Are Reverse-Bias Instabilities in Perovskite Solar Cells Due to Electrochemistry or Pre-Existing Defects?

Perovskite solar cells (PSCs) boast impressive power conversion efficiencies, but they often exhibit rapid and severe failure under even moderate reverse bias voltages. This instability is a major obstacle in their path to commercialization since reverse bias is experienced in real-world conditions when series-connected cells become shaded. This effect is relevant in all solar cell technologies and is well known in the industry; many commercial solar modules implement bypass diodes to allow current to flow around the shaded cells or strings. In the case of PSCs, however, their low reverse-bias damage threshold --- often as small as |-2| V --- is too low to effectively implement this strategy. To date, perhaps the most popular explanation of this instability involves conductive filament formation through the perovskite film, enabled by redox reactions between the perovskite elements and metal electrodes. In this framework, conductive filaments rapidly form and lead to shunts, resulting in irreversible and complete loss of power conversion efficiency. Several research groups have thus proposed and demonstrated solutions which involve arresting ion diffusion between the perovskite and the metal contact, for example, by using dense and thick metal-oxide barrier layers. This has led to seemingly stable reverse-bias behavior with reverse-bias breakdown voltages exceedingly approximately |-10| V. In this talk, we show experimental results that suggest an alternative degradation mechanism related to the uniformity and coverage of the hole transport layers (HTL) and electron transport layers (ETL). In many ways, this hypothesis is reassuring because it indicates simpler fabrication cleanliness-related routes to stabilize PSC performance. Using a combination of electroluminescence imaging, photoluminescence imaging, microscopy and thermal imaging, we show evidence that preexisting film defects cause abrupt reverse-bias degradation events in perovskite solar cells. Following this, we fabricate a series of perovskite-free HTL-ETL diodes to demonstrate that reverse-bias resilience may be governed largely by HTL and ETL coverage, and protrusions in the underlying transparent conductive oxide. We make two important observations: 1) these perovskite-free HTL-ETL diodes exhibit rapid and spontaneous shunting, strongly resembling that in a complete perovskite solar cell, when film coverage is insufficient; 2) these same perovskite-free HTL-ETL diodes exhibit remarkable reverse-bias resilience when film coverage and conformality is sufficient. This intuition is then applied to complete perovskite solar cells, where we find that TCO texture, HTL and ETL coverage, and perovskite morphology all notably affect reverse-bias stability. This work provides a potentially new perspective to PSC instabilities and highlights several fabrication-related features that must be considered while pushing perovskite solar cells toward commercialization.