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Universal energy-level alignment of molecules on metal oxides

Nature Materials volume 11, pages 76–81 (2012)Cite this article

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Abstract

Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides—representing a broad diversity in electronic properties—with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.

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Figure 1: Energy levels at oxide/molecule interfaces.
Figure 2: Universal energy-level alignment trends.
Figure 3: Valence bands of transition-metal oxide classes.
Figure 4: Energy levels of transition-metal oxides and organic semiconductors.
Figure 5: Origin of energy-level-alignment trend.
Figure 6: Tuning energy alignment through electron chemical potential.

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Change history

  • 01 December 2011

    In the version of this Article originally published online, equation (2) was incorrect. These errors have been corrected in all versions of the Article.

  • 06 December 2011

    In the version of this Article originally published online, equation (2) was incorrect and the parameter definitions incomplete. These errors have been corrected in all versions of the Article.

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Acknowledgements

We thank J. Nogami, C. Mims and G. Walker for their helpful advice. We also thank D. Grozea for his technical assistance, and E. Nolan for his help editing the manuscript. We would also like to thank R. D’Souza-Greiner for proofreading the manuscript. The financial support for this research was provided by the NRC-NSERC-BDC Nanotechnology Initiative (NNBPJ364261-07). Z-H.L. is the Canada Research Chair in Organic Optoelectronics, Tier I.

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Authors and Affiliations

  1. Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto, Ontario M5S 3E4, Canada

    Mark T. Greiner, Michael G. Helander, Wing-Man Tang, Zhi-Bin Wang, Jacky Qiu & Zheng-Hong Lu

  2. Department of Physics, Yunnan University, 2 Cuihu Beilu, Yunnan, Kumming 650091, China

    Zheng-Hong Lu

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  1. Mark T. Greiner
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Contributions

M.T.G. planned the study, carried out XPS and UPS measurements and wrote the manuscript. M.G.H. assisted in XPS and UPS experiments, as well as method validation, and participated in discussion. W-M.T. prepared samples and assisted with atomic force microscopy measurements. Z-B.W. and J.Q. took part in discussion and assisted in sample preparation. Z-H.L. supervised the research, and gave direction in writing the manuscript.

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Correspondence to Mark T. Greiner.

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Greiner, M., Helander, M., Tang, WM. et al. Universal energy-level alignment of molecules on metal oxides. Nature Mater 11, 76–81 (2012). https://doi.org/10.1038/nmat3159

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