Charge and Thermoelectric Transport in Metal Halide Perovskite Semiconductors

Abstract

Metal halide perovskites exhibit optoelectronic properties that hold significant potential for applications in photovoltaics, light-emitting diodes, photodetectors, X-ray detectors, and field-effect transistors (FETs). Rapid and substantial enhancements in the performance of perovskite-based optoelectronic devices have been achieved, that recent records indicate power conversion efficiency (PCE) of single-junction solar cells exceeding 26%, and FET mobility surpassing 50 cm2V-1s-1. These advancements in metal halide perovskites have been paralleled by efforts to understand the material's fundamental chemistry and physical properties from a bottom-up perspective, providing extensive theoretical optimization guidance. In this thesis, efforts to improve device performance have been paralleled by a focus to understand the underlying charge transport physics in these lead-based and tin-based perovskite systems. In Chapter 3, we identified a strong causal relationship between metal contact reaction and non-ideal FET characteristics in CsFAMAPbI3 perovskite FETs. Prolonged channel bias leads to enhanced n-type doping at the contact interface, resulting in increased device conductivity. This research also extended to investigating electroactive A-site molecules on charge transport in alkyl-diammonium 2D/3D perovskite FETs (Chapter 4.1), and pure n=1 2D perovskite diodes (Chapter 4.2). Carbazole alkylammonium molecules were found to accept transferred holes and increase electron-hole pair separation, consequently enhancing charge carrier mobility in the out-of-plane direction. Tin-based perovskites have gained significant attention since 2022, as they are found to be less susceptible to ion migration and exhibit p-type FET mobility exceeding 10 cm2V-1s-1. Here, we optimized tin perovskite compositions with mixed A-site cations and de-doping additives, achieving p-type FET mobility above 2 cm2V-1s-1 at room temperature. However, the reported high FET mobility exceeding 10 cm2V-1s-1 in these tin-based perovskites is sometimes controversial. In Chapter 4.3, we conducted a comprehensive analysis of the impact of device geometry on mobility extraction and demonstrated more reliable mobility results using gated four-point probe measurements on tin perovskite FETs. In Chapter 5, we conducted a comprehensive analysis of charge transport characterization in tin-based perovskite systems. We precisely measured the hole concentration and mobility of CsSnI3 films across varying grain sizes and Pb-substitution ratios. A transition in charge transport mechanisms was observed, shifting from predominantly band-like in larger grains to being dominated by grain boundary effects in smaller grains and films alloyed with Pb. Films with the largest grains exhibited a Hall effect mobility of 60 cm2V-1s-1 at room temperature and between 100-160 cm-2V-1s-1 at low temperatures, while maintaining a constant hole density of 1.6x10-19 cm-3. This transition is thoroughly explained using a mixed transport model that integrates band-like, grain boundary thermal activation, and grain boundary site semi-metallic or impurity scattering mechanisms. To assess their thermoelectric performance and probe their electronic structure, we measured the temperature-dependent Seebeck coefficient. From a Seebeck coefficient of 85 μV K-1 in CsSnI3 we derived a DOS effective mass of 0.25 to 0.3me, which remains consistent regardless of temperature and grain size and is slightly higher in samples with Pb-substitution. Finally, with thermal conductivity measurements, our best CsSnI3 film exhibited a high thermoelectric power factor of 118 μW m-1K-2 and a thermoelectric figure-of-merit of 0.11 at room temperature. We demonstrate that enhancing sample grain size and crystallinity is a viable strategy for optimizing the thermoelectric performance of tin perovskites.