In situ TEM observation of the heat–induced degradation of single– and triple–cation planar perovskite solar cells

Benefiting from the structural stability of the formamidium and Cs cation by tuning the methylammonium while partially replacing iodine with Br, the triple–cation perovskite solar cells (PSCs) have shown improved power conversion efficiency (PCE) and thermal stability, compared to single–cation PSCs. However, the thermal stability of the triple–cation perovskite is not fully understood, which limits the further development of device performance and stability of the PSCs. This paper reports the thermal–induced effects on the photovoltaic performances for Cs_x(FA_yMA_(1–y))_(1–x)Pb(I_zBr_(1–z))₃ (Cs/FA/MA)–PSCs as compared to MAPbI₃ (MA)–PSCs, and further investigate the degradation phenomena directly by using real–time in situ transmission electron microscopy (TEM). This in situ TEM observation shows the different degradation phenomena in the MA– and Cs/FA/MA–PSCs, and confirms the correlation of its effects on the device performance. Furthermore, analyses of the elements and crystal structures reveal distinct differences in the decomposed products between MA– and Cs/FA/MA–PSC, and these relevant thermal–induced degradation mechanisms are also discussed in detail. This study will potentially help understanding of the thermal degradation of PSCs, and can be used for the future development of high–performance and stable PSCs.