Tomoya Sato

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.

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.

Electronic structures and chemical states of methylammonium lead triiodide thin films and the impact of annealing and moisture exposure

Methylammonium lead triiodide (CH3NH3PbI3) is the fundamental material used in perovskite solar cells, and its electronic properties have, therefore, attracted a great deal of attention as a potential key to highly efficient solar cell performance. However, the deterioration of perovskite solar cells when exposed to high temperature and humidity remains a serious obstacle to the materials use, and the clarification of the degradation mechanisms has been keenly anticipated. In this study, the valence electronic structures and depth-dependence of the chemical states of CH3NH3PbI3 thin films are investigated using ultraviolet photoelectron spectroscopy and excitation energy dependent X-ray photoelectron spectroscopy. Additionally, the effects of high temperature and a moisture rich atmosphere on the CH3NH3PbI3 thin films are examined. It is confirmed that the high temperature and moist atmosphere facilitate the oxidation of CH3NH3PbI3, whereas the Pb:I stoichiometry of the CH3NH3PbI3 thin films is found to ...

Density of states evaluation of an insulating polymer by high-sensitivity ultraviolet photoemission spectroscopy

Although the contact electrification of insulating polymers has been widely used in various technologies, the mechanism of electrification is still not well understood and several models have been proposed to explain the mechanism. Some of the models assume the existence of bandgap states that can store or release electrons to charge the polymer; however, the density of states in the bandgap region is not well examined. In this study, an approach to directly measure the density of state of insulating polymers using hν-dependent high-sensitivity ultraviolet photoemission spectroscopy is proposed. Demonstration of the approach to a representative insulating polymer, nylon-6,6, is reported with the estimation of the charge density and charge penetration depth as a function of the work function difference.

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.