Narrow-bandgap acceptors with suppressed exciton thermalization loss for highly efficient perovskite–organic tandem solar cells

The development of high-performance perovskite–organic tandem solar cells has been impeded by the scarcity of efficient narrow-bandgap small-molecule acceptors. Realizing such materials requires overcoming the energy gap law to minimize the energy loss and simultaneously optimizing the donor–acceptor phase separation. Here, to address these challenges, we present an asymmetric halogen heavy-atom modification strategy, synthesizing a series of narrow-bandgap small-molecule acceptors, namely, E3-2Cl, E3-2Br and E3-2I. The heavy-atom effect restricts molecular backbone vibrations and suppresses reorganization energy, thereby enhancing the luminescence efficiency and reducing the non-radiative energy loss, which collectively contribute to a higher open-circuit voltage. In parallel, this approach strengthens terminal-mediated intermolecular interactions, leading to well-defined donor–acceptor double fibril morphology and improved phase separation in the blend films. As a result, the organic solar cells based on E3-2Cl deliver a remarkably low energy loss of 0.488 eV and a power conversion efficiency of 20.7%, ranking among the highest for low-energy-loss organic solar cells. Furthermore, by integrating the E3-2Cl-based organic rear sub-cell with a wide-bandgap perovskite front cell, we demonstrate a perovskite–organic tandem solar cell with an open-circuit voltage of 2.18 V and a power conversion efficiency of 28.2% (certified, 27.5%) under an aperture area exceeding 1 cm². This work establishes a viable molecular design strategy for developing low-energy-loss narrow-bandgap acceptors, paving the way for high-efficiency perovskite–organic tandem solar cells.