Mechanism of an AZO-coated FTO film in improving the hydrogen plasma durability of transparent conducting oxide thin films for amorphous-silicon based tandem solar cells

An Al-doped ZnO (AZO)/F-doped SnO ₂ (FTO) film structure in an amorphous silicon solar cell (a-Si:H) was studied under exposure to hydrogen (H) plasma treatment via a simple spray pyrolysis. A radio-frequency magnetron sputtering method was used to deposit the FTO film first and the subsequent AZO overlayer on top of the FTO. The results of this approach suggest that the AZO film acts as a protection layer for the underlying FTO film and thus provides an excellent interface that protects the film from the direct bombardment with H ⁺ ions and radicals. Even under exposure to H-plasma, the optical (UV-Vis) and electrical properties (Hall measurement) of the AZO/FTO double layer film show improvement in plasma stability in comparison with a single FTO film. Without the AZO overlayer, the reduction of SnO ₂ to metallic Sn and sub-oxidized SnO can occur in the FTO film. However, after a post-annealing treatment at 400 °C, the degradation of the FTO can be reversed by reoxidizing the Sn-O bonds. This interpretation of the H ⁺ ion diffusion and the Sn-O redox process is confirmed via XPS and SIMS analysis. This investigation provides an opportunity to study the H ⁺ ion diffusion mechanism in a metal oxide matrix in the presence of plasma and thermal effects. The study thus provides an alternative to the conventionally used FTO-based Si solar cells.