Vibronic exciton model for low bandgap donor-acceptor polymers

Area of Science:

• Materials Science
• Theoretical Chemistry
• Condensed Matter Physics

Background:

• Conjugated polymers with low bandgaps are crucial for optoelectronics.
• Understanding their excited state properties, including absorption spectra, is key to material design.

Purpose of the Study:

• To introduce and validate a vibronic exciton model for low bandgap donor-acceptor conjugated polymers.
• To elucidate the excited state band structure and absorption spectra, particularly the near-IR absorption band.

Main Methods:

• Development of a vibronic exciton model Hamiltonian in a diabatic basis.
• Inclusion of Frenkel-like, charge-transfer (CT), and vibronic couplings.
• Application of the model to the polymer PffBT4T-2DT.

Main Results:

• The model reveals J-aggregate behavior in donor-acceptor repeat units, enhanced in polymers.
• Significant enhancement of the 0-0 vibronic peak in the near-IR band due to J-aggregate behavior.
• Near-IR band comprises approximately equal contributions from CT and Frenkel-like excitations.
• Simulated absorption spectrum for PffBT4T-2DT quantitatively matches experimental data.

Conclusions:

• The vibronic exciton model accurately describes the excited state properties of low bandgap conjugated polymers.
• J-aggregate behavior significantly influences the optical properties, enhancing near-IR absorption.
• The model provides a framework for designing polymers with tailored optoelectronic characteristics.