Bithiazole-based copolymer with deep HOMO level and noncovalent conformational lock for organic photovoltaics

Abstract

To match with the state-the-art-of non-fullerene acceptor for polymer solar cells (PSCs), it is urgent to develop high performance wide-bandgap (WBG) copolymer donors with deep highest occupied molecular orbital (HOMO) level and highly planar backbone to further promote the device efficiency. A new WBG copolymer PBDTBTz, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithi ophene))-alt-(5,5’-(-4,4’-dinonyl-2,2’-bithiazole))], which is based on benzodithiophene (BDT) as donor unit and 4,4’-dinonyl-2,2’-bithiazole (BTz) as acceptor unit is prepared for non-fullerene PSCs. Due to the strong electron-withdrawing effect, BTz unit can dramatically lower the HOMO level of PBDTBTz to −5.60 eV. Notably, double noncovalent conformational locks (N⋯S) of BTz are formed in the backbone to reduce the steric hindrance and favor a highly planar geometry for efficient charge transport and molecular packing. As a result, the device based on PBDTBTz as donor and 3,9-bis(2-methylene-((3-(1,1-dic-yanomethylene)-6,7-difluoro)-indanone))-5,5,11, 11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d']-s-indaceno[1,2-b:5,6-b']dithiophen e(IT-4F) as acceptor afforded a high open circuit voltage (Voc) of 0.92 V, which is the highest value for IT-4F-based PSCs reported so far. Furthermore, the device operated well with a negligible driving force (ΔEHOMO) of as small as 0.06 eV. These results revealed that combination of electron-withdrawing bithiazole and double noncovalent conformational lock of N⋯S is a promising molecular design concept of polymer donor with deep and planar structure for high performance PSCs.