Thermal stability analysis of WOx and MoOx as hole-selective contacts for Si solar cells using in situ XPS

Transition metal oxides with a high work function can be used as hole-selective materials for c-Si solar cells. WOx and MoOx can have work function as high as >6 eV which has attracted increasing interests in the c-Si PV community. Apart from the optical and electrical properties of such materials, their thermal stability is of crucial importance for the application of these contacts in high-efficiency silicon solar cells. This work investigates the thermal stability of WOx and MoOx films with and without a-Si:H interface passivation layer using in situ X-ray photoelectron spectroscopy (XPS). The samples were annealed and measured inside the XPS chamber under high vacuum in the 300 K to ≥ 900 K temperature range. This semi-quantitative XPS analysis shows that the oxidisation state of tungsten reduces from +6 to +5 for annealing temperatures > 600 K, while molybdenum is significantly less stable and its oxidation state reduces for all investigated annealing temperatures. When using a-Si:H interface passivation layer it is found that hydrogen diffusing from the a-Si:H enhances the degradation of the metal's oxidation state at high temperature. Besides, the passivation level of the film stack a-Si:H/WOx showed a maximum lifetime of 3.5 ms (at 1×1015 cm-3) after annealing at 500 K, and for the case of a-Si:H/MoOx annealing resulted in a decrease of the minority carrier lifetime.