Anil Kumar Bharwal

Laura Manceriu
email: laura.manceriu@uliege.be

Celine Olivier

Abdelfattah Mahmoud

Cristina Iojoiu

Thierry Toupance

Carmen M Ruiz

Marcel Pasquinelli

David Duché

Jean-Jacques Simon

Catherine Henrist

Fannie Alloin

Remarkable 8.3% efficiency and extended electron lifetime towards highly stable semi-transparent iodine-free DSSCs by mitigating the in-situ triiodide generation

Remarkable 8.3% efficiency and extended electron lifetime towards highly stable semi-transparent iodine-free DSSCs by mitigating the in-situ triiodide generation

Introduction

Since the groundbreaking work by Honda and Fujishima in the 1972s, photoelectrochemical cells gained more and more success and popularity [START_REF] Fujishima | Electrochemical photolysis of water at a semiconductor electrode[END_REF]. In 1991, O'Regan and Grätzel reported the fabrication of dye-sensitized solar cells (DSSCs) with the standard I -/I3 -liquid electrolyte and a Ru-based dye [START_REF] O'rdgan | A low-cost high-efficiency solar cell based on dye-sensitized colloidal TiO2[END_REF]. In recent years, DSSCs have gained significant attention thanks to their academic and industrial development [START_REF] Cao | Direct contact of selective charge extraction layers enables high-efficiency molecular photovoltaics[END_REF][START_REF] Vaghasiya | Dual functional hetero-anthracene based single component organic ionic conductors as redox mediator cum light harvester for solid state photoelectrochemical cells[END_REF][START_REF] Sonigara | A smart flexible solid state photovoltaic device with interfacial cooling recovery feature through thermoreversible polymer gel electrolyte[END_REF][START_REF] Devadiga | Recent progress in dye sensitized solar cell materials and photo-supercapacitors: A review[END_REF][START_REF] Kokkonen | Advanced research trends in dye-sensitized solar cells[END_REF][START_REF] Stojanović | The Rise of Dye-Sensitized Solar Cells: From Molecular Photovoltaics to Emerging Solid-State Photovoltaic Technologies[END_REF][START_REF] Vlachopoulos | New approaches in component design for dyesensitized solar cells[END_REF][START_REF] Muñoz-García | Dye-sensitized solar cells strike back[END_REF][START_REF] Aftabuzzaman | In-depth understanding of the energy loss and efficiency limit of dye-sensitized solar cells under outdoor and indoor conditions[END_REF][START_REF] Kim | Highly efficient gel electrolytes by end group modified PEGbased ABA triblock copolymers for quasi-solid-state dye-sensitized solar cells[END_REF]. A typical DSSC is usually made up of three components: a dye coated-TiO2 electrode that absorbs sun or artificial light, and generates photoexcited electrons, a redox electrolyte for the dye regeneration, and hole transport, and a counter electrode to recover the reduced form of the redox mediator [START_REF] O'rdgan | A low-cost high-efficiency solar cell based on dye-sensitized colloidal TiO2[END_REF][START_REF] Zhang | A molecular photosensitizer achieves a V oc of 1.24 V enabling highly efficient and stable dye-sensitized solar cells with copper (II/I)-based electrolyte[END_REF][START_REF] Vaghasiya | Efficient power generating devices utilizing low intensity indoor lights via non-radiative energy transfer mechanism from organic ionic redox couples[END_REF][START_REF] Enaganti | Dye-sensitized solar cells as promising candidates for underwater photovoltaic applications[END_REF]. In particular, the electrolyte properties impact strongly the device stability and efficiency [START_REF] Wu | Electrolytes in Dye-Sensitized Solar Cells[END_REF][START_REF] Masud | Polymer gel electrolytes based on PEG-functionalized ABA triblock copolymers for quasi-solid-state dye-sensitized solar cells: Molecular engineering and key factors[END_REF][START_REF] Kim | PAN-Based Triblock Copolymers Tailor-Made by Reversible Addition-Fragmentation Chain Transfer Polymerization for High-Performance Quasi-Solid State Dye-Sensitized Solar Cells[END_REF]. For a long time, great efforts have been made to achieve efficiencies over 11% for DSSCs fabricated with standard I -/I3 -electrolyte [START_REF] Chen | Highly efficient light-harvesting ruthenium sensitizer for thin-film dye-sensitized solar cells[END_REF][START_REF] He | In situ ligand-free growth of TiO2-escapsulated Au nanocomposites on photoanode for efficient dye sensitized solar cells[END_REF][START_REF] Pang | Heterogeneous FeNi3/NiFe2O4 nanoparticles with modified graphene as electrocatalysts for high performance dye-sensitized solar cells[END_REF]. A benchmark power conversion efficiency (PCE) of 13% was achieved with new redox couples based on Co-complexes liquid electrolytes associated with porphyrin dye [START_REF] Mathew | Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers[END_REF][START_REF] Yella | Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency[END_REF][START_REF] Chen | Double Fence Porphyrins that are Compatible with Cobalt (II/III) Electrolyte for High-Efficiency Dye-Sensitized Solar Cells[END_REF]. Eventually, the PCE of DSSCs has progressively increased from 7% to 14.3% [START_REF] O'rdgan | A low-cost high-efficiency solar cell based on dye-sensitized colloidal TiO2[END_REF][START_REF] Kakiage | Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxyanchor dyes[END_REF][START_REF] Ji | 14.2% Efficiency Dye-Sensitized Solar Cells by Co-sensitizing Novel Thieno [3, 2-b] indole-Based Organic Dyes with a Promising Porphyrin Sensitizer[END_REF]. However, practical applications of DSSCs based on these liquid electrolytes are restricted due to solvent leakage and evaporation, not to mention the counter electrode corrosion [START_REF] Jiang | High temperature stability of dye solar cells[END_REF].

To counterbalance those issues, solid-state DSSCs (ss-DSSCs) are fabricated by replacing the liquid electrolytes with inorganic semiconductors, such as CuI, CuSCN, or CsSnI3-xFx [START_REF] O'regan | A solid-state dye-sensitized solar cell fabricated with pressure-treated P25-TiO2 and CuSCN: analysis of pore filling and IV characteristics[END_REF][START_REF] Hagfeldt | Dye-sensitized solar cells[END_REF][START_REF] Lee | All-solid-state dye-sensitized solar cells with high efficiency[END_REF], or hole-transporting materials, such as P3HT, Spiro-OMeTAD, or amorphous Cu(II/I) [START_REF] Fabregat-Santiago | Electron transport and recombination in solid-state dye solar cell with spiro-OMeTAD as hole conductor[END_REF][START_REF] Zhang | High-performance solid-state organic dye sensitized solar cells with P3HT as hole transporter[END_REF][START_REF] Cao | 11% efficiency solid-state dye-sensitized solar cells with copper (II/I) hole transport materials[END_REF]. However, the efficiency of ss-DSSCs is lower than liquid-state DSSCs due to the surface limited penetration of solid electrolytes into the mesoporous TiO2 and shortened electron-free path. In recent years, the use of quasi-solid-state electrolytes (QSSEs) based on ionic liquids (ILs) [START_REF] Bousrez | Ionic Liquid-Based Dye-Sensitized Solar Cells-Insights into Electrolyte and Redox Mediator Design[END_REF][START_REF] Ma | Highly efficient and stable ionic liquid-based gel electrolytes[END_REF] and poly(ionic liquid)s (PILs) [START_REF] Thomas | Grafted PEO polymeric ionic liquid nanocomposite electrospun membrane for efficient and stable dye sensitized solar cell[END_REF][START_REF] Lin | A novel multifunctional polymer ionic liquid as an additive in iodide electrolyte combined with silver mirror coating counter electrodes for quasi-solid-state dye-sensitized solar cells[END_REF] have shown great potential for preparing quasi-solid state DSSCs. Essentially, PILs as QSSEs are very exciting since they have similar properties to ILs with improved mechanical durability and dimensional control of a polymer [START_REF] Lee | Poly (ionic liquid) s for dye-sensitized solar cells: a mini-review[END_REF]. PILs are typically solid or viscous liquids depending on their structure, glass transition temperature (Tg), and molecular weight. Due to these unique properties, PILs are used as electrolytes in DSSCs in various forms e.g. as additives [START_REF] Lin | A novel multifunctional polymer ionic liquid as an additive in iodide electrolyte combined with silver mirror coating counter electrodes for quasi-solid-state dye-sensitized solar cells[END_REF][START_REF] Tseng | Synthesis of a series of novel imidazolium-containing ionic liquid copolymers for dyesensitized solar cells[END_REF] or membranes [START_REF] Jeon | Quasi-solid-state polymer electrolytes based on a polymeric ionic liquid with high ionic conductivity and enhanced stability[END_REF][START_REF] Pang | Electrospun membranes of imidazole-grafted PVDF-HFP polymeric ionic liquids for highly efficient quasi-solidstate dye-sensitized solar cells[END_REF][START_REF] Thomas | Porous membrane of polyindole and polymeric ionic liquid incorporated PMMA for efficient quasi-solid state dye sensitized solar cell[END_REF][START_REF] Thomas | Grafted PEO polymeric ionic liquid nanocomposite electrospun membrane for efficient and stable dye sensitized solar cell[END_REF] in liquid electrolytes, in the pure solid form [START_REF] Chi | Highly efficient I2-free solid-state dye-sensitized solar cells fabricated with polymerized ionic liquid and graft copolymer-directed mesoporous film[END_REF][START_REF] Chi | A shape-and morphology-controlled metal organic framework template for high-efficiency solid-state dye-sensitized solar cells[END_REF], as well as in blends with wellmatched ILs [START_REF] Chi | Highly efficient I2-free solid-state dye-sensitized solar cells fabricated with polymerized ionic liquid and graft copolymer-directed mesoporous film[END_REF][START_REF] Wang | Highly efficient solid-state dyesensitized solar cells based on hexylimidazolium iodide ionic polymer electrolyte prepared by in situ low-temperature polymerization[END_REF][START_REF] Bharwal | Ionic-Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State Dye-Sensitized Solar Cells[END_REF].

In the last few years, numerous efforts aiming at improving the stability of the DSSCs by using polysiloxane-based electrolytes have been made [START_REF] Cipolla | An ion conductive polysiloxane as effective gel electrolyte for long stable dye solar cells[END_REF][START_REF] Pujiarti | Poly (ionic-liquid) from imidazoline-functionalized siloxane prepared by simple sol-gel route for efficient quasi-solid-state DSSC[END_REF][START_REF] Čolović | Amphiphilic POSS-based ionic liquid electrolyte additives as a boost for dye-sensitized solar cell performance[END_REF]. In our previous work, we systematically studied and investigated the photovoltaic properties of different polysiloxanebased PILs blends with ILs, such as 1-methyl-3 propylimidazolium-iodide (MPII) and 1methyl-3-propyl-imidazolium-Bis(trifluoromethane)sulfonimide (MPITFSI), and non-ionic liquids such as ethylene carbonate (EC) [START_REF] Bharwal | Ionic-Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State Dye-Sensitized Solar Cells[END_REF][START_REF] Bharwal | Tuning bimodal porosity in TiO2 photoanodes towards efficient solid-state dyesensitized solar cells comprising polysiloxane-based polymer electrolyte[END_REF][START_REF] Bharwal | Bimodal titanium oxide photoelectrodes with tuned porosity for improved light harvesting and polysiloxane-based polymer electrolyte infiltration[END_REF]. However, all these polymer electrolytes contained iodine (I2) which affected the durability and limited the performance of the DSSCs due to the I2 sublimation and incident light absorption, respectively. Furthermore, when present in the electrolyte, I2 leads to the formation of extra I3 -that causes photodegradation, counterelectrode corrosion, and dye desorption [START_REF] Rong | Monolithic quasi-solid-state dye-sensitized solar cells based on iodine-free polymer gel electrolyte[END_REF]. The different I2 concentrations in electrolytes influence the charge transportation and electron recombination dynamics in DSSCs [START_REF] Lan | Effects of iodine content in the electrolyte on the charge transfer and power conversion efficiency of dye-sensitized solar cells under low light intensities[END_REF]. Ali et al. [START_REF] Ali | Elimination of 50% Iodine and Excellent Performance of Dye-Sensitized Solar Cell Enabled by TEMPO Radical Dendrimer-Iodide Dual Redox Systems[END_REF] showed that the removal of 50% of I2 from the QSSCs significantly improved both the stability and the PCE of DSSCs. The first I2-free DSSC was reported by Wang et al. [START_REF] Wang | Solid-state composite electrolyte LiI/3-hydroxypropionitrile/SiO2 for dye-sensitized solar cells[END_REF] with a PCE of 5.5%, maintaining about 70% of its initial photonic efficiency after one-month storage. Another approaches for fabricating stable solid-state DSSC was the utilization of I2free PILs in combination with different hierarchical TiO2 photoanodes, a combination necessary for avoiding a trade-in of specific surface and electrolyte penetration efficiency.

Since then, there have been few attempts to prepare I2-free DSSCs to improve stability and PCE. Yu et al. [START_REF] Yu | Iodine-free nanocomposite gel electrolytes for quasi-solid-state dye-sensitized solar cells[END_REF] reported DSSCs based on I2-free liquid electrolytes gelled with 15 wt.% TiO2 which reached a PCE of 8.0%. Although the PCE value of the resulting I2-free DSSCs was slightly lower than that of I2-based DSSCs (8.32%), its long-term at-rest stability was improved. Ahn et al. [START_REF] Ahn | Direct Assembly of Preformed Nanoparticles and Graft Copolymer for the Fabrication of Micrometer-thick, Organized TiO2 Films: High Efficiency Solid-state Dye-sensitized Solar Cells[END_REF] reported the use of PVC-g-POEM templated, 6 µm thick, mesoporous TiO2 as photoanode in combination with a solid I2-free poly((1-(4-ethenylphenyl)methyl)-3butyl-imidazolium iodide) (PEBII) reaching 7.1 % efficiency. The same authors reported a slightly higher PCE of 7.4 % for honeycomb-like organized, 8.5 µm thick TiO2 photoanodes in the presence of PEBII [START_REF] Ahn | Honeycomb-Like Organized TiO2 Photoanodes with Dual Pores for Solid-State Dye-Sensitized Solar Cells[END_REF]. By imbedding double-shell TiO2 nanosheet-covered SnO2 hollow spheres in the same templated TiO2 photoanode the same group managed to improve the dye loading and light-harvesting characteristics reaching 8.2 % efficiency [START_REF] Ahn | Hierarchical Double-Shell Nanostructures of TiO2 Nanosheets on SnO2 Hollow Spheres for High-Efficiency, Solid-State, Dye-Sensitized Solar Cells[END_REF]. However, the improved efficiency is more related to the increased light harvesting than to improved electrolyte penetration since there is no evolution in the FF values. Roh et al. [START_REF] Roh | High efficiency solid-state dyesensitized solar cells assembled with hierarchical anatase pine tree-like TiO2 nanotubes[END_REF] reported 8.0% efficiency by using the same solid electrolyte, the PEBII, in combination with a much thicker tree-like nanostructured TiO2 photoanode. Although the organized TiO2 photoanodes in these studies ensured a high enough specific surface area for efficient dye-loading and good enough porosity for the solid electrolyte penetration, the opaque design does not facilitate their future building integration. Thus, new trends in designing transparent DSSCs were developed for maximizing the harvested light and facilitating their building integration. In 2011, Lunt et al. [START_REF] Lunt | Transparent, near-infrared organic photovoltaic solar cells for window and energy-scavenging applications[END_REF] first introduced a fullerene-based organic planar heterojunction transparent photovoltaic technology, showing up to 57% of total transparency and 1.7% PCE. Very recently, Alkarsifi et al. [START_REF] Alkarsifi | Organic-inorganic doped nickel oxide nanocrystals for hole transport layers in inverted polymer solar cells with color tuning[END_REF] demonstrated the attractive color-tuning properties of organic planar heterojunction based on non-fullerene acceptor ITIC-4F and PBDB-T-2F donor. Henceforth, many efforts have been made in color-tunable and transparent photovoltaic technologies [START_REF] Kini | Latest Progress on Photoabsorbent Materials for Multifunctional Semitransparent Organic Solar Cells[END_REF][START_REF] Wang | Colorful Perovskite Solar Cells: Progress, Strategies, and Potentials[END_REF][START_REF] Ren | A blue photosensitizer realizing efficient and stable green solar cells via color tuning by the electrolyte[END_REF][START_REF] Yang | Expanded Phase Distribution in Low Average Layer-Number 2D Perovskite Films: Toward Efficient Semitransparent Solar Cells[END_REF]. The optical properties of DSSCs can be modified by tuning several parameters such as the dye, the electrolyte, or the electrodes thicknesses [START_REF] Roy | Color comfort evaluation of dye-sensitized solar cell (DSSC) based building-integrated photovoltaic (BIPV) glazing after 2 years of ambient exposure[END_REF][START_REF] Huaulmé | Photochromic dye-sensitized solar cells with lightdriven adjustable optical transmission and power conversion efficiency[END_REF][START_REF] Naim | Transparent and Colorless Dye-Sensitized Solar Cells Exceeding 75% Average Visible Transmittance[END_REF][START_REF] Grifoni | Toward Sustainable, Colorless, and Transparent Photovoltaics: State of the Art and Perspectives for the Development of Selective Near-Infrared Dye-Sensitized Solar Cells[END_REF]. Lately, Chalkias et al. [START_REF] Chalkias | Spectral engineering of semi-transparent dye-sensitized solar cells using new triphenylamine-based dyes and an iodine-free electrolyte for greenhouse-oriented applications[END_REF] developed a binary approach to fabricating semi-transparent DSSCs by using organic dyes and I2-free liquid electrolytes. The resulting I2-free DSSCs exhibited transmittance up to 55%, offering high IPCE (85%) in the whole blue and green part of the visible spectrum.

Recently, we reported improved performance and stability (in both ambient and accelerated aging conditions) of I2-free DSSCs fabricated with TiO2 electrodes grown on flexible Ti substrate and polysiloxane ionic conductors as electrolytes [START_REF] Bharwal | Plasticized I2-free polysiloxane ionic conductors as electrolytes for stable and flexible solid-state dye-sensitized solar cells[END_REF]. We assumed that the in-situ generation of I3 -ions in I2-free DSSCs was sufficient for the DSSC operation. In 2005, Seo et al. [START_REF] Seo | In-situ analyses of triiodide formation in an iodine-free electrolyte for dye-sensitized solar cells using electro-diffuse-reflection spectroscopy (EDRS)[END_REF] presented the first electro-diffuse-reflection spectroscopy study on the I3 - in-situ generation of I2-free DSSCs during working conditions. Prehal et al. [START_REF] Prehal | Persistent and reversible solid iodine electrodeposition in nanoporous carbons[END_REF] used in-situ Raman spectroscopy to successfully demonstrate the reversible formation of I3 -, I5 -, and I2 in the carbon nanopores produced by the oxidation of I -.

In this work, we focused on optimizing the polymer electrolyte composition for a standard randomly-organized TiO2 photoanode such as to ensure a similar conversion efficiency but enhanced long-term stability of semi-transparent I2-free quasi solid-state DSSCs. In addition, we have employed Raman spectroscopy to validate the in-situ generation of I3 -ions in I2-free polymer electrolytes alone and embedded in liquid-state DSSCs.

Experimental section

Materials

The FTO glass substrate (TEC15) and the TiO2 colloidal pastes (18 NR-AO) were purchased from Dyesol. Transparent TiO2 electrodes (13 µm) were purchased from Greatcellsolar. Ti-Nanoxide R/SP paste, Platisol platinum solution, the N719 dye, and the Surlyn spacer (60 µm)

were bought from Solaronix. Acetonitrile, iodine (I2), 1-butyl-3-methylimidazolium iodide, guanidine thiocyanate, 4-tert-butylpyridine, valeronitrile, ethanol, titanium tetrachloride, ethylene carbonate (EC) and LiClO4 were all obtained from Sigma Aldrich.

Fabrication of PILs-based electrolytes

The synthesis procedure of PILs and ILs is reported in our previous work. [START_REF] Bharwal | New polysiloxane bearing imidazolium iodide side chain as electrolyte for photoelectrochemical cell[END_REF] The PILs, poly(1-N-methylimidazolium-pentylpolydimethylsiloxane)iodide having different ionicity and viscosity (denoted to PS1, PS2, and PS3 in this work), and the ILs, 1-methyl-3

propylimidazolium-iodide (MPII) and 1-methyl-3-propyl-imidazolium-Bis(trifluoromethane)sulfonimide (MPITFSI) were synthesized as reported elsewhere [START_REF] Bharwal | New polysiloxane bearing imidazolium iodide side chain as electrolyte for photoelectrochemical cell[END_REF].

The polymer electrolytes (with and without I2) were prepared by mixing the three synthesized PILs with the ILs or the EC. Electrolytes based on ILs (with and without I2) were also prepared.

Appropriate weights of the components (PILs, ILs, EC, and I2) were dissolved in a very small amount of acetonitrile/valeronitrile (85/15, v/v) and then continuously stirred at 50 o C until complete dissolution (Table S1). The "reference" liquid electrolyte consisted of 0.6 M 1-butyl-3-methylimidazolium iodide, 0.03 M I2, 0.1 M guanidine thiocyanate and 0.5 M 4-tertbutylpyridine in acetonitrile/valeronitrile (85/15, v/v).

Fabrication of DSSC

Prior to use, commercially TiO2 photoanodes were sintered at 450 o C for 30 min.

Additionally, a scattering layer (Dyesol 18 NR-AO) was deposited by spin-coating. In this work three types of photoanodes were used: the commercial-13 µm-TiO2 photoanodes (marked as TiO2 photoanodes), the commercial-13 µm TiO2 photoanodes with an additional scattering layer of 4 µm (marked as TiO2/SL photoanodes), and TiCl4-treated commercial-13 µm TiO2

photoanodes with additional scattering layer of 4 µm (marked as TiCl4-TiO2/SL photoanodes).

The overall thicknesses of TiO2/SL photoanodes were kept at approximately 17 µm (13 µm of mesoporous photoanode + 4 µm of scattering layer). The TiCl4 treatment of the TiO2/SL photoanodes was carried out by soaking the photoanodes in a 0.04 M TiCl4 aqueous solution at 70 °C for 30 min. After rinsing in water and ethanol, the photoanodes were dried and sintered again in the air for 30 min at 450 °C. All freshly sintered photoanodes were immersed in N719 alcoholic solution at room temperature for 24 h. Finally, the dye-sensitized photoanodes were assembled in a sandwich configuration with a Pt counter-electrode and surlyn as a spacer. The electrolytes were vacuum backfilled into the device through the hole in the counter electrode.

Before sealing, the device was heated at 100 o C for 5 min on a hot plate, followed by vacuum drying overnight to ensure the complete removal of volatile solvent. Finally, the hole was sealed with surlyn and a microscope cover glass.

Characterization techniques

The TiO2 photoanode thicknesses and effective electrolyte infiltration were determined by scanning electron microscopy (FEG-SEM XL30, FEI). The pore-filling was asserted by crosssection energy-dispersive X-ray spectroscopy (EDX) elemental mapping of the device.

Photocurrent-voltage (J-V) characteristics of the devices were measured by the solar simulator (from Newport Spectra-Physics) coupled with a Keithley 2400 Source Meter under a simulated Air Mass 1.5G solar spectrum (irradiance: 100 mWcm -1 , i.e. 1 sun). The area exposed to light was 0.159 cm 2 . Incident Photon to Charge Carrier Efficiency (IPCE) data was collected in the 300-800 nm range using a Xe lamp associated with a monochromator (Triax 180, Jobin Yvon).

The integrated Jsc value was calculated from the IPCE based on ASTM G173-03 solar spectral irradiance (Global tilt Wm -2 nm -1 ) [START_REF]Solar Spectra[END_REF]. The open-circuit voltage decay (OCVD) measurements were achieved by illuminating the DSSC device with a white LED and then observing the voltage decay by the oscilloscope (Tektronix MDO3024) when the light source is turned off.

Electrochemical impedance spectroscopy (EIS) was done using a BioLogic SP-200 potentiostat (Science Instrument), under dark conditions at room temperature, in the 10 mHz -100 kHz frequency range by applying a bias equal to the Voc. The obtained data were analyzed with the EC-lab software. Raman microprobe measurements were done using a HR800-UV Horiba-Jobin Yvon spectrometer coupled with an Olympus metallographic microscope. The measured spectra were obtained using as excitation the red line of the He-Ne laser (λ = 632.8 nm). The transmittance spectra of the devices and the diffusive reflectance spectra of the photoanodes were recorded using a UV-visible spectrophotometer (Perkin Elmer UV-Vis Spectrometer Lambda 14P). The absorbance spectra of the electrolytes were recorded using a UV-Vis spectrophotometer (Shimadzu 1600). A Biologic SP50 electrochemical workstation was utilized to assess the redox and the mass transfer capacity of the electrolytes through cyclic voltammetry (CV) and Tafel polarization measurements, respectively. The CV measurements for investigating reduction and oxidation processes in ILs and PILs/ILs were performed at 5 mV s -1 scan rate by using a three-electrode configuration with Pt (diameter = 3 mm) as a working electrode, Pt as a counter electrode, and Ag wire as a Pseudo-reference electrode. The measurement was carried out on neat ILs or PILs/ILs without any solvent and I2 in Glove-box.

The CV measurements were also performed at 50 mV s -1 scan rate by using a three-electrode setup with Pt as counter and working electrodes and Ag/AgCl (sat KCl) from Biologic (Eo = 0.197 vs NHE) as the reference electrode. The electrolyte was obtained by diluting 20 µL of each electrolyte with 20 mL ACN containing 0.1M LiClO4. The Tafel polarization plots were recorded at a scan rate of 10 mV s -1 by using a Pt-Pt parallel symmetric cell (distance in between = 2 mm) with the same electrolyte in between.

Results and discussion

Electrolyte design

All three PILs we have chosen have different ionic functionality (Fig. 1), leading to different ionic and physical properties as shown in Table S1 [START_REF] Bharwal | New polysiloxane bearing imidazolium iodide side chain as electrolyte for photoelectrochemical cell[END_REF]. The ionic conductivity of the neat ILs, PILs, PILs/ILs, and PIL/EC (with no solvent and I2) was calculated from the bulk resistance obtained from the complex impedance plot as shown in Fig. S1 [START_REF] Sonigara | A smart flexible solid state photovoltaic device with interfacial cooling recovery feature through thermoreversible polymer gel electrolyte[END_REF]. Considering these previous results, we selected only a few of the quasi solid-state electrolytes (PILs, PILs/IL, and PILs/EC)

for the present study. Indeed, for DSSC fabrication, two ILs (MPII and MPITFSI) and a nonionic liquid (EC) were used for the electrolytes preparation. The MPII acts as a source of I - species whereas the MPITFSI mainly improves the conductivity.

The CV curves of the neat MPII are shown in Fig. S2a-b. In oxidation, due to the high concentration of the I -, an oxidation wall can be noticed and the reduction of generated I3 -is observed in the reverse scan. Regarding the stability in reduction, at around +600 mV, the reduction of I3 -traces present in the electrolyte (a little oxidation of the electrolyte via O2) is observed. At -1.1 V, the reduction of the imidazolium cation, due to its acidic proton, is observed [START_REF] Garcia | Room temperature molten salts as lithium battery electrolyte[END_REF]. During the return sweep, several oxidation processes can be observed, associated with the species obtained by reduction, but no oxidation can be obtained if the scan was stopped at -1 V vs Ag, indicating the purity of the used ILs.

As reported previously in our group [START_REF] Bharwal | Ionic-Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State Dye-Sensitized Solar Cells[END_REF][START_REF] Bharwal | New polysiloxane bearing imidazolium iodide side chain as electrolyte for photoelectrochemical cell[END_REF], moderately (PS2) and highly (PS3) functionalized polymers show good compatibility with the imidazolium-based ILs and EC when added in large proportion, whereas PS1 is only miscible with MPITFSI due to its lower ionic functionality and hydrophobic character. However, we have demonstrated that in PS2/MPII/I2 electrolytes the DSSCs performances were higher (PCE = 5.6%) due to the higher availability of I -ions [START_REF] Bharwal | Ionic-Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State Dye-Sensitized Solar Cells[END_REF]. Therefore, this blend was further investigated in this work. Concerning the addition of EC, we opted for the most concentrated PIL, the PS3 which is rich in I -as the addition of solvent reduces the ionic concentration. Furthermore, for this type of electrolyte, good photovoltaic performances are associated with the creation of ionic/non-ionic phaseseparated domains that favor I -percolation and rapid diffusion [START_REF] Nguyen | Nanostructured multi-block copolymer single-ion conductors for safer highperformance lithium batteries[END_REF][START_REF] Wang | Poly (ionic liquid) Bridge Joining Smectic Lamellar Conducting Channels in Photoelectrochemical Devices as High-Performance Solid-State Electrolytes[END_REF]. As shown in Fig. 1, for the highly substituted PS3/EC blends the hydrophilic domains are better defined and more continuous, facilitating the ion flow despite its high viscosity.

Photovoltaic performance of ionic liquid-based electrolytes

The J-V characteristics and photovoltaic parameters (Voc: open-circuit voltage; Jsc: shortcircuit photocurrent; FF: fill factor; PCE: overall power conversion efficiency) of the DSSCs using different I2-based and I2-free ILs and the commercial 13-μm TiO2 photoelectrode are shown in Fig. 2a and Table 1. As expected, the DSSC with MPITFSI (D3) did not generate a photocurrent since it had no redox mediator, whereas by adding I2 (D3I) a little amount of I -was formed, resulting in a working device (Table 1 and Fig. 2a). As expected, the large increase in I -concentration, using the MPII/I2 electrolyte (D2I) generated much higher Voc and Jsc than its MPITFSI/I2 counterpart. Interestingly, the omission of I2 from MPII drastically improved Jsc and Voc probably due to the difference between Fermi level of TiO2 and redox potential of electrolyte [START_REF] Rong | Monolithic quasi-solid-state dye-sensitized solar cells based on iodine-free polymer gel electrolyte[END_REF] but led to a reduced FF due to a higher charge transport resistance associated with the insufficient amount of I3 - [START_REF] Yu | Iodine-free nanocomposite gel electrolytes for quasi-solid-state dye-sensitized solar cells[END_REF][START_REF] Seo | In-situ analyses of triiodide formation in an iodine-free electrolyte for dye-sensitized solar cells using electro-diffuse-reflection spectroscopy (EDRS)[END_REF]. It has been demonstrated that the imidazolium-based IL could be used as the sole charge transfer mediate [START_REF] Rong | Efficient monolithic quasi-solid-state dye-sensitized solar cells based on poly (ionic liquids) and carbon counter electrodes[END_REF]. Without I2, I3 -can be formed by the reaction of I -with the oxidized dye (dye + ) (step 1), and then I -ions were regenerated by a reduction process at the counter electrode (step 2).

I -+ I2 I3 - polyiodides
The enhanced Voc for I2-free MPII is correlated with the suppression of the dark current at the TiO2 /electrolyte interface (Fig. 2b). The presence of extra I2 in the electrolytes leads to the formation of polyiodides such as I5 -and I7 -which can recombine with the electrons from the photoanode conduction, resulting in an increase of the dark current. Zaban et al. [START_REF] Zaban | Determination of the electron lifetime in nanocrystalline dye solar cells by open-circuit voltage decay measurements[END_REF] reported that the charge recombination process and electron transport properties play an important role in improving Voc and Jsc. It has been reported that the OCVD measurements can provide the information about recombination process and electron lifetime in solar devices [START_REF] Perkhun | High-Efficiency Digital Inkjet-Printed Non-Fullerene Polymer Blends Using Non-Halogenated Solvents[END_REF][START_REF] Mohammadnezhad | Gold nanoparticle decorated carbon nanotube nanocomposite for dye-sensitized solar cell performance and stability enhancement[END_REF]. As shown in Fig. 2c, the I2-free device (D2) had a milder decay measured as compared to the I2based device (D2I), indicating a reduced charge recombination effect. The electron lifetime (τe) is defined by using equation 1 [START_REF] Mohammadnezhad | Role of carbon nanotubes to enhance the long-term stability of dye-Sensitized solar cells[END_REF]:

𝜏 𝑒 = - 𝑘 𝐵 𝑇 𝑞 / ( 𝑑𝑉 𝑜𝑐 𝑑𝑡 ) (1)
Where kB is the Boltzmann constant, T is the absolute temperature in kelvin and q is the elementary charge. The results showed that the calculated electron lifetime is higher for the I2free device (D2) as compared to I2-based (Fig. 2d), indicating the reduced surface traps and charge recombination. Developing highly transparent DSSCs makes them suitable for greenhouse-oriented and BIPV windows applications, and provides better aesthetics [START_REF] Roy | Color comfort evaluation of dye-sensitized solar cell (DSSC) based building-integrated photovoltaic (BIPV) glazing after 2 years of ambient exposure[END_REF][START_REF] Naim | Transparent and Colorless Dye-Sensitized Solar Cells Exceeding 75% Average Visible Transmittance[END_REF][START_REF] Chalkias | Spectral engineering of semi-transparent dye-sensitized solar cells using new triphenylamine-based dyes and an iodine-free electrolyte for greenhouse-oriented applications[END_REF]. The transmittance spectra of D2 was blue-shifted with the report to D2I (Fig. 2e) as a sign of improved light harvesting, which is in perfect agreement with the enhanced Jsc. To examine the visible light absorption of MPII/I2 and MPII electrolytes (both 10x diluted with acetonitrile/valeronitrile), the UV-VIS spectra were measured. As shown in Fig. S3, the I2-free MPII looks colorless and absorbs less visible light compared to the one with I2 [START_REF] Wu | Electrochemical characterization of a novel iodine-free electrolyte for dye-sensitized solar cell[END_REF]. Fig. 2f also showed that the D2 exhibited higher diffuse reflectance than the D2I, indicating that light penetrated deeper into the structure of dye-sensitized TiO2 leading to multiple internal reflections and thus more effective charge photogeneration.

We used Raman spectroscopy to investigate the iodide speciation in MPII and MPII/I2 electrolytes and corresponding devices under constant cell illumination. As shown in Fig. 3a the MPII/I2 electrolytes exhibited a peak at around 116 cm -1 representing the symmetric stretch vibration of I 3 -[56,86] (noted with Peak 1) and a second prominent peak centered at around 152 cm -1 corresponding to the presence of polyiodides (noted as Peak 2) [START_REF] Tanaka | Polyiodide solid-state dye-sensitized solar cell produced from a standard liquid I-/I 3-electrolyte[END_REF]. There was no indication of the presence of molecular I2 which bands should appear around 180 cm -1 [START_REF] Tanaka | Polyiodide solid-state dye-sensitized solar cell produced from a standard liquid I-/I 3-electrolyte[END_REF].

The Raman spectra of I2-free MPII electrolyte showed no bands confirming the absence of I 3 -.

In which concerns the corresponding devices, the 116 cm -1 band was identified in both devices and particularly in the I2-free MPII, indicating the in-situ generation of I 3 -. A peak centered at 146 cm -1 (noted as Peak 3 in Fig. 3a) could be assigned to the N719 sensitized TiO2 [START_REF] Mao | Direct Dynamic Evidence of Charge Separation in a Dye-Sensitized Solar Cell Obtained under Operando Conditions by Raman Spectroscopy[END_REF].

Increasing the measurement time seemed to not affect the Peak 1 and Peak 3 evolution (Fig. 3b) indicating that the I 3 -was constantly generated and regenerated as well as the polyiodides whose contribution was observed at around 170 cm -1 (peak 4) [START_REF] Mao | Direct Dynamic Evidence of Charge Separation in a Dye-Sensitized Solar Cell Obtained under Operando Conditions by Raman Spectroscopy[END_REF]. The evolution of the Peak 3/Peak 1 ratio confirmed that an equilibrium, not far from the initial state, of the I 3 -flow inside the device, was quickly established through the Grotthuss-type electron exchange mechanism [START_REF] Kubo | Quasi-solid-state dye-sensitized TiO2 solar cells: effective charge transport in mesoporous space filled with gel electrolytes containing iodide and iodine[END_REF].

Contrarily, in the case of the D2I device, Peaks 1 strongly evolved with the report to Peak 3 (Fig. 3c) as the analysis time was increased. The intensity of Peak 1 with the report to Peak 3 markedly decreases in the first 8 to 10 min (Fig. 3d) indicating an initial system overload with I 3 -. It is very likely that part of the I 3 -species no longer interacted with the dye but were completely reduced to I -. The absence of polyiodides in the iodide-containing might be due to the same spontaneous reduction process encouraged by a possible functioning in over operando conditions (Voc D2I < Voc D2, Table 1) [START_REF] Mao | Direct Dynamic Evidence of Charge Separation in a Dye-Sensitized Solar Cell Obtained under Operando Conditions by Raman Spectroscopy[END_REF].

Photovoltaic performance of polyionic liquid-based electrolytes

Then, DSSCs prepared with the PS1, PS2, and PS3 PILs electrolytes were investigated. First of all, devices (D4, D4I, D5, and D5I) fabricated with the least viscous PS1 result in poor efficiencies and the addition of MPITFSI further degrades the efficiency by lowering the I - concentration despite the higher ionic conductivity (Table S2 and Fig. S4a). The I2-free device (D4) showed the highest, still modest, efficiency in agreement with the higher photon harvest and conversion efficiency (Fig. S4b) and lower dark current (Fig. S5a). The PS2 and PS3 are very viscous (106 and 1071 Pa s, respectively), having slow diffusion coefficients, which represents the main drawback in applications like DSSCs [START_REF] Papageorgiou | On the relevance of mass transport in thin layer nanocrystalline photoelectrochemical solar cells[END_REF]. We tried to improve this aspect by blending them with MPII and EC [START_REF] Bharwal | Ionic-Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State Dye-Sensitized Solar Cells[END_REF]. As shown in Fig. 4a and Table 2, PCE values of 3.42% and 4.50% were achieved for the PS2/MPII/I2 and PS3/EC/I2 devices, respectively when using the same commercial 13-μm TiO2 photoelectrode. Higher Voc and Jsc values were calculated for the I2-free D6 and D7 devices in comparison with their counterparts in part due

to the increase of the potential difference between the TiO2 fermi level and the electrolyte redox level, but also due to the improved light-harvesting once the I2 absorption was removed (Fig. S3b). The higher Voc of the I2-free D7 device compared to D6 is related to the polymer electrolyte's higher ionic conductivity (Table S1) and thus lower viscosity which improved interface contact. The device D7 based on the I2-free PS3/EC showed the highest Jsc and Voc (Table 2 and Fig. 4a), a FF of 60%, resulting in the highest PCE, 5.03%, and improved transparency (Fig. S3b).

The solid nature of PS2 and PS3 was not changed either with the addition of MPII or EC (Fig. S3a). 2 for DSSC codes.

As previously mentioned the Voc and Jsc should increase when I2 is removed from the electrolyte. Indeed, the highest Voc value is obtained for the I2-free electrolytes (Table 2), which agrees with the dark current diminution (Fig. S5b) but also with the electrochemical results.

Upon blending the MPII to PS2, the slope of the oxidation of I -in I3 -is lower for PS2/MPII due to the higher viscosity (Fig. S6).

To prove our Voc increase hypothesis, we have evaluated the redox capacity of our electrolytes through CV. The CV curves of PS2/MPII and PS2/MPII/I2 were prepared in 0.1 M LiClO4 in ACN. (Fig. S7a) reveal the presence of two anodic peaks at around 0.7/0.9V and 0.4/0.5V vs. Ag/Ag + (Epa 1 and Epa 2 , respectively in Table S3) corresponding to the I -/I 3 -and I 3 -/I 2 redox reactions.

The Epa 1 and Epa 2 and the corresponding cathodic Epc 1 and Epc 2 calculated values were used to determine the electrolyte formal redox potential E0 vs NHE = (Epa + Epc)/2 + 0.197. The values are gathered in Table S3 and also represented in Fig. 5 for the I2-free and I2-containing PS2/MPII electrolytes. Indeed, the Voc of I2-free PS2/MPII electrolyte is higher than its iodinerich counterpart and higher than the standard iodide electrolyte, explaining the higher Jsc and PCE values. The more anodic redox potential of the PS2-based electrolytes also enhances the dye regeneration rate. Tafel polarization curves were performed in a Pt-Pt symmetrical cell containing the same electrolyte and allowed determining the exchange (J0) and diffusion-limited (Jlim) current densities as indicated in Fig. S7b. The limiting current is attributed to the maximum ionic carrier transfer and allows calculating the apparent diffusion coefficient Dapp using the following equation [START_REF] Sonigara | A smart flexible solid state photovoltaic device with interfacial cooling recovery feature through thermoreversible polymer gel electrolyte[END_REF]:

𝐷 𝑎𝑝𝑝 = 𝑑𝐽 𝑙𝑖𝑚 (2𝑛𝐹𝐶) -1 (2)
In equation 2, d is the electrodes gap (2 mm), n is the number of electrons involved in the reduction of the I3 -reaction which is 2, F is the Faraday's constant (96485.4 C mol -1 ) and C is the concentration of I3 -. The electrolyte/Pt charge transfer resistance Rct can be determined based on J0 values by using the following equation [START_REF] Rafique | Cost effective dye sensitized solar cell based on novel Cu polypyrrole multiwall carbon nanotubes nanocomposites counter electrode[END_REF]:

J0 = RT/nFRct (3)
Here n is the number of electrons transferred which is 2; F is the Faraday constant, R is the gas constant (0.082 L⋅atm⋅K -1 ⋅mol -1 ) and T is the absolute temperature (291.15 K). Since we used the same Pt counter-electrodes the Rct values will reflect the differences in the electrolyte ionic transfer properties. The Jlim, J0, Dapp, and Rct values are listed in Table 3.

Table 3. Charge transfer characteristics of the PS2/MPII and PS2/MPII/I2 electrolytes.

As expected, the iodine-rich polymer has a higher Jlim due to the higher concentration of generated I 3 -. The removal of iodine does not severely impact the mass transfer properties of the PS2/MPII electrolyte as the Dapp parameter is only slightly decreased (Table 3). The lower J0 and thus higher Rct values are a reflection of the lower ionic conductivity of the I2-free PS2/MPII [START_REF] Bharwal | Ionic-Liquid-like Polysiloxane Electrolytes for Highly Stable Solid-State Dye-Sensitized Solar Cells[END_REF]. Nevertheless, the more anodic redox potential of the I2-free electrolyte (Fig. 5)

increases Voc outcomes these effects leading to an increased PCE for the device based on I2free PS2/MPII electrolyte (Table 2). The Rct values evolution from Tafel measurements was confirmed through electrochemical impedance spectroscopy (EIS).

The Nyquist and Bode phase plots of the DSSCs comprising polymer electrolytes are shown in Fig. 4b-c and4d, respectively. The EIS spectrum of the DSSC is comprised of three signatures in the studied frequency range (10 mHz to 100 kHz) [START_REF] Wang | Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells[END_REF] and was fitted with the circuit illustrated in the inset of 4. The sheet resistance (Rs) comprises the contacts/wires and electrolyte resistance. The lower Rs value of I2-based electrolytes for DSSCs explained the slightly better FF [START_REF] Yu | Iodine-free nanocomposite gel electrolytes for quasi-solid-state dye-sensitized solar cells[END_REF][START_REF] Chalkias | Spectral engineering of semi-transparent dye-sensitized solar cells using new triphenylamine-based dyes and an iodine-free electrolyte for greenhouse-oriented applications[END_REF][START_REF] Bharwal | Plasticized I2-free polysiloxane ionic conductors as electrolytes for stable and flexible solid-state dye-sensitized solar cells[END_REF][START_REF] Yu | Efficient dye-sensitized solar cells employing highly environmentally-friendly ubiquinone 10 based I 2-free electrolyte inspired by photosynthesis[END_REF]. A lower R1 and R2 for the I2-containing electrolytes versus the I2-free counterparts was correlated to the higher concentration of I -/I3 -at the photoanode interface [START_REF] Yu | Iodine-free nanocomposite gel electrolytes for quasi-solid-state dye-sensitized solar cells[END_REF][START_REF] Chalkias | Spectral engineering of semi-transparent dye-sensitized solar cells using new triphenylamine-based dyes and an iodine-free electrolyte for greenhouse-oriented applications[END_REF]. Nevertheless, this can be a negative point for the DSSCs as excess I3 -can induce parasitic/recombination reactions. However, the extremely large R3 value of I2-free electrolytes-based DSSCs indicated a very little amount of I3 -ions as the EIS measurements were done in dark conditions [START_REF] Yu | Iodine-free nanocomposite gel electrolytes for quasi-solid-state dye-sensitized solar cells[END_REF][START_REF] Chalkias | Spectral engineering of semi-transparent dye-sensitized solar cells using new triphenylamine-based dyes and an iodine-free electrolyte for greenhouse-oriented applications[END_REF]. The effective electron lifetime (τe) was calculated from the bode phase plot (Fig. 4d) based on the lower frequency range peak by using equation 4 [START_REF] Xia | Influence of doped anions on poly (3, 4-ethylenedioxythiophene) as hole conductors for iodine-free solidstate dye-sensitized solar cells[END_REF]:

𝜏 𝑒 1 2л𝑓 𝑚𝑎𝑥 ( 4
)
Where fmax is the frequency of the peak. The I2-free DSSCs have a superior electron lifetime than the one with I2containing counterparts (Table 4). Likewise, the Voc decay rate (Fig. 4e) and the calculated electron lifetime (Fig. 4f) of the DSSCs with I2-free electrolyte were much higher than for the ones with I2-based DSSCs, in agreement with the EIS measurements. This further confirms that the charge recombination is suppressed by the removal of I2 from polymer electrolytes with a better effect for the D7 device which is our champion device. The decrease in charge recombination is also consistent with the diminution of the measured dark current (Fig. S5b). We have disassembled the D7 device upon the completion of the above-referred examination of the PV performances. The cross-section SEM analysis (Fig. S8a) confirmed that the polymer infiltrated thoroughly the pores of TiO2 (grain boundaries not visible) and evenly covers the entire TiO2 photoanode, ensuring good interfacial contact, excellent porefilling, and thus efficient dye regeneration. The EDX spectrum (Fig. S8b) confirmed the presence of titanium (Ti), silicon (Si), and iodine (I) through the entire cross-section, while the mapping illustrated in Fig. S8c confirmed the electrolyte penetration across the TiO2 photoanode thickness.

Influence of scattering layer

So far, we assessed the photovoltaic performance of the DSSCs based on the PS3/EC (D7) and PS3/EC/I2 (D7I) electrolytes with thicker photoelectrode. A light scattering layer (SL) of 18NR-AO of about 4 µm was deposited on top of the 13 µm photoelectrode to enhance the thickness and the light-harvesting. The cross-sectional SEM image in Fig. 6a showed that the TiO2/SL photoanode is around 17 µm thick. By adding the SL, the diffuse reflectance of the photoanode was increased (Fig. 6b). The J-V characteristics of all the devices made with TiO2/SL photoanode and different electrolytes are shown in Fig. 6c, and their respective photovoltaic parameters are briefed in Table 5. In DSSCs, the impact of SL is largely retrieved in the Jsc value [START_REF] Lan | Formation of size-tunable dandelion-like hierarchical rutile titania nanospheres for dye-sensitized solar cells[END_REF][START_REF] Mustafa | Review on the effect of compact layers and light scattering layers on the enhancement of dye-sensitized solar cells[END_REF]. The ss-DSSCs with TiO2/SL photoanodes showed superior PCE as compared to pristine TiO2 photoanodes (Table 2 vs. Table 5). The Jsc is increased upon increasing the thickness, with a slight decrease in the FF. Notably, the device D7-SL (I2-free PS3/EC in combination with TiO2/SL photoanodes) showed the highest Jsc (14.94 mA cm -2 ) corresponding to a 1.2 times improvement in efficiency compared to the device D7 (with commercial TiO2 photoanodes). As shown in Fig. 6d, the maximum bandwidths of the IPCE spectra for PS3/EC DSSC (D7) were nearly the same as those of the PS1 DSSC (D4) but the plateau was much higher, which is consistent with the improved Jsc. For this device, the highest Voc value was obtained which corresponded to a reduced dark current (Fig. S5c). 4 for DSSC codes.

Influence of TiCl4 treatment

TiCl4 post-treatment of TiO2 photoanode significantly improves the photovoltaic performance of liquid electrolyte-based DSSCs as shown in Fig. S9. One of the reasons for improved performance is attributed to the increased surface area of the photoanode as a result of TiCl4 treatment, resulting in more dye loading and hence improved light-harvesting efficiency (Fig S10) [START_REF] Lee | Effects of TiCl4 treatment of nanoporous TiO2 films on morphology, light harvesting, and charge-carrier dynamics in dye-sensitized solar cells[END_REF]. To optimize the ss-DSSC performance, the TiO2/SL photoanodes of the D7 and D7I devices were subjected to a post-treatment with aqueous 0.04M TiCl4 at 70 o C for 30 min. The corresponding photovoltaic parameters are listed in Table 6 and Fig. 7a. The performance of the DSSC containing I2-free highly substituted PS3/EC (D7-SL-TiCl4) electrolytes was, boosted, yielding the highest efficiency ever reported for this type of electrolytes, 8.30%. In comparison, the device D7I-SL-TiCl4 with PS3/EC/I2 shows a relatively lower PCE of 6.50%. The combined effect of TiCl4 and SL resulted in shifting the entire IPCE curve upward (Fig. 6d vs Fig. 7b). The performance improvements could be attributed to the improved connectivity between the TiO2 grains, a shift in the conduction band edge of the TiO2, reduced defect traps, and inhibition of electron recombination due to TiCl4 treatment (Fig. S5d) [START_REF] Lee | Effects of TiCl4 treatment of nanoporous TiO2 films on morphology, light harvesting, and charge-carrier dynamics in dye-sensitized solar cells[END_REF][START_REF] Sommeling | Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films in dye-sensitized solar cells[END_REF][START_REF] Park | Enhanced efficiency of dye-sensitized solar cells through TiCl4-treated, nanoporous-layer-covered TiO2 nanotube arrays[END_REF]. We also believe that the extra dye-loading capacity provided by the TiCl4 treatment, (Fig. S10) lead to an improved charge carriers photogeneration. It is also worth mentioning that ultrasound soldering of the electrode contacts improved the electric contacts, which resulted in DSSCs with high FF and PCE. The Voc decay rate (Fig. 7c) and calculated electron lifetime (Fig. 7d) of the device D7I-SL-TiCl4 were greater than its I2-containing counterpart. The optimized D7-SL-TiCl4 device under varied light intensity illumination (1 sun to 0.1 sun). Fig. 7e shows that the device maintained its performance even at low light intensity illumination (Table S4), with a maximum PCE value reaching 9.1% under 0.3 sun i.e. 30 mW cm -2 . DSSCs are mostly designed for indoor applications. Therefore, we studied the long-term stability of our best devices, D7-SL-TiCl4 versus the D7I-SL-TiCl4 by aging them in ambient conditions for 26 months. Astonishingly, D7-SL-TiCl4 displayed excellent long-term stability, retaining up to 84% of its initial efficiency after 26 months, whereas the D7I-SL-TiCl4 device retained only 23% (Fig. 7f and Fig. S11). This result further confirms the enhancement of the long-term stability of DSSC when fabricated with I2-free polysiloxane electrolytes. We examined the influence of SL thickness of two different SLs (18NR-AO and R/SP) on the photovoltaic output. The side-view images shown in Fig. S12 indicate that the 18NR-AO comprises small nanorods (~20 nm) mixed with large crystals (200-400 nm) and the R/SP paste contains only large nanoparticles (>100 nm) as also reported by Lan et al., [START_REF] Lan | Formation of size-tunable dandelion-like hierarchical rutile titania nanospheres for dye-sensitized solar cells[END_REF]. Table S5 and Fig. S13 present the photovoltaic parameters showing the maximum performance obtained for 17 µm thickness for both AR-NO and R/SP SL which is attributed to the higher dye uptake (Fig. S10) and improved light-harvesting harvesting due to the superior light scattering (Fig. S12c-d). Note that the thicker TiO2/3SL (19 µm) shows much lower performance than the others due to mass transportation limitations [START_REF] Wang | Significant influence of TiO2 photoelectrode morphology on the energy conversion efficiency of N719 dye-sensitized solar cell[END_REF]. stability of the I2-based and I2-free DSSC over 26 months. See Table 5 for DSSC codes.

Tables S6, S7, and S8 display the yearly evolution of the photovoltaic parameters of the allstate DSSCs based on polysiloxane, I2-free, and PIL electrolytes, respectively. The careful examination of these data, reveals that the PCE reported in this work is higher than the efficiency reported for all-state I2-free PILs and polysiloxane-based electrolytes, as highlighted in Fig. 8.

Conclusion

A series of I2-free polysiloxane-based poly(ionic liquids), having different ionic functionality and ionic liquids, as well as their blends, were successfully implemented as polymer electrolytes for quasi solid-state DSSCs. In this work, we have proved that exclusion of I2 was necessary to diminish the polyiodide originated recombination, and thus enhance the transparency, photovoltaic performance, and device stability. Raman spectroscopy was used to prove the in-situ generation and efficient regeneration of I3 -in I2-free DSSCs but also to indicate the participation of polyiodide species in the charge transfer mechanism. Likewise, through this method, we identified the presence of excess I3 -and its imperfect regeneration explaining the lower performance and stability of the I2-containing devices. We have also shown through electrochemical analysis that the iodine removal increased the Voc contributing to a more efficient charge photogeneration and dye regeneration.

Through electrolyte optimization, we have found that the highly substituted I2-free PS3 polymer electrolyte plasticized with ethylene carbonate reached the highest reported efficiency (8.3 % and 9.1% under 1 and 0.3 sun illumination), excellent long-term stability (only 16% efficiency loss after 26 months aging) and improved transparency when tested with a traditional TiCl4-treated randomly-organized TiO2 with scattering layer.

We may, hence, anticipate that this work will trigger further interest in I2-free polymer electrolytes and thus consolidate our efforts to develop stable and fully transparent efficient solar energy conversion devices while being extendable to other energy-related applications such as solar energy storage, photo-chargeable batteries.

Fig. 1 .

 1 Fig. 1. (a) Schematic illustration at nanoscale level of the ions percolation paths through the

Fig. 2 .

 2 Fig. 2. J-V curves of the DSSCs comprising a 13-μm TiO2 photoelectrode in combination with

Fig. 3 .

 3 Fig. 3. (a) Raman spectra of MPII, MPII/I2, D2, and D2I recorded after 2 mins of laser

Fig. 4 .

 4 Fig. 4. (a) J-V of DSSCs comprising a 13-μm TiO2 photoelectrode in combination with I2-

Fig. 5 .

 5 Fig. 5. Energy level diagram for the PS2/MPII and PS2/MPII/I2 electrolytes.

Fig. 4b .

 4b The first (high frequency) and second (middle frequency) semicircles correspond to the charge-transfer resistances at the counter electrode/electrolyte interface (R1 and C1 are the resistance and capacitance of the corresponding double layer) and at the photoanode/electrolyte (R2 and C2 are the resistance and capacitance of the corresponding double layer) interfaces, while the third one in the low-to the diffusion of the electrolyte (R3 and C3 are the electrolyte diffusion resistance and capacitance). The values of the circuit elements obtained through fitting are gathered in Table

Fig. 6 .

 6 Fig. 6. (a) Cross-sectional SEM image showing double-layer structure TiO2/SL photoelectrode;

Fig. 7 .

 7 Fig. 7. (a) J-V and (b) IPCE curves with integrated Jsc values of DSSCs comprising a TiCl4-

Fig. 8 .

 8 Fig. 8. Time evolution of the photovoltaic efficiency for the all-state DSSCs based on PILs,

Table 1 .

 1 Photovoltaic parameters of the DSSCs on the 13-μm TiO2 photoelectrode in combination with liquid, I2-based, and I2-free ILs-based electrolytes. The photovoltaic performance is the result of averaging 3-4 devices and standard deviation values are also included.

	Device code	Electrolyte	Voc (V)	Jsc (mA cm -2 ) FF (%) Average
						PCE (%)
	D1	Reference	0.66 ±	12.25 ± 0.42 68.37 ±	5.49 ±
		(liquid)	0.007		0.26	0.22
	D2	MPII	0.65 ±	13.02 ± 0.38	55 ±	4.64 ±
			0.01		0.97	0.25
	D2I	MPII/I2	0.56 ±	9.87 ± 0.75	60 ±	3.28 ±
			0.01		0.74	0.31
	D3	MPITFSI	0	0	0	0
	D3I	MPITFSI/I2	0.39 ±	0.82 ± 0.04	51 ±	0.16 ±
			0.015		2.17	0.016

Table 1

 1

	for DSSC codes.

Table 2 .

 2 Photovoltaic parameters of the DSSCs on the 13-μm TiO2 photoelectrode in combination with I2-based and I2-free PS2 and PS2/MPII based electrolytes

	Device	Electrolyte	Voc (V)	Jsc (mA cm -2 ) FF (%)	PCE (%)
	D6	PS2/MPII	0.67 ±	10.94 ± 0.38	61 ±	4.47 ± 0.25
			0.007		0.74
	D6I	PS2/MPII/I2	0.63 ±	8.44 ± 0.45	64 ±	3.42 ± 0.31
			0.01		2.09
	D7	PS3/EC	0.71 ±	11.68 ± 0.45	60 ±	5.03 ± 0.32
			0.01		1.17
	D7I	PS3/EC/I2	0.65 ±	10.77 ± 0.45	65 ±	4.50 ± 0.17
			0.01		0.16

Table 4 .

 4 Parameters were obtained by fitting the EIS spectra of the DSSCs incorporating different electrolytes.

	Device	Electrolyte	Rs (Ω)	R1 (Ω) R2 (Ω)	R3 (Ω)	fmax (Hz)	τe (ms)
	D6	PS2/MPII	33	9	20	53	0.81	197
	D6I	PS2/MPII/I2	31	4	9	3	12.87	13
	D7	PS3/EC	33	10	21	40	0.53	300
	D7I	PS3/EC/I2	30	7	17	4	7.27	22

Table 5 .

 5

	J-V characteristics of DSSCs comprising a 17-μm TiO2/SL photoelectrode
	combination with I2-based and I2-free PS2/MPII and PS3/EC -based electrolytes.
	Device	Electrolyte	Voc (V)	Jsc (mA cm -2 )	FF (%)	PCE (%)
	D6-SL	PS2/MPII	0.63 ±	14.03 ± 0.36	60 ±	5.30 ±
			0.004		1.39	0.23
	D6I-SL	PS2/MPII/I2	0.59 ±	11.31 ± 0.55	61 ±	4.10 ±
			0.003		0.91	0.21
	D7-SL	PS3/EC	0.69 ±	14.94 ± 0.16	58 ±	5.97 ±
			0.005		0.83	0.16
	D7I-SL	PS3/EC/I2	0.61 ±	13.34 ± 0.59	62 ±	5.04 ±
			0.007		0.76	0.13

Table 6 .

 6 Photovoltaic parameters of the DSSCs containing PS3/EC and PS3/EC/I2 in combination with TiCl4-TiO2/SL photoanodes.

	Device	Electrolyte	Voc (V)	Jsc (mA cm -2 )	FF (%)	PCE (%)
	D7-SL-TiCl4	PS3/EC	0.70 ±	19.40 ± 1.89	61 ±	8.30 ±
			0.004		1.77	0.27
	D7I-SL-TiCl4	PS3/EC/I2	0.64 ±	15.35 ± 0.45	66 ±	6.50 ±
			0.01		0.35	0.30

Acknowledgments

The research was supported by ARC4-Énergies Région, Rhône-Alpes. The authors would also like to acknowledge IDS FunMAT (International Doctorate School in Functional Materials) and the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" n° AN-10-LABX-44-01 for financial support. Electron microscopy studies were performed at the CAREM facility (Celluled'Appui à la Recherche et à l'Enseignement en Microscopie -ULiège).