Electronic structure analysis and density functional study of an alternating donor/acceptor polymer

Abstract

Conducting polymers have attracted great attention for the last two decades because of their potential applications in many fields. One of the major goals for the scientists who are studying in this field is to synthesize conducting polymers with extremely low band gap. Such polymers would be intrinsically conducting, and thus eliminating the need for doping. Several powerful approaches are proposed to synthesize low band gap polymers. One of them is the donor/acceptor concept, which was proposed by Havinga and his co-workers. In their model, they attached regularly alternating electron withdrawing and electron donating groups to the carbon backbone. By connecting these groups, they aimed to decrease the lowest unoccupied molecular orbital (LUMO) level, and to increase the highest occupied molecular orbital (HOMO) level of the polymer respectively. By this way they claimed that one could obtain very low band gap conducting polymers by introducing the alternating donor/acceptor groups, where the electronegativity difference between these groups is highest. Huang and Pickup synthesized donor/acceptor copolymers of 3,4- ethylenedioxythiophene (EDOT) which is considered as electron donating group and 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CDM) as electron withdrawing group. By changing the monomer ratio in the copolymers, they decreased the band gap to less than 0.16 eV, however, the intrinsic conductivity was about 10-3 S/cm. Although a very low band gap was achieved, the conductivities of copolymers were still low. To understand this behaviour and also to check the validity of donor/acceptor concept, we performed theoretical studies for these systems. Monomer through hexamer of EDOT, oligomers for 1:1 copolymer of EDOT and CDM were optimized using density functional theory. Ionization potentials, electron affinities, energy gaps, and band widths of the polymers were obtained by extrapolation. The lowest band gaps are calculated for copolymer poly-CDM-EDOT and poly-CDM. Band gaps of the polymers are found to agree well with the experimental values. When we combine two monomers (EDOT: electron-rich, CDM: electrondeficient) in one to one ratio and in different ratios in order to form co-oligomers, the highest occupied molecular orbitals of co-oligomers are averaged between the HOMO levels of parent homo-oligomers. Lowest unoccupied molecular orbitals of cooligomers are also averaged between the LUMO levels of parent homo-oligomers; however, averaged LUMO levels of co-oligomers are more close in energy to the LUMO level of CDM oligomer. We also found that poly-CDM and poly-CDMEDOT have very flat conduction bands. These narrow bands are attributed to the localized states, which decrease the mobility of n-type carriers in these bands consistent with the experimental findings. Overall, we concluded that donor/acceptor concept can be used to decrease the band gap while sacrificing the dispersion of valence and conduction bands. However, band broadening does not occur in alternating donor/acceptor polymers as donor/acceptor approach suggested.