Impact of processing atmosphere on nanoscale properties of highly efficient Cs_0.05MA_0.05FA_0.9PbI₃ perovskite solar cells

The fabrication process of triple-cation-halide organic inorganic perovskites must be tightly controlled to make high-efficiency solar cells. After precursor deposition, the amount of oxygen and moisture during the annealing process is important but not always well-monitored and understood. In this study, Cs_0.05MA_0.05FA_0.9PbI₃ perovskite films were annealed in different environments, namely N₂, O₂ and air, to systematically explore the relationship between the evolution of PbI₂, the grain boundary band bending and the optoelectronic properties. We find higher amounts of PbI₂ after air annealing, accompanied by an increased number of grain boundaries that show downward band bending. Photoluminescence measurements showed that absorbers annealed in the absence of air or O₂ (<i>i.e.</i> N₂ environment) exhibit the best optoelectronic properties, which however did not translate to the highest <i>V</i>_OC of the devices. Drift-diffusion simulations show that the interface between the perovskite and the Spiro-OMeTAD is very sensitive to the defect density. Consequently, the higher amount of PbI₂ is likely to passivate some of the interface defects, which means better translation of the opto-electronic absorber quality into open-circuit voltage. Although this strategy was adequate for the perovskite/Spiro-OMeTAD solar cell architecture that was used in this study, our results show that an even better way would be to grow perovskites without intentional incorporation of air or oxygen, which reduces PbI₂ and grain boundary band bending, allowing higher quasi Fermi-level splitting. This layer would need to be combined with an optimized hole extraction layer with improved band alignment.