Hybrid Functional DFTB Parametrizations for Modeling Organic Photovoltaic Systems

Area of Science:

• Computational Chemistry
• Materials Science
• Organic Photovoltaics

Background:

• Density Functional Tight Binding (DFTB) is a computationally efficient quantum chemical method, but its application in organic photovoltaics (OPV) has been hindered by inadequate parameterization.
• Existing DFTB methods struggle with self-interaction errors in density functional theory (DFT) functionals that lack long-range correction, limiting their accuracy for OPV materials.

Purpose of the Study:

• To develop and validate new DFTB parametrizations specifically for OPV applications, addressing limitations of existing methods.
• To enable accurate prediction of ground- and excited-state properties, including charge-transfer mechanisms, in realistic OPV systems.

Main Methods:

• Developed new DFTB parametrizations using hybrid functionals (B3LYP, CAM-B3LYP) for elements crucial to OPV (H, C, N, O, F, S, Cl).
• Employed Bayesian optimization to refine unoccupied shell eigenenergies.
• Validated parametrizations against DFT references for 12 OPV molecules and investigated excited-state properties using real-time time-dependent DFTB (real-time TD-DFTB).

Main Results:

• The new DFTB parametrizations demonstrated consistent performance compared to DFT references for ground-state properties (geometries, frontier molecular orbitals).
• Real-time TD-DFTB simulations revealed charge-transfer (CT) excitations in dimers and explored the impact of alkyl side-chains on photoinduced CT.
• The developed parameters accurately capture electronic and repulsive interactions for key OPV elements.

Conclusions:

• The novel DFTB parametrizations significantly enhance the applicability of DFTB for simulating OPV materials.
• This work provides a reliable computational tool for studying complex phenomena in OPV, such as band alignments and charge-transfer dynamics at interfaces.
• The improved method facilitates the design and optimization of next-generation organic photovoltaic devices.