Microdisplays have been used in various applications such as beam projectors, view finders of digital cameras, projection TVs, night vision for military use, and augmented reality/virtual reality ...(AR/VR) devices. Organic light-emitting diode (OLED) microdisplays have attracted much attention as main displays of glass-type and head-mounted-type AR/VR devices due to their rich colors, high contrast ratio, fast response time, small form factor, and high resolution. This review investigates the device, process, pixel circuit, and panel technologies for OLED microdisplays. In addition, the technology status and issues of OLED microdisplays are discussed.
Thin-film transistor (TFT)-driven full-color organic light-emitting diodes (OLEDs) with vertically stacked structures are developed herein using photolithography processes, which allow for ...high-resolution displays of over 2,000 pixels per inch. Vertical stacking of OLEDs by the photolithography process is technically challenging, as OLEDs are vulnerable to moisture, oxygen, solutions for photolithography processes, and temperatures over 100 °C. In this study, we develop a low-temperature processed Al
O
/SiN
bilayered protection layer, which stably protects the OLEDs from photolithography process solutions, as well as from moisture and oxygen. As a result, transparent intermediate electrodes are patterned on top of the OLED elements without degrading the OLED, thereby enabling to fabricate the vertically stacked OLED. The aperture ratio of the full-color-driven OLED pixel is approximately twice as large as conventional sub-pixel structures, due to geometric advantage, despite the TFT integration. To the best of our knowledge, we first demonstrate the TFT-driven vertically stacked full-color OLED.
Colloidal quantum dots (QDs) stand at the forefront of a variety of photonic applications given their narrow spectral bandwidth and near-unity luminescence efficiency. However, integrating ...luminescent QD films into photonic devices without compromising their optical or transport characteristics remains challenging. Here we devise a dual-ligand passivation system comprising photocrosslinkable ligands and dispersing ligands to enable QDs to be universally compatible with solution-based patterning techniques. The successful control over the structure of both ligands allows the direct patterning of dual-ligand QDs on various substrates using commercialized photolithography (i-line) or inkjet printing systems at a resolution up to 15,000 pixels per inch without compromising the optical properties of the QDs or the optoelectronic performance of the device. We demonstrate the capabilities of our approach for QD-LED applications. Our approach offers a versatile way of creating various structures of luminescent QDs in a cost-effective and non-destructive manner, and could be implemented in nearly all commercial photonics applications where QDs are used.
The optical properties of the materials composing organic light‐emitting diodes (OLEDs) are considered when designing the optical structure of OLEDs. Optical design is related to the optical ...properties, such as the efficiency, emission spectra, and color coordinates of OLED devices because of the microcavity effect in top‐emitting OLEDs. In this study, the properties of top‐emitting blue OLEDs were optimized by adjusting the thicknesses of the thin metal layer and capping layer (CPL). Deep blue emission was achieved in an OLED structure with a second cavity length, even when the transmittance of the thin metal layer was high. The thin metal film thickness ranges applicable to OLEDs with a second microcavity structure are wide. Instead, the thickness of the thin metal layer determines the optimized thickness of the CPL for high efficiency. A thinner metal layer means that higher efficiency can be obtained in OLED devices with a second microcavity structure. In addition, OLEDs with a thinner metal layer showed less color change as a function of the viewing angle.
Organic light-emitting diode (OLED) microdisplays have received great attention owing to their excellent performance for augmented reality/virtual reality devices applications. However, high pixel ...density of OLED microdisplay causes electrical crosstalk, resulting in color distortion. This study investigated the current crosstalk ratio and changes in the color gamut caused by electrical crosstalk between sub-pixels in high-resolution full-color OLED microdisplays. A pixel structure of 3147 pixels per inch (PPI) with four sub-pixels and a single-stack white OLED with red, green, and blue color filters were used for the electrical crosstalk simulation. The results showed that the sheet resistance of the top and bottom electrodes of OLEDs rarely affected the electrical crosstalk. However, the current crosstalk ratio increased dramatically and the color gamut decreased as the sheet resistance of the common organic layer decreased. Furthermore, the color gamut of the OLED microdisplay decreased as the pixel density of the panel increased from 200 to 5000 PPI. Additionally, we fabricated a sub-pixel circuit to measure the electrical crosstalk current using a 3147 PPI scale multi-finger-type pixel structure and compared it with the simulation result.
In this letter, we discuss the temperature distribution of light-illuminated Si substrates with time and the successful fabrication of high-performance, solution-processed indium-oxide thin-film ...transistors within an annealing time of 2 min using a high-power flash lamp under ambient conditions. The precursor films are completely converted into oxide films within 30 s using flash lamp annealing (FLA). As a result, we obtained the high performance of indium-oxide TFTs with the mobility of impressive 38.9 cm 2 V -1 s -1 with the on/off ratio of ~104 for the irradiation time of 2 min and the mobility of 10.3 cm 2 V -1 s -1 with the on/off ratio of ~5 × 10 6 for the irradiation time of 10 s for three times, which is the highest mobility ever reported using FLA. This result will be helpful for breaking the barrier in the mass production of the next-generation semiconductors.
We demonstrated highly efficient inverted bottom-emission organic light-emitting diodes (IBOLEDs) using tin dioxide (SnO2) nanoparticles (NPs) as an electron injection layer at the interface between ...the indium tin oxide (ITO) cathode and the organic electron transport layer. The SnO2 NP layer can facilitate the electron injection since the conduction band energy level of SnO2 NPs (−3.6 eV) is located between the work function of ITO (4.8 eV) and the lowest unoccupied molecular orbital (LUMO) energy level of typical electron transporting molecules (−2.5 to −3.5 eV). As a result, the IBOLEDs with the SnO2 NPs exhibited a decrease of the driving voltage by 7 V at 1000 cd/m2 compared to the device without SnO2 NPs. They also showed a significantly enhanced luminous current efficiency of 51.1 cd/A (corresponds to the external quantum efficiency of 15.6%) at the same brightness, which is about two times higher values than that of the device without SnO2 NPs. We also measured the angular dependence of irradiance and electroluminescence (EL) spectra in the devices with SnO2 NPs and found that they had a nearly Lambertian emission profile and few shift in EL spectrum through the entire viewing angles, which are considered as remarkable and essential results for the application of OLEDs to display devices.
We investigated the optical reflectance, surface roughness, and sheet resistance of aluminum (Al)/titanium nitride (TiN), titanium (Ti), and tungsten (W) layers on Si substrates, which are available ...in the CMOS foundry, for organic light-emitting diode (OLED) microdisplays. The devices with different metal anode layers exhibited different hole-injection properties and OLED performances, owing to the different optical and electrical properties of metal anode layers. Based on the OLED characteristics, the Al/TiN layer was selected as an anode layer for OLED microdisplays. A green monochromatic OLED microdisplay panel was designed and implemented using the 0.11-μm CMOS process. The density of pixels was ~2 351 pixels per inch and the panel's active area was 0.7 in in diagonal. The resolution of the panel was 1 280 × 3 × 1 024, corresponding to SXGA. The panel was successfully operated, and the maximal luminance was ~460 cd/m 2 .
Microdisplays based on organic light‐emitting diodes (OLEDs) have a small form factor, and this can be a great advantage when applied to augmented reality and virtual reality devices. In addition, a ...high‐resolution microdisplay of 3000 ppi or more can be achieved when applying a white OLED structure and a color filter. However, low luminance is the weakness of an OLED‐based microdisplay as compared with other microdisplay technologies. By applying a tandem structure consisting of two separate emission layers, the efficiency of the OLED device is increased, and higher luminance can be achieved. The efficiency and white spectrum of the OLED device are affected by the position of the emitting layer in the tandem structure and calculated via optical simulation. Each white OLED device with optimized efficiency is fabricated according to the position of the emitting layer, and red, green, and blue spectrum and efficiency are confirmed after passing through color filters. The optimized white OLED device with color filters reaches 97.8% of the National Television Standards Committee standard.
The subpixel size of high-resolution organic light-emitting diode (OLED) microdisplays measures a few micrometers. To achieve a full-color microdisplay, the current preference is to use ...photolithography technology on a white OLED instead of using a fine metal mask for patterning. The application of photolithography techniques allows for the creation of microcavity structures or color filters (CFs) to enhance color purity in each emitted color. This paper focuses on investigating specialized OLED device structures for high-resolution microdisplays. The characteristics of each structure, including efficiency and color gamut, are thoroughly discussed. The structure, combining microcavity and CF, achieves the widest color gamut, while microcavity structures show higher efficiency compared to CF-only structures. Tandem structures are also considered for improved efficiency and stability.
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