Novel efficient accptor1-acceptor2 type copolymer donors: Vinyl induced planar geometry and high performance organic solar cells

Abstract

Accptor1-acceptor2 (A1-A2) type polymer donors are attractive to down-shift the energy levels, due to the double acceptor units with strong electron affinity. The fruitful A units provide a plenty of opportunities to enrich the types of polymer donors. Nevertheless, the reported A1-A2 type polymer donors always suffer from the low short-circuit current, ascribing to their unsatisfactory spectral absorption. In this work, two novel A1-A2 type wide bandgap copolymer donors, PV2TC-BDD and PV2TC-FTAZ, were designed and synthesized by a new acceptor unit A1, 2-butyl-1-octyl carboxylate substituted thiophene-vinyl-thiophene (V2TC) with two different acceptor units A2 (BDD and FTAZ), respectively. The small and simple vinyl unit can effectively reduce the steric hindrance and improve the planarity of molecules, deepen the HOMO energy level, enhance the light absorption and facilitate charge transport. The two copolymer donors show a deep-lying HOMO level of −5.50 eV for PV2TC-BDD and −5.35 eV for PV2TC-FTAZ. The absorption bands of the copolymers are observed to distinctly red-shift than those of the reported A1-A2 type copolymers based on carboxylate substituted thiophene. As a result, when blended with IT-4F, both of A1-A2 type copolymers obtain an improved Jsc of 18.83 mA cm−1 for PV2TC-BDD and 20.49 mA cm−1 for PV2TC-FTAZ, which is the highest value for the A1-A2 type copolymer-based devices. The PV2TC-BDD-based OSC also achieves a high power conversion efficiency (PCE) of 11.50%, which is an outstanding performance of the devices based on A1-A2 type polymers. These results demonstrate that introducing the new acceptor unit V2TC to construct high efficiency A1-A2 type copolymers is an effective strategy to develop high performance polymer donors for OSCs.