Intramolecular donor-acceptor regioregular poly(3-hexylthiophene)s presenting octylphenanthrenyl-imidazole moieties exhibit enhanced charge transfer for heterojunction solar cell applications

Intramolecular donor-acceptor structures prepared by covalently binding conjugated octylphenanthrenyl-imidazole moieties onto the side chains of regioregular poly(3-hexylthiophene)s exhibit lowered bandgaps and enhanced electron transfer compared to the parent polymer, e.g., conjugation of 90 mol% octylphenanthrenyl-imidazole moieties onto poly(3-hexylthiophene) chains reduces the optical bandgap from 1.91 to 1.80 eV, and the electron transfer probability is at least twice as high as that of pure poly(3-hexylthiophene) when blended with [6,6]-phenyl-C-61-butyric acid methyl ester. The lowered bandgap and the fast charge transfer both contribute to much higher external quantum efficiencies, thus much higher short-circuit current densities for copolymers presenting octylphenanthrenyl-imidazole moieties, relative to those of pure poly(3-hexylthiophene)s. The short-circuit current density of a device prepared from a copolymer presenting 90 mol% octylphenanthrenyl-imidazole moieties is 13.7 mA . cm(-2) which is an increase of 65% compared to the 8.3 mA . cm(-2) observable for a device containing pure poly(3-hexylthiophene). The maximum power conversion efficiency of this particular copolymer is 3.45% which suggest that such copolymers are promising polymeric photovoltaic materials.