Reverse-Bias Testing of Perovskite Cells to Inform Bypass-Diode Design

Perovskite-based PV devices promise the combination of low cost and high efficiency. While much effort has been spent characterizing perovskite stability at the cell level, less has been dedicated to module-level stability. Nonuniform illumination of modules can cause failure by forcing shaded cells to operate in reverse bias to pass current generated by illuminated cells. Bypass diodes are employed in conventional PV to re-route this current, sometimes shared across multiple cells to reduce cost. Furthermore, we envision a cell stack with thin-film diodes integrated into each cell. To understand how to best employ bypass diodes in perovskite modules, it is important to understand the cell-level electrical requirements to prevent damage. Here, we characterized the reverse bias behavior of spin-coated and thermally-evaporated single-junction and tandem perovskite-Si devices. We measured breakdown voltages and currents, and the amount of effective shading time cells could survive. The devices were also subjected to reverse-bias conditions and in-situ characterized via DLIT to identify local failure modes. We find that reverse-bias breakdown and irreversibly damaging conditions depend on both the device architecture and perovskite deposition. Solution-coating perovskite results in non-uniformities where breakdown occurs. Conformal evaporated perovskite layers have more robust and consistent reverse bias properties, and integration into perovskite-Si tandems results in higher breakdown voltages. We find that even in the conditions cells would experience when shaded and bypassed by a diode, some damage still occurs after hours of shading time. These findings have big implications for bypass diode integration into perovskite-based modules - setting limits on what conditions the cells can experience when considering stringing bypass diodes, providing design rules for integrating on-cell bypass diodes, and guiding use cases based on realistic shading scenarios.