Crossover from Hopping to Band-Like Charge Transport in an Organic Semiconductor Model: Atomistic Nonadiabatic Molecular Dynamics Simulation.

Samuele Giannini,Antoine Carof,Jochen Blumberger

doi:10.1021/acs.jpclett.8b01112

Abstract

The mechanism of charge transport (CT) in a 1D atomistic model of an organic semiconductor is investigated using surface hopping nonadiabatic molecular dynamics. The simulations benefit from a newly implemented state tracking algorithm that accounts for trivial surface crossings and from a projection algorithm that removes decoherence correction-induced artificial long-range charge transfer. The CT mechanism changes from slow hopping of a fully localized charge to fast diffusion of a polaron delocalized over several molecules as electronic coupling between the molecules exceeds the critical threshold V ≥ λ/2 (λ is the reorganization energy). With increasing temperature, the polaron becomes more localized and the mobility exhibits a “band-like” power law decay due to increased site energy and electronic coupling fluctuations (local and nonlocal electron–phonon coupling). Thus, reducing both types of electron–phonon coupling while retaining high mean electronic couplings should be part of the strategy toward discovery of new organics with high room-temperature mobilities.

Highlights

F orming the active layers of organic field effect transistor, light-emitting diodes, and organic photovoltaics, organic semiconductors (OSs) have enabled disruptive technologies in the plastics electronics industry and in the renewable energy sector
Propagating the carrier wave function along a chain of sites described by a model Hamiltonian, both predicted a decrease in mobility with increasing temperature in the large electronic coupling regime, similar to polaronic band-like transport but without assuming delocalized carriers
While Troisi’s work emphasized the crucial role of thermal fluctuations of electronic coupling,[14,21] Wang concluded that thermal site energy fluctuations are the major source of polaron localization and mobility decay in this regime.[15]

Summary

The Journal of Physical Chemistry Letters

Letter nuclear degrees, was treated (no feedback,[14,17] Ehrenfest,[20] fewest switches surface hopping (FSSH)[15,16]). Simulation details and force field parameters, determination of diffusion constants and fitting equations, one figure showing MSD and IPR time series for different temperatures and coupling regimes, discussion of the influence of the decoherence time on charge mobility along with one figure comparing various decoherence times and one figure on the internal consistency of the method, master equation model and employed rate constant, and site energy and electronic coupling fluctuations alongside a figure showing such fluctuations for high coupling and two different temperatures as well as a table summarizing these latter results (PDF)

■ ACKNOWLEDGMENTS

■ REFERENCES