Thin Film a-Si/c-Si1-xGex/c-Si Heterojunction Solar Cells: Design and Material Quality Requirements

Abstract

a-Si:H/crystalline-Si1-xGex/c-Si heterojunction solar cells (HIT cells) are simulated and fabricated for the first time. Cells with junction layers consisting of Si, Si0.75Ge0.25, and Si0.59Ge0.41 are compared to study the effect of increasing Ge concentration. The results show a Voc drop from 0.6V for Si cells to 0.4V forSi0.59Ge0.41, consistent with the reduction in bandgap. The measured Jsc increases from ~18.5 mA/cm2 for Si cells to 20.3 mA/cm2 for theSi0.59Ge0.41 cells,for one light pass. Simulations suggest that the measured Jscfor the Si0.59Ge0.41 based solar cells is limited by a low lifetime. In order for Si1-xGexbased cells to exceed the efficiency of Si, simulations indicate that Ge percentages larger than 40% and lifetimes above 1 μs are required.