Quantification of light-enhanced ionic transport in lead iodide perovskite thin films and its solar cell applications

Abstract

Ionic transport in organometal halide perovskites is of vital importance because it dominates anomalous phenomena in perovskite solar cells, from hysteresis to switchable photovoltaic effects. However, excited state ionic transport under illumination has remained elusive, although it is essential for understanding the unusual light-induced effects (light-induced self-poling, photo-induced halide segregation and slow photoconductivity response) in organometal halide perovskites for optoelectronic applications. Here, we quantitatively demonstrate light-enhanced ionic transport in CH3NH3PbI3 over a wide temperature range of 17–295 K, which reveals a reduction in ionic transport activation energy by approximately a factor of five (from 0.82 to 0.15 eV) under illumination. The pure ionic conductance is obtained by separating it from the electronic contribution in cryogenic galvanostatic and voltage-current measurements. On the basis of these findings, we design a novel light-assisted method of catalyzing ionic interdiffusion between CH3NH3I and PbI2 stacking layers in sequential deposition perovskite synthesis. X-ray diffraction patterns indicate a significant reduction of PbI2 residue in the optimized CH3NH3PbI3 thin film produced via light-assisted sequential deposition, and the resulting solar cell efficiency is increased by over 100% (7.5%–15.7%) with little PbI2 residue. This new method enables fine control of the reaction depth in perovskite synthesis and, in turn, supports light-enhanced ionic transport. Using light to excite ions in perovskite thin films can improve the conductivity and synthetic deposition of low-cost solar cells. Organometal halide perovskites have a suitable bandgap for photovoltaics and are compatible with solution processing, but tend to degrade after long exposure to sunlight. A team led by Qing Zhao from Peking University now reports that excited state ionic transport is the key to understanding perovskite’s poor photostability. Through video snapshots and quantitative conductivity extractions, their analysis revealed that illumination drops the energy barrier needed to activate ionic transport by almost five fold—an enhancement that may induce disorder of electronic structure in the solar cell over time. Intriguingly, the light-enhanced ionic transport can also catalyze removal of metal halide precipitates during thin film annealing in sequential deposition reaction, boosting the device efficiency from 7.5 to 15.7% after just 10 minutes of light exposure.