Multi-Color Light-Emitting Diodes

Abstract

Multi-color light-emitting diodes (LEDs) with various advantages of color tunability, self-luminescence, wide viewing angles, high color contrast, low power consumption, and flexibility provide a wide range of applications including full-color display, augmented reality/virtual reality technology, and wearable healthcare systems. In this review, we introduce three main types of multi-color LEDs: the organic LED, colloidal quantum dots (CQDs) LED, and CQD–organic hybrid LED. Various strategies for realizing multi-color LEDs are discussed including red, green, and blue sub-pixel side-by-side arrangement; vertically stacked LED unit configuration; and stacked emitter layers in a single LED. Finally, according to their status and challenges, we present an outlook of multi-color devices. We hope this review can inspire researchers and make a contribution to the further improvement of multi-color LED technology.

1. Introduction

Light-emitting diodes (LEDs) are semiconductor optoelectronic devices that emit light when electrons and holes recombine under applied voltage [1,2,3,4]. Multi-color LEDs with various advantages of color tunability, self-luminescence, wide viewing angles, high color contrast, and low power consumption are considered as the new-generation full-color displays used in televisions, laptops, mobile phones, and augmented reality/virtual reality technology [3]. In addition, the excellent flexibility and thin thickness of multi-color LEDs provide a wider range of applications in emerging optoelectronic devices including folding full-color displays, smartwatches, and wearable healthcare systems [5]. Furthermore, multi-color LED integration with the sensor is a promising and innovative way to visualize the electronic output signal of sensors to different visible colors, which has great application prospects in various applications such as real-time electrocardiogram monitors and electronic skin sensors [6,7].

Organic LEDs (OLEDs) are a type of LED in which the emissive electroluminescent layer is a film of the organic compound [7,8,9,10,11,12]. In 1950, French chemist André Bernanost for the first time discovered the electroluminescent properties of organic compounds; since then, tremendous progress has been made in the development of OLED technology [4,13,14]. Up to now, OLEDs have been mass-produced and commercialized, and successfully applied in either rigid or flexible OLED displays and excellent solid-state lighting [15,16]. Due to the mature application foundation of monochrome OLEDs for years, multi-color OLEDs with various lateral/vertical integration configurations have been fairly widely researched [17,18,19,20].

In recent years, colloidal quantum dots (CQDs) have been considered to be promising visible emitter materials to replace organic luminescent materials due to their inherent luminescent properties, including tunable emission wavelength, narrow spectral bandwidth, and high photoluminescence quantum yield [21,22,23,24,25]. Using CQDs as the emissive electroluminescent layer of LEDs, CQD-based LEDs (QLEDs) have stimulated a great deal of research interest. QLEDs possess unique merits of high color saturation, tunable emission color, high brightness, and simple solution processability [26,27,28]. Recent advances have enabled monochromatic QLEDs to exhibit excellent performance. For example, the external quantum efficiency (EQE) of red, green, and blue QLEDs is higher than 20%, 22%, and 18%, respectively [27,29,30,31]. On this basis, researchers began to aim for high-performance multi-color QLEDs to achieve high-resolution full-color QLED displays [32,33,34,35].

However, the unstable blue QLEDs with a much shorter lifetime (T₅₀ lifetime of 200 h at an initial brightness of 1000 cd m⁻²) than the red (T₅₀ lifetime of 26,500 h at 1000 cd m⁻²) and green ones (T₅₀ lifetime of 25,000 h at 1000 cd m⁻²) restrict the development of full-color QLEDs [27,29,30,31]. By substituting the blue QLEDs with relatively stable blue OLEDs, a CQD–organic hybrid multi-color LED has been proposed combining the excellent saturation of QLEDs and high stability of OLEDs [36,37].

There are many reviews about introducing monochrome OLEDs or QLEDs aiming to improve the device performance such as luminance, EQE, and lifetime. In early reviews, Vanessa Wood et al. summarized the features of various QLEDs whose structures were classified by different charge transport layers, outlined the challenges, and discussed the directions of QLED research [38]. Recently, Gloria Hong et al. made an overview of the history and development of the emitter materials of OLEDs including fluorescence, phosphorescence, and thermally activated delayed fluorescence, and explored the direction of new-generation luminescent materials [39]. From the perspective of solid-state lighting applications, Ramchandra Pode classified OLED devices based on the emission mechanism of emitting materials, and they paid attention to the low manufacturing costs and energy consumption of OLED technologies [40]. In addition, many reviews have been reported to research the luminous mechanism, summarize the device performance, and discuss the future development of monochrome OLEDs or QLEDs [41,42,43,44,45,46,47,48,49,50,51,52,53,54]. While reviews of monochrome LEDs are comprehensive, there is little focus on multi-color LEDs.

Many studies have reported various methods to realize multi-color LEDs, such as lateral/vertical LED integration configuration, and optimized multi-color emission layers. It is necessary to summarize and analyze the features of different methods, which are of great significance for the further improvement of multi-color LED technology. Recently, Bernard Geffroy et al. made an overview of full-color OLEDs from the perspective of different materials and discussed the development of multi-color OLED display technology [13]. Yizhe Sun et al. focused on the red/green/blue pixel patterning methods for realizing full-color QLED displays including inkjet printing, contact and transfer printing, and photolithography technologies [55]. In order to study the electroluminescence (EL) characteristics of multi-color QLEDs, the EL mechanisms, exciton formation process, and nonradiative processes were discussed in a review by Qilin Yuan et al., which has guiding significance to the optimal structure design [56]. In recent years, various types of multi-color LEDs, such as multi-color OLEDs, QLEDs, and CQD/organic hybrid LEDs, have appeared successively. However, to our knowledge, existing reviews about multi-color LEDs just focus on a single type of multi-color LED, such as multi-color OLEDs or QLEDs. There are few comparative summaries of various types of multi-color LEDs including multi-color OLEDs, QLEDs, and CQD/organic hybrid LEDs in one review. In addition, previous reviews have not systematically summarized and compared the different structures of multi-color LEDs, preventing a more comprehensive understanding of multi-color LED technology.

Thus, in this review, we introduce in detail three main types of multi-color LEDs: multi-color OLEDs, multi-color QLEDs, and multi-color CQD–organic hybrid LEDs. It is mainly divided into three representative device structures: (1) red, green, and blue sub-pixel side-by-side arrangement, (2) vertically stacked LED unit configuration, and (3) stacked emitter layers in a single LED. Progress in multi-color devices is summarized in Figure 1. Aside from this, the application of multi-color LEDs in flexible and wearable fields is briefly introduced. Finally, we present existing challenges and a vision for the future of multi-color devices. This review provides a more comprehensive summary, more detailed classification, and more diverse possible applications of multi-color LEDs compared with previous reviews, which is beneficial for researchers to contribute to the further improvement of multi-color technology.

2. Types of Multi-Color Devices

After recent developments, there are many ways to implement multi-color devices, such as multi-colored OLEDs, QLEDs, and CQD–organic hybrid LEDs. In the following sections, we describe these types of multi-color LEDs, introduce representative works, and analyze their features. The summarization of multi-color devices is shown in Table 1.

2.1. Multi-Color OLEDs

OLEDs are expected to play a critical role in solid-state lighting and display application, owing to their unique properties, such as a wide range of colors, excellent luminance, ultrathin thickness, and flexibility. There are different ways to realize multi-color OLED devices. Firstly, the red, green, and blue OLEDs are arranged in parallel to achieve a wide color gamut [13,57]. However, this method has an intrinsic deficiency of a low pixel density. Secondly, the vertical stack OLED configuration has an improved pixel density and high efficiency. However, the structure of the device is relatively complex, leading to poor stability. Thirdly, the emission of OLEDs with multiple emission layers is selectively activated by different voltages [18,28,58,59,60,61,62,63]. Still, the transition of the complex exciton region between the two emission layers tends to reduce the device’s efficiency.

2.1.1. Side-by-Side Structure

Combining the two, three, or four single-color OLED units in the side-by-side arrangement was proposed, driven by circuitry comprising a thin-film transistor (TFT) and capacitors that can address each pixel independently. The devices exhibit good optical performances since the light directly emits from respective units.

In 2003, C. David Muller et al. used a polymer with photoresist properties to make red/green/blue side-by-side OLED units via solution processing, as shown in Figure 2a [17]. The polymers are the classes of oxetane-functionalized spirobifluorene-co-fluorene, and they can be crosslinked photochemically to produce insoluble polymer networks in desired areas. A photograph of a red, green, and blue device is shown in Figure 2b. The obtained red-, green-, and blue-emitting units possess high luminous efficiency (LE) of about 1, 7, and 3 cd A⁻¹, respectively (Figure 2c).

Red, green, and blue color patterning is achieved via the sequential vacuum deposition of red, green, and blue materials through fine metal mask technology. However, the technique has several inherent limitations, such as difficulties in mask-to-substrate overlay alignment and in making micrometer-level dimensional accuracy masks, resulting in a low manufacturing yield and poor display resolution. In 2015, Yoshitaka Kajiyama et al. developed a maskless red/green/blue color patterning technique based on the diffusion of luminescent dopant molecules, which overcomes challenging issues in multi-color OLED display manufacturing arising from shadow mask limitations (Figure 2d) [64]. The EL images and corresponding EL spectra of the successfully fabricated red, green, and blue OLEDs side by side on one substrate are displayed in Figure 2e,f, respectively.

2.1.2. Vertically Stacked OLEDs

Compared with side-by-side red/green/blue OLED units to make a multi-color display, vertically stacked OLED units enable smaller pixel size and more fill factor, resulting in a threefold increased pixel density of the display. However, because of the light loss and the resistance increase in the semitransparent central electrode, the brightness of multi-color OLEDs inevitably decreases and a high drive voltage is needed.

Two-Unit Stacked OLEDs

In 1997, P.E. Burrows and S.R. Forrest et al. at Princeton University first proposed a new type of display pixel in which the two OLED units of red, green, or blue emission were placed in a vertically stacked geometry to mix red, green, or blue colors (Figure 3a) [65]. For example, independently blue and red emission OLED units are stacked and connected by the central magnesium–silver–indium tin oxide (Mg-Ag-ITO) electrode. The central electrode works as a common carrier injection layer of two units, and the color continuously changes from deep red to blue depending on the voltage bias ratio. On this basis, in 2009, C.J. Liang et al. achieved highly efficient multi-color emission with two-unit stacked OLEDs (Figure 3b) [9]. They found that the thin organic film underlying the ITO layer would be harmed by the magnetron sputtering process; thus, they solely adopted an aluminum–gold (Al-Au) dual film with suitable thickness as the inter-mediate electrode and, to improve the balance of electrons and holes injection, at both ends of the electrode they increased lithium fluoride (LiF) as the electron injection buffer layer and F₄TCNQ-doped m-MTDATA as the hole injection buffer layer.

However, adopting a traditional metallic conductor as an intermediate layer does not only reduce transparency but also influences stability. An effective way to reduce the driving voltages of tandem structures is by replacing them with a charge generation layer (CGL). In 2013, Yongbiao Zhao et al. from Nanyang technological university inserted a p-type layer of molybdenum trioxide (MoO₃) between hole transport layer TCTA constituting an n-i-p-i-n heterojunction symmetric structure (Figure 3d) [66]. When applying an alternating current (AC) signal, the n-i-p junction and p-i-n junction would switch on alternatively, and the device-emitted light mixed with two stacked OLED units. The correlated color temperature (CCT) of this device could be changed from 7575 to 2773 K, and the Commission Internationale de l’Eclairage (CIE) was varied from (0.16, 0.32) to (0.61, 0.38). Although the efficiency of carrier injection has been promoted, the diffusion of dopants would deteriorate the properties of devices. In 2022, Qian Chang et al. used two purely organic materials HAT-CN and CuPc composing a p-n type CGL as shown in Figure 3c; this device not only had better current efficiency but also realized color-tunable stacked OLEDs in a single direct current (DC) signal. When applying a driving voltage, p-n type CGL would generate two type carriers, and two OLED units emitted light simultaneously; the CE and EQE of obtained QLEDs can reach up to 46.3 cd A⁻¹ and 15.1%, respectively [67].

Figure 3. Two−Unit Stacked OLEDs. (a) Schematic cross−section of the layers of a red−blue tunable OLED. Reprinted with permission from Ref. [65]. 1996, American Institute of Physics. (b) The energy level diagram shows the highest occupied molecular orbital (HOMO) and the deeper lowest unoccupied molecular orbital (LUMO) levels, and carrier flows in OLEDs. Reprinted with permission from Ref. [9]. 2009, Elsevier. (c) The working principle of the tandem device. Reprinted with permission from Ref. [67]. 2022, Elsevier. (d) Schematic n-i-p-i-n structure of the OLEDs Reprinted with permission from Ref. [66]. 2013, Elsevier. (e) Fabrication steps of the proposed color−tunable OLED. The o−IBOLED is first deposited with a fine mask, followed by depositing the b−NBOLED without the fine mask. Reprinted with permission from Ref. [12]. 2013, Elsevier. (f) The operation mechanism of top orange and bottom blue tandem LEDs coupled with energy−level alignment under DC forward, DC reverse, and AC fields. Reprinted with permission from Ref. [68]. 2016, The Royal Society of Chemistry. (g) The schematic architecture of the color−tunable OLED device. Reprinted with permission from Ref. [19]. 2017, Springer Nature. (h) CIE coordinates of the three color-tunable devices calculated from their EL spectra of the tandem OLED cells with material combinations of green−blue, red−blue, and red−green operated by applying an AC signal with different pulse offsets. Reprinted with permission from Ref. [19]. 2017, Springer Nature. (i) Schematic illustration of the architecture of the investigated color−tunable AC/DC OLEDs. Reprinted with permission from Ref. [20]. 2015, Springer Nature. (j) Photographs showing an AC/DC OLED sample upon application of a DC bias (blue emission), a DC bias with reversed polarity (yellow emission), and an AC voltage with a frequency of 50 Hz (white emission). Reprinted with permission from Ref. [20]. 2015, Springer Nature.

Figure 3. Two−Unit Stacked OLEDs. (a) Schematic cross−section of the layers of a red−blue tunable OLED. Reprinted with permission from Ref. [65]. 1996, American Institute of Physics. (b) The energy level diagram shows the highest occupied molecular orbital (HOMO) and the deeper lowest unoccupied molecular orbital (LUMO) levels, and carrier flows in OLEDs. Reprinted with permission from Ref. [9]. 2009, Elsevier. (c) The working principle of the tandem device. Reprinted with permission from Ref. [67]. 2022, Elsevier. (d) Schematic n-i-p-i-n structure of the OLEDs Reprinted with permission from Ref. [66]. 2013, Elsevier. (e) Fabrication steps of the proposed color−tunable OLED. The o−IBOLED is first deposited with a fine mask, followed by depositing the b−NBOLED without the fine mask. Reprinted with permission from Ref. [12]. 2013, Elsevier. (f) The operation mechanism of top orange and bottom blue tandem LEDs coupled with energy−level alignment under DC forward, DC reverse, and AC fields. Reprinted with permission from Ref. [68]. 2016, The Royal Society of Chemistry. (g) The schematic architecture of the color−tunable OLED device. Reprinted with permission from Ref. [19]. 2017, Springer Nature. (h) CIE coordinates of the three color-tunable devices calculated from their EL spectra of the tandem OLED cells with material combinations of green−blue, red−blue, and red−green operated by applying an AC signal with different pulse offsets. Reprinted with permission from Ref. [19]. 2017, Springer Nature. (i) Schematic illustration of the architecture of the investigated color−tunable AC/DC OLEDs. Reprinted with permission from Ref. [20]. 2015, Springer Nature. (j) Photographs showing an AC/DC OLED sample upon application of a DC bias (blue emission), a DC bias with reversed polarity (yellow emission), and an AC voltage with a frequency of 50 Hz (white emission). Reprinted with permission from Ref. [20]. 2015, Springer Nature.

For the practical application, there are issues regarding the complicated and expensive manufacturing process that need to be addressed. In 2013, Jiang et al. from Hong Kong University also fabricated a color-tunable device based on the two stacked OLED units without fine mask alignment, as shown in (Figure 3e) [12]. Although adjusting the polarity of the AC signal, the CIE coordinates are varied from (0.21, 0.23) to (0.57, 0.37). The color rendering index (CRI) is 54.2 when the CIE is (0.38, 0.29). The solution process for the polymer emitters and the interface materials can result in a much simpler and cheaper fabrication process. In 2016, Sung Hwan Cho et al. combined two tandem OLEDs by sharing a polymer electrode as a charge injection layer through solution process methods (Figure 3f) [68]. The maximum current efficiency (CE) and luminance of obtained OLEDs are 2.5 cd A⁻¹ and 1300 cd m⁻², respectively. In 2017, Fei Guo et al. introduced highly conducting and transparent silver nanowires as the intermediate charge injection layer between two OLED units (Figure 3g) [19]. When a low-frequency square-shaped AC signal with positive and negative potentials is applied to the tandem cells, the two diodes can alternatingly turn on and emit pulses of the two primary colors. The CIE coordinates of the three color-tunable devices with material combinations of green–blue, red–blue, and red–green operated by applying an AC signal with different pulse offsets are shown in Figure 3h.

Another way to achieve multi-color emission with two OLED units is to introduce a yellow emission. In 2015, Markus Frobe et al. demonstrated a new device that could emit color from deep blue to warm white to saturate yellow by independently addressing a fluorescent blue emission unit and a phosphorescent yellow emission unit (Figure 3i,j) [20]. The leading way is the AC/direct current (DC) method by changing the pulse width and pulse height of the positive and negative cycles. The highest power efficiency (PE) values of OLED reach 36.8 lm W⁻¹ at warm white color coordinates.

Three-Unit Stacked OLEDs

Three-unit stacked OLED devices are proposed due to the better color gamut and higher CRI compared with the two-unit stacked ones. However, the higher driving voltages for independently controlling three units are inevitably applied. Aside from this, serious color distortion issues appear caused by the micro-cavity effect.

In 2017, Mi Jin Park et al. successfully fabricated highly efficient three-stacked OLEDs based on the optical simulation results (Figure 4a); the measured spectrum was almost identical to the simulated device as shown in Figure 4b, showing an EQE of 49.4%, a PE of 33.4 lm W⁻¹, and a CRI of 93 (Figure 4c) [18]. In 2018, Hyunkoo Lee et al. also fabricated multi-color OLEDs with a vertical stack of three primary colors [61]. To reduce the driving voltage, they inverted the middle green OLED which shared metal electrodes with the bottom and the top OLEDs (Figure 4d). The driving voltages of blue, green, and red emission units are around 5.3 V, 2.8 V, and 4.4 V, respectively, at 1000 cd m⁻² (Figure 4e). The CCT could be easily changed from warm white to cool white and a CRI of 88.7 could be reached. Furthermore, they also optimized the thickness of the hole transport layers (HTLs) and the electron transport layer (ETL) to alleviate the micro-cavity effect. As a result, the color distortion issues are alleviated and the EQE of blue, green, and red emission units can reach 11.1%, 10.9%, and 9.6%, respectively (Figure 4f).

2.1.3. Stacked Emitter Layers

Stacking various emitter layers with different emission wavelengths is a direct and simple way to achieve multi-color emission compared with side-by-side and vertically stacked OLED units. In this way, multiple colors can be obtained by changing the bias voltages. To make carriers’ injection more balanced and emission of OLEDs more efficient with stacked emission layers, the energy level structure should be carefully designed and the energy transfer between the host and dopant should be considered.

Single Emission Layer

The multi-color OLED with a single emission layer (S-EML) has a simple structure, and it can be fabricated by an all-solution method suitable for commercial applications. By doping multi-emission wavelength materials into the host emission layer, the different colors can be emitted by changing the bias of voltage. In addition, the emission spectrum is influenced by the doped ratios; therefore, the concentration of dopants should be considered and optimized.

In 1994, J. Kido and K. Hongawa et al. first proposed the white OLEDs by using a single emitter layer doped with three fluorescent dyes [69]. The fluorescent dyes including blue-emitting 1,1,4,4-tetraphenyl-1,3-butadiene, green-emitting coumarin 6, and orange-emitting DCM 1 were doped into the poly(9-vinylcarbazole) (PVK) layer (Figure 5a). The obtained OLEDs possessed high luminance of 3400 cd m⁻² at a driving voltage of 14 V and an excellent LE of 0.85 lm W⁻¹.

Compared with fluorescent materials, OLEDs with phosphorescent materials as emission layers possess higher efficiency. In 2004, Brian W et al. doped phosphorescent materials of red-emitting PQIr, green-emitting Ir(ppy)₃, and blue-emitting FIr₆ into the inert host UGH2 (Figure 5b) [70]. The CIE chromaticity coordinates of obtained multi-color OLEDs vary from (0.43, 0.45) at 0.1 mA cm⁻² to (0.38, 0.45) at 10 mA cm⁻². In addition, the maximum PE could be up to 42 lm W⁻¹, and the CRI is as high as 80. In 2009, Qi Wang et al. used FIrpic as the blue-emission doping phosphorescent material and (fbi)₂Ir(acac) as an orange one [71]. The redundant excitons are formed on the host of mCP, and then the energy is transferred to FIrpic via the Forster- or Dexter-type transfer mechanism, leading to the blue emission (Figure 5c). Thus, the charge balance is improved and the common energy losses are eliminated. The performance of OLED is enhanced with PE of 42.5 lm W⁻¹, EQE of 19.3%, and CE of 52.8 cd A⁻¹.

Although the performance of OLEDs with phosphorescent materials as emission layers has been greatly improved, the rare metal in phosphorescent materials increases the cost of preparation. Recently, OLEDs based on thermally activated delayed fluorescence (TADF) have received wide attention due to both high efficiency and low cost. In 2015, Huang et al. fabricated color-tunable OLEDs through the solution-processed method with TCAC as the host material, FIrpic as the blue dopant, Os(btfp)2(pp2b) as the red dopant, and Os(bptz)2(dppb) as the yellow dopant of light-emitting layers (EMLs) (Figure 5d) [72]. The EQE of multi-OLEDs is up to 13.6%, as shown in Figure 5e. In addition, the OLEDs exhibit variation in the color of CIE from (0.52,0.40) to (0.36,0.38) (Figure 5f). In 2017, Jun-Yi Wu et al. reported a T-P (TADF-phosphorescence hybrid white OLED using the TADF material of DMAC-TRZ as blue dopants, and the phosphorescent materials of Ir(dpm)PQ2 as red dopants [62]. The obtained OLEDs possess excellent performance of EQE of 12.32% and PE of 18.1 lm W⁻¹. To further improve the efficiency of the devices, in 2020, Ding D et al. demonstrated an all-TADF-doped multi-color OLED [73]. The EML consists of yellow TADF dopants (4CzTPNBu) and blue TADF ones (ptBCzPO2TPTZ). The low-energy-gap yellow dopants with the shallower HOMO and LUMO could form the hole and electron traps for carrier and exciton capture, therefore further hampering the charge exchange-based exciton migration (Figure 5g). The devices possess high PE of 55.1 lm W⁻¹, excellent EQE of 23.6%, and maximum luminance beyond 30000 cd m⁻², as shown in Figure 5h. At 1000 cd m⁻², the OLEDs with different blue TADF doping concentrations of 1.0%, 1.5%, 2.0%, and 3.0% emit cool white, pure white, and warm white lights with the CIE coordinates of (0.28, 0.34), (0.34, 0.36), (0.41, 0.42), and (0.46, 0.44) and the CCT of 8332, 5152, 3563, and 2883 K, respectively (Figure 5i).

Double Emission Layers

Multi-color OLEDs with a double emission layer (D-EML) concept were first introduced by X. Zhou et al. in 2002 [74]. For OLEDs with S-EMLs, electrons or holes tend to accumulate at the interface of the EML/ETL or EML/HTL due to the large energy barrier. The high density of accumulated carriers leads to the quenching of triplet excitons. In contrast, the D-EML structure can significantly avoid carrier accumulation at the interface by widening the triplet excitons’ generation zone, leading to better device performance.

In 2004, Gufeng He et al. fabricated an efficient OLED with D-EML structure through doping the same phosphorescent material Ir(ppy)₃ into both TCTA and TAZ emission hosts (Figure 6a) [75]. Because the HOMO level of the TCTA was much lower than that of Ir(ppy)₃, a part of the holes would be captured by the Ir(ppy)₃; in this way, excitons not only formed on the interface of TCTA and TAZ, but also could be combined in the Ir(ppy)₃ sites directly. This structure increased the recombination area, resulting in an excellent performance with PE of 64 lm W⁻¹ at 1000 cd m⁻² and EQE of 19.3% (Figure 6b). To broaden the range of colors, in 2010, Sebastian Reineke et al. embedded the blue and green dopants in a common host of TPBi, and the red one in the host of TCTA to achieve D-EML-structured multi-color OLEDs (Figure 6c) [76]. The PE of obtained OLEDs can reach 90 lm W⁻¹ at the brightness of 1000 cd m⁻², and the EQE can reach 34% (Figure 6d). In 2016, Xuming Zhuang et al. reported a four-color OLED device employing a D-EML structure with a blue host/orange dopant and green host/red dopant; they avoided using the structure of only the dopant as the emitting molecules, and achieved the most broad range of spectra with the D-EML structure, exhibiting a high CRI of 92, EQE of 23.3%, and PE of 63.2 lm W⁻¹ (Figure 6e,f) [59].

Multiple Emission Layers

To further improve the balance of the carrier injection, it is necessary to develop color-tunable OLEDs with multiple emission layers. In 2016, Ping Chen et al. from Jilin University fabricated OLEDs with double blue emission layers and an orange ultrathin layer between them [60]. TCTA with high hole transport mobility and TPBi with high electron transport mobility were blended as the mixed host for the blue phosphorescent emitter (Figure 7a). Two different hosts expand the exciton recombination zone, which leads to charge balance. Normalized EL spectra of the device at different voltages from 3 V to 9 V are shown in Figure 7b. The peak PE can be above 40.8 lm W⁻¹, and a CRI of 62 and CE of 39.8 cd A⁻¹ can be achieved. In 2018, Gyeong Woo Kim et al. also adopted the blue–yellow–blue multiple EML structure while they selected the TADF material as a blue emitter (Figure 7c), achieving a high EQE of 23.1% at the CIE coordinate of (0.324, 0.337) [77]. Normalized EL spectra of devices are presented in Figure 7d. Although sandwiching single-color emission layers between two blue emission layers has obtained stable multi-color emission, it is essential to increase three or more emitters to achieve wider luminescent spectra. In 2019, Baiqian Wang et al. fabricated a relatively stable white OLED with two emission layers of both blue dopants, and three ultrathin layers of red, orange, and green emission layers in the middle (Figure 7e) [63]. The obtained OLEDs have ambipolar charge carrier transport properties between EMLs. Normalized EL spectra of the device at different driving voltages are shown in Figure 7f. The performance of OLEDs is excellent with CRI reaching 94 and a maximum PE of 33.4 lm W⁻¹.

2.2. Multi-Color QLEDs

CQDs have been considered promising visible emitter materials to replace organic luminescent materials due to their inherent luminescent properties, including tunable emission wavelengths, narrow spectral bandwidths, and solution–process compatibility [24,25,27]. Since the first demonstration of CQD-based LEDs (QLEDs) in 1994, the performance of monochromatic QLEDs has been significantly improved [78]. Based on this, recently, researchers began to aim for high-performance multi-color QLEDs to obtain high-resolution full-color QLED displays.

2.2.1. Side-by-Side Structure

The realization of multi-color QLEDs is early achieved by the side-by-side patterning of red/green/blue CQDs onto the pixelated display panel. The cross-contamination issues of the red/green/blue pixels could be solved through the photolithography approach, inkjets method, or transfer printing process replacing the spin-coating method used to fabricate monochrome displays. However, the fill factors and pixel density are inevitably sacrificed due to the lateral integration configuration. Aside from this, other key figures of merit such as operation lifetime, film uniformity, and interfacial charge transport efficiency are probably deteriorated because of incompatibility with lithographical chemicals or coffee ring effects.

In 2011, Tae-Ho Kim et al. demonstrated large-area, full-color CQD displays driven by oxide-based thin-film transistor (TFT) arrays through side-by-side nano-transferring red/green/blue CQDs (Figure 8a) [79]. Solvent-free transfer printing associated with kinetic control and interfacial chemistry enables printed CQD films to exhibit excellent morphology, well-ordered CQD structure, and clearly defined interfaces. The fluorescence micrograph of the transfer-printed red/green/blue CQD stripes onto the glass substrate excited by 365 nm ultraviolet (UV) radiation is shown in Figure 8b. A 4-inch full-color active matrix CQD display with a resolution of 320 × 240 pixels can be realized (Figure 8c). Figure 8d shows an optical image of simultaneous red/green/blue EL emission from the pixelated area of QDs during operation.

However, nonuniformity over the deposited area of the printing process still exists. The development of new fabrication technologies for efficient high-resolution and large-area patterning of CQD devices is required. In 2018, Han-Lim Kang et al. proposed efficient and simple patterning technologies that employ locally controlled surface tailoring of constitutional functional layers via photochemical deactivation routes (Figure 8e) [32]. The patterning area of each CQD layer was observed in the photoluminescence (PL) images as a continuous and rectangular region (20 µm × 80 µm) when excited by 356 nm UV radiation (Figure 8f). The obtained multi-color QLED devices possess a maximum luminescence of 1950 cd m⁻² and a CE of 2.9 cd A⁻¹.

Although the technologies for high-resolution and large-area patterning of CQD devices have been improved, the relatively high cost limits the practical application. An optimized low-cost photolithography process seems to be a suitable practical patterning approach. In 2020, Wenhai Mei et al. demonstrated a sacrificial layer-assisted patterning (SLAP) approach, which could be applied in conjunction with photolithography to fabricate high-resolution, full-color side-by-side CQD patterns (Figure 8g) [34]. A 500-ppi, full-color, passive, matrix QLED prototype with no color impurities in the subpixels was successfully fabricated via this process (Figure 8h). The obtained QLED has a high color gamut of 114% (National Television Standards Committee (NTSC)) (Figure 8i).

2.2.2. Vertically Stacked QLEDs

Instead of directly patterning EMLs, the combination of white QLEDs and color filters can also realize red/green/blue side-by-side color pixels to avoid the problem of chemical incompatibility caused by photolithography and the printing process. Vertically stacking red, green, and blue QLED units to form a tandem structure offers a practical solution for obtaining white QLEDs with high CE and a long lifetime, combining with color filters to realize high-resolution full-color displays. However, the luminance of QLEDs is markedly weakened because over 2/3 of the emitted light is absorbed by color filters.

In 2017, Heng Zhang et al. demonstrated all-solution-processed three-unit (red/green/blue) white tandem QLEDs for the first time (Figure 9a) [80]. The tandem devices are achieved by serially connecting the red bottom sub-QLED, the green middle sub-QLED, and the blue top sub-QLED using the inter-connecting layer based on the Zn_0.9Mg_0.1O/poly(ethylenedioxythiophene): polystyrenesulfonate (ZnMgO/PEDOT: PSS) heterojunction (Figure 9b). The three-unit white QLEDs exhibit evenly separated red, green, and blue emissions and red/green/blue colors can be easily recovered using conventional color filters. A peak CE of 4.75 cd A⁻¹, EQE of 2.0%, and a high luminance of 4206 cd m⁻² are obtained, as shown in Figure 9c.

2.2.3. Stacked Emitter Layers

Developing color-tunable QLEDs with stacked emitter layers could circumvent the limitation of red/green/blue side-by-side color pixels. Because the full color is attained in a single color-tunable pixel instead of three red/green/blue pixels, the pixel density and fill factor of a display with color-tunable pixels can be enhanced three times. This is essential to improve the resolution of full-color QLED displays. Earlier studies began by vertically stacking multiple emission layers to achieve multi-color QLEDs. However, the fabrication of QLEDs with multiple emission layers is complex, which is difficult for repeatable and large-scale preparation. Some researchers have found that mixed CQDs as a single emission layer can also achieve highly efficient multi-color emission controlled by bias, which is closer to the practical application.

Double or Multiple Emission Layers

Patterning and stacking variously colored CQDs (red, orange, green, blue) in the exciton recombination zone could be an efficient method to acquire multi-colored QLEDs. However, for the realization of full-color displays with QLEDs, the development of a fabrication process for the deposition of homogeneous and uniform CQD layers over a large area with patterning capability is still necessary. In 2010, Wan Ki Bae et al. fabricated the all-CQD multi-layer films via a layer-by-layer assembly method using electrostatic interactions between each layer through the sequential deposition of oppositely charged CQDs onto the substrates (Figure 10a) [81]. The exciton recombination zone was investigated by monitoring the EL spectral change with the introduction of sensing layers within the all-CQD multi-layer films. The total EL emission comes mostly at the top CQD monolayer, adjacent to the ETL layers (∼90%), and partially at the second CQD monolayer from the top (Figure 10b).

Although large-area practical multi-colored QLEDs are realized, the accumulated charges at the interfaces are always inevitable in the QLED devices. In 2021, Ting Wang et al. found that the charge accumulation issue could be addressed by employing the alternating current (AC) driving mode in a QLED, and they successfully fabricated multi-color QLEDs with bilayer stacked emissive layers composed of red and green CQDs (Figure 10c) [82]. The emission color of the AC QLED can be tuned by both the polarity and amplitude of the driving voltage (Figure 10d,e). The efficient electron/hole injection to the emission helps to form excitons and reduces the turn-on voltage.

Single Emission Layer with mixed CQDs

Although multi-color light emission is achieved by stacking double or multiple emission layers, the alternatingly consecutive solvent treatments inevitably damage the prior-deposited CQD films and degrade interfacial carrier transport efficiency. By using a single emission layer with mixed red/green/blue CQDs as light-emitting materials, highly efficient multi-color QLEDs that can emit full colors under different driving bias voltages are achieved.

In 2015, Ki-Heon Lee et al. reported all-solution-processed bi- and trichromatic QLEDs, where red, green, or blue CQD-mixed EMLs are sandwiched with poly(9-vinlycarbazole) (PVK, HTL) and zinc oxide nanoparticles (ZnO, ETL) with a standard device architecture (Figure 11a,b) [33]. PL decay dynamics of a homogeneously distributed red/green/blue CQD-mixed solid film were first examined, showing the shortened lifetimes for blue and green QDs and the lengthened lifetime for red ones as a result of Forster resonant energy transfer (FRET) among different band gap CQD emitters (Figure 11c). On this basis, in 2022, Ge Mu et al. achieved color-tunable QLEDs with the largest color variation controlled by bias among existing multi-color QLEDs by optimizing the mixing ratio of red, green, and blue CQDs [35]. Normalized PL spectra of mixed red/green/blue CQDs used for the fabrication of full-color tunable QLEDs with different formulations are shown in Figure 11d–f. At the optimal mixing ratio of red, green, and blue CQDs, full-color tunable QLEDs exhibit wide color variation ranging from the CIE chromaticity coordinates of red (0.649, 0.330) to orange (0.453, 0.389) to yellow (0.350, 0.347) to green (0.283, 0.305) to blue (0.255, 0.264) upon increasing voltages from 2 V to 9 V (Figure 11g–i). In addition, the fabricated multi-color QLEDs show high luminance approaching 10³ cd m⁻² and a superior EQE of 13.3%.

2.3. CQD and Organic Hybrid LED

Although several methods are proposed to obtain high-performance multi-color QLEDs, the realization of high-resolution full-color QLED displays remains challenging. This is because the blue QLEDs are unstable, with a short T₅₀ lifetime of 200 h at an initial brightness of 1000 cd m⁻², which is much shorter than those of red and green QLEDs of 26,500 and 25,000 h, respectively. However, blue OLEDs are relatively stable and have been applied in displays for years. By substituting the blue QLEDs with blue OLEDs, a hybrid device promises to achieve a multi-color LED with high saturation and high stability, which is beneficial for realizing high-resolution full-color displays.

In 2020, Heng Zhang et al. stacked a yellow QLED with a blue OLED using a transparent, indium–zinc oxide (IZO), intermediate connecting electrode to achieve a full-color tunable hybrid tandem LED (Figure 12a) [37]. By varying the driving AC signals, the device could emit red, green, and blue primary colors as well as arbitrary colors covering a 63% National Television System Committee (NTSC) color triangle (Figure 12b). Both the EQE of the blue OLED and the yellow QLED are high at 5.6% and 11.2%, respectively, and the maximum brightness of the blue OLED and yellow QLED is 9359 and 51,590 cd m⁻², respectively, showing that the introduction of IZO has no bad effect on the device performance (Figure 12c,d). The brightness/efficiency of the red and green emissions according to correlating the emission color with the voltage is shown in Figure 12e.

The tandem LED structure is relatively complex; thus, in 2022, Suhyeon Lee et al. developed a simpler method to fabricate a CQD–organic hybrid device [36]. They deposited an organic blue common layer (BCL) through a common mask over the green and red QLEDs (Figure 12f). The commonly deposited BCL is not only a blue EML but also an ETL for the red and green QLED sub-pixels (Figure 12g). Although the blue EML was shared, the CIE color coordinates of the red and green QLEDs were negligibly changed and thus we can keep the advantages of excellent color purity of CQDs (Figure 12h).

Table 1. Summarization of Multi-color Devices.

Year	Type		Maximum Brightness (cd m−2)	EQE (%)	PE (lm W−1)	CE (cd A−1)	CIE	Ref.
2003	OLED	Side-by-Side Structure	-	-	-	7 2.9	Green blue	[17]
2014	OLED	Vertically Stacked OLEDs	-	10.5	14.4	-	-	[83]
2015	OLED	Vertically Stacked OLEDs	-	3.4	36.8	-	(0.44, 0.45)	[20]
2016	OLED	Vertically Stacked OLEDs	1300	-	-	2.5	(0.326, 0.381)	[68]
2017	OLED	Vertically Stacked OLEDs	43,594	12.32	18.1	28.8	(0.38, 0.44)	[62]
2018	OLED	Vertically Stacked OLEDs	-	-	18.1	-	(0.375, 0.395)	[61]
2018	OLED	Vertically Stacked OLEDs	-	49.4	33.4	-	(0.467, 0.423)	[18]
2022	OLED	Vertically Stacked OLEDs	5748.4	15.1	46.3	42.9	(0.247, 0.579)	[67]
2004	OLED	Single Emission Layer	-	12	42	-	(0.43, 045)	[70]
2009	OLED	Single Emission Layer	-	19.3	42.5	52.8	(0.33, 0.39)	[71]
2015	OLED	Single Emission Layer	-	13.6	14.5	22.5	(0.36, 0.38)	[72]
2020	OLED	Single Emission Layer	30,000	23.6	55.1	52.7	(0.34, 0.36)	[73]
2004	OLED	Double Emission Layers	-	19.3	64	-	-	[75]
2010	OLED	Double Emission Layers	-	34	90	-	(0.45, 0.47)	[76]
2016	OLED	Double Emission Layers	-	23.3	63.2	-	(0.433, 0.458)	[59]
2016	OLED	Multiple Emission Layers	11,000	-	40.8	39.8	(0.32, 0.39)	[60]
2018	OLED	Multiple Emission Layers	-	23.1	59.0	-	(0.324, 0.337)	[77]
2019	OLED	Multiple Emission Layers	23,730	-	33.42	32.74	(0.391, 0.471)	[63]
2020	QLED	Side-by-Side Structure	247,000	22.9	-	9.8	(0.16, 0.77)	[34]
2017	QLED	Vertically Stacked QLEDs	4206	2.0	0.46	4.75	(0.30, 0.44)	[80]
2015	QLED	Stacked Emitter Layers	1170	0.6	0.6	0.9	(0.33, 0.253)	[24]
2022	QLED	Stacked Emitter Layers	1000	13.3	-	-	(0.283, 0.305)	[35]
2020	Hybrid LED	Vertically Stacked Structure	107,000	26.02	20.31	-	(0.34, 0.36)	[37]
2022	Hybrid LED	Vertically Stacked Structure	7735	8.6	-	-	(0.67, 0.30)	[36]
2022	Hybrid LED	Vertically Stacked Structure	24,911	13.7	-	-	(0.16, 0.76)	[36]

Table 1. Summarization of Multi-color Devices.

Year	Type		Maximum Brightness (cd m−2)	EQE (%)	PE (lm W−1)	CE (cd A−1)	CIE	Ref.
2003	OLED	Side-by-Side Structure	-	-	-	7 2.9	Green blue	[17]
2014	OLED	Vertically Stacked OLEDs	-	10.5	14.4	-	-	[83]
2015	OLED	Vertically Stacked OLEDs	-	3.4	36.8	-	(0.44, 0.45)	[20]
2016	OLED	Vertically Stacked OLEDs	1300	-	-	2.5	(0.326, 0.381)	[68]
2017	OLED	Vertically Stacked OLEDs	43,594	12.32	18.1	28.8	(0.38, 0.44)	[62]
2018	OLED	Vertically Stacked OLEDs	-	-	18.1	-	(0.375, 0.395)	[61]
2018	OLED	Vertically Stacked OLEDs	-	49.4	33.4	-	(0.467, 0.423)	[18]
2022	OLED	Vertically Stacked OLEDs	5748.4	15.1	46.3	42.9	(0.247, 0.579)	[67]
2004	OLED	Single Emission Layer	-	12	42	-	(0.43, 045)	[70]
2009	OLED	Single Emission Layer	-	19.3	42.5	52.8	(0.33, 0.39)	[71]
2015	OLED	Single Emission Layer	-	13.6	14.5	22.5	(0.36, 0.38)	[72]
2020	OLED	Single Emission Layer	30,000	23.6	55.1	52.7	(0.34, 0.36)	[73]
2004	OLED	Double Emission Layers	-	19.3	64	-	-	[75]
2010	OLED	Double Emission Layers	-	34	90	-	(0.45, 0.47)	[76]
2016	OLED	Double Emission Layers	-	23.3	63.2	-	(0.433, 0.458)	[59]
2016	OLED	Multiple Emission Layers	11,000	-	40.8	39.8	(0.32, 0.39)	[60]
2018	OLED	Multiple Emission Layers	-	23.1	59.0	-	(0.324, 0.337)	[77]
2019	OLED	Multiple Emission Layers	23,730	-	33.42	32.74	(0.391, 0.471)	[63]
2020	QLED	Side-by-Side Structure	247,000	22.9	-	9.8	(0.16, 0.77)	[34]
2017	QLED	Vertically Stacked QLEDs	4206	2.0	0.46	4.75	(0.30, 0.44)	[80]
2015	QLED	Stacked Emitter Layers	1170	0.6	0.6	0.9	(0.33, 0.253)	[24]
2022	QLED	Stacked Emitter Layers	1000	13.3	-	-	(0.283, 0.305)	[35]
2020	Hybrid LED	Vertically Stacked Structure	107,000	26.02	20.31	-	(0.34, 0.36)	[37]
2022	Hybrid LED	Vertically Stacked Structure	7735	8.6	-	-	(0.67, 0.30)	[36]
2022	Hybrid LED	Vertically Stacked Structure	24,911	13.7	-	-	(0.16, 0.76)	[36]

3. Flexible and Wearable Multi-Color Devices

In the era of artificial intelligence, flexible and wearable devices become more and more popular in our daily life. With the development of comfortable and stretchable electrodes and substrates, flexible displays based on deformable OLEDs and QLEDs have been widely developed. Wearable optoelectronics typically require light sources that can be manufactured as thin films, such as OLEDs and QLEDs. By integrating flexible multi-color LED with stretchable electronic devices, invisible signals such as temperature and heart rate can be converted into visible colors, which could be used in various types of wearable sensors and healthcare systems.

In 2017, Ja Hoon Koo et al. reported a wearable electrocardiogram monitor via integrating sensors, a carbon nanotube signal amplifier, and ultrathin voltage-dependent color-tunable OLEDs (Figure 13a) [6]. Multi-color OLEDs are used for the colorimetric display of the retrieved electrocardiogram signals. By optimizing the structure of multi-OLEDs, the wearable OLEDs exhibit electrocardiogram-dependent color changes from dark red to pale red, to white, to sky blue, and finally to deep blue. The ultrathin design enables the device to conform to the curvilinear and dynamic surface of human skin and exhibit excellent stability and reliability after repeated deformations (Figure 13b,c).

Although OLED-based optoelectronic devices have successfully realized wearable applications, current OLED technologies require high voltage and lack the power needed for wearable photodynamic therapy (PDT) applications. In 2020, Yongmin Jeon et al. presented a parallel-stacked multi-color OLED with high power, more than 100 mW cm⁻², at low voltage (<8 V) (Figure 13d) [7]. The parallel-stacked OLED with color tuning is realized through OLED color combination, and a high brightness of over 30,000 cd m⁻² is obtained, below 8.5 V (Figure 13e). Confirming its potential application to PDT, the measured singlet oxygen generation ratio of the parallel-stacked OLED is found to be 3.8 times higher than the reference OLED (Figure 13f).

4. Challenges and Perspectives

Overall, the types, characteristics, and possible wearable applications of multi-color LEDs are introduced in detail. The realization of multi-color LEDs is early achieved by the side-by-side patterning of red, green, and blue sub-pixels onto the pixelated display panel. The devices exhibit good optical performances since the light directly emits from respective units through TFT addressing each pixel independently. However, the fill factors and pixel density are inevitably sacrificed due to the lateral integration configuration. Vertical stacking LED configuration could circumvent the limitation of red/green/blue side-by-side color pixels. Because the full color is attained in a single color-tunable pixel instead of three red/green/blue pixels, the pixel density and fill factor of a display with color-tunable pixels can be enhanced three times. However, because of the light loss and the resistance increase in the semitransparent central electrode, the brightness of multi-color LEDs inevitably decreases, and a high voltage is needed to drive multiple LED units. Stacking various emitter layers with different emission wavelengths in a single LED is a direct and simple way to achieve multi-color emission compared with side-by-side and vertically stacked LED units. However, the transition of the complex exciton region between the two emission layers tends to reduce the device’s efficiency.

Therefore, different types of multi-colored LEDs have their respective advantages and disadvantages. Developing particular strengths and circumventing their weaknesses to be applied to appropriate fields is challenging. Although multi-color LED devices have made many remarkable signs of progress in the past few years, there are still some important problems that need further consideration to meet application requirements.

Firstly, the efficiency (such as EQE, PE, CE) of the multi-color LEDs is still too low to meet the needs of practical applications. For example, the EQE that gives the ratio of extracted photons over injected charges of most multi-colored LEDs is below 20%. This may be because the efficiency of monochrome LEDs such as blue LEDs still needs to be improved. In addition, the multi-color LED with a complex structure formed by stacking multiple LED units or various emitter layers leads to inadequate recombination of charge carriers. Thus, luminous materials with excellent properties need to be developed and the device structure to achieve an efficient and balanced carrier injection should be optimized. In addition, designing the optical structure to utilize the microcavity effect could further maximize the efficiency of multi-color LEDs.

Secondly, the high-stability device with a long lifetime is critical for practical applications, but few multi-color LEDs focus on it. Therefore, more attention should be paid to the lifetime of multi-color LEDs in future research. A blue LED emitter is the most unstable emitter, and blue LEDs mostly are required to create a full-color LED display. Thus, it is necessary to develop a commercially efficient phosphorescent blue emitter. In addition, ingeniously designing the multi-color device structure employing mature high-stability red and green LEDs so that it does not require a blue emitter to achieve a full-color display is also a solution.

Thirdly, although multi-color LEDs can realize color tunability controlled by the bias voltage, the range of color variation is still limited, and no arbitrary color within full color can be achieved. In addition, the sensitivity of color variation to voltage should also be considered. When sensitive multi-color LEDs and sensors are integrated to construct optoelectronic devices, little changes in signal intensity can achieve significant variations in colors. Therefore, in the future, it is necessary to expand the degree of the color change in multi-color LEDs tuned by bias and improve the sensitivity of LED color changing with voltage.

Finally, the challenge for flexible multi-color LED devices is to ensure that they operate reliably after cycle bending. In addition, encapsulation technology that withstands long-term bending is also critical to practical application. Therefore, more attention should be paid to the stability of flexible multi-color LEDs after cycle bending and developing relevant encapsulation technology.