Influence of the Carbon Concentration on (p) Poly-SiC_x Layer Properties With Focus on Parasitic Absorption in Front Side Poly-SiC_x/SiO_x Passivating Contacts of Solar Cells

Abstract

Passivating contacts based on polycrystalline silicon (poly-Si) on an interfacial oxide are limited by parasitic absorption, which may be reduced by incorporation of foreign elements in the poly-Si layer. In this study, the influence of carbon incorporation in the concentration range of 6.9–21.5 at% on boron-doped polycrystalline silicon carbide (poly-SiCx) layer properties is investigated and interpreted in the context of an application as full-area passivating contact on the front side of a solar cell. For constant annealing parameters, higher carbon concentrations reduce the crystallinity of the layers. A high crystallinity in turn is confirmed to be a key parameter for the application in a solar cell as it ensures both low resistivity as well as low parasitic absorption. Low recombination current densities in the range of 7.2–12.2 fA/cm2 are determined for all layers on interfacial oxides on planar surfaces, whereas the differences are rather related to variations in the boron concentration than to the carbon concentration or the deposition parameters. A reduction of the (p) poly-SiCx layer thickness down to 10 nm would yield a parasitic absorption current density of 1.13 ± 0.13 mA/cm2. Using this value and the lowest measured recombination current density, a simple model predicts a theoretical solar cell efficiency limit of 26.7 ± 0.2%.