Characterising Nanostructure and Understanding its Influence on Phase Stability and Performance in Halide Perovskites
Halide perovskites possess exceptional characteristics for the next generation of low-cost optoelectronic applications. Photovoltaic (PV) devices fabricated from perovskite absorbers exhibit certified power conversion efficiencies exceeding 25.5% in single-junction devices and 29.5% in tandem configurations. Two of the fundamental challenges to address before perovskites can be widely commercialized relate to device performance and phase stability: 1) In spite of their high performance, perovskites still exhibit a sizeable density of deep sub-gap non-radiative trap states, which create local variations in photoluminescence that fundamentally limit device performance. Understanding the origin and nature of these traps is critical to eliminate performance losses and yield devices operating close to their theoretical limits. 2) Formamidinium (FA)-based perovskites are ideal candidates for commercial applications due to high thermal stability and bandgap of ~1.48 eV. However, FAPbI3 is challenging both to fabricate and stabilize as the desirable cubic phase is only stable at temperatures greater than 150° C and at room temperature transitions to wide bandgap hexagonal polytypes, which are not useful for PV. Alloying FA with Cs+ and/or MA on the A site of the ABX3 perovskite structure has proven a promising strategy for stabilizing FAPbI3-like cubic structures at room temperature but comes with an unwanted shift to higher bandgap and compromised thermal stability. The mechanism of stability afforded by alloyed approaches are unclear but elucidating them will facilitate the fabrication of pure-FAPbI3 perovskites free of cationic additives. Answers to both challenges necessitate nanoscopic insight. This thesis develops methodologies to investigate local, correlated, structural and photophysical properties in beam-sensitive halide perovskite materials. Utilising Scanning Electron Diffraction, I first show that deep trap states in perovskite materials are strongly correlated with the presence of phase impurities on nm length scales. I then show that these very phase impurities, which are hexagonal wide-bandgap perovskite polytype structures or PbI2, seed material degradation under operational conditions. Finally, I show that alloyed FAPbI3-like perovskites, contrary to popular belief, are non-cubic, exhibiting slight octahedral tilting at room temperature. This octahedral tilting, induced by cationic additives, frustrates the transition between the cubic photoactive and hexagonal wide-bandgap phases. Local regions of a perovskite film without a tilted structure rapidly transition to the nm sized hexagonal phase impurities that cause deep traps and seed material degradation.