Double Hole Transport Layers Enable 20.42% Efficiency Organic Solar Cells by Aggregation Control of Self‐Assembled Molecules on Cobalt Salt Surfaces

Abstract 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 3+ /Co 2+ ratio and hydroxyl groups. This meticulous design fine‐tuned the assembly behavior of self‐assembled molecules, resulting in the transition from spherical aggregates to a more uniform worm‐like morphology. Additionally, the electrical and optical properties are optimized to passivate surface defects and enhance the wettability of organic solvents, leading to improved hole extraction and reduced interfacial charge carrier recombination losses. Consequently, an OSC with Cu‐Co/4P as the HTL exhibited the highest power conversion efficiency of 20.42% (certified 20.20%). The characteristic universality and stability make the Cu‐Co/4P HTL a potential candidate for widespread applications, particularly in providing rationalized guidance to further enhance the performance of OSCs.