Measuring Energy Gaps of Organic Semiconductors by Electron Energy Loss Spectroscopies

Abstract

Herein is explored the use of electron energy loss due to π–π* transition to measure energy gaps of organic semiconductors. Sources of kinetic electrons studied include an external electron gun (reflection electron energy loss spectroscopy (REELS)) and an internal core shell excitation by X‐rays (i.e., X‐ray photoelectron spectroscopy (XPS)). To obtain the bandgap accurately, a data analysis method is proposed to extract the optical bandgap from the π–π* inelastic electron energy loss spectra. This method uses a Gaussian function to fit experimental data yielding a peak width w and position E0. The energy gap of an organic semiconductor, i.e., the onset of the π–π* transition peak, can then be calculated by . Through examination of 11 organic semiconductors, it is found that the bandgaps measured by REELS agree well with optical bandgaps. The bandgaps measured by XPS, however, do not always agree with optical gaps. This indicates that the XPS π–π* inelastic peak in some material systems may convolute with other core shell processes such as shake‐up. In addition, it is shown that all key energy structures of an organic semiconductor can be measured concurrently by REELS and ultraviolet photoemission spectroscopy.