Coexistence of band-like and thermally activated charge transport through nuclear tunneling effect in organic semiconductors

Feng Xu

doi:10.1063/5.0047898

Abstract

In this study, the coexistence of band-like and thermally activated charge transport in organic semiconductors is demonstrated through the first-principles calculation of the electric field dependent charge mobilities of TIPS-pentacene, which strongly deviates from the semi-classical Marcus theory. The nuclear tunneling hopping rates simultaneously exhibit band-like and thermally activated conduction for the downward and upward hoppings, respectively, which explains the puzzling observation of the transition from thermally activated transport to band-like transport with increasing field strength. As the hopping rates are very sensitive to the site-energy fluctuations induced by defect configurations, the transition from band-like transport to thermally activated transport with increasing energetic disorder is found. The results suggest that the interplay of the quantum-mechanical effect and disorder is of particular importance in the understanding of the temperature and field dependence of mobility in organic materials.

Highlights

In order to explain the band-like charge transport where charge carriers are localized on at most a few molecules,18 both intermolecular and intramolecular mechanisms have been suggested
An often overlooked fact in the discussion of band-like conduction in organic semiconductors is its coexistence with thermal-activated conduction.33 6,13-bispentacene (TIPS-pentacene) is a rare example of such conduction
Its hole mobilities are band-like at high electric fields but thermally activated at low fields, while charges are found localized by optical charge modulation spectroscopy (OCMS) in both cases

Summary

Introduction

In order to explain the band-like charge transport where charge carriers are localized on at most a few molecules,18 both intermolecular and intramolecular mechanisms have been suggested. Models beyond Fermi’s golden rule (e.g., the second-order scitation.org/journal/adv expansion of the reduced density matrix approach) have been developed, which reproduce the experimental temperature dependence of charge mobilities of typical organic semiconductors and show that the Marcus approach leads to incorrect trends because it cannot account for tunneling effects dominating at low temperatures.31,32

Results

Conclusion