Radical doped hole transporting material for high-efficiency and thermostable perovskite solar cells

To date, the most efficient PSCs use lithium bis(trifluoromethanesulfonyl)imide/oxygen p-doped organic semiconductors as hole-transporting layers (HTL), which are often prone to unsatisfactory reproducibility and long-term instability. Herein, we report a new p-doping strategy for a wide range of small-molecular and polymeric organic semiconductors with free radicals that enable reproducible enhancement in the conductivity and tuning of the work function. We explored the doping mechanism and found that the free-radical initiator benzoyl peroxide promotes the generation of chlorine radicals from 1,1,2,2-tetrachloroethane, which steadily oxidizes organic semiconductors. With a fine tuning of the counter ions, we achieved an efficiency of 21.1% for small-area (0.16 cm(2)) PSCs and 16.8% for larger-area (10 cm(2)) modules. Moreover, the solar cells showed significantly enhanced stability against thermal and humidity stress, which we attribute to the use of pure HTLs devoid of doping by-products and other impurities.