Toward a better physical understanding of a-Si:H/c-Si heterojunction solar cells

Despite energy conversion efficiencies exceeding 22%, current understanding of the physics behind heterojunction solar cells remains incomplete. The role of hydrogen and ion bombardment during the plasma deposition as well as the influence of an epitaxial layer remains a subject of debate. Our results suggest that hydrogen plays a key role in the fabrication of high efficiency heterojunction solar cells. We show that ion bombardment is not as detrimental as is often thought. Moreover we find that an epitaxial layer is not necessarily harmful to the cell's Voc. We propose a criterion linking the layer's epitaxy and the cells' performance. To further investigate the role of the H2 plasma, we carry out in situ ellipsometry measurements on various kinds of c-Si wafers. The effects of this H2 plasma strongly depend on the resistivity of the c-Si wafer, suggesting that plasma conditions must be tuned to optimize cell efficiency according to the c-Si resistivity.