Surpassing 90% Shockley-Queisser V_OC Limit in 1.79 eV Wide-Bandgap Perovskite Solar Cells Using Bromine-Substituted Self-Assembled Monolayers

All-perovskite tandem solar cells (TSCs), composed of a 1.7-1.9 eV wide-bandgap (WBG) top cell and a 1.2-1.3 eV narrow-bandgap (NBG) bottom cell, hold the promise of surpassing the efficiency limit of single-junction solar cells. However, a significant challenge in enhancing the efficiency of these TSCs is the substantial open-circuit voltage (V_OC) loss in the WBG subcell, where the hole transporting layer (HTL) plays a crucial role. In this study, we employed a bromine-substitution strategy to synthesize a novel self-assembled monolayer (SAM), (4-(3,11-dibromo-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (DCB-Br-2), as the HTL for the 1.79-eV WBG perovskite solar cells (PSCs). The bromine in DCB-Br-2 donates a pair of non-bonded electrons to uncoordinated Pb²⁺ ions or halide vacancies, passivating defects of the perovskite bottom layer and reducing interfacial non-radiative recombination. DCB-Br-2 also improves energy level alignment, facilitating faster hole extraction. As a result, the optimized WBG solar cell demonstrates an impressive V_OC of 1.37 V, surpassing 90% of the Shockley-Queisser limit with only a 0.42-V V_OC loss. This represents the highest reported V_OC value for PSCs with similar bandgaps. Combined with a 1.25-eV NBG subcell, this enabled a two-terminal all-perovskite TSC with a power conversion efficiency of 27.70%. This study presents an innovative approach to minimizing the V_OC loss in WBG subcells through the rational design of the SAM, thus advancing the development of high-performance tandem devices.