Performance enhancement of perovskite solar cells using NH4I additive in a solution processing method

Abstract

A two-step sequential deposition has been widely adopted to fabricate a perovskite film of planar heterojunction organo-lead halide perovskite solar cells. However, in this method complete conversion of Lead iodide (PbI2) into CH3NH3PbI3 is limited by the diffusion of CH3NH3I into PbI2 film, thus limits current density. Here, we report a facile method for enhancing perovskite film morphology by introducing a small amount of ammonium iodide (NH4I) as a halogen additive into PbI2 solution, which can react with PbI2 and forms nanostructured platelet structure that helps to diffusion of CH3NH3I, for further improving CH3NH3PbI3 film morphology. Under the optimized NH4I additive concentration of 5 wt%, the best power conversion efficiency (PCE) is 17.4% achieved, which is an improvement of 14% compared to that of the original device without the additive. SEM studies reveal that addition of NH4I into PbI2 films improves the porosity of the films, which helps to improve the perovskite film morphology by increasing the grain size. The enhancement of CH3NH3PbI3 film morphology is useful for increasing the optical absorption of perovskite film thereby increasing short-circuit current density, whence an improvement in the efficiency of the perovskite solar cell devices. The perovskite solar cells fabricated by this method preserved high crystallinity, negligible hysteresis, reliable reproducibility and improved stability.