Enhanced Photovoltaic Performance of Amorphous Donor–Acceptor Copolymers Based on Fluorine-Substituted Benzodioxocyclohexene-Annelated Thiophene

摘要

Donor-acceptor (D-A) type pi-conjugated copolymers with crystalline behavior have been extensively investigated as donor semiconductors in organic photovoltaics (OPVs). On the other hand, the development of high-performance amorphous donor materials is still behind. The amorphous donor copolymer DTS-C-0(F-2) consisting of dithieno[3,2-b:2',3'-d]silole (DTS) donor unit and the recently developed fluorine-substituted naphtho[2,3-c]thiophene-4,9-dione (C-0(F-2)) acceptor unit shows moderate photovoltaic performance upon blending with PC71BM. In this work, to enhance the hole-transporting characteristics, a 3-hexylthiophene (HT) spacer unit is integrated into the conjugated backbone, resulting in a new amorphous copolymer DTS-HT-C-0(F-2). The strong electron-accepting nature of C-0(F-2) allows the introduction of the HT spacer without affecting the frontier orbital energies and thus the D-A character. Without using solvent additives and thermal annealing, OPVs based on DTS-HT-C-0(F-2) and [6,6]-phenyl-C71-butyric acid methyl ester PC71BM show an improved power conversion efficiency of 9.12%. Investigation of the device physics unambiguously reveals that the hole mobility of the copolymer in the blend is increased by an order of magnitude by the introduction of HT, while keeping an amorphous film nature, leading to higher short-circuit current density and fill factor. These results demonstrate the realization of high-performance OPVs based on amorphous active layers.