Origin of the efficiency improvement in pre-annealed P3HT/PCBM solar cells with LiF/Al electrodes

In this letter, the origin of the efficiency improvement in pre-annealed P3HT/PCBM solar cells with LiF/Al electrodes is investigated. No definite LiF layer is observed in the devices undergoing LiF layer annealing. There exists a nanoscopic difference of interfacial behavior and morphology of active layers near LiF/Al layers in LiF/Al pre-annealed and post-annealed devices. The LiF can diffuse into the active layer in the period of LiF layer annealing. Due to penetration of LiF into the active layer, the crystallinity of P3HT decreased. The annealing process of LiF thin layer causes diffusion of LiF, leading to decline of device performances.