# Performance-Enhancing Sulfur-Doped TiO2 Photoanodes for Perovskite Solar Cells
## 3 Results
### Linear Sweep Voltantmetry (LSV) and Photocurrent-Voltage (J-V) Curve Studies  
The photo-current response of the undoped and S-doped TiO\({}_{2}\) ETLs fabricated by spin-coating and sintered at 450 \({}^{\circ}\)C were evaluated using linear sweep voltammetry. The measurements were performed in the dark and under visible light-irradiated environment. The samples were soaked in 0.5 M Na\({}_{2}\)SO\({}_{4}\) solution at a scan rate of 20 mV/s, and the results are displayed in Figure 6a. At 1.0 V, the photo-current densities of the un-doped and S-doped samples were 0.0611 mA/cm\({}^{2}\) and 0.0695 mA/cm\({}^{2}\) (13.75% increase), respectively. Hence, the incorporation of S in the TiO\({}_{2}\) lattice increased the number of charge carriers (electron/hole pairs) in the photo-catalytic reaction of the thin film [23]. A previous study has reported similar responses, in which an increase in the photo-current to a certain extent was observed when the S content was increased, and a higher loading would be detrimental towards the photo-current readings due to the charge recombination of the electron-hole pair [31]. Thus, the incorporation of S as dopant could serve as a beneficial component to increase the photo-current in the thin film.  
Figure 4: Fluorescence emission spectra of undoped and S-doped TiO\({}_{2}\) photoanode thin film.  
Perovskite solar devices with different photo-anodes were fabricated in high-humidity environment as described in the experimental procedure. Figure 6b (inset) shows the schematic diagram of the fabricated planar structured PSC in a relatively high humidity (40-50%). The photocurrent-voltage (J-V) characteristic curves were observed under 100 mW cm\({}^{-2}\) (1 sun) illumination. Figure 6b shows the J-V curve of the heterojunction perovskite solar devices based on the FTO/S-doped TiO\({}_{2}\) or TiO\({}_{2}\)/CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\)/spiro  
Figure 5: FESEM morphological images of the EDX elemental spectra and elemental mapping of Ti, O and S of the un-doped (**a**,**c**,**e**) and (**b**,**d**,**f**) S-doped TiO\({}_{2}\).  
Figure 6: (**a**) LSV responses of the un-doped and S-doped TiO\({}_{2}\) photo-anode in the dark and visible light-irradiated environment and (**b**) photo-current–voltage curve of the perovskite solar devices based on the S-doped TiO\({}_{2}\) and TiO\({}_{2}\) measured under standard AM1.5G illumination and the planar PSC structure (inset).  
MeOTAD/Ag for the best device. The un-doped TiO\({}_{2}\) sample (reference device) produced a short-circuit density (JSC) of 13.1 mA cm\({}^{-2}\), open circuit voltage of 0.982 mV, fill factor of 39.88% and PCE of 5.1%. By contrast, the S-doped perovskite device was improved with device performance of J\({}_{\text{SC}}\) of 13.9 mA cm\({}^{-2}\), open circuit voltage of 0.997 mV, fill factor of 43.18% and PCE of 6.0%. The values of best device and device average over 10 made samples are summarised in Table 1. The device equipped with S-doped TiO\({}_{2}\) had better J\({}_{\text{SC}}\), FF and PCE compared with the un-doped TiO\({}_{2}\), but the V\({}_{\text{OC}}\) was relatively the same. The improvement in the J\({}_{\text{SC}}\) of the S-doped perovskite device was attributed to the increase in the electron collection capacity of the charge carrier and photo-current density, as described by Figure 6a, as reported in a previous study [1]. Additionally, Table 1 shows that the charge transfer resistance (R\({}_{\text{CT}}\)) in the S-doped TiO\({}_{2}\) perovskite device was lower than that of the un-doped TiO\({}_{2}\) device. The lower resistance could facilitate higher electron transfer in the perovskite device, resulting in the higher overall PCE.