Tandem solar cells based on high-efficiency c-Si bottom cells: Top cell requirements for >30% efficiency

Tandem solar cells based on crystalline silicon present a practical route toward low-cost cells with efficiencies above 30%. Here, we evaluate a dual-junction tandem configuration consisting of a high-efficiency c-Si bottom cell and a thin-film top cell based on low-cost materials. We show that the minimum top cell efficiency required to reach 30% tandem efficiency ranges from 22% for a bandgap of 1.5 eV to 14% for a bandgap of 2 eV. We investigate these limits using a simple model for a four-terminal tandem to identify the material requirements for the top cell in terms of optical absorption, electronic bandgap, carrier transport, and luminescence efficiency. In particular, we show that even relatively low-quality earth-abundant semiconductor materials with luminescence efficiencies of 10^-5 and diffusion lengths below 100 nm are compatible with tandem cell efficiencies above 30%. Introducing light trapping in the top cell can increase the efficiency beyond 32% and reduce the required diffusion length below 50 nm. This analysis establishes clear research targets for high-bandgap semiconductor materials and novel thin-film solar cell concepts that can be combined with existing c-Si technology. Such tandem approaches could enable the rapid development of a new generation of low-cost high-efficiency cells.