Oxygen vacancy modulation of nanolayer TiO_x to improve hole-selective passivating contacts for crystalline silicon solar cells

Carrier-selective passivating contacts in crystalline silicon (c-Si) solar cells have expanded from doped silicon films to non-silicon wide-bandgap materials to reduce parasitic absorption and production costs. Titanium oxide (TiO_x) has emerged as one of the most promising materials and has achieved high performance in c-Si solar cells. In this work, TiO_x is explored as a passivation interlayer in hole-selective contacts rather than conventional electron-selective contacts. Theoretical calculations and experimental results demonstrate that negative charges and shallow states in TiO_x, derived from oxygen vacancies (V_O), enhance surface passivation and assist hole tunneling, respectively. As a strategy to modulate V_O, forming gas annealing is performed to further improve hole selectivity. By incorporating the TiO_x passivation interlayer into MoO_x-based c-Si solar cells, we achieve an improved efficiency and stability of the device, with the highest efficiency of 21.28%. This work advances the understanding of TiO_x as a promising material to enhance hole selectivity for c-Si solar cells.