More junctions is not always better: mapping optimal bandgap partners for tandem and triple-junction solar cells

Abstract

The detailed balance (DB) efficiency limit serves as a fundamental framework for evaluating the potential power conversion efficiency (PCE) of photovoltaic (PV) devices. While single-junction (1J) solar cells exhibit a theoretical efficiency peak of ∼30% under standard illumination conditions, multijunction architectures such as tandem (2J) and triple-junction (3J) solar cells can exceed 44% and 50%, respectively. This work systematically explores the DB efficiency limits of multijunction solar cells, incorporating radiative coupling effects and optimizing bandgap partner selections. Our contour plot analyses reveal the impact of bandgap partner choices on efficiency gains and highlight key design trade-offs in junction count and sub-cell configuration. We find that while optimized 3J devices theoretically outperform their 2J and 1J counterparts, experimental reports often fall short of these theoretical limits due to suboptimal bandgap partner selection, particularly in all-perovskite systems. The study underscores the importance of achieving at least a 5% efficiency gain per added junction to justify increased complexity in 2J and 3J devices. Furthermore, we discuss how radiative coupling can enhance multijunction device performance, particularly for tandem configurations below the optimal bandgap partner line. Our findings provide insights for optimizing PV device design, balancing efficiency improvements with practical material and fabrication constraints.