Donor–acceptor conjugated polymers based on cyclic imide substituted quinoxaline or dibenzo[a,c]phenazine for polymer solar cells

Abstract

A series of donor–acceptor type of conjugated polymers were designed and synthesized via palladium catalyzed Stille copolymerization. These copolymers consisted of the electron-donating benzodithiophene or indacenodithiophene units and the electron-deficient cyclic imide substituted quinoxaline derivatives. It was noted that all copolymers exhibited typical dual-absorption characteristics in both solution and as thin films with an optical band gap in the range of 1.61–1.72 eV. In comparison with the copolymers based on the electron-deficient unit of 5,9-di(thien-2-yl)-2,3-diphenyl-7-octyl-6H-pyrrolo[3,4-g]quinoxaline-6,8(7H)-dione (TPQD) as the electron-accepting unit, the resultant copolymers based on the enlarged coplanar N-octyl-10,13-di(thien-2-yl)-2,7-dioctyldibenzo[a,c]phenazine-11,12-dicarboxylic imide (TBPDI) unit exhibited an obviously enhanced absorption coefficiency. All polymers showed relatively deep highest occupied molecular orbital energy levels ranging from −5.40 to −5.52 eV. It was also realized that the hole mobility of copolymers based on TBPDI is much higher than those based on the TPQD unit as measured by the space charge limited current method. Polymer solar cells with a device architecture of ITO/PEDOT:PSS/polymer : PC₇₁BM/Ca/Al demonstrated that TBPDI-based copolymers exhibited higher performances than TPQD-based copolymers. The best device performance with a power conversion efficiency of 4.60% was achieved by using the indacenodithiophene-alt-TBPDI copolymer as the photoactive layer. Further optimization of devices by introducing a cathode interfacial layer and solvent vapor annealing in chloroform led to an impressive power conversion efficiency of 5.58%. These observations indicated that the developed TBPDI unit that has enlarged coplanarity can be a promising building block for the construction of highly efficient conjugated polymers for solar cell applications.