Two-Dimensional Conjugation Extended CH-Series Acceptors with a Distinctive A–D–A Character

Conspectus As one of the most important indicators for evaluating photovoltaic devices, the power conversion efficiencies (PCEs) for the first-class organic solar cells (OSCs) have reached the level of ∼20%, but they still lag far behind that of over 25% for their inorganic counterparts. With the similar if not better fill factor and short-circuit current, this wide gap of PCEs should be fundamentally attributed to the greatly larger nonradiative energy losses in OSCs, which are usually above 0.2 eV for OSCs but only 0.03–0.04 eV for high-performance inorganic solar cells. Note that the stubbornly severe nonradiative recombination in OSCs is associated with multiple characteristics of organic light-harvesting molecules, such as intrinsically large exciton binding energies and small relative dielectric constants, defective intermolecular packing networks, or more crystal defects caused by the flexibility of large organic molecular skeletons, nonideal nanoscale film morphologies, and so on. All the factors above require that rational design of light-harvesting molecules should be carried out not only at single molecule but also at aggregation levels if further dramatic improvement of PCE is to be achieved for OSCs. In this Account, we will first expound the unique merits of acceptor–donor–acceptor (A–D–A) type light-harvesting materials in frontier orbital distribution, energy level tuning, and intermolecular packings, meanwhile revealing the dominant role of A–D–A type molecules in facilitating charge transfer/transport, suppressing energy loss, and improving photovoltaic performance of OSCs eventually. In light of the conspicuous superiority of A–D–A type molecules, a convincing conclusion can be made that further exploration of novel A–D–A type light-harvesting materials is crucially important to shrink the PCE gap between OSCs and inorganic solar cells. Second, our recent studies for a really exciting A–D–A type molecular platform (CH-series) will be discussed comprehensively, involving various high-performance nonfullerene acceptors (NFAs) with small molecular, dimer-like, and polymerized architectures. Note that the most distinctive feature of CH-series NFAs is two-dimensional (2D) conjugation extension, especially for central units. Therefore, the favorable effects of 2D conjugation extension of these molecules on their fundamental physicochemical properties, intermolecular packing modes, blended film morphologies, photovoltaic parameters, and energy losses of resulting OSCs will be fully discussed. Abiding by the unveiled design rules of high-performance A–D–A type NFAs, the highest PCE of approaching 20% has been achieved for OSCs based on CH-series molecules. The evolution path of previous OSCs is based on traditional materials such as that of PCBM, ITIC, Y6, etc. could be one lesson; CH-series molecules are very likely to offer a great platform capable of achieving record-breaking OSCs along with much decreased energy losses, especially considering their wide and various structural modification possibilities. Finally, despite the rapidly surging PCEs of OSCs, there are still several insurmountable hurdles when attempting to break through bottlenecks existing in OSCs. Therefore, we propose some perspectives that can be further conducted on CH-series NFAs, which may conquer the great challenge of too large energy losses and thus boost OSCs toward commercial applications further.