Organic photovoltaic materials consist of electron donors and electron acceptors. In the past years, fullerenes and their derivatives have been the dominant electron acceptor materials in organic solar cells (OSCs). As fullerene acceptors have some drawbacks, such as weak absorption in the visible spectral region and limited energy level variability, nonfullerene electron acceptors have received increasing attention. However, OSCs based on nonfullerene acceptors exhibit much lower power conversion efficiencies (PCEs) than their fullerene counterparts. Development of high-performance nonfullerene acceptors is a challenge in this field.
Zhan group firstly proposed the concept of “fused-ring electron acceptor (FREA)”, and designed and synthesized a series of FREAs. These FREAs present some advantages: 1) They have high electron mobilities similar to those of fullerenes; 2) They exhibit strong and broad absorption, especially in the 700–800 nm range, and can match with wide-bandgap donor polymers to achieve complementary absorption; 3) They have tunable energy levels and thus match with various high-performance electron donors; 4) Their solubility, crystallinity and film morphology can be tailored; 5) Their synthesis is easy to scale up.
In 2015, Zhan group reported some novel electron acceptors based on extended fused-ring core, such as indacenodithiophene and indacenodithieno[3,2-b]thiophene. OSCs based on blends of the widely used polymer donor PTB7-TH and these acceptors (e.g., ITIC) exhibited PCEs up to 6.8%, a new record for fullerene-free OSCs at that time. The PTB7-TH: ITIC-based OSCs exhibited even better performance than the control devices based on PTB7-TH: PCBM. See: Adv. Mater., 2015, 27, 1170–1174 (cited 100 times, ESI Hot Paper, ESI Highly Cited Paper); Energy Environ. Sci., 2015, 8, 610-616 (cited 70 times, ESI Hot Paper, ESI Highly Cited Paper); Energy Environ. Sci., 2015, 8, 3215-3221.
Very recently, through molecular engineering, Zhan group designed and synthesized novel planar FREAs with alkyl and thienyl side-chains, which exhibited higher electron mobility and better light absorption than ITIC. OSCs based on blends of the wide-bandgap polymer donor PDBT-T1 and these acceptors exhibited PCEs up to 9.6%, which is a new record for fullerene-free OSCs. See: J. Am. Chem. Soc., 2016, 138, 2973?2976; J. Am. Chem. Soc., 2016, DOI: 10.1021/jacs.6b02004. Now FREAs designed by Zhan group are commercial available. Many well-known research groups across the world have already utilized these FREAs to fabricate high-efficiency OSCs, which rival the high-performance single junction OSCs based on fullerene acceptors.
This research work was supported by the National Nature Science Foundation of China and the Ministry of Science and Technology of China.