Study and Interpretation of short-term temporary reverse-bias degradation in wide-bandgap FAPbBr3 perovskite solar cells

Perovskite Solar Cells (PSCs) are among the prime candidate technologies for next-generation PV applications where the shortcomings of silicon PV are manifested, due to their potential performance, tunable and adaptable properties. Both intrinsic and extrinsic factors still hinder their promotion from lab-scale to industry-scale and their subsequent commercial success. Among these, stability under reverse-bias is paramount to ensure adequate long-term reliability in real-world operating conditions; reverse-bias stability is relevant for scenarios such as shading instances in a PV module (for which it is unwanted sporadic and systematic) or photodetection applications (for which it is deliberate and systematic). The study of reverse-bias stability in wide-bandgap semi-transparent FAPbBr3 PSCs is hereby presented. Step stress results show that short-term and temporary degradation is triggered below -1.5 V, reaching full extent around -3 V; degradation continues to worsen below -3 V, until permanent and irreversible breakdown occurs around -7.5 V, leading to a visible distributed burn-mark on the cell surface which is mostly due to the intensity of the current flow. Constant stress results show that longer exposure to reverse-bias at values up to -1.5 V does not significantly impact cell performance, while a reverse-bias of -3 V is instead sufficient to cause short-term degradation, which is also observed to be apparently fully recoverable. The observed degradation appears as a shunt-like phenomenon and pertains to the open-circuit voltage, while short-circuit current remains unaffected. These outcomes lead to an interpretation which involves the temporary relocation of ions and corresponding vacancies form the bulk of the perovskite material towards opposite interfaces. This generates a severe compensation of the energy band bending across the active layer, thus causing a temporary compensation of the internal potential and a shunt-like charge-transfer across the PVK/ETL interface; after removing the reverse-bias, this mechanism eventually ceases, and full recovery occurs.