Thin film morphology and charge carrier mobility of diketopyrrolopyrrole based conjugated polymers

A series of narrow bandgap donor-acceptor (DeA) type p-conjugated polymers consisting of thiophene (Th)- or furan (Fu)-flanked diketopyrrolopyrroles (DPPs) as acceptor units and thieno[3,2-b] thiophene (TT) or didodecyloxybenzo[1,2-b; 3,4-b] dithiophene (BDT) moieties as donor units were synthesized. Cyclic voltammetry and UV-vis absorption studies revealed that both the heteroatom substitution in the aromatic end-groups of DPPs and variations in the chemical backbone influenced the bandgap of these polymers. The TT-based polymers showed a better thermal stability than their BDT analogs. P(ThDPP-TT) with better backbone planarity among the polymers being studied was found to organize in edge-on fashion on silicon/silicon dioxide surface, while all others showed a mixed type of both face-and edge-on arrangements. A substantial reduction in the thin film order and packing of the polymer chains was observed as the terminal thiophene units of DPP were replaced with furan. However, the furyl substituted polymer exhibited relatively high charge carrier mobility possibly due to improved conduction along the polymer backbone. The charge carrier transport properties of these polymers in organic field effect transistors (OFETs) were found to be influenced by incorporated donor units. Polymers with BDT-donor group exhibited unipolar hole transport, while those with TT units were ambipolar in nature. The present study demonstrates a systematic investigation of the structure-property relationships of DeA type DPP based p-conjugated copolymers which may enlighten better design strategy of polymers with novel chemical structures in the future. (C) 2015 Elsevier Ltd. All rights reserved.