Origin of thermal induced variation in performance and negative reactance of inorganic CsPbI2Br perovskite solar cells

Abstract

Commercial viability of perovskite solar cells (PSCs) requires stable performance under daily temperature fluctuations. While previous research has extensively examined organic-inorganic hybrid PSCs at temperatures below 85°C, understanding charge carrier behavior in PSCs at higher temperatures remains insufficiently characterized, a critical knowledge gap for deploying PSCs in extreme environments such as satellites. This study investigates charge-carrier and ion-transport mechanisms, and associated degradation processes, in all-inorganic mixed-halide CsPbI₂Br PSCs with an inverted PIN architecture subjected to thermal cycling between 25°C and 105°C using in-situ photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and in-situ impedance spectroscopy (IS). Thermal cycling induces temperature-dependent halide migration and lattice expansion that transiently improve carrier transport and device efficiency but also lead to structural degradation and the formation of non-photoactive secondary phases, defining a narrow window of stable operation. These ionic and structural changes account for inverted current–voltage hysteresis, progressive performance loss, and negative reactance in the impedance response under thermal stress. In-situ optoelectronic and structural measurements link bandgap shifts, lattice parameter changes, and surface halide redistribution to these behaviors. A four-component extended Matryoshka nested-ladder equivalent circuit separates overlapping electro-ionic processes, relating thermally activated iodide and bromide migration and their activation energies to transport characteristics and impedance features. The results establish a mechanistic connection between ionic dynamics, lattice adaptation, and device performance, guiding halide-composition and lattice-stabilization strategies to extend the operational window of inorganic PSCs for high-temperature and extreme-environment applications.