Parasitic Absorption in Polycrystalline Si-layers for Carrier-selective Front Junctions

We investigate the optical properties of n- and p-type polycrystalline silicon (poly-Si) layers. We determine the optical constants n and k of the complex refractive index of polycrystalline silicon by using variable-angle spectroscopic ellipsometry. Moreover, we investigate the effect of different doping levels in the poly-Si on free carrier absorption (FCA). Thereby, we demonstrate that the FCA in poly-Si can be described by a model developed for crystalline silicon (c-Si) at a first approximation. The optical properties of hydrogenated amorphous silicon layers (a-Si:H) are also investigated as a reference. With ray tracing simulations the absorption losses of poly-Si and of the a-Si:H layers are quantified with respect to the film thickness. Based on this approach we find that the short-circuit current density losses due to parasitic absorption of poly-Si layers are significantly lower when compared to a-Si:H layers of the same thickness. For example the short-circuit current density loss due to a 20 nm thick p-type poly-Si layer is around 1.1 mA/cm2, whereas a 20 nm thick p-type a-Si:H layer leads to a loss of around 3.5 mA/cm2.