Transition metal oxides as hole-transporting materials in organic semiconductor and hybrid perovskite based solar cells

Organic polymer solar cells (OSCs) and organic-inorganic hybrid perovskite solar cells (PSCs) have achieved notable progress over the past several years. A central topic in these fields is exploring electronically efficient, stable and effective hole-transporting layer (HTL) materials. The goal is to enhance hole-collection ability, reduce charge recombination, increase built-in voltage, and hence improve the performance as well as the device stability. Transition metal oxides (TMOs) semiconductors such as NiO _x , CuO _x , CrO _x , MoO _x , WO₃, and V₂O₅, have been widely used as HTLs in OSCs. These TMOs are naturally adopted into PSC as HTLs and shows their importance. There are similarities, and also differences in applying TMOs in these two types of main solution processed solar cells. This concise review is on the recent developments of transition metal oxide HTL in OSCs and PSCs. The paper starts from the discussion of the cation valence and electronic structure of the transition metal oxide materials, followed by analyzing the structure-property relationships of these HTLs, which we attempt to give a systematic introduction about the influences of their cation valence, electronic structure, work function and film property on device performance.