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Highly efficient and wide-color-gamut organic light-emitting devices based on multi-scale optical design
Published in Gin Jose, Mário Ferreira, Advances in Optoelectronic Technology and Industry Development, 2019
Organic Light-Emitting Devices (OLEDs) are now widely recognized as a potential application for high-quality flat-panel displays and general lighting. The near 100% internal quantum efficiency of OLEDs has been achieved using phosphorescent or thermally assisted fluorescent materials (Baldo et al., 1999). However, the External Quantum Efficiency (EQE) of conventional devices remains at 20–25% because of poor light extraction efficiency (Tanaka et al., 2007). One of the reasons for this are the quite large losses induced by the Surface Plasmon (SP) polariton, which is the direct interaction between a metal cathode and evanescent wave in near-field of vertical dipole emission (Neyts, 1998). In results, the optical energy of the propagation wave is restricted to a half of the total emission energy. We have found that the light extraction efficiency can be enhanced by using a high refractive index substrate coupled with a micro-lens array and micro-cavity structure (Mikami, 2011). In addition, light extraction efficiency of organic light-emitting devices has improved by using a thin film stacked cathode structure, consisting of semi-transparent metal and an Optical Buffer (OB) layer (Mikami, 2016). In this paper, we propose an external micro-cavity coupled with SP in the cathode, for the improvement of color purity as well as the emission efficiency. The relationship between the external micro-cavity and SP resonance will be discussed from experimental results and theoretical analysis.
Optoelectronics – solid state optical devices
Published in David Jiles, Introduction to the Electronic Properties of Materials, 2017
The market for flat panel displays has grown from typically tens of millions of dollars per year in the 1980s to about twenty billion dollars a year today. These flat panel displays come in a variety of different forms, the most important of which are active matrix liquid crystal displays (AMLCDs), electroluminescent displays (ELDs), field emission displays (FEDs) and colour plasma displays (CPDs) [24]. Flat panel displays provide one of the many examples of how a diverse variety of materials and electronic technologies are integrated to produce a device with widespread applications in the display of digital information. The construction and features of some of the principal types of flat panel displays are shown in Fig. 12.12.
Display Systems
Published in Lars-Ingemar Lundström, Digital Signage Broadcasting, 2013
Plasma flat-panel displays consist of small pixel-sized cells containing ionized gas that glows when an electrical field is built up around it by a grid of electrodes. Each pixel actually consists of three fluorescent lamps: one red, one green, and one blue (Figure 2.7).
Thermodynamic, optical and switching parameters of a ferroelectric liquid crystalline material having SmA*-SmC*-SmBh* phase sequence
Published in Phase Transitions, 2018
Ashwani Kumar Singh, Amir Iqbal, Upendra Bahadur Singh, Roman S. Dabrowski, Ravindra Dhar
The structure of SmC* phase is lamellar and the molecules within the layers are tilted at a temperature-dependent angle (θ) from the layer normal which is termed as tilt angle. The non-chiral smectic C phase has a monoclinic symmetry and belongs to the point group C2h. It has a mirror plane and a two-fold axis perpendicular to it. The mirror plane is given by the smectic layer normal (k) and the SmC director i.e. the tilt plane. If the constituent molecules are chiral, then the chirality breaks the mirror symmetry and the structure is left with only one symmetry axis C2. The chirality, together with the piezo and flexo-electric effects, induces precession of the tilt direction from the layer to layer resulting in plane spontaneous polarization. This leads to the formation of helicoidal structure with the axis parallel to the smectic layer normal and hence the net spontaneous polarization of bulk sample becomes zero. An electric field applied perpendicular to the helix unwinds helicoidal structure and gives rise to linear electro-optic effects [4–11]. SmC* phase shows sub micro-second and bi-stable switching properties of surface stabilized ferroelectric liquid crystal (SSFLC) [12–16]. FLCs have been studied extensively in the last four decades due to the interesting electro-optic effects which make them capable for optical displays. Fast response time and wide viewing angles make them attractive for use in the flat panel displays.
Fabrication of vertically aligned liquid crystal cell without using a conventional alignment layer
Published in Liquid Crystals, 2018
Masanobu Mizusaki, Yohei Nakanishi, Satoshi Enomoto
Liquid crystal displays (LCDs) are the most popular type of flat-panel displays and are used in television sets, notebook computers, smartphones, tablets, car navigations, digital signage and so forth because they have features such as high resolution, low power consumption and thinness. So far, the LCDs have usually used a twisted nematic (TN) mode [1,2], but the TN mode has disadvantages of narrow viewing angle and low contrast ratio. Therefore, other modes with wide viewing angle and high contrast ratio such as in-plane switching mode [3], fringe-field switching mode [4], multidomain vertical alignment (MVA) mode [5] and patterned vertical alignment (PVA) mode [6] have been developed. Among these modes, vertical alignment (VA) modes such as the MVA and PVA modes have a significantly high contrast ratio because liquid crystal (LC) molecules are vertically aligned, which induce little retardation. To achieve VA, VA layers, which are mainly made from polyimides having side chains, are usually prepared on a pair of substrate [7,8]. The preparation of the VA layers usually requires large amount of solvent, high–temperature operation for post-baking and cleaning process [9].
A switching threshold programmable high-linearity transimpedance amplifier for OLED pixel current mismatch measurement
Published in Journal of Information Display, 2020
Organic Light Emitting Diodes (OLED) display has become the most popular choice for modern mobile and large flat-panel display devices. One challenge of OLED display is to keep the brightness of pixels uniform across the panel. It is well known that this problem stems from the uncertainty of threshold voltage of mass-produced Thin Film Transistors (TFTs) that are used in an OLED driver circuit. To address this issue, many techniques have been reported to improve the uniformity by active compensation circuits [1,2]. Conceptually, the compensation process consists of (1) measuring the non-uniformity of the pixel current and (2) applying appropriate voltage or current adjustment in the OLED pixel driver. Therefore, to use this compensation technique, measuring the mismatch current in each OLED pixel is essential. Several current measurement circuits have been reported [3,4] but a current measurement circuit that is optimized for an OLED display system needs to be developed. Due to large number of pixels on the panel, OLED systems typically use multitude of transimpedance amplifiers (TIAs) to measure all pixel currents during a given time frame. When such TIAs are used to measure the current from a pixel using a current source as a driver circuit, the switching threshold voltage of the TIA input sets the source voltage of the current source. Therefore, if the TIA’s switching threshold voltage has uncertainty due to its own random device variation, it will add extra mismatch to the inherent pixel driver mismatch. To resolve this issue, this paper proposes a switching threshold programmable TIA topology for OLED pixel current measurement where the switching threshold can be digitally programmable.