Fused-ring Electron Acceptors for Organic Solar Cells

Nonfullerene organic solar cell (OPV) is one of the hottest frontiers in chemistry and materials science. China has been leading this hot frontier, and Chinese researchers have made great contribution to this research field. We proposed a novel concept - fused-ring electron acceptor (FREA), established a brand-new, high-performance nonfullerene acceptor system, and invented star molecule ITIC. These FREAs show some advantages: (1) they have high electron mobility similar to those of fullerenes; (2) they exhibit strong and broad absorption, especially in the 700 - 1000 nm range, and can match with wide-bandgap donor materials to achieve complementary absorption; (3) their energy level can be tuned and thus they match with various high-performance electron donors; (4) their crystallinity and film morphology can be tuned; (5) their synthesis is easy to scale up. Our original and pioneering work has received extensive attention. FREAs are now commercial available. Many well-known research groups across the world have already utilized these FREAs to fabricate high-efficiency OPV. FREA-based OPV now has achieved a power conversation efficiency of 13% -14%, surpassing the fullerene counterpart. Moreover, the FREA-based OPV exhibits better device stability than the fullerene-based counterpart. The emergence of ITIC-like FREAs has overturned predominant position of fullerene acceptor in OPV and is inaugurating the nonfullerene OPV era. In this review, we summarize our progress of FREA design and application in OPV, and give an outlook of the FREAs. We first introduce the background information, concept and working mechanism of OPV; then the advantages and disadvantages of fullerene acceptors are compared with nonfullerene acceptors; and the concept and merits of FREAs are finally discussed. The main text focuses on fused-ring core engineering, electron-withdrawing group engineering and side chain engineering, and we emphasize effects of electron-donating fused-ring cores, electron-withdrawing end groups and side chains on solubility, crystallinity, energy levels, absorption spectra, electron mobilities and photovoltaic properties of FREAs.