Rational Design of Medium-Bandgap Perovskite Solar Cells for Triple-Junction Si Tandems

Although perovskite-based triple-junction tandem solar cells have a higher theoretical maximum efficiency than the double-junction counterparts, their actual performances are not only far behind the theoretical one but also worse than the double-junction cells. One of the major issues limiting their performances is that the overall tandem current density is limited by the middle cell with a bandgap energy higher than the optimum value. In this study, we propose a comprehensive design rule of the middle cell specifically optimized for triple-tandem applications. We investigated the thickness effect of medium-bandgap perovskite and electron-transporting layers, especially focusing on the spectral responses to the filtered incident light in order to maximize the middle-cell photocurrent density and thus the overall tandem current density. This triple-tandem-specific designing of the middle cell leads to a high current density of 11 mA/cm² for the perovskite/perovskite/Si 3J tandem solar cell, and its conversion efficiency could be further increased to as high as 24.96% after additional interfacial defect passivation by PDAI₂.