Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions

John E. Anthony,Alessandro Troisi,Guillaume Schweicher,Steffen Illig,Lang Jiang,Henning Sirringhaus,Yves Henri Geerts,Stephen G. Yeates,Mark Nikolka,Alexander S. Eggeman,Chris Warwick

doi:10.1038/ncomms10736

Abstract

Thermal vibrations and the dynamic disorder they create can detrimentally affect the transport properties of van der Waals bonded molecular semiconductors. The low-energy nature of these vibrations makes it difficult to access them experimentally, which is why we still lack clear molecular design rules to control and reduce dynamic disorder. In this study we discuss the promising organic semiconductors rubrene, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothio-phene and 2,9-di-decyl-dinaphtho-[2,3-b:20,30-f]-thieno-[3,2-b]-thiophene in terms of an exceptionally low degree of dynamic disorder. In particular, we analyse diffuse scattering in transmission electron microscopy, to show that small molecules that have their side chains attached along the long axis of their conjugated core are better encapsulated in their crystal structure, which helps reduce large-amplitude thermal motions. Our work provides a general strategy for the design of new classes of very high mobility organic semiconductors with a low degree of dynamic disorder.

Highlights

Thermal vibrations and the dynamic disorder they create can detrimentally affect the transport properties of van der Waals bonded molecular semiconductors
It has been shown that charge carriers in these systems can be delocalized over a small number of molecules, resulting in narrow bands of spatially extended electronic states that allow for a far more efficient charge transport mechanism than charge hopping between molecularly localized orbitals[1]
Being a fingerprint of thermal vibrations, the diffuse features clearly weaken at low temperature measurements[13]

Summary

Introduction

Thermal vibrations and the dynamic disorder they create can detrimentally affect the transport properties of van der Waals bonded molecular semiconductors. The first step towards a high-mobility organic semiconductor is to start with a system that has an intrinsically high transfer integral J, allowing the charge carrier to form delocalized states[2]. It is believed that in the highest mobility systems available to date, device optimization has been pushed to an extent that wave function delocalization is not limited any more by static charge carrier traps but by thermal fluctuations in the transfer integrals, which is referred to as dynamic disorder[6,7]. An increasing number of high-performance organic semiconductors have been reported in the literature to exhibit a band-like temperature dependence of the mobility near room temperature[8,9] This indicates that dynamic disorder is already one of the major factors limiting charge transport. Dynamic disorder can be reduced by increasing the force constants between molecules, which helps to avoid large-amplitude thermal vibrations in the first place[10]

Methods

Results

Conclusion