Conductive polymers have received vast attention to be adopted as a transparent electrode, based on their flexibility and ease of fabrication in thin-films via a solution-based process. Thanks to its thermal and chemical stability, PEDOT:PSS has been widely used as a part of solar cells and LEDs, especially the hole transport layer. However, the high sheet resistance of PEDOT:PSS thin film induces efficiency drag. Therefore, other works have been dedicated to enhancing this property, mainly via secondary doping. This work reports adapting hot-casting to the fabrication of DMSO-doped PEDOT:PSS thin film which yields a significant decrease in the sheet resistance. From atomic force microscopy imaging and sheet resistance measurements, it can be concluded that hot-casting yields a decrease in resistance and surface smoothening, and both of them seem to be due to interconnection among the grain cores. The further application to the hole transport layer of the CH3NH3PbI3 perovskite solar cell was examined and resulted in increased power conversion efficiency. Notably, the hot-casting temperature at which the sheet resistance was minimized did not exactly match the temperature at which the power conversion efficiency maximized. This seems to be due to the morphology smoothening effect of the film.