Deconstruction-assisted perovskite formation for sequential solution processing of Cs_0.15(MA_0.7FA_0.3)_0.85PbI₃ solar cells

Organic-inorganic hybrid lead halide perovskite has shown to be one of the best light-harvesting materials for solar cell in the last decade. However, there still is needed a deeper understanding of phase and film formation for better control of device fabrication. In this work, we visualise the formation mechanism of Cs_0.15(MA_0.7FA_0.3)_0.85PbI₃ perovskite by the sequential spin-coating method and how changes in the dispensing timing and substrate motion affect the formation process and properties of the final film quality. In particular, this is the first time that we are able to visualise and identify the different stages of the film formation: i) "initial formation"; ii) "perovskite deconstruction" and iii) "perovskite re-crystallisation". This particularly applies to films that are sequentially spin-coated and involve the use of dimethyl sulfoxide (DMSO) as the "deconstruction" is caused by the formation of intermediate-DMSO-complex. These findings are validated by FTIR and XRD measurements. Comparison among processes also suggests that motion causes an earlier onset of deconstruction, which will lead to a slower re-crystallisation resulting in better quality perovskite film with less non-perovskite phase. This can be achieved by motion dispensing and dynamic processing (where there is no stoppage between the two sequential steps). Reasons for the earlier onset of deconstruction are the higher kinetic energy supplied by the dynamic process. This work has provided more insights into the complex stages involved in perovskite conversion specific to sequential processing. The knowledge will aid future process optimisation for better device fabrication.