Wide bandgap D-A copolymers with same medium dithieno [2,3-e;3′2′-g]isoindole-7,9 (8H) acceptor and different donors for high-performance fullerene free polymer solar cells with efficiency up to 14.76%

Abstract

Three new donor–acceptor conjugated polymers incorporating same dithieno [2,3-e;3′2′-g]isoindole-7,9 (8H) (DTID) as medium acceptor and different donors, i.e., dithienosilole (DTS) PDTID-DTS (P1), dithieno-benzene (DTB) PDTID-DTB (P2) and dibenzothiophene (BDT) PDTID-BDT (P3) as a donor have been synthesized and explored for non-fullerene based BHJ polymer solar cells (PSCs). These copolymers had optical band gaps of 1.83–2.07 eV and HOMO energy levels of −5.37 to −5.67 eV. The P2 consists of a DTB donor unit that results in a deeper HOMO level of −5.67 eV, which may be due to the weak electron-donating nature of DTB. Pairing with the narrow bandgap non-fullerene acceptor BThIND-Cl, the P1, P2, and P3-based PSCs showed overall power conversion efficiency of 14.76 %, 7.22 % and 13.13 %, respectively. The PSCs based on P2 and P1 showed the highest and lowest values of open-circuit voltage, respectively, consistent with their HOMO energy levels. The lowest value of PCE for P2 may be associated with the negative HOMO offset between the P2 and BThIND-Cl, which hamper the hole transfer from the HOMO of BThIND-Cl to P2, resulting in a low value of short circuit current and fill factor. The higher value of PCE of 14.76 % for P1 may be related to the broader absorption profile of the active layer and balanced charge transport, more appropriate nanoscale phase separation, and compact π-π stacking distance, leading to the high value of short circuit current and fill factor.