Solution-processed tungsten oxide with Ta5+ doping enhances the hole selective transport for crystalline silicon solar cells

Transition metal oxides (TMOs) with high work function (WF) are typically used for hole selective layers for crystalline silicon (c-Si) heterojunction solar cells. Among them, tungsten oxide (WO3−x) exhibits a wide bandgap and high thermal stability; however, the photoelectrical conversion efficiency (PCE) of solar cells based on WO3−x lags behind its counterparts (MoO3−x and V2O5−x). Moreover, the films are usually deposited by vacuum methods, which makes it difficult to modulate the WF or oxygen vacancies (VO) to improve the hole selectivity. In this work, a low cost, solution-processed WO3−x film with ion doping is deposited as a hole selective layer for p-Si heterojunction solar cells. Experimental results and density functional theory calculations demonstrate that Ta5+ tends to replace W5+ and strengthens the W–O binding, which facilitate the reduction of VO and increase the WF of WO3−x films, resulting in the increase of band bending of the c-Si surface and the improvement of hole selectivity. The solar cells with a structure of p-Si/Ta5+:WO3−x/Ag have achieved a remarkable PCE of 18.67%, which is the highest efficiency obtained for WO3−x/c-Si heterojunctions based on solution processing up to now. This work also paves the way for extending the doping strategy of WO3−x to other optoelectronic device applications.