Structural and thermal stability of metal halide perovskite solar cells

Abstract

Metal halide perovskite solar cells were crowned the top-performing emerging photovoltaic technology with a certified maximum efficiency of 25.2 % in 2021. Their facile low-cost processability and wide optoelectronic properties make them both a powerful contender against traditional crystalline silicon cells and the perfect ally in tandem configurations. However, their rapid degradation upon operational conditions, including long-term exposure to temperature oscillations, represents a major obstacle to their commercialization. Hence, finding perovskite compositions that are structurally stable within the operational temperature range is crucial for the development of market-viable photovoltaic panels. This thesis firstly explores the validity of the most popular method used in the literature to assess structural stability in perovskites: integral breadth (IB) XRD microstrain analysis. It unveils how IB techniques are unsuitable for the extraction of microstrain in perovskite thin films since the results breach the Wilson-Stokes approximation constraint: parallel hkl planes do not report equivalent values of microstrain as expected. This phenomenon persists even when using instrumental error minimized scan conditions, suggesting the presence of a microstrain gradient across the order-of-diffraction. Additionally, B-site mixing is explored as a delayer of the thermal decomposition of triple cation mixed halide perovskites, creating a symbiosis between high entropy and structural stability. Partially replacing Pb2+ with Sb3+ in multi-mixed systems improves their heat endurance without compromising their structural stability as proven by XRD and an in-house designed in-situ colourimetry test kit. The complex interplay between the defect-prone optoelectronic properties and the improved structural strength of these heterovalent perovskites is also investigated in solar cells using different charge transporting materials. Overall, this thesis highlights the importance the structural stability has in the thermal endurance of perovskites, suggesting the distribution of microstrain both between crystallographic orientations and within their order-of-diffraction as a playing factor in the pursuit of long-lasting operational devices.