Back contact formation for p‐type based a‐Si:H/c‐Si heterojunction solar cells

Even if p-type silicon is a more common PV material, heterojunction solar cells on p-type c-Si are less popular than on n-type. In turn, it has been difficult to achieve high efficiency with double-sided heterojunction with intrinsic thin layer structure on p-type c-Si. The double-sided silicon heterojunction solar cell is more appropriate for n-type crystalline silicon wafers than for p-type c-Si ones because of larger band offset at the valence band edge between the amorphous and crystalline semiconductors with respect to the conduction band edge. Indeed, it represents a large barrier for majority carrier holes flowing through to the back contact. In turn at the backside, the small conduction band offset provides a much less effective mirror for the minority carrier electrons. We have found an alternative way to obtain a quasi-ohmic contact promoting the formation of a CrSi film on top of p-a-Si:H layer. This reduces the problem of low p-a-Si:H doping value and the high activation energy, leading to a better carrier collection. In this work we present a detailed investigation of the p-c-Si/i-a-Si:H/p-a-Si:H contact, comparing the experimental transport measurements with numerical model of the stacked structure. We verify the effectiveness of p-c-Si/i-a-Si:H/p-a-Si:H as back side contact on the heterojunction solar cell performances. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)