A novel 2,7-linked carbazole based “double cable” polymer with pendant perylene diimide functional groups: preparation, spectroscopy and photovoltaic properties

Abstract

The preparation and chemical analysis of a ‘double-cable’ conjugated polymer comprising a backbone of alternating 2,7-linked carbazole repeat units with covalently attached perylene diimide (PDI) substituents and dithienyl repeat units is presented. The backbone of the new “double-cable” polymer P1 acts as an electron donor while the pendant PDI substituents act as electron acceptors. In order to investigate the influence of the PDI moieties on the polymer backbone as well as to elucidate their photophysical interaction, three reference-compounds were also prepared and analysed: a polymer with the same backbone as that of P1 but without PDI substituents (P2) and two PDI derivatives with branched alkyl side chains (PDI 2 with 12-tricosanyl substituents and PDI 1 with 3-pentyl substituents). We find that polymer P1 shows pronounced electron transfer from the polymer backbone to the PDI chromophore units covalently bound to it, resulting in highly efficient quenching of excitons and strong suppression of fluorescence in solutions and in thin films. The existence of long-lived polaronic states resulting from exciton dissociation on P1 is confirmed using steady state photo-induced absorption spectroscopy. Despite the improved exciton quenching yield shown by P1 over a blend of P2/PDI, photovoltaic devices fabricated from P1 have a low external quantum efficiency (EQE) of around 0.43% at 410 nm; a value that is smaller than that from a conventional BHJ device of P2/PDI which is found to have a peak external quantum efficiency of 3.7% at 463 nm. We tentatively ascribe the reduced EQE of the double-cable polymer to geminate recombination of charge-carriers as a result of poor charge transport and a complete lack of donor acceptor phase separation.