Comparative Intrinsic Thermal and Photochemical Stability of Sn(II) Complex Halides as Next-Generation Materials for Lead-Free Perovskite Solar Cells

Here, we present a systematic study of the thermal and photochemical degradation pathways for a series of complex tin-based halides ASnX3 (X = I, Br) with organic (CH3NH3+, H2NCHNH2+) and inorganic (Cs+) univalent A-site cations. Thin films of tin-based perovskites were exposed to continuous light soaking and/or thermal annealing in the dark under an inert atmosphere, which simulate pragmatic anoxic operation conditions of solar cells with the absorber layer isolated from the (re)action of oxygen and moisture by appropriate encapsulation. Using a set of complementary techniques such as optical spectroscopy, atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectra, we have elucidated that hybrid tin halide perovskites undergo rapid thermal and light-induced degradation with the complete elimination of organic cations and the formation of some volatile decomposition products and Sn(IV) halide species. On the contrary, all-inorganic compositions comprising CsSnBr3 and, particularly, CsSnI3 showed a much superior thermal and photochemical stability with respect to both light and elevated temperatures. Unfortunately, all investigated complex tin halides suffer from heat- and light-induced Sn(II) disproportionation with the formation of Sn(IV) species and, presumably, metallic Sn0. This facile disproportionation and chemical degradation pathway reduces dramatically the intrinsic stability of Sn(II) complex halides and limits their potential for practical applications. While this problem can be addressed using additional stabilizing additives and crystal-lattice-engineering approaches, the analysis of the comprehensive sets of our results solidifies further rational design approaches for the development of lead-free absorbers for inorganic perovskite-based solar cells with enhanced stability for efficient and durable photovoltaic systems.