Heuristic Rule of Thumb for Tandem Solar Cells and Perspectives for the Future

Abstract

Photovoltaics has gained significant interest as renewable electricity source. For cost reduction, maximizing efficiency is paramount. Tandem solar cells, combining two absorbers with different band gaps, offer improved solar spectrum utilization. Hereby, a two-terminal configuration simplifies the layer structure but demands current matching. While the search for new, especially top absorber materials continues, we focus on fundamental tandem solar cell principles to emphasize key requirements. We analyze how top-cell transparency affects bottom-cell performance in stacked tandems. Imperfect transmission impacts the bottom cell, but in a current-matched device, even the top-cell efficiency. To match the bottom-cell performance, 50% top-cell transmission and 50% efficiency relative to the single bottom cell are required, but higher values to surpass it. Subgap transparency remains a critical challenge, underscoring the need for top-cell efficiencies approaching those of the bottom cell. To relax particularly current-matching constraints, concepts like luminescent coupling, wavelength-selective intermediate reflectors, or bifacial illumination may be considered. Operating under light concentration further enhances efficiency and better justifies tandem fabrication costs. Looking ahead, bifacial tandem concentrator cells, in a three-terminal configuration and combined with spectrum-splitting optics to reduce optical losses and improve adaptability to variable illumination, offer an innovative pathway.