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Abstract Narrow bandgap mixed lead-tin perovskites are critical for efficient all-perovskite multi-junction solar cells, but their poor stability under operating conditions represents a major barrier to implementation. In this work, we explore the causes of this instability under combined heat and light stress (ISOS L-2 conditions). The phase, absorbance, morphology, and background hole density in lead-tin perovskite films are observed to be stable beyond the usual timescales associated with device degradation. We measure a moderate increase in non-radiative recombination during stressing, but device simulations demonstrate that this can only account for a small portion of the observed steady-state performance loss. Variable rate current-voltage scanning of devices instead reveals an increasing impact of mobile ions to be the major cause of early-time performance degradation. This impact is found to be significantly mitigated by selecting an alternative hole transport layer. Over longer aging times, we also identify the growth of impurity phases as well as hole transport material-dependent changes in the electronic properties of the perovskite. By quantifying the impact of these changes on device performance, we identify the most dominant degradation pathway at each aging time for different device architectures, defining a clear direction for future stability improvements.
Rombach et al. (Mon,) studied this question.