A Protocol for Fast Prediction of Electronic and Optical Properties of Donor–Acceptor Polymers Using Density Functional Theory and the Tight-Binding Method

The ability of donor–acceptor
(D–A) type polymers
to control the positions of the highest occupied (HOMO) and lowest
unoccupied (LUMO) molecular orbitals makes them a popular choice for
organic solar cell applications. The alternating D–A pattern
in a monomer leads to a weak electronic coupling between the constituent
monomers within the polymer chain. Exploiting the weak electronic
coupling characteristics, we developed a method to efficiently calculate
(1) the electronic properties and (2) the optical gap of such polymer
chains. The electronic properties (HOMO and LUMO energies, ionization
potential, electron affinity, and quasiparticle gap of an oligomer
of any length up to an infinitely long polymer) of the D–A
polymers are predicted by combining density functional theory calculation
results and a tight-binding model. The weak electronic coupling implies
that the optical gap of the polymer is size-independent, and thus,
it can be calculated using a monomer. We validated the methods using
a set of 104 polymers by checking the consistency where the electronic
gap of a polymer is larger than the optical gap. Furthermore, we establish
relationships between the results obtained from more accurate, yet
slower methods (i.e., B3LYP functional, singlet-ΔSCF) with those
obtained from the faster counterparts (i.e., BLYP functional, triplet-ΔSCF).
Leveraging the found relationships, we propose a way in which the
electronic and optical properties of the polymers can be calculated
efficiently while retaining high accuracy. The use of the tight-binding
model combined with the approach to estimate more accurate results
based on less expensive simulations is crucial in the applications
where a large volume of computations needs to be carried out efficiently
with sufficiently high accuracy, such as high-throughput computational
screening or training a machine-learning model.