Effect of backbone structures on photovoltaic properties in naphthodithiophene-based copolymers

A "zigzag" naphthodithiophene-based copolymer, poly[4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b′]dithiophene-2, 7-diyl-alt-1,3-(5-heptadecan-9-yl)-4H-thieno[3,4-c]pyrrole-4,6-dione] (P1) is synthesized and its properties are compared to "linear" naphthodithiophene-based copolymer, poly[4,9-bis(2-ethylhexyloxy)naphtho[2,3-b: 6,7-d′]dithiophene-2,7-diyl-alt-1,3-(5-heptadecan-9-yl)-4H-thieno[3,4-c] pyrrole-4,6-dione] (P2). The field-effect carrier mobilities and the optical, electrochemical, and photovoltaic properties of the copolymers are systematically investigated. The results suggest that the backbone of the copolymer structure significantly influences the band gap, electronic energy levels, carrier mobilities, and photovoltaic properties of the resultant thin films. In this work, the zigzag naphtho[1,2-b:5,6-b′]dithiophene-based copolymer displays a good hole mobility and a high open-circuit voltage; however, polymer solar cells in which the linear naphtho[2,3-b;6,7-d′] dithiophene-based copolymer is used as the electron donor material perform better than the cells prepared using the zigzag naphtho[1,2-b:5,6-b′] dithiophene-based copolymer. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 305-312 Compared to the zigzag naphthodithiophene (NDT)-based copolymer, the linear NDT-based copolymer exhibits a low bandgap, a broad absorption spectrum, and enhanced short-circuit currents, while the zigzag NDT-based copolymer shows an improved hole mobility and a lower highest occupied molecular orbital energy level, which would lead to a higher open-circuit voltage in polymer solar cells, as compared to the linear NDT-based copolymer.