In situ x-ray photoelectron emission analysis of the thermal stability of atomic layer deposited WO_x as hole-selective contacts for Si solar cells

WO_x is one of the most promising high work function materials to be used as hole-selective materials for c-Si solar cells. Apart from the optical and electrical properties of such materials, their thermal stability is of crucial importance for the potential application of these contacts. The thermal stability of plasma-enhanced atomic layer deposited WO_x is investigated with and without an a-Si:H interface passivation layer. Time-of-flight secondary ion mass spectroscopy reveals that the as-deposited WO_x films contain H resulting from the W precursor. In situ x-ray photoelectron spectroscopy under high vacuum in the 300 to 900 K temperature range shows that tungsten starts degrading from W⁺⁶ to W⁺⁵ for temperatures >600 K. The work function is found to be stable up to temperatures of 600 K. Subsequently, hydrogen diffusion from a-Si:H decreases the work function of WO_x and enhances the degradation of tungsten's oxidation state. Fourier transform infrared spectroscopy confirms the reduction in the hydrogen content in the thin film stack after annealing at 600 K. Besides, the passivation level of the film stack a-Si:H/WO_x showed a maximum lifetime of 3.5 ms (at 1 × 15 cm^-3) after annealing at 500 K. The results are of key importance for the integration of these novel contacts in high-efficiency silicon solar cells.