Improving the stability and decreasing the trap state density of mixed-cation perovskite solar cells through compositional engineering

In order to become a commercially viable technology, perovskite solar cells will require both high efficiencies and long-term stability. Many of the highest efficiency devices reported to date have used complex perovskite formulations synthesized from PbI2, formamidinium iodide, PbBr2, and CH3NH3Br; the addition of a small amount of CsI to this composition can further improve the device efficiency. Unfortunately, previous work has also shown that the methylammonium cation can not only make the perovskite more hygroscopic, it also renders it susceptible to photochemical decomposition; additionally, mixed-halide perovskite compositions have been shown to suffer from a phenomenon known as halide segregation. Here we show that both PbBr2 and CH3NH3Br can be omitted from Cs-containing perovskite devices with little change in device efficiency. We also show that replacing these components with PbI2 and guanidinium iodide leads to a decrease in the trap state density and an increase in the open-circuit voltage. Importantly, the mixed-cation perovskite shows improved moisture resistance and photochemical stability in both thin films and devices.