Thermally Driven Polaron Transport in Conjugated Polymers

TL;DR

This paper models charge-polaron transport in conjugated polymers using a hybrid quantum-classical approach, revealing temperature-dependent diffusion and hopping mechanisms influenced by thermal fluctuations and Landau-Zener dynamics.

Contribution

It introduces a combined quantum-classical simulation framework for polaron transport, highlighting the role of thermal fluctuations and Landau-Zener processes in temperature-dependent dynamics.

Findings

At low T, polaron diffusion is activationless and quasidiabatic.

Rising T induces a crossover to activated hopping between diabatic states.

Transport mechanisms are governed by Landau-Zener type dynamics.

Abstract

We present a hybrid quantum-classical simulation of charge-polaron transport in conjugated polymers. The charge, which couples to the angular rotations of the monomers, is modeled via the time-dependent Schr\"odinger equation, while the monomers are treated classically via the Ehrenfest equations of motion. In addition, the system is thermalized by assuming that the monomers are subject to Brownian fluctuations modeled by the Langevin equation. Charge coupling to the monomer rotations localizes the particle into a Landau polaron, while the thermal fluctuations of the monomers causes polaron dynamics. The emergent low-energy scale of the model is the polaron reorganization energy, $E_r$, and thus $T_r = E_r/k_B$ is a convenient scale for the low-temperature dynamics. We investigate two types of dynamics -- both relevant for temperatures $T < T_r$. In the lower temperature regime the system remains in the same quasidiabatic state, corresponding to activationless polaron diffusion as the polaron crawls stochastically along the chain. As the temperature is raised, however, there is a cross-over to an additional activated transfer process which corresponds to hopping between diabatic states. We show that these processes exhibit Landau-Zener type dynamics. We note that as our model is general, it equally applies to exciton-polaron (i.e., energy) transport in conjugated polymers, and to charge and exciton polaron transport in quasi one-dimensional molecular stacks.