Achieving High Doping Concentration by Dopant Vapor Deposition in Organic Solar Cells.

Compared with the interfacial doping, molecular doping in bulk heterojunction (BHJ) is a more direct but challenging approach to optimize the photovoltaic performance in organic solar cells (OSCs). One of the main obstacles for its success is the low doping concentration because of the morphological damage. Starting from the phase diagram analysis, we discover that the unpreferred good miscibility between the p-type dopant and the acceptor leads to incorrect dopant dispersion, which reduces the achievable doping content. To overcome this, we use sequential doping by vapor annealing instead of blend solution doping, and we achieve the high doping concentration without sacrificing the blend film morphology. Benefiting from the undamaged film, we fulfill improved photovoltaic performance. Our positive results reveal the feasibility of high-level doping in complex organic BHJ films. It is believed that doping at high concentration potentially enlarges the extent of tunable range on electronic properties in OSCs and indicates greater improvement for device performance.