Analysis of external and internal disorder to understand band-like transport in n-type organic semiconductors

TL;DR

This paper investigates how internal and external disorder affect charge transport in n-type organic semiconductors, demonstrating that molecular design can enable band-like transport despite disorder influences.

Contribution

It provides a comprehensive experimental and theoretical analysis showing that tailored molecular design can mitigate disorder effects and promote band-like transport in n-type organic semiconductors.

Findings

Internal dynamic disorder causes transient localization of charge carriers.

External disorder can be controlled by dielectric properties of the gate insulator.

Molecular design can reduce disorder, enabling band-like transport.

Abstract

Charge transport in organic semiconductors is notoriously extremely sensitive to the presence of disorder, both internal and external (i.e. related to the interactions with the dielectric layer), especially for n-type materials. Internal dynamic disorder stems from large thermal fluctuations both in intermolecular transfer integrals and (molecular) site energies in weakly interacting van der Waals solids and sources transient localization of the charge carriers. The molecular vibrations that drive transient localization typically operate at low-frequency (< a-few-hundred cm-1), which renders it difficult to assess them experimentally. Hitherto, this has prevented the identification of clear molecular design rules to control and reduce dynamic disorder. In addition, the disorder can also be external, being controlled by the gate insulator dielectric properties. Here we report on a comprehensive study of charge transport in two closely related n-type molecular organic semiconductors using a combination of temperature-dependent inelastic neutron scattering and photoelectron spectroscopy corroborated by electrical measurements, theory and simulations. We provide unambiguous evidence that ad hoc molecular design enables to free the electron charge carriers from both internal and external disorder to ultimately reach band-like electron transport.