LiI doping of mixed-cation mixed-halide perovskite solar cells: Defect passivation, controlled crystallization and transient ionic response

We investigate lithium iodide (LiI) doping of mixed-cation mixed-halide (MCMH) perovskites as a method to improve film morphology and optoelectronic properties, reduce defect-related recombination losses and limit ion migration. The optimized Li-doped devices achieved 21.5% power conversion efficiency (PCE) compared to 20.4% for undoped control devices. Furthermore, optimized Li-doped devices show minimized current-voltage hysteresis across a wide range of scan rates compared to control devices, which displayed significant scan-rate dependent hysteresis. We show, using a combination of experimental analysis and numerical device simulations, that the improved device performance and transient response at optimum doping concentration are consistent with reduced non-radiative recombination in the perovskite. However, increasing the LiI dopants beyond the optimum concentration alters the ionic properties of the perovskite absorber and changes the transient response. We also find that LiI addition results in a downward shift of the valence band and a corresponding transition from a weakly n-type to p-type perovskite material. These findings reveal the beneficial role of LiI dopant at low concentrations can improve shelf-life and minimize recombination-active defects, which are favourable for designing high-performance and stable mixed-cation mixed-halide perovskite devices.