High-Efficiency Crystalline-Si Solar Cells with Screen-Printed Front-Side Metallization: A Percolation Model to Explain the Current Path

Abstract

Resistive losses corresponding to the front-side metallization limit the efficiency of Si solar cells. At the front-side contact, the Si emitter is covered by a glass layer that is less than 1 μm thick embedded with Ag colloids to volume fraction >20%. Bulk Ag fingers are arranged on top of the glass layer. A similar microstructure is found for both n-type and p-type cells showing high efficiency. The Ag colloids constitute current filaments with reduced resistance in the glass layer, thereby introducing a percolative current which is the basis of the proposed model. This model is new and differs from the classical percolation model in its direct reliance on the macroscopic resistance of these filaments, and in considering the matrix as semiconducting rather than insulating. For periodically arranged Ag colloids of fixed diameter, the percolative limit of 13% in two dimensions (2D) and 15% in three dimensions (3D) depends only on the volume fraction of colloids but not their size. The resistance of randomly arranged and sized Ag colloids confirms the analytical results. The model explains quantitatively, consistent with microstructural analyses, why low contact resistances are found in solar cells with high colloid density. The introduced percolation model is also relevant for other systems in which metallic precipitates are found in a semiconducting matrix.