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

Abstract

Transition metal oxides (TMOs) with high work function (W_F) are typically used for hole selective layers for crystalline silicon (c-Si) heterojunction solar cells. Among them, tungsten oxide (WO_3−x) exhibits a wide bandgap and high thermal stability; however, the photoelectrical conversion efficiency (PCE) of solar cells based on WO_3−x lags behind its counterparts (MoO_3−x and V₂O_5−x). Moreover, the films are usually deposited by vacuum methods, which makes it difficult to modulate the W_F or oxygen vacancies (V_O) to improve the hole selectivity. In this work, a low cost, solution-processed WO_3−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 V_O and increase the W_F of WO_3−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⁺:WO_3−x/Ag have achieved a remarkable PCE of 18.67%, which is the highest efficiency obtained for WO_3−x/c-Si heterojunctions based on solution processing up to now. This work also paves the way for extending the doping strategy of WO_3−x to other optoelectronic device applications.