The effect of the thickness of a ZnO: AI back reflector on the performance of p-i-n thin-film solar cells

The absorption loss in the n/Al interface of a p-i-n thin- film solar cell is high due to the higher extinction coefficient of the Al used as a back reflector. The insertion of a thin ZnO layer in the n-a-Si : H/Al interface of a solar cell with Glass/textured SnOa : F/p-a-SiC : H/buffer layer/i-a-Si : H/n-a-Si : H/Al structure minimizes the absorption loss as ZnO has a lower extinction coefficient (less than 0.5) at longer wavelengths. In this article, the dependence of various electrical characteristics, such as the resistivity, the carrier concentration, the mobility and the optical transmittance, of ZnO : Al films on the thickness and the impact of the thickness of a ZnO : Al film used as a back reflector on the performance of solar cells are reported. The ZnO:Al films are prepared by using a rf magnetron sputtering system. For a 140-nm-thick ZnO : Al layer, the short-circuit current (I _sc) increases by 12.7 % in comparison to that for the pure Al-metal back-reflector case. I _sc further increases by 23 % when a 180-nm ZnO : Al layer is used, and the conversion efficiency of the solar cell is around 10.26 % for the 180-nm-thick ZnO : Al. The cell efficiency depends on the film thickness, The crystal quality of the deposited film is improved as the film thickness is increased.