Anthony Perthué

Thérèse Gorisse

Hugo Santos

Didier Bégué

Agnès Rivaton
email: agnes.rivaton@uca.fr

Guillaume Wantz
email: guillaume.wantz@enscpb.fr

Influence of traces of oxidized polymer on the performances of bulk heterojunction solar cells

Organic Photovoltaic (OPV) is facing with the key challenge of making efficient and stable devices with organic materials, which are susceptible to undergo photodegradation in the presence of atmospheric oxygen. It is therefore essential to determine to which extent a minimal oxidation for the donor material used in the active layer results in solar cell's performance loss. Here we mainly focused on thieno [3,4-b]thiophene-alt-benzodithiophene polymer (PTB7) to be compared to well known poly(3-hexylthiophene) (P3HT). The complexity of the PTB7 chemical structure, based on benzodithiophene (BDT) and thienothiophene (TT) alternation and flanked with alkoxy and alkyl side-chains, necessitates the re-investigation of the first step of the photooxidative process. It is shown that neither intrinsic photochemical process nor the presence of an alkoxy side-chain are critical for the photostability. The high initial sensibility of PTB7 in photooxidative conditions is related to singlet oxygen attack on the conjugated backbone giving rise to the formation of carbonylated species. In a second part, it is shown that traces of PTB7 oxidation, resulting from processing or very short irradiation under ambient air, results in a significant drop in solar cells performance. We also show that the easiness of PTB7 photooxidation is higher than the one of P3HT in line with the higher instability of PTB7-based solar cells. The novel bottom-up approach implemented in this work reveals the importance of traces of polymer oxidation products in altering the solar cells efficiency. The use of unstable materials is susceptible to play a key role in the poor initial performances and/or reduced lifetimes of organic solar cells.

Introduction

Organic solar cells (OSC) technology enables the development of flexible, lightweight and transparent modules. [1][2][3][4] There have been intense research efforts devoted to increase the ability of OSC to convert sunlight with high power conversion efficiency (PCE). 5,6 In recent years, research work was mainly focused on the improvement of the absorbing material used in the active layer with the development of the so-called low band-gap copolymers based on the alternation of donor-acceptor moieties. 7 PTB7 a thieno [3,4-b]thiophene-alt-benzodithiophene polymer is one of the most famous low band-gap copolymers with achieved efficiencies of up to 9%. [8][9] The backbone structure of PTB7 is complex, based on the combination of two units: thienothiophene (TT) and benzodithiophene (BDT). The BDT unit is flanked by two alkoxy sidechains whereas the side-chain of the TT unit is linked to the backbone by an ester group (Figure 1). Breakthroughs in OSCs stability remain necessary before large scale production of this promising cost-effective renewable electricity generation technology. Beyond successful attempts to address the stability issues such as using inverted architectures, stable electrode materials or ultra-barrier encapsulation, the ultimate OSCs lifetime is limited by the stability of their organic active components. Some studies have revealed a relatively low stability of devices based on PTB7 and of PTB7 deposits under illumination. [10][11] Recent results evidenced the role played by the polymer chemical structure in the UV-degradation resistance under inert atmosphere of bulk heterojunction solar cells. 12 Environmental stress like air exposure combined with illumination drastically increases the instability of organic materials and related devices. It should be noted that a wide variety of parameters, including the molecular weight and the dispersity of the polymer 13,14 , the nature of solvent used for film deposition or the use of high boiling point solvent additives 15 , were shown to dramatically impact the photochemical stability of active layers. Therefore, with the aim of making efficient and stable devices with readily photooxidable polymers, a key issue is the understanding of the effects of a small amount of oxidized polymer, incorporated in the active layer of solar cells. On top of that, little is known on the photodegradation mechanisms responsible for the polymer early step degradation. In what operating conditions is the polymer unstable? What is the vulnerability point of the chemical sructure? Which transient species are involved in the photodegradation process? Here are some of the questions that we try to answer in this study.

Photodegradation is in essence the effect of light on the material (intrinsic photochemistry), the combination of oxygen and/or water with light exposure (photo-oxidation) being susceptible to drastically impact the intrinsic photochemical processes (also called photolytical processes). 16 In the recent years, several research groups have investigated the PTB7 underlying degradation mechanisms by experimental or modelling approaches, without reaching a consensus. Indeed, due to the complexity of the chemical structure of PTB7 (and the same is true for most of the recent push-pull low band-gap copolymers) numerous and sometimes conflicting mechanisms were suggested to account for the PTB7 photochemical reactivity. Darlatt et al. 17 and Shah et al. 18 have focused on the intrinsic light induced degradation of PTB7 and suggested different processes based on experimental study and Density Functional Theory (DFT) modelling respectively. In the same way, different mechanisms were proposed by Kettle et al. 19 Son et al.

11

, Alem et al. 20 and Razell-Hollis et al. 21 to explain the photodegradation of PTB7 when light exposure is performed at ambient air (photo-oxidation).

Answering the crucial issue, dealing with the effect of donor material degradation on OSCs performances, is quite challenging: all the components of the multilayer architecture of OSCs are prone to degrade. Studies focused on the stability of devices based on PCDTBT (poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2thienyl-2',1',3'-benzothiadiazole)], another well-known conjugated polymer, suggested that PCDTBT degradation within the active layer of devices occurs under operating conditions, and that higher polymer molecular weights and inverted architectures enhance the stability of devices. 22 But it is crucial to investigate further, and it is what we propose in this work using PTB7 as prototypical low bandgap polymer.

In the first part of this paper, we reconsider the mechanisms proposed for PTB7 degradation 11,[17][18][19][20][21] based on new experimental results obtained by combining IR analysis of thin films with DFT modelling and with the effect of chemically generated singlet oxygen. We also compared PTB7 to another polymer with a slight different chemical structure: PTB7-Th (also known as PBDTTT-EFT or PTB7-Th in various reports, see Figure S1). On PTB7-Th, the BDT side chain is linked with a thiophene instead of an ether to the backbone. The effect of PCBM (phenyl-C 61 -butyric acid methyl ester) fullerene derivative on PTB7 photodegradation is also reconsidered. In a second part, we implement a new bottom-up approach 23 to directly evaluate the impact of PTB7 deposit and minutes photo-oxidation on the efficiency of devices. The same experiment was also performed and compared with poly(3hexylthiophene) P3HT, the work-horse of polymer photovoltaic research. Finally, we show that the oxidation products, even in very low concentration, significantly impact the performances of solar cells.

Results and discussion

Photochemical instability of PTB7. The irradiation of conjugated polymer thin films and tracking the photobleaching over time, i.e. the rate of absorption loss, allows to quickly screen their photostability. [24][25] The loss of absorption is related to a loss of conjugation resulting from photochemical reactions. Whether irradiation is performed in absence of oxygen (hereafter called photolysis) or under ambient air (hereafter called photooxidation), the photobleaching rate allows to quickly evaluate the intensity of different degradation mechanisms: intrinsic light degradation resulting from the light exposure alone or the so-called photooxidative degradation in the presence of oxygen.

Absorption spectra of PTB7 deposits, irradiated either under ambient conditions in air or encapsulated in a glass tube sealed under secondary vacuum were periodically measured to track the decrease of the visible absorption with time (see ESI, Figure S2).

Irradiation in vacuum-sealed glass tubes was performed in order to simulate the conditions of ultra-barrier encapsulated solar cells and to address the issue of intrinsic photo-degradation.

In order to compare the photobleaching rates between the two different environments, the normalized absorbance versus time is plotted in Figure 1. As currently observed for other conjugated polymers [26][27][28] PTB7 essentially undergoes a very slow decrease of its visible absorption in deoxygenated environment while fast absorption loss, within hours, occurs in the presence of oxygen. The first crucial issue is to determine the underlying degradation mechanisms and the corresponding Achilles' heel of the PTB7 chemical structure. Please do not adjust margins Please do not adjust margins Darlatt et al. studied by ultraviolet photoelectron spectroscopy (UPS) and S 2p X-ray photoelectron spectroscopy (S 2p XPS) the effect of the irradiation wavelength on the PTB7 stability, in conditions where oxygen and moisture are absent. 17 They concluded that photons of wavelengths > 320 nm provoke no significant degradation (at shorter irradiation wavelength, PTB7 degradation would only concern the S atom of the structure). UPS and XPS are very sensitive methods but, as surface analysis techniques, they probe only the first few nanometres of exposed deposits. In our experiment the encapsulating glass filters the radiations below 320 nm. Nevertheless, Figure 1 clearly shows that prolonged exposure in deoxygenated atmosphere provoked a decrease of the UV-visible absorbance of the PTB7 deposits revealing a reduction of the conjugation length. IR spectra reported hereafter (Figure 2) confirm that prolonged exposure in deoxygenated atmosphere induces a decrease of the band at 1568 cm -1 (C=C stretching vibration of the polymer backbone) related to the conjugated skeleton. UV-Vis and IR (Infrared) spectroscopy probes the whole thickness of irradiated films (~100 nm). The remaining question is which part of the polymer, if there is one, could be specifically responsible for this evolution?

Shah et al. used ab initio simulations to investigate the intrinsic light induced degradation of PTB7 at the atomistic level. 18 They have modelled some reversible and irreversible degradation pathways, all of them being linked to the alkoxy side-chain on the BDT unit. Irreversible processes would lead to the formation of defects such as changes in the alkyl side-chain structure, formation of -OH and C=O groups. The involvement of the two first is not observed in the IR spectra reported in Figure 2. As we clearly observe no modification in the alkyl stretching vibration domain (3000-2700 cm -1

) and no increase of absorbance in the hydroxyl domain (3700-3200 cm -1

), the occurrence of these theoretical rearrangements (if they exist) is fairly low. Concerning the formation of new C=O groups, the benzoquinone species suggested by Shah et al. 18 are usually observed at ~ 1660 cm -1

. [START_REF] Lin-Vien | The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules[END_REF] No clear evolution is observed on the IR spectra. The formation of these benzoquinone species is either not significant or eventually overlapped by the pristine ester band. The carbonyl region shows the ester vibration on the TT unit of the pristine structure (1680-1750 cm -1

). To get more insight in the evolutions provoked by exposure in this domain, we subtracted spectra (irradiated -pristine). The modifications observed are characteristic of a shift of the ester vibration bands to higher wavenumbers. It is worth noting that the same evolution was observed with PTB7-Th submitted to irradiation in deoxygenated atmosphere and therefore cannot be related to a hypothetical modification of the alkoxy group which is only present in the PTB7 structure. The shift of the ester band to higher wavenumbers was also observed under every stress conditions (temperature and light w/ or w/o oxygen) for both PTB7 and PTB7-Th (see Figure S3 in ESI and Figure 4a presented later), and the spectroscopic modifications observed during photo-oxidation under 18 O 2 presented thereafter (Figure 4b) allow us to assign the ester band modification to an intrinsic re-organization. The shift of the ester band to higher wavenumbers could suggest that its chemical environment is modified: possible interaction with the F atom could come into play.

29

Coming back to the alkoxy group (-OR) of PTB7, Kettle et al. suggested that light exposure provoked the homolytical scission of the BDT-OR bond, even in the absence of O 2 . 19 We were able to calculate at the DFT level of theory the energy of the BDT-OR bond which is only of 55 kcal.mol -1 thus suggesting that all the photons of  < 520 nm are able to provoke the homolytic scission of the BDT-OR bond (see also Table S2 in supporting information). It is usually observed that when pendant groups are cleaved from the main chain, part of them is susceptible to migrate out of the film in the gas phase. [START_REF] Tournebize | [END_REF] But this process is not significantly occurring in our irradiation conditions, as no modifications in the alkyl side chain stretching vibration domain (2950 -2850 cm -1

) can be observed even after prolonged exposure (2000 h) as shown in Figure 2. To explore this issue, we have submitted PTB7-Th films to irradiation in the same conditions. Data reported in Table S2 clearly show that the scission of the alkyl side chain requires more energy in the case of PTB7-Th (109 kcal mol -1

) compared to PTB7, but still may be cleaved by UV photons (< 330 nm). Curves reported in Figure 3 show that the decrease of the visible absorbance occurs at the same rate in PTB7 and PTB7-Th deposits. Interestingly the presence of PCBM slows down the PTB7 and PTB7-Th absorbance loss, and the same effect was previously monitored when P3HT and P3HT:PCBM films were submitted to prolonged exposure in the absence of oxygen. 31 One could hypothesize that PTB7 and PTB7-Th have the same evolution as the one postulated for P3HT irradiated in deoxygenated atmosphere 32 : the homolytical scission of the sidechain could be followed by addition on the conjugated backbone. Such saturation reaction which induces a reduction of the conjugation length of the pi-conjugated system could account for This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins the decrease in the visible absorbance observed in the UV-visible spectra. The stabilising effect provided by the fullerene derivative in polymer:blend active layer was then ascribed to its ability to scavenge free radicals, hindering thus the radical-based degradation mechanism of polymers.

Should we conclude that the degradation observed in deoxygenated conditions is only related to these supposed intrinsic photo-degradation processes? It cannot be ruled out that oxygen traces are still remaining in the polymer films especially in our conditions of encapsulation and analysis which inevitably involve the contact with atmospheric oxygen. To infirm or confirm this hypothesis, the influence that the (re)encapsulation process may have on the introduction of oxygen was assessed. To do this, polymer films were encapsulated in glass tube under secondary vacuum and analyzed after 2000 h of exposure, with no intermediate steps. Interestingly, an Absorbance loss of only 6.5% was observed after completion of 2000 h of photolysis to be compared with the 12% obtained for the same batch of sample sealed in the exact same conditions but opened and re-sealed multiple times during the 2000 h of irradiation (see Figure 3 bottom). The different extents of degradation in the two cases indicate that each time the tube is opened to analyse the sample, a certain amount of oxygen is introduced and trapped in the polymer film. Samples analysed at several irradiation steps have been in contact with higher amounts of oxygen, thus explaining the higher absorption loss. It is therefore possible to conclude that contrary to what might have been feared on the basis of theoretical calculations, the presence of O atom between the side chain and the BDT units is not the crucial point that renders PTB7 vulnerable under light exposure in deoxygenated atmosphere. Our main conclusion is that the chemical structure of PTB7 is intrinsically resistant to light exposure in optimal encapsulation conditions. However, oxygen traces in combination with light probably provoke the tiny chemical structure evolutions observed upon photolysis experiments. Therefore, if atmospheric oxygen has such a huge effect on PTB7, it seems appropriate to elucidate the reason why. This is the subject of the following section.

Previous studies devoted to the photooxidation of PTB7 suggested an attack by singlet oxygen, but the postulated oxidation site and resulting products were different. Thanks to sensitization techniques performed in solution combined with molecular orbital calculations, Son et al. suggested that singlet oxygen attacks the TT moieties of the polymer, and proposed that the thienyl ring directly linked to the polymer main chain is most likely the site of a [2 + 4] cycloaddition.

11

. Alem et al. proposed on their side that singlet oxygen can react with the BDT unit of PTB7 according with the wellknown behaviour of anthracene with 1 O 2 20 Razzell-Hollis et al.

studied the photooxidative degradation of PTB7 using Raman spectroscopy combined with DFT modelling. 21 Short exposure in the laser beam of the Raman apparatus provoked a shift of a band attributed to BDT unit without intensity decrease. This observation has led the authors to suggest that the initial oxidation of PTB7 involves the hydroxylation of the BDT unit of PTB7 without absorbance decrease as a result of singlet oxygen attack. It is worth recalling that hydroxyl groups formation was not directly observed and that the UV-Vis spectra were not recorded in parallel to the Raman spectra. Additionally, no detailed mechanism was proposed to explain how singlet oxygen attack provokes PTB7 hydroxylation.

To get deeper information in the photooxidative mechanism, we tackled the problem from the direct observation of the effect of short time exposures using both UV-vis monitoring and IR spectroscopy which is a very efficient method to track the formation and identification of oxidation products of polymers. We also performed thermooxidation experiments as well as exposure in 18 O 2 atmosphere, and used a new method allowing to assess singlet oxygen involvement in the photo-oxidation of conjugated materials. 33 Photo-oxidation (Suntest device, samples temperature ~37°C) and thermo-oxidation (ventilated oven at 100°C) of PTB7 deposits induce a progressive loss of the UV-visible absorbance, indicating that the pi-conjugated backbone is broken during ageing (see ESI, Figures S2 andS4). Photo-oxidation and thermo-oxidation also result in rapid and noticeable modifications in the IR spectrum of PTB7. Upon ageing, a rapid disappearance of the various functional groups of PTB7 is observed. At the same time, new absorption bands in the carbonyl, hydroxyl and fingerprint regions develop. Figure 4a (subtracted spectra, direct spectra are reported in ESI, Figure S5) compares the IR modifications for PTB7 samples thermooxidized in air at 100 °C or photooxidized in Suntest device in air. As observed previously upon irradiation in deoxygenated atmosphere, photo-and thermo-oxidation induce the rearrangement of the ester group. The increase of absorbance observed after long duration exposure (> 3 h) in the hydroxyl region (3200 cm -1

) could be attributed to the OH vibration of carboxylic acids. Their C=O vibration is most likely overlapped by the initial ester band of the TT moiety as confirmed below with 18 O 2 photo-oxidation experiments (Figure 4b). Photo-and thermo-oxidation also lead to the formation Please do not adjust margins Please do not adjust margins of numerous bands in the fingerprint region, as well as the decrease of the intensity of all the PTB7 bands (conjugated skeleton and sidechains). But the most noteworthy aspect of very short duration photooxidation (1 h -3 h) is the formation of two bands at 1630 and 1660 cm -1

, whereas no increase of absorbance in the hydroxyl domain is observed. The band at 1630 cm -1 is not observed upon thermo-oxidative degradation. We have checked that post-thermal treatment at 100°C does not decrease the intensity of this band. It is then possible to conclude that its formation involves pure photochemical processes or photochemically produced transient species. The 1630 cm -1 band appears in the domain of carbonyl products and/or double bonds. To determine which explanation is the most likely, we performed the same photo-oxidation experiment but under ). This band is overlapped by the ester vibration and thus it is not observed in ambient air ( 16O 2 ) irradiation conditions. This information confirms our previous attribution of an OH vibration (3200 cm -1

) to carboxylic acids. The ester vibration bands modification on its side is clearly not modified by the irradiation under 18 O 2 , thus confirming that this evolution results from an intrinsic structural modification of the ester moieties. To complete the analysis of the IR spectra evolution, the decrease of the alkyl bands intensity (2850-2950 cm -1

) is correlated with the detection of low molecular weight oxidation products in the gas phase (see ESI, Figure S6 and Table S1). Moreover, the oxidation of the sulfur atom of the thiophene moieties leads to the formation of sulfinic ester as shown by the formation of a band at 620 cm -1 after long photooxidation duration (> 30 h) (see Figure S7 in ESI).

Finally, it was necessary to evaluate if the carbonyl products created at 1630 cm -1 are assigned (or not) to singlet oxygen attack on PTB7 structure. Figure 4c shows the IR spectra of PTB7 chemically treated with singlet oxygen according to a new protocol that has been recently developed. [START_REF] Perthué | [END_REF] Clearly, singlet oxygen attack result in the development of the band at 1630 cm -1 thus allowing to conclude that i) singlet oxygen attack leads to the formation of carbonyl products and not hydroxyl adducts, ii) the very first step of PTB7 photooxidation involves singlet oxygen attack.

- Please do not adjust margins Please do not adjust margins Singlet oxygen is a dienophile and an electrophile which is going to attack the electron-rich regions of PTB7. In order to probe the most probable regions of the molecule susceptible to undergo an oxygen attack, we have calculated and plotted the Condensed Fukui Functions (CFF) for both molecules (PTB7 and PTB7-Th structures). In zeroth order, it would be enough to calculate the electronic density of the HOMO orbital but this approach does not take into account any orbital relaxation. The CFF are able to describe how the attack probability upon a definite reaction is distributed on the molecular coordinates. Three CFF exist, f + , f -and f 0 , describing, respectively, the easiness of the molecule to undergo an electrophilic, nucleophilic or a radial attack. Generally speaking, for a molecule having N electrons in its ground state, these functions are calculated as follows:

where q k is the charge of the k-th atom of the molecule for a cationic (N-1) or anionic (N+1) electron states. Oxygen being an electrophile, we need to estimate the propensity of the molecule to suffer such electrophilic attack. In this way, plotting f -can indicate the most probable regions of the molecule to do so. Thus, one can note from Figure 5 that the BDT donor group is the one that is most probable to be attacked by singlet oxygen in PTB7 and in PTB7-Th structure. Our conclusion obtained by modelling is in line with the one of Razell-Hollis et al.. 21 We have also calculated the electrophilicity of each one of PTB7 and PTB7-Th structures in an attempt to quantify their photo-oxidation easiness. The full details can be found in the ESI, but briefly: the electrophilicity can be understood as the ratio between the eagerness of a molecule to capture an additional electron from its environment (chemical potential) and its resistance to back-transfer it. In this way, the higher the electrophilicity of a compound is, the higher also is its resistance to an electrophile attack. Our calculations show that PTB7-Th and PTB7 have very similar electrophilicities which very little differences do not allow us to conclude about which molecule is the least probable to undergo an electrophile attack.

The identification of by-products is essential to validate a proposal of reaction mechanism. Based on modelling of the Raman spectra of PTB7 deposits irradiated in pure oxygen or in situ in the Raman apparatus, Razell-Hollis et al. suggested that oxygen attack on the BDT unit of PTB7 results in the mono-and di-hydroxylation of the thiophene rings of the BDT unit. 21 In our conditions representative of outdoor exposure we observe the formation of carbonyl products, not hydroxyl groups. Carbonylated species identified on the IR spectra at 1630 cm -1 in Figure 4 probably result of the decomposition of highly unstable primary products issue from singlet oxygen attack (typically endoperoxide (R-O-O-R'), hydroperoxyde (R-O-O-H…). [35][36] What is certain, however, is that carbonylated species identified on the IR spectra at 1630 cm -1 are not issue from mono-or dihydroxylated structures modelled by Razell-Rollis et al. 21 as these structures can be considered as rather stable in our mild irradiation conditions. While it has now been proven that PTB7 degradation is initiated by singlet oxygen attack one can ask about the role of the free-radical chain oxidation route which is generally thought to lead the polymer photo-oxidation and starting with a hydrogen abstraction. It appears that the carbonyl products observed at 1630 cm -1 does not accumulate much in the film but other degradation products observed at 1660 and 1790 cm -1 continue to be formed. Those products are willingly attributed to find their origin either from the free-radical chain oxidation mechanism, which can take place in parallel to the singlet oxygen reactivity, or from the next steps occurring after singlet oxygen reactivity on PTB7. Two action modes are known concerning singlet oxygen reactivity: the first one being the formation of endoperoxide species and the second one being the formation of a hydroperoxide. Both mechanisms can lead to the formation of radicals which will in turn start a free-radical oxidation route by hydrogen abstraction on the side chain of the polymer, finally conducting to the oxidation of the sulphur atoms of the thiophenic moieties (see Figure S7) as reported for many conjugated polymers. 25,30,37 . Our results differ from the literature on one point though: the PCBM effect. It appears that, in our irradiation conditions, PCBM increases the PTB7 and PTB7-Th photostability which could validate the involvement of radical process inhibited by the radical scavenging properties of the fullerene cage (see Figure 4d). Interestingly Razzell-Hollis et al. 21 and Tremolet de Villers et al. 38 observed no photo-stabilizing effect of PC 71 BM on respectively PTB7 and PTB7-Th photo-oxidation. The degradation conditions were different in both cases: for the former, pure oxygen and the laser beam of the Raman apparatus were used and remaining di-iodooctane (DIO) as processing co-solvent might have drastically impacted PTB7 stability. 15,39,40 For the latter, the thickness of the polymer deposits compared to the one of the blend (twice the thickness) may have played a significant role in the photo-oxidation rate as previously reported. Another possibility is that PC 61 BM used in the present study has a much better stabilizing

Please do not adjust margins Please do not adjust margins effect due to a higher electron affinity as this goes hand-in-hand with a lower rate of blend photobleaching according to Hoke et al..

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Importantly, the data reported in Figure 4d shows that the photooxidation rate of PTB7 and PTB7-Th are roughly the same, thus indicating that the presence of a thiophenyl group or O atom linking the side chain to the BDT unit does not play a crucial role. However, DFT modelling reported in ESI show that the link between the BDT unit and the alkyl chain plays a key role in the oxidability of the side-chain versus a chain radical oxidation. Clearly the presence of the thiophenyl group in PTB7-Th renders the H atom on the CH 2 in alpha position quite labile. Therefore, all these results suggest that the key point in low band-gap polymers based on the BDT-TT alternation is their easiness to be attacked by singlet oxygen, and that the nature of the side chain plays a minor role.

Effect of PTB7 oxidation traces on OSC efficiency. Finally, the important question to be addressed is to determine if small amounts of oxidation products, unavoidable because of the easiness of PTB7 photooxidation, could alter the electronic properties of the devices. The additional issue here is to determine if it is the amount of oxidation products or their chemical structure that plays the most important role. It was therefore decided to compare the effect of very short photooxidation duration of PTB7 to the one of P3HT as the photooxidative degradation of this latter does not involves singlet oxygen. [START_REF] Perthué | [END_REF][42][43] To determine the impact of PTB7 and P3HT oxidation traces on OSC performances, we have implemented a bottom-up approach depicted in a previous paper.

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Thin films of PTB7 and P3HT were first exposed to different photooxidation duration times: from 0 to 20 min for PTB7, and from 0 to 60 min for P3HT. The duration is longer for P3HT as this polymer is more resistant to photooxidation compared to PTB7 in terms of photobleaching. This is clearly observed in Figure 6 (left) which shows the decrease of the UV-Vis absorbance of P3HT and PTB7 deposits submitted to short duration photooxidation. The irradiation duration was limited so that the absorbance decrease is minimal. It is noteworthy that after 3 min / 20 min exposure for PTB7 / P3HT respectively the absorbance decrease of the deposit is lower than 2%. In addition, the corresponding level of oxidation is so low that oxidation products were barely observed to be formed for PTB7 by IR spectroscopy (see Figure S8 in ESI).

After irradiation, polymer thin films were re-dissolved and then blended with PC 61 BM and used to produce inverted bulk heterojunction solar cells. The efficiencies (PCE) of OSCs processed with partially photooxidized P3HT and partially photoxidized PTB7 deposits are presented in Figure 6 , open-circuit voltage (Voc) of 0.52 V and 0.72 V, and fill factor (FF) of 0.57 and 0.71 respectively with P3HT and PTB7. We can observe that quite different photovoltaic performances are obtained when polymers were marginally photooxidized. Indeed, the PCE undergoes a rapid drop: a 20% decrease of the PCE is observed when solar cells are made with PTB7 and P3HT photooxidized 3 and 20 min respectively; PCE is close to zero after 20 min of photo-oxidation in the case of PTB7 whereas 60 min of photooxidation in the case of P3HT provokes only less than 50% decrease. Importantly the similarity of the kinetics reported in Figure 6 left (polymer oxidation tracking) and right (devices performances) suggest that the efficiency of OSC performance is clearly related to the level of polymer oxidation. Note that the scales in Figure 6 (left and right) are very different: between 100% and 90% for Absorbance loss (left) and between 100% and 0% for Photovoltaic characteristics (right).

To determine whether the chemical structure of oxidation products plays a role in the alteration of the PV properties of the devices, we have reported below the evolution of the main photovoltaic parameters: fill factor (FF), short-circuit current (Jsc) and opencircuit voltage (Voc) of solar cells based on photooxidized PTB7 and photooxidized P3HT (Figure 7). The loss of short circuit current is the most detrimental origin of loss of efficiency. The loss of absorbance is only minimal as can be seen on Figure 6 (e.g. lower than 2% after 3 min / 20 min exposure for PTB7 / P3HT respectively). It cannot explain on its own the drop of Jsc. As a consequence, the traces of oxidized polymer (P3HT or PTB7) are acting as traps or recombinaison centers. In addition, PTB7 photooxidation involves volatile formation (Figure S6 and Table S1 in ESI) as well as polymer backbone scissions 15

. Such a reduction in molecular weight could also contribute to the decrease of photovoltaic performances, as it has been shown in various studies. 13,44,45 It is important to notice that here only the polymer was partially oxidized on purpose prior device fabrication, meaning the PC 61 BM was pristine. We also conducted experiments in which the whole BHJ blend PTB7:PCBM (or P3HT:PCBM) was exposed to gradual photooxidation prior to device fabrication following the same experimental procedure. The results are that the photooxidation of the blend gave the same trend of results (Figure S9) as when the polymer only was oxidized. As said previously blends are also sensitive to oxidation, the presence of fullerene does not completely inhibit the formation of photooxidized products. On top of that, in a previous study, we have shown on a similar protocol that the photooxidized PCBM is also a killer of solar cell performances. 23 Here, we guess that the blends are containing both: traces of photooxidized polymer and traces of photooxidized This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins PCBM. However, it is not stronger with blend. This is the first time it is shown that degradation of performance of polymer solar cells could be related to the polymer oxidation itself as a newly evidence mechanism of degradation in a real operating device.

Conclusions

We have shown that PTB7 is steadily oxidized when exposed to the combination of light and oxygen. This vulnerability is coming from the facility of the BDT unit in BDT-TT alternation structures to be attacked by singlet oxygen, thus leading to the formation of carbonylated species. Due to the high "oxidizability" of PTB7, our results suggest that the involvement of intrinsic photochemical processes that can be theoretically modelled should be considered as an epiphenomenon versus the effect of oxidation traces during fabrication or upon operating conditions. A key result highlighted by this study is the demonstration that polymer oxidation traces, resulting from indoor processing or minutes photooxidation, have a dramatic negative impact on the efficiency of PTB7 based solar cells. The bottom-up approach implemented in this work clearly emphases the key role of traces of polymer oxidation product on solar cells efficiency. This degradation mechanism is scarcely ever put forward due to a lack of sensitive methods. Further investigations may be needed to better identify the exact role of polymer oxidation defects in the photovoltaic mechanism. This study should guide the synthesis of novel low band gap polymers that are more resistant to photooxidation, and more particularly not attacked by singlet oxygen, allowing participating in maintaining OSC efficiency over long lifetimes. As we determined that traces of polymer oxidation product act as an important loss mechanism, with the actual materials, ultrabarrier encapsulation is so far mandatory to maximize the actual solar cells durability.

O 2 with an isotopic purity of 97 % was obtained from Sigma-Aldrich.

Sample preparation

Thin film preparation. Deposits on KBr substrates and solar cells were prepared through spin coating. Large surface area glass plates coated with PTB7, PCBM or PTB7-PCBM blend were prepared by bare-coating according to a method recently described.

23

Device processing. ITO-coated glass was washed through ultrasonic treatment in soap / de-ionized water solution, in pure de-ionized water and in isopropanol (15 min each) and subjected to UV-ozone for 10 min to complete the cleaning procedure. The electron transfer layer consists of a 40-nm-thick ZnO sol-gel layer: zinc acetate dihydrate, dissolved with monoethanolamine (both at 0.15 mol l -1

) in absolute ethanol, was spun coat at 2000 rpm for 60 s on the ITO substrate and annealed at 180°C for 1 h. The samples were then transferred into a water-and oxygen-free environment (glove-box). The blends were prepared following the same procedure as described above. After the blend deposition, the substrates were transferred to a thermal evaporator, where the top electrode, 7 nm of MoO 3 and 70 nm of Ag, was deposited at 3x10 -6 mbar. The final device area was 10 mm², as defined by a shadow mask. Thermal ageing. Thermo-oxidation was conducted in a ventilated oven at a temperature of 100 °C.

Ageing

1 O 2 treatment. PTB7 and PTB7-Th were submitted to singlet oxygen in chloroform solutions, at ambient temperature and in the dark as already reported for other semi-conducting organic materials.

33

Singlet oxygen was generated through the cycloreversion reaction

Please do not adjust margins Please do not adjust margins occurring at ambient temperature of dimethylnaphthalene endoperoxide which was synthesized according to Turro et al.

46

Characterizations

Spectroscopy. UV-visible spectra were obtained with a UV2600 spectrophotometer equipped with an integrating sphere from Shimadzu.

Infrared (IR) transmission spectra were obtained using a Thermo Scientific Nicolet 6700 spectrophotometer purged with dry air at a resolution of 4 cm -1 by summing 32 scans.

HS-SPME-GC-MS.

HS-SPME-GC-MS (headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography/mass spectrometry (GC-MS)) was employed to collect and analyze the volatile compounds created upon photooxidation of PTB7 films, irradiated in sealed vials. The procedure has been detailed for studies concerning other polymer matrices.

47

Photovoltaic performances. Current density vs voltage curves were measured for each device using a Keithley 2400 analyser under an AM1.5 solar simulator with a Xenon lamp calibrated with a radiometer (IM1400) at 100 mW cm -². The power conversion efficiency (PCE), fill-factor (FF), short circuit density current (Jsc) and open circuit voltage (Voc) were extracted from these curves. For each condition, 2 substrates, each with four cells with 10 mm² of surface area, were fabricated. Because a few devices malfunctioned, the average and standard deviations were calculated with between 6 and 8 values.

Modelling. Molecular geometries were optimised within the B3LYP/def2-TZVP Density Functional Theory (DFT) level of theory using the D3BJ dispersive correction to account for long-range corrections in the unrestricted wave-function formalism. The RIJCOSX approximation with the def2-TZVP/C auxiliary basis set was used throughout the calculations. Energy and gradients had their convergence criteria set to very tight. All the calculations were performed using the Orca 3.0.3 software. Whenever needed, molecular orbitals were plotted using a 0.02 isosurface value. The Molecular Electrostatic Potential (MEP) was used to map the Total Electron Density (TED) using a 0.001 isosurface value. Plotting was performed for differences in electrical charge from -2E-2 to 2E-2e. Time-dependent Density Functional Theory (TDDFT) was used in the same level of theory to calculated the UV-VIS electronic transitions as well as to probe the energies of the first excited states. 30 roots were calculated and the analysis was performed by using both Differences of Transition Densities (DTD) and Natural Transition Orbitals (NTO). Finally, calculations of the ionic states were done by performing single point calculations on the ground-state geometry (i.e., in the vertical approximation). Whenever the Bond Dissociation Energies (DBE) were calculated, the same level of theory was used except for the choice of the basis set, for which a 6-31G** one was used for such calculations. These DBE calculations were performed in the adiabatic approximation (i.e., once broken apart, each fragment had its geometry optimized in the same level of theory).

Notes and references

Figure 1 .

 1 Figure 1. Normalized absorbance loss (maximum absorbance) of PTB7 films submitted to irradiation in ambient air (photooxidation, red symbol) and in deoxygenated atmosphere (photolysis, black symbol). Inset shows the chemical structure of PTB7.

1 Figure 2 .

 12 Figure 2. Infrared spectra of PTB7 submitted to photolysis; direct spectrum of pristine PTB7 (top); subtracted spectra: spectrum of PTB7 photolyzed 2000 h -pristine PTB7 spectrum (0 h) (bottom).

Figure 3 .

 3 Figure 3. Normalized Absorbance loss (maximum absorbance) of PTB7, PTB7-Th, PTB7:PCBM and PTB7-Th:PCBM deposits submitted to irradiation in deoxygenated atmosphere (top). Normalized absorbance loss for PTB7 films in photolysis experiment with or without multiple sealant apertures (bottom).

Figure 4 .

 4 Photooxidation d)

Figure 5 .

 5 Figure 5. PTB7 (left) and PTB7-Th (right) plotting of f -Condensed Fukui Function indicating the most probable regions of the molecule to undergo an electrophilic attack by singlet oxygen

Figure 6 .

 6 Figure 6. (left) Normalized Absorbance loss (between 100% and 94%) of P3HT and PTB7 deposits submitted to short duration photooxidation. (right) Photovoltaic properties (between 100% and 0%) of solar cells based on partially oxidized PTB7 or partially oxidized P3HT.

Figure 7 .

 7 Figure 7. Normalized photovoltaic characteristics (between 100% and 0%) of solar cells based on previously photo-oxidized PTB7 from 0 to 20 min (left) or P3HT from 0 to 60 min (right).

  Pa) before exposure in the Suntest. Short irradiations were also conducted by enclosing the samples in a borosilicate tube filled with a slight overpressure of synthetic air composed of 20 % of either

	Irradiation. Photodegradation experiments were conducted in a
	Suntest CPS/XLS Atlas device. A xenon lamp from ATLAS (NXE1700)
	configured at 750 W m	-2 in the UV-visible domain (300-800 nm) was
	provided with it. IR irradiation and UV photons below 300 nm were
	cut off thanks to the so-called "daylight filter" from Atlas. A cryostat
	allows maintaining the Black Standard Temperature (BST) at 60 °C
	which corresponds to about 37°C in the irradiation chamber.
	Irradiations in absence of oxygen were conducted by encapsulating
	the samples in borosilicate tubes sealed under secondary vacuum
	(10	-6 18 O 2 or	16 O 2 and 80 % of N 2 .

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

Acknowledgements

This work was supported by the Agence Nationale de la Recherche (ANR) through the PROGELEC 2013 program « HELIOS » (ANR-13-PRGE-0006).

Conflicts of interest

There are no conflicts to declare.

Methods

Materials

PTB7 and PTB7-Th were purchased from 1-Materials. PCBM was kindly provided by ARMOR collaborators. Chlorobenzene (CB), orthodichlorobenzene (ODCB), chloroform, absolute ethanol, diiodooctane and Zinc di-hydrate acetate were obtained from Sigma Aldrich. MoO 3 and Ag were purchased from Neyco and the ITO-coated glass from Visiontek.


Aurélien Tournebize

Dargie Deribew

Alberto Gregori
email: gregori.alberto@gmail.com

Roger C Hiorns

Andreas Distler

Hans-Joachim Egelhaaf

Christine Lartigau-Dagron

Ahmed Allal

Agnès Rivaton

Heiko Peisert

Thomas Chassé

Influence of material migration on the mechanical integrity of inverted organic solar cells

Keywords: Stability, ISOS-D3, delamination, PCBM diffusion

We have identified modifications of material distributions in inverted organic solar cell structures during accelerated aging processes that are potentially linked to the loss of the mechanical integrity of the devices. Non-encapsulated devices were treated with ISOS-D-3 ageing (the so called damp heat exposure at 85 °C/85% room humidity in the dark). After performing pull-off tests, the exposed surfaces were analyzed by X-ray photoemission spectroscopy and atomic force microscopy. For fresh devices, the results revealed that mechanical failure occurs close to the PEDOT:PSS/P3HT:PCBM interface, in agreement with the literature. However, after a short exposure to damp heat, XPS investigations indicate an enrichment of PCBM at the PEDOT:PSS/P3HT:PCBM interface and a change in the PEDOT:PSS surface composition. Both phenomena may explain the rapid drop in device performances and the increased tendency towards delamination.

Introduction

Delamination is a known issue for organic solar cells [START_REF] Jørgensen | Stability of polymer solar cells[END_REF][START_REF] Lloyd | Influence of the hole-transport layer on the initial behavior and lifetime of inverted organic photovoltaics[END_REF]. The adhesion between several layers based on metals, and organic and inorganic materials is sensitive to mechanical deformation occurring during solar cell fabrication and the operating lifetime. Moreover, diffusion of water and oxygen [START_REF] Adams | Water ingress in encapsulated inverted organic solar cells: Correlating infrared imaging and photovoltaic performance[END_REF] induces a material dependent degradation [START_REF] Grossiord | Degradation mechanisms in organic photovoltaic devices[END_REF], which can furtherly undermine the device mechanical stability. Although device encapsulation strongly mitigate these effects [START_REF] Kim | A study for degradation of flexible organic photovoltaic modules via damp-heat test: By accessing individual layers of the module[END_REF], the evolution of the layers physico-chemical properties still plays a fundamental role in long-term device stability [START_REF] Sapkota | Promising long-term stability of encapsulated ITO-free bulk-heterojunction organic solar cells under different aging conditions[END_REF][START_REF] Madogni | Comparison of degradation mechanisms in organic photovoltaic devices upon exposure to a temperate and a subequatorial climate[END_REF]. Poly (3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), is one of the most widely used hole-transport materials in organic solar cells. In devices where hygroscopic PEDOT:PSS is used as hole transporting layer (HTL), a particular concern is the diffusion of water molecules resulting in a loss of photovoltaic properties and mechanical integrity [START_REF] Grossiord | Degradation mechanisms in organic photovoltaic devices[END_REF]. This delamination of the PEDOT:PSS layer increases electrical resistance and affects the electric field distribution at the PEDOT:PSS/photo-active interface, resulting in a hindrance of charge extraction processes [START_REF] Adams | Water ingress in encapsulated inverted organic solar cells: Correlating infrared imaging and photovoltaic performance[END_REF][START_REF] Kawano | Degradation of organic solar cells due to air exposure[END_REF]. The subsequent deterioration of various PV parameters, in particular the fill factor (FF), can be observed from the changes in the shape of the current density -voltage (J-V) curves. Specifically, changes occur in the values of the series (RS) and shunt resistances (RSH). A high RS is expected to lower the FF and with it the maximum power point (Vmpp) of the cell.

Recently, we investigated the adhesive properties of non-encapsulated, inverted OSCs based on three different p-type polymers and two PEDOT:PSS formulations [START_REF] Gregori | The role of donor polymer and PEDOT:PSS formulation on adhesion processes in inverted organic solar cells[END_REF], using a controlled pull-off test. With any given combination of active layer and PEDOT:PSS formulation, the fracture plane was near the interface between the two. To gain insight into the adhesive failure and the effect of the manufacturing processes, Dupont et al. investigated the interlayer adhesion in inverted polymer solar cells with respect to the deposition technique be it spin, spray or slot-die coating, of the active layer [START_REF] Dupont | The effect of anneal, solar irradiation and humidity on the adhesion/cohesion properties of P3HT:PCBM based inverted polymer solar cells[END_REF][START_REF] Dupont | Interlayer adhesion in roll-to-roll processed flexible inverted polymer solar cells[END_REF]. They observed the same adhesive failure for all deposition techniques, located at the interface between the active layer (P3HT:PCBM) and the hole transporting layer (PEDOT:PSS), suggesting a material-dependent mechanism. Moreover, they investigated the influence of temperature, solar irradiation, and humidity on these properties. While the thermally induced changes in morphology, specifically vertical phase segregation, lead to an improved adhesion in the case of annealing [START_REF] Balcaen | Thermal cycling effect on mechanical integrity of inverted polymer solar cells[END_REF] and solar irradiation, the combined effect of stress and humidity greatly accelerated the decohesion rate in the hygroscopic PEDOT:PSS layer through a moisture-assisted mechanism [START_REF] Dupont | Decohesion Kinetics of PEDOT:PSS Conducting Polymer Films[END_REF].

In the present work, we examine the change in materials composition at the weakest mechanical point of non-encapsulated inverted devices, degraded according to the ISOS-D-3 protocol [START_REF] Reese | Consensus stability testing protocols for organic photovoltaic materials and devices[END_REF], also known as damp-heat test (85 °C/85% relative humidity, rH, in the dark). The layers were deposed by doctor blading on glass ITO, with the following layer stack ITO/ZnO/P3HT:PC60BM/PEDOT:PSS/Ag, and Clevios TM HTL Solar as PEDOT:PSS. The evolution of both electrical parameters and devices mechanical integrity was monitored by means of JV and pull-off test. The exposed surfaces were then analyzed by XPS and AFM respectively to obtain the chemical composition and the surface topography to identify the fracture plane.

Experimental

OSCs fabrication, characterization and degradation

Inverted devices with the structure glass/ITO/ZnO/P3HT:PC60BM/PEDOT:PSS/Ag were fabricated using standard procedures. First, the glass/ITO substrates were cleaned in an ultrasonic bath of acetone and propan-2-ol for 10 min. Following cleaning, the substrates were dried and treated by UV-ozone for 10 min. A 5 wt% ZnO nanoparticle suspension in ethanol was coated on top of the cleaned glass/ITO substrate and then annealed in air at 140 °C for 10 min. The photoactive layer, 250 nm of P3HT:PC60BM obtained from using a 1:0.8 w/w solution in xylene:1-methylnaphthalene (7:1, v/v stirred overnight) coated on top of the ZnO layer.

PEDOT:PSS (Clevios TM HTL Solar) was coated directly from the commercial aqueous dispersion on top of the active layer. All layers were coated in air using a doctor blade. The devices were then completed by thermally evaporating 300 nm of Ag electrode on a customized mask. Before the evaporation of the metal electrode, the photoactive layer was annealed (N2, 140 °C, 5 min). For degradation, devices were exposed to damp heat in a climate chamber (85 °C/85% humidity) for a specific period of time. For light currentvoltage measurements, cells were illuminated with a simulated solar spectrum from Oriel solar simulator at 100 mW cm -2 .

The J-V characteristics of the devices were recorded using a Keithley 2400 SMU in combination with a Keithley 7001 Multiplexer system and custom software.

Pull-off test

The use of the pull-off test for the characterization of the mechanical stability of OPV devices has been presented and thoroughly described in a previous publication [START_REF] Gregori | The role of donor polymer and PEDOT:PSS formulation on adhesion processes in inverted organic solar cells[END_REF]. For this set of experiments, we used a circular probe with a diameter of 12 mm. The epoxy adhesive (SADER® from Bostik S. A.) was prepared before each measurement. The approaching speed of the probe was 50 µm s -1 , with a sampling frequency of 100 Hz. The force sensor used was a TME F108TC (1-100 N, ±0.05 N). The pulling speed was 10 µm s -1 , with a sampling frequency of 100 Hz. The pictures were taken with a Canon IXUS 132 and images were analyzed using ImageJ [15]. The test was performed on 5 different areas of samples fabricated on different substrates in the same batch, at different degradation stages (0, 0.5 and 8 h).

Surface analyses

XPS measurements were carried out using a multi-chamber UHV-system (base pressure 2 x10 -10 mbar), equipped with a SPECS Phoibos 150 cylindrical hemispherical analyser and a monochromatic Al K source (Focus 500 monochromator, XR50m X-ray source, h(Al Kα) = 1486.74 eV). A Nanoscope IIIa atomic force microscope (AFM) from Veeco Instruments was used for surface topography measurements.

Results and discussion

Degradation and device performances

Inverted devices (glass/ITO/ZnO/P3HT:PC60BM/PEDOT:PSS/Ag) were fabricated and immediately exposed to damp heat (85 °C/85% rH in the dark, ISOS-D3 [START_REF] Reese | Consensus stability testing protocols for organic photovoltaic materials and devices[END_REF]), while monitoring the evolution of the J-V curves and of the photovoltaic parameters with time (reported in Fig. 2). As the devices were not encapsulated, the damp heat exposure induced a rapid drop in device efficiencies. From Fig. 2 b-e), we can clearly distinguish two degradation steps:

i)

A rapid loss of 70% of the PCE occurs during the first hour of DH exposure, due to a simultaneous decrease in Jsc (by 51%) and FF (by 38%), while Voc remains relatively stable, showing a variation of around 7%.

ii) Subsequently, the degradation is driven by a loss of Voc and mainly Jsc, while the FF is almost stable. and shunt (RSH) resistances. The approximate values of RS and RSH are extracted by fitting the semi-logarithmic dark-current density curves using the general equivalent circuit model for organic solar cells [START_REF] Lee | Enhanced diode characteristics of organic solar cells using titanium suboxide electron transport layer[END_REF] (Fig. 3) at forward bias (i.e. near the saturation region) and at the ohmic part of the J-V curve (near the origin), respectively. RS increases from 2.37 ± 0.01 Ω cm 2 of fresh devices to 5.70 ± 0.01 Ω cm 2 after 5 h of damp heat ageing, while RSH decreases by one order of magnitude after only 0.5 h. This behavior is clearly visible in the J-V curves under illumination. It is well known that the fill factor, as well as the JSC, are largely influenced by series and parallel resistances [START_REF] Gupta | Correlating reduced fill factor in polymer solar cells to contact effects[END_REF]. Indeed, the FF decreases from an initial value of 57.0 ± 0.7 % to 43 ± 3 %, corresponding to a 25% reduction, after 1 h of ageing, while the JSC drops by almost 50%, from 7.8 ± 0.7 to 4.0 ± 0.5 mA cm -2 . While the reduction in the VOC is initially less pronounced (7% after 1 h), it accelerates in the second stage. The values obtained for the devices aged 8 h are not reported because they were, by that point, electrically dead.

Mechanical failure analysis

We followed the evolution of the device adhesive properties by means of the pull-off test. In Fig. 4, we report the stress at break values of fresh (0 h) and degraded (0.5 and 8 h) samples. We observe a slight drop (<22%) in the stress at break after 0.5 h and for longer times the value stays constant. The sensitivity of the measurement is also strongly affected; we attribute it to a water-based diffusion mechanism, which happens with an edge-to-center direction or through pin-holes [START_REF] Adams | Water ingress in encapsulated inverted organic solar cells: Correlating infrared imaging and photovoltaic performance[END_REF][START_REF] Feron | Spatially resolved photocurrent measurements of organic solar cells: Tracking water ingress at edges and pinholes[END_REF], and leads to this wider range of values depending on the relative position of the tested area. Moreover, the relatively large size of the probe (1.1 cm 2 ) further contributes to this effect.

Identification of the fracture path

The pull-of test separates the devices in two parts, an upper and a lower, whose surfaces represent the weakest mechanical plane. The upper parts were subjected to XPS and AFM analyses to determine the atomic composition and the topography surface, respectively.

Fracture path of the fresh sample

In a prior work, we analyzed the fracture path of fresh devices with the same architecture by means of XPS [START_REF] Gregori | The role of donor polymer and PEDOT:PSS formulation on adhesion processes in inverted organic solar cells[END_REF]. The atomic composition observed in the lower part corresponded to a pure active layer of P3HT:PC60BM, while the upper part was mainly composed of PEDOT:PSS, with a small contribution from the active-layer materials. Thus, the PEDOT:PSS/P3HT:PC60BM interface was interpreted as the mechanically weak point of the device, in agreement with previous results reported in the literature [START_REF] Dupont | Interlayer adhesion in roll-to-roll processed flexible inverted polymer solar cells[END_REF][START_REF] Dupont | Adhesion properties of inverted polymer solarcells: Processing and film structure parameters[END_REF]. In the present study,

Damp-heat time (h)

in good line with our prior work, the survey spectra of the fresh sample (Fig. 5, blue curve) is a combination of PEDOT:PSS with traces of P3HT:PCBM [START_REF] Gregori | The role of donor polymer and PEDOT:PSS formulation on adhesion processes in inverted organic solar cells[END_REF]. It should be noted though that the presence of silver from the top electrode is indicated.

Fracture path of the aged samples

In Fig. 5, we show the change in the XPS spectra, the atomic composition, with the corresponding photograph of the sample following damp heat (DH) exposure. Detailed data of atomic composition, obtained from the intensity of the respective core level spectra, are presented in Table 2. Upon visual inspection, we observed a yellowing of the sample surface with increasing DH exposure time. After 0.5 h and 8 h of DH exposure, the XPS data clearly show a carbon enrichment of the fracture surface, while the S2p decreases. This can be explained by the presence of active layer materials, especially PCBM. However, we can note that the relative oxygen content (9.9%) slightly increases up to 11.2% after 0.5h of DH exposure, which cannot be explained only by a PCBM enrichment. We will discuss this point later when considering the evolution of the S2p signal. To derive more information, it is necessary to analyze the core level signals in more detail. Fig. 6 focuses on the C1s signals of fresh and 0.5 h aged samples, comparing them with, respectively, PEDOT:PSS and PCBM references. The main peak at ~284.5 eV in both cases is related to C-C bonds, whereas the high binding energy (BE) shoulder in the C1s spectrum of PEDOT:PSS is ascribed to C atoms in PEDOT, which are bonded to oxygen atoms. Thus, the height of this shoulder depends on both the PEDOT:PSS mixing ratio at the surface and the presence of additional carbon compounds. Features at BEs higher than 287 eV are ascribed to highly oxidized carbon species or so-called shake-up satellites.

In Fig. 6.a, we see that the signal for the upper part of the fresh sample shows the typical high BE shoulder of the PEDOT:PSS reference, although the relative intensity of this component is weaker. After a 0.5 h DH exposure, the shoulder has almost disappeared, and the shape of the C1s signal is similar to that of the PCBM reference (Fig. 6.b). Also, signals at higher BE, typical for PCBM (arising from shake-up structures and the methyl ester group) become visible. Thus, we conclude that PCBM molecules diffuse toward the active layer/HTL interface as a consequence of the damp-heat treatment. The question arises, if the PEDOT:PSS mixing ratio is also changed at the active layer/PEDOT:PSS interface. In Fig. 7.a, we compare the S2p signal of the fresh sample to the one of the 0.5 h DH. We notice that the fresh one is quite similar to the PEDOT:PSS, with an approximate 2:1 ratio between PSS and PEDOT, with an additional feature at low binding energy. An S2p3/2 signal at 161.1 eV corresponds to metal sulfide [START_REF] Smart | XPS of Sulphide Mineral Surfaces : Metaldeficient , Polysulphides , Defects and Elemental Sulphur[END_REF], such as Ag2S [START_REF] Jain | An XPS study of the early stages of silver photodiffusion in Ag/a-As2S3 films[END_REF],

meaning that Ag has diffused from the electrode into the PEDOT:PSS layer [START_REF] Rösch | Investigation of the degradation mechanisms of a variety of organic photovoltaic devices by combination of imaging techniques -The ISOS-3 inter-laboratory collaboration[END_REF]. In order to confirm this, we performed a fitting of the S2p signal (Fig. 7.b). It shows that the upper part of the fresh sample is mainly composed of PEDOT:PSS, with a small contribution of Ag2S. After 0.5 h of DH exposure, the S2p signal becomes very noisy due to the low amount of sulfur (2.2%), caused by carbon enrichment. However, some intensity in the range of PEDOT/P3HT-S2p (162-165 eV) and Ag2S (161 eV) is visible, whereas the PSS-S2p signal at 167-169 eV is below the detection limit. This finding can be explained either by a PEDOT enrichment or by traces of P3HT [START_REF] Hintz | Reversible and Irreversible Light-Induced p-Doping of P3HT by Oxygen Studied by Photoelectron Spectroscopy[END_REF] at the fracture plane. The change in atomic composition (Table 2) suggests that the first explanation is more likely. Indeed, we have observed a slight increase of relative oxygen content after a 0.5h of DH exposure that cannot be consistent with a P3HT enrichment (P3HT bears no oxygen atoms while PEDOT has 11.1% of oxygen atoms). Figure S1 shows the O1s signal of the fresh and 0.5h exposed samples. For the fresh sample, the two signals can be assigned to the oxygen belonging to PEDOT (531.8 eV) and PSS (530.6 eV). After 0.5h of DH exposure, we observe an almost complete disappearance of the PSS-related shoulde at the low binding energy. On the other hand, the enhanced intensity at the high binding energy side of the peak (> ~532 eV) may be assigned to additional adsorbed water on the polymer surface.

The presence of doped P3HT is unlikely as p-doping of P3HT requires the combined oxygen and light exposure, which is not the case here [START_REF] Hintz | Reversible and Irreversible Light-Induced p-Doping of P3HT by Oxygen Studied by Photoelectron Spectroscopy[END_REF]. We also performed a topographic survey of the exposed surface by AFM (Fig. 8). The surfaces are all quite flat, with no particular features except for the upper part after 8 h of DH.

Indeed, in the top right corner of the 20x20 m image, some structures, possibly PCBM crystallites [START_REF] Campoy-Quiles | Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends[END_REF], are visible. Despite this and the chemical composition change, the surface roughness, expressed by the root mean square (RMS), slightly changes from 15.7 nm (0 h) to 14.3 nm (0.5 h), to the final value of 19.6 nm (8 h).

Tentative degradation mechanism

The rapid loss in stage i) is the result of materials migration at the HTL/active layer interface, while in stage ii) H2O becomes predominant with a stronger reduction of VOC, caused by a mismatch in the work function of the electrodes [START_REF] Brabec | Origin of the open circuit voltage of plastic solar cells[END_REF], and a further, slower reduction in JSC. This two-stage mechanism is due to the experimental conditions, in particular to our choice of performing the damp heat test on non-encapsulated devices. Since the OSCs were not exposed to light, we consider that oxidation (irreversible degradation) and p-doping (reversible degradation) are non-dominant degradation mechanisms compared to the moisture effect. We have observed that a DH exposure induces a rapid loss of device performances concomitant with a loss of mechanical integrity. Focusing on the 0.5 h exposure, the PCE and stress at break decreases of 27 and 22%, respectively. We propose a tentative mechanism to explain the loss of performance and mechanical integrity after such a short exposure. Through the XPS analysis of the fracture surface of fresh samples, we observe the expected PEDOT:PSS finger print, with traces of active layer and the presence of silver/silver-sulfur complexes. A short DH exposure 0.5 h leads to a drastic modification of the fracture plan, which is richer in PC60BM and poorer in PSS.

Balcaen et al. [START_REF] Balcaen | Thermal cycling effect on mechanical integrity of inverted polymer solar cells[END_REF] observed an increase in adhesive energy of encapsulated devices under thermal stress, due to the formation of a PEDOT/P3HT intermixed layer. Dupont et al. [START_REF] Dupont | Decohesion Kinetics of PEDOT:PSS Conducting Polymer Films[END_REF] observed that the cohesion rate of the PEDOT:PSS layer is dependent on the synergistic effect of relative humidity, temperature, and mechanical stress. In our experiment, the devices were neither encapsulated nor underwent mechanical stresses. The different testing techniques (pull off vs. cantilever beam) and conditions (85°C/85% rH vs. thermal cycling or temperature/humidity/mechanical stresses) make it harder to compare our results. Under the described experimental condition, we did not assist to the formation of an intermixed interfacial layer [START_REF] Bruner | Decohesion Kinetics in Polymer Organic Solar Cells[END_REF] (and corresponding increased adhesion), but to the PCBM diffusion toward the AL/HTL interface. The thermally activated Oswalt ripening [START_REF] Kouijzer | Predicting morphologies of solution processed polymer:Fullerene blends[END_REF][START_REF] Treat | Interdiffusion of PCBM and P3HT reveals miscibility in a photovoltaically active blend[END_REF] depends on PCBM diffusion and eventually leads to the formation of large aggregates [START_REF] Morse | The effect of polymer solubilizing side-chains on solar cell stability[END_REF], as we observed in the AFM images (Fig. 8). A thin layer of PCBM explains the yellowing, and contributes, through the corresponding evolution of the BHJ vertical phase segregation, to both the current density and the mechanical integrity losses. These findings are in good agreement with previous work [START_REF] Kim | A study for degradation of flexible organic photovoltaic modules via damp-heat test: By accessing individual layers of the module[END_REF][START_REF] Norrman | Degradation Patterns in Water and Oxygen of an Inverted Polymer Solar Cell[END_REF], where a similar evolution of phase segregation and PV properties was observed for both encapsulated and non-encapsulated devices.

The presence of small molecules (PC60BM) at a polymer/polymer interface may contribute to the loss of adhesion, reducing possible interchain interdigitation and charge transfer/coordination complex between PEDOT:PSS and P3HT [START_REF] Dupont | Adhesion properties of inverted polymer solarcells: Processing and film structure parameters[END_REF]. The migration of PCBM towards the interface and the consequent phase separation account for the increase in RS, while the decrease in RSH is most probably due to the diffusion of Ag inside the layer stack [START_REF] Madogni | Comparison of degradation mechanisms in organic photovoltaic devices upon exposure to a temperate and a subequatorial climate[END_REF].

Conclusion

Exposure to damp heat (85 °C/85% rH in the dark) of non-encapsulated solar cell devices has led to a drastic degradation of performances. Pull-off tests on fresh and degraded devices always showed a breaking point near the PEDOT:PSS/P3HT:PCBM interface. The aged samples, however, showed reduced adhesion strength and an enrichment of PCBM at the breaking point.

Moreover, the interfacial composition of PEDOT:PSS layer changed in terms of an enhanced PEDOT to PSS ratio. These observations are assumed to be the reason behind an increased tendency towards delamination as well as a rapid device performance loss under these conditions.

Figure 1 .

 1 Figure 1. Experimental protocol including the exposure to damp heat (DH): Step 1: complete structure to

Figure 2 .

 2 Figure 2. Evolution of PV performance of solar cells of the architecture

Figure 3 .

 3 Figure 3. Dark JV curves of fresh and aged (damp heat, 85 °C/85% rH) solar cell devices.

Figure 4 .

 4 Figure 4. Evolution of the stress at break for fresh and degraded devices upon damp heat with time.

Figure 5 .

 5 Figure 5. XPS survey spectra of the upper parts of fresh and degraded devices upon damp heat. Photographs and

Figure 6 .

 6 Figure 6. a) C1s signals of the fresh delaminated sample (blue) and PEDOT:PSS as reference (black); b) C1s

Figure 7 .

 7 Figure 7. a) S2p signal of the fresh and 0.5 h DH exposed samples compared to PEDOT:PSS reference; b) peak

Figure 8 .

 8 Figure 8. AFM topographic images of fresh and DH exposed upper parts at different scales: 20x20 µm, 10x10

Figure 9 .

 9 Figure 9. Proposed mechanism of delamination in inverted devices based on P3HT:PC60BM/PEDOT:PSS exposed

Table 1 .

 1 PV parameters of fresh and aged (damp heat, 85 °C/85% rH) solar cell devices (average values on 8-16 devices, according to the degradation stage).

	Damp heat exposure time [h]	VOC [V]	JSC [mA cm -2 ]	FF [%]	PCE [%]	RS [Ω cm 2 ]	RSH [kΩ cm 2 ]
	0	0.54 ± 0.01	7.8 ± 0.7	57.0 ± 4	2.4 ± 0.2	2.37 ± 0.01	11.4
	0.5	0.54 ± 0.01	6.3 ± 0.7	51 ± 5	1.7 ± 0.3	2.74 ± 0.01	0.75
	1.0	0.50 ± 0.03	4.0 ± 0.5	43 ± 3	0.8 ± 0.2	3.61 ± 0.02	0.34
	2.0	0.48 ± 0.02	3.4 ± 0.3	44 ± 3	0.7 ± 0.1	4.79 ± 0.02	0.64
	5.0	0.40 ± 0.02	2.0 ± 0.2	40 ± 4	0.33 ± 0.06	5.70 ± 0.01	0.40

Table 1

 1

summarizes the evolution of the photovoltaic parameters and of the series (RS)

Table 2 .

 2 Atomic composition of the upper part at different degradation stages (1-3) and the corresponding reference layers (4-7).

			C1s	S2p	O1s	Na1s	Ag3d	C/S	C/O
	1	Fresh	79.6	7.2	9.9	-	3.3	11.0	8.0
	2	0.5 h	84.7	2.2	11.2	-	2.0	38.5	7.6
	3	8 h	93.9	1.8	3.9	-	0.4	52.2	24.1
	4	PEDOT:PSS	69.5	6.0	24.0	0.5	-	11.6	2.9
	5	P3HT:PC60BM 90.7	6.9	2.4	-	-	13.1	37.8
	6	P3HT	91.3	8.7	-	-	-	10.5	-
	7	PCBM	90.9	-	9.1	-	-	-	10.0

Acknowledgements

The research leading to these results has received funding from the European Union Seventh Framework Program (FP7/2011) under grant agreement ESTABLIS no. 290022.

Appendix A: Supporting Information

Influence of material migration on the mechanical integrity of inverted organic solar cells

Aurélien Tournebize 1,4,5 , Dargie Deribew 2 , Alberto Gregori* 3 , Roger C. Hiorns 3 , Andreas Distler 2# , Hans-Joachim Egelhaaf 2# , Christine Lartigau-Dagron 3 , Ahmed Allal 3 , Agnès Rivaton, 4,5 Heiko Peisert 1 , and Thomas Chassé 1