Greene Dissertation Award Winner: Characterising nanostructure and understanding its influence on phase stability in halide perovskites
ORAL · Invited
Abstract
Halide perovskites possess exceptional characteristics for the next generation of low-cost optoelectronic applications. Photovoltaic (PV) devices fabricated from perovskite absorbers already exhibit certified power conversion efficiencies exceeding 25.5% in single-junction devices and 32% in tandem configurations. However a number of challenges remain before perovskites can be widely commercialized. For instance, in spite of their high performance, perovskites still exhibit a sizeable density of deep sub-gap non-radiative trap states1,2 that lead to local variations in photoluminescence and ultimately limit device performance. Understanding the origin and nature of these traps is critical to further reduce losses and yield devices operating at close to their theoretical limits. In this talk, we use correlated nanoscale characterisation methodologies to explore the nanoscopic landscape of beam-sensitive halide perovskite materials. Utilising Scanning Electron Diffraction, in particular, we show that deep trap states in perovskite materials are correlated with the presence of nm scale phase impurities1 that include hexagonal wide bandgap perovskite polytypes and PbI2. We then show that these same phase impurities that are responsible for performance losses, seed material degradation in the perovskite absorber layer under operational conditions6. Finally, we show that state-of-the-art alloyed formamadinium lead iodide-like perovskites are non-cubic on average, exhibiting slight octahedral tilting at room temperature. This octahedral tilting, induced by cationic additives, frustrates the transition between the photoactive and hexagonal perovskite phases7 thus providing an intrinsic barrier to phase impurity formation. However, local regions of a perovskite film without octahedral tilting can rapidly transition to the nm sized hexagonal phase impurities that cause deep traps and seed material degradation.
–
Publication: 1. T. A. S. Doherty, A. J. Winchester et al., Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites. Nature. 580, 360–366 (2020).<br>2. D. W. de Quilettes et al., Impact of microstructure on local carrier lifetime in perovskite solar cells. Science. 348, 683–686 (2015).<br>3. Saliba, M. et al. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science 9, 1989–1997 (2016).<br>4. Lu, H. et al. Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells. Science 370, (2020).<br>5. Jeong, J. et al. Pseudo-halide anion engineering for a-FAPbI 3 perovskite solar cells. Nature 592, 381–385 (2021).<br>6. Macpherson, S. et al. Local Nanoscale Phase Impurities are Degradation Sites in Halide Perovskites. Nature 1–3 (2022) doi:10.1038/s41586-022-04872-1.<br>7. Doherty Tiarnan A. S. et al. Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases. Science 374, 1598–1605 (2021).<br>
