A wide-bandgap copolymer donor with a 5-methyl-4H-dithieno[3,2-e:2',3'-g]isoindole-4,6(5H)-dione unit

Wide-bandgap copolymer donors with fused-ring accept-or units (FAUs) present excellent performance in non-fullerene organic solar cells due to their complementary light-absorption with nonfullerene acceptors,deep the highest occupied molecular orbital (HOMO) levels and high hole mobilities1-4].A bunch of FAU-based copolymer donors were developed in recent years,such as PM6s],PM76],PBQx-TCI7],PTQ108],PBQ69],P2F-EHp10],D1611],L1-S12],D1813,14]and D18-CI1s,16].They delivered ＞16％ power conversion effi-ciencies (PCEs) in solar cells.To develop good FAUs is the key toward efficient FAU-based copolymer donors.A good FAU generally has a strong electron-withdrawing character that leads to a low HOMO level and a high open-circuit voltage(Voc),and a relatively large molecular plane that facilitates poly-mer stacking and enhances hole mobility.Recently,we de-veloped copolymer donors D18 and D18-CI by using dithi-eno3',2':3,4;2",3":5,6]benzo1,2-c]1,2,5]thiadiazole (DTBT)unit13] (Fig.1(a)).Thanks to the strong electron-withdrawing property and the rigid and extended molecular plane of DTBT,D18 and D18-CI deliver outstanding PCEs up to 18.69％13-16].The success of D18 polymers stimulated us to design more high-performance copolymer donors with nov-el FAUs.In this work,we designed a wide-bandgap copoly-mer donor P1 by using a fused-ring imide building block,5-methyl-4H-dithieno3,2-e:2',3'-g]isoindole-4,6(5 H)-dione(MDTID).Compared with the thiadiazole moiety in DTBT,the imide moiety in MDTID is more electron-withdrawing.The density functional theory (DFT) calculations show that MDTID has deeper HOMO and the lowest unoccupied molecular orbit-al (LUMO) levels than DTBT,suggesting the stronger electron-accepting capability of MDTID (Fig.1(a)).DFT calculations also indicate that MDTID leads to a deeper HOMO for P1 than that of D18,thus benefiting Voc (Fig.S1).