Stabilizing the Efficiency Beyond 20% with a Mixed Cation Perovskite Solar Cell Fabricated in Ambient Air under Controlled Humidity

Perovskite solar cells have evolved to have compatible high efficiency and stability by employing mixed cation/halide type perovskite crystals as pinhole-free large grain absorbers. The cesium (Cs)-formamidium-methylammonium triple cation-based perovskite device fabricated in a glove box enables reproducible high-voltage performance. This study explores the method to reproduce stable and high power conversion efficiency (PCE) of a triple cation perovskite prepared using a one-step solution deposition and low-temperature annealing fully conducted in controlled ambient humidity conditions. Optimizing the perovskite grain size by Cs concentration and solution processes, a route is created to obtain highly uniform, pinhole-free large grain perovskite films that work with reproducible PCE up to 20.8% and high preservation stability without cell encapsulation for more than 18 weeks. This study further investigates the light intensity characteristics of open-circuit voltage (V-oc) of small (5 x 5 mm(2), PCE > 20%) and large (10 x 10 mm(2), PCE of 18%) devices. Intensity dependence of V-oc shows an ideality factor in the range of 1.7-1.9 for both devices, implying that the triple cation perovskite involves trap-assisted recombination loss at the hetero junction interfaces that influences V-oc. Despite relatively high ideality factor, perovskite device is capable of supplying high power conversion efficiency under low light intensity (0.01 Sun) whereas maintaining V-oc over 0.9 V.