Letter: Novel approaches to realize high‐resolution AMOLED display

Two different approaches to realize high‐resolution active‐matrix organic light‐emitting device (AMOLED) display were delivered. By adopting specific organic light emitting diode (OLED) structure with pre‐pattern electrode and the utilization of color filter, we successfully simplify the fabrication process with fine metal mask (FMM)‐free or one‐FMM solutions. Each approach was demonstrated with a 4.4″ panel with 413 ppi pixel density based on real stripe RGB. Both panels possessed low power consumption, low reflectivity, and superior NTSC performance. Because the utilization of FMM was avoided or reduced, higher production yield, higher throughput, and lower cost could be achieved. Therefore, these two approaches are very promising for mass production of high‐resolution AMOLED display.     

Fine metal mask material and manufacturing process for high‐resolution active‐matrix organic light‐emitting diode displays

Manufacturing fine metal mask (FMM) is one of the biggest hurdles to realize the ultra‐high definition (UHD) grade AMOLED displays for smartphone and augmented reality (AR). We have developed the state‐of‐the‐art material and processing technology to achieve 800ppi or higher‐resolution FMMs. The Invar thinning and the thermal damage‐free laser ablation process realized us achieving the FMM for UHD displays.     

Discharge efficiency of a 25‐in. high‐resolution SMPDP

Abstract— A novel round subpixel and triangle‐arrangement shadow‐mask plasma‐display panel (SMPDP) suitable for high‐resolution displays has been investigated. The discharge efficiency of this high‐resolution SMPDP and the AC coplanar PDP (ACCPDP) has been calculated separately. The variance of the discharge efficiency with pressure and xenon content will be reported. Results indicate that the SMPDP can reach a higher efficiency for high‐resolution displays than conventional ACCPDPs.     

A high resolution multi‐view 3D display using switchable liquid crystal lens

A 4.4‐inch 2D/3D switchable full high definition (FHD) six‐view 3D display with 3D resolution greater than 170 ppi has been accomplished. In addition to adopting low temperature polysilicon technology (LTPS), which is most suitable for high resolution displays, a new RGBW pixel arrangement using four‐square sub‐pixels has been devised. In 2D, a resolution greater than 500 ppi, accompanied with high luminance, has been achieved. A new liquid crystal lens (LCL) has been exploited for 2D/3D switching. By employing a special multielectrode structure and dedicated manufacturing process, an optical focal ratio less than 20%, which is essential for low 3D cross talk for a six‐view 3D display, has been attained by adopting the LCL. In the vertical direction of the display, there is no cross talk increase when the viewing position is changed because of the new pixel structure. The strong focal strength of the LCL combined with a revised high‐density multi‐view design give rise to a wide 3D viewing angle greater than 20 degrees in the horizontal direction and minimum cross talk less than 10%.     

Active‐matrix organic light‐emitting diode displays with indium gallium zinc oxide thin‐film transistors and normal,inverted, and transparent organic light‐emitting diodes

Abstract— Active‐matrix organic light‐emitting diode (AMOLED) displays have gained wide attention and are expected to dominate the flat‐panel‐display industry in the near future. However, organic light‐emitting devices have stringent demands on the driving transistors due to their current‐driving characteristics. In recent years, the oxide‐semiconductor‐based thin‐film transistors (oxide TFTs) have also been widely investigated due to their various benefits. In this paper, the development and performance of oxide TFTs will be discussed. Specifically, effects of back‐channel interface conditions on these devices will be investigated. The performance and bias stress stability of the oxide TFTs were improved by inserting a SiO^x protection layer and an N²O plasma treatment on the back‐channel interface. On the other hand, considering the n‐type nature of oxide TFTs, 2.4‐in. AMOLED displays with oxide TFTs and both normal and inverted OLEDs were developed and their reliability was studied. Results of the checkerboard stimuli tests show that the inverted OLEDs indeed have some advantages due to their suitable driving schemes. In addition, a novel 2.4‐in. transparent AMOLED display with a high transparency of 45% and high resolution of 166 ppi was also demonstrated using all the transparent or semi‐transparent materials, based on oxide‐TFT technologies.     

A flexible OLED display with an OTFT backplane made by scalable manufacturing process

Abstract— A full‐color top‐emission AMOLED display driven by an organic TFT backplane manufactured using a scalable, lift‐off‐free, and shadow‐mask‐free process has been developed. It was shown that cost‐efficient copper can be used for S/D electrodes. The display has shown no significant degradation over a storage life of more than 10 months and operation over 25 hours during which the display is bent over 10,000 times.     

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.     

Oxide‐TFT technologies for next‐generation AMOLED displays

Abstract— High‐mobility high‐reliability low‐RC‐delay oxide TFTs have been developed. Their performances are good enough for AMOLED displays even for the large‐sized super‐high‐resolution, or high‐frame‐rate displays. In this paper, the status of oxide‐TFT development and the issues for the mass‐production of next‐generation AMOLED displays will be discussed, and three types of AMOLED displays using different oxide materials and TFT structures will be demonstrated.     

System design for a wide‐color‐gamut TV‐sized AMOLED display

Abstract— By using current technology, it is possible to design and fabricate performance‐competitive TV‐sized AMOLED displays. In this paper, the system design considerations are described that lead to the selection of the device architecture (including a stacked white OLED‐emitting unit), the backplane technology [an amorphous Si (a‐Si) backplane with compensation for TFT degradation], and module design (for long life and low cost). The resulting AMOLED displays will meet performance and lifetime requirements, and will be manufacturing cost‐competitive for TV applications. A high‐performance 14‐in. AMOLED display was fabricated by using an in‐line OLED deposition machine to demonstrate some of these approaches. The chosen OLED technologies are scalable to larger glass substrate sizes compatible with existing a‐Si backplane fabs.     

Principle and validation of digital driving for active matrix organic light emitting diodes displays

The feasibility of digital driving has been investigated, which may be an enabling technology for low‐cost, power‐efficient and high‐quality active matrix organic light emitting diodes displays (AMOLED displays). The theoretical requirements therefore are analyzed. Methods for avoiding imaging artifacts are proposed. Results based on two prototypes are presented showing that digital drive is feasible for state‐of‐the‐art manufacturing technology with comparable image quality to analog‐driven AMOLED displays but at lower power consumption and lower production cost.     

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.     

PECVD‐based nanocrystalline‐silicon TFT backplanes for large‐sized AMOLED displays

Abstract— A 14.1‐in. AMOLED display using nanocrystalline silicon (nc‐Si) TFTs has been developed. Nanocrystalline silicon was deposited using conventional 13.56‐MHz plasma‐enhanced chemical vapor deposition (PECVD). Detailed thin‐film characterization of nc‐Si films was followed by development of nc‐Si TFTs, which demonstrate a field‐effect mobility of about 0.6–1.0 cm²/V‐sec. The nc‐Si TFTs show no significant shift in threshold voltage when over 700 hours of constant current stress is applied, indicating a stable TFT backplane. The nc‐Si TFTs were successfully integrated into a 14.1‐in. AMOLED display. The display shows no significant current decrease in the driving TFT of the 2T‐1cap circuit because the TFTs are highly stable. In addition to the improved lifetime of AMOLED displays, the development of nc‐Si TFTs using a conventional 13.56‐MHz PECVD system offers considerable cost advantages over other laser and non‐laser polysilicon‐TFT technologies for large‐sized AMOLEDs.     

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.     

Real RGB printing AMOLED with high pixel per inch value

A 403‐ppi (pixel per inch) real RGB active‐matrix organic light‐emitting diode (AMOLED) was fabricated without using fine metal mask (FMM). Organic light‐emitting materials were printed on array panel, with a novel process that has the advantages of low cost and high pixel density. The uniformity of the organic light‐emitting diodes (OLEDs) was improved with carefully tuning printing parameters, and good panel performance was achieved. The key to such a process was the design of the hexagonal patterning of the pixels. The image quality of the panel with this hexagonal pixel arrangement was evaluated with a bmp format Pixel Layout Simulation picture.     

High resolution photolithography for direct view active matrix organic light‐emitting diode augmented reality displays

High‐resolution RGB organic light‐emitting diode frontplane is a key enabler for direct‐view transparent augmented reality displays. In this paper, we demonstrate 1250 ppi passive displays and semi‐transparent active displays. Organic light‐emitting diode photolithography can provide pixel density above 1000 ppi while keeping effective emission area high because of high aperture ratio. Patterns with 2 μm line pitch were successfully transferred to emission layers, indicating possible further pixel density scaling. Lifetime after patterning, key parameter enabling industrialization, is above 150 h (T90 at 1000 nit).     

Amorphous‐oxide TFT backplane for large‐sized AMOLED TVs

Abstract— Amorphous‐oxide thin‐film‐transistor (TFT) arrays have been developed as TFT backplanes for large‐sized active‐matrix organic light‐emitting‐diode (AMOLED) displays. An amorphous‐IGZO (indium gallium zinc oxide) bottom‐gate TFT with an etch‐stop layer (ESL) delivered excel lent electrical performance with a field‐effect mobility of 21 cm²/V‐sec, an on/off ratio of >10⁸, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous‐oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold‐voltage (VTH) compensation pixel circuits, especially in the negative state. A full‐color 19‐in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19‐in. AMOLED display. The AMOLED display with a‐IGZO TFT array is promising for large‐sized TV because a‐IGZO TFTs can provide a large‐sized backplane with excellent uniformity and device reliability.     

Development of a 25‐in. SVGA SMPDP

Abstract— A newly developed 25‐in. SVGA ACPDP containing a shadow mask (SMPDP) is presented. Some modifications have been introduced into the fabrication technique. Instead of using conventional Ag in a thick‐film process, an aluminum bus electrode without ITO was fabricated on the front substrate, which was made by using a thin‐film process. Due to this improvement, excellent uniformity of the dielectric layer has been achieved. Only five necessary process steps are required during the manufacturing process of the front/rear substrates for the SMPDP. The panel capacitance was reduced by over 20% by adopting an ITO‐less structure, which enables high‐speed addressing for large‐capacity displays. The color purity was improved by applying a high Xe content.     

Full‐color AMOLED with RGBW pixel pattern

Abstract— A full‐color AMOLED display with an RGBW color filter pattern has been fabricated. Displays with this format require about one‐half the power of analogous RGB displays. RGBW and RGB 2.16‐in.‐diagonal displays with average power consumptions of 180 and 340 mW, respectively, were characterized for a set of standard digital still camera images at a luminance of 100 cd/m². In both cases, a white‐emitting AMOLED was used as the light source, and standard LCD filters were used to provide the R, G, and B emission. The color gamuts of these displays were identical and the higher overall efficiency of the RGBW format results from two factors. First, a large fraction of a typical image is near neutral in color and can be reproduced using the white sub‐pixel. Second, the white sub‐pixel in an RGBW AMOLED display is highly efficient because of the absence of any color filter. The efficiency of these displays can be further enhanced by choosing a white emitter optimized to the target display white point (in this case D65). A two‐emission layer configuration based upon separate yellow and blue‐emitting regions is shown to be well suited for both the RGBW and RGB formats.     

High‐resolution and high‐brightness full‐colour “Silicon Display” for augmented and mixed reality

High‐brightness micro‐LED display bonded onto silicon backplane has been successfully demonstrated. The 0.38‐inch full‐colour active matrix LED microdisplay system consists of 352 × 198 pixels. Each pixel is 24 μm square composed of red, green, and blue (RGB) subpixels corresponding to a pixel resolution of 1053 ppi. Quantum‐dot materials are formed on III‐nitride blue micro‐LED array to convert blue light into red and green for full‐colour operation. We have confirmed that this microdisplay, which we call “Silicon Display” has wide colour gamut exceeding 120% of sRGB. We describe the advantage of this colour‐converting approach for the full‐colour micro‐LEDs. Progress toward higher resolution is also described. Brightness of more than 30 000 cd/m² has been confirmed at a driving current density of 4 A/cm² for 3000 ppi blue monochrome micro‐LED prepared for full‐colour Silicon Display. We believe our “Silicon Display” is ideally suited for near‐to‐eye displays for augmented and mixed reality.     

Ultra‐high‐resolution 1058‐ppi OLED displays with 2.78‐in size using CAAC‐IGZO FETs with tandem OLED device and single OLED device

We fabricated new 2.78‐in 1058‐ppi organic light‐emitting diode (OLED) displays. The displays used OLED devices with a tandem structure and a single structure and a field effect transistor (FET) using c‐axis aligned crystalline In–Ga–Zn–O (CAAC‐IGZO) for an active layer and employing the 1.5‐µm rule over a glass substrate. Even in the displays with such high resolution exceeding 1000 ppi, crosstalk that was observed in the lower luminance region was suppressed. The displays achieved high color reproducibility and reduced viewing angle dependence.