How Pinholes in Perovskite Films Create Stealthy ETL-HTL Diodes that Slightly Reduce Efficiency and Dramatically Enhance Reverse Bias Shunting

Solution processing metal halide perovskites has advantages for scaling up manufacturing, but reproducibility and reliability are complicated by physics dictating coating (e.g. surface tension and dewetting). Void defects and pinholes form via tension created by volume reduction during the drying phase. We systematically investigate the origin and impacts of ‘void defects’ and reveal five common defect formation mechanisms that can result in points where the intrinsic perovskite absorber is missing. At most of these sites, metal- electron transport material (ETM) -hole transport material (HTM)- metal diodes exist. We determined how much current can pass through these defect diodes by making diodes without the perovskite layer and measuring the current-voltage curve. We found that the current turned on between 0.4 and 0.8 V depending on which contact layers we used. Dark Lock-in Thermography (DLIT) of a perovskite solar cell shows localized regions begin to heat under forward bias, but only above the turn-on voltage of transport-layer diodes indicating diode-like defect response in real devices. An important consequence of this behavior is that the defect diodes do not pass current near 0 V where the shunt resistance is typically measured by taking the slope of the current-voltage curve. They do, however, pass significant current at the maximum power point and the open circuit voltage. Consequently, they have a more detrimental effect on device performance than most people realize. These defects have even more important consequences when the cells are operated in reverse bias, which can happen when a panel is partially shaded. These defects enable break down at lower voltages and shunt current, resulting in local ohmic heating, followed by the melting and propagation of burning metal shunts across the surface. This effect produces catastrophic degradation of the solar module. It is therefore critical to mitigate such defects for widespread implementation of perovskite PV. We demonstrate optical profilometry and PL-mapping as high-throughput detection methods for these defects as well as improved particle control and solution wetting as possible preventative measures. Elimination or passivation of these defects will prevent localized loss and enhance long-term reliability of perovskite solar cells, paving the way for successful commercial perovskite solar cell production.