Surface‐light‐emitting transistors based on vertical‐type metal‐base organic transistors

Abstract— Light‐emitting transistors having a metal‐base organic transistor (MBOT) structure demonstrate both the function of an organic thin‐film transistor (OTFT) and organic light‐emitting diode (OLED). The MBOT is a vertical‐type organic transistor having a simple structure composed of organic/metal/organic layers demonstrating high‐current and low‐voltage operation. The light‐emitting MBOT was fabricated simply by inserting additional layers of hole‐transporting and emissive materials used in the OLED into the col lector layer. The device showed perfect surface emission similar to an OLED. A luminance modulation of 370 cd/m² was observed at a collector voltage of 20 V and a base voltage of 3 V. This device can be applied to an OLED display device to increase the numerical aperture or reduce the required current of the TFT backplane.     

Achieving a foldable and durable OLED display with BT.2020 color space using innovative color filter structure

A foldable active‐matrix organic light‐emitting diode display capable of enduring very severe folding and environmental impact was obtained using symmetric panel stacking with an innovative design of color filter structure. The display was subjected to in situ folding cycle under an ambient test condition of 60°C/90% relative humidity, and no performance degradation was found for the display and the touch function during and after the test. In addition, 95% coverage of BT.2020 color space was obtained without additional power consumption compared with that of the panel with National Television System Committee (NTSC) color space.     

Development of 8‐in. oxide‐TFT‐driven flexible AMOLED display using high‐performance red phosphorescent OLED

An 8‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display driven by oxide thin‐film transistors (TFTs) has been developed. In‐Ga‐Zn‐O (IGZO)‐TFTs used as driving devices were fabricated directly on a plastic film at a low temperature below 200 °C. To form a SiO_x layer for use as the gate insulator of the TFTs, direct current pulse sputtering was used for the deposition at a low temperature. The fabricated TFT shows a good transfer characteristic and enough carrier mobility to drive OLED displays with Video Graphic Array pixels. A solution‐processable photo‐sensitive polymer was also used as a passivation layer of the TFTs. Furthermore, a high‐performance phosphorescent OLED was developed as a red‐light‐emitting device. Both lower power consumption and longer lifetime were achieved in the OLED, which used an efficient energy transfer from the host material to the guest material in the emission layer. By assembling these technologies, a flexible AMOLED display was fabricated on the plastic film. We obtained a clear and uniform moving color image on the display.     

A 5.5‐inch full HD foldable AMOLED display based on neutral‐plane splitting concept

Splitting of the mechanical neutral plane is a promising concept for foldable displays because it reduces the folding stress in each layer of the display. We verified the splitting concept experimentally and revealed a linear relation between the relative position of the neutral plane and the logarithm of the adhesive's elastic modulus. As the modulus decreased, the position of the neutral plane approached that of perfect splitting. On the basis of the neutral‐plane splitting concept, we developed 5.5‐inch full high‐definition foldable active matrix organic light‐emitting diode (AMOLED) displays, which endured 150 k inward folding cycles and 150 k outward folding cycles with folding radii of 3 and 5 mm, respectively. This study is expected to improve the flexibility of designing foldable AMOLED displays, enabling better balance of the portability versus practicality trade‐off in mobile displays.     

Development of a color‐tunable polychromatic organic‐light‐emitting‐diode device for roll‐to‐roll manufacturing

A flexible vertically stacked flexible polychromatic color‐tunable OLED has been developed by means of low resistive intermediate electrode technology. The polychromatic OLED has a capability to show 16 million colors with 105% National Television Committee Standard (NTSC) color reproduction. The device can produce arbitrary shape with arbitrary colors, suitable for artistic expressions, just as many as those used in information displays. Independently controlled red, green, and blue light‐emitting layers are stacked vertically. With conventional indium tin oxide technology, because of the temperature restriction, it was quite difficult to achieve low resistance on plastic substrate. The reported numbers were all more than 80 Ω/□. According to the surface mobility control using Fick's law analysis, low sheet resistance 7.34 Ω/□ on plastic film was developed. At first, flexible 7.17 cm² transparent OLED was fabricated for the performance confirmation of transparent electrode. And then polychromatic color‐tunable OLED with the same size were successfully fabricated on plastic. With optical length optimization for each color stack of polychromatic OLED, more than 100% color reproduction in National Television Committee Standard was achieved by stack design. The polychromatic device can be used for colored illumination, as well as for organic‐light‐emitting display pixels for three times emission than conventional pixel design. The device is fabricated on plastic substrate so that the polychromatic organic‐light‐emitting‐diode device is manufacturable with roll‐to‐roll production line.     

High‐resolution OLED display with remarkably low power consumption using blue/yellow tandem structure and RGBY subpixels

A top‐emission organic light‐emitting diode (OLED) with a microcavity structure combined with a blue/yellow tandem structure was developed. A high‐resolution active‐matrix OLED display with the world's lowest level of power consumption using the tandem OLED with red, green, blue, and yellow subpixels was fabricated.     

Composite silver electrode for double‐sided‐emitting stacked organic light‐emitting diode

Abstract— A reflective composite silver electrode is proposed and characterized as the middle electrode of a stacked organic light‐emitting diode (OLED) with double‐sided light emission. The proposed electrode is composed of a thermally evaporated stack of LiF (1 nm)/Al (3 nm)/Ag (70 nm) layers. The LiF/Al and the plasma‐treated Ag of the electrode function well as the respective cathode and anode of the bottom‐ and top‐emitting stacked OLEDs, with both being of the non‐inverted type. Power efficiencies of 10.3 and 12.1 lm/W at 100 cd/m² have been measured for bottom‐ and top‐emitting OLEDs, respectively, using dye doping. The stacked OLED having this bipolar middle electrode can be constructed as a two‐terminal‐only device, allowing for simpler driving schemes in double‐side‐emitting passive‐/active‐matrix OLED displays.     

A 5.8‐in. phosphorescent color AMOLED display fabricated by ink‐jet printing on plastic substrate

Abstract— A flexible phosphorescent color active‐matrix organic light‐emitting‐diode (AMOLED) display on a plastic substrate has been fabricated. Phosphorescent polymer materials are used for the emitting layer, which is patterned using ink‐jet printing. A mixed solvent system with a high‐viscosity solvent is used for ink formulation to obtain jetting reliability. The effects of evaporation and the baking condition on the film profile and OLED performances were investigated. An organic thin‐film‐transistor (OTFT) backplane, fabricated using pentacene, is used to drive the OLEDs. The OTFT exhibited a current on/off ratio of 10⁶ and a mobility of 0.1 cm²/V‐sec. Color moving images were successfully shown on the fabricated display.     

Flexible high‐resolution full‐color top‐emitting active‐matrix organic light‐emitting diode display

We developed a high‐performance 3.4‐in. flexible active‐matrix organic light‐emitting diode (AMOLED) display with remarkably high resolution using an oxide semiconductor in a backplane, by applying our transfer technology that utilizes metal separation layers. Using this panel, we also fabricated a prototype of a side‐roll display for mobile uses. In these AMOLED displays, a white OLED combined with a color filter was used in order to achieve remarkably high resolution. For the white OLED, a tandem structure in which a phosphorescent emission unit and a fluorescent emission unit are serially connected with an intermediate layer sandwiched between the emission units was employed. Furthermore, revolutionary technologies that enable a reduction in power consumption in both the phosphorescent and fluorescent emission units were introduced to the white tandem OLED.     

Recent development of soluble hole injection material for OLED display

We developed soluble hole injection materials and inks, named ELsource, that can be used as hole injection layer in organic light‐emitting diode (OLED) display. OLED is an optical device. Therefore, we developed the hole injection materials with optical properties necessary for an optical device.     

A foldable OLED display with an in‐cell touch sensor having embedded metal‐mesh electrodes

A foldable organic light‐emitting diode display integrating a touch sensor is fabricated. The touch sensor has an in‐cell structure where metal‐mesh sensor electrodes are formed in a counter substrate. It is demonstrated that touch on the entire panel surface, including a bent portion, is detected and that the touch panel operates correctly after 100,000 folding operations with a radius of curvature of 5 mm.     

High‐efficiency OLED microdisplay with microlens array

A high‐efficiency organic light‐emitting diode (OLED) microdisplay has been developed with some new technologies including microlens array. We focused on the improvement of the out‐coupling efficiency and achieved three times higher efficiency as compared with conventional OLED. By using our developed technologies, it is possible to improve the maximum luminance from 1600 to 5000 cd/m² while maintaining same lifetime.     

Highly efficient single‐unit white OLED device with emission from both singlet and triplet excitons

We succeeded in developing a single‐unit hybrid organic light‐emitting diode (OLED) device with efficient light emission from both a phosphorescent layer and a fluorescent layer. The single‐unit hybrid OLED achieved a power efficiency higher than that of a two‐unit hybrid tandem OLED with phosphorescent and fluorescent layers.     

Realization of high efficacy white organic light emitting diode by controlling internal light absorption and angular distribution

We investigated highly efficient organic light emitting diode (OLED) with advanced optical designs of organic layers to convert evanescent mode (internal absorption) into guided light and micro structure to extract the specifically distributed guided light dominated by wide angular substrate mode. White OLED device based on these optical designs realized high efficacy of 133 lm/W and external quantum efficiency of 56 % at 1000 cd/m².     

Realization of RGB colors from top‐emitting white OLED by electron beam patterning

We demonstrate the realization of red, green, and blue colors from top‐emitting white organic light‐emitting diode (OLED) for display applications. In our approach, red, green, and blue colors are realized by microcavity‐based mode selection from the spectrum of a white OLED. For the tuning of individual microcavities, the OLED hole transport layer is patterned by an electron beam process.     

Advanced compensation technologies for large‐sized UHD OLED TVs

In this paper, we present novel organic light‐emitting diode (OLED) display panel compensation technologies for large‐sized ultra‐high‐definition OLED TVs considering variations of threshold voltage, mobility, channel size, OLED efficiency, and OLED uniformity. Using these technologies, we have successfully launched 55‐, 65‐ and 77‐in. ultra‐high‐definition OLED TVs.     

A 5291‐ppi organic light‐emitting diode display using field‐effect transistors including a c‐axis aligned crystalline oxide semiconductor

C‐axis‐aligned crystalline‐oxide semiconductor field‐effect transistor (CAAC‐OS FET) can be scaled down to a width and a length of 60 nm. We fabricated an organic light‐emitting diode (OLED) display with more than 5000 ppi, which is required in virtual reality (VR) display applications, using CAAC‐OS FETs as the backplane.     

Development of a liquid crystal with fine‐pitch light‐source display (LFD) using an organic light‐emitting diode (OLED)

Abstract— A novel display system, refered to as an LFD (liquid crystal with fine‐pitch light‐source display) is proposed. In an LFD, an auxiliary light source patterned with a fine pitch is attached to a reflective liquid‐crystal display (LCD), and a light shield is formed on the observer's side of the light source. A vertical‐alignment LCD (VA‐LCD) is attached as the reflective LCD, and an organic light‐emitting diode (OLED) is attached as the fine‐pitch light source. An LFD can produce bright, high‐contrast images under any ambient light. A test sample was built and its display characteristics confirmed.     

High‐mobility self‐aligned top‐gate oxide TFT for high‐resolution AM‐OLED

High‐mobility and highly reliable self‐aligned top‐gate oxide thin‐film transistor (TFTs) were developed using the aluminum reaction method. Al diffusion to the oxide semiconductor and homogenization of the oxygen concentration in the depth direction after annealing were confirmed by laser‐assisted atom probe tomography. The high mobility of the top‐gate TFT with amorphous indium tin zinc oxide channel was demonstrated to be 32 cm²/V s. A 9.9‐in. diagonal qHD active‐matrix organic light‐emitting diode (AM‐OLED) display was fabricated using a five‐mask backplane process to demonstrate an applicable solution for large‐sized and high‐resolution AM‐OLEDs.     

Characterization and compensation of OLED aging in a digital AMOLED system

Organic light‐emitting diode (OLED) aging is the root cause for image sticking artifact and considered as the toughest problem besides the low yield problem of active‐matrix organic light‐emitting diode (AMOLED) displays. Digital driving can eliminate Mura artifact and allow a similar yield like LCD. However, it is more prone to OLED aging than analog driving, so that the lifetime will become shorter. In this paper, we pursue the approach to measure the pixel current and compensate OLED I–V drift. Information gained from electrical measurements during the lifetime of the display may be correlated to electro‐optical drift, particularly the current efficiency. The aging model has to consider the dependence of I–V drift and efficiency loss on the operation point/voltage for each subframe. Specific compensation algorithms have been developed. Two AMOLED prototypes (1.5 and 2.8 in.) were validated. Burned‐in pattern can be compensated, so the concept has been proven as effective. With the method described in this paper, digital AMOLED may reach a similar and even significantly higher lifetime than an analog AMOLED.