Tiarnan Doherty¹,Stuart Macpherson¹,Satyawan Nagane¹,Dominik Kubicki²,Affan Iqbal¹,Elizabeth Tennyson¹,Francesco Simone Ruggeri³,Sofiia Kosar⁴,Andrew Winchester⁴,Young-Kwang Jung⁵,Yu-Hsien Chiang¹,Kyle Frohna¹,Miguel Anaya¹,Keshav Dani⁴,Aron Walsh⁶,Paul Midgley¹,Samuel Stranks¹

University of Cambridge¹,University of Warwick²,Wageningen University & Research³,Okinawa Institute of Science and Technology⁴,Yonsei University⁵,Imperial College London⁶

Abstract

University of Cambridge¹,University of Warwick²,Wageningen University & Research³,Okinawa Institute of Science and Technology⁴,Yonsei University⁵,Imperial College London⁶

Halide perovskite materials exhibit promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25.5 % in single-junction devices and 29.5% in tandem devices. Formamadinium (FA) lead iodide (FAPbI3) and FA-rich perovskites are preferred for photovoltaic applications, but their widespread adoption is hindered by the rapid degradation of the desirable corner-sharing cubic (3C) phase to an undesirable wide bandgap, face-sharing, hexagonal (2H) phase under ambient conditions. Alloying FA with small amounts of Cs+ and methylammonium (MA) on the A site cation of the ABX3 perovskite structure has proven a promising strategy for stabilizing photoactive perovskite phases. For example, photovoltaic devices fabricated with compositions such as Cs0.05FA0.78MA0.17Pb(I0.83Br0.17)3 (triple cation) perovskites have achieved high device efficiencies with greatly enhanced reproducibility and ambient stability 1. Recently, there has also been renewed interest in methods to stabilize pure FAPbI3 through strategies such as the incorporation of MA via treatment with methylammonium thiocyanate vapour 2, addition of formamidinium formate 3 or methylammonium formate 4. The mechanism of improved stability obtained from these approaches is generally explained as either originating from a tuning of the Goldschmidt tolerance factor towards the perfect cubic perovskite structure via cation mixing in the case of Triple Cation perovskites 1,5, by templating growth of the corner-sharing cubic structure in stabilized-FAPbI3perovskites2,4, or by reducing intrinsic defect density 3. The key tenet in all these explanations is that the final photoactive perovskite material, regardless of stabilization approach, is a cubic perovskite structure and that this is the structure that should be pursued for optimal stability and performance. Here, we reveal that rather than being cubic, FA-rich perovskites exhibit a slight degree of octahedral tilting that frustrates the transition from the corner sharing photoactive phase to hexagonal photoinactive phases6. Local regions of stabilised FA-rich perovskite films that do not exhibit octahedral tilting, transition to nanoscale hexagonal phase impurities that are rich in performance limiting defect clusters7. Finally, we show that by modulating the magnitude of octahedral tilting through compositional modification in FA-rich perovskites, the density of these sinister hexagonal phase impurities, and thus overall film photostability, can be tuned. Combined, our talk provides insight into the common origin of performance losses and photostability issues in FA-rich perovskite materials as well as a pathway to mitigate them8. 1. Saliba, M. et al. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science 9,(2016). 2. Lu, H. et al. Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells. Science 370, (2020). 3. Jeong, J. et al. Pseudo-halide anion engineering for α-FAPbI 3 perovskite solar cells. Nature 592, 381–385 (2021). 4. Hui, W. et al. Stabilizing black-phase formamidinium perovskite formation at room temperature and high humidity. Science 371, 1359–1364 (2021). 5. Kim, J. Y., Lee, J.-W., Jung, H. S., Shin, H. & Park, N.-G. High-Efficiency Perovskite Solar Cells. Chem. Rev.120, 7867–7918 (2020). 6. Doherty Tiarnan A. S. et al. Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases. Science 374, 1598–1605 (2021). 7. Doherty, T. A. S. et al. Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites. Nature 580, 360–366 (2020). 8. Macpherson, S. et al. Local Nanoscale Phase Impurities are Degradation Sites in Halide Perovskites. Nature1–3 (2022).