Theoretical investigation of approaches for obtaining narrow band gaps in conducting polymers

Abstract

Over the last few years, there has been a great deal of research interest in developing organic conjugated polymers with narrow energy band gaps. Narrow band gap polymers would be intrinsically conducting, and thus eliminating the need for doping. There are several approaches for the construction of low band gap systems. Copolymerization of aromatic and o-quinoid heterocycles, minimization of bond length alternation, copolymerisation of donor and acceptor moieties might be most important factors for the lowering the band gap. The main aim of this work is to determine the reasons for low band gaps and to analyse the major effects, separately. Recently a number of low band gap systems were synthesized. These systems consist of aromatic donors and quinoid acceptors. To analyse the behaviour of donor/acceptor systems, we performed theoretical studies for these systems. We chose thieno [3,4-b] pyrazine (ThP), F-substituted thieno [3,4-b] pyrazine (F-ThP) and quinoxaline (Qx) as an acceptor and thiophene (Th) and pyrrole (Py) as donor units. Monomer through octamer of ThP, monomer through tetramer of F-ThP and Qx, were optimised by using density functional theory (DFT). All calculations were performed with the Gaussian 98 program. For ThP-Th and ThP-Py co-oligomers, energy levels were calculated for monomer through tetramer and, for F-ThP-Py, monomer through trimer data were used. Ionization Potentials (IPs), electron affinities (EAs), band gaps, and band widths of polymers were obtained by extrapolation. IPs and EAs are taken as negative HOMO and LUMO energies. The order of the band gaps of the homo-polymers is found: F-PThP (1.1 eV) < PThP(1.4 eV) < PQx.(2.4 eV). Band gaps of the systems are found to agree well with the experimental results. We increased the acceptor strength of PThP by adding fluorine units. The band gap is decreased by adding fluorine, however, the valence band width decreased, too. According to the donor-acceptor concept, we predict narrow band gaps and wider band widths for donor-acceptor co-polymers. However, for ThP-Th and ThP-Py copolymers, the band gaps are smaller than those of the homo polymers, but there is no increase on conduction band width. ThP-Th and ThP-Py have the same band gap (1.2 eV). The stronger donor pyrrole does not lead to a smaller band gap and wider band widths than the weaker donor thiophene. Thus, there is a contradiction with donor-acceptor concept qualitatively, and also quantitatively. The results show that, there is no certain correlation between band gaps and inner bond lengths. The bond length alternation (BLA) data are not consistent with the band gap results. Due to our results, the geometrical mismatch between aromatic and quinoid repeat units forces geometry distortions that impose smaller band gaps. Overall, we concluded that the situation is more complicated than simple concepts imply and further studies are required before a final conclusion can be reached.