Actively transparent display with enhanced legibility based on an organic light-emitting diode and a cholesteric liquid crystal blind panel

Hybrid-Type Transparent Organic Light Emitting Diode with High Contrast Using Switchable Windows

Transparent organic light emitting diode (OLED) display is one of the most promising devices among next-generation information displays because of beneficial characteristics, such as self-emissive and optically clear properties. Nevertheless, in conventional transparent OLED display devices, there are serious intrinsic problems in terms of the transmittance in the dark state because of empty windows in the cell, so the contrast ratio of the transparent OLED display would be deteriorated even though it can exhibit excellent bright state. In general, the transparent mode using the OLED device applies an empty area in each pixel because an emitting device could never reveal the background image, so the transparent OLED should contain the empty area in the pixel for transparent images. This may cause the optical degradation in the dark state. To solve this problem, we propose hybrid-type transparent OLED display modes that apply a liquid crystal (LC) to the transparent window part of the empty space. In this paper, we applied two dichroic LC modes- which use an electrically controlled birefringence (ECB) mode (Heilmeier type) for the polarized mode and a cholesteric LC mode (Guest-Host mode) for the non-polarized mode-to the empty area. In each hybrid mode, we have observed optical performance, including the transmittance in the dark/bright state, contrast ratio and response time as a function of cell parameters. As a result, we confirmed that the dark state and the contrast ratio could be improved by applying the proposed modes without serious decay of the transmittance in the bright state.

Performance evaluation of micro lens arrays: Improvement of light intensity and efficiency of white organic light emitting diodes

This paper proposes a unique method to improve light intensity and efficiency of white organic light emitting diodes (OLEDs) by engraving micro lens arrays (MLAs) on the outer face of the substrate layer. The addition of MLAs on the substrate layer improves the light intensity and external quantum efficiency (EQE) of the OLEDs. The basic OLED model achieved an EQE of 14.45% for the effective refractive index (ERI) of 1.86. The spherical and elliptical (planoconvex and planoconcave) MLAs were incorporated on the outer face of the substrate layer to increase the EQE of the OLEDs. The maximum EQE of 17.30% was obtained for Convex-1 (elliptical planoconvex) MLA engraved OLED where the ERI was 1.70. In addition, Convex-1 MLA engraved OLED showed an improvement of 3.8 times on the peak electroluminescence (EL) light intensity compared to basic OLED. Therefore, Convex-1 MLA incorporated OLED can be considered as a potential white OLED because of its excellent light distribution and intensity profile.

In situobservation of dynamic pitch jumps of in-planar cholesteric liquid crystal layers based on wavelength-swept laser

We report in situ observation of dynamic pitch jumps in cholesteric liquid crystal (CLC) layers that depend on the applied electric field. A high-speed and wide bandwidth wavelength-swept laser is used as an optical broadband source to measure the dynamic pitch jumps. We could not observe the dynamic pitch jump in the quasi-static pitch variation. Instead, we carry out two driving methods, a normal driving and an overdriving method, in order to measure the dynamic pitch jump in the CLC cell. For the case of normal driving, it has been confirmed that the reflection band from the measurement region is discontinuously shifted by movement of the defect wall. The reflection band was compressed and recovered before the band moved, but the dynamic pitch jump of the helix could not be observed. For the case of overdriving, however, it was possible to observe the unwinding of the helix during the dynamic pitch jump. The entire dynamic pitch jump process in the CLC cell could be observed by measuring the transmission spectra from the CLC cell by varying the applied electric field. We confirm that the entire reaction time with the overdriving method was about 800 ms, which was shorter than with the normal driving method. This study contributes to the development of fast in-plane switching research and the development of new CLC devices.

Enhancement of absorption and haze with hybrid anchoring of dye-doped cholesteric liquid crystals

We demonstrated that hybrid anchoring of dye-doped cholesteric liquid crystals (ChLCs) could be used for the simultaneous control of haze and transmittance. Hybrid anchoring of ChLCs can be obtained by the vertical anchoring at one substrate surface and planar anchoring at the other substrate surface. In a ChLC cell with hybrid anchoring, the LCs near the planar alignment layer are in the planar state, while those near the vertical alignment layer are in the focal-conic state. In the initial opaque state, the incident light can be absorbed by the LC mixture in the planar state and scattered by the LCs in the focal-conic state. The ChLC cell with hybrid anchoring exhibited lower transmittance and haze value than those of a focal-conic state in a ChLC cell with vertical anchoring. The cell can be switched to the transparent state without use of a complicated drive scheme at a driving voltage significantly lower than that for a double-layered cell.

Mechanism of electrically induced photonic band gap broadening in polymer stabilized cholesteric liquid crystals with negative dielectric anisotropies

We experimentally observed that the photonic band gap (reflection band) of polymer stabilized cholesteric liquid crystals with negative dielectric anisotropies can be greatly broadened under DC electric fields. We explored the underlying mechanism. We found that the dispersed polymer network moved when DC voltages were applied across the liquid crystal cell. The motion of the polymer network stretched the helical pitch of the liquid crystal on one side of the cell and compressed the helical pitch on the other side of the cell. We proposed a phenomenological theory to explain the motion of the polymer network and the effect of the polymer network on the helical pitch, and this theoretical prediction agreed well with the experimental results.