Soluble phenanthrenyl-imidazole-presenting regioregular poly (3-octylthiophene) copolymers having tunable bandgaps for solar cell applications

We have used Grignard metathesis polymerization to successfully synthesize a series of regioregular polythiophene copolymers that contain electron-withdrawing and conjugated phenanthrenyl-imidazole moieties as side chains. The introduction of the phenanthrenyl-imidazole moieties onto the side chains of the regioregular polythiophenes increased their conjugation lengths and thermal stabilities and altered their bandgap structures. The bandgap energies, determined from the onset of optical absorption, could be tuned from 1.89 eV to 1.77 eV by controlling the number of phenanthrenyl-imidazole moieties in the copolymers. Moreover, the observed quenching in the photoluminescence of these copolymers increases with the number of phenanthrenyl-imidazole moieties in the copolymers, owing to the fast deactivation of the excited state by the electron-transfer reaction. Both the lowered bandgap and fast charge transfer contribute to the much higher external quantum efficiency of the poly (3-octylthiophene)-side-chain-tethered phenanthrenyl-imidazole than that of pure poly(3-octylthiophene), leading to much higher short circuit current density. In particular, the short circuit current densities of the device containing the copolymer having 80 mol % phenanthrenyl-imidazole, P82, improved to 14.2 mA cm(-2) from 8.7 mA cm(-2) for the device of pure poly(3-octylthiophene), P00, an increase of 62 %. In addition, the maximum power conversion efficiency improves to 2.80 % for P82 from 1.22 % for P00 (pure P30T). Therefore, these results indicate that our polymers are promising polymer photovoltaic materials.