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

► The pre and post-annealing for P3HT/PCBM solar cells with LiF/Al electrodes investigate. ► High-resolution transmission electron microscopy and secondary ion mass spectroscopy are used. ► The LiF thermal annealing process causes the diffusion of LiF. ► LiF/Al post-annealed processes degrade the device performance.