Achieving Efficient Polymer Solar Cells Based on Near-Infrared Absorptive Backbone Twisted Nonfullerene Acceptors through a Synergistic Strategy of an Indacenodiselenophene Fused-Ring Core and a Chlorinated Terminal Group

To expand the light-absorption regions of main-chain contorted nonfullerene acceptors (NFAs), three backbone twisted small molecules (i-IDSe, i-IDSe-4F, and i-IDSe-4Cl) were prepared, characterized by adopting an indacenodiselenophene (IDSe) segment as the electron-donating unit and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile derivatives as the end-capping units. Thanks to the strong electron-giving properties of IDSe unit, all three NFAs show red-shifting absorption spectra in the long-wavelength areas, leading to relatively small bandgaps, 1.54, 1.52, and 1.47 eV for i-IDSe, i-IDSe-4F, and i-IDSe-4Cl, respectively. Despite having deepened molecular orbital levels, the acquired fluorinated molecule (i-IDSe-4F) and chlorinated compound (i-IDSe-4Cl) exhibit broadened absorption regions and enhanced extinction coefficients relative to the nonhalogenated counterpart i-IDSe. Moreover, the blend based on PBDB-T and i-IDSe-4Cl has better charge collection and exciton dissociation properties, inhibitive trap-assisted recombination, and fast exciton splitting characteristics. Therefore, a high efficiency of 12.39% was acquired in the PBDB-T:i-IDSe-4Cl-based device, which was superior to the 8.16% of i-IDSe- and 11.99% of i-IDSe-4F-related devices. This work demonstrates the significance of the adoption of a strong electron-donating fused-ring core and the chlorinated end-capping group have in the construction of backbone twisted NFAs for high-performance polymer solar cells.