Fully Solution-Processed Tandem White Quantum-Dot Light-Emitting

Jun 12, 2018 - Solution-processed electroluminescent tandem white quantum-dot ... loss of brightness after light passes through a matched color filter...
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Fully Solution-Processed Tandem White Quantum-Dot Light-Emitting Diode with an External Quantum Efficiency Exceeding 25% Congbiao Jiang,†,§ Jianhua Zou,†,‡,§ Yu Liu,† Chen Song,† Zhiwei He,† Zhenji Zhong,† Jian Wang,† Hin-Lap Yip,† Junbiao Peng,*,† and Yong Cao† Downloaded via KAOHSIUNG MEDICAL UNIV on July 10, 2018 at 09:48:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ Guangzhou New Vision Optoelectronic Technology Co., Ltd., Guangzhou 510730, China S Supporting Information *

ABSTRACT: Solution-processed electroluminescent tandem white quantum-dot light-emitting diodes (TWQLEDs) have the advantages of being low-cost and high-efficiency and having a wide color gamut combined with color filters, making this a promising backlight technology for highresolution displays. However, TWQLEDs are rarely reported due to the challenge of designing device structures and the deterioration of film morphology with component layers that can be deposited from solutions. Here, we report an interconnecting layer with the optical, electrical, and mechanical properties required for fully solution-processed TWQLED. The optimized TWQLEDs exhibit a state-of-theart current efficiency as high as 60.4 cd/A and an extremely high external quantum efficiency of 27.3% at a luminance of 100 000 cd/m2. A high color gamut of 124% NTSC 1931 standard can be achieved when combined with commercial color filters. These results represent the highest performance for solution-processed WQLEDs, unlocking the great application potential of TWQLEDs as backlights for new-generation displays. KEYWORDS: quantum dots, light-emitting diodes, interconnecting layer, high efficiency, backlight

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(VR) displays. To exploit the unique properties of QDs to achieve ultrathin, flexible, and high-resolution displays, the method of incorporating individually controllable white emission pixels with color filters is proposed. This design has been successfully applied in curved OLED TV and microOLED displays.7,8 Compared with white organic LEDs (WOLEDs), white quantum-dot LEDs (WQLEDs) have a much narrower emission bandwidth, which can not only extend the color gamut to a wider range but also reduce substantial attenuation and loss of brightness after light passes through a matched color filter (CF).9−11 The first QD-based light-emitting diode was demonstrated in 1994, and rapid progress in electroluminescent (EL) performance has since been achieved through continual improvements in material synthesis and device optimization.12−17 The state-of-the-art external quantum efficiencies (EQEs) of red, green, and blue QLEDs are comparable with

olloidal semiconductor quantum dots (QDs) can be engineered to achieve high photoluminescent (PL) efficiency, outstanding photostability, and tunable emissions with good color saturation properties, making them promising visible emitters for display and lighting applications.1−3 QDs have recently been applied to improve liquid crystal display (LCD) technology by using particularly red and green QDs as down-converters, with blue lightemitting diodes (LEDs) as backlights, to improve color purity and the color gamut of LCD displays.4 However, as liquid crystal is required in this QD-enhanced LCD technology, it is challenging to achieve the ultrathin, ultralight, ultrafast, and flexible characteristics of new-generation displays. QD LEDs with an active matrix of individually controllable self-emitting pixels (AMQLEDs) could meet this challenge. AMQLED displays have been fabricated by transfer printing and inkjet printing techniques, showing a wide color gamut of 109% of the National Television Systems Committee (NTSC 1931) standard.5,6 While the inkjet printing technique has a cost advantage, it is difficult to achieve a sufficiently high definition to satisfy the requirements of portable device and virtual reality © 2018 American Chemical Society

Received: March 27, 2018 Accepted: June 12, 2018 Published: June 12, 2018 6040

DOI: 10.1021/acsnano.8b02289 ACS Nano 2018, 12, 6040−6049

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Figure 1. Architecture of tandem devices and a schematic diagram of the charge generation and injection mechanism. (a) Structure of tandem red QLEDs; (b) structure of TWQLEDs; (c) schematic diagram of the charge generation and injection mechanism.

those of OLEDs.13−15,18−20 However, most significant works have focused on improving monochrome QLEDs and only a few reports on WQLEDs, although these are technologically more important. The highest current efficiency (CE) of WQLEDs fabricated based on a mixed red, green, and blue QD emission layer is only 21.8 cd/A, which is inferior to the performance of commercial WOLEDs.9,21 Furthermore, the EL spectra of this mixed-color QD-based WQLED inevitably change with applied bias, due to the charge migration between QDs with different band gaps.9 To circumvent this problem, the emission layer of the different color QDs in the devices must be separated in space, which can be achieved using a tandem structure with stacked red-, green-, and blue-emitting layers. Indeed, such tandem designs have been successfully used to boost device efficiency and stabilize EL spectra in WOLEDs.22 In tandem LEDs, two or more light-emitting units (LEUs) are connected through an interconnecting layer (ICL). When bias is applied, multiple electron−hole pairs can be injected through the electrodes and charge generation layer into the different LEUs; therefore, it is possible to simultaneously emit multiple photons, which results in an increase in CE.23,24 The generation and recombination of excitons also occur on each emission layer, which prevents the close range charge transfer between different QDs; therefore, the variation in EL spectra recorded at different driving voltages can be reduced effectively.25,26 The design of proper ICLs is critical to achieving highperformance tandem LEDs, as they play an important role in tuning the electron and hole carriers’ generation and injection properties in the devices. For example, an extremely efficient charge generation has been successful demonstrated by the contact between an organic hole transport layer (HTL) and a transition metal oxide (TMO) layer, such as MoO3, WO3, and V2O5.27−29 Therefore, an electron transferred from the highest occupied molecular orbital (HOMO) of the adjacent HTL to the conductive band (CB) of the TMO is energetically favorable due to the very deep lying conduction bands of TMOs, which means that TMOs can work as a hole injection

layer (HIL) in the device. A HTL with a HOMO level matched to the HOMO or valence band (VB) of the HIL in the LEUs is chosen in this case to facilitate efficient hole injection. Providing efficient electron injection to the LEU usually requires inserting an electron injection layer (EIL) with tailored LUMO or CB on the other side of the TMO layer. Thus, the expected ICLs consist of an EIL and HIL to efficiently facilitate charge injection. Successful ICLs formed by a vacuum deposition process are BPhen:Cs2CO3/WO3 and BPhen:Mg/MoO3, which have been used to construct very efficient tandem OLEDs.28,29 However, creating an efficient ICL from solution processes remains challenging, as it requires the development of a combination of component layers that can be processed from orthogonal solvents without degrading the underlayers while maintaining efficient charge generation and injection functions. A high work function, aqueous-processed conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), is the most-used p-type material for forming the ICL in tandem devices.30−35 It is typically used in conjunction with other n-type metal oxides such as zinc oxide (ZnO), titanium oxide (TiO2), or Mg-doped ZnO (ZnMgO) to create the desired p/n junctions.25,31,33,36 Nevertheless, the acidic nature of PEDOT:PSS makes it susceptible to reacting with metal oxides, causing degradation of the contact interface and device performance. Kido et al. recently addressed this by inserting a pH-neutral PEDOT:PSS between the acidic PEDOT:PSS and ZnO nanoparticle layers to circumvent the chemical reaction and solvent penetration problems in tandem OLEDs.33 A similar strategy has also been used to create a solution-processed ICL based on WO3/PEDOT:PSS/ZnO in a polymer tandem OLED.25 In addition, a thin metal film inserted between the ETL and PEDOT:PSS HTL to improve recombination efficiency has been reported in solar cells.32 Chen et al. also reported a simplified bilayer ICL based on a ZnMgO/acid PEDOT:PSS structure for TWQLEDs. However, the deteriorated surface morphology after multiple solvent treatments led to a relatively low CE of 4.57 cd/A in 6041

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Figure 2. Characteristics of ZnO/PMA bilayer ICL. (a) Cross-sectional STEM image of TWQLEDs; (b) quantitative material distribution by EDS line scanning; (c) PL spectra of ITO/QDs/ZnO/PMA before and after p-Xy rinsing; (d) transmittance of ZnO, PMA, and ZnO/PMA films; (e) current density−voltage (J−V) curves of red QLEDs with various structures. Device A: ITO/PEDOT/TFB/red QD/ZnO/Ag. Device B: ITO/PEDOT/TFB/red QD/ZnO/PMA/Ag. Device C: ITO/ZnO/PMA/TFB/red QD/ZnO/Ag.

the devices,37 suggesting that the development of PEDOT:PSS replacement in the ICL for solution-processed tandem LEDs is required. In this work, an inorganic ZnO nanoparticle and polyoxometalate phosphomolybdic acid (PMA) bilayer structure was used as an efficient solution-processed ICL for TWQLEDs. The proposed structure can not only be processed from orthogonal solvents to form a robust ICL but also protect the QD layer underneath from being eroded by subsequent solvent processing. This allows tandem devices with over 10 functional layers to be constructed, including three LEUs composed of red-, green-, and blue-emitting QD layers. Fully solution-processed TWQLEDs were successfully presented with the proposed ICL, achieving state-of-the-art EL performance with peak values of 60.8 cd/A in CE, 27.4% in EQE, and over 200,000 cd/m2 in luminance. In addition, the TWQLEDs hold a high EQE of over 20% at a wide luminance range from 10 000 to 100 000 cd/m2 with a minimal color variation (ΔCIE (Commission International de I’Eclairage)) of (0.014, 0.030). To the best of our knowledge, these results represent the highest CE and EQE reported in solution-processed WQLEDs.9,37 By further combining with a commercial CF, saturated RGB colors can be produced, resulting in a color gamut as high as 124%, relative to the NTSC 1931 standard.

erosion of the bottom layers. The ICL must therefore not only be processed by orthogonal solvents but also form a robust layer with suitable solvent resistance to protect the bottom light-emitting layers from degrading when the upper lightemitting layers are deposited. We report an ICL design comprising a ZnO and PMA for efficient charge injection and transfer into adjacent LEUs to achieve this, and the device structure is depicted in Figure 1a and b. ZnO is selected as the electron injection layer due to its good energy-level alignment with QDs and good resistance, which can prevent solvent penetration.3,38 The function of PMA is equivalent to MoO3, which has a high work function (WF) and has been widely used in solution-processed solar cells and OLEDs/QLEDs as hole extraction and injection layers, respectively.39−41 Figure 1c gives the energy-level diagram of red QDs/ZnO/PMA/ HTL/red-QDs, which is the heterointerface between adjacent LEUs and shows the charge generation and injection mechanism. Because this ICL is composed of n-type ZnO with low WF on its one side and n-type PMA with high WF on its other side, the electrons will spontaneously transfer from a conduction band of ZnO to that of PMA leading to a large interface dipole, which causes high upward vacuum level shift toward the PMA side.22 Due to very deep lying CB of PMA, the electron is energetically transferred from the HOMO of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) to the CB of PMA, and a hole is formed on the HOMO of TFB when PMA contacts the TFB layer.27−29 Under forward bias, the electron can smoothly transfer from the CB of PMA to that of ZnO because of shifting of the vacuum level,

RESULTS AND DISCUSSION Characteristics of the ICL. The key criterion for constructing solution-processed multilayer tandem devices is to ensure each layer can be deposited subsequently without 6042

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Figure 3. Electric characteristics of tandem red QLEDs and the reference device. (a) Current density−voltage (J−V) curves. Inset: luminance−voltage (L−V) curves. (b) Luminance−current density (L−J) curves. (c) CE−J curves. (d) EL spectra recorded at 5 mA/cm2.

due to its wide band gap, which is beneficial for better light outcoupling and EL performance. The charge injection and transport property of ICL was also tested using the structure of ITO/HIL/TFB/red QD/ETL/ Ag, and the current densities versus voltage (J−V) characteristics are shown in Figure 2e. In the following results, control device A uses PEDOT:PSS as an HIL and ZnO as an ETL, while device B and device C employ an ICL as the ETL and HIL respectively. Details of the device fabrication method are given in the Experimental Section. All three devices show J−V curves with a visible ohmic regime and a trap-limited conduction regime, as reported elsewhere.42,43 In addition, all devices have an onset voltage of charge injection at about 2.0 V, which indicates that the ICL can inject both electrons to QDs and holes to TFB layers38,44 and that ohmic contact was formed between ICL/QDs and ICL/TFB interfaces. However, it can be noticed that devices with a PMA layer have a relatively low current density at high driving voltage comparing with control device A, which is due to poorer conductivity caused by gradual reduction from Mo(V) to Mo(IV) of the MoO3 unit in PMA during the thermal annealing process, which is required to form a robust PMA film.40 As mentioned, morphology deterioration is the main problem in solution-processed tandem devices and strongly affects EL performance. Therefore, before the device performance study we first studied the surface morphology of films in the multilayer devices. We selectively studied the surface morphology of the ZnO layers in the different positions of the tandem cells to investigate the morphology evolution in the devices. The surface roughness of the corresponding ZnO layers in the single-, double-, and triple-junction devices is

then inject into the CB of the QD layer of the bottom unit, while the hole on the HOMO of the TFB layer moves in the opposite direction and injects into the valence band of the QD layer in the top unit, as shown in Figure 1c. Both ZnO and PMA can be subsequently processed from alcoholic solvents forming a well-defined interface. TFB dissolved in p-xylene (p-Xy) is then used as a hole transport layer for all of the LEUs in the tandem cell. The QD layer is deposited from octane, which indicates that the QDs may also be susceptible to washing away by other nonpolar solvents such as p-Xy. Indeed, a QD layer rinsed by p-Xy did show degradation in its light-emitting quality, as shown in Supplementary Figure S1. Therefore, the ICL layers play a vital role in protecting the bottom QD layer when depositing the TFB layer from p-Xy, which eventually enables the successful fabrication of the whole tandem cell from solutions. The quality of the layered structure of the TWQLED was examined using a cross-sectional scanning transmission electron microscope (STEM) and energy-dispersive X-ray spectroscopy (EDS) line scanning images for compositional analysis, which are shown in Figure 2a and b, respectively. The results clearly show that distinct interfaces are formed between each layer in the tandem cell, which further confirms that the proposed ICL and device processes are suitable for making high-quality multilayer devices. The steady-state PL intensity of the red QD layer coated with an ICL before and after rinsing with p-Xy is shown in Figure 2c, and the identical PL spectrum obtained suggests that the ICL can fully protect the QDs’ emissive layer from being damaged. Furthermore, as shown in Figure 2d, the ZnO/PMA layer has a high transmittance of over 95% in the entire visible light region, 6043

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Figure 4. EL characteristics of tandem white devices. (a−c) EL spectra evolution of W1, W2, and W3 with luminance; (d) CIE coordinate variation for W1, W2, and W3 with luminance; (e) angular distributions of the emission of W3 at a luminance of 10 000 cd/m2; (f) lighting on image of W3 at an operation voltage of 15 V.

illustrated in Supplementary Figure S2. After fabricating the first LEU with ZnO as the fourth layer, its root-mean-square (Rms) roughness was only around 1.10 nm, while the Rms roughness of ZnO as the eighth and 12th layers in the second and third LEUs continued to maintain low values of 0.97 and 1.09 nm, respectively. The smooth surfaces suggested that the component layers could be processed from solutions with high uniformity, and the results are in good agreement with the cross-sectional TEM study, showing abrupt interfaces across each layer. Tandem Red QLEDs. To evaluate the performance of the tandem QLEDs, we first fabricated pure red-emitting tandem devices with the structure ITO/PEDOT:PSS/TFB/red-QDs/ ZnO/PMA/TFB/red-QDs/ZnO/Ag. In addition to the ITO

anode and Ag cathode, all eight component layers in this double-junction tandem device were deposited by spin coating. For comparison, single-junction devices mimicking the bottom LEU with the structure ITO/PEDOT:PSS/TFB/red-QDs/ ZnO/Ag and the top LEU with the structure ITO/PMA/TFB/ red-QDs/ZnO/Ag were also fabricated. Details of the device fabrication method are given in the Experimental Section. The EL performances of these three devices are shown in Figure 3 and summarized in Table S1. As illustrated in the J−V characteristics in Figure 3a, the top LEU exhibits a higher operation voltage than the bottom LEU, which is attributed to poorer conductivity than in the PEDOT:PSS layer used in the bottom LEU-based devices.40 The EL performance of the red tandem QLED was tested, and the J−V characteristics are also 6044

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ACS Nano shown in Figure 3a in conjunction with the theoretical J−V curve, calculated by adding the bottom and top reference LEUs. The two J−V curves matched near perfectly, suggesting that the ICL provides very efficient charge injection and transport properties. A minimal voltage drop occurred at the junction, which was not surprising, as we had already found that an ohmic junction was formed, as in Figure 2e. In addition, as shown in Figure 3b, the luminance of the tandem device is equal to the sum of the bottom and top LEUs at a given current density, indicating that negligible light absorption loss is presented in the ICL. Similarly, the CE of the tandem device is also equal to the sum of the two reference devices. For example, the tandem device exhibits a peak CE of 27.9 cd/ A, which is almost the sum of 15.1 and 12.7 cd/A exhibited by the bottom and top LEUs at a current density of about 120 mA/cm2. The tandem red QLEDs also show a very slight efficiency roll-off at high luminance. For example, a relatively high CE of about 27.7 cd/A is obtained at a high luminance of 50 000 cd/m2, making it viable within a wide illumination window range. Finally, the EL spectra of the tandem device and the bottom and top reference devices were recorded and are shown in Figure 3d. These reveal an identical saturated red emission with a central wavelength of 618 nm. Tandem White QLEDs. To achieve the white emission, triple-junction tandem QLEDs were fabricated by stacking the RGB LEUs in the sequence of bottom blue LEU, middle green LEU, and top red LEU to avoid reabsorption. As shown in Figure S3a of UV−vis absorption and PL spectra of blue, green, and red QD films, blue emission could be absorbed by the red QD film since there is a relatively large overlap between the PL spectrum of the blue QD film and the absorption spectrum of the red QD film, which would reduce the EL efficiency. Thus, the blue LEU was arranged at the bottom, which is close to the light output side, while the red LEU is close to the cathode to reduce the absorption of blue emission by the red QD film. The fully solution-processed TWQLED is fabricated with 12 layers, with a structure of ITO/ PEDOT:PSS/TFB/blue-QDs/ZnO/PMA/TFB/green-QD/ ZnO/PMA/TFB/red-QD/ZnO/Ag, and labeled as W1. The fabrication details are given in the Experimental Section. The EL spectrum of W1 is shown in Figure 4a. We observed strong green and red emissions but a very weak blue emission. We attribute this to the low efficiency of the blue LEU in the W1 device compared with the green and red LEUs. As the blue QD has a larger band gap, a larger hole injection barrier is presented at the TFB/QD interface, which causes the low level of efficiency and the weak blue emission in the TWQLED, as shown in the energy-level diagram of the TWQLEDs in Supplementary Figure S4. To improve the device performance of the blue LEU, poly(9-vinylcarbazole) (PVK), with a deeper HOMO than that of TFB, was chosen to reduce the hole injection barrier.14,45 We thus investigated the EL performance of the single blue LEU based on TFB, PVK, and TFB/PVK double layers as HTL. The architecture of the single blue LEU was ITO/PEDOT:PSS/HTL/blue-QDs/ZnO/Ag, and the EL performances of the corresponding devices are summarized in Supplementary Figure S5 and Table S2. Compared with the TFB-based devices, the CEs of the PVK- and PVK/TFB-based devices increased from 0.29 to over 1.20 cd/A. In addition, the blue LEU with bilayers of HTL exhibited the best EL performance, with a lowest VT (the voltage at a luminance of 1 cd/m2) of 3.0 V and a peak CE of 1.24 cd/A. We attribute the improvement to the stepwise energy level of the TFB/PVK

bilayer, which further reduces the hole injection barrier. In addition, the higher hole mobility of TFB also makes it more efficient in hole transport compared with the single PVK layer.13,45 Also, the bilayer HTL of TFB/PVK was applied to the red QLED, but its EL efficiency is just slightly improved compared to that of the blue one. The EL characteristics of the red QLED with different HTLs are exhibited in Supplementary Figure S6, and the CE increases from 15.1 cd/A to 17.0 cd/A when using TFB/PVK as the bilayer HTL. The slight enhancement of EL efficiency of the red QLED might be due to the different energy level structure between the red and blue QDs. Considering the slight enhancement of the EL efficiency and complicated fabrication process when using the bilayer HTL, the single TFB layer was finally adopted as the HTL for red and green LEUs in the TWQLEDs. Consequently, to improve the performance of the TWQLEDs, devices consisting of the bottom blue LEU with PVK and TFB/PVK as the HTL were fabricated and named W2 and W3, respectively. As expected, both W2 and W3 displayed stronger blue emissions than that of W1, as depicted in Figure 4b and c. The intensity of the blue light in W2 decreased sharply with increasing luminance, causing the Commission Internationale de l’Eclairage (CIE) coordinate to shift from (0.358, 0.364) to (0.402, 0.405) when the luminance increased from 10 000 to 100 000 cd/m2 (Figure 4d), resulting in a large color variation at different luminance. Conversely, the EL spectra of W3 were more stable with increasing luminance, and the CIE coordinate shifted slightly from (0.407, 0.400) to (0.405, 0.388) when the luminance increased from 25 000 to 100 000 cd/m2. The color variation was only (0.002, 0.012) within a wide luminance range. The better stabilization of the EL spectra of W3 can be attributed to the lower efficiency in the rolling-off of the blue LEU in W3. It is noticed that W2 exhibits a large change of the EL spectra with different driving voltages, which is quite different from those of W1 and W3 (Figure 4). The significantly reduced blue emission of W2 with increasing driving voltages might be due to high roll-off of the blue LEU (Figure S5 and Table S2). For the relative increase of green emission in W2, the inefficient charge generation and separation at the PMA/TFB interface was the principal reason. Because at low driving voltages, hole injection of the blue LEU is inefficient due to the large energy barrier between PEDOT and PVK (Figure S4), the injected electrons might accumulate at the interface between PVK and the blue QDs. The accumulated electrons might impede the charge generation and separation at the PMA/TFB interface, leading to inefficient hole injection from TFB into the green QDs and weak green emission, which might be the reason that the green emission of W2 shows relatively weaker emission compared with that of W3 (Figure 4b and c). With increasing the driving voltages, the hole injection of the blue LEU is enhanced and the accumulated electrons at the interface of PVK and blue QDs were reduced, resulting in the improvement of hole injection from TFB into the green QDs and the increase of the green emission as well. It is noticed that the W2 and W3 exhibit almost identical EL spectra at high driving voltages (e.g., at a brightness of 40 000 cd/m2) (Figure 4b and c), implying that both holes and electrons can be efficiently injected into RGB QDs at high driving voltages. Moreover, in TWQLEDs, RGB emission intensity at a given current density is proportional to the EQE of the corresponding LEU, and the higher the EQE, the stronger the emission intensity. Thus, the peak intensities in the EL spectrum of W3 are decreasing in the 6045

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Figure 5. Electric characteristics of tandem WQLEDs. (a) J−V−L curves; (b) CE−J curves; (c) EQE−J curves; (d) histogram of maximum CE of W3 for 17 devices from three batches.

Table 1. Performance of Tandem WQLEDs device

current density (mA/cm2)

voltage (V)

brightness (cd/m2)

CE (cd/A)

PE (lm/W)

EQE (%)

W1

20 50 100 200 20 50 100 200 20 50 100 200

16.0 17.8 18.8 19.4 17.6 19.0 20.0 20.6 15.2 16.8 17.6 18.0

8200 23 600 51 200 104 800 10 040 26 100 58 200 100 500 9400 28 100 63 000 115 000

38.69 46.07 50.98 55.37 47.70 53.18 55.89 54.61 45.58 52.14 57.53 60.46

7.59 8.13 8.51 8.96 8.51 8.79 8.78 8.32 9.42 9.75 10.26 10.55

14.35 17.09 18.68 21.04 21.99 23.93 24.15 23.26 19.53 22.52 25.60 27.39

W2

W3

CIE (x, y) (0.427, (0.423, (0.437, (0.441, (0.358, (0.371, (0.385, (0.402, (0.421, (0.407, (0.404, (0.405,

0.542) 0.534) 0.525) 0.515) 0.364) 0.378) 0.391) 0.405) 0.422) 0.400) 0.390) 0.388)

a given operation voltage. The improvement is attributed to the enhancement of the hole current in the TFB/PVK bilayer HTL-based blue LEU. In addition, W3 shows ultrahigh brightness over 200 000 cd/m2, which is 2 times higher than that of W1. The maximum CEs of these three types of TWQLEDs are 56.3, 56.2, and 60.8 cd/A, respectively, with peak EQEs of 21.6%, 24.4%, and 27.4% (Figure 5b and c). To the best of our knowledge, these results provide the highest values of white-emitting devices based on quantum dots, as can been seen from Supplementary Table S3. Further details of the performance of the three tandem WQLEDs are given in Table 1. To investigate the reliability and reproducibility of the fully solution-processed TWQLEDs, more than 17 devices of each W1, W2, and W3 were fabricated from different batches, and their maximum CEs were recorded. As shown in Figure 5d and

order of red, green, and blue, the same as the EQE decreasing order. For example, according to the current efficiency of each LEU, the EQEs of RGB emission of W3 are estimated to be 7.9%, 6.1%, and 2.7%, respectively, at a brightness of 5000 cd/ m2 (12 mA/cm2). Even so, warm white light emission can be achieved from W3 due to the lower sensitivity of human eyes to red light. Figure 4e shows the angular dependence of the radiance for W3, illustrating that the light emission of the tandem device agrees well with the Lambertian assumption. Thus, the forward-viewing CE was recorded without correction.33,38 The photo of the TWQLED (W3) under operation in Figure 4f reveals a uniform white light emission. Finally, the J−V−L curves of the W1, W2, and W3 tandem devices are summarized in Figure 5a to compare their overall electrical properties. W3 exhibits the highest current density at 6046

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vacuum baking oven. Before depositing PEDOT:PSS, the ITO substrates were treated with oxygen plasma for 10 min, and then a 30 nm thick PEDOT:PSS was spin-coated on the ITO anode as HIL followed by a 15 min annealing treatment at 150 °C. The TFB (10 mg/mL in p-Xy) and red QDs (20 mg/mL in octane) were spincoated on the HIL in sequence with thicknesses of 30 and 25 nm, respectively, and baked at 150 °C for 5 min in a glovebox. Sequentially, a 40 nm thick ZnO layer was coated on red QDs and annealed at 150 °C for 10 min. The PMA as an HIL for the top LEU was spin-coated on the ZnO film of the bottom LEU with a thickness of 20 nm and baked at 150 °C for 5 min. The depositions of TFB, red QDs, and ZnO layers were then the same as the bottom LEU. Finally, the Ag as a metal cathode was deposited by vacuum thermal evaporation with a thickness of 200 nm. Fabrication of TWQLEDs. The device structure of TWQLEDs was ITO/PEDOT:PSS/HTL/blue-QD/ZnO/PMA/TFB/greenQD/ZnO/PMA/TFB/red-QD/ZnO/Ag (Figure 1b) with an active area of 2 × 2 mm2. Before fabrication, the ITO substrate treatment process was the same as that of tandem red QLEDs. The PEDOT:PSS as an HIL for the blue LEU of 30 nm was coated onto the ITO and dried at 150 °C for 15 min. For the fabrication of W1 and W2, 30 nm thick TFB (10 mg/mL in p-Xy) and PVK (8 mg/mL in chlorobenzene) were coated on PEDOT:PSS, followed by a coating of 20 nm thick blue QDs, and baked at 150 °C for 5 min. To fabricate W3, TFB (8 mg/mL in p-Xy) of 25 nm was first deposited on the HIL and baked at 150 °C for 5 min, and then PVK (2 mg/mL in 1,4dioxane) of 5 nm was coated onto the TFB. In sequence, the 20 nm thick blue QDs were coated on the PVK and dried at 150 °C for 5 min. The middle green and top red LEUs were then stacked onto the bottom blue LEU using the same process used for the fabrication of the top LEU of tandem red QLEDs. Here, the green QDs were deposited on the TFB at 20 nm to achieve a high EL performance. Finally, Ag was deposited as the metal cathode by vacuum thermal evaporation, with a thickness of 200 nm. Device Measurement. The PL and EL spectra were recorded using FL3C-111 (HORIBA Instruments Inc.) and a fiber optic spectrometer (Ocean Optics USB 2000). The thickness and surface roughness were recorded using Dektak XT and Bruker Multimode 8. The cross-sectional STEM image was taken with a FIB-TEM Helios Nanolab 450S. The transmittance was measured with an HP 8453E UV−visible spectrophotometer. The EL performance was measured after being encapsulated using epoxy glue and a glass cover. The J− V−L characteristics and CIE coordinates were measured by a Keithley 2400 source meter and a Konica Minolta Chroma Meter CS-200. Angular distributions of the emission were measured by a Zolix Chame-LuSA100. The image of lighting on the tandem white device was taken by a Canon EOS 550D.

Figure S7, an average peak CE of 55.4 cd/A with only a 5.0% relative standard deviation (RSD) was observed from W3. The low RSD data indicate that the solution-processed multilayer tandem WQLEDs have very good reproducibility. Besides, all three types of TWQLEDs show good operation stability at a constant current with an initial brightness over 3000 cd/m2, as shown in Figure S8. Owing to lower operation voltage, the lifetime of W3 reaches about 11.5 h from 6.5 h of W1, about a 1.7 times enhancement. Finally, by further combining the TWQLEDs with commercial color filters, primary RGB emissions can be reproduced and a high color gamut of 124% can be achieved, as shown in Supplementary Figure S9, which is much better than that obtained from commercial fullcolor OLEDs.46,47

CONCLUSION In summary, fully solution-processed, high-efficiency, and highbrightness tandem QLEDs were successfully fabricated. A bilayer structure of ZnO/PMA was introduced as the interconnection layer for the tandem QLEDs. This ICL not only provides good solvent resistance to protect the bottom QD light-emitting layers and prevent them from being destroyed during the solution deposition processes but also has excellent charge injection and transport properties. In addition, the high visible transmittance of the ICL minimizes the parasitic light absorption within the devices, leading to highly efficient tandem red QLEDs with a CE and EQE almost equal to the sum of the individual bottom and top singlejunction red-emitting reference devices. By further stacking RGB LEUs in series with the proposed ICL, the optimized TWQLED exhibited the highest reported CE and EQE of 60.4 cd/A and 27.3%, respectively, at a luminance of 100 000 cd/ m2. The TWQLED holds a high EQE exceeding 20% at a wide luminance ranging from 10 000 to 100 000 cd/m2. A wide color gamut of 124% (NTSC 1931) can also be achieved by combining TWQLEDs with commercial color filters, suggesting promising applications as a plane backlight source in highdefinition displays. EXPERIMENTAL SECTION Materials. ITO-coated glass with a sheet resistance of 15−20 Ω per square was purchased from China Southern Glass Holding Corp.; PEDOT:PSS (Clevios P VP AI 4083) from Heraeus Electronic Materials Division; TFB from American Dye Source, Inc.; and PVK and PMA from Sigma-Aldrich. The red and green QDs with alloyed core−shell CdSe/ZnS, capped with oleic acid ligand, were purchased from Mesolight Inc. The diameter is about 11 ± 1.5 nm for red and 10 ± 1.3 nm for green, and the quantum yield is about 80% and 85% for red and green, respectively. The blue QDs with core−shell ZnCdS/ZnS, capped with oleic acid, were purchased from Guangdong Poly Optoelectronics Co., Ltd. The diameter is about 13 ± 1.6 nm and quantum yield is about 80%. The UV−vis absorption and PL spectra of red, green, and blue QDs are shown in Figure S3a in the Supporting Information. The zinc oxide nanoparticles with a size of 3−5 nm dispersed in ethanol with a mass concentration of 30 mg/mL were purchased from Guangdong Poly Optoelectronics Co., Ltd. The UV−vis absorption and TEM image are demonstrated in Figure S9 in the Supporting Information. All solvents with purities of 99.9% are from Sigma-Aldrich. Fabrication of Tandem Red QLEDs. The device structure of tandem red QLEDs was ITO/PEDOT:PSS/TFB/red-QD/ZnO/ PMA/TFB/red-QD/ZnO/Ag (Figure 1a) with an active area of 2 × 2 mm2. Before device fabrication, the ITO substrates were thoroughly cleaned in sequence in an ultrasonic bath of acetone, isopropyl alcohol (IPA), detergent, and deionized water and dried in a

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b02289. Additional information (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Jianhua Zou: 0000-0002-5262-7062 Jian Wang: 0000-0001-5629-6793 Hin-Lap Yip: 0000-0002-5750-9751 Junbiao Peng: 0000-0003-1671-2750 Author Contributions §

C. Jiang and J. Zou equally contributed.

Notes

The authors declare no competing financial interest. 6047

DOI: 10.1021/acsnano.8b02289 ACS Nano 2018, 12, 6040−6049

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