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.

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Published in Nature Communications, v. 7, article no. 10270, p. 1-9.

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This work was primarily supported by the National Science Foundation under Grant No. DMR-0706011. Part of this work was carried out in the Characterization Facility, University of Minnesota, which received partial support from NSF through the MRSEC program under Grant No. DMR-1420013.

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Supplementary Information: Supplementary Figures 1-3, Supplementary Tables 1-3, Supplementary Methods and Supplementary References