Decoding non-fullerene acceptors for organic solar cells: a theoretical structure-property-performance perspective

Organic solar cells (OSCs) have become promising next-generation photovoltaic technologies because of their flexibility, solution processability, and light weight. However, the use of traditional fullerene-based acceptors has been hampered by poor absorption, poor tunability, and energy losses that require the fabrication of sophisticated materials like non-fullerene acceptors (NFAs). The purpose of this review is to provide multi-faceted theoretical insight into the structure-property-performance relationship in NFAs with emphasis on how electronic structure, optical behavior, charge transport and eventual device efficiency can be impacted under the influence of molecular design. It is analyzed systematically through theoretical and computational methods such as density functional theory (DFT), time-dependent DFT (TD-DFT), molecular dynamics simulations and machine learning (ML) methods. Such techniques are used to measure important descriptors including HOMO-LUMO energy levels, bandgap, reorganization energy, dipole moment, exciton binding energy and molecular packing. The work shows that optoelectronic properties can be accurately tuned using rational molecular engineering via donor-acceptor architectures, end-group modification, core extension and side-chain optimization. State-of-the-art NFAs, especially Y-series acceptors, feature narrow bandgaps, strong near-infrared absorption, lower energy loss and efficient charge separation which result in much higher power conversion efficiencies on the order of 20%. Morphology and intermolecular interactions are found to be key factors that determine dynamics of charge transport and recombination. The combination of theoretical knowledge and experimental results generates a coherent model of predicting and optimizing NFA performance. The review indicates that future multiscale and data-driven modeling strategies will be central to enhancing the design of next-generation NFAs that will lead to highly efficient, stable and commercially viable organic solar cells.