Akiko Hirao

Effect of polymer matrices on charge transport in molecularly doped polymers

Hole mobilities have been measured in diethylamino‐benzaldehyde‐diphenylhydrazone (DEH)‐doped polymers as a function of the polymer composition, temperature, and electric field. It was found that the electric field dependence on the mobility was a strong function of the binder polymer employed. In all cases, the electric field dependence E can be described as exp(βE1/2T−1). β2 is related to the density of photoinduced spins in the molecularly doped polymers, which are related to the interaction between DEH and the polymer.

Relationship between Mobility and Diffusion Coefficient in Molecularly Doped Polymers

A theoretical photocurrent transient equation for a thin or thick sample was derived from the charge transport equation. This equation has been fitted to measured photocurrent transients of molecularly doped polymers in order to obtain the actual relationship between the mobility (µ) and the diffusion coefficient (D). The relationship between the mobility and the diffusion coefficient has been derived from the Langevin equation which takes into account the random electric field resulting from the randomly located and the randomly oriented electric dipoles. A time correlation function of the fluctuating electric field necessary for calculating the diffusion coefficient is assumed to be 2σE2exp (-|t|/τ), where σE is the dispersion of the fluctuating electric field, which is proportional to the electric dipole moment, and τ is the correlation time of the electric field fluctuation. The result, D=τσE2 µ2, was consistent with the actual relation, D∝µ2.

25.4: High-Brightness Large-Area White OLED Fabricated by Meniscus Printing Process

We demonstrated high-brightness large-area, white organic light-emitting diode (OLED) consisting of printing-processed organic semiconductor layers. Meniscus printing process was applied to the substrate with 2 μm-high stripe-shape auxiliary electrodes. The OLED panel showed white emission all over the whole emitting area of 58 mm × 52 mm, high average luminance of 10,000 cd/m2, luminance uniformity of 40 %, and high luminous flux of 95 lm.

Photoinduced electron transfer and electron-mediating systems from aromatic amines to triplet states of C60 and C70 in the presence of a viologen dication

Photoinduced electron transfer between fullerenes (C60 and C70) and various aromatic amines (AAs) in the absence and presence of a viologen dication has been studied by the transient absorption method in the visible and near-IR regions. Electron-transfer takes place from AAs to the triplet states of fullerenes (3C60* and 3C70*) giving the anion radicals of fullerenes (C60*- and C70*-) and the radical cations of AAs (AA*+). The rate constants and efficiencies of electron transfer are quite high, because of the high electron-donor abilities of AAs as their low oxidation potentials indicate. The absorption bands of AA*+ appeared also in the near-IR region indicating that the radical-cation center (hole) delocalizes over the entire region of each AA. On addition of an octylviologen dication (OV2+) to C60/C70-AA systems, the electron-mediating process from C60*- and C70*- to OV2+ occurs yielding the viologen radical cation (OV*+) with longer lifetime.

Writing a grating in the photorefractive polymers with no applied field

Writing a refractive index grating by anomalous carrier diffusion without applying an external electric field has been achieved in a photorefractive polymer whose D/μ value is larger than that derived from Einsteins law. A grating was written in a photorefractive polymer by anomalous diffusion.

Carrier generation in fullerenes

The efficiency of carrier generation by the zerographic discharge technique has been measured on four films; a molecularly C60-doped polymer film, a molecularly C70-doped polymer film, an aggregate C60 film, and an aggregate C70 film. The yield for molecularly C70-doped polymer was three orders of magnitude larger than that for the film of molecularly C60-doped polymer. The photon energy dependence of efficiency for the aggregate films differed from that for the molecularly doped polymer films. The carrier generation efficiency spectrum for the aggregate films showed peaks corresponding to the Frenkel type excitons. The sensitivity of a photoreceptor fabricated with C70 was 0.5(cm2/(mu) J) at wavelengths in the 420nm-670nm region.

New method for obtaining drift mobility and diffusion coefficient and their relation in photorefractive polymers

A new easy method for obtaining a drift mobility and a diffusion coefficient from a nondispersive time-of-flight transient has been developed. Nondispersive transients are described well in the theoretical photocurrent equation (PTE) based on the fact that a carrier packet drifts at a constant velocity and is spread by diffusion, the top electrode acts as a reflecting and partially absorbing wall, and the counter electrode acts as an absorbing wall. The fitting of the PTE to photocurrent transients gives the mobility and the diffusion coefficient (D) simultaneously. These are suitable characteristic values for descriptions of carriers transport because they do not show the thickness dependence and the negative field dependence in a low electric field. The mobility that sometimes shows the thickness dependence and the negative field dependence in a low electric field, however, has usually been measured from the time of the intersection of the asymptotes to the plateau and trailing edge of the transients. In order to obtain (mu) a from photocurrent transients by a simple method, we have tried to describe t0 and tail-broadening parameter W as functions of (mu) a and D, where W is defined as (t1/2 - t0)/t1/2 and t1/2 is the time at which the current is a half of that in the plateau region. The dependences of calculated (mu) k and W on the electric field and the sample thickness agreed well with those of the experimental data. These results verify the PTE and suggest that (mu) a and D can be calculated from t0 and W. We also report that the diffusion coefficient is proportional to the power of 2 of the mobility. This result agrees with a theory based on the Langevin equation which describes motions of carriers in a fluctuated field.

Mobility enhancement by adding fullerene C70 to molecularly doped polymers

Saiwai-ku, Kawasaki 210-8582, JAPAN The drift mobility and the diffusion coefficient of molecularly doped polymers (MDPs) are measured by fitting the theoretical equation to the observed time—of—flight photocurrent transient. The 70% mobility enhancement is observed when 0.2 wt.% fullerene C70 is added to the diphenylamino— benzaldehyde diphenyl hydrazone (DPH) doped polystyrene. We consider this is due to the interaction between C0 and DPH which form the charge transfer complex. Keywords: molecularly doped polymer, diffusion, drift mobility, time—of—flight experiment.

Persistent Enhanced Conductivity Induced by Light Irradiation in Hydrazone-Polycarbonate Dispersions

Photoinduced spins with extended lifetime have been observed in hydrazone-polycarbonate dispersions. The presence of the spins changed the electrical characteristics of the dispersions. Measurements of electrical current associated with thermal equilibration after photoexcitation in dispersions of diethylamino-benzaldehyde diphenylhydrazone (DEH) in polycarbonate have been carried out. Sandwiched layers of the dispersions were excited with absorbed light. After withdrawal of the light, a voltage was applied and the current was measured. Experimental data provide evidence that photoinduced spins are derived from trapped carriers.

Effect of Substrates on Recording Properties of Blue-Sensitive Photopolymers

Effects of substrates on recording properties of blue-sensitive photopolymers are reported for the first time. Polycarbonate substrate lowers M/#. Threshold energy are enlarged for polycarbonate substrate. Introduction of SiO2 layer improves the recording properties.