Low band‐gap modulation of isoindigo‐based copolymers toward high open‐circuit voltage of polymer solar cells. Issue 16 (11th June 2013) - Wan, Meixiu
ABSTRACT: Designing low band‐gap‐conjugated polymers coupled with low HOMO levels attracts great attention in the field of polymer solar cells (PSCs). By using donor–acceptor (D‐A) copolymerization strategy, we designed and synthesized a series of low band‐gap copolymers with deep HOMO levels via introducing an isoindigo (IID) acceptor unit in the copolymers with the donor unit of fluorene (F) (PIID‐F), carbazole (Cz) (PIID‐Cz), thiophene (Th) (PIID‐Th), dithiophene (DTh) (PIID‐DTh), or dithienosilole (DTS) (PIID‐DTS). The HOMO level of the copolymers, measured by electrochemical cyclic voltammetry, varies from −5.3 eV to −5.8 eV, depending on different donor units in the copolymers. However, the LUMO levels of all the copolymers are fixed at about −3.6 eV, which is mainly determined by IID acceptor unit due to its strong electron‐withdrawing ability. The new results will provide an effect help in designing IID based molecular structures. Among the copolymers, PIID‐DTS has a low band gap of 1.58 eV and possesses a low‐lying HOMO energy level of −5.33 eV. The PSCs based on PIID‐DTS as donor and PC70 BM as acceptor exhibited a high open‐circuit voltage ( V oc ) of 0.93 V and a primary power conversion efficiency of 2.45%. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3477–3485 Abstract : The HOMO levels based on IID unit‐conjugated polymers could be easily tuned from −5.84 eV to −5.33 eV by increasing electron‐donating abilities from fluorene, carbazole, thiophene, dithiophene to dithienosilole. However, the LUMO levels of all the copolymers are found to be fixed at about −3.6 eV. The polymer solar cells based on PIID‐DTS as donor and PC70 BM as acceptor exhibited a high V oc of 0.93 V and a primary power conversion efficiency of 2.45%.