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Measuring the electron affinity of organic solids: an indispensable new tool for organic electronics

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Abstract

Electron affinity is a fundamental energy parameter of materials. In organic semiconductors, the electron affinity is closely related to electron conduction. It is not only important to understand fundamental electronic processes in organic solids, but it is also indispensable for research and development of organic semiconductor devices such as organic light-emitting diodes and organic photovoltaic cells. However, there has been no experimental technique for examining the electron affinity of organic materials that meets the requirements of such research. Recently, a new method, called low-energy inverse-photoemission spectroscopy, has been developed. A beam of low-energy electrons is focused onto the sample surface, and photons emitted owing to the radiative transition to unoccupied states are then detected. From the onset of the spectral intensity, the electron affinity is determined within an uncertainty of 0.1 eV. Unlike in conventional inverse-photoemission spectroscopy, sample damage is negligible and the resolution is improved by a factor of 2. The principle of the method and several applications are reported.

Energy level diagram of low-energy inverse photoemission spectroscopy, LEIPS (left). A beam of low-energy electrons with the kinetic energy E k is focused onto the sample surface, and photons hν emitted owing to the radiative transition to unoccupied states are detected. From the onset of the spectral intensity, the electron affinity E A is determined. The electron affinities of typical organic semiconductors determined using LEIPS (right).

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Notes

  1. In organic semiconductors, the valence states (unoccupied states) correspond to the valence bands (conduction bands) in inorganic semiconductors. Since the edges of the valence states (unoccupied states) originate from the highest occupied molecular orbital, HOMO, (the lowest unoccupied molecular orbital, LUMO) of the constituent molecules, they are often referred to the HOMO level (LUMO level).

  2. Strictly speaking, the energy released depends on the timescale. On the timescale of the processes in IPES, only the electronic process is usually considered.

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  1. Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan

    Hiroyuki Yoshida

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  1. Hiroyuki Yoshida
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Correspondence to Hiroyuki Yoshida.

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Yoshida, H. Measuring the electron affinity of organic solids: an indispensable new tool for organic electronics. Anal Bioanal Chem 406, 2231–2237 (2014). https://doi.org/10.1007/s00216-014-7659-1

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