Metal grid contact design of four-terminal tandem solar cells

Tandem solar cells comprised of a III-V material on Si offer a path to efficiencies exceeding 30%. The four-terminal architecture, in which each of the two sub-cells is electrically independent, allows for especially high energy yields, as it is not limited by current-matching. Each sub-cells' contacts cause optical and electrical losses, however; and a careful optimization is required to exploit the full potential of this architecture. In this study, we explore the potential efficiency gains of an example 21.3% GaAs-on-Si four-terminal tandem [1] by re-designing the metal grid contact geometry. We optimize and model the contact losses and predict a potential efficiency increase of 3.4%. We demonstrate the importance of careful metal grid design and the benefits of using energy yield as a figure of merit when design contacts for a practical 4T tandem subject to realistic operating conditions.