Theory and models for electronic dynamics in organic type II semiconductors

University Of Houston, Houston TX

Investigators

Abstract

Eric Bittner from the University of Houston is supported in an award by the Chemical Theory, Models and Computational Methods program in the Division of Chemistry and by the Condensed Matter and Materials Theory program in the Division of Materials Research to conduct research in charge and energy transport in photo-excited materials of interest for solar energy conversion. The goal of this work is to understand the behavior of electrons, nuclei, and light interacting in organic polymers, particularly those where the repeating parts of the polymers have small defects or are strongly disordered or come into contact with other materials. This work is to gain insight and advance materials design in energy applications and devices, e.g., plastic devices that generate electricity from sunlight. While quantum mechanical processes in these systems occur on an ultra short (10^-15 s) time scale, they directly influence device behavior emerging on longer time scales (mesoscales). However, lack of detailed understanding and control of these mesoscale processes has thus far prevented researchers from fully exploiting these effects. To this end, this research seeks understanding of how emergent phenomena arise due to interactions beyond on longer time and length scales. The Bittner group's work in this field is the development of advanced theoretical methods for studying charge and energy transfer processes initiated by the absorption of photons. The electronic and optical properties of conjugated macromolecules are due to the delocalized pi-electron systems involving strong electron correlations and coupling of electrons to molecular structure in oorganic semiconductors. This project explores how quantum coherence and quantum entanglement both impact the initial charge separation following photoexcitation and help to suppress subsequent recombination of charges once they have formed. The theoretical approach combines the use of lattice models to describe the mesoscale structure of polymer/fullerene bulk heterojunction interafaces -- parameterized using ab initio and experimental data -- and time-convolutionless master equation methods to describe non-adiabatic interstate electronic transition as driven by phonons. This approach is being benchmarked against a series of model donor-bridge-acceptor molecules originally studied by Closs and Miller in their classic work that confirmed the Marcus inverted regime. Other components include developing theoretical models for excitons/phonon polaritons in organic aggregates. Work is continuing in polariton Bose Condensation in J-aggregatea, especially in light of recent experiments by Bulovic's group at MIT. An additional aspect of the work is in the photophysics of excitons in confined geometries, where one can exploit the Purcell effect to prepare states that are otherwise inaccessible to experimental probes.