Abstract 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 CH 3 NH 3 PbI 3 (MAPbI 3 ) 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 (MAPbI 3 , 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 MAPbI 3 and spiro-OMeTAD in a solar cell. It was confirmed that, at high temperatures (85 °C), I − and CH 3 NH 3 + (MA + ) diffused into the spiro-OMeTAD layer in the form of CH 3 NH 3 I (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.