Organic–inorganic halide perovskites are promising as the light absorber of solar cells because of their efficient solar power conversion. An issue frequently occurring in perovskite solar cells (PSCs) with a hole transport layer of N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) is a quick performance degradation at high temperature. Herein, it is discovered that postdoping of the spiro-OMeTAD layer by iodine released from the perovskite layer is one possible mechanism for the high-temperature PSC degradation. Iodine doping leads to the highest occupied molecular orbital level of the spiro-OMeTAD layer becoming deeper and, therefore, induces the formation of an energy barrier for hole extraction from the perovskite layer. It is demonstrated that it is possible to suppress the high-temperature degradation by using an iodine-blocking layer or an iodine-free perovskite in PSCs. These findings will guide the way for the realization of thermally stable perovskite optoelectronic devices in the future.
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