Microcrystalline silicon tunnel junctions for amorphous silicon-based multijunction solar cells

The formation of tunnel junctions for applications in amorphous silicon (a-Si:H) based multifunction n-i-p solar cells has been studied using real time optics. The junction structure investigated in detail here consists of a thin (similar to 200 Angstrom) layer of n-type microcrystalline silicon (mu c-Si:H) on top of an equally thin layer of p-type mu c-Si:H, the latter deposited on thick (similar to 2000 Angstrom) intrinsic a-Si:H. Such a structure has been optimized in an attempt to obtain single-phase mu c-Si:H with a high crystallite packing density and large grain size for both layers of the tunnel junction. We have explored the conditions under which grain growth is continuous across the p/n junction and conditions under which renucleation of n-layer grains can be ensured at the junction. One important finding of this study is that the optimum conditions for single-phase, high-density mu c-Si:H n-layers are different depending on whether the substrate is a mu c-Si:H p-layer or is a H-2-plasma treated or untreated a-Si:H i-layer. Thus, the top-most mu c-Si:H layer of the tunnel junction must be optimized in the multijunction device configuration, rather than in single cell configurations on a-Si:H i-layers. Our observations are explained using an evolutionary phase diagram for a-Si:H and mu c-Si:H film growth versus thickness and H-2-dilution ratio, in which the boundary between the two phases is strongly substrate-dependent.