显示性能进展：AR、VR、OLED

The emerging augmented reality (AR) and virtual reality (VR) applications continue to drive the development of the near-eye display (NED) or head-mounted display (HMD) techniques. Various research and development efforts are being made to enhance the traditional performance factors of the AR and VR NEDs, such as field of view (FOV) and angular resolution. The recent researches, however, also focused on various features enabling realistic and comfortable image presentation, including vergence-accommodation conflict (VAC) mitigation, hard-edge occlusion, and vision correction. Table 1 shows the features of the recently reported or commercialized NEDs.

The angular resolution of a NED is defined by the number of pixels in a unit degree. The direct approach to enhancing the angular resolution is to increase the pixel density of the display panel. The trade-off relationship between the FOV and the angular resolution, however, makes it difficult to achieve a wide FOV and a high angular resolution simultaneously at a given pixel density of the display panel . A notable research work in 2019 reported the foveated displays. Motivated by the different angular resolution of the human visual system in the central vision (around 60 pixels per degree) and the peripheral vision (around 30 pixels per degree) , the foveated displays present high-angular-resolution images only within the eye gaze area while maintaining a low angular resolution in the peripheral area, reducing the total system resolution requirement. Several techniques have been reported in 2019 to achieve the foveated image presentation and dynamic change of the foveated area according to the tracked eye gaze direction, which are summarized in Table 2.

The usual AR and VR NEDs optically form virtual images at a fixed distance while the distance perceived by the user is varied by the disparity in the stereoscopic image pair presented to the two eyes. The difference between the optical and perceived distances causes the VAC, which hinders natural and comfortable viewing in both AR and VR applications. In the AR case, the different focal blur between the real objects and the virtual images also deteriorates the AR experience. VAC mitigation has been an active research field in academia for a decade. Since recently, however, industry has also been making considerable efforts to develop practical solutions for VAC. Various techniques, including the multiplane display , varifocal display, extendeddepth-of-focus display, light field display, and holographic displays , have been reported in 2019, and their features are summarized in Table 3.

Research on the synthesis of colloidal nanocrystal quantum dots (QDs) and their application to quantum dot light-emitting diodes (QLEDs) has attracted great attention for a decade due to such QDs’ unique optical and electrical properties. The photoluminescence quantum yield (PLQY) of the QDs have been approaching unity owing to the advances in the QD synthesis methods. As a result, the performance of QLEDs has also been increasing, as plotted in Figure 1. It is noticeable that the number of papers reporting InP-based QLEDs is currently rapidly increasing.

Fig1

The device performances of the soluble OLEDs have been steadily improved every year. In particular, progress in the blue device performances was noticed in 2019. The lifetime of the blue soluble OLEDs is approaching that of the vacuum-deposited blue OLEDs. The EQE and lifetime data of soluble OLEDs are summarized in Table 9. (Figure 3).

The device performances of PhOLEDs have not been largely improved, but significant advances in the bluedevice lifetime have been reported. For the first time, an over 1000 h device lifetime was reported in the deep-blue PhOLEDs’ EQE. The lifetime data of PhOLEDs are summarized in Table 8.

The progress of the quantum dot light-emitting diode (QLED) device performances was remarkable in 2019. In particular, the Cd-free quantum dot (QD) technology is catching up with the Cd-based QD technology. Although the QLED performances of the Cd-free green and blue QDs are inferior to those of the Cd-based QDs, they will be upgraded in the near future. In the field of organic light-emitting diodes (OLEDs), the lifetime of the blue devices is not long enough, but the development of the narrowband blue emitters opened a new way of upgrading the device performances of the blue OLEDs. The continuous exploration of the narrowband organic emitters in the red, green, and blue colors will further promote the efficiency and lifetime of the OLEDs. The augmented reality (AR) and virtual reality (VR) displays have also been significantly advanced. The active applications of liquid crystal (LC) or metahologram-based global positioning system (GPS) devices to achieve a wide field of view (FOV), a high compact form factor, and enhanced vergence-accommodation conflict (VAC) mitigation were notable in 2019. Foveated displays are also being studied actively, and their implementations in various system configurations have been reported. Research on the new image combining devices and optical systems is expected to continue to enhance not only the key performance factors like the FOV, angular resolution, and form factor but also new features like VAC mitigation and hard-edge occlusion.