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

Doping of bis(trifluoromethane)sulfonimide (LiTFSI) and 4-tert-butylpyridine (TBP) in 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) is currently considered irreplaceable for fabricating 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. Herein, we report a strategy of doping with an organic molten salt, cyclohexylamine trifluoroacetic acid (CYTFA), to stabilize doped Spiro-OMeTAD for high-performance PSCs. We found that Li⁺ diffusion and TBP volatilization were effectively suppressed through strong interactions of dissociated CY⁺ and TFA⁻ acting on TBP and Li⁺. Moreover, CYTFA-doped Spiro-OMeTAD exhibits an order of magnitude increase in hole mobility and matched energy levels with perovskites. As a result, a solar cell with a power conversion efficiency of 25.80% was achieved, 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.