Perovskite solar cells (PSCs) tend to degrade rapidly under reverse bias, a condition that arises during partial shading. At voltages below −2 V, current forced backward through shaded cells produces localized hotspots and thermal runaway, leading to catastrophic breakdown. Unlike silicon solar cells, where bypass diodes mitigate reverse bias, PSCs remain highly vulnerable due to structural weaknesses in their active layers, making it essential to pinpoint the mechanisms driving this failure.
Image credit: Joule
A recent study, led by the McGehee group at the University of Colorado Boulder in collaboration with the National Renewable Energy Laboratory (NREL), identifies nanoscale to microscale defects - particularly pinholes in the perovskite film - as the principal trigger of breakdown events. These defects are introduced during the solution-processing fabrication method, which is prone to creating gaps and thin spots due to film inhomogeneity. Although such pinholes have only a minor impact on overall power conversion efficiency, they represent weak points where localized heating and failure originate under stress conditions.
Advanced characterization techniques, including electroluminescence imaging, scanning electron microscopy, laser-scanning confocal microscopy, and video thermography, provided compelling visual evidence that these defects are the focal points of reverse-bias degradation. In thermographic images, defective sites appear as bright hotspots, while electroluminescence imaging reveals them as dark regions, together offering a robust diagnostic toolkit for mapping breakdown pathways.
To isolate and study these effects with greater precision, the team fabricated miniature perovskite devices with active areas as small as 0.032 mm², comparable in width to two human hairs. Their small size allowed nearly defect-free films to be produced, offering a clear comparison with larger-area devices. Unlike their defect-ridden counterparts, these virtually perfect cells withstood hours of reverse bias without undergoing catastrophic breakdown, demonstrating that material uniformity is key to resilience. Importantly, investigations of perovskite-free transport-layer diodes further confirmed that metal migration and filament formation played little role in the abrupt failure. Instead, the primary cause was direct shorting between electrodes at pinhole sites, where the perovskite layer failed to provide complete insulation.
In response, the researchers explored strategies to eliminate interlayer contact at defect sites. Atomic layer deposition of tin oxide proved highly effective, not because it blocked ion migration, but rather because it acted as a reliable physical barrier preventing electrode-to-electrode bridging. This finding underscores that the future of stable PSCs lies in fabrication techniques that maximize film uniformity and eliminate nanoscale pinholes entirely. Cleaner deposition processes and advanced thin-film engineering therefore represent the most viable path forward.
By revealing the precise mechanism of low-voltage reverse-bias breakdown, this work addresses a long-standing uncertainty in perovskite research. It makes clear that defect suppression, rather than intrinsic material instability, governs device reliability in shaded operation. The implications could be significant: by adopting fabrication methods that yield smooth, pinhole-free films and by reinforcing transport layers against inter-electrode shorting, it may be possible to engineer PSCs that rival the stability of silicon while surpassing them in efficiency and affordability. The study marks a potential step toward turning perovskite photovoltaics from laboratory prototypes into durable commercial devices.
