Evolution of defects during the degradation of metal halide perovskite solar cells under reverse bias and illumination

The efficiency and stability of perovskite solar cells are essentially determined by defects in the perovskite layer, yet their chemical nature and linking with the degradation mechanism of devices remain unclear. Here we uncover where degradation occurs and the underlying mechanisms and defects involved in the performance degradation of p–i–n perovskite solar cells under illumination or reverse bias. Light-induced degradation starts with the generation of iodide interstitials at the interfacial region between the perovskite and both charge transport layers. While we observe trap annihilation of two types of iodide defect at the anode side, we find negatively charged iodide interstitials near the cathode side, which we show to be more detrimental to the solar cell efficiency. The reverse-bias degradation is initialized by the interaction between iodide interstitials and injected holes at the interface between the electron transport layer and the perovskite. Introducing a hole-blocking layer between the layers suppresses this interaction, improving the reverse-bias stability. The efficiency of perovskite solar cells decreases over time, yet the underlying mechanisms are unclear. Ni et al. observe charged iodide interstitial defects within the device layers and how they contribute to the efficiency degradation when the cell is operated under illumination or reverse bias.