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We propose a spectrum-dependent mechanism for the oxidation of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), which is commonly used in perovskite solar cells as the hole transport layer. The perovskite layer plays different roles in the Spiro-OMeTAD oxidization for various spectral ranges. The effect of oxidized Spiro-OMeTAD on the solar cell performance was observed and characterized. With the initial long-wavelength illumination (>450 nm), the charge recombination at the TiO2/Spiro-OMeTAD interface was increased due to the higher amount of the oxidized Spiro-OMeTAD. On the other hand, the increased conductivity of the Spiro-OMeTAD layer and enhanced charge transfer at the Au/Spiro-OMeTAD interface facilitated the solar cell performance.
Wang et al. (Wed,) studied this question.