Emerging lead halide perovskite materials have enormous potential for a range of optoelectronic devices, such as solar cells, light emitting diodes, transistors and lasers. However, the large-scale commercialization of these technologies will depend on the ability of the active material to be stable under environmental and operating conditions. Perovskite can be degraded by many factors such as humidity, light, oxygen and temperature. More recently, electrical stress has also been shown to be detrimental for the stability of perovskite solar cells (PSCs). However, the origin of such degradation, which has been attributed to either ion migration or superoxides formation, is not clear and needs further investigations. Besides, there is no study on perovskite devices with the inverted structure although it can be radically different from the standard structure as the electric field is opposite in that case and other types of interlayers are used. Inverted PSCs have several advantages over standard top-anode devices such as less hysteresis and lower processing temperatures, while record efficiencies are getting comparable to the standard architecture. In this work, the electrical bias-induced degradation of inverted perovskite solar cells in the dark is systematically investigated in four different environments, which allowed us to conclude that humidity coupled with electrical bias results in fast degradation of CH₃NH₃PbI₃ into PbI₂.

Micro-Raman and photoluminescence show that the degradation starts from the edge of the cell due to moisture ingress. By using novel local Raman-transient photocurrent measurements, we were able to probe local ion migration at the degraded region and non-degraded region with micro-meter resolution and found that the formation of PbI₂ can passivate perovskite by reducing ion migration. The degradation is far from uniform across different grains as revealed by secondary electron hyperspectral imaging, an advanced microscopy technique which allows to probe the composition of individual grain from the cross-section. By using potential step chronoamperometry, we also found that the humidity – mediated bias degradation agrees well with the increased density of mobile ion defects.

The unique combination of a number of established methods with several novel analytical tools from nano-scale to cell level demonstrates the impact played by ion migration on the bias degradation of inverted perovskite solar cells. Importantly, these advanced techniques will be useful for studying perovskite solar cells in general, while the dark bias degradation has significant insight to other perovskite based (opto-)electronic devices.