Dependencies Between poly-Si Composition and Solar Cell Performance of poly-Si/SiO_x Passivating Contacts

Abstract

In this thesis, dependencies between incorporation of foreign elements in the poly-Si layer and solar cell performance of poly-Si/SiO_x passivating contacts are explored. For both dopant polarities a difference in total and electrically active dopant concentrations is observed. If applied on the front side of a solar cell, poly-Si/SiO_x contacts based on phosphorus doped poly-Si layers are rather limited by free carrier absorption and based on boron doped poly-Si layers are rather limited by detrimental strong dopant in-diffusion. Incorporation of the light elements oxygen, nitrogen and carbon widens the optical band gap and thus lowers parasitic absorption in the poly-Si layer at the expense of a higher resistivity. Both effects are mediated by a lower crystallinity compared to pure poly-Si layers without incorporated light elements. Therefore, the crystallinity was identified as key parameter for solar cell performance if applied on the front side of a solar cell. All investigated layer types yield an excellent passivation quality in terms of contact recombination current densities down to J_c = 2.3 fA/cm² and selectivities up to S₁₀ = 15.2. The maximum achievable efficiency for an application of the investigated layers in an actual solar cell is estimated to 24.2% for a phosphorus doped poly-Si layer on the rear side and a boron doped poly-SiO_xN_y layer on the front side with thicknesses of 100 nm. This limit may be increased by 2%_abs if the layer thicknesses are reduced to 30 nm.