Improvement in image quality of a 5.8‐in. OTFT‐driven flexible AMOLED display

Abstract— The image quality of an OTFT‐driven flexible AMOLED display has been improved by enhancing the performance of OTFTs and OLEDs. To reduce the operating voltage of OTFTs on a plastic film, Ta²O⁵ with a high dielectric constant was used as a gate insulator. The organic semiconductor layer of the OTFT was successfully patterned by a polymer separator, which is an isolating wall structure using an organic material. The OTFT performance, such as its current on/off ratio, carrier mobility, and spatial uniformity on the backplane, was enhanced. A highly efficient phosphorescent OLED was used as a light‐emission device. A very thin molybdenum oxide film was introduced as a carrier‐injection layer on a pixel electrode to reduce the operating voltage of the OLED. After an OTFT‐driven flexible AMOLED display was fabricated, the luminance and uniformity on the display was improved. The fabricated display also showed clear moving images, even when it was bent at a low operating voltage.     

A flexible full‐color AMOLED display driven by OTFTs

Abstract— Organic thin‐film‐transistor (OTFT) technologies have been developed to achieve a flexible backplane for driving full‐color organic light‐emitting diodes (OLEDs) with a resolution of 80 ppi. The full‐color pixel structure can be attained by using a combination of top‐emission OLEDs and fine‐patterned OTFTs. The fine‐patterned OTFTs are integrated by utilizing an organic semiconductor (OSC) separator, which is an insulating wall structure made of an organic insulator. Organic insulators are actively used for the OTFT integration, as well as for the separator, in order to enhance the mechanical flexibility of the OTFT backplane. By using these technologies, active‐matrix OLED (AMOLED) displays can be driven by the developed OTFT backplane even when they are mechanically flexed.     

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.     

Low‐temperature fabrication of 5‐in. QVGA flexible AMOLED display driven by OTFTs using olefin polymer as the gate insulator

Abstract— A 5‐in. QVGA flexible AMOLED display driven by OTFTs has been fabricated at a low temperature of 130°C. A polyethylene naphthalate film was used as the flexible substrate and an olefin polymer was used as the gate insulator for the OTFT. This layer was formed by spin‐coating and baking at 130°C. Pentacene was used as the organic semiconductor layer. The OTFT performance to drive the flexible display with QVGA pixels in terms of current on/off ratio, carrier mobility, and spatial uniformity on the backplane have been obtained. Phosphorescent and fluorescent OLEDs were used as light‐emitting devices on a flexible display. Those layers were formed by vacuum deposition. After the flexible display was fabricated, a clear and uniform moving image was obtained on the display. The display also showed a stable moving image even when it was bent.     

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.     

Top‐emission AMOLED display driven by organic TFTs with semiconductor layer patterned by inkjet process

We have developed an inkjet process for laying down an organic semiconductor layer in organic thin‐film transistors (OTFTs). The organic semiconductor crystallinity was improved by adjusting the contact angles of the bank, the gate insulator, and the source/drain electrodes. The threshold voltage of the OTFT was controlled by means of several surface treatments of the silicon dioxide gate insulator. The OTFTs showed a high mobility of 2.5 cm²/Vs and uniform threshold voltages of ?0.4 ± 0.7 V. We also fabricated a 4‐in., 80‐ppi active‐matrix organic light‐emitting diode on a glass substrate that showed good luminance uniformity and high moving picture quality.     

Fabrication of flexible organic light‐emitting‐diode display with a flexible color filter driven by In—Ga—Zn—Oxide TFTs

Abstract— A novel flexible active‐matrix organic light‐emitting‐diode (OLED) display fabricated on planarized stainless—used‐steel substrates with a resolution of 85 dpi in a 4.7‐in. active area has been demonstrated. Amorphous indium—gallium—zinc—oxide thin‐film transistors were used as the backplane for the OLED display with high device performance, high electrical stability, and long lifetime. A full‐color moving image at a frame frequency of 60 Hz was also realized by using a flexible color filter directly patterned on a plastic substrate with a white OLED as the light source.     

5.8‐inch QHD flexible AMOLED display with enhanced bendability of LTPS TFTs

By applying the curve‐type thin film transistor (TFT) with longitudinal strain, TFT parameters do change little down to the 2R bending. The mobility variation range reduces down to 4% compared with 28% of the line‐type channel with transverse strain. The smaller variation is preferred for a high quality display. We clarified that majority carrier's effective mass and scattering rate are dominant factors influencing the bended TFT's performance, which can be controlled by the strain orientation and channel shape. This understanding and improvement was embedded in the 5.8″ flexible QHD active matrix organic light emitting diode panel with multi edge curvature of Galaxy S8. Through this achievement, we made our flexible premium active matrix organic light emitting diode panels more performable, reliable, and highly productive in small R bending circumstance.     

A 5‐in. flexible ferroelectric liquid‐crystal display driven by organic thin‐film transistors

Abstract— An organic thin‐film‐transistor (OTFT) driven color flexible ferroelectric‐liquid‐crystal (FLC) display with 160 × 120 pixels and a resolution of 50 ppi has been developed. The flexible FLC was fabricated on a pentacene‐OTFT array using printing and lamination techniques. To drive the display at a fast driving speed, an OTFT was developed with a short channel length having a large current output. The fabricated OTFT array with a channel length of 5 μm exhibits a carrier mobility of 0.3 cm²/V‐sec and an ON/OFF ratio of over 10⁷ at a low drain voltage of ?6 V. A field‐sequential‐color system with a flexible backlight unit was also developed and used to drive the display. Color moving images were successively shown on the 5‐in. display using an active‐matrix driving technique of the OTFT.     

12.1‐in. WXGA AMOLED display driven by InGaZnO thin‐film transistors

Abstract— A full‐color 12.1‐in.WXGA active‐matrix organic‐light‐emitting‐diode (AMOLED) display was, for the first time, demonstrated using indium‐gallium‐zinc oxide (IGZO) thin‐film transistors (TFTs) as an active‐matrix backplane. It was found that the fabricated AMOLED display did not suffer from the well‐known pixel non‐uniformity in luminance, even though the simple structure consisting of two transistors and one capacitor was adopted as the unit pixel circuit, which was attributed to the amorphous nature of IGZO semiconductors. The n‐channel a‐IGZO TFTs exhibited a field‐effect mobility of 17 cm²/V‐sec, threshold voltage of 1.1 V, on/off ratio >10⁹, and subthreshold gate swing of 0.28 V/dec. The AMOLED display with a‐IGZO TFT array is promising for large‐sized applications such as notebook PCs and HDTVs because the a‐IGZO semiconductor can be deposited on large glass substrates (larger than Gen 7) using the conventional sputtering system.     

Power saving through state retention in IGZO‐TFT AMOLED displays for wearable applications

We present a qHD (960 × 540 with three sub‐pixels) top‐emitting active‐matrix organic light‐emitting diode display with a 340‐ppi resolution using a self‐aligned IGZO thin‐film transistor backplane on polyimide foil with a humidity barrier. The back plane process flow is based on a seven‐layer photolithography process with a CD = 4 μm. We implement a 2T1C pixel engine and use a commercial source driver IC made for low‐temperature polycrystalline silicon. By using an IGZO thin‐film transistor and leveraging the extremely low off current, we can switch off the power to the source and gate driver while maintaining the image unchanged for several minutes. We demonstrate that, depending on the image content, low‐refresh operation yields reduction in power consumption of up to 50% compared with normal (continuous) operation. We show that with the further increase in resolution, the power saving through state retention will be even more significant.     

A novel TFT‐OLED integration for OLED‐independent pixel programming in amorphous‐Si AMOLED pixels

Abstract— The direct voltage programming of active‐matrix organic light‐emitting‐diode (AMOLED) pixels with n‐channel amorphous‐Si (a‐Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n‐channel a‐Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn‐on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active‐matrix OLED displays with a‐Si technology and conventional bottom‐anode OLEDs.     

Fast voltage‐driving scheme for AMOLED displays based on internal feedback

Abstract— A new driving scheme for active‐matrix organic light‐emitting diodes (AMOLED) displays based on voltage programming is proposed. While conventional voltage drivers have a trade‐off between speed and accuracy, the new scheme is inherently fast and accurate. Based on the new driving scheme, a fast pixel circuit is designed using amorphous‐silicon (a‐Si) thin‐film transistors (TFTs). As the simulation results indicate, this pixel circuit can compensate the threshold‐voltage shift (VT shift) of the driver transistors. This pixel can be programmed in just 10 μsec, and it can compensate the threshold‐voltage shifts over 5 V with an error rate of less than 5% for a 1 ‐μA pixel current.     

Wearable 4‐in. QVGA full‐color‐video flexible AMOLEDs for rugged applications

Abstract— Work on the world's first wrist‐worn communications device built on a flexible, low‐power‐consumption full‐color AMOLED using phosphorescent OLEDs is presented. The device offers the wearer the ability to see high‐information‐content video‐rate information in a thin‐and‐rugged‐form‐factor 4‐in. QVGA display, conformed around a human wrist.     

Development of a 10.4‐in. UXGA display using low‐temperature poly‐Si technology

A new 10.4‐in.‐diagonal display with UXGA resolution (1600 H × 1200 V pixels) using low‐temperature polysilicon (poly‐Si) TFTs has been developed for notebook‐PC applications. The source drive technique uses integrated selector switches, which decreases the number of tape carrier packages (TCPs) for a poly‐Si TFT‐LCD and increases the connection pitch of the TCPs to the glass substrate. In this paper, we present a new display configuration and fabrication process.     

A 3.0‐in. 308‐ppi WVGA AMOLED with a top‐emission white OLED and color filter

Abstract— A 3.0‐in. 308‐ppi WVGA top‐emission AMOLED display with a white OLED and color filters, driven by LTPS TFTs demonstrating a color gamut of >90% and a Δ(u′,v′) of <0.02 is reported. A white‐emission source with a unique device structure was developed using all fluorescent materials and yielded efficiencies of 8.45% and 16 cd/A at 4000 nits with CIE color coordinates of (0.30, 0.32).     

A fully integrated 1‐in. AMOLED display using current feedback based on a five‐mask LTPS CMOS process

Abstract— In the past, a five‐mask LTPS CMOS process requiring only one single ion‐doping step was used. Based on that process, all necessary components for the realization of a fully integrated AMOLED display using a 3T1C current‐feedback pixel circuit has recently been developed. The integrated data driver is based on a newly developed LTPS operational amplifier, which does not require any compensation for Vth or mobility variations. Only one operational amplifier per column is used to perform digital‐to‐analog conversion as well as current control. In order to achieve high‐precision analog behavior, the operational amplifier is embedded in a switched capacitor network. In addition to circuit verification by simulation and analytic analysis, a 1‐in. fully integrated AMOLED demonstrator was successfully built. To the best of the authors' knowledge, this is the first implementation of a fully integrated AMOLED display with current feedback.     

A 13.5‐in. Quad‐FHD flexible CAAC‐OS AMOLED display with long‐life OLED device structure

In this study, the device structure of a white tandem organic light‐emitting diode (OLED) was changed to control the emission area and thereby achieve less luminance decay. A long‐life 13.5‐inch 4 K flexible c‐axis‐aligned crystal oxide semiconductor (CAAC‐OS) active‐matrix OLED with less color shift and high resolution was fabricated using this long‐life white OLED, transfer technology, and a CAAC‐OS field‐effect transistor.     

Development of a high‐resolution RGBW flexible display using a white organic light‐emitting diode with microcavity structure and that of a side‐roll touch panel

In this study, white organic electroluminescent devices with microcavity structures were developed. A flexible high‐resolution active‐matrix organic light‐emitting diode display with low power consumption using red, green, blue, and white sub‐pixels formed by a color‐filter method was fabricated. In addition, a side‐roll touch display was developed in combination with a capacitive flexible touch screen.     

Development of 55‐in. 8K AMOLED TV based on coplanar oxide thin‐film transistors and inkjet printing process

In this paper, we presented 55‐in. 8K4K AMOLED TV employing coplanar oxide thin‐film transistor (TFT) backplane, top emissive inkjet‐printing organic light‐emitting diode (OLED) device, gate driver on array (GOA), and compensation technologies. It is so far the largest prototype AMOLED TV fabricated by using inkjet printing process with 8K resolution. It shows the stunning display quality, thanks to the high resolution and fast refresh frequency. It proves that the inkjet printing process is not only cost competitive but also can deliver premium display.