In Situ Multimodal Analysis of Metal Halide Perovskite Film Formation and Degradation

The recent developments in the metal halide perovskite solar cells (PSC) were able to achieve power conversion efficiencies comparable to silicon solar cells. This astounding performance was achieved due to advances in perovskite crystallisation, engineering both electron and hole transport layers as well as interface engineering to minimise losses. Despite achieving an impressive performance, PSC still face significant challenges for outdoor implementation due to limited reliability. Although there are many external factors at play such as humidity, temperature and even the prolonged sun exposure, the interfaces between the halide perovskite absorber layer and adjacent charge transport films play a big part in the inherent stability of the cell components.<br/>Here, we studied the effect of external stressors such as the exposure to air and humidity on double cation, Cs0.3FA0.7Pb(Br0.2I0.8)3 and triple cation, Cs0.05(MA0.17FA0.83)0.95Pb(Br0.2I0.8)3 perovskites as well as perovskite crystallisation by in-situ Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) and photoluminescence (PL) spectroscopy measurements. We carried out GIWAXS measurements whilst depositing perovskite on different substrates to study perovskite crystallisation, where we were able to detect differences in perovskite crystallisation. Furthermore, to advance our understanding of degradation, we carried out real-time GIWAXS and concomitant PL measurements as the perovskite degrades under different conditions where it was possible to detect different perovskite crystal structures, present heterogeneously, diminish with time as the PbI2 concentration increased at the interface. We observed that different conditions triggered unique defect routes with different reaction kinetics. In air, we observed phase instabilities such as the breakdown of both double and triple cation perovskites into non-perovskite organic iodide species (CH3NH2I) as well as the delta phase of cesium iodide CsI. These phase instabilities were not present under 100% humidity without oxygen. Whilst the phase instabilities were present in air for both double cation and triple cation, we observed no intrinsic phase instabilities when 2D interlayer (4-FPEAI) was deposited on top of the perovskite. For 100% humidity, we observed partially reversible electron transfer from I- to Pb2+ leading to irreversible Pb0 formation for double and triple cation perovskite cells.