# Performance-Enhancing Sulfur-Doped TiO2 Photoanodes for Perovskite Solar Cells
## 2 Materials and Methods  
Titanium diisopropyloxide bis(acetylacetonate) (75 wt% in isopropanol), thiourea (\(\geq\)99.0%), absolute ethanol (99.95%), acetylacetone (\(\geq\)99%), lead iodide (PbI\({}_{2}\), 99%), lithium bis(trifluoromethylomenthanesulfonyl)imide (Li-TPSI, 99%), 4-tert-butylpyridine (tBP, 96%), N,N-dimethylformamide (DMF, 99.8%), 2,20,7,70-tetrakis-(N,N-dip-methoxyphenylamine)-9,90-siprobiluorene (spiro-OMeTAD, SHT-263, Solarpur), tris(2-(1H-pyrazol-1-yl)-4-tert-butyl-pyridine) cobalt(III) tri[bis(trifluoromethylomethane)sulfonimide] (FK209 Co(III) TFSI, 98%) and anhydrous acetonitrile (99.8%) were obtained from Sigma Aldrich and used as received. Prior to the deposition of photoanode layer, FTO-coated glass (FTO, TEC15, 15 \(\Omega\)/sq, Great-cell Solar Materials) was first etched to design the desired pattern of electrode using zinc powder and 3M HCL. Next, the FTO substrate is cleaned using ultrasonic bath for 10 min, in the following order: soap (2% Hellmanex in water), acetone then propanol (99.5%), to remove the organic residue on the surface of FTO-glass substrate. In this modified sol-gel method, S-doped TiO\({}_{2}\) photoanode is prepared by first mixing 1.2 mL acetyl acetone, 7 mL TTIP, 35 mL absolute ethanol and calculated amounts of thiourea as sulfur source. To evaluate to effect of sulfur doping concentration, the amount of thiourea is prepared with 5%, 10% and 15% mol thiourea compared to TTIP. The precursor solution was then magnetically stirred for 180 min to produce a clear and homogeneous solution. A blank precursor solution was also prepared without the thiourea as control. The deposition of photoanode was carried out using spin coating method. 3 layers of TiO\({}_{2}\) photoanode was spin-coated at 3000 rpm with drying process at 120 \({}^{\circ}\)C for 10 min after each layer deposition. Afterwards, the TiO\({}_{2}\) photoanode was sintered at 450 \({}^{\circ}\)C to complete the deposition. The nomination of all sample relates to the sulfur doping content of the initial precursor solution. To investigate the optical properties of the photoanode thin film, room temperature absorbance measurement (300 to 700 nm) is recorded using a UV-Vis spectrophotometer (Lambda 35, PerkinElmer). The XRD diffractogram of the photoanode thin film were analysed using X-ray diffractometer model Bruker D8 Advance (20, 10-70\({}^{\circ}\)). The FTIR spectrum of TiO\({}_{2}\) photoanodes were evaluated using FTIR spectrometer (Shimadzu--840 s). Fluorescence spectroscopy was used to assess the charge recombination behaviours of TiO\({}_{2}\) photoanodes. The fluorescence spectroscopy analysis was carried out using a Perkin Elmer Luminescence spectrometer (LS 55) with a thin film holder accessory, at room temperature and 300 nm as excitation wavelength. Morphological study of the TiO\({}_{2}\) photoanodes were carried out using field-emission scanning electron microscopy (FESEM) were analysed using FE-SEM SUPRA VP55 equipped with an energy dispersive X-ray (EDX) spectroscopy for elemental detection and mapping. The photoelectrochemical (PEC) properties of the TiO\({}_{2}\) photoanodes were determined via linear sweep voltammetry (LSV) using an electrochemical workstation (Metrohm Autolab), simulated AM 1.5 at a calibrated intensity 100 mWcm\({}^{-2}\) at NTP conditions. TiO\({}_{2}\) samples as the working electrode is immersed in Na\({}_{2}\)SO\({}_{4}\) solution (0.5 M), Pt as counter electrode and Ag/AgCl as the reference electrode. The scanning rate is kept at 20 mVs\({}^{-1}\) (\(-\)0.4 to 1.4 V vs. Ag/AgCl). The perovskite solar devices were prepared by depositing the perovskite absorber layer, hole transporting layer and silver contact, consecutively. The perovskite absorber layer is deposited using a 2-step method where in the first layer, 100 uL of PbI\({}_{2}\) solution consisting of PbI\({}_{2}\) powder (507 mg) in DMF (1 mL) and tBP (100 uL) is spin coated on the TiO\({}_{2}\) photoanode at 3000 rpm for 30 s then annealed at 100\({}^{\circ}\)C for 10 min.