Hiroumi Kinjo

Influence of the direction of spontaneous orientation polarization on the charge injection properties of organic light-emitting diodes

A tris(7-propyl-8-hydroxyquinolinato) aluminum [Al(7-Prq)3] film shows negative giant surface potential (GSP) because of spontaneous orientation polarization. The polarity of this film is opposite to those of tris-(8-hydroxyquinolate) aluminum films. In Al(7-Prq)3-based organic light-emitting diodes, negative GSP leads to the positive interface charge and governs the electron injection and accumulation properties. In addition, a high resistance to the electron injection at the Al(7-Prq)3/Ca interface is suggested possibly because of the negative polarization charge at the interface. These results show the importance of orientation polarization in controlling the charge injection and accumulation properties and potential profile of the resultant devices.

Determination of the highest occupied molecular orbital energy of pentacene single crystals by ultraviolet photoelectron and photoelectron yield spectroscopies

The electronic structures of pentacene single crystals (SCs) were elucidated by ultraviolet photoelectron spectroscopy (UPS) and photoelectron yield spectroscopy (PYS). An asymmetric HOMO peak profile of the pentacene SCs obtained by UPS exhibits a close similarity to the k-projected density-of-states of the valence band that has been predicted by a theoretical calculation [H. Yoshida and N. Sato, Phys. Rev. B 77, 235205 (2008)]. The ionization energy of the pentacene SCs is successfully determined to be 4.95 (± 0.03) eV which is evidently greater than that of the bulk films of pentacene [4.90 (± 0.02) eV].

Significant relaxation of residual negative carrier in polar Alq3 film directly detected by high-sensitivity photoemission

Tris(8-hydroxyquinoline)aluminum (Alq3) has been widely applied as a good electron-injecting layer (EIL) in organic light-emitting diodes. High-sensitivity photoemission measurement revealed a clear photoemission by visible light, although its ionization energy is 5.7 eV. This unusual photoemission is ascribed to Alq3 anions captured by positive polarization charges. The observed electron detachment energy of the anion was about 1 eV larger than the electron affinity reported by inverse photoemission. This difference suggests that the injected electron in the Alq3 layer is energetically relaxed, leading to the reduction in injection barrier. This nature is one of the reasons why Alq3 worked well as the EIL.

Effects of the ambient exposure on the electronic states of the clean surface of the pentacene single crystal

ABSTRACT Understanding of the electronic structures is indispensable for complete elucidation of the charge carrier behaviors in organic semiconductors. Although recent progress enabling accurate photoemission demonstrations of organic single crystals has greatly promoted such understanding, it had been achieved merely on partially oxidized surfaces by exposure to ambient conditions. In this study, we successfully prepared an oxide-free surface of the pentacene single crystal (PnSC) by cleavage in vacuum. X-ray and ultraviolet photoelectron spectroscopy measurements on the PnSC clean surface revealed improved energetic homogeneity of the C1s level and highest-occupied state in comparison to those of the partially oxidized surface.

Photoelectron Yield Spectroscopy for Organic Materials and Interfaces

Photoelectron yield spectroscopy (PYS), in which total photoelectron yield is recorded as a function of incident photon energy, has been widely applied to determine the ionization energy of various organic electronic materials. PYS has some advantage complimentary to conventional photoelectron spectroscopy; (i) measurement environment is not limited to vacuum, (ii) sample charge-up problem is practically negligible, (iii) high sensitivity is available in vacuum, and so on. Thus, PYS is a powerful method to explore the electronic structures of organic materials and interfaces in practical situation. In this chapter, first we describe the basic principle and experimental setup of PYS. Then the applications to various organic materials and interfaces are described with the results of combined application of PYS and high sensitivity photoemission spectroscopy.

1015 cm−3 eV−1 level detection of density of states of a p-type polymer by hν-dependent high-sensitivity ultraviolet photoemission spectroscopy

We propose a method, called h?-dependent high-sensitivity ultraviolet photoemission spectroscopy, to observe the density of states (DOS) in a very wide range from HOMO to extremely weak gap states (1022 to 1015 cm?3 eV?1 in density of states). The method was applied to a p-type semiconducting polymer. A series of spectra for h? = 4.4?7.7 eV were recorded, and the DOS was obtained by overlapping the spectral part with a similar line shape between adjacent photon energy spectra to eliminate the photon energy dependence of the photoionization cross section. This method can be applied to both organic and inorganic materials, providing useful information about the DOS of functional materials.

Energetic Relaxation of Anions in OLED Films Directly Observed by High Sensitivity Photoemission: Impact of Orientation Polarization on Electron Injection

OLED-related materials often show orientation polarization in evaporated films. High sensitivity photoemission revealed that these positive polarization charges energetically stabilize the anions. This indicates that orientation polarization is significant to improve electron injection.

Electronic Structure of Chlorophyll a Solution Investigated by Photoelectron Yield Spectroscopy

Various bio-related processes are driven by electron transfer reactions. Therefore the electronic structures of bio-molecules in their living environment are keys of their functionalities. One significant example photosynthesis which has attracted much attention due to urgent necessity of clean energy source. In this study, we carried out photoelectron yield spectroscopy (PYS) measurements to demonstrate the electronic structures of oligomerizedChl-a molecules, which is known as an essential reaction center of the photosystem in general green plants, under the atmospheric environment. The ionization energies of the Chl-a aggregates are successfully derived.