New Experimental Method to Precisely Examine the LUMO Levels of Organic Semiconductors and Application to the Fullerene
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New Experimental Method to Precisely Examine the LUMO Levels of Organic Semiconductors and Application to the Fullerene Derivatives Hiroyuki Yoshida1,2 Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.
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JST PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan.
E-mail: [email protected] ABSTRACT Inverse-photoemission spectroscopy (IPES) in the near-ultraviolet range is a new tool for investigating the LUMO levels of organic materials. Previous IPES methods have had two serious weaknesses, i.e. low energy resolution and sample damage to organic materials. In the present method, on the other hand, the irradiation damage to the organic sample is significantly reduced by decreasing the kinetic energy of electrons below the damage threshold. The energy resolution of the instrument is improved by a factor of two to 0.3 eV by using multilayer band pass filters. Acceptor materials widely used in organic photovoltaic cells, C60 and phenyl-C61-butyric acid methyl ester (PC61BM), are measured with this new technique to determine the electron affinities. INTRODUCTION In the research of organic photovoltaic cells, lack of precise information about the lowest-unoccupied molecular orbital (LUMO) energy levels of acceptor materials is an important issue, since the open circuit voltage correlates with the difference in energy levels between the highest occupied molecular orbital (HOMO) level of the donor and the LUMO level of the acceptor [1,2]. Although the HOMO levels have been extensively examined by photoemission spectroscopy (PES), little is known about the LUMO levels of organic materials, impeding further analysis of the charge separation, electron transport and electron collection mechanisms. The LUMO levels of organic materials have been determined by cyclic voltammetry, from the optical gap added to the HOMO level, and inverse photoemission spectroscopy (IPES). The reduction potential determined in solution may be different from that in solid. The optical gap is smaller by 0.2 up to 1 eV [3, 4] making the estimated electron affinities larger than the true values. IPES is therefore an ideal tool to determine the LUMO levels because it examines the electron transport levels in the solid samples. IPES can be regarded as the time reversed process of PES. The electrons are introduced to the sample surface and the photons emit by the radiative transition to the unoccupied states. The LUMO levels of solid samples are determined from the photon intensity at different electron
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or photon energies. The large drawback of IPES is its low cross section. According to the theoretical study, the cross section of IPES is 3-5 orders of magnitude smaller than that of PES [5].
Thus the photon intensities are typically very low and high incident electron fluxes are
required. Almost all of the IPES measurements on organic materials are carried out in the vacuum ultraviolet range using a bandpass photon detector first developed by Dose [6,7]. In this experimental setup, 10 eV photons are
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