High-resolution Active-matrix Organic Light-emitting Diode Research Articles

(Invited) Short Channel Coplanar Amorphous-Indium-Gallium-Zinc-Oxide Thin-Film Transistors on Polyimide Substrate for High Resolution Displays

High-resolution active-matrix organic light-emitting diode (AMOLED) thin-film transistor (TFT) backplane with lower power consumption is needed for augmented reality (AR) applications. The near-to-eye format of AR terminals needs facing several technical challenges, such as higher optical efficiency, wider field of view, lighter weight, and smaller size to accelerate widespread applications. For these reasons, downscaling of TFTs with amorphous-indium-gallium-zinc-oxide (a-IGZO) or LTPS is required. The channel length should be less than 1 μm and the flexible substrate is better than glass because of light weight and very thin structure. TFT technologies on polyimide (PI) substrate can support the applications for foldable and rollable displays. For flexible AMOLED, PI layer of 10 to 20 um is widely used for foldable displays. The PI layer is coated by slot coating on the top of a carrier glass with a buffer layer of carbon nanotubes (CNTs) and graphene oxide (GO) mixture coated by spray. This layer can reduce the adhesion force between PI and glass and thus mechanical detachment can be possible. The CNTs are conducting so that it can release the electrostatic discharge (ESD) damage because CNTs/GO layer exists on the bottom side of the PI substrate. The PI substrate is covered by SiO2 and ZrAlOx layers (five layers in total starting with SiO2) by plasma enhanced chemical vapor deposition and spray pyrolysis, respectively, which acts as a gas barrier. The amorphous ZrAlOx film by spray coating can be used as a high-k gate insulator and also a gas barrier. The threshold voltage of a-IGZO TFTs tends to shift toward negative gate voltage with decreasing the channel length. In this work, a ZrAlOx passivation layer is introduced for the short channel, coplanar a-IGZO TFT. A very thin ZrAlOx layer is deposited by spray pyrolysis on the top of a-IGZO TFT at the substrate temperature of 350 ℃ before interlayer deposition. The a-IGZO TFT with a very thin ZrAlOx layer exhibits the stable threshold voltage dependence with decreasing the channel length. The 0.8 μm channel length a-IGZO TFT with ZrAlOx layer by spray pyrolysis at the substrate temperature of 350 ℃ exhibits the field-effect mobility of 7.09 cm2 V-1 s-1, the threshold voltage of -0.6 V, and on/off current ratio of 3.3 × 107. The formation of Zr-O bonds on the a-IGZO surface region reduces the carrier concentration in the a-IGZO TFT offset region, between source/drain contact and the channel, and blocks the carrier diffusion into the channel. This is the first reported self-aligned coplanar a-IGZO TFT under 1 μm channel length on a flexible substrate. Figure 1

One-Step Etching Characteristics of ITO/Ag/ITO Multilayered Electrode in High-Density and High-Electron-Temperature Plasma.

This paper presents the dry etching characteristics of indium tin oxide (ITO)/Ag/ITO multilayered thin film, used as a pixel electrode in a high-resolution active-matrix organic light-emitting diode (AMOLED) device. Dry etching was performed using a combination of H2 and HCl gases in a reactive ion etching system with a remote electron cyclotron resonance (ECR) plasma source, in order to achieve high electron temperature. The effect of the gas ratio (H2/HCl) was closely observed, in order to achieve an optimal etch profile and an effective etch process, while other parameters—such as the radio frequency (RF) power, ECR power, chamber pressure, and temperature—were fixed. The optimized process, with an appropriate gas ratio, constitutes a one-step serial dry etch solution for ITO and Ag multilayered thin films.

Pixel Circuit With Parallel Driving Scheme for Compensating Luminance Variation Based on a-IGZO TFT for AMOLED Displays

This work proposes a new pixel structure based on amorphous indium-gallium-zinc-oxide thin-film transistors (a-IGZO TFTs) and a parallel addressing scheme for high-resolution active-matrix organic light-emitting diode (AMOLED) displays. The proposed circuit compensates for the nonuniformity of luminance that is caused by shifts in the threshold voltage ( ${{V}}_{{\rm{TH}}}$ ) and mobility of driving TFTs. Measurement results verify that the parallel addressing scheme successfully extends the compensation time and accurately detects the ${{V}}_{{\rm{TH}}}$ of the driving TFT. Moreover, the proposed circuit reduces the variations of OLED luminance from more than 83% to less than 13% when the ${{V}}_{{\rm{TH}}}$ and mobility of driving TFT shifts by 1 V and 30%, respectively, and the ${{V}}_{{\rm{TH}}}$ of OLED varies from 0 to 0.9 V.

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.

A Simple Pixel Structure Using Polycrystalline-Silicon Thin-Film Transistors for High-Resolution Active-Matrix Organic Light-Emitting Diode Displays

A simple pixel structure comprising two transistors and one capacitor and a novel driving method are proposed for small-sized and high-resolution active-matrix organic light-emitting diode displays on polycrystalline-silicon thin-film transistor (TFT) backplane. The proposed pixel increases the yield due to small number of TFTs and improves image quality by compensating threshold voltage variations of driving TFTs. The proposed pixel is designed as test pixels with 353 pixels per inch, and the measured emission current error range of -74.2% -60.6% is improved to the range of -2.5% -2.1% when the proposed pixel is used.

A New 4T0.5C AMOLED Pixel Circuit With Reverse Bias to Alleviate OLED Degradation

A new 4T0.5C circuit is proposed to achieve a high-resolution active-matrix organic light-emitting diode (OLED) display with the capability of alleviating OLED degradation, while the panel nonuniformity is compensated by an already-proposed system. The designed circuit is built on two adjacent pixels with eight transistors and one capacitor, thus equivalently as a 4T0.5C pixel circuit in occupied layout area. In this circuit, a single storage capacitor is shared by the two adjacent pixels to minimize the entire layout area. The minimized pixel size is compatible to a conventional 2T1C pixel circuit. Along with this 4T0.5C circuit, a new alternative drive scheme is developed to address the panel. The scheme offers the function of automatic reverse biasing on OLEDs to alleviate degradation. The designed circuit is fabricated, and the performance of OLED degradation is successfully validated.

New Pixel Circuit Compensating Poly-si TFT Threshold-voltage Shift for Driving AMOLED

This study presents a novel pixel circuit that uses only n-type low-temperature polycrystalline-silicon (poly-Si) thin-film transistors ((LTPS-TFTs) to simplify the fabrication process of active-matrix organic light-emitting diode (AMOLED) displays. The proposed pixel circuit consists of five switching TFTs, one driving TFT (DTFT), and two capacitors. The output current and the OLED anode voltage error rates are about 3% and 0.7%, respectively. Thus, the pixel circuit can realize uniform output current with high immunity, to the poly-Si TFT threshold voltage deviation. The proposed novel pixel design has great potential for use in large-size, high-resolution AMOLED displays.

38.2: 302-ppi High-Resolution AMOLED using Laser-Induced Thermal Imaging

Samsung SDI has developed 2.6-inch full color VGA Active Matrix Organic Light-Emitting Diode (AMOLED) display with the world's highest resolution of 302ppi using Laser Induced Thermal Imaging (LITI) method with the top emission structure. A voltage compensating pixel circuit with 2μm design-rule in low-temperature poly-Si (LTPS) thin film transistors (TFTs) was introduced to compensate the variation of the threshold voltage (Vth) in order to improve the image quality. Moreover, an aperture ratio of ∼40% was obtained for each pixel even though the sub-pixel pitch was 28um. The luminance for the white color was 200cd/m2 and color gamut was 74% compared to national television standard committee (NTSC) standards. These results show the possibility of high-resolution AMOLED display incorporating the LTPS technology and LITI method for digital mobile era applications in the near future.