Progress of display performances: AR, VR, QLED, and OLED

In 2019, the device performances of the display technologies were largely advanced by the development of new materials and of the device architecture and driving scheme. The recent progress in the areas of virtual reality (VR), augmented reality (AR), quantum dot light-emitting diode (QLED), and organic light-emitting diode (OLED) is comprehensively summarized and discussed in this paper.


Recent progress of augmented reality and virtual reality
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.

FOV and angular resolution
The FOV of a NED is the angular size of the displayed virtual image. Generally, a large FOV is desirable to cover the FOV of the human visual system, which reaches 160 degrees when the eye rolling is considered [15]. Although the maximum FOV of the commercialized VR NED is about 170 degrees, which is achieved by using double display devices [11], the typical FOV of the commercialized VR NED is limited to 110 degrees [16]. The effort to enhance the FOV for the VR NEDs is usually focused on the development of new lens optics [17,18] [19,20]. The use of the geometric-phase (GP) lens has also been reported to achieve over 80 degrees FOV by enabling transmissiontype configuration [9,21]. 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 [22]. 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) [23], 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 [13,[24][25][26], which are summarized in Table 2.

Focal cue
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.

Various functionalities
Along with the research on the fundamental performance factors, including the FOV, angular resolution, and VAC mitigation, various techniques enabling more realistic and comfortable viewing have also been reported in 2019. One example is the hard-edge occlusion of real objects by Polarization multiplexing using GP lenses [28,29] Varifocal Mechanical axial shift of the display panel [13] Varifocal Freeform Alvarez lens [30] Light field Hybrid presentation of 2D and 3D images using an LC lens array to enhance the perceived resolution [31] Holographic Full color using a single SLM and grating [32] Holographic Foveated hologram synthesis for NEDs [33,34]

Holographic
Hologram synthesis from light field data with enhanced resolution over a holographic stereogram [35,36] the displayed virtual images in the AR NEDs. The usual AR NEDs only add the light for the virtual images to the light coming from real objects without proper blocking, which makes the displayed virtual images translucent. The hard-edge occlusion techniques block the light from real objects at the virtual image position, enabling more realistic image presentation and enhancing the image contrast. Although the first high-edge occlusion technique has been demonstrated nearly two decades ago [37], the recent works in 2019 report a more compact form factor [38] and variable distance occlusion [39]. Another example is the AR NEDs with vision prescription lenses [40]. For the widespread use of the AR and VR NEDs, comfortable wearing is important. The compact AR NEDs with vision prescription lenses make it easy to wear the AR NEDs on top of the vision-correcting glasses, making the AR NED experience comfortable and more practical. A subtractive AR NED has also been reported in 2019 [41]. While most AR NEDs work in additive mode, the subtractive AR NED presents images by subtracting the virtual image portion from the incoming real object light, making the virtual images more apparent in bright ambient light conditions. Table 4 summarizes these works. Occlusion-capable Double-pass configuration using polarization BS [38] Occlusion-capable Varifocal image + occlusion using tunable lenses [39] Prescription AR Prescription lens implemented in the NED; compact form factor [40] Subtractive AR Spatial color filtering using phase SLM; passive image illumination by environmental light [41]

Progress of QLEDs
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.

Cd-based QLEDs
Cd-based QDs have the advantages of narrow emission (FWHM < 30 nm) and high stability. With these advantages, the external quantum efficiency (EQE) of Cd-based QLEDs is continuously increasing. Recently, > 30% EQE was reported through shell growth control and modification of the surface ligands of the red-emitting Cd-based QDs [45]. As shown in Figure 1 and Table 5, the maximum EQEs of red-, green-, and blue-emitting QLEDs reached 30.9, 25.04, and 19.5%, respectively [45,51,54].
From the viewpoint of brightness, a QLED exhibiting an extremely high value of > 1,600,000 cd/m 2 was achieved by improving the heat dissipation through the use of sapphire as a substrate as sapphire has higher thermal conductivity than the glass substrate [52].

InP-based QLEDs
Colloidal QDs without any Cd atom is of interest due to the toxicity of Cd. For the red and green emitters, InP is the most widely investigated. The performance of InP-based QLEDs has been rapidly improved of late. A > 20% EQE was reported for the red-emitting InP-based QLEDs in November 2019, by removing the defective oxide layer at the surface of the InP core and engineering the shell thickness and ligand length [61]. This work also reported the high brightness of ∼ 100,000 cd/m 2 as well as enhanced operational lifetime. Bright and efficient green-emitting InP QLEDs with narrow spectral bandwidths (FWHM < 40 nm) were also reported by introducing a hole-suppressing interlayer with a top emission structure [59], and by adopting the composition-tailored ZnMgO nanoparticles as the ETL [66]. These results are brightening the prospects of using Cd-free QDs for the practical QLEDs. The device performances of the selected InP-based QLEDs are summarized in Table 6.

Demonstration of active-matrix QLED displays
The implementation of active-matrix QLEDs (AMQLEDs) is highly meaningful for the realization of QLED displays. A few companies and research groups have demonstrated monochromic or full-color AMQLEDs [67][68][69][70][71][72][73], as shown in Figure 2. For the monochromatic AMQLEDs, the QD layer can be formed    through spin-coating. For the fabrication of full-color AMQLEDs, however, QD patterning methods are among the essential technologies. Several QD patterning methods have been reported, such as pick-and-place transfer and inkjet printing [69,71,73]. The size of the display screen is also increased gradually; a well-known display company showed its 14-inch full-color AMQLED display driven by indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) in 2018. Only Cd-based QDs have been used to date, but the InP-based AMQLEDs are expected to be demonstrated in near future.

Fluorescent OLEDs
In the field of fluorescent OLEDs, most of the past development efforts were focused on the research on singlet exciton harvesting fluorescent OLEDs rather than on the traditional fluorescent OLEDs. The progress of the fluorescent blue OLEDs was marginal. In the red, green, and blue singlet harvesting fluorescent OLEDs, the EQE is close to or above 20%. Therefore, the potential of the   singlet exciton harvesting fluorescent OLEDs as highefficiency OLEDs has been established. Moreover, the lifetime was also largely advanced by the material and device engineering. In 2019, remarkable advances in the blue thermally activated delayed fluorescence (TADF) performances have been made. The EQE of the blue TADF OLEDs exceeding 30% with a deep-blue color coordinate was first demonstrated, and it was even better than that of the blue phosphorescent OLEDs. The lifetime data of the TADF OLEDs, however, are still limited. The device performances of the fluorescent OLEDs achieved in 2019 are summarized in Table 7.

Phosphorescent OLEDs (PhOLEDs)
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.

Soluble OLEDs
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).

Outlook
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.

Disclosure statement
No potential conflict of interest was reported by the author(s). He has been working on the acquisition, processing, and display of three-dimensional information using holography and light field techniques.