Hybrid Functional DFTB Parametrizations for Modeling Organic Photovoltaic Systems

Density functional tight binding (DFTB) is a quantum
chemical simulation
method based on an approximate density functional theory (DFT), known
for its low computational cost and comparable accuracy to DFT. For
several years, the application of DFTB in organic photovoltaics (OPV)
has been limited by the absence of an appropriate set of parameters
that adequately account for the relevant elements and necessary corrections.
Here we have developed new parametrizations using hybrid functionals,
including B3LYP and CAM-B3LYP, for OPV applications within the DFTB
method in order to overcome the self-interaction error present in
DFT functionals lacking long-range correction. These parametrizations
encompass electronic and repulsive parameters for the elements H,
C, N, O, F, S, and Cl. A Bayesian optimization approach was employed
to optimize the free atom eigenenergies of unoccupied shells. The
effectiveness of these new parametrizations was evaluated by a data
set of 12 OPV donor and acceptor molecules, showing consistent performance
when compared with their corresponding DFT references. Frontier molecular
orbitals and optimized geometries were examined to evaluate the performance
of the new parametrizations in predicting ground-state properties.
Furthermore, the excited-state properties of monomers and dimers were
investigated by means of real-time time-dependent DFTB (real-time
TD-DFTB). The appearance of charge-transfer (CT) excitations in the
dimers was observed, and the influence of alkyl side-chains on the
photoinduced CT process was explored. This work paves the way for
studying ground- and excited-state properties, including band alignments
and CT mechanisms at donor–acceptor interfaces, in realistic
OPV systems.