Aryl amine substituted low energy gap carbazole polymers: preparation and photovoltaic properties

The preparation of a series of donor/acceptor copolymers comprising alternating 2,7-linked 3,6-dimethyl-9-arylamino-carbazole units and 5,7-bis-(thiophen-2-yl)-2,3-diphenyl-thieno[3,4-b]pyrazine units is presented. The triarylamine substituents are attached to the carbazole repeat units either through the para- or meta-positions. The effect of the linkage position of triaryl amine substituents in this series of polymers as well as the effect of substitution of the phenyl groups at the 2,3-positions of 5,7-bis-(thiophen-2-yl)-2,3-diphenyl-thieno[3,4-b]pyrazine repeat units with alkoxy-substituents are investigated in this work. The polymers were characterized by NMR spectroscopy, UV-Vis absorption spectroscopy and fluorescence spectroscopy, and their molecular weights were estimated using gel permeation chromatography. Polymers P1–P4 absorb light up to 900 nm and have energy gaps ranging from 1.37 to 1.40 eV. Photovoltaic cells with ITO/PEDOT:PSS/P2:PC70BM(1/4, w/w)/Al showed an open circuit voltage of 0.60 V under white light illumination, power conversion efficiency of 0.67% and short circuit current of 3.6 mA cm−2.