Light cycling as a key to understanding the outdoor behaviour of perovskite solar cells

Forecasting the real-world stability of perovskite solar cells (PSCs) using indoor accelerated tests is a significant challenge on the way to commercialising this highly anticipated PV technology. The lack of outdoor data and considerable magnitude of meta-stability effects (or reversible changes) in PSCs' performance over the day-night cycle makes it particularly challenging to correlate results of the commonly utilised light-soaking ageing test with outdoor experiments. Here we show the variety of short-term and long-term ageing behaviours by testing PSCs of various architectures under constant and intermitted light indoors and exposing them to natural conditions outdoors. We demonstrate that it is impossible to predict the results of a light cycling test from a continuous light test without prior knowledge of the ageing patterns for a particular device architecture. Cycling the light does not necessarily lead to an increased lifetime as expected due to dark time recovery. Instead, it sometimes reveals a different degradation behaviour resulting in a drastic lifetime reduction. The presence of various degradation patterns for different PSCs implies that an accelerated ageing with constant light experiment is no "worst-case scenario" and thus cannot replace the light cycling test nor can it reproduce the real-world scenarios. Furthermore, we show unique sets of weeks-to-years-long outdoor series on different PSCs highlighting the monumental importance of accounting for the meta-stability effects when analysing PSC outdoor data as opposed to simply following evaluation routines developed for silicon-based devices. In particular, meta-stability complicates the decoupling of the effects of environmental conditions from the cell's ageing behaviour and can result in large artefacts. A varying degree of saturation of reversible processes also results in unusual strong seasonality documented for PSCs, with summer representing favourable conditions for some PSCs' energy generation compared to winter, despite higher temperatures. For the first time, the decisive impact of meta-stable processes on the outdoor performance and stability of perovskite solar cells is demonstrated, with data from over two years in the field, which is the longest outdoor exposure of PSCs reported so far to the best of our knowledge. The correlation between the outdoor results and those from the light cycled experiments is evident. The long-term outdoor performance and stability of Perovskite Solar Cells showed several features that stem from the device's meta-stability. To rationalize this behavior we need indoor ageing experiments with cycled light.