Efficiency Roadmap for Evolutionary Upgrades of PERC Solar Cells by TOPCon: Impact of Parasitic Absorption

Passivating contacts created via a thin interfacial oxide and a highly doped polysilicon layer, e.g., the TOPCon technology, are on the verge of being implemented in solar cell mass production. Investment decisions rely on R&D to identify the most promising implementation option, meaning a trustworthy roadmap based on predicted performance gains. This article shows how to thoroughly quantify the performance potential via numerical simulation, focusing on an evolutionary upgrade of a busbarless p-type bifacial passivated emitter rear cell (PERC) technology. We specifically highlight the need to consider not only the electrical gains of passivating contacts, but also the associated optical losses due to parasitic absorption in the polysilicon layers for front and rear illumination. The influence of free-carrier absorption in polysilicon on the solar cell optics is characterized on experimental test structures in order to verify our optical simulation model. Introducing TOPCon fully at the rear and also locally aligned to the front fingers can boost the PERC efficiency by approximately 1%abs. The final device is strongly limited by losses in the p-type c-Si bulk and phosphorus-doped front emitter. Consequently, the presented evolutionary TOPCon upgrades may well be of increased relevance for future improved p-PERC cells, as an alternative to the current focus on n-type TOPCon cells with boron emitter.