Oxoammonium cation of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) was used as an oxidizing dopant of triaryl amines to efficiently and almost quantitatively generate radical cations of the amines or a hole carrier. The doped-triaryl amines yielded an amorphous and homogeneous layer without any residual oxidant or neutral TEMPO molecule through its sublimination or warming the layer. The TEMPO cation-doped spiro-OMeTAD [tetrakis(dimethoxyphenylamine)spirobifluorene] produced a high hole mobility of 2 × 10-4 cm2/Vs. The perovskite solar cell fabricated with the TEMPO cation-doped or residual dopant-free spiro-OMeTAD as the hole-transporting layer displayed a photo-conversion efficiency of 20.1% with durability.
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