Stabilizing doped Spiro-OMeTAD by organic molten salt for efficient and stable perovskite solar cells

Bis(trifluoromethane)sulfonimide (LiTFSI) and 4-tert-butylpyridine (TBP) have been currently suggested to be irreplaceable in classic doped 2,2′,7,7′-tetrakis (N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) for high-performance perovskite solar cells (PSCs). However, the stability of Spiro-OMeTAD was demonstrated to be seriously limited by the Li+ diffusion of LiTFSI and the volatilization of TBP. Here, we report an organic molten salt, Cyclohexylamine Trifluoroacetic acid (CYTFA), doping strategy to stabilize doped Spiro-OMeTAD for high-performance PSCs. We found the Li+ diffusion and the TBP volatilization were effectively suppressed through strong interaction by the dissociated CY+ and TFA- acting on TBP and Li+. Moreover, the CYTFA doped Spiro-OMeTAD exhibits an order of magnitude increase in hole mobility and matched energy levels with perovskite. As a result, a solar cell with a power-conversion efficiency of 25.80% was achieved with maintaining 96% and 80% of the initial efficiency for 500 hours at 55°C and 55% humidity and for 470 hours at the maximum power point, respectively.