Extrinsic Movable Ions in MAPbI3 Modulate Energy Band Alignment in Perovskite Solar Cells

Ionic movement is considered awful in perovskite solar cells (PSCs) for relating with the hysteresis and long-term instability. However, the positive role of ions to enhance the energy band bending for high performance PSC is always overlooked, let alone reducing the hysteresis. In this work, LiI is doped in CH3NH3PbI3. It is observed that the aggregation of Li+/I- tunes the energy level of the perovskite and induces n/p doping in CH3NH3PbI3, which makes charge extraction quite efficient from perovskite to both NiO and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) layer. Therefore, in NiO/LiI doped perovskite/PCBM solar cells, Li+ and I- modulate the interface energy band alignment to facilitate the electron/hole transport and reduce the interface energy loss. On the other hand, n/p doping enlarges Fermi energy level splitting of the PSCs to improve the photovoltage. The performance of LiI doped PSCs is much higher with reduced hysteresis compared to the undoped solar cells. This work highlights the positive effect of selective ionic doping, which is promisingly important to design the stable and efficient PSCs.