Relationship between ion migration and interfacial degradation of CH₃NH₃PbI₃ perovskite solar cells under thermal conditions

Organic-inorganic hybrid perovskite solar cells (PSCs) have been extensively studied because of their outstanding performance: a power conversion efficiency exceeding 22% has been achieved. The most commonly used PSCs consist of CH₃NH₃PbI₃ (MAPbI₃) with a hole-selective contact, such as 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spiro-bifluorene (spiro-OMeTAD), for collecting holes. From the perspective of long-term operation of solar cells, the cell performance and constituent layers (MAPbI₃, spiro-OMeTAD, etc.) may be influenced by external conditions like temperature, light, etc. Herein, we report the effects of temperature on spiro-OMeTAD and the interface between MAPbI₃ and spiro-OMeTAD in a solar cell. It was confirmed that, at high temperatures (85°C), I^- and CH₃NH₃⁺ (MA⁺) diffused into the spiro-OMeTAD layer in the form of CH₃NH₃I (MAI). The diffused I^- ions prevented oxidation of spiro-OMeTAD, thereby degrading the electrical properties of spiro-OMeTAD. Since ion diffusion can occur during outdoor operation, the structural design of PSCs must be considered to achieve long-term stability.