Phonon-Induced Localization of Excitons in Molecular Crystals from First Principles

Area of Science:

• Solid-state physics
• Materials science
• Quantum chemistry

Background:

• Exciton spatial extent is crucial for molecular system photophysics and optoelectronics.
• Phonons influence exciton localization and delocalization, but microscopic mechanisms remain unclear.
• Understanding exciton-phonon coupling and nuclear fluctuations is key.

Purpose of the Study:

• Investigate phonon-induced exciton (de)localization in solid pentacene.
• Clarify the formation of localized exciton states and the role of vibrations.
• Determine the impact of quantum and thermal nuclear fluctuations on exciton behavior.

Main Methods:

• First-principles calculations using density functional theory (DFT).
• Ab initio GW-Bethe-Salpeter equation (GW-BSE) for electronic excitations.
• Finite-difference and path integral techniques for nuclear motion and anharmonicity.

Main Results:

• Zero-point nuclear motion uniformly localizes excitons in pentacene.
• Thermal motion adds localization for Wannier-Mott-like excitons.
• Anharmonic effects drive temperature-dependent localization, hindering delocalization.

Conclusions:

• Exciton localization is dominant in pentacene due to nuclear motion.
• Anharmonicity plays a significant role in temperature-dependent exciton behavior.
• Conditions for achieving delocalized excitons require further exploration.