organic material. .....................................................................................................................

Figure 3 -

 3 Figure 3-14: typical J-V characteristics .....................................................................

Figure 4 - 1 :

 41 Figure 4-1: Effect of the DMSO (red), Zonyl (blue) and their mixture (yellow; DMSO amount is kept constant at 5 wt.% as Zonyl concentration is varied) on the electrical properties of PEDOT:PSS as a function of additives concentration in solution: (a) conductivity, (b) carrier concentration (inset: schematic illustration of Hall effect measurement configuration), (c) mobility measured by Hall effect. Inset in (c) presents Hall mobility on a logarithmic scale. d) Comparison resistance measured by the Hall effect (red) and 4-points probe (blue). .........................................................

Figure 4 - 2 :

 42 Figure 4-2: Variation of (a) Young's modulus and (b) Stress strain measurement with respect to the additive composition for Zonyl (blue), DMSO (red) and the mixture of Zonyl with 5 wt% DMSO (yellow). ..........................................................................

Figure 4 - 3 :

 43 Figure 4-3: Conductivity measurement on 15-25μm thick freestanding films. .........

Figure 4 - 4 :

 44 Figure 4-4: UV-Visible Transmittance measurement on thin film spin coated at 1000 rpm in presence of (a) Zonyl and (b) Zonyl and DMSO. (c) TCE figure of merit(FoM) with respect to the additive composition for Zonyl (blue), DMSO (red) and the mixture of Zonyl with 5 wt% DMSO (yellow). (d) Sheet resistance-transmittance (550 nm) plot for highly conducting and stretchable PEDOT:PSS films (1 wt% Zonyl and wt% DMSO) and its comparison with commercial ITO and previous reports using the same additives.[3,13] ....................................................................................................

  Figure 4-4: UV-Visible Transmittance measurement on thin film spin coated at 1000 rpm in presence of (a) Zonyl and (b) Zonyl and DMSO. (c) TCE figure of merit(FoM) with respect to the additive composition for Zonyl (blue), DMSO (red) and the mixture of Zonyl with 5 wt% DMSO (yellow). (d) Sheet resistance-transmittance (550 nm) plot for highly conducting and stretchable PEDOT:PSS films (1 wt% Zonyl and wt% DMSO) and its comparison with commercial ITO and previous reports using the same additives.[3,13] ....................................................................................................

Figure 4 - 5 :

 45 Figure 4-5: Topographic AFM images of PEDOT:PSS films prepared (a) with no additives, (b) with DMSO (5 wt%), (c) with Zonyl (1 wt%) and (d) with a mixture of 5 wt% DMSO and 1 wt% Zonyl. Dimensions 2μm x 2μm. (e) The root mean squared (RMS) roughness values as a function of the additives content (Zonyl in blue, DMSO in red and the mixture in yellow). ..............................................................................

Figure 4 - 6 :

 46 Figure 4-6: Topographic AFM images of pristine PEDOT:PSS, with DMSO, with Zonyl and for the mixture of x wt% Zonyl and 5 wt% DMSO. Dimensions 2μm x 2μm. Height distribution regarding the weight percent of the additives content. ......

Figure 4 - 7 :

 47 Figure 4-7: 2D GIWAXS images during spin coating for 0 and 300 s with respect to the DMSO (top) and Zonyl (bottom) concentration. Kapton tape was used to prevent spattering of the solution ............................................................................................

Figure 4 - 8 :

 48 Figure 4-8: GIWAXS image during annealing at 110°C for 0 and 300 s with respect to the DMSO (top) and Zonyl (bottom) concentration .................................................

Figure 4 - 9 :

 49 Figure 4-9: Time-resolved GIWAXS performed in situ during (a) spin-coating at rpm and (b) annealing at 110C of a drop-cast film. (c) In situ thickness measurement of spin-cast films during annealing from 24 to 50C. From top to bottom: PEDOT:PSS formulation with no additives, 5 wt% DMSO, 1 wt% Zonyl, and 1 wt% Zonyl/5 wt% DMSO. .................................................................................................

Figure 4 -

 4 Figure 4-10: a) In situ GIWAXS measurement during annealing at 60°C of spin coated thin film made of PEDOT:PSS pristine. Snapshot b) at t = 0 s and c) at t = 70 s. .................................................................................................................................

Figure 4 - 11 :

 411 Figure 4-11: Time-resolved GIWAXS during annealing: first row, integrated intensity from time-resolved; second row, evolution of the main peaks position between 12 to 22 nm -1 , and last row, intensity versus time(s) at 0, 1, 5 and 10 wt% DMSO respectively from left to right column .......................................................................

Figure 4 - 12 :

 412 Figure 4-12: In situ thickness at high temperature (110°C) .......................................

Figure 4 - 13 :

 413 Figure 4-13: 2D GIWAXS images for PEDOT:PSS films with (a) no additives, (b) 5 wt% DMSO, (c) 1 wt% Zonyl, and (d) 1 wt% Zonyl/5 wt% DMSO. Full integration linecuts for PEDOT:PSS thin films with (e) DMSO and (f) Zonyl in comparison with the mixture. Crystalline characteristics, including (g) displacement of center maximum of PEDOT pi-stacking scattering feature and (h) relative crystallinity of PSS domains. .............................................................................................................

Figure 4 - 14 :

 414 Figure 4-14: Contact angle measurements of water on PEDOT:PSS films in the presence of co-solvent (DMSO) and surfactant (Zonyl) ............................................

Figure 4 - 15 :

 415 Figure 4-15: Schema of additives effects and fibrils structuration of PEDOT:PSS (PH1000) films...........................................................................................................

Figure 5 - 1 :

 51 Figure 5-1: Chemical structure of the surfactant Zonyl and the compounds used to form the mPEG network. ...........................................................................................

Figure 5 - 2 :

 52 Figure 5-2: Normalized FTIR spectra and thermal polymerization of mPEG: a) in the absence and b) in the presence of Zonyl. The alkene is characterized at 1640 cm -1 and almost entirely disappeared after annealing. The peak of ester shifts from 1715 to 1720 cm -1 during thermal annealing at 110 °C. Both peaks are characteristic of PEGM and PEGDM polymerization. The ratio of the peak area gives the percent of polymerization before and after annealing. ...............................................................

Figure 5 - 3 :

 53 Figure 5-3: Effect of the methanol treatment on the conductivity of PEDOT:PSS:Zonyl (pink) and PEDOT:PSS:mPEG (blue). ......................................

Figure 5 - 4 :

 54 Figure 5-4: Effect of additive content on the electrical and transport properties of PEDOT:PSS in the presence of Zonyl, mPEG or both after treatment: a) conductivity, b) ionization potential, c) carrier density, as well as d) Hall mobility (Zonyl in pink, mPEG in blue and in yellow, the mixture of mPEG with 1 wt% Zonyl). .................

Figure 5 - 5 :

 55 Figure5-5: Theoretical PEDOT content change with the increase of additives by content (%) and the number of additives. The first additive corresponds to mPEG addition. In case of two additives, we change the additive content of only one additive (mPEG). We suppose that Zonyl was kept at 1 wt%. Without any additive, PEDOT represents almost only a third of the annealed film. The addition of an additive decrease in its proportion. With two additives, the PEDOT content is only 16%. ....

Figure 5 - 6 :

 56 Figure 5-6: Transmission spectra of PEDOT:PSS film in the presence of: a) Zonyl, b) mPEG and c) mPEG and 1 wt% Zonyl. Glass signal was subtracted........................

Figure 5 - 7 :

 57 Figure 5-7: Effect of the additive content on a) the transmittance at 550nm, b) the resistance measurement and c) the FoM. ...................................................................

Figure 5 - 8 :

 58 Figure 5-8: Evolution of the crack-onset strain (CoS) with the additive content. .....

Figure 5 - 9 :

 59 Figure 5-9: Stress-strain characteristic curves of PEDOT:PSS microfilm in the presence of a) Zonyl, b) mPEG,and c) mPEG and 1wt% Zonyl. d)Young's moduli. Note that 5 wt% Zonyl gives Young's modulus as low as 32 MPa, and this concentration is the limit where self-supported films could be manipulated. At higher concentration, Zonyl/PEDOT:PSS films were extremely viscous and not tractable.

Figure 5 - 10 :

 510 Figure 5-10: Strain-dependence of electrical response: relative resistance of PEDOT:PSS film mixed with (a) Zonyl and (b) mPEG in the presence of 1 wt% Zonyl (b). The black dot line shows the theoretical R/R0 for an isotropic materials with Poisson's ratio of 0.35. The corresponding relative conductivity is plotted in c) for Zonyl and d) for mPEG/Zonyl. ............................................................................

Figure 5 - 11 :

 511 Figure 5-11: Optical microscope images of PEDOT:PSS thin film with a fixed amount of Zonyl (horizontally specified). The films were coated on top of PDMS and subjected to a mechanical strain as vertically indicated. ...........................................

Figure 5 - 12 : 173 Figure 5 - 13 : 174 Figure 5 - 175 Figure 5 - 15 : 176 Figure 5 - 177 Figure 5 - 17 : 178 Figure 5 - 18 : 179 Figure 5 - 19 : 180 Figure 5 - 20 : 180 Figure 5 - 21 : 183 Figure 5 - 22 : 184 Figure 5 -

 512173513174517551517651775171785181795191805201805211835221845 Figure 5-12: Optical microscope images of PEDOT:PSS thin film with a fixed amount of mPEG and 1 wt% Zonyl. The quantity of mPEG content is horizontally specified. The films were coated on top of PDMS and subjected to a mechanical strain as vertically indicated. .................................................................................................... 173 Figure 5-13: Electrical response under 0%-120%-0% strain of PEDOT:PSS film spincoated on PDMS. The additive content was varied for: a) Zonyl only, and b) mPEG in presence of 1 wt% Zonyl. .......................................................................................... 174 Figure 5-14: a) Electrical response under stretch-and-release cycles at the indicated strain measured in the motorized and automated stage of film-coated on PDMS with PEDOT:PSS containing 1 wt% Zonyl, b) corresponding optical microscope images after cycling 250 times the films at different strain as indicated on the pictures and c) AFM images for 250 cycles at 40% strain. ................................................................ 175

Figure 6 - 1 :

 61 Figure 6-1: Illustration of the strategies to improve the mechanical reliability of blend solar cells via fullerene crosslinking or elastomer blending. .....................................

Figure 6 - 2 :

 62 Figure 6-2: Chemical structures of the cross-linker molecules and elastomers. ........

Figure 6 -

 6 Figure 6-3: a) Scheme of reaction between bis-azide and PCBM molecule under heat treatment. ...................................................................................................................

Figure 6 - 4 :

 64 Figure 6-4: Thermal gravimetric analysis and DSC plots of a) P3HT and PCBM, and b-d) cross-linkers PEG3N3 and PEGnN3 at different temperatures. ...........................

Figure 6 - 5 :

 65 Figure 6-5: DSC analysis of a) PEG3N3 mixed with either P3HT (1:1) or PCBM (1:1), or P3HT:PCBM (1:1:1)..............................................................................................

Figure 6 - 6 :

 66 Figure 6-6: FTIR spectra of a) the precursors before cross-linking. b) Kinetic of crosslinking via monitoring P3HT:PC61BM:PEG3N3 after thermal annealing at 150°C. c) Temperature effect on P3HT:PC61BM:PEG3N3, for 5min. d) Kinetic of P3HT:PC61BM:C12N3 and e)P3HT:PC61BM:PEGnN3 after thermal annealing at 150°C. f) Dependence of the temperature on the crosslinking of P3HT:PC61BM:PEGnN3 during 5 min of curing. .......................................................

Figure 6 - 7 :

 67 Figure 6-7: Micrographs of P3HT, PCBM, and P3HT:PCBM in the presence of additives as indicated in the picture after annealing ..................................................

Figure 6 - 8 :

 68 Figure 6-8: AFM images of P3HT:PCBM blended with SEBS ................................

Figure 6 - 9 :

 69 Figure 6-9: Microscopic images of PCBM aggregation in P3HT:PCBM layer in the presence of additives at 10 wt% before and after aging for 60 h at 150°C................

Figure 6 -

 6 Figure 6-10: UV-vis absorbance spectra obtained of a) casted PCBM, P3HT, and P3HT:PCBM (1:0.8) blend and b) annealed P3HT:PCBM film in the presence of additives at 50 wt%. ...................................................................................................

Figure 6 -

 6 Figure 6-11: a) Two-dimensional GIWAXS image of P3HT:PCBM thin film and b) the resulting integration. ............................................................................................

Figure 6 - 12 :

 612 Figure 6-12: Effect of the additives on 2D GIWAXS diffraction pattern: a) fully integrated, b) out-of-plane integrated and c) in-plane integrated signal ....................

Figure 6 - 13 :

 613 Figure 6-13: 2D grazing incidence X-ray diffraction patterns of annealed P3HT:PCBM blend with additives ............................................................................

Figure 6 -

 6 Figure 6-14: a) Example of azimuthal integration of (100) P3HT lamella peak. b) The relative degree of aggregation to neat P3HT:PCBM of (100) peak...........................

Figure 6 -

 6 Figure 6-15: a) Average Young's modulus according to the weight percent of additives added into P3HT:PCBM blend prior to annealing. b) The elastic modulus was deducted from stress-strain curves with 50 wt% additive content. ....................

Figure 6 - 16 :

 616 Figure 6-16: Temperature dependence of a) storage modulus and b) Tan δ for the P3HT:PCBM:additive composites (1:0.8:0.4), i.e., in the presence of 50 wt% additive and c) for SEBS in bulk or mixed in P3HT:PCBM thick film. d) Summary of the glass temperature. ...............................................................................................................

Figure 6 - 17 :

 617 Figure 6-17: Photovoltaic performances of P3HT:PC61BM-based OPV in the function of the additive content. ...............................................................................................

Figure 6 - 18 :

 618 Figure 6-18: Photovoltaic curves associated with P3HT:PC61BM solar cells in the presence of additives. .................................................................................................

Figure 6 -

 6 Figure 6-19: PCE10:PC71BM (1:1.5) solar cell: a) chemical structures, b) energy band diagram, c) absorption spectra, and d) J-V characteristic curve. ...............................

Figure 6 -

 6 Figure 6-20: PCE10:PC71BM:SEBS morphology characterized by optical microscopy (top row) and atomic force microscopy (bottom row). ..............................................

Figure 6 - 21 :

 621 Figure 6-21: Absorption spectra of PCE10:PC71BM film mixed with SEBS elastomer. ....................................................................................................................................

Figure 6 -

 6 Figure 6-22: a) Mass loss of PCE10:PC71BM under vacuum at 50°C after MeOH treatment. b) Characterization stress-strain after vacuum drying at 50°C for 90 min. c) Young's moduli by stress-strain or buckling methods with different amount of SEBS (wt%/wt PC71BM). .....................................................................................................

Figure 6 - 23 :

 623 Figure 6-23: Evolution of the photovoltaic characteristics of PCE10:PC71BM blend in the function of SEBS additive content. ......................................................................

Figure 6 -

 6 Figure 6-24: a) chemical structures of PCE13 and IT-4F. b) UV-visible absorption spectra of PCE13, IT-4F, and PCE13:IT-4F (1:1) blend. c) J-V characteristics curves of PCE13:IT-4F based OPV. .....................................................................................

Figure 6 -

 6 Figure 6-25: a) Optical morphology and b) UV-visible absorption spectra of PCE13:IT-4F mixed with SEBS elastomer................................................................

Figure 6 - 26 :

 626 Figure 6-26: Determination of Young's moduli: a) stress-strain curves of PCE13:IT-4F with 0, 10, 25, and 50 wt% SEBS and b) comparison of Young's moduli by buckling method or direct measurements by stress-strain. ........................................

Figure 6 - 27 :

 627 Figure 6-27: Photovoltaic performances of PCE13:IT-4F with different amounts of SEBS elastomer. ........................................................................................................

Figure 6 - 28 :

 628 Figure 6-28: Effect of SEBS on the a) initial elastic modulus and b) initial power conversion efficiency .................................................................................................

Figure 6 -

 6 Figure 6-29: I-V characteristics of P3HT:PC61BM with the exchange of the electrode/substrate and solvent. In red, ITO; in blue, stretchable TCE on glass and in yellow, stretchable TCE on PDMS. ...........................................................................

Figure 6 - 30 :

 630 Figure 6-30: Dependence of spin coating velocity versus the power conversion efficiency (PCE) and associated I-V curves: A-B) ITO/Glass; C-D) PEDOT:PSS/Zonyl/DMSO mixture/Glass and on E-F) PEDOT:PSS/Zonyl/DMSO mixture/PDMS. The curves are measured for the first irradiation at 100 mW.cm -2 .. 226

Figure 1 - 1 :

 11 Figure 1-1: Structure of an organic solar cell (a) bi-layer heterojunction type and (b) bulk heterojunction type. The donor is represented in pink and acceptor in yellow.

  devote an up-to-date listing of the strategies elaborated to induce stretchability. Recent developed stretchable solar cells are exploited and intensively compared. Finally, several benefits from organic and inherent stretchable materials are highlighted, bringing us our research goals.Chapter 3 provides the specific methods and characterizations that are required for this type of solar cell. Introduction to mechanical, electronic, performance characterizations employed in the following chapters is outlined.Chapter 4 deals with the fabrication of stretchable transparent conducting electrodes (TCEs) by the addition of additives into PEDOT:PSS electrodes. The role of additives on self-assembly, morphology, and transport is intensively described. The addition of DMSO and Zonyl into PEDOT:PSS is considered, and their combination leads to excellent electrical, optical, and mechanical properties. In situ X-ray scattering is combined with charge transport and morphology studies to provide new insights into the intricate, hierarchical core-shell microstructure formed in highly conductive soft, transparent PEDOT:PSS electrodes in the presence of Zonyl and DMSO.Chapter 5 analyses the mechanical properties of PEDOT:PSS based TCEs in the presence of poly(ethylene glycol) (mPEG) network and Zonyl. The fabrication of a mechanically robust and intrinsically stretchable transparent electrode is shown. A small amount of additives (<1%) presents good transparency and electrical properties.For these low concentrations, mechanical sensors are made while higher quantity allows a high stretchability. A rigorous comparison between surfactant (Zonyl) and a three-dimensional poly(ethylene oxide) matrix is examined. The electrodes are characterized using the stress-strain measurement for Young's modulus determination, atomic force microscopy, Hall Effect measurement, resistance measurement, and ultraviolet visible transmission. The crosslinking additive shows, in general, a good electrical response along with a great ductility. Resistance under strain is tested to exploit the stretchability. This promising material is an excellent candidate for a wide range of applications like solar cell systems and wearable sensors. Chapter 6 discusses the potential effects of introducing a third component into the active layer on mechanical and photovoltaic properties. In this study, two kinds of tertiary components are used for the photoactive layer: either a crosslinker for fullerene-based blend or using an elastomer to improve the stability and mechanical compliance of the solar cell. For the cross-linking approach, three different diazide-type additives are tested 1,2-diazidododecane (C12N3); 1,11-diazido-3,6,9-trioxaunedecane (PEG3N3) and poly(ethylene glycol) bisazide (PEGnN3). For elastomeric composite, SEBS (Styrene Ethylene Butylene Styrene Block Copolymer) and PDMS (Polydimethylsiloxane) are characterized. The effect of each additive on the ductility and photovoltaic performance blend is studied. SEBS matrix shows the best compromise for photovoltaic devices. Three different acceptor/donor blends are characterized P3HT:PC61BM, PCE10:PC71BM, and PCE13:IT4F in the presence of SEBS.

Figure 2 - 1 :

 21 Figure 2-1: A) Schema of bending material and definition of the curvature radius. B) Evolution of interest in flexible and stretchable electronics per year.

Figure 2 - 2 :

 22 Figure 2-2: Schema of deformations. Stretchable material can be bent, shaped, twisted, or stretched by mechanical strength.

Figure 2 - 4 :

 24 Figure 2-4: A) Viability of bioelectronics systems with harvesting technologiesdetermined by power density consumption and generation.[START_REF] Connor | Stretchable Bioelectronics for Medical Devices and Systems[END_REF] B) Range of Young's modulus regarding the materials. The numbers in brackets are in Pa for soft, MPa for tough, and GPa for hard materials, respectively.[42]

Figure 2 - 5 :

 25 Figure 2-5: Structures based on the buckling process. A) Drawing of buckling strategy principle by deposition on thin material on the soft elastomer. AFM images of a buckling-induced structure of silicon membrane on B) uniaxially andC) biaxially pre-strained substrate.[120] D) SEM image of single-crystal Si ribbons on elastomeric substrate.[119]

Figure 2 - 6 :

 26 Figure 2-6: Illustration of GaAs ribbons formed at the controlled oxidation patterned surface of PDMS. (A) Schematic of process and (B) SEM images of thefinal ribbons[103,123]

Figure 2 - 9 B

 29 Figure 2-9 B gives insight into mechanical behavior. Bao's group described another

Figure 2 - 10 :

 210 Figure 2-10: Schematic example of conjugated polymer engineering for improving the stretchability intrinsically.

  's modulus of fused and non-fused thiophenes, shown in Figure 2-11. They compared regioregular P3HT and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2b]thiophene) (pBTTT) annealed or as-cast, which integrates a fused thienothiophene

Figure 2 - 13 :

 213 Figure 2-13: Determination of the molecular weight dependence of P3HT characterized by Stress-Strain curved by film-on-water (FOW) technique.[241]

Figure 2 - 14 :

 214 Figure 2-14: Copolymerization of A) poly(3-alkylthiophene),[111] and B) PDPP2FT-seg-2T[113] for tunable tensile modulus and photovoltaic performance.

Figure 2 - 15 :

 215 Figure 2-15: Effect of the spacer incorporation in DPP-x on mobility andelasticity.[244,247]

Figure 2 -

 2 Figure 2-17: A) Chemical structure of DPP-based co-polymer with cross-linkable sidechains and PDMS crosslinker. B) The network formed after crosslinking. C) Measured mobility of the non-crosslinked and crosslinked polymers. D) Effect ofthe crosslinking on the Young's modulus.[272]

Figure 2 -

 2 Figure 2-18: Stress-alignment of P3HT and effect on mobility. On the left, a schema of P3HT film: A) at rest, P3HT is randomly oriented with a preferential edge-on arrangement. B) Under stress, there is a reorientation of P3HT nanocrystals with the alignment of the backbone with the strain direction.Characterization of carrier mobility with the applied strain: C) Effect of the strain on the mobility and the stretching direction with the transistor channel (parallel and perpendicular to the channel). D) Evolution of anisotropic mobilitywith the strain.[277]

  -19 B.

Figure 2 - 19 :

 219 Figure 2-19: Stretchable inorganic: A) Silicon based[282] and B) GaAs based solar cells.[34] C) Stretchable methylammonium lead trihalide based perovskite solar cell.[285] D) Stretchable dye-sensitized solar cell.[129]

Figure 2 -

 2 Figure 2-20 B.

  -20 C. With a similar approach, Chen et al. incorporated poly(dimethylsiloxane-co-methyl phenethylsiloxane) (PDPS), into poly(6-fluoro-2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-dyl-alt-thiophene-2,5diyl) (TQ-F) : poly((N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl)-alt-5,5'-(2,2'-bithiophene)) (P(NDI2OD known as N2200

Figure 2 -

 2 Figure 2-21 A-B andTable 2-1 summarize the stretchable solar cells examined under

Figure 2 -

 2 Figure 2-21: A) Trend of power conversion efficiency for stretchable solar cells on mechanical compliance achieved by intrinsic (blue) or extrinsic (green) strategies from the literature. The distinction between the type of device has been made with symbol shape round (•) for organic, diamond (◊) for DSC, triangle (∆) for perovskite, and star (☆) for inorganic devices. B) Evolution over the years of power conversion efficiency (PCE) and of the strain (inset).

  while the regioregularity leads to an improvement of π-stacking and crystallinity for a mode of better carrier transport. Besides the competition between mechanical softness and electronic properties, the right balance could be designed to improve the elasticity of a conjugated polymer without loss of electrical mobility. The stretchable conducting polymer should have a flexible backbone, structural randomness by copolymerization, an optimal regioregularity, relatively high MW, and a crosslinked network. Basic molecular engineering is required to find the perfect combination of stretchability and electrical performances to provide optimized materials for the next generation of stretchable optoelectronic devices.The fabrication of stretchable solar cells makes it possible to meet the need for a selfpowered device in clothing, textiles, and bioelectronics. The recent development in stretchable organic electronics may increase the development of stretchable OPVs. The intermediate layers, such as the electron and hole transport layers, play an important

Figure 3 - 1 : 3 . 2

 3132 Figure 3-1: Chemical structure of the component used in this work

Chapter 4 :

 4 different quantities of Zonyl (0 to 15 wt%) and DMSO (0 or 5 wt%) were incorporated into a commercial solution of PH1000 before casting. Thus, all the concentrations given are expressed hereafter in mass fraction or mass percent (noted wt%) compared to the total weight of the solution. The mixture was stirred for 2-3 h before coating on pre-cleaned glass for optoelectronic measurements. Glass cleaning consists in consecutive ultrasonic bath withs soap, deionized water, acetone, and isopropanol, following by a 15 min UV-Ozone treatment. Afterward, the solution of PEDOT:PSS containing the additives was spin-coated at 1000 rpm for 50 s on different substrates (glass or PDMS). Finally, the coated films were annealed on a hot plate at 110°C for 30 min.Chapter 5: mPEG solution was obtained by mixing PEGM, PEGDM, and K2S2O8 with 100:10:1 weight ratio. Then, in a commercial solution of PH1000, different quantities of Zonyl, mPEG solution, and 5 wt% DMSO were incorporated. The mixture was stirring for 2-3h and was either spin-coated at 1000 rpm for 50 s[2][3][4][5][6][7][8] For these reasons and from our experience, all solutions include 5 percent weight dimethyl sulfuxide (DMSO). mPEG solution was obtained by mixing PEGM, PEGDM, and K2S2O8 with 100:10:1 weight ratio (PEGM:PEGDM:K2S2O8). Then, in a commercial solution of PH1000, different quantities of Zonyl, mPEG, and 5 wt% DMSO are incorporated into the solution. The mixture was stirring for 2-3h and was either spin-coated at 1000rpm during 50sec on glass or PDMS, or either drop cast at 50 °C on glass, depending on the following characterizations. The films were annealed on the hot plate at 110 °C for 30 min and cooled down at room temperature. The electrodes were rinsed three times with methanol and annealed at 110 °C for 10 min, before further testing. Table3-1 shows the theoretical ratio of conductive PEDOT after annealing. We consider that DMSO was entirely evaporated and does not contribute in the change of solid content. The incorporation of Zonyl or mPEG into PEDOT:PSS solution decreases mainly the conductive solid in the dry state film, supposedly that a total polymerization of mPEG precursors occurs and that the methanol rincing does not remove any component. Table 3-1: Theoretical solid contents in percent after annealing and representation of insulating part (PSS and additives) and conducting solid (PEDOT).

  P3HT:PCBM blend (1:0.8 ratio) was dissolved in chlorobenzene at 60 °C for 3 h. The final concentrations were 15 mg/mL of P3HT and 12 mg/mL of PCBM. The solution was cooled down at room temperature, and different amounts of additive were incorporated in chlorobenzene into P3HT:PCBM solution. The ratio of additive was determined in weight percent (wt%) compared to the acceptor (PCBM) content. These solutions were either spin-coated or drop-casted, depending on the aimed characterization. Table3-2 displays the solid content after annealing at 140 °C in the function of the amount of additives introduced. Table 3-2: Representative P3HT:PCBM:X solid content in percent after annealing. PCE10:PC70BM, PBDB-T-SF:IT-4F, and PBDB-T-2F:IT-4F blend solutions were prepared differently. PCE10:PC70BM (1:1.5) was dissolved in chlorobenzene in the presence of 3% vol. 1,8-diiodooctane (DIO) at 70 °C for 2 h. A serie of additives were introduced to the acceptor:donor mixture: 1,12-diazidododecane (C12N3), 1,11diazido-3,6,9-trioxaundecane (PEG3N3), poly(ethylene glycol) bisazide (PEGnN3) SEBS (Styrene Ethylene Butylene Styrene Block Copolymer) and PDMS(polydimethylsiloxane). The additives were mixed at a ratio between 0 to 50 wt%/wt% PC70BM, keeping the final concentrations at 10 mg/mL for PCE10 and 15 mg/mL PC70BM. In the case of PBDB-T-SF:IT-4F (1:1), the mixture was dissolved at the concentration of 7 mg/mL in chlorobenzene and 0.5% vol DIO and stirred at 100 °C for 1 h. When the effect of additives was studied, we introduced a PBDB-T-SF:IT-4F concentrated solution and adjusted to 7 mg/mL with an additive solution whose the ratio was between 0 to 50 wt%/wt% IT-4F. PBDB-T-2F:IT-4F (1:1, w/w, 20mg/mL) were mixed with chlorobenzene and 1,8-diiodooctane (DIO) (1% vol) in presence of additives.

  The thickness was measured by a profilometer with average values of 60 ± 5 nm. P3HT:PCBM mixture with additives, was deposited by spin coating at 500 rpm during 20 s. We kept the cells in a Petri dish overnight. Then, annealing was done at 140 °C for 10 min. PBDB-T-2F:IT-4F (1:1, w/w, 20mg/mL) mixture was spin-coated (2000 rpm for 60 s) on top of AI4083. The active layer was annealed at 100 °C for 10 min. PBDB-T-SF:IT-4F (1:1) mixture was spin-coated at 2000 rpm for 60 s, no annealing was required. (2-(1,10-phenanthrolin-3-yl)naphth-6-yl)diphenylphosphine (Phen-NaDPO) as cathode interfacial material (0.5 mg/mL in isopropanol) was spin-coated at 2000 rpm for 30 s on top of the active layer. Finally, we evaporated Aluminum metal (100 nm). The area was 8 mm 2 .

Figure 3 - 3 :

 33 Figure 3-3: Schematics of A) two-points probe and B) four-points probe for sheet resistance measurement.

𝑅 𝐻 = - 1 𝑒𝑛 1 𝑒𝑝

 11 , we have n-type doping and oppositely, 𝑅 𝐻 = for p-type doping.

Figure 3 -

 3 6 represents the curve obtained by the tensile test. The stress-strain curve defines the mechanical behavior of a material under stress with different regimes characterized by several points. For small deformations between O and A, the linear regime represents the elasticity behavior where the material is stretched and reversibly recovers.

Figure 3 - 6 :

 36 Figure 3-6: Typical stress-strain curve

Figure 3 - 9 :

 39 Figure 3-9: Apparatus used for resistance measurement under strain.

  -11a), qz and qxy are, in reality, not orthogonal to each other. Reshaping reconstructs the GIWAXS image into the actual q space coordinates with a missing wedge (Figure3-11b). Fit2D software integrates these images into the 1D plot of intensity versus q (Figure3-11c). The evolution of structure during the spin coating and annealing has been performed in situ via the collection of hundreds of 2D GIWAXS images during a fixed interval of time. The scattering of these images is radially integrated and transformed into I (scattering intensity) vs. q plots. The accumulation over time of all of the data collected can be converted into a 2D time-resolved intensity map (Figure3-11d).

Figure 3 - 11 :

 311 Figure 3-11: GIWAXS data collection of (a) raw 2D GIWAXS image, reshaped as (b), and converted into (c) 1D intensity plot. The accumulation of hundreds of graphs over time gives a (d) time-resolved GIWAXS image

Figure 3 -

 3 Figure 3-12 shows the relation between the shape and azimuthal distribution of

  Photon emission from PESA allows low excitation energy from 3.4 to 6.2 eV.

Figure 3 -

 3 photoelectron yield slope, as illustrated in Figure3-13B.

Figure 3 - 13 :

 313 Figure 3-13: PESA technique: A) Photoelectric effect principle exposed to the band diagram of metal or semiconductor and B) typical photoelectron spectra for organic material.

Figure 4 -

 4 Figure 4-1a depicts the conductivity () behavior of solid-state PEDOT:PSS films in

Figure 4 - 1 :

 41 Figure 4-1: Effect of the DMSO (red), Zonyl (blue) and their mixture (yellow; DMSO amount is kept constant at 5 wt.% as Zonyl concentration is varied) on the electrical properties of PEDOT:PSS as a function of additives concentration in solution: (a) conductivity, (b) carrier concentration (inset: schematic illustration of Hall effect measurement configuration), (c) mobility measured by Hall effect. Inset in (c) presents Hall mobility on a logarithmic scale. d) Comparison resistance measured by the Hall effect (red) and 4-points probe (blue).

1 . 15

 115 GPa for pristine PEDOT:PSS (0 wt% DMSO or Zonyl). The addition of Zonyl plasticizes the film and drastically reduces the tensile modulus, with 1 wt% Zonyl decreasing it by a factor of 4 to ca. 338 MPa. By contrast, addition of DMSO, even in large amounts, has a marginal effect on the mechanical properties of the conducting film. The mixture of Zonyl and DMSO improves the plasticity of the film, further reducing the Young's modulus of the film to <100 MPa for 1-2 wt% of Zonyl, in an apparently synergistic manner. We can advance that this is due to the plasticizing effect of Zonyl combined with microstructural and morphological changes likely due to the addition of DMSO, which will be investigated in detail later.

Figure 4 - 2 :

 42 Figure 4-2: Variation of (a) Young's modulus and (b) Stress strain measurement with respect to the additive composition for Zonyl (blue), DMSO (red) and the mixture of Zonyl with 5 wt% DMSO (yellow).

Figure 4 - 3 :

 43 Figure 4-3: Conductivity measurement on 15-25μm thick freestanding films.

  -1a). By contrast, addition of DMSO (red), increases the FoM to 55 for 3 wt% DMSO (Figure4-4c) and remains nearly constant for up to 15 wt% DMSO, producing a potentially viable albeit rigid transparent conducting electrode (Figure4-2a). The FoM measured for the mixture of additives for different concentrations of Zonyl with 5 wt% DMSO follows a predictable trend, decreasing slowly and monotonously with addition of Zonyl. However, the FoM remains superior to 35 for up to 2 wt% Zonyl in the additive mixtures.

Figure 4 - 4 :

 44 Figure 4-4: UV-Visible Transmittance measurement on thin film spin coated at 1000 rpm in presence of (a) Zonyl and (b) Zonyl and DMSO. (c) TCE figure of merit (FoM) with respect to the additive composition for Zonyl (blue), DMSO (red) and the mixture of Zonyl with 5 wt% DMSO (yellow). (d) Sheet resistancetransmittance (550 nm) plot for highly conducting and stretchable PEDOT:PSS films (1 wt% Zonyl and 5 wt% DMSO) and its comparison with commercial ITOand previous reports using the same additives.[3,13]

Figure 4 - 5 :

 45 Figure 4-5: Topographic AFM images of PEDOT:PSS films prepared (a) with no additives, (b) with DMSO (5 wt%), (c) with Zonyl (1 wt%) and (d) with a mixture of 5 wt% DMSO and 1 wt% Zonyl. Dimensions 2μm x 2μm. (e) The root mean squared (RMS) roughness values as a function of the additives content (Zonyl in blue, DMSO in red and the mixture in yellow).

Figure 4 - 6 :

 46 Figure 4-6: Topographic AFM images of pristine PEDOT:PSS, with DMSO, with Zonyl and for the mixture of x wt% Zonyl and 5 wt% DMSO. Dimensions 2μm x 2μm. Height distribution regarding the weight percent of the additives content.

Figure 4 - 7 :Figure 4 - 8 :Figure 4 - 9 :

 474849 Figure 4-7: 2D GIWAXS images during spin coating for 0 and 300 s with respect to the DMSO (top) and Zonyl (bottom) concentration. Kapton tape was used to prevent spattering of the solution

Figure 4 -

 4 Figure 4-10: a) In situ GIWAXS measurement during annealing at 60°C of spin coated thin film made of PEDOT:PSS pristine. Snapshot b) at t = 0 s and c) at t = 70 s.

Figure 4 - 11 :

 411 Figure 4-11: Time-resolved GIWAXS during annealing: first row, integrated intensity from time-resolved; second row, evolution of the main peaks position between 12 to 22 nm -1 , and last row, intensity versus time(s) at 0, 1, 5 and 10 wt% DMSO respectively from left to right column

Figure 4 - 12 :

 412 Figure 4-12: In situ thickness at high temperature (110°C)

from 18 .

 18 1 to 18.6 nm -1 , corresponding to a contraction of 3.47 and 3.39 Å. Mixing of Zonyl and DMSO results in less contraction of the lattice. PSS spacing does not change in the presence of DMSO or Zonyl. The relative crystallinity of PEDOT (PSS) domains increased by 10% (20%) with addition of DMSO (Zonyl). The relative crystallinity increase was more modest for the case of additive mixtures, increasing 4% for PEDOT and 11% for PSS.

Figure 4 -

 4 Figure 4-13: 2D GIWAXS images for PEDOT:PSS films with (a) no additives, (b) 5 wt% DMSO, (c) 1 wt% Zonyl, and (d) 1 wt% Zonyl/5 wt% DMSO. Full integration linecuts for PEDOT:PSS thin films with (e) DMSO and (f) Zonyl in comparison with the mixture. Crystalline characteristics, including (g) displacement of center maximum of PEDOT pi-stacking scattering feature and (h) relative crystallinity of PSS domains.

  -14), reinforcing the notion that much of the Zonyl is incorporated into the PEDOT:PSS fibrils rather than decorating their outer surface at low content. A large fraction of Zonyl (>1 wt%) extends slowly the distribution of the surfactant outside the interfibers indicating by an when using Zonyl only and the loss of conduction pathway can explain the one order of magnitude lower hall mobility when Zonyl is used as the additive.Based on the above discussion, we propose a mechanism of formation of fibrils in the presence of polar solvent and surfactant additives, as shown in Figure4-15. In the presence of DMSO, core-shell fibrils are nucleated due to selective solubility of PSS, phase segregation, and aggregation of PEDOT, leading to a highly aggregated PEDOT core and a dispersed PSS. In the presence of Zonyl, core-shell fibrils are also formed due to Zonyl's affinity with PSS and its amphiphilic nature and hydrophobic/hydrophilic interactions with PEDOT and PSS, respectively. However, PEDOT and PSS are still co-solubilized in water and they solidify simultaneously, with a limited driving force for segregation. The core-shell fibrils are elongated but do not form a highly aggregated PEDOT core. When both additives are present, DMSO helps Zonyl to separate and arrange PEDOT and PSS into a core-shell fibril with Zonyl present both at the interface and between fibrils. The fibrils are well-connected electrically through conducting bridges.

Figure 4 - 15 :

 415 Figure 4-15: Schema of additives effects and fibrils structuration of PEDOT:PSS (PH1000) films.

Figure 5 - 1 :

 51 Figure 5-1: Chemical structure of the surfactant Zonyl and the compounds used to form the mPEG network.

  we use the same formulation together with other additives. Specifically, conductive PEDOT:PSS electrodes were fabricated by adding Zonyl, as a surfactant, and mPEG precursors into commercial Clevios PH1000 PEDOT:PSS dispersion. The weight fraction or additive content was calculated from the additive weight divided by the total weight of the solution (𝑎𝑑𝑑𝑖𝑡𝑖𝑣𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (

Figure 5 - 2 :

 52 Figure 5-2: Normalized FTIR spectra and thermal polymerization of mPEG: a) in the absence and b) in the presence of Zonyl. The alkene is characterized at 1640 cm -1 and almost entirely disappeared after annealing. The peak of ester shifts from 1715 to 1720 cm -1 during thermal annealing at 110 °C. Both peaks are characteristic of PEGM and PEGDM polymerization. The ratio of the peak area gives the percent of polymerization before and after annealing.

Figure 5 - 3 :

 53 Figure 5-3: Effect of the methanol treatment on the conductivity of PEDOT:PSS:Zonyl (pink) and PEDOT:PSS:mPEG (blue).

Figure 5 -

 5 4 shows the effect of the additive content on electrical and transport properties.The addition of Zonyl into PEDOT:PSS solution, as plotted in Figure5-4a, leads to a decrease of conductivity. We attribute this to the insulating character of Zonyl, which decreases the overall conductivity of PEDOT:PSS due to the dilution effect. Figure5-4b-dshows Hall effect and ionization potential measurements of the film as well. A similar trend is observed for the Hall mobility, which is found to reduce from 0.38 cm 2 /Vs, at 0 wt% Zonyl, to 0.09 cm 2 /Vs, at 10 wt% Zonyl, whereas the carrier density remains relatively constant at 1.7×10 22 cm -3 regardless of Zonyl content. Moreover, the ionization potential (IP) reduces from 4.84 to 4.54 eV (IP = -0.3 eV) with increasing the weight fraction of Zonyl from 0 to 15 wt%. To summarize, the addition of Zonyl has a detrimental effect on the charge transport properties of the PEDOT:PSS layers.

Figure 5 - 4 :

 54 Figure 5-4: Effect of additive content on the electrical and transport properties of PEDOT:PSS in the presence of Zonyl, mPEG or both after treatment: a) conductivity, b) ionization potential, c) carrier density, as well as d) Hall mobility (Zonyl in pink, mPEG in blue and in yellow, the mixture of mPEG with 1 wt% Zonyl).

Figure 5 -

 5 5, we plot the theoretical PEDOT content left after annealing, assuming that: (1) DMSO evaporates, and (2) the addition of a second additive (i.e., Zonyl) is kept at 1 wt%.Although the mixture of PEDOT:PSS:mPEG:Zonyl is less conducting than the binary composition of PEDOT:PSS:mPEG, the conductivity remains remarkably high despite the low PEDOT:PSS content. To this end, the Hall Effect measurements (Figure5-4 cd) indicate a slight increase of the carrier density with increasing additive content. Thus, charge transport analysis suggests an average effect of Zonyl and mPEG when added into PEDOT:PSS.

Figure 5 - 5 :

 55 Figure 5-5: Theoretical PEDOT content change with the increase of additives by content (%) and the number of additives. The first additive corresponds to mPEG addition. In case of two additives, we change the additive content of only one additive (mPEG). We suppose that Zonyl was kept at 1 wt%. Without any additive, PEDOT represents almost only a third of the annealed film. The addition of an additive decrease in its proportion. With two additives, the PEDOT content is only 16%.

Figure 5 -

 5 6 shows the transmission spectra of the films across the ultraviolet-visible part of the electromagnetic spectrum, where the parasitic absorption of the substrate (i.e., glass) has been subtracted. The addition of the second additive, Zonyl, into PEDOT:PSS tends to increase the transmittance slightly, from 94.1% at 0 wt% to 95.7% at 15 wt% with the film becoming more transparent. We attribute this to the dilution of PEDOT:PSS, which is the only component absorbing in the visible range.

Figure 5 - 6 :

 56 Figure 5-6: Transmission spectra of PEDOT:PSS film in the presence of: a) Zonyl, b) mPEG and c) mPEG and 1 wt% Zonyl. Glass signal was subtracted.

Figure 5 - 7 :

 57 Figure 5-7: Effect of the additive content on a) the transmittance at 550nm, b) the resistance measurement and c) the FoM.

Figure 5 -

 5 8 shows the CoS of PEDOT:PSS with Zonyl. The increase of the additive content results in an improvement of the ability to undergo strain without crack formation. The crack-onset of Zonyl-containing formulations varies logarithmically with the additive content, from 21 to 70% strain, by merely changing the amount from 0.5 to 15 wt% Zonyl. PEDOT:PSS formulations containing a fixed 1wt% Zonyl and an increasing wt% of mPEG exhibit a similar behavior but with even higher CoS, demonstrating that the combination of the two additives is beneficial for stretchability. Interestingly, a CoS quasi-plateau appears for compositions between 5 to 15 wt% of mPEG with the CoS values ranging from 75 to 84%.

Figure 5 - 8 :

 58 Figure 5-8: Evolution of the crack-onset strain (CoS) with the additive content.

containing 1 wt%

 1 Zonyl and 5 wt% mPEG exhibit the best properties with a FoM value of >35 and a corresponding CoS of 75%. The good properties are attributed to the synergistic effects of the two additives, where on the one hand, Zonyl enables strong adhesion of the TCE on PDMS, whilst the presence of mPEG leads to the remarkably high mechanical crack-onset due to the high stretchability of the PEDOT:PSS:mPEG film.

Figure 5 - 9 :

 59 Figure 5-9: Stress-strain characteristic curves of PEDOT:PSS microfilm in the presence of a) Zonyl, b) mPEG, and c) mPEG and 1wt% Zonyl. d)Young's moduli. Note that 5 wt% Zonyl gives Young's modulus as low as 32 MPa, and this concentration is the limit where self-supported films could be manipulated. At higher concentration, Zonyl/PEDOT:PSS films were extremely viscous and not tractable.

Figure 5 -

 5 10a-b show electrical responses as a function of the applied strain from 0 to 120%, i.e. above the CoS of the electrodes. The relative resistance, R/R0, where R is the measured resistance, and R0 the resistance at zero strain, is related to the applied strain. The recorded electrical responses indicate a two-stage response for both Zonyl and Zonyl/mPEG-containing PEDOT:PSS compositions: Initially, (stage-i) the R/R0 resistance exhibits small changes until crack-onset is reached, however, once the first cracks are formed, the relative resistance increases with increasing concentration and the strain (stage-ii).

Figure 5 - 10 :

 510 Figure 5-10: Strain-dependence of electrical response: relative resistance of PEDOT:PSS film mixed with (a) Zonyl and (b) mPEG in the presence of 1 wt% Zonyl (b). The black dot line shows the theoretical R/R0 for an isotropic materials with Poisson's ratio of 0.35. The corresponding relative conductivity is plotted in c) for Zonyl and d) for mPEG/Zonyl.

  Zonyl concentrations. The different crack formation behavior is responsible for the R/R0 evolution, for which a sharp increase above CoS is observed. The slope of R/R0 versus strain beyond CoS decreases with the addition of Zonyl (Figure5-10a). Indeed, electrodes containing 5 wt% Zonyl show only a 6-fold increase in R/R0 at 120% strain, while reducing Zonyl concentration to 0.5 wt% increases R/R0 by 360-fold because of the detrimental effect of the cracks. Increasing the Zonyl concentration further reduces crack formation even more. Such electrodes feature microscopic cracks and maintain better electrical conductivity than films with long fissures.

Figure 5 - 11 :

 511 Figure 5-11: Optical microscope images of PEDOT:PSS thin film with a fixed amount of Zonyl (horizontally specified). The films were coated on top of PDMS and subjected to a mechanical strain as vertically indicated.

Figure 5 - 12 :

 512 Figure 5-12: Optical microscope images of PEDOT:PSS thin film with a fixed amount of mPEG and 1 wt% Zonyl. The quantity of mPEG content is horizontally specified. The films were coated on top of PDMS and subjected to a mechanical strain as vertically indicated.

Figure 5 - 13 :

 513 Figure 5-13: Electrical response under 0%-120%-0% strain of PEDOT:PSS film spin-coated on PDMS. The additive content was varied for: a) Zonyl only, and b) mPEG in presence of 1 wt% Zonyl.

Figure 5 -

 5 14a displays the durability of electrical response during stretch-and-release cycling at different strains using a linear stage with a speed of 2.5 mm/s. The relative resistance of PEDOT:PSS:Zonyl(1 wt%) films does not change significantly after 250 cycles at 20% strain and at 40% only a slight increase is observed in the first cycles after crack-onset is reached. At 60% elongation per cycle, the generated stress provokes cracks on the surface and the relative resistance increases by 4-fold. An irreversible behavior is also observed during the cycling: the relative resistance gradually increases with the number of cycles, which can be explained by increasing crack formation, perpendicularly to the stretching direction.

Figure 5 -

 5 Figure 5-14: a) Electrical response under stretch-and-release cycles at the indicated strain measured in the motorized and automated stage of film-coated on PDMS with PEDOT:PSS containing 1 wt% Zonyl, b) corresponding optical microscope images after cycling 250 times the films at different strain as indicated on the pictures and c) AFM images for 250 cycles at 40% strain.

Figure 5 -

 5 14b-c depicts the surface of the film after cycling at 40% strain by optical microscopy and atomic force microscopy (AFM). The microscope image shows a rough surface with apparent micrometer-long cracks.

Figure 5 - 15 :

 515 Figure 5-15: Comparison of resistance measurement made in parallel or perpendicular to the stretching direction. The film was fabricated with 1 wt% Zonyl and PEDOT:PSS.

Figure 5 -

 5 16c demonstrates that the films remain cracks-free after 250 cycles at 60% strain. No electrical fatigue is recorded at 60% stretch-and-release cycling for more than 5000 cycles (Figure5-16b).

Figure 5 -

 5 Figure 5-16: a) Evolution of the relative resistance at different strains as denoted in the legend. b) R/R0 and σ/σ0 vs. number of stretch-and-release cycles at 60% strain. c) Corresponding microscope images and d) AFM image. The films were made with PEDOT:PSS, 5 wt% mPEG and 1 wt% Zonyl.

Figure 5 -Figure 5 -

 55 Figure 5-17c and under strain in Figure 5-17d. The incorporation of Zonyl surfactant

Figure 5 - 17 :

 517 Figure 5-17: AFM phase images (2μm x 2μm) of PEDOT:PSS in the presence of 1 wt% Zonyl at a) 0% strain and b) 20% strain, c-d) their corresponding AFM topography images (2μm x 2μm) and their height distributions in e).

Figure 5 -

 5 Figure 5-18a shows the phase AFM images of the PEDOT:PSS films made with 5 wt%

Figure 5 - 18 :Figure 5 -

 5185 Figure 5-18: AFM images of PEDOT:PSS mixed with 5 wt% mPEG with 1 wt% Zonyl at different strain as indicated on the picture: a) phase and b) height images, and c) their height distributions. 2μm x 2μm

Figure 5 - 19 :

 519 Figure 5-19: Schematics illustrating the mechanisms of strain-induced PEDOT:PSS rearrangement of (1) PEDOT:PSS:Zonyl fibres network, and (2) nanoconfinement of PEDOT:PSS into mPEG-based matrix.

Figure 5 - 20 :

 520 Figure 5-20: Storage modulus and tan delta of pure mPEG network in function of the temperature.

FF decreases further due

  to higher resistive losses and/or change in the electrode energetics. A similar behavior is observed for cells based on poly[(2,6-(4,8-bis(5-(2ethylhexylthio)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8dione)] called PBDB-T-SF and IT-4F BHJ (Figure 5-22 and Table 5-2).

Figure 5 - 21 :

 521 Figure 5-21: ITO-free organic solar cells using standalone PEDOT:PSS-based electrodes: a) chemical structure of PBDB-T-2F and IT-4F. b) Schematic of the cell device architecture employed. c) Dependence of PCE on the transparent electrode composition. d) Set of representative J-V characteristics for the various solar cells shown in (c).

Figure 5 -

 5 Figure 5-22: PBDB-T-SF:IT-4F solar cell: a) chemical structure of PBDB-T-SF and IT-4F, b) schematic of the device structure, c) electrode comparison regarding the power conversion efficiencies, and d) the corresponding J-V characteristics curves.

Figure 5 -

 5 Figure 5-23: PEDOT:PSS-based mechanical sensors: a) Schematic of manual finger bending. Initially, a strip is attached with copper tape (pink) to a finger in straight position. When the finger bends (right image), the strip follows a radius of curvature, which determine the strain undergone. Relative resistance measurement in real-time for small stretching, using a test fixture attached to a finger, containing b) 0.5 wt% Zonyl and c) 0.5 wt% mPEG/1 wt% Zonyl.

Figure 6 -

 6 photoactive layer: 1,12-diazidododecane (C12N3), 1,11-diazido-3,6,9-trioxaundecane

Figure 6 -

 6 2 shows the corresponding chemical structures. C12N3 compound was synthesized by reacting NaN3 with 1,12-dibromododecane (see Chapter 3). Wantz and Derue have patented C12N3

Figure 6 - 1 :

 61 Figure 6-1: Illustration of the strategies to improve the mechanical reliability of blend solar cells via fullerene crosslinking or elastomer blending.

Figure 6 - 2 :

 62 Figure 6-2: Chemical structures of the cross-linker molecules and elastomers.

Figure 6 - 3 :

 63 Figure 6-3: Scheme of reaction between bis-azide and PCBM molecule under heat treatment.

Figure 6 -

 6 Figure 6-4 demonstrates the thermal change of P3HT, PCBM, and cross-linkers. PCBM

Figure 6 - 4 :

 64 Figure 6-4: Thermal gravimetric analysis and DSC plots of a) P3HT and PCBM, and b-d) cross-linkers PEG3N3 and PEGnN3 at different temperatures.

Figure 6 - 5 :

 65 Figure 6-5: DSC analysis of a) PEG3N3 mixed with either P3HT (1:1) or PCBM (1:1), or P3HT:PCBM (1:1:1).

Figure 6 -

 6 Figure 6-6b. The intensity of the azide peak (νN3 = 2090 cm -1 ) was reduced by 70 %

Figure 6 - 6 :

 66 Figure 6-6: FTIR spectra of a) the precursors before cross-linking. b) Kinetic of cross-linking via monitoring P3HT:PC61BM:PEG3N3 after thermal annealing at 150°C. c) Temperature effect on P3HT:PC61BM:PEG3N3, for 5min. d) Kinetic of P3HT:PC61BM:C12N3 and e)P3HT:PC61BM:PEGnN3 after thermal annealing at 150°C. f) Dependence of the temperature on the crosslinking of P3HT:PC61BM:PEGnN3 during 5 min of curing.

Figure 6 -

 6 Figure 6-7 shows the micro-texture change in the presence of additives into

Figure 6 - 7 :Figure 6 - 8 :

 6768 Figure 6-7: Micrographs of P3HT, PCBM, and P3HT:PCBM in the presence of additives as indicated in the picture after annealing

Figure 6 - 9 :

 69 Figure 6-9: Microscopic images of PCBM aggregation in P3HT:PCBM layer in the presence of additives at 10 wt% before and after aging for 60 h at 150°C.

Figure 6 - 10 :

 610 Figure 6-10: UV-vis absorbance spectra obtained of a) casted PCBM, P3HT, and P3HT:PCBM (1:0.8) blend and b) annealed P3HT:PCBM film in the presence of additives at 50 wt%.

Figure 6 -

 6 11 corresponds to the GIWAXS 2D image obtained for P3HT:PCBM thin film. The series of scattering patterns along with qz corresponding to crystallized P3HT lamellar stacking with a main edge-on orientation. The Bragg peaks rising at qz = 3.8, 7.5, 11.5 nm -1 correspond to P3HT

Figure 6 -

 6 Figure 6-15: a) Average Young's modulus according to the weight percent of additives added into P3HT:PCBM blend prior to annealing. b) The elastic modulus was deducted from stress-strain curves with 50 wt% additive content.

Figure 6 - 17 :

 617 Figure 6-17: Photovoltaic performances of P3HT:PC61BM-based OPV in the function of the additive content.

Figure 6 - 18 :

 618 Figure 6-18: Photovoltaic curves associated with P3HT:PC61BM solar cells in the presence of additives.

efficiency 6 . 2 . 6 . 1

 6261 PCE10:PC71BM Based on the blending strategy, SEBS additive was introduced to poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-3-fluorothieno[3,4b]thiophene-2-carboxylate] (PTB7-Th or PCE10) and[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend, also referred as PCE10:PC71BM blend. Figure6-19shows the chemical structure, the energy band diagram, the absorption spectra, and J-V curve associated with PCE10:PC71BM blend at a weight ratio of 1:1.5. PCE10 polymer has a low bandgap (1.6 eV), with a broard absorption between 500 and 800 nm with two peaks at 645 and 700 nm. The solar spectrum between 300 and 800 nm is covered in a complementary manner by PC71BM blended with PCE10 (Figure6-19c).The solar cell layout is ITO/PEDOT:PSS/PCE10:PC71BM/LiF/Al. This BHJ blend demonstrates a PCE of up to 10% (Jsc = 17.3 mA.cm -2 , Voc = 0.8 V, FF = 60.0 %), in agreement with several reports.[59][60][61]

Figure 6 - 19 :

 619 Figure 6-19: PCE10:PC71BM (1:1.5) solar cell: a) chemical structures, b) energy band diagram, c) absorption spectra, and d) J-V characteristic curve.

Figure 6 -

 6 20 shows the resulting morphology observed by optical microscope and atomic force microscopy (AFM) after spin-coating. Between 0 to 25 wt% SEBS, no drastic change is seen at the microscale.The addition of 50 wt% reveals a micro-phase separation of the polymers blend. AFM additionally characterized the morphology of the BHJ thin film. The roughness and the domain increase with SEBS content. The phase-separated thin film shows nanodomains at 10 wt% to micro-domains at 25 wt%. This suggests that the introduction of SEBS increases phase-separation. In comparison, the phase-separation in PCE10:PC71BM blend is less noticeable than P3HT:PC61BM for the same content of SEBS (Figure6-8). On the contrary, the topography indicates relatively good miscibility between SEBS and PCE10:PC71BM blend.

Figure 6 -

 6 Figure 6-20: PCE10:PC71BM:SEBS morphology characterized by optical microscopy (top row) and atomic force microscopy (bottom row).

Figure 6 -

 6 Figure 6-21 displays the normalized absorption spectra of PCE10:PC71BM thin film

Figure 6 - 21 :

 621 Figure 6-21: Absorption spectra of PCE10:PC71BM film mixed with SEBS elastomer.

Figure 6 -

 6 22ashows the mass loss during annealing under vacuum. A 25 % mass loss is observed after 1h, and no change is perceived after that time. This suggests that thick PCE10:PC71BM film requires at least an hour drying prior tensile test. After applying 90 min annealing under vacuum, stress-strain characteristics were recorded at fixed weight percents of SEBS. The stress-strain curves are shown in Figure6-22b. Finally, the Young's modulus deduced by the slope at the origin of the stress-strain curve is plotted in Figure6-22e, and compared to the buckling method. The two methods show similar values. At 0 wt% SEBS the film displays an elastic modulus of around 350 MPa compared to 450 MPa by buckling method. Our results are in accordance with the

Figure 6 - 23 :

 623 Figure 6-23: Evolution of the photovoltaic characteristics of PCE10:PC71BM blend in the function of SEBS additive content.

  Poly[(2,6-(4,8-bis(5-(2-ethylhexylthio)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PCE13 or PBDB-T-SF) was blended with 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2b:5,6-b']dithiophene (IT-4F). This produces PCE13:IT-4F blend, whose chemical structures are given in Figure 6-24a.Figure 6-24b displays the absorption spectra of the polymers. The low bandgap donor polymer (PCE13) exhibits an absorption peak onset at 684 nm corresponding to an optical band gap of 1.8 eV, while IT-4F acceptor presents at peak onset at 786 nm. PCE13:IT-4F at a weight ratio of 1:1, covers the absorption spectra of the two component fully. The ITO/PEDOT:PSS/PCE13:IT-4F/DPO/Al solar

Figure 6 -

 6 Figure 6-24: a) chemical structures of PCE13 and IT-4F. b) UV-visible absorption spectra of PCE13, IT-4F, and PCE13:IT-4F (1:1) blend. c) J-V characteristics curves of PCE13:IT-4F based OPV.

Figure 6 -

 6 Figure 6-25: a) Optical morphology and b) UV-visible absorption spectra of PCE13:IT-4F mixed with SEBS elastomer.

Figure 6 - 26 :

 626 Figure 6-26: Determination of Young's moduli: a) stress-strain curves of PCE13:IT-4F with 0, 10, 25, and 50 wt% SEBS and b) comparison of Young's moduli by buckling method or direct measurements by stress-strain.

Figure 6 -

 6 Figure 6-27 illustrates the impact of SEBS on the photovoltaic performances of PCE13:IT-4F. For this system, Jsc decreases slightly from 21.7 to 19.4 mA/cm 2 with a

Figure 6 - 27 :Figure 6 -

 6276 Figure 6-27: Photovoltaic performances of PCE13:IT-4F with different amounts of SEBS elastomer.

Figure 6 - 28 :

 628 Figure 6-28: Effect of SEBS on the a) initial elastic modulus and b) initial power conversion efficiency

Figure 6 - 30 :

 630 Figure 6-30: Dependence of spin coating velocity versus the power conversion efficiency (PCE) and associated I-V curves: A-B) ITO/Glass; C-D) PEDOT:PSS/Zonyl/DMSO mixture/Glass and on E-F) PEDOT:PSS/Zonyl/DMSO mixture/PDMS. The curves are measured for the first irradiation at 100 mW.cm -2

  adopted those strategies for our research for the modification of the transparent electrode and the active layer in an organic solar cell. The introduction of additives into PEDOT:PSS solution and their effects on the electrical, electronics, optical and mechanical properties were discussed (Chapters 4 and 5). Indeed, we demonstrate, via the addition of DMSO and Zonyl into EDOT:PSS, that it is possible to produce highly transparent conducting electrodes (FoM > 35) with low Young's modulus and high tested to exploit the stretchability of up to 120% elongation. Over 5000 repeated stretched-and-released cycles at different strains are also carried out to evaluate the resistance to fatigue and robustness. The interpenetration of PEDOT:PSS into the mPEG network shows an interconnected morphology with the beneficial synergetic effect of stretchability and conductivity. The common materials demonstrate an excellent capability to detect small motions of the finger, which is suitable for stretchable on-skin sensors.

Table 2 -

 2 1: Review of stretchable solar cells tested under deformation in the literature: experienced strain, power conversion efficiency before (PCE0) and after (PCE) stretching, type of stretching (extrinsic when the structure is patterned or intrinsic when using soft materials), Young's modulus of the active semiconductor (E), crackonset (CoS), number of cycles in case of mechanical durability test and associated reference. Different methods have been used to determine Young's modulus by: 1) stress-strain, 2) buckling based method, 3) nanoindentation, 4) PeakForce mapping as indicated in Young's modulus column by a superscript number. ................................

Table 3 -

 3 1: Theoretical solid contents in percent after annealing and representation of insulating part (PSS and additives) and conducting solid (PEDOT). ..........................

Table 3 -

 3

2: Representative P3HT:PCBM:X solid content in percent after annealing.100

Table 4 -

 4

1: Transparent, stretchable, conductive PH1000 based electrode in the literature ..................................................................................................................... Table 4-2: Quantitative microstructural analysis of PEDOT domains from GIWAXS data on PEDOT:PSS films. ........................................................................................ Table 4-3: Quantitative microstructural analysis of PSS domains from GIWAXS data on PEDOT:PSS films. ................................................................................................

Table 5 -

 5

1: Summary of the important operating characteristics extracted from ten PBDB-T-2F:IT-4F solar cells for each electrode composition. ................................. Table 5-2: Photovoltaic characteristics of PBDB-T-SF:IT-4F solar cells. Average of 10 cells. ......................................................................................................................

Table 6 -

 6

1: Summary of GIWAXS peak fitting results. The additives were introduced at 50 wt% compared to PCBM, i.e. P3HT:PCBM:additive at 1:0.8:0.4. Lamellar stacking was taken from (100) peak, and the π-π stacking was extracted from (

010

) peak. ...........................................................................................................................

Table 6 -

 6 2: Mechanical properties of P3HT:PCBM with 50 wt% additive. The letter E states for elastic modulus. Tα is the alpha transition temperature extracted from DMA data at the maximum of the peak. ..............................................................................

Table 6 -

 6

3: Summary of solar cell performances. In brackets, numbers summarized the average of 10 cells. .................................................................................................... Table 6-4: Best photovoltaic characteristic of PCE10:PC71BM on different electrode/substrates couple. The best-measured cell is displayed while the average of 15 cells is writing in brackets for the first illumination. ............................................

Table 2

 2 -1 specified not only the deformation strain and the power conversion efficiency but as well, the associated reference, structure and mechanical properties such as Young's modulus and crackonset. Here, we differentiate intrinsically and extrinsically stretchable systems.Inventoried stretchable solar cells are equally studied with either intrinsic or extrinsic strategies. Surprisingly, although inorganic, perovskite, and dye-sensitized solar cells have mostly higher power conversion efficiency, few studies reported stretchable solar cells. Over 80% of research on stretchable solar cells focused on organic-based devices, as demonstrated in Figure2-21 A by round symbols.

Table 2

 2

-1.

Table 2 -1: Review of stretchable solar cells tested under deformation in the literature: experienced strain, power conversion efficiency before (PCE0) and after (PCE) stretching, type of stretching (extrinsic when the structure is patterned or intrinsic when using soft materials), Young's modulus of the active semiconductor (E), crack-onset (CoS), number of cycles in case of mechanical durability test and associated reference. Different methods have been used to determine Young's modulus by: 1) stress-strain, 2) buckling based method, 3) nanoindentation, 4) PeakForce mapping as indicated in Young's modulus column by a superscript number.

 2

	Structure	Strain (%)	PCE (%)	PCE0 (%)	PCE/ PCE0	Type of stretchability	E (GPa)	CoS (%)	# cycles Ref.
	Inorganic
	SiμP/PDMS composite	40	3.21 3.59	0.9	Extrinsic -Tensile strain	130 1			[283]
	SiμP/PDMS	40	4.42 4.42	1.0	Extrinsic -Tensile strain	130 1			[283]
	GaAs	20	12.5 13.00	1.0	Extrinsic -Tensile strain			500	[34]
	Perovskite
	CH3NH3PbI3	50	7.80 12.00 0.65	Extrinsic -Compression	100 2			[285]
	DSCs
	TiO2/MWCNT	20	6.35 7.05	0.9	Extrinsic -Tensile strain			50	[129]
	OPV
	P3HT:PC61BM	18.5 1.20 1.33	0.9	Extrinsic -Compression		22.2	11	[182]
	P3HT:PC61BM	10	0.01 0.59	0.0	Intrinsic -Tensile strain	2.0 2	3		[112]

  . The resistance R12,34 is defined as 𝑅 12,34 =

	𝑉 34
	𝐼 12

Table 4 -1: Transparent, stretchable, conductive PH1000 based electrode in the literature

 4

					Young's
	Compositions	R (Ω.sq -1 )	T (%)	FoM	modulus	Conditions	Ref
					(GPa)
		630 x10 3
		[55]
	PH1000

Table 4 -

 4 2 and Table 4-3 for PEDOT and PSS domains, respectively. The stacking distance, d, can be estimated from the reciprocal space position of diffraction as 𝑞 =

2𝜋 𝑑

Table 4 -2: Quantitative microstructural analysis of PEDOT domains from GIWAXS data on PEDOT:PSS films.

 4

	DMSO (wt%)	FWHM (nm -1 )	Center Max (nm -1 )	d spacing (Å)	Relative Crystallinit y	CCL (nm)	number of stacked molecules	face-on amount
	0	4.94	18.02	3.49	1.00	1.27	3.7	0.38
	1	4.59	18.10	3.47	1.03	1.37	3.9	0.38
	5	4.45	18.49	3.40	1.10	1.41	4.2	0.39
	Zonyl
	(wt%)
	0	4.94	18.02	3.49	1.00	1.27	3.7	0.38
	0.5	4.57	18.05	3.48	1.00	1.37	3.9	0.39
	1	4.27	18.07	3.48	1.06	1.47	4.2	0.40
	Zonyl in
	Mixture
	(wt%)
	0	4.45	18.49	3.40	1.10	1.41	4.2	0.39
	0.5	3.94	18.42	3.41	1.02	1.59	4.7	0.38
	1	3.78	18.20	3.43	1.04	1.66	4.8	0.39

Table 4 -3: Quantitative microstructural analysis of PSS domains from GIWAXS data on PEDOT:PSS films.

 4

	DMSO (wt%)	FWHM (nm -1 )	Center Max (nm -1 )	d spacing (Å)	Relative Crystallinit y	CCL (nm)	number of stacked molecules	face-on amount
	0	3.55	13.10	4.79	1.00	1.77	3.7	0.38
	1	3.72	13.17	4.77	1.02	1.69	3.5	0.38
	5	4.49	13.22	4.75	1.01	1.40	2.9	0.38
	Zonyl
	(wt%)
	0	3.55	13.10	4.79	1.00	1.77	3.7	0.38
	0.5	3.60	13.12	4.79	1.15	1.74	3.6	0.41
	1	3.85	13.13	4.78	1.19	1.63	3.4	0.43
	Zonyl in
	Mixture
	(wt%)
	0	4.49	13.22	4.75	1.01	1.40	2.94	0.39
	0.5	4.62	13.24	4.74	1.07	1.36	2.87	0.43
	1	4.52	13.26	4.74	1.11	1.39	2.93	0.44

Table 5 -1: Summary of the important operating characteristics extracted from ten PBDB-T-2F:IT-4F solar cells for each electrode composition.

 5

	Electrode	Jsc (mA/cm 2 )	Voc (V) FF (%) PCE (%) PCEmax (%)
	ITO	20.48 (±0. 67)	0.879 (± 0.006)	70.0 (± 2.6)	12.60 (± 0.56)	13.4
	1 wt% Zonyl	20.73 (±0.53)	0.808 (± 0.020)	59.8 (± 3.9)	10.01 (± 0.72)	11.1
	5 wt% mPEG	20.83 (±0.37)	0.813 (± 0.015)	62.9 (± 4.0)	10.64 (± 0.60)	11.7
	5 wt% mPEG + 1 wt% Zonyl	19.99 (±0.35)	0.815 (± 0.023)	65.9 (± 5.2)	10.72 (± 0.65)	11.9

Table 5 -2: Photovoltaic characteristics of PBDB-T-SF:IT-4F solar cells. Average of 10 cells.

 5

	Electrode	Jsc (mA/cm 2 )	Voc (V) FF (%) PCE (%) PCEmax (%)
	ITO	18.22 (±1.00)	0.928 (± 0.064)	47.0 (± 4.6)	7.90 (± 0.63)	9.6
	1 wt% Zonyl	16.69 (± 2.97)	0.779 (± 0.034)	49.6 (± 3.6)	6.44 (± 1.05)	8.1
	5 wt% mPEG	16.98 (±0.68)	0.854 (± 0.027)	53.5 (± 2.5)	7.77 (± 0.60)	8.8
	5 wt% mPEG + 1 wt% Zonyl	16.35 (±0.91)	0.823 (± 0.023)	49.4 (± 3.4)	6.65 (± 0.83)	8

Table 6 -1: Summary of GIWAXS peak fitting results. The additives were introduced at 50 wt% compared to PCBM, i.e. P3HT:PCBM:additive at 1:0.8:0.4. Lamellar stacking was taken from (100) peak, and the π-π stacking was extracted from (010) peak.

 6

			Lamellar stacking	π-π stacking
	Additive	MW (g.mol -1 )	d-spacing (nm)	CCL (nm)	d-spacing (nm)	CCL (nm)
	w/o additive	-	1.65	11.8	0.37	3.7
	C12N3	252	1.66	11.5	0.38	3.8
	PEG3N3	244	1.65	11.3	0.38	4.1
	PEGnN3	1100	1.65	11.5	0.38	3.6
	SEBS	-	1.65	11. 7	0.38	4.9
	PDMS	-	1.67	14.3	0.38	4.9

Table 6 -3: Summary of solar cell performances. In brackets, numbers summarized the average of 10 cells.

 6 If we change the chemical nature of the active layer, PCE10:PC71BM based solar cell shows a PCE around 9% as previously discussed. The interface properties between the electrode and PCE10:PC71BM may require an optimization of the spin coating velocity for each electrode/substrate pair. Thickness depends on the spin velocity. Faster is the speed; thinner is the coated film. Velocity dependence of the active layer has been studied in Figure6-30 and summarized in Table6-4. On ITO/glass, a maximum of 9.7%

	Substrate	Electrode	Jsc (mA/cm 2 )	Voc (V)	FF (%)	PCE (%)
	Glass	ITO	10.73 (9.7)	0.613 (0.61)	60.4 (61.0)	4 (3.6)
	Glass	PH1000:Zonyl:DMSO	11.43 (11.3)	0.566 (0.55)	57.2 (50.0)	3.7 (3.1)
	Glass/PDMS PH1000:Zonyl:DMSO	11.54 (11.1)	0.348 (0.34)	29.2 (28.8)	1.2 (1.1)

Table 6 -4: Best photovoltaic characteristic of PCE10:PC71BM on different electrode/substrates couple. The best-measured cell is displayed while the average of 15 cells is writing in brackets for the first illumination.

 6

	Substrate	Electrode	Velocity (RPM)	Jsc (mA/cm 2 )	Voc (V) FF (%) PCE (%)
			750	17.93 (17.3)	0.803 (0.80)	65.5 (65.0)	9.4 (9.0)
			1000	18.18 (17.8)	0.803 (0.80)	66.3 (63.8)	9.7 (9.1)
	Glass	ITO	1500 2000	17.90 (17.1) 16.65 (16.4)	0.807 (0.81) 0.807 (0.81)	63.5 (64.3) 66.4 (63.7)	9.2 (8.8) 8.9 (8.4)
			3000	12.63 (12.6)	0.803 (0.80)	65.4 (61.6)	6.6 (6.2)
			4000	13.45 (9.7)	0.789 (0.79)	64.0 (61.6)	6.8 (4.7)
			500	20.88 (19.9)	0.711 (0.67)	41.0 (37.1)	6.1 (5.0)
			750	17.98 (17.4)	0.759 (0.71)	57.5 (40.9)	7.8 (5.1)
	Glass	PH1000:Zonyl:DMSO	1000 1500	17.32 (16.8) 18.78 (13.6)	0.743 (0.66) 0.719 (0.70)	56.7 (39.0) 35.3 (35.3)	7.3 (4.4) 4.8 (3.3)
			2000	11.85 (11.2)	0.617 (0.73)	64.6 (41.3)	4.7 (3.4)
			2500	12.58 (11.1)	0.741 (0.64)	34.5 (31.4)	3.2 (2.3)
			500	13.94 (12.7)	0.647 (0.65)	23.6 (22.9)	2.1 (1.8)
	Glass/PDMS PH1000:Zonyl:DMSO	750 1000	13.57 (12.7) 16.51 (13.4)	0.805 (0.69) 0.770 (0.65)	20.8 (21.6) 27.2 (26.3)	2.3 (1.9) 3.5 (2.3)
			1500	17.44 (16.6)	0.813 (0.75)	30.1 (28.7)	4.3 (3.6)

ACKNOWLEDGEMENTS

First, I would like to thank the King Abdullah University of Science and Technology (KAUST) and the Ecole Doctorale Sciences et Ingénierie de l'Université de Cergy-Pontoise, for their financial supports, which allow me to pursue this Ph.D. I want to thank my advisors -Cédric Plesse, Fabrice Goubard, Aram Amassian, and Thomas Anthopoulos. Cédric Plesse for his good supervision, availability, reliability and kindness. Fabrice Goubard for his encouragement, supports, and disponibility. role in adhesion and charge extraction in OPV. Ethoxylated polyethylenimine (PEIE) and PEDOT:PSS Clevios Al 4083 have assumed the role of intermediate layers. [33,35] However, their mechanical properties and their effect of the device are not well studied for intrinsically stretchable solar cells. The development of new stretchable transport layers could be a key parameter to overpass the low power conversion efficiency. For instance, PTB7-Th:PC71BM shows a large degradation of PCE (6%) for a stretchable architecture compared to the standard rigid configuration (>10% PCE). [300,301] softening effect. The alpha temperature decreases in presence of diazide cross-linker. This result suggests that a plasticization occurs and responsible for the softening effect.

In the case of rubber, the softening seems more related to the incorporation of soft rubber with low interaction with the other components of the layer. While the elastomer blending leads to softer film, crosslinking strategy does not disturb the phase-separation keeping high photovoltaic performance. Despite this, the elastomer blending, such as SEBS, provides a versatile strategy enhancing the mechanical properties such as Young's modulus, adaptable also to non-fullerene BHJ. With the fast progress in OPV, this kind of method would easily find its place due to its low-cost, high adaptability, and high mechanical compliance. Besides some engineering issues to build a fully stretchable solar cell that needs to be solved, our investigation demonstrates the efficacy of elastomer blending in P3HT:PC61BM, PCE10:PC71BM, and PCE13:IT-4F.

LIST OF ABBREVIATIONS

2 𝐹 (Eq. 3-4) where F is a form factor directly related to the distribution of contacts. These expressions are significantly simplified if the geometry of the sample and contacts are symmetrical, as in Figure 3-4B. As a result of the symmetry, the resistance becomes

Hall Effect measurement

Edwin Hall revealed the Hall effect in 1879. [13] It gives access to three crucial electrical parameters: the square resistance Rs, the charge density, and the carrier mobility μd. The principle of the Hall Effect on a semiconductor rod is represented in Figure 345. A magnetic field Bz is applied vertical and perpendicular to the sample (in the z-direction), which is traversed by current density i, in the x-direction. sample were also aligned with the incident X-ray beam, and an incidence angle of 0.17 o was used for in situ experiments. In situ measurements and the spin coating process were initiated together and lasted 300 s, using a time resolution of 0.5 to 1 s for in situ measurements. 2D GIWAXS images were transformed with a geometric correction and two-dimensional intensity reshaping using GIXGUI software.

In situ ellipsometry

Spectroscopic ellipsometry characterizes optical properties of thin films by measuring the change of polarization after reflection on the sample of the light. The change of polarization is measured by the ratio of the reflection coefficient of p-polarized (Rp) and s-polarized (Rs). This can be expressed as the amplitude (tan(Ψ)) of the reflection and the phase shift difference (Δ) as: [22] 𝜌 = 𝑅 𝑝 𝑅 𝑠 = tan Ψ 𝑒 𝑖Δ (Eq. 3-22) where (ρ) is the complex reflectance ratio. Ellipsometry technique records the sample's properties, such as the complex refractive index thickness or dielectric constants. In situ ellipsometry refers to the acquisition of overtime during the growth of the film.

PEDOT:PSS films was monitored using spectroscopic ellipsometry (M-2000XI, J. A.

Woollam Co., Inc) measurements in reflection mode. In situ measurement monitored the thinning process during annealing. This technique works when the solution thickness is within 10 microns.

Photoelectron Spectroscopy in Air (PESA)

The ionization potential was determined by photoemission spectroscopy in the air (PESA) using PESA system AC-2 from RKI instruments Inc. This technique has the advantage of being operational in the air with short measuring time compared to ultrahigh vacuum techniques such as ultraviolet photoelectron spectroscopy (UPS). PESA

Morphology, microstructure and self-assembly of PEDOT:PSS film.

To explain the mechanisms by which the additives so drastically modify the transport and mechanical properties of PEDOT:PSS film, we turn our attention to inspect the morphology, microstructure and thin film formation from solution. Topographic atomic force microscopy (AFM) images in Figure 4-5 provide evidence of important morphological changes in the presence of additives. The pristine film is rather featureless and appears to be smooth (Figure 4-5a). Addition of DMSO promotes formation of a densely packed network of short and straight fibrils in the plane of the film (Figure 4-5b), consistent with previous reports. [31,58] Addition of Zonyl also leads to formation of fibrils (Figure 4-5c), but the latter tend to be longer, exhibit many twists, and tend to be less densely packed. Combination of the two additives (Figure 4-5d) straightens, lengthens and widens the fibrils and yields an intermediate density of prominent fibrils with good interconnectivity. It is worth recalling that the latter film contains as much Zonyl as PEDOT:PSS, which explains why the fibrils tend to be a bit more spaced apart. We have calculated the root mean squared (RMS) surface roughness with respect to different additive amounts (Figure 4-5e; AFM images in Figure 456).

The surface roughness is unchanged with addition of small to moderate amounts of DMSO, but increases from 1.3 to 2.7 nm with the addition of 15 wt% DMSO. Zonyl addition increases the surface roughness (2.2 nm) up to a concentration of 2 wt%, then decreases. We believe the excess low energy fluorosurfactant segregates to the surface, providing an additional smoothing effect by burying fibrils and filling the gaps between them. In the case of mixtures with 5 wt% DMSO, the roughness increases sharply to 4 nm for small amounts of Zonyl, surpassing the roughness of DMSO-only and Zonylonly films, then decreases sharply for >2 wt% Zonyl. This quantitative increase of increase of hydrophobicity and a smoothing effect detected on AFM roughness measurement. The importance of the interfacial incorporation of Zonyl, as opposed to surface coating of fibrils, cannot be overstated in the context of conducting materials and the need for a percolating pathway. Zonyl and DMSO both promote phase separation and fibril formation, resulting in cores-shell fibrils which can be electronically connected, for instance, through exposed PEDOT and PEDOT:PSS sections of the fibrils, to form a conducting network. The Zonyl surfactant can thus mechanically soften the fibrils, but can also mechanically bridge adjacent fibrils without encapsulating them with an insulator. This may explain partly why with DMSO only, fibrils appear quite a bit narrower than with Zonyl or Zonyl/DMSO mixture. With Zonyl and DMSO both present, the crystalline correlation length (CCL) of PEDOT and PSS increases, possibly

indicating thicker cores and shells in addition to higher crystallinity. This would then contribute to fibril thickness in addition to the presence of the amphiphilic Zonyl at the interface and partly at the surface too. The fact that carrier concentration is nearly the same for Zonyl-only and DMSO-only formulations indicates that the doping is similar at the local molecular level. However, the reduced crystalline order in PEDOT regions role in the mechanical properties. [62] During stretching, the stress is transferred at the interface of PEDOT:PSS and the deformable mPEG matrix where the energy is absorbed. The stress is transferred from the nanofibers to mPEG matrix via the interfaces, which increases the ductility of the composite electrode. At high strain loading (>80%), even for high concentration (≥5 wt%) of mPEG, the stress transfer cannot be assured anymore at the interface, and the matrix fails. The decohesion effect provokes the initiation of the cracks at the interface, which irreversibly propagate and manifest as long gaps. Although nanoconfinement explains the elastic properties of the composite electrodes, yet increasing the elasticity often results in a decrease in conductivity. The rise of the conductivity may be due to the increased interconnections between the PEDOT:PSS chains' network. The formation of an interconnected network enhances electrical percolation explaining the enhancement of carrier mobility. On the other hand, the introduction of (physical or chemical) cross-linking points in the polymer produces a network in which PEDOT:PSS chains do not move freely. The creation of the three-dimensional system preserves the elastic characteristics with good recoverability as no buckling is observed (Figure 5-16c). The high durability of over 5000 cycles at a strain of 60% (Figure 5-16b) is a strong indication of the presence of an interconnected network.

Domain transformation is another possible mechanism that could explain the elasticity and the enhancement of the conductivity during stretching. Lee et al. showed that there is a strain-induced reorganization of the pristine PEDOT:PSS. [59] They attributed the net decrease of electrical resistivity under stress to the local enlargement of PEDOT cores minimizing the electronegative interactions between the PSS chains. In our case, AFM images do not show a domain rearrangement. Therefore, we believe that in the case of our PEDOT:PSS:mPEG electrodes, a combination of both nanoconfinement and interconnection mechanisms can explain the excellent stress dissipation and excellent charge transport characteristics. These synergistic mechanisms can explain the higher crack-onset and Young's modulus compared to PEDOT:PSS:Zonyl mixtures.

Applications

Transparent electrode for solar cell

To test the viability of the PEDOT:PSS:mPEG:Zonyl transparent conductive electrodes for practical applications, we fabricated organic solar cells using a binary bulk The power conversion efficiency (PCE) of this BHJ system has recently been reported to exceed 13.2%. [64] The fabrication details are given in chapter 3, Materials and Methods. 6. CHAPTER 6. Versatile methods for improving the mechanical properties of the photoactive layer in an organic solar cell

Introduction

Organic photovoltaic (OPV) devices have received much attention in the past two decades due to their low-cost and ease of fabrication using low-cost and scalable solution processing methods such as roll-to-roll. The research in this area went mostly toward improving the power conversion efficiency (PCE), and the recent progress has demonstrated the best at 17% PCE. [1] Besides, they certainly offer other advantages such as portability, lightweight, flexibility, and to some extent, stretchability. One aspect of material design, which is rarely considered but essential, is mechanical compliance, as several interesting other potential applications require elasticity like foldable smartphones, human skin, and wearable electronic textiles. Krebs et al. have actively focused their work on the degradation of OPV and the viability of its commercialization. [2][3][4][5][6] Interestingly, they found that one of the major problems of OPV prototype tested in the field with children was a mechanical failure. [4] Excellent performance along decent mechanical deformation could grant the long-term durability of the device. Stretchability can be induced by applying architectural modifications to a device, or by manipulating materials chemistry and blending. [7][8][9][10] However, with the fast progress in the organic bulk heterojunctions (BHJ) system, an adaptable method is now required to satisfy the new conjugated blends as well. To minimize costs and avoid the time-consuming and complexity associated with the chemical alteration of new polymers, the development of a general method for stretchable BHJ would be more appropriate.

Crosslinking is a multipurpose tool to link molecules together by chemical bonds or bridges under a stimulus (ultraviolet irradiation or heat treatment). This strategy polymer, which is not surprising. On the other hand, the elastomer blending tends to increase the CCL promoting phase separation of P3HT:PCBM, as confirmed by the optical image. In the presence of cross-linkers, the intensity of the PCBM ring increases, and a small shift to lower q value is perceived from 14.0 to 13.7 nm -1 for C12N3 and 13.8 nm -1 for PEG3N3. This could be due to the cross-linking effect on PCBM. PCBM scattering center does not shift in the presence of elastomers. However, the crystallinity of PCBM decreases in the embedded film with SEBS and PDMS. The relative degree of aggregation to neat P3HT:PCBM of (100) peak

Mechanical Properties

Young's Modulus

On the contrary to the common opinion that organic materials are ductile, P3HT:PCBM is stiff (2-3 GPa) [18,[52][53][54] and not the most suitable for plastic electronics. The Young's moduli of P3HT:PCBM mixed with additive are plotted in Figure 6-15 and were deduced by strain-stress curves. The pristine P3HT:PCBM shows a Young's modulus of 2.1 GPa. This is comparable to the literature. [18,[52][53][54] The diazide cross-linker components decrease the elastic modulus of the P3HT:PCBM. The addition of 50 wt% crosslinker decreases P3HT:PCBM modulus by 2.4-fold for C12N3, 2.9-fold for PEG3N3, and 4.6-fold for PEGnN3. Similarly, with elastomer additives, the elastic modulus of P3HT:PCBM film drops from 2.1 GPa to 0.3 GPa in the presence of 50 wt% SEBS, and to 0.7 GPa for 50 wt% PDMS. Two effects due to the incorporation of than the cross-linkers. On the other hand, the blending of P3HT:PCBM with elastomers promotes the elasticity to 54% strain with 50 wt% SEBS. This suggests an excellent softening effect of SEBS, as previously seen for P3HT:SEBS composite. [31] of the layer [55,56] (Figure 6-16a). After this transition, the material keeps softening with no stable rubbery plateau observed, indicating a creep behavior. As far as films with additives are concerned, two types of behavior can be observed. For small molecular weight additives (C12N3 and PEG3N3) only one transition occurs also, in the same temperature range than neat P3HT:PCBM but with a less pronounced modulus drop.

For macromolecular additives (PEGnN3, SEBS and PDMS), an additional transition can be observed at lower temperature (-40 and -20°C), corresponding to the α relaxation of the macromolecular additives themselves. Interestingly, with increasing the temperature above the transition of P3HT:PCBM phase, a rubbery plateau is reached for all three samples indicating that no creep is observed in these conditions, on the opposite of neat films and films modified with low molecular wight additives. From the variation of loss tangent depicted in Figure 6-16b, it can be seen that no modification of the α temperature is observed for the sample with SEBS, demonstrating poor interaction between SEBS and P3HT:PCBM phase, due to the heterogeneity of the composite in accordance with the micrographs (figure 678). On the other side, a single glass transition is observed with all other compositions with a shift to lower temperature and a broader transition range, attesting the miscibility of the additives with semiconductor polymer. Plasticizer addition results in a progressive decrease of the α relaxation, acting as a lubricant in polymer chains. [57] Accordingly, the glass temperature depression occurring (Figure 6-16d) attests to the softening effect, as suggested by the Young's modulus.

Photovoltaic performances

Photovoltaic performances were measured in order to investigate the influence of additives on the photovoltaic efficiency. elastic modulus reported between 0.1-0.4 GPa for neat PCE10. [65] Elastic moduli decrease up to 60 MPa by stress-strain and 77 MPa by buckling with 50 wt% SEBS.

This decrease represents between 80 to 85 % reduction of initial Young's modulus.

Buckling method leads to a Young's modulus of the same order of magnitude compared to stress-strain method. SEBS blending offers an excellent softening effect. The strain at break increases by 4-fold with 50 wt% SEBS compared to the pristine, demonstrating an increase of the stretchability. These results are coherent compared to the previous study with P3HT:PC61BM presence of SEBS. PDMS, the photovoltaic performances drop, leading to low PCE. This may be due to possible surface defects (confirmed with the large decrease of FF value) and a high reflection index. The removal of the glass substrate in the electrode/PDMS system was not tested here, as the system is extremely fragile, and the contact was difficult. Subsequently, the fabrication of conducting, transparent, and stretchable electrodes based on PEDOT:PSS was reported using an innovating approach combining an interpenetrated poly(ethyleneglycol) (mPEG) and a surfactant (Zonyl). This later assures the deposition on elastomer and improves the ductility of PEDOT:PSS, while the presence of a three-dimensional mPEG network offers excellent conductivity, elasticity, and mechanical recoverability. In this investigation, the additive materials display attractive properties such as conductivity, transparency, stretchability, among others, and state-of-the-art performance that makes it attractive for a wide range of applications. The conductivity is enhanced up to a value of 1230 S/cm with a transparency of up to 95%T at 550 nm. These electrodes are tested in ITO-free solar cells and show power conversion efficiency up to 12.5%. Resistance under strain is recommend that the introduction of an insulating ternary component into the acceptor:donor blend should be optimized and restricted to 25 wt% to beneficiate of the elasticity without the deleterious effect in the transport. From this study, the addition of SEBS elastomer appears to the most favorable for stretchable application. SEBS not only improves the ductility and elasticity, it also straightforwardly dissolves and does not require any post-treatment (on the contrary to the cross-linkers and PDMS blend).

For these reasons, we introduced SEBS into other bulk-heterojunction blends: PCE10:PC71BM and PCE13:IT-4F. Similarly to P3HT:PCBM, the addition of SEBS into those blends affects positively the Young's modulus and improving the overall elasticity. Nevertheless, the fill factor and the current density decreases with the SEBS content, while the VOC is unchanged. This behavior translates a linear decay of the PCE with the amount of SEBS. Below 25 wt% SEBS, the photovoltaic performances remain under 20% loss, i.e. keeping more than 80% of its initial PCE and improving by 3-fold the Young's modulus. These results show that it is possible to tailor the mechanical properties with the PCE depending on the targeted application. Moreover, the implantation of the stretchable active layer on top of stretchable electrode showed their potential for fully stretchable solar cell and the engineering difficulties that can occur when the ITO electrode on glass are switched to a soft electrode on PDMS.

Recommendations

In our study, we preconize versatile implant methods that can be adapted to a different system and establishing excellent property-performance relationships. In general, extending the portfolio of additives for PEDOT:PSS based electrode and BHJ blend will bring reliable conclusions to this emerging field and for numerous applications.

The results obtained with SEBS in the case of a P3HT:PC61BM mixture, allowed us to validate our protocol. Additionally, the incorporation of elastomer into the BHJ blend future research in this field should include the study of the mechanical and electrical properties of Ag NW film and its integration into a photovoltaic device.

Finally, the stability of the cells was not one of the main concerns in this work but remains a critical aspect for potential commercialization. Thermal and under real exposure conditions aging studies need to be carried out and could give a favorable argument to the use of additives such as SEBS for OPVs. This would help to solve the degradation mechanisms that remain elusive. Furthermore, the encapsulation of the solar cell could be deployed as a potential strategy to enhance stability.

APPENDICES: Publications

The following publications are to be/were made during this Ph.D. work: