Anion exchange strategies in radical doped hole transporting material for high-efficiency perovskite solar cells

Perovskite solar cells (PSCs) have received significant attention due to their superior photovoltaic conversion capability. To date, most efficient PSCs use p-doped organic semiconductors as hole-transporting layers (HTL). Many evidences show that the generally used p-doping system, i.e. lithium bis(trifluoromethane)sulfonimide and air exposure threaten the reproducibility and stability of PSCs. Recently, free radicals doped HTL have been used to fabricate efficient PSCs without using anying harmful dopants. Herein, we found that the low doping efficiency of radials is due to a weak Madelung potential. We developed a spontaneous anion exchange to stabilize the radical doped HTL via introducing an additional anion into the host-guest system. Conductivity and interfacial charge transfer are significantly enhanced by regulating the size of anions. We found that the conductivity and device performance is increased when the size of the anion increases. As a result, a champion efficiency of 20.7% under AM 1.5G 1-sun irradiation is achieved, which is comparable with the conventionally doped HTL. Moreover, the solar cells showed significantly enhanced stability against the humidity stress, which we attribute to the use of HTLs devoid of environmental sensitive additives.