Strain effects on the work function of an organic semiconductor.

Yanfei Wu,Hong Li,Chad Risko,Greg Haugstad,Geoffrey A Rojas,Sergei V Kalinin,Annabel R Chew,Gjergji Sini,Alberto Salleo,Jean-Luc Brédas,C Daniel Frisbie,Alex Belianinov

doi:10.1038/ncomms10270

Abstract

Establishing fundamental relationships between strain and work function (WF) in organic semiconductors is important not only for understanding electrical properties of organic thin films, which are subject to both intrinsic and extrinsic strains, but also for developing flexible electronic devices. Here we investigate tensile and compressive strain effects on the WF of rubrene single crystals. Mechanical strain induced by thermal expansion mismatch between the substrate and rubrene is quantified by X-ray diffraction. The corresponding WF change is measured by scanning Kelvin probe microscopy. The WF of rubrene increases (decreases) significantly with in-plane tensile (compressive) strain, which agrees qualitatively with density functional theory calculations. An elastic-to-plastic transition, characterized by a steep rise of the WF, occurs at ∼0.05% tensile strain along the rubrene π-stacking direction. The results provide the first concrete link between mechanical strain and WF of an organic semiconductor and have important implications for understanding the connection between structural and electronic disorder in soft organic electronic materials.

Highlights

Establishing fundamental relationships between strain and work function (WF) in organic semiconductors is important for understanding electrical properties of organic thin films, which are subject to both intrinsic and extrinsic strains, and for developing flexible electronic devices
In soft organic semiconductors that serve as the active components in many organic electronic devices[8,9,10,11,12], tensile and compressive strains modify the material electronic properties and function
Our measurements focus on p-type rubrene single crystals, which serve as a model material platform for many fundamental studies of organic semiconductor physics due to their exemplary transport properties, that is, the highest reproducible chargecarrier mobilities to date have been achieved in single crystal rubrene field-effect transistors[26]

Summary

Introduction

Establishing fundamental relationships between strain and work function (WF) in organic semiconductors is important for understanding electrical properties of organic thin films, which are subject to both intrinsic and extrinsic strains, and for developing flexible electronic devices. In soft organic semiconductors that serve as the active components in many organic electronic devices[8,9,10,11,12], tensile and compressive strains modify the material electronic properties and function. By adhering thin rubrene crystals onto substrates with coefficients of thermal expansion (CTEs) distinctly different from rubrene and varying the temperature, we systematically induce large and controlled tensile or compressive strains in rubrene crystals and quantify the elastic portion by X-ray diffraction. The corresponding WF of rubrene, measured by temperature-dependent scanning Kelvin probe microscopy (SKPM), is found to increase (decrease) with the in-plane tensile (compressive) strain. We find that the onset of tensile plastic strain leads to even larger increases of WF with strain These findings constitute a definitive link between structural deformation and electronic disorder in a model organic semiconductor

Methods

Results

Conclusion