Heterojunction interfaces play a crucial role in charge carrier transport, influencing the overall photovoltaic performance of organic solar cells (OSCs). Despite the importance, advancements in interfacial engineering, especially in optimizing the microstructure and nanomorphology, have not kept pace with research on photoactive layers. In the study, a strategy is explored to control the self-assembly growth of alcohol-soluble Me-4PACz (4P) used as a hole transport layer (HTL) in OSCs. The surface architecture is modified of inorganic Co salts via Cu doping and UV-ozone treatments, creating a smooth top surface with an increased Co