Introduction

Flexible displays have received significant attention owing to their potential applications to mobile and wearable electronics1,2 such as smartphones, automotive displays, and wearable smart devices. Displays with flexible form factors have thin, lightweight, and nonbreakable characteristics, which enable the fabrication of displays on curvilinear surfaces and allow their shapes to be transformed.3,4 In 2008, Nokia announced Morph, an innovative mobile display concept with flexible, bendable, and interactive features. This was followed by the development of early prototypes of flexible e-paper. In 2013, Samsung Electronics demonstrated the first curved television (TV) based on organic light-emitting diodes (LEDs) that feature a wide field of view, high color purity, and outstanding contrast. Two years later, the same company reported a smartphone with the curved edge display (Galaxy S6 edge) that used a curved organic LED display panel with touch sensors to improve the user interface and device design.

Although non-flat displays are being marketed, the currently available commercialized displays are mostly bent displays whose shape cannot be changed. The next-generation display, by contrast, will be transformable into various forms.5,6,7 Figure 1 shows representative examples of future interactive displays with flexible form factors. Smart glasses and/or smart lens (Fig. 1a) will enable augmented reality, displaying information additively over the natural scene behind the glasses or lens. It will be possible to display vital signs (e.g., blood pressure, pulse, respiration rate, and body temperature) or other health information measured by a wearable sensor in real-time through smart watches (Fig. 1b). LEDs fabricated in the form of yarns can be woven into fabric and cloth for wearable displays (Fig. 1c). Ultrathin displays will be attached to the human skin in the form of electronic tattoos (Fig. 1d). Bendable displays can be utilized as foldable tablets whose screen sizes are tunable (Fig. 1e). Moreover, transparent flexible displays can be used for smart windows or digital signage, which can display digital information overlapped on the background view (Fig. 1f).

Fig. 1
figure 1

Future of flexible and wearable displays. a Smart glasses or smart lens. b Smart watch with wearable biosensors. c fabric display. d Ultrathin electronic tattoo. e Bendable and foldable display. f transparent smart windows

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A major technological goal in the research field of such next-generation displays is to develop LEDs with mechanical deformability as well as excellent device performances.8 Inorganic LEDs (iLEDs) have shown high brightness (106–108 cd m−2) and low turn-on voltages (<2 V) and have been used to develop flexible LED arrays. However, thick and brittle active layers (~micrometers) limit their flexibility, and the point-array design cannot realize a high-resolution display (Table 1, i).9,10 Organic LEDs (OLEDs; Table 1, ii) and polymer LEDs (PLEDs; Table 1, iii) have become a hot research topic because the self-luminous active layer simplifies the device structure, which can drastically reduce the entire thickness of the display.11,12 Recently, LG Electronics introduced a 66-inch ultra-large TV at SID 2017, which reduced the total panel thickness down to 1 mm. However, the flexibility of the current OLED display is still limited by thick encapsulation layers (e.g., it allows bending rather than folding or stretching). It is essential to develop thin encapsulation layer which endures continuous bending stress and effectively prevents oxidation of organic active layer, organic charge transport layer and thin metal electrode. The long-term electroluminescence (EL) stability and achievement of high color purity also remain practical challenges of OLEDs.

Table 1 Representative characteristics of different types of light-emitting didoes
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Recently, quantum dot LEDs (QLEDs) have received great attention because of their outstanding color purity (full-width-at-half-maximum (FWHM) ~30 nm), high brightness (up to ~200,000 cd m−2), low operating voltage (Vturn-on < 2 V), and easy processability (Table 1, iv).13,14,15 The high thermal and air stabilities of the inorganic quantum dots (QDs) enables the enhanced lifetime and durability of the display. In addition, the recent advances in patterning techniques have made it possible to achieve ultrahigh-resolution full-color (red, green, and blue; RGB) QLED array, which could not be implemented with the conventional display processing technologies (e.g., shadow masking in OLEDs). More detailed characteristics of the aforementioned LEDs with different types of light emitting layers (e.g., inorganic, organic, polymer, and QD) are summarized in Table 1.

Here, we thereby focus on QLEDs among the various types of light-emitting devices toward the next generation flexible displays. First, we review the synthesis strategies for highly efficient and stable QD emitters, the device structure engineering and operation principles of each structure, and the patterning technologies for full-color QD displays. Then we describe the recent technological advances in QLEDs such as flexible white QLEDs and transparent version of flexible QLEDs. Finally, this review introduces integrated systems using flexible QLEDs as informative displays for wearable electronics and concludes with a brief outlook.

Material design for efficient QLEDs

It is worthwhile to introduce the recent development of QLEDs, because it is highly related to the advance of flexible/wearable QLEDs. In this section, we discuss materials chemistry of QDs, which enables effective operation of QLEDs. QDs have many advantages in display applications, originating from the quantum-confinement effect.16,17,18,19,20,21,22,23,24 For example, emission wavelengths of CdSe QDs can be tuned by changing their size to emit the entire visible light (Fig. 2a).25 In addition, QDs based on various semiconductor materials provide broad spectral window and chemical versatility (Fig. 2b, c).26,27,28,29,30,31,32,33,34 High color purity is also an important characteristic for display applications. CdSe QDs show very sharp emission spectra (FWHM of ~30 nm) and wide color gamuts compared to the conventional standard emission spectra of the commercialized high-definition television (Fig. 2d).35,36,37

Fig. 2
figure 2

Material designs for efficient QLEDs. a Photograph of solutions containing CdSe QDs of various sizes, showing emission color covering the entire visible light range. b Spectral range of QDs with various semiconductor materials. c PL spectra of CdSe/ZnS and PbS/CdS core/shell QDs. d CIE chromaticity diagram showing the color gamut of CdSe QDs in comparison with that of conventional high-definition television standards. e Scheme showing the band structure of core/shell heterojunctions. f Scheme comparing the carrier dynamics of core/shell QDs with thick and thin shells. g Time-resolved PL spectra of QDs under various conditions. The inset shows the change in the carrier lifetime as a function of applied voltage. h Comparison of EQEs of QLEDs with CdSe/CdS core/shell QDs and CdSe/CdS/Zn0.5Cd0.5S core/shell/shell QDs. i, j, Current density–luminance–voltage characteristics (i), current efficiency and EQE (j) of QLEDs using CdSe/ZnS QDs with CdS-rich (blue) and ZnSe-rich (red) intermediate shells. k Energy band diagram, schematic, and TEM images of double heterojunction nanorods. l Current and power efficiencies of QLEDs based on double heterojunction nanorods as a function of luminance. The inset show a photograph comparing double heterojunction nanorod QLEDs and conventional core/shell QLEDs. m Energy diagram of InP@ZnSeS QLEDs. n EL spectrum and photograph of flexible InP@ZnSeS QLEDs on a polyethersulfone substrate. o TEM images and corresponding schematic images of Cu–In–S/ZnS QDs with controlled shell thickness. p EL spectra and corresponding images of Cu–In–S/ZnS QLEDs as a function of applied voltage. Figure reproduced with permission from: a ref. 25 © 2010 Wiley; c ref. 26 © 2010 Nano Reviews; f ref. 46 © 2012 NPG; g, h ref. 47 © 2013 NPG; i, j ref. 52 © 2015 NPG; k ref. 53 © 2014 NPG; l ref. 54 © 2015 ACS; m, n ref.69 © 2013 ACS; o, p ref. 73 © 2016 ACS

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Type I core/shell QDs are commonly employed in QLEDs (Fig. 2e), because overcoating the core QDs with wide band gap shell materials passivates surface defects and confines excitons to the cores.38,39,40,41,42,43 This leads to significant improvements in stability and the photoluminescence quantum yield (PLQY) that is directly proportional to the external quantum efficiency (EQE = n × χ × ηPL × ηoc). For example, CdSe/ZnS QDs exhibit high PLQY of 70–95%, which is an order of magnitude higher than that of bare QDs. However, the increase in PLQY does not guarantee the enhanced EL performance. Auger recombination of charged QDs and/or interparticle energy transfer between different QDs reduce the EL efficiency. These processes are dramatically affected by the structure of the core/shell interfaces, and consequently the structure modification of the core/shell QDs has become an important issue.

The simplest way of controlling the core/shell structure is to modify the shell thickness, which affects significantly the carrier dynamics of QDs as well as their stability. QDs with thick shells are less blinking (or nonblinking) because of either suppression of charge fluctuations or enhancement of the PLQY of charged QDs (Fig. 2f).44,45,46 The enhanced PL dynamics in thick-shell QDs can improve device performance significantly. As shown in Fig. 2g, QDs in devices are easily charged by excessive charge carriers (electrons in this case). Thicker shell helps to suppress charging of QDs during light emission of QLEDs, which results in the improved EL efficiency (Fig. 2h).47,48,49,50,51 The composition of the core/shell interface is also important for the carrier injection and recombination. Recently, two types of CdSe/ZnS core/shell QDs with similar PLQY and bandgaps were compared based on the composition of their core/shell interface: QDs with ZnSe-rich intermediate shells and QDs with CdS-rich intermediate shells (Fig. 2i, j).52 ZnSe-rich QDs demonstrated superior EL properties, which were attributed to the low carrier injection barrier of ZnSe-rich QDs. Currently, the composition of core/shell interface has been controlled roughly, and its impact on the EL mechanism has yet to be fully understood. A major limiting factor in these efforts is the lack of characterization methods that can determine precisely the three-dimensional composition distribution of core/shell QDs.

Structure engineering of QDs improves not only carrier dynamics but also other factors such as light outcoupling. For example, QLEDs based on double-heterojunction nanorods show unexpected enhancement of EL performance (maximum brightness = 76,000 cd m−2, peak EQE = 12%) (Fig. 2k, l).53,54,55 In this structure, two CdSe emitters are directly connected to CdS nanorods and the remaining surface of CdSe is passivated by ZnSe. Remarkably, the obtained peak EQE (12%) was higher than the expected upper limit (8%) considering their PLQY (40%). It was suggested that the shape anisotropy and directional band offsets of the double-heterojunction nanorods could improve light outcoupling.

Along with the recent progress, there have been growing concerns regarding the use of QDs containing the Cd element, which is very harmful for the human body and environment.56,57,58,59,60,61,62 This issue become more significant in flexible/wearable displays, where the device is in direct contact with the human body. For instance, the European Union’s Restriction of Hazardous Substances Directive regulates the use of Cd-based compounds in consumer electronics. Although several approaches such as encapsulation, or minimization of Cd concentration by composition control have been reported, it is clear that the development of efficient heavy-metal-free QLEDs is ultimately required for the commercial success of flexible/wearable QLEDs.

III-V InP QDs are promising alternatives to Cd-chalcogenide QDs because their bandgaps cover the entire visible range (bulk bandgap = 1.34 eV) with outstanding PLQYs.63,64,65,66 Although there have been several reports on InP QLEDs, a gap remains in terms of the performance of InP QLEDs and that of Cd-chalcogenide QLEDs,67,68 which can be attributed to the lack of fundamental understanding of the EL process of InP QDs. Recent progress on QLEDs based on green InP core–shell QDs is encouraging (Fig. 2m, n); record EQE69 = 3.4%, record brightness70 = 10,490 cd m−2. Another important approach is the utilization of I–III–VI semiconductor QDs (Fig. 2o, p).71,72,73,74 A critical drawback of I–III–VI QDs is their broad emission spectra (FWHM: ~100 nm), which removes the major advantage of QDs (i.e., high color purity due to narrow spectra) in display applications.75

Device structure and operation principles of QLEDs

Basically, the device structure of flexible/wearable QLEDs are largely adopted from the general QLEDs with several modifications to achieve high deformability. The general structure of QLEDs consists of an anode, electron transport layers (ETLs), QD layers, hole transport layers (HTLs), and a cathode (Fig. 3a). The working mechanism of QLEDs is as follows: (i) electrons and holes are injected from electrodes into charge transport layers (CTLs); (ii) the carriers are injected into QDs from CTLs; and (iii) radiative recombination of the injected carriers takes place in QDs (Fig. 3b). The performance and stability of QLEDs are largely dependent on the choice of CTL materials. Good CTLs should have the high carrier mobility and balance the electron/hole injections well.

Fig. 3
figure 3

Device structure and operation principles of QLEDs. a Representative device structure of quantum dot light-emitting diodes (QLEDs). b Energy diagram of a typical QLED showing charge injections from the anode and cathode. c Four representative device types of QLEDs according to the type of charge transport layer (CTL): (i) organic polymer CTL, (ii) all-organic CTL (e.g., TPD and TPBi), (iii) all inorganic CTL (e.g., n-type and p-type GaN), and (iv) organic (e.g., TFB)–inorganic (e.g., ZnO) CTL. d, e Progress in the performance of QLEDs in terms of peak external quantum efficiency (EQE) (d) and maximum brightness (e). The data are categorized according to the type of CTL (From type (i) to (iv) in c). Data for d are taken from references. (i) organic single CTLs: 76, 77, 78, 110, (ii) all-organic CTLs: 79, 80, 82, 83, 84, (iii) all-inorganic CTLs: 85, 86, 88, (iv) hybrid CTLs: 52, 89, 92, 93, 94, 95, 96. Data for panel e are taken from references: (i) organic single CTLs: 76, 77, and 110, (ii) all-organic CTLs: 79, 80, 82, and 83, (iii) all-inorganic CTLs: 86, (iv) hybrid CTLs: 48, 52, 89, 92, 93, 94, and 98

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According to the type of CTLs employed, the structure of QLEDs can be categorized into four different types (Fig. 3c): (i) organic/QD bilayer; (ii) all-organic CTLs; (iii) all inorganic CTLs; and (iv) organic–inorganic CTLs. Their device performances in terms of the peak EQE and maximum brightness are summarized in Fig. 3d, e. Owing to its simplicity, the type (i) structure (organic/QD bilayer) was commonly used in the earliest QLEDs.76,77,78 However, the electron injection was poorly controlled, and the leakage current was significant (maximum brightness ~100 cd m−2, EQE <0.01%) because of the absence of ETLs and poor physical separation of QDs and CTLs. To solve these issues, the type (ii) device (QD layers sandwiched between organic HTLs and ETLs) was proposed.79,80,81,82,83,84 The peak EQE of the initial type (ii) device was ~0.5% and has been improved to 6% (Fig. 3d, e).

Inorganic materials are among the most important choices for CTLs (type (iii) QLEDs; all-inorganic CTLs) owing to their high electrical conductivity and good stability against environmental factors such as oxygen and moisture. In the initial study, QD layers were sandwiched between p-type and n-type GaN (EQE <0.01%).85 In the following works, QLEDs with all-inorganic CTLs composed of metal oxides (e.g., ZnO, SnO2, ZnS, NiO, and WO3) were demonstrated.86,87,88 These devices exhibited superior stability under long-term usage and high-current-density conditions, which would be potentially beneficial for future flexible display applications. However, the overall device performance was poor as a result of the degradation of QDs during the harsh deposition process of inorganic layers.

The type (iv) structure (usually organic HTLs and inorganic ETLs) was developed to take advantage of both inorganic and organic CTLs. Although their performance did not improve significantly (EQE of ~0.2%) in the initial work 89, an important breakthrough was made by introducing ZnO nanoparticles as ETLs.90,91,92,93,94,95,96 ZnO exhibits excellent electron mobility even in the form of nanoparticles, and no significant damage occurs to the underlying QD layers during the introduction of these nanoparticles in the device because they are solution processable. Currently, because of their excellent EL performance (see Fig. 3d, e for comparison), type (iv) devices using ZnO nanoparticles as ETLs have become a standard in the QLED research48,52,97,98 including flexible devices. Another important benefit of these devices is the ultrathin form factor (hundreds of nanometers) of overall layers, which makes them suitable for flexible displays. For example, one of the recent studies demonstrated the highly deformable wearable QLEDs whose total thickness is less than 3 μm including device parts and double-layered encapsulation layers.99 More information on ultrathin encapsulation layers for flexible QLEDs is available in the following section on wearable quantum dot displays.

Patterning technology of QDs for full-color displays

Significant efforts have been made to achieve high-resolution full-color QD-based displays, including flexible ones. Especially, wearable and/or portable electronics integrated with flexible display need high resolution and full color form to present vivid visual information within limited space. As the display technology has evolved, the resolution of TVs has reached the ultra-high definition (UHD; 3840 × 2160) level, and that of smartphone has become as high as ~800 pixels per inch (ppi). For instance, the pixel resolution of Xperia XZ premium (Sony) is 807 ppi. To show natural and clear images, the demand for the higher resolution displays is expected to increase further, as displays with the finer pixel resolution can express more vivid images. In case of the head-mount displays or virtual-reality displays, where flexible displays can be applied, implementation of even higher resolution displays is required to project three-dimensional synaptic images by magnifying the original two-dimensional images. There are two major approaches to patterning and integrating differently colored QDs with high resolutions onto the display panel—transfer printing100 and inkjet printing.101

Since the as-synthesized colloidal QDs are dispersed in a solution phase, the spin-casting process was typically used in the early QLED studies, which led to monochromatic light-emitting devices. Later, a structured elastomeric stamp (usually a poly(dimethylsiloxane) (PDMS) stamp) was used to make pixelated QD patterns. In 2008, the Bawendi and Bulovic´ group reported QLEDs patterned with lines and spaces, prepared by directly spin-casting the QD solution onto a structured stamp.102 Later, the researchers at the Samsung Advanced Institute of Technology (SAIT) developed a kinetically controlled transfer-printing technology.97 The spin-coated QD film was contacted with a structured stamp, quickly picked up from the self-assembled monolayer-treated donor substrate, and released onto the desired substrate (Fig. 4a). As a result of the pressure applied by the stamp (Fig. 4b), the printed QD layer showed less vacancies and cracks after transfer printing than the QD patterns formed by other patterning methods. This well-packed QD layer led to low leakage current and improved charge transport, as shown in the current density—voltage (J–V) curve (Fig. 4c). Using this transfer printing method, a 4-inch full-color flexible display with 320 × 240 pixels was successfully demonstrated.

Fig. 4
figure 4

Patterning techniques for multicolor QLEDs. a Scheme illustrating pick-and-release transfer printing using structured stamp. b SEM image of QD morphology before (left) and after (right) transfer printing. c J–V characteristics of QLEDs fabricated by transfer printing. d Four-inch full-color QD display. Inset shows the EL of transfer printed RGB pixels. e Scheme illustrating intaglio transfer printing. f Multiple transferred hierarchical RGB patterns. Bottom images show the SEM image and PL image of magnified RGB patterns. g Images of E-jet printing. The right image shows E-jet printed dual-color QD patterns. h SEM image of monolayered 400 nm line pattern. Right images show the magnified view of the E-jet printed QD line. Figure reproduced with permission from: a–d ref. 97 © 2011 NPG; e, f ref. 99 © 2015 NPG; g, h ref. 104 © 2015 ACS

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Although many patterning technologies were developed, extremely high resolution and printing yields could not be achieved. Therefore, to obtain pixels with higher definitions and yields, Choi et al. proposed an intaglio transfer-printing technique that could pattern a 2460 ppi red-green-blue (RGB) QD array.99 Unlike the technique base on structured stamps, the QD patterns were defined when the stamp, fully inked with QDs, was contacted to and released from the intaglio trench made on the Si wafer (Fig. 4e). The difference in the interfacial energies (EQD-trench >> EQD-stamp) enabled a much higher transfer yield without cross-contamination between adjacent pixels of the differently colored QDs than the previous transfer printing approaches (Fig. 4f).

In addition to the transfer printing method, inkjet printing has also attracted significant attention since it can form the desired patterns without photo and metal-shadow masks.103 However, the conventional inkjet printing methods are not suitable for the fine patterning of QD films because the additives used to improve the dispersion of QDs act as barriers for efficient charge transport and degrade the electrical performance of the QLEDs. However, efforts are being made to solve these issues. Recently, the Rogers group reported fine QD patterns (~5 μm) obtained using electrodynamic jet (E-jet) printing (Fig. 4g, h).104 E-jet printing uses the electric field to eject the QD ink with a narrow width, and the resulting QD pattern shows uniform line thicknesses. Using this printing method, red and green QD pixels are formed with a resolution up to that of the commercial display.

Flexible white QLEDs

White LEDs (WLEDs) are widely utilized as large-area lighting devices and/or backlight sources of the display panel. Arrays of inorganic white LEDs are being used, but point-emission, rather than areal emission, causes the areal non-uniformity. Organic white LEDs are considered as a good alternative, but many issues such as lifetime and cost still exist. Therefore, recently colloidal QDs are used as an emission component of WLEDs owing to their desirable properties, including high quantum yield, size-tunable emission spectrum, narrow emission bandwidth, and photo/thermal stability.13,14,15 Considerable efforts have been devoted toward realizing highly efficient QD-based WLEDs.

First, color-converting WLEDs composed of blue/UV light sources and smaller bandgap QDs in the polymer matrix were reported.105 Jang et al. reported a 46-inch TV panel using liquid crystal display with a WLED backlight by integrating red (CdSe/CdS/ZnS/CdSZnS) and green (CdSe/ZnS/CdSZnS) QDs with blue inorganic LEDs.106 However, the color-converting WLEDs showed generally low quantum efficiencies because of the reabsorption of high-energy photons by small bandgap QDs, internal photoscattering, photobleaching, and imbalanced charge carriers. In addition, the broad emission spectra of the conventional light sources reduced the luminous efficacy and showed low color rendering index (CRI).

To enhance the CRI and efficiency of WLEDs, EL-based white QLEDs were developed by using a mixture of QDs with different colors (Fig. 5a).107 In 2007, the Bawendi and Bulovic´ group reported a white EL device using a monolayer of randomly mixed QDs.108 The EL spectrum was easily tunable by controlling the mixing ratio of RGB QDs, and the white QLED showed improved EQE and CRI of 0.36% and 81, respectively. The human eye can easily perceive light with wavelengths between 440 and 650 nm, and, therefore, tuning the emission spectra within this range improves the CRI value. Bae et al. controlled the emission spectrum of white QLEDs, by precisely adjusting the mixing ratio of the QDs of different colors (Fig. 5b, c).109 A narrow bandwidth for the QD emitter (<30 nm) increases the color purity of monochromatic QD emission, but it also causes a wide spectral gap between the emission spectra of different colors and lowers the CRI value of WLEDs. To solve this issue, the number of emission peaks can be increased. This results in the more completely filled visible spectrum and a higher CRI value. The CRI value increases dramatically from 14 to 93, as the number of types of mixed QDs is increased from two (blue and yellow QD) to four (blue, cyan, yellow, and red). The white QLEDs based on randomly mixed QDs have advantages such as easy processing and cost reduction, but the inter-particle energy transfer between QDs of different colors induces low current efficiency, poor EQE, and red-shifted EL.110 Therefore, the mixing ratio and the mixed structure of the different QDs should be precisely optimized to obtain balanced white EL.

Fig. 5
figure 5

Flexible white QLEDs. a Device structure (left) and corresponding energy diagram (right) of a white QLED with mixed RGB QD emitting layers. b, c EL of flexible white QLEDs (b) and corresponding EL spectra (c). d Scheme illustrating the transfer-printing process for a QD monolayer. Structure of white QLED with multiple stacked QD layers and corresponding cross-sectional TEM image. Scale bar, 10 nm. e PL spectrum of B/G/B/R white QLEDs under various applied voltages. f TRPL decay of multistacked QD films with different layer structures. g Flexible white QLED with RGB patterns prepared using pick-and-release transfer printing. Inset shows the RGB pixels of the white QLED. h RGB patterns for white QLEDs using intaglio transfer printing. i Ultrathin white QLED array attached to human skin. j TRPL spectra of a blue QD layer and white QLEDs composed of RGB pixels (pixelated white QLED; PWQLED) and mixed QD layer (MWQLED). Figure reproduced with permission from: a–c ref. 109 © 2014 Wiley; d–f ref. 111 © 2013 NPG; g ref. 97 © 2011 NPG; h–j ref. 99 © 2015 NPG

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To enhance the EL efficiency, QD monolayers stacked layer-by-layer were introduced by the SAIT group using the pick-and-place transfer-printing technique (Fig. 5d).111 By adjusting the stacked order of the RGB QD monolayers, the nonradiative energy transfer (e.g., G → R) was prevented, and true white EL could be achieved (Fig. 5e, f). However, the vertically stacked QD layers inevitably showed interparticle energy transfer112,113 (e.g., G → R or B → R) because the QD layers of different colors were stacked in the direction of charge injection. Moreover, the EL spectrum was blue-shifted as the applied bias was increased because of the increased EL contribution of the larger bandgap QDs at the higher bias condition.

The white QLED based on the pixelated RGB QD array can solve these problems (Fig. 5g–j). Recently the Kim group reported a high-resolution RGB pixel array (>2400 ppi) using the intaglio transfer-printing method (Fig. 5h, i).99 As shown in Fig. 5j, the carrier lifetime of the pixelated QD layer and that of the blue QD layer excited at the same wavelength (440 nm) were similar, but the carrier lifetime of the RGB mixed layer was much shorter because of the Förster energy transfer between QDs in the mixed QD layer. This result shows that pixelated RGB WQLEDs are more efficient than WQLEDs using mixed QDs. If the transistors individually controlled the EL of the RGB QD pixels, the pixelated QLED would show even higher performances under various luminances.

Flexible transparent QLEDs

Fabrication of transparent displays suitable for windows, glasses, and transparent housewares would increase the range of display applications significantly by allowing the projection of visual information onto a background without affecting its original appearance and background views.114 In particular, flexible transparent displays enable novel curved display applications such as the smart car window, wearable smart watch, and public signage display. Until now, however, the EL performances of flexible transparent displays were significantly lower than those of their nontransparent counterparts,115 mainly as a result of the constraints in transparent electrodes that require high conductivity, high transparency, and proper energy levels for effective charge injections simultaneously.116 Table 2 summarizes the optical and electrical performances of previously reported transparent QLEDs including transparency, current efficiency, and device lifetime.

Table 2 Representative characteristics of transparent QLEDs
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To achieve flexibility in transparent LEDs, thin metal films (e.g., Au, Ag, Ca/Ag, and Al) have been used as the semi-transparent electrode (Fig. 6a).117,118,119 Reducing the thickness of the metal film from ~100 nm to less than ~10 nm maintains the original wavelengths of light emission. However, the metal film unfortunately sacrifices the device transparency particularly when it comes to low resistance electrodes. In fact, the transparency of semi-transparent QLEDs is less than 60%, and it gets lower as the viewing angle increases (Fig. 6b). Currently, graphene is an attractive candidate material for the next generation of transparent electrodes owing to its ultrathin thickness, high transparency, and low resistance.120,121,122,123,124,125,126,127 Seo et al. reported fully transparent QLEDs using Au-nanoparticle (NP)-doped graphene and Ag-nanowire (NW)-decorated graphene as anode and cathode, respectively (Fig. 6c).128 The intercalation of Au NPs and Ag NWs into the graphene layers effectively modulates the energy level of the electrode, while maintaining the high transparency and low sheet resistance (Fig. 6d). To prevent the contamination of the underlying emitting layers, the engineered graphene electrode was formed using the dry transfer printing method instead of the conventional scooping process. However, the transferred graphene layers showed high sheet resistance because of the high contact resistance, which consequently decreased the EL properties of the QLEDs, including the high turn-on voltage and low brightness (Fig. 6e).

Fig. 6
figure 6

Flexible transparent QLEDs. a, b Device structure (a) and the corresponding transparency (b) of semi-transparent QLEDs. Inset of (b) shows the EL of semitransparent QLED. c–e Exploded view (c), transparency (d), and L–V properties (e) of graphene based transparent QLEDs. Inset of (e) shows J–V properties of the same device. f, g, Device structure (f), transparency (g; top), and J–V–L characteristics (g; bottom) of Ag NWs-based transparent QLEDs. h Schematic representation of TCO-based transparent QLEDs with TCO buffer layers. i Schematic representation of ITO-based transparent QLEDs with alumina-modified ZnO ETL layer. j, k Ultrathin flexible transparent QLEDs with foldable (j) and stretchable (k) form factors. Figure reproduced with permission from: a, b ref. 118 © 2014 RSC; c–e ref. 128 © 2014 ACS; f, g ref. 135 © 2015 NPG; h ref. 115 © 2016 Wiley; i–k ref. 50 © 2017 Wiley

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Ag NWs represent another good candidate for transparent electrodes.129,130,131,132 The percolated assembly of ultralong Ag NWs provides low sheet resistance (<10 Ω  sq−1),133,134 while maintaining high transparency owing to their highly porous structure. As Ag NWs are easily deposited on the target surface by spin casting or doctor blading, the Ag NW-based QLEDs can be cost-effective and highly flexible. For instance, the Zhang group reported solution-processed transparent QLEDs with an Ag NW cathode, which showed high luminance (~25,000 cd m−2) and high transparency (70%) (Fig. 6f, g).135 Although graphene and Ag NWs have shown meaningful advances, their device performances need to be improved further.

Transparent-conducting oxides (TCOs) have been the most widely utilized transparent electrodes over the past decades. However, the fabrication of TCO-based transparent top electrode remains challenging because of the mechanical and/or chemical damages to the underlying emitting materials during the harsh deposition process (e.g., sputtering).136 Pre-deposition of the thick inorganic buffer layers and successive sputtering process of the top TCO electrode have been employed to prevent the damages to the QD layer and formation of unwanted conducting paths between CTLs (Fig. 6h).115 However, the transparent light-emitting devices still show low EL characteristics compared with the nontransparent counterparts because of imbalanced charge carriers within the devices. Moreover, the thick ETL and/or inorganic buffer layers increase the stiffness, thereby decreasing the flexibility of the QLEDs. In 2017, the Kim group reported the engineered ETL structure composed of ZnO NPs and ultrathin alumina overlayers (Fig. 6i).50 Modification of inorganic ETL structure with the 2 nm-thick alumina overlayer effectively protected the emitting layer and balanced electron/hole injection into QDs, thus resulting in highly transparent (84% over visible range) and bright (~43,000 cd m−2) QLEDs. They also reported foldable and stretchable transparent QLEDs, prepared using the parylene-epoxy double layer as encapsulation and the buckled device structure (Fig. 6j, k). The ultrathin flexible transparent QLEDs showed highly stable EL during 1000 cycles of the bending test because the designed ETL layer did not increase the overall thickness of the QLEDs, and fragile ITO electrodes were located in the pseudo-neutral mechanical plain. These ultrathin and flexible transparent QLEDs can be incorporated onto the surface of a variety of curved objects, which can be a step forward to a smart Internet of things (IoTs).

Wearable quantum dot displays

One of the most promising applications of flexible QLEDs is the wearable display. Skin-mounted electronics have provided new routes for advanced wearable diagnostic/therapeutic solutions.137,138,139,140,141,142,143,144,145,146 However, significant challenges still remain for the wearable displays that can show the monitored data from the wearable sensors to the user in real time. One of the major challenges for ideal wearability is the thickness and stiffness of the conventional flexible displays.147 The high water/air stability of QLEDs enables a much thinner encapsulation layer compared to organic LEDs, which increases the flexibility of the device dramatically.

Flexible QLEDs have been commonly manufactured based on ITO electrodes patterned on a flexible PET substrate, whose thickness was in the range of hundreds of micrometers.148 Owing to the thick substrate and the fragile ITO electrode, the minimum bending radius of the display was limited to several tens of millimeters. For highly flexible QLEDs, the Demir group reported a sticker-like top-emitting QLED using a Kapton polyimide film as a substrate and a thin metal film as a semitransparent electrode (Fig. 7a, b).149 The high stability of the polyimide film against thermal/solvent exposure enabled high-temperature annealing during QLED fabrication, and the thin Ag electrode (18 nm) acted as a semitransparent top electrode, which could endure mechanical deformations. The thin-film–type QLED was easily deformed and laminated on curved surfaces of various objects, including the edge of a thin plate and the chest of a mascot doll (Fig. 7c).

Fig. 7
figure 7

Wearable quantum dot display. a, b Schematic illustration (a) and energy diagram (b) of the sticker-type top-emission QLED film. c Curved QLEDs on the edge of a thin steel plate (left) and on curvilinear surface of a toy (right). Inset of the left image shows a large-area QLED on a flexible substrate (20 mm × 25 mm). d, e Schematic illustration (d) and cross-sectional SEM image (e) of the tattoo-like wearable QLED. Inset of (e) shows the magnified device structure. f Ultrathin tattoo-like QLEDs laminated on human arm. g Waterproof properties of ultrathin QLEDs. h Ultrathin QLED display (left: rolling, right: crumpling). i Sequential images of the QLED display with different operating voltages. Figure reproduced with permission from: a–c ref. 149 © 2014 ACS; d–g ref. 99 © 2015 NPG; h, i ref. 51 © 2017 Wiley

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For wearable displays, it is essential to establish a biocompatible ultrathin encapsulation layer. Choi et al. reported parylene-epoxy double-layered ultrathin QLEDs for electronic tattoo-like displays (Fig. 7d).99 The FDA (Food and Drug Administration) approved biocompatible parylene-C film forms a good interface with skin and protects it from rashes or itching. The ultrathin epoxy layer also prevents any damage to the parylene film during the sputtering process of the bottom ITO electrode. The thickness of the double-layered encapsulation is ~1.2 μm, and the overall thickness of the QLED is ~2.6 μm (Fig. 7e). As the fragile ITO electrode is located near the neutral mechanical plane, where tensile and compressive strains are compensated, the ultrathin QLED can be freely deformed without mechanical fractures, even on soft human skin (Fig. 7f). Even in the wavy deformed state with a radius of curvature of the micrometer scale, the peak strain applied to the flexible QLED is less than the fracture strain of ITO electrode (~2.2%), that allows highly deformable flexible QLED.50 In addition, the ultrathin encapsulation layer makes the device waterproof, which effectively protects the wearable QLED under high humidity conditions (Fig. 7g). By applying a passive matrix array design, the wearable QLED displays diverse information even in rolled and crumpled objects (Fig. 7h).51 Figure 7i shows sequential images displayed on the epidermal QLED. The passively operating wearable QLEDs minimize power consumption and suppress overheating owing to the line-by-line passive matrix operation process, which ensures the safe operation of the wearable display on human skin.

Flexible QLEDs integrated with other electronic devices

In this section, we discuss flexible QLEDs integrated with other electronic components such as sensors, memories, controllers, and Bluetooth units for the next-generation portable and/or wearable type of electronic/optoelectronic systems.150,151,152,153,154 The flexible form factor of the integrated electronic system will provide new design platform for the wearable display.

One interesting application based on flexible QLEDs is a smart pressure-sensitive display, which measures, stores, and displays external mechanical deformations in real time. Son et al. integrated MoS2-based resistive random-access memory (ReRAM) devices and pressure sensors with a QLED array.155 The measured data from the pressure sensor were first stored in the MoS2 ReRAM array, and subsequently the written data were displayed visually through the QLED array (Fig. 8a, b). The wearable QLEDs could be integrated with a multiplexed transparent touch sensor array as an input port of user’s intentions (Fig. 8c). The ultrathin QLED can be also integrated with transparent force touch sensors (i.e., pressure sensors and touch sensors) (Fig. 8d).156 The soft integrated electronic system could be laminated on the human skin by van der Waals force alone, and operated stably even in the deformed state (Fig. 8e). These system-level integration examples confirm the possibility of novel wearable electronic systems integrated with wearable displays.

Fig. 8
figure 8

Wearable QLEDs integrated with other electronic devices. a, b Cross-sectional device structure (a) and schematic illustration (b) of the QLED display integrated with MoS2 ReRAM. c, d Schematic representation (c) and image (d) of transparent and wearable force touch sensor array integrated with a skin-mounted QLED. Inset of (d) shows the QLEDs under dark conditions. e Twisted wearable devices on ultrathin QLEDs. f Epidermal PPG sensor composed of QLED and QD PD. g PPG signal from the QD-based epidermal PPG sensor. h, i Schematic illustration (h) and photograph (i) of the ultrathin QLED display integrated with wearable electronics. Inset of (i) shows an illustration of the measurement setup. j Integrated wearable electronic system subjected to external heat (left) and running wearer (right). Skin-mounted QLED shows the measured information. Figure reproduced with permission from: a, b ref. 155 © 2016 Wiley; c–e ref. 156 © 2017 Wiley; f, g ref. 158 © 2017 ACS; h–j ref. 51 © 2017 Wiley

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The wearable QLED, i.e., another application example of the flexible QLED, can work as a light source for wearable light-based biosensors.157 In 2017, Kim et al. reported wearable photoplethysmographic (PPG) sensors that combined stretchable QLEDs and QD photodetectors.158 The graphene-based transparent electrode provided extreme bendability for the QD-based LEDs and PDs. The QLED was transferred onto a prestrained elastomer to form a buckled structure, and displayed 70% stretchability. For the PPG sensors, the stretchable QLEDs and PDs were attached around the fingertip, side by side, and performed as a light source and detector, respectively (Fig. 8f). The absorption spectrum changes could be well correlated with the pulse. The wearable PPG sensor also measured accurately fine changes in pressure (Fig. 8g). Such an optoelectronic device composed of QD-based LEDs and PDs can be utilized for diverse wearable sensor applications such as human motion detection and/or heart rate measurement.

Another application example of QLEDs in fully integrated wearable electronics is a flexible printed circuit board (FPCB), which integrates the QLED display, touch sensors, microcontroller modules, wireless units, other physical sensors, and power sources (Fig. 8h–j).51 The touch sensor is co-embedded with a QLED display, while maintaining an ultrathin form factor (5.5 μm). The touch interface provides an interactive user interface by changing the sensing mode in the QLED display (Fig. 8i). The 8 × 8 ultrathin QLED passive matrix array laminated on the human arm can display temperature and step information measured from the wearable sensors (Fig. 8i, right) in real time (Fig. 8j). This fully integrated wearable QLED display can provide new insights for advanced wearable healthcare electronic systems.

Conclusions

In this review, we discussed the recent developments and cutting-edge technologies in flexible and wearable QLEDs. Tremendous efforts have been made to improve the EL performance of QLEDs by optimizing device structures and QD synthetic methods. Despite several challenges (e.g., long-term device lifetime, low EL efficiencies for blue emissions, toxicity of Cd-based QDs), QLEDs exhibit unique characteristics that surpass other types of LEDs, such as high color purity, high brightness with low turn-on voltage, high resolution RGB array patterning, and ultrathin form factors. These advantages make QLEDs promising for the next-generation display applications, particularly in the field of flexible/wearable electronics. Recent advances in QD processing, encapsulation technology, and unconventional device/system designs have resulted in even faster development of flexible QLEDs. With these technological progresses, QLEDs can be successfully applied to more advanced devices such as flexible white QLEDs and highly transparent flexible QLEDs. Each key technology of unconventional QLEDs provide many opportunities in novel electronics and optoelectronics. It has been recently demonstrated that these QLEDs can be successfully integrated with various wearable electronic devices, including wearable sensors, data storage modules, touch interfaces, and flexible wireless data transfer devices for fully integrated systems. In the future, other home applications and mobile electronics will be connected wirelessly and wearable displays will visualize information for users. These technological advances shed light on the promising future of flexible QLEDs and related next-generation displays.

Data availability

The data that support the findings of this study are available from the corresponding author on request.