Strain in Halide Perovskites: Characterisation, Crystallography, Consequences

Abstract

Optoelectronic devices such as solar cells, LEDs, and X-ray detectors making use of halide perovskites as the light absorbing or emitting material have experienced a dramatic increase in efficiency in recent years. As a result, halide perovskite-based solar cells are now starting to make their way out of laboratories and into the commercial market. However, there is still room for efficiency improvements, and in many of the devices that do boast high efficiencies, long-term stability remains a thorny issue for the field. Strain is known to affect the performance of a range of semiconductors, and halide perovskites are no exception. However, the precise links between strain and optoelectronic properties in these materials appeared obscure, with a number of studies in the literature presenting conflicting results. In this thesis, after setting out my motivation in Chapter 1, I make use of a range of synchrotron-based X-ray diffraction techniques to characterise strain in halide perovskites. As introduced in Chapters 2 & 3, this crystallographic approach is combined with photoluminescence (PL) measurements as a probe of optoelectronic performance with the view to understand the consequences of strain in these materials. I primarily employ coherent diffraction techniques, for example using Bragg coherent diffraction imaging in Chapter 4 to image internal strain distributions in halide perovskite devices with nanoscale resolution. In so doing, I find intra-grain and grain-to-grain strain distributions to be remarkably heterogeneous, and I identify dislocations in full solar cell device stacks. Chapter 5 then demonstrates that in situ illumination induces striking dislocation migration. Extending my coherent diffraction imaging approach to Bragg ptychography in Chapter 6 allows me to spatially correlate increased tensile strain with red-shifted PL spectra and shorter PL lifetimes in a single crystal of CsPbBr3 purposely isolated from other possible causes of optoelectronic heterogeneity. Finally, pivoting to diffuse scattering measurements, in Chapter 7 I uncover dynamic domains of locally tilted lead-halide octahedra in CsPbBrxCl3−x using a novel big-box combined Bragg scattering and pair distribution function refinement procedure. Off the back of the results presented in the following pages, I have tried to indicate where the crystallographic insights can inform device fabrication methods, with the aim of realising more efficient and longer-lasting halide perovskite-based devices. However, the techniques and analysis described below can be applied to many functional materials systems and devices far beyond the realms of halide perovskites.