Shunji Imamura

Photocarrier generation, injection, and trapping at the interface in a layered organic photoconductor: Metal‐free phthalocyanine/molecularly doped polymer

We have studied photocarrier generation and injection at the interface in a double‐layered organic photoconductor consisting of a charge generation layer (CGL) of vacuum‐deposited phthalocyanine film and a charge transport layer (CTL) of p‐diethylaminobenzaldehyde‐1,1‐diphenyl hydrazone doped polymer film. The photocarrier generation efficiency in the CGL was measured by the photoacoustic method. The transport and trapping of holes in the CTL and at the CGL/CTL interface were studied by xerographic discharge measurements and time‐of‐flight photoconductivity measurements. The photogeneration efficiency in the CGL is affected by the hydrazone concentration in the CTL. At high hydrazone concentrations, the photogeneration efficiency means the quantum efficiency of photocarrier generations controlled by geminate and nongeminate recombination in the CGL. At low hydrazone concentrations, the photogeneration efficiency is affected by the injection and trapping of holes at the CGL/CTL interface. Xerographic and t...

Reversible light-induced change in gap states in molecularly doped polymers studied by xerographic dark-discharge measurements

Abstract A new reversible light-induced change in gap states for charge emission in molecularly doped polymers is studied by time-resolved analysis of xerographic dark-discharge. In dark-rested films, the dark-discharge of the surface potential is due to the field-assisted thermal generation of holes from a discrete localized state. Long exposure to light decreases the saturated surface potential and increases the dark-discharge rate of the surface potential. During light exposure, the gap state profile of hole-generation centers is changed from a discrete localized state to continuous states with a broad energy distribution. The rest in the dark reverses the process thermally.

Xerographic studies of charge trapping in layered organic photoconductors

Charge trapping in layered organic photoconductors consisting of a charge generation layer (CGL) and a charge transport layer (CTL) was studied by xerographic residual potential measurements. The residual potential builds up during repeated corona charging and light exposure cycles of photoconductors. The negative residual potential is proportional to the square of the CGL thickness and is linearly proportional to the CTL thickness. The square dependence is caused by the bulk trapping of electrons in the CGL, and the linear dependence is due to the negative charges on the CTL surface. From experimental results, it is concluded that the electron trapping in the CGL enhances the trapping of holes at the CGL/CTL interface, and consequently a fraction of negative corona charges remains on the CTL surface even after light exposure. The buildup of the residual potential during xerographic cycling is initiated by the electron trapping in the CGL. On the other hand, the decay rate of the residual potential after ...

Carrier transport in amorphous chalcogenide multilayer structures

Abstract Coplanar transient photoconduction in multilayer structures of amorphous Se and Se1−xTex was studied by time-of-flight photoconductivity measurement. Optical absorption spectra of the multilayers show an excess absorption near the band tail compared with those of the single-layers of Se1−xTex. Transient photocurrent shows an anomalous two-peak shape which can be divided into two components. We concluded that one of the components, which is considered as a representative feature of photoconduction in multilayer structures, arises from the thermal release of carriers trapped at interfaces in multilayer structures.

Effect of polymer matrix on a reversible light-induced change in charge acceptance and retention of molecularly doped polymers

We have studied the effects of a polymer matrix on charge acceptance and retention of polymers doped with hole‐transport molecules in order to understand the nature of traps in molecularly doped polymers (MDPs). Prolonged illumination decreases charge acceptance and retention of MDPs. A light‐induced change in charge acceptance depends on the kind of polymer binder. A remarkable change is observed in acidic polymer binders or basic polymer binders exhibiting broad absorption spectra. In MDPs manifesting a remarkable light‐induced change, the transient photocurrent pulse shape becomes dispersive after prolonged light exposure. The experimental results show that the shallow charge emission centers increase during light exposure. These shallow charge emission centers are metastable states formed by charge transfer between the transport molecule and the polymer matrix during light exposure.

A photoacoustic study of photoinjection processes in double‐layered organic photoconductors

Photoinjection efficiencies of holes from the carrier generation layer (CGL) into the carrier transport layer (CTL) in double‐layered organic photoconductors were measured by using a photoacoustic technique. The photoinjection efficiency depends on the concentration of transporting molecules doped in the polymer of the CTL. At low molecular concentrations, the injection efficiency is mainly limited by the low ability to transport carriers in the CTL. The concentration dependence of the photoinjection efficiency is found to be similar to that of hopping transport processes in the CTL. At high molecular concentrations, the photoinjection efficiency is limited by the free‐carrier generation process in the CGL.

Measurement of photocarrier injection efficiencies at the interface in double layered organic photoconductors

Abstract Photocarrier injection efficiencies from the carrier generation layer into the transport layer in double-layered photoconductors were measured by using a photoacoustic technique. Photoinjection efficiencies were derived by measuring the decrease in photoacoustic signal when an electric field was applied to the samples. A linear relationship was observed between the logarithm of the photoinjection efficiencies and the square root of the applied electric field. Photocarrier injection efficiencies are limited by a barrier between the carrier generation layer and the carrier transport layer.

THREE‐PHOTON PHOTOELECTRIC EFFECT IN K3Sb

Cubic response in photoelectric emission from K3Sb illuminated by a Nd‐glass laser was investigated. Comparison of energy distribution of emitted electrons under the laser irradiation and monochromatic irradiation of natural light (3000–3500 A) showed that most of the photoelectrons excited by simultaneous absorption of three laser photons were changed in energy distribution by collision with valence electrons.

Photocarrier generation and injection at the interface in double‐layered organic photoconductors

Photocarrier generation and injection processes in double‐layered organic photoconductors consisting of carrier generation layer (CGL) and carrier transport layer (CTL) were studied by means of photoacoustic and xerographic time‐of‐flight methods. In the photoacoustic method, the photocarrier generation efficiency in the CGL was derived by measuring the decrease in photoacoustic signal due to carrier recombination in the CGL. On the other hand, in the xerographic method, the ratio of the number of holes emitted into the CTL to that of absorbed photons in the CGL was obtained from the temporal change in surface voltage under light pulse irradiation. From comparing two experimental results, we evaluated the injection efficiency of holes at the CGL/CTL interface. The injection efficiency is determined by the trapping of holes at the CGL/CTL interface.

Charge trapping in a layered organic photoconductor studied by photoacoustic and xerographic discharge measurements

The influence of charge trapping on photocarrier generation in a layered organic photoconductor consisting of a charge generation layer (CGL) and a charge transport layer was studied by means of photoacoustic and xerographic discharge techniques. The photocarrier generation efficiency in the CGL was derived by measuring the decrease in the photoacoustic (PA) signal when an electric field was applied to the sample. Prolonged light exposure under a high applied field increased the photocarrier generation efficiency and the negative xerographic residual potential. After the termination of the applied field, the PA signal reverted to the initial value very slowly. The recovery time of the PA signal after the termination of the applied field agreed well with the decay time of the xerographic residual potential. The increase in the photocarrier generation efficiency and the negative residual potential are due to the trapping of electrons in the CGL.