High performance conjugated terpolymers as electron donors in nonfullerene organic solar cells

Abstract

We demonstrate the synthesis of three π-conjugated terpolymers based on the nonconventional molecular design strategy D1-D2-D1-A comprising two different multi-fused ladder-type arene electron donating moieties [(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl (DTS) as D1 and thienyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) derivatives as D2] and an electron withdrawing unit [5,6-difluoro-2,1,3-benzothiadiazole (DFBT) as A]. The implementation of these materials as electron donors is explored in high performance near infrared non-fullerene acceptor (NFA) organic solar cells utilizing the benchmark low bandgap NFA IEICO-4F. The triple bulk heterojunction blend systems provide the basis for a detailed structure-property-performance relationship in terms of BDT's thienyl substitution (alkyl, alkylthio and fluoro) by investigating the correlations between the molecular energy level alignments, performance, and device physics of OSCs. The alkylthienyl-BDT based π-conjugated terpolymer [P(DTS-BDT-DFBT)] exhibits the best photovoltaic performance delivering a power conversion efficiency of 10% with a high short circuit current density of 22.7 mA cm-2. The combination of optoelectronic measurements and morphological analyses revealed the suppression of field dependent charge recombination in P(DTS-BDT-DFBT):IEICO-4F as compared to alkylthiothienyl-BDT [P(DTS-BDTS-DFBT):IEICO-4F] and fluoro-substituted thienyl-BDT [P(DTS-BDTF-DFBT):IEICO-4F] based OSC devices.