Simultaneous Substitution of Silicon and Fluorine Atom on Donor-Acceptor Copolymers for Photovoltaic Applications

Abstract

To investigate the effect of silole-containing and fluorination on the photovoltaic performance of polymer solar cells, two conjugated donor–acceptor copolymers have been synthesized via Stille coupling reaction. Both donor–acceptor alternating copolymers consisted of 4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene as the donor unit and nonfluorinated or fluorinated 2,1,3-benzooxadiazole (BO) as the acceptor unit, along with a thiophene group as the π-bridge between the donor and acceptor unit. For comparison, the copolymer composed of the 4,4-diethylhexyl-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) donor unit and the BO acceptor unit has also been prepared. UV–vis spectra of the three copolymers display a panchromatic absorption ranging from 300 to 1100 nm. The optical band gap obtained from the Tauc relation for the CPDT-containing polymer is 1.54 eV. With regard to the silole-containing copolymers, the optical band gaps for nonfluorinated copolymer and fluorinated copolymer are reduced to 1.46 and 1.42 eV, respectively. In comparison with both nonfluorinated polymers, the photovoltaic performance of fluorinated polymer is significantly improved due to the increased J sc and enhanced V oc. Based on the indium tin oxide (ITO) coated glass/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/polymer:phenyl-C61-butyric acid methyl ester/LiF/Al device structure, the optimal device efficiency obtained from the fluorinated copolymer at room temperature under the illumination of AM 1.5 is 3.74%, with a high V oc up to 0.87 V.