Modeling Parasitic Absorption in Silicon Solar Cells with a Near-Surface Absorption Parameter
One drawback of passivating contacts in crystalline silicon solar cells is the current loss due to parasitic absorption within the involved material layers. When employed on an illuminated side of the cell, the full spectrum of the incident light will be partly absorbed before reaching the silicon bulk. Additionally, near-infrared (NIR) absorption can substantially reduce the cell's NIR spectral response (SR) also when employed on the non-illuminated side. As those losses are hard to measure directly, optical modeling is crucial for their quantification. This paper presents an extension to an analytical light-trapping model to account for parasitic absorption in the near-surface region via a new parameter Appp (absorbed fraction per perpendicular pass). We test the model by analyzing i) reflectance measurements on samples with varying doping profiles, ii) SR measurements on TOPCon solar cells with varying back-side poly-silicon layers, and iii) SR measurements of a bifacial silicon heterojunction cell. We show that the model can well be calibrated by fitting a single value for Appp to reflectance measurements. This enables a quantification of the parasitic absorption loss without requiring knowledge of all layer's optical properties. The model also is able to predict parasitic absorption in TOPCon cells when knowing the thickness and doping density of the poly-Si layer. Having proven the usefulness of Appp to represent parasitic absorption as a single-valued quantity, we suggest Appp as a third figure of merit for the quality of a passivating contact, next to the recombination parameter J0c and the contact resistivity rc.