High-Efficiency Ternary Organic Solar Cells Enabled by Synergizing Dicyanomethylene-Functionalized Coumarin Donors and Fullerene- Free Acceptors

The small-molecule-based donor/acceptor blend is an important component in bulk-heterojunction organic solar cells (OSCs). Although a variety of small-molecule donors have been developed in recent years, only a few of them have reached efficiencies over 15% using fullerenefree acceptors in OSCs. Therefore, it is important to develop high-performing molecular donor materials with ideal molecular orbital energy levels, complementary absorption to the acceptors, and good charge-transporting character. Herein, the design and development of two wide band gap functional coumarin-based donor materials C1-CN and C2-CN are presented, which are synthesized efficiently using readily accessible starting materials, an important criterion for the realization of cost-effective large area OSCs. The associated effect of excellent optoelectronic and thermal properties and frontier orbital energy levels enabled the application of the synthesized donors in solution-processed fullerene-free OSCs. Ternary OSCs fabricated with C1CN:DBTBT-IC:Y6 displayed efficiencies up to 15.28%, one of the highest values reported for all-small-molecule OSCs with a low energy loss of 0.43 eV.