Mechanism of Organic Solar Cell Performance Degradation upon Thermal Annealing of MoO_x

In this work we focus on P3HT:PC61BM bulk heterojunction (BHJ) devices with MoO3 at the hole extraction side of the BHJ, which relies on the formation of a strong dipole at the BHJ/MoO3 interface, as a reference system that has been extensively studied. We have observed, depending on when the annealing is performed during device fabrication, that the device performance either increased or decreased due to formation of a sharp or relatively diffuse interface, respectively, due to diffusion of MoOx into the BHJ. The measured strength of the dipole at this interface following thermal annealing correlated well with the width of the interface and device performance, with the sharper interface resulting in a stronger dipole and in improved device performance. This is expected to be a general phenomenon for evaporated coatings onto polymeric BHJ, regardless of the polymers involved.