Pendant group effects on the optical and electrical properties of carbazole-diketopyrrolopyrrole copolymers

Two kinds of novel carbazole and diketopyrrolopyrrole based conjugated copolymers, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(3,6-dithien-2- yl-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione)] (P1) and poly[N-9′-(3,5-bis(octyloxy)phenyl)-2,7-carbazole-alt-5,5-(3, 6-dithien-2-yl-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione)] (P2), were synthesized via a Suzuki cross coupling reaction. The optical and electrical properties of these polymers, in which the pendent groups on the carbazole moiety were modified, were investigated. Aryl substituted P2 shows a narrower band gap (1.61 eV), higher hole mobility (4.4 × 10^-3 cm ² V^-1 s^-1) and more planar backbone structure than alkyl substituted P1. The modification of side groups significantly affected their surface morphologies. The device performance of solar cells based on these polymers and a fullerene acceptor was characterized.