Resolving Hysteresis in Perovskite Solar Cells with Rapid Flame-Processed Cobalt-Doped TiO2

To further increase the open-circuit voltage (V-oc) of perovskite solar cells (PSCs), many efforts have been devoted to doping the TiO2 electron transport/selective layers by using metal dopants with higher electronegativity than Ti. However, those dopants can introduce undesired charge traps that hinder charge transport through TiO2, so the improvement in the V-oc is often accompanied by an undesired photocurrent density-voltage (J-V) hysteresis problem. Herein, it is demonstrated that the use of a rapid flame doping process (40 s) to introduce cobalt dopant into TiO2 not only solves the J-V hysteresis problem but also increases the V-oc and power conversion efficiency of both mesoscopic and planar PSCs. The reasons for the simultaneous improvements are two fold. First, the flame-doped Co-TiO2 film forms Co-O-v (cobalt dopant-oxygen vacancy) pairs and hence reduces the number density of Ti3+ trap states. Second, Co doping upshifts the band structure of TiO2, facilitating efficient charge extraction. As a result, for planar PSCs, the flame doping of Co increases the efficiency from 17.1% to 18.0% while reducing the hysteresis from 16.0% to 1.7%. Similarly, for mesoscopic PSCs, the flame doping of Co increases the efficiency from 18.5% to 20.0% while reducing the hysteresis from 7.0% to 0.1%.