Structure-Property Relationships in Amorphous Thieno[3,2-b]thiophene-Diketopyrrolopyrrole-Thiophene-Containing Polymers

We present calculations of electronic structure properties of disordered conducting polymers containing thieno[3,2b]thiophene, diketopyrrolopyrrole, and thiophene. Atomistic force field parameters for the polymer were optimized to minimize the difference between the ab initio and empirical potential energy surfaces and their corresponding first derivatives. These new force fields are employed to propagate the nuclear dynamics, and the equilibrium trajectories are sampled for subsequent electronic structure calculations. We found that the fluctuations of the bulk density of states are negligibly small and do not vary significantly with the length of the backbone and the side-chains. The localization length near the band gap is between 8 and 12 angstrom, which is about half of the length of the monomer and significantly less than the length of the extended polymer (similar to 200-400 angstrom). This indicates that the orbital localization is not affected by the length of the polymer. The inter-chain excitonic couplings are usually smaller than 5 meV, suggesting that the transport mechanism across chains is described by incoherent hopping, and excitons mainly move along the chain. Furthermore, thermal fluctuations cause the evolution of the excitons along the chain. Characterization of the relationships between the geometric disorder of the polymers and the distributions of the lowest excited states reveals that the low-energy excitons tend to localize in regions that are more planar and less folded. However, some excitons are also spread over defects. Thus, our theoretical calculations and the new force fields provide a direct route for characterizing the structure-property relationships and helpful information for constructing more realistic models for the exciton dynamics study of this class of polymeric materials.