Loss analysis of emitter-wrap-through silicon solar cells by means of experiment and three-dimensional device modeling

Abstract

Currently, the emitter-wrap-through (EWT) design of Si solar cells is being intensively investigated as a potential candidate for cheap, low-quality Si materials. So far, experimentally achieved energy conversion efficiencies have stayed unexpectedly far below the expectations of common device theory. Therefore, we analyze fabricated EWT cells in detail and refine device theory to account for the limiting loss mechanisms present only in EWT cells. By means of rigorous three-dimensional numerical device modeling, we show that the fill factor (FF) is significantly reduced, primarily due to a effect we call the via-resistance induced recombination enhancement effect. The FF is only secondarily reduced by the resistive losses in the vias where the emitter is wrapped through the cell. This implies that lowering the base resistivity will improve cell efficiency more effectively than lowering the resistance in the vias. Our simulations predict that the EWT design with a nonpassivated rear emitter and a homogeneous emitter diffusion leads to an efficiency improvement of about 1% absolute, as compared to the common front-junction design. This is so for excess carrier lifetimes in the bulk between 1 and 100 μs, which means also for multicrystalline cells.