In order to develop a printable organic thin film transistor with high performance, it is required to develop not only printable materials and a printing process but also a suitable device structure for it. We have newly designed an organic field effect transistor with a diagonal configuration of source and drain electrode, named as the Top and bottom contact (TBC) configuration. It has several advantages due to its unique structure. For example, it can be prepared by a simple stacking process without any micro-machining process or related photolithography procedures. This is thus suitable for applying the simple printing technique such as a screen-printing. In the proposed structure, source and drain electrodes are arranged diagonally across the active layer. Therefore, the channel length can be controlled by the deposited active layer thickness. In this study, we have prepared the pentacene transistor with the TBC configuration. By inserting the extra insulator layer, leakage current between the top and bottom electrode was remarkably reduced. The output current
density was about two orders larger than the conventional organic transistor with a top contact configuration. These high performances are mainly due to the improvement of the carrier injection efficiency owing to the short channel length (ca.0.5μm).
The strong coupling between exciton and photon modes in a conjugated polymer-based semiconductor microcavity was observed. The σ-conjugated poly(bis(p-butylphenyl)silane) (PBPS) thin films were inserted between metal and dielectric stack mirrors to form the microcavity structure. Change of the PBPS film-thickness (from 80 to 140 nm) allowed the cavity photon resonance to be tuned in the free-exciton transition. The expected anti-crossing behavior was observed at room temperature in the reflection spectra and the vacuum Rabi-splitting energy was found to be about 430 meV. This giant value is almost the same as the expectations of transfer matrix reflectivity calculations performed with optical constants (refractive index: n, extinction coefficient: k) derived from a Kramers-Kronig analysis of the PBPS absorption spectrum. Angle-dependent photoluminescence measurements were performed in each PBPS-based microcavity. In the microcavity with a 120nm-PBPS layer, the polariton emission that displays almost no blueshifts with angle was observed, a desirable feature for potential display applications.
The process of electroluminescence (EL) decay of devices made by a trivalent europium (Eu) complex, Eu(DBM)3(phen)(DBM=dibenzoylmethane, phen=phenanthroline), dispersed in poly(N-vinycarbazole) (PVK), has been studied. The decay process exhibits the characteristics of the time constant being independent of the intensity of pulsed drive voltage and the EL intensity having nearly single exponential dependence on the decay time. The decay process is considered as the monomolecular recombination process. By analyzing the process, we believe that the charged carrier concentrations (involving both holes and electrons) at initial state (the decay time t=o) do not influence the time constant, indicating that injected holes and electrons within PVK film are of unbalance seriously. From this point of view, we discussed an efficient device based on Eu(DBM)3 (phen) dispersed in polymer structure.
Survey of dye materials for the emission layer in the multilayer organic electroluminescent (EL) device is discussed in terms of emission color and fluorescent efficiency. Organic semiconductors for the electron or the hole transport layer in the EL device are proposed for preparing stable homogeneous thin layer. Requirement of accomplishing the confinement of the singlet excitons generated by the recombinations of injected electrons and holes is discussed by using three layer EL devices with extremely thin bimolecular emission layer. Then the emphasis is laid on the size effects in three layer EL device with double heterojunctions on the spontaneous emission. Variations of the intensity and pattern of outer emission through semitransparent ITO glass substrate with the spacing between the emission layer and the metallic electrode are discussed theoretically and experimentally. And variation of the fluorescent lifetime or the radiative decay rate with the spacing is also discussed theoretically and experimentally.
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