Structure–Property Relationships in Amorphous Thieno[3,2‑<i>b</i>]thiophene–Diketopyrrolopyrrole–Thiophene-Containing Polymers

We present calculations
of electronic structure properties of disordered
conducting polymers containing thieno­[3,2-b]­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 Å, which is about half of the length of the monomer
and significantly less than the length of the extended polymer (∼200–400
Å). 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.