Structural, Thermodynamic and Photo-Physical Properties of Cesium Lead Halide Perovskites
Authors:   , Dastidar Subham , Fafarman Aaron T. 
Published: July 2021

Recently, the emergence of metal halide perovskite solar cells as a contender for next-generation photovoltaics constitutes a paradigm shift in the field of solution-processed semiconductors. Increased understanding of the fundamental optoelectronic, and structural properties of these materials and advances in thin film processing over the last few years have led to a meteoric rise in power conversion efficiencies, exceeding 23%, now rivaling much more mature commercial technologies. While the vast majority of halide perovskite studies involve the prototypical organic-inorganic hybrid compound methylammonium lead iodide (MAPbI3), the volatile and hygroscopic organic cation is detrimental to its chemical stability. Thus, finding an alternative material system, which can mitigate these issues without diminishing the desirable optoelectronic properties, is necessary. This thesis focuses on understanding the thermodynamic and photo-physical properties of a promising all-inorganic alternative, perovskite-phase cesium lead iodide (CsPbI3). The perovskite ('black') phase of CsPbI3 spontaneously forms at elevated temperature (>320 °C). At room temperature (ambient condition) the thermodynamically favored phase is a non-perovskite structure ('yellow'), not useful for photovoltaic application. To develop a fundamental understanding of phase transitions in CsPbI3, and quantify the thermodynamic landscape of phase change, herein spectroscopy and calorimetry techniques have been employed. A reversible enthalpy difference of 14.2 (± 0.5) kJ/mol between the two phases is observed which is offset only by the similarly large entropic favorability of the desired phase. It has also been shown that by rapid cooling (quenching) the black phase of CsPbI3 can be kinetically trapped in a metastable state. It is demonstrated herein that the mechanistic role of atmospheric moisture in the phase transition is catalytic rather than enthalpic. These fundamental physicochemical insights are implemented to design stable derivatives. To enhance the phase stability of the black-CsPbI3 towards atmospheric moisture a chloride doping strategy has been devised and implemented herein. The conventional synthetic routes do not allow a significant amount of chloride doping due to limited miscibility of chloride in a CsPbI3 host lattice. A combined solution- and solid-state method has been adopted. Colloidal nanocrystals of pure CsPbCl3 and CsPbI3 are co-deposited and the resulting nanocrystal solid is subsequently fused into a polycrystalline thin film of CsPbI3-xClx by chemically-induced, room-temperature sintering. This approach ensures nanometer-scale mixing even at compositions that potentially exceed the bulk miscibility of the two phases. From theoretical calculations an atomistic insight into the formation of the mixed crystal phase has been obtained. In order to accept CsPbI3 as an alternative to MAPbI3, the optoelectronic parameters need to be comparable. Transient Absorption Spectroscopy (TA) and Time Resolved Terahertz Spectroscopy have been employed to reveal the recombination kinetics of CsPbI3. By developing a 1D finite difference diffusion-recombination model the recombination rate constants are estimated and the role of carrier diffusion deconvoluted. A nearly identical bimolecular rate constant, a fundamental property of the semiconducting materials, between CsPbI3 the most highly optimized MAPbI3 films in the literature has been observed. Such resemblance without the presence of molecular dipole infers that the organic cation does not provide any fundamental advantage in the slow radiative recombination kinetics of the hybrid perovskites.