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 Al₂O₃/SiN_x 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.

We fabricated highly efficient iridium(III) bis[(4,6-di-fluorophenyl)-pyridinato-$\text{N},\text{C}{2}^{\prime }$ ] picolinate doped inverted bottom-emission blue phosphorescent organic light-emitting diodes, with an electron injection layer of zinc oxide (ZnO) nanoparticles (NPs). The ZnO NPs layer lowers the turn-on voltage by about 4 V and significantly enhances the efficiency. The device with ZnO NPs shows peak efficiencies of 16.5 cd/A and 8.2%, about three times higher than those of the device without ZnO NPs. Since the ZnO NPs layer has a wide band gap, good electron transporting properties and low work function, it can be utilized as an effective electron injection layer with good transparency.

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.