Temperature dependence of electroluminescent properties of Cz doped PVCz double-layered EL devices has been studied. Luminance is more than 3000cd/m^2 at 110K and 500cd/m^2 at 300K. Temperature dependence of carrier mobility and barrier height at negative electrode has been studied by considering current-voltage characteristics. The temperature dependence of the hole mobility in Cz doped PVCz has been analyzed and the activation energy for carrier hopping has been derived. The tunneling carrier injection at negative electrode has been clearly observed and the barrier height has been determined.
PVCz double-layered organic EL devices with negative electrodes using magnesium, strontium and silver metals have been fabricated and dependence of electroluminescent characteristics on the cathode metal has been studied. The device structure is such as ITO/PVCz/PVCz: perylene/Metal. Luminance of 1000 cd/m^2 has been obtained for the devices without electron transport molecules in the layer adjacent to the negative electrode. Dependencies of threshold voltage of electroluminescence emission and barrier height at negative electrode on the metal work function have been clearly observed from the luminance-voltage characteristics and from the charge transport analysis based on Fowler-Nordheim plot, respectively.
A gallium complex (GaMq_2Cl) consisting of two 2-methyl-8-hydroxy-quinolines (Mq) and a chlorine was synthesized and used for the fabrication of organic EL devices. The photoluminescent peak of GaMq_2Cl at 492 nm was as strong in intensity as that of tris(8-hydroxyquinolinato)aluminum (Alq_3). The organic EL devices using GaMq_2Cl as an emitting material showed green luminance of 10, 490 cd/m^2. When it was used as an electron transport material in a rubrene doped device, an organic EL device with a high luminance of 27, 700 cd/m^2 was obtained.
The presence of impurity ions in liquid crystal displays has long been blamed for degrading of devices. We have investigated the behavior of ions in bulk of the liquid crystal (LC) and in the interface between the LC and the alignment layer, using electro-optical, electrochemical and ζ-potential measurements. We report the effect of impurity ions in Nematic LC cells with asymmetric alignment layers made by different process. When a symmetric external electric field is applied to a Nematic LC cell, the internal dc voltage is built up, and the flicker is observed. This is a possible cause for impurity ions being introduced by the applied electric field and disturbing the absorption-desorption equilibrium.
The transients of the transmittance and the voltage holding ratio(VHR) of a liquid crystal cell were measured by the bipolar pulse driving in order to clarify the influence of residual DC voltage to image-sticking effects. The pulse voltage with the condition of half-tone image driving was applied to the LC cell involving ions after the DC voltage application to induce the residual DC voltage. The resultant transmittance of the cell was lower than that of the unstressed level with increasing VHR as a function of DC application time. The drop of the transmittance is considered to be not caused by residual DC but the increase of the VHR due to decrease of mobile ion density.
Reported is switching of bistable PDLCs with dual frequency drive. Low frequency voltage pulses (2.5kHz/100 V/>0.25 s) result in the transparent memory state. High frequency pulses (100kHz/80V/0.5s) return to the opaque memory state; which is a time useful for practical applications, e.g.still picture. Applying repeatedly high frequency voltage pulses recover the initial scattered state, which is different from above two states, for reproducible switching. Polymer content and surface treatment of substrates drastically affect the PDLC droplet formation and bistability without holding voltage.
A holographic polymer dispersed liquid crystal (HPDLC) device is useful for color reflective LCD because it requires neither polarizer nor color filter. HPDLC devices with high reflectivity have been prepared by controlling the phase separation temperature of the HPDLC precursor. The acceleration of the phase separation is one of the guides for HPDLC with high reflectivity. In addition to this, a multi-layered HPDLC device and a HPDLC device having two gratings in a pixel have also been prepared.