A buffer-layer/a-SiO_x:H(p) window-layer optimization for thin film amorphous silicon based solar cells

Amorphous silicon based (a-Si:H-based) solar cells with a buffer-layer/boron doped hydrogenated amorphous silicon oxide (a-SiO _x:H(p)) window-layer were fabricated and investigated. In the first part, in order to reduce the Schottky barrier height at the fluorine doped tin oxide (FTO)/a-SiO_x:H(p) window-layer heterointerface, we have used buffer-layer/a-SiO_x:H(p) for the window-layer, in which boron doped hydrogenated amorphous silicon (a-Si:H(p)) or boron doped microcrystalline silicon (μc-Si:H(p)) is introduced as a buffer layer between the a-SiO _x:H(p) and FTO of the a-Si:H-based solar cells. The a-Si:H-based solar cell using a μc-Si:H(p) buffer-layer shows the highest efficiency compared to the optimized bufferless, and a-Si:H(p) buffer-layer in the a-Si:H-based solar cells. This highest performance was attributed not only to the lower absorption of the μc-Si:H(p) buffer-layer but also to the lower Schottky barrier height at the FTO/window-layer interface. Then, we present the dependence of the built-in potential (V_bi) and blue response of the devices on the inversion of activation energy (ξ) of the a-SiO _x:H(p), in the μc-Si:H(p)/a-SiO_x: H(p) window-layer. The enhancement of both V_bi and blue response is observed, by increasing the value of ξ. The improvement of V_bi and blue response can be ascribed to the enlargement of the optical gap of a-SiO _x:H(p) films in the μc-Si:H(p)/a-SiO_x:H(p) window-layer. Finally, the conversion efficiency was increased by 22.0%, by employing μc-Si:H(p) as a buffer-layer and raising the ξ of the a-SiO _x:H(p), compared to the optimized bufferless case, with a 10 nm-thick a-SiO_x:H(p) window-layer.