# Effects of Doped Hole-Transporting Layers on Perovskite Solar Cell Performances
## 4 Conclusions  
A novel processing technology to improve the stability of PSCs using doping hole-transporting layers was examined. The optimal doping rate was 2 wt% of DIO to PEDOT: PSS and this doping enhanced the PCE of the device by about 10%. The electrostatic effect of the dopant on the device performances was studied by changing the dopant from DIO to DCO. Replacing the atoms in the dopant from diiodide to dichloride changed the ion density of the HTL (because of differences in the electronegativity between chlorine and iodine) and resulted in an electrostatic interaction change between the halide atoms of the dopant and the lead ions of the perovskite crystal. Due to the molecular weight differences between iodide and chlorine, the same dopant mole ratio to PEDOT was reduced to 1 wt% and this composition showed the best device performance. The device performances were within the experimental error range, so it could be concluded that the electrostatic effect was a very weak influence on the device performances. The surface roughness effect on the device performances was examined using AFM. Doped HTL produced a smoother surface, resulting in a smoother perovskite surface. According to the XRD diffraction peak intensity study, the perovskite crystallinity formed on the doped HTL (both DIO and DCO) was higher. The conductivity of the HTL was  
Figure 7: PCE changes as a function of time to obtain LT80. (**a**) The 0.09 cm\({}^{2}\) cell exposure to 100 mW/cm\({}^{2}\) solar simulator light at a relative humidity of 35% without encapsulation, (**b**) the 0.36 cm\({}^{2}\) cell exposure to 100 mW/cm\({}^{2}\) of solar simulator light at a relative humidity of 35% without encapsulation, (**c**) the 0.09 cm\({}^{2}\) cell stored at a relative humidity of 35% without device encapsulation, and (**d**) the 0.36 cm\({}^{2}\) cell stored at a relative humidity of 35% without device encapsulation. measured using a four probe method and was higher for doped devices. Combining these electrostatic, surface roughness, and conductivity results, the enhancement of device performance was mainly caused by smoother surface formation and an increase in conductivity rather than the electrostatic interaction. Long-term stability of the device was tested by measuring the time required for the PCE to fall below 80% of the initial value. DIO doping appeared to be slightly better than DCO doping on device performance. When a 200 W microwave was used to anneal PEDOT: PSS film, the processing time was reduced to 1/30 th of that when thermal annealing was used without losing device performance. A lower power (50 W) microwave treatment was used for the post-annealing of the fully fabricated devices and it turned out to vaporize traces of moisture and prolong the lifetime of the device. Stability enhancement was studied using a larger area cell; the doped devices showed much more enhanced device lifetime compared with that for un-doped devices. According to larger size cell studies, the device lifetime was decreased as the cell area increased; however, as the size of the cell increased, the extent of lifetime enhancement through doping increased. These enhancements of long-term stability were attributed to the passivation process. The partially electro-negative halogen (i.e., 1S\({}^{-}\) and Cl\({}^{8-}\)) in DIO and DCO could interact with the Pb\({}^{2+}\) of the perovskite crystal while the hydrophobic aliphatic moiety (i.e., partially positive octyl groups) can passivate the surface of perovskite thin film from moisture. This passivation effect through the surface modification of the perovskite grains could be beneficial for the long-term stability of devices and is well studied [14; 25; 26; 27; 28]. Methodology, M.J.K. ; investigation, H.L. and J.Y.J. ; writing--review and editing, supervision, funding acquisition, J.-W.Y. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Research Foundation of Korea (NRF) with grant funded by the Korean government (MSIT) (Grant No.