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%.