Effects of Fluorination and Side Chain Branching on Molecular Conformation and Photovoltaic Performance of Donor-Acceptor Copolymers

The performance of donor-acceptor type conjugated polymers in photovoltaic devices is strongly dependent on the morphology of the thin film, which can be controlled by the substitution of various side groups on to the conjugated backbone. We investigate the effects of fluorination of the acceptor unit, and differences in the symmetry of alkyl side chain branching on the resulting photovoltaic device efficiency. Chemical variants of the donor-acceptor copolymer poly{4,8-bis[(triisopropylsilyl)ethynyl)benzo[1,2-b:4,5-b']-dithiophene-alt-2-(heptadecan-9-yl)-4,7-bis(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole} (PBDTBTz) are considered. Using Raman spectroscopy we find that the benzodithiophene-thiophene (BDT-T) interunit bond dihedral torsion is minimized by using a symmetrically branched alkyl side chain on the benzotriazole (BTz) unit. This leads to an increased quality of molecular packing, resulting in the highest hole mobility (similar to 3.8 X 10(-3) cm(2)/(V s)) and overall device efficiency (similar to 5.5%). By contrast, fluorination of the BTz unit is detrimental to device performance. In this case, while the BTz-T interunit bond dihedral angle is reduced, the neighboring BDT-T interunit bond dihedral is increased, and the pi-pi packing distance is increased (from 4.05 to 4.60 angstrom) in the neat film. We conclude that, while the fluorination tends to planarize the polymer backbone, it also introduces repulsive fluorine-hydrocarbon interactions, which oppose close molecular packing, and hinder efficient charge generation in the photovoltaic blend.