Principle and application of low energy inverse photoemission spectroscopy: A new method for measuring unoccupied states of organic semiconductors

Abstract

Information about the unoccupied states is crucial to both fundamental and applied physics of organic semiconductors. However, there were no available experimental methods that meet the requirement of such research. In this review, we describe a new experimental method to examine the unoccupied states, called low-energy inverse photoemission spectroscopy (LEIPS). An electron having the kinetic energy lower than the damage threshold of organic molecules is introduced to a sample film, and an emitted photon in the near-ultraviolet range is detected with high resolution and sensitivity. Unlike the previous inverse photoemission spectroscopy, the sample damage is negligible and the overall resolution is a factor of two improved to 0.25 eV. Using LEIPS, electron affinity of organic semiconductor can be determined with the same precision as photoemission spectroscopy for ionization energy. The instruments including an electron source and photon detectors as well as application to organic semiconductors are presented. Keywords low-energy inverse photoemission spectroscopy; unoccupied state; electron affinity; organic semiconductor; © 2015. This manuscript version is made available under the Elsevier user license

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High‐Performance Solar Cells

“…Inverse photoemission spectroscopy has been used for this purpose, although the high‐energy electron beam can damage organic materials, preventing accurate evaluation of the EA. Recently, Yoshida developed low‐energy inverse photoemission spectroscopy (LEIPS), which can be used to measure the EA of organic semiconductors more accurately . LEIPS prevents damage by reducing the beam energy to less than 5 eV, which leads to high sensitivity measurements because emission in the near‐UV region is detected.…”

Complete description of ionization energy and electron affinity in organic solids: Determining contributions from electronic polarization, energy band dispersion, and molecular orientation

“…Recently, we have developed an experimental method called low-energy inverse photoemission spectroscopy (LEIPS) [24] which is an advanced version of the inverse photoemission spectroscopy (IPES). We have so far determined A s of several organic materials with a precision of 0.1 eV [24][25][26][27][28][29][30][31][32]. Further, we have demonstrated that the precisely determined A s as well as I s allows us to distinguish the contribution of the polarization energy to the energy level change induced by the thermal crystallization of organic film of [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) [33].…”

Electron Transport in Bathocuproine Interlayer in Organic Semiconductor Devices

“…[24][25][26] In this technique, electrons having a kinetic energy below 5 eV are introduced to the sample surface, and photons emitted due to the radiative transition to the unoccupied states are detected.…”