Improved Efficiency and Enhanced Color Quality of Light-Emitting

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Improved efficiency and enhanced color quality of light emitting diodes with quantum dot and organic hybrid tandem structure Heng Zhang, Yuanxiang Feng, and Shuming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07303 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016

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ACS Applied Materials & Interfaces

Improved Efficiency and Enhanced Color Quality of Light-Emitting Diodes with Quantum Dot and Organic Hybrid Tandem Structure

Heng Zhang, Yuanxiang Feng, Shuming Chen* Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, 518055, P. R. China [email protected] Abstract Light-emitting diodes based on organic (OLEDs) and colloidal quantum dot (QLEDs) are widely considered as next generation display technologies because of their attractive advantages such as self-emitting and flexible form factor. The OLEDs exhibit relatively high efficiency, but their color saturation is quite poor compared with that of QLEDs. In contrast, the QLEDs show very pure color emission but their efficiency is lower than that of OLEDs currently. To combine the advantages and compensate the weaknesses of each other, we propose a hybrid tandem structure which integrates both OLED and QLED in a single device architecture. With ZnMgO/Al/HATCN inter-connecting layer, hybrid tandem LEDs are successfully fabricated. The demonstrated hybrid tandem devices feature high efficiency and high color saturation simultaneously; for example, the devices exhibit maximum current efficiency and external quantum efficiency of 96.28 cd/A and 25.90%, respectively; meanwhile, the full width at half maximum of the emission spectra is remarkably reduced from 68 nm to 44 nm. With the proposed hybrid tandem structure, the color gamut of the displays can be effectively increased from 81% to 100% NTSC. The results indicate that the advantages of different LED technologies can be combined in a hybrid tandem structure.

Keywords:

tandem

structure;

organic

light-emitting

light-emitting diodes; color purity; inter-connecting layer

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ACS Paragon Plus Environment

diodes;

quantum

dot


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1. Introduction Light-emitting diodes based on organic (OLEDs) and colloidal quantum dot (QLEDs) have been attracting intensively interests because of their potential applications in next generation displays.1-9 They have many advantages over traditional display technologies like self-emitting, high efficiency, pure emission color, simple structure, easy fabrication and excellent flexibility, which make them promising candidates for future displays.10-16 OLEDs have been developed for several decades, and the device performances have been improved greatly. For example, the external quantum efficiency (EQE) of the OLEDs has approached 30% by using efficient phosphorescent emitters like Ir(ppy)2(acac)

17-18

or Ir(ppy)2tmd 19. However,

the color saturation of the OLEDs is relatively poor because the emission spectra of the organic emitters are usually wide with a full width at half maximum (FWHM) typically larger than 60 nm.20 To narrow the FWHM and thus improve the color saturation, microcavity structure is commonly used.21-22 However, the emission spectra of microcavity OLEDs are angularly dependent,23-24 which is detrimental to the viewing characteristics of the displays. While OLEDs are struggling to achieve high color saturation, QLEDs inherently exhibit very high color purity. The FWHM of the spectra of QLEDs typically is smaller than 30 nm, and thus displays based on QLEDs exhibit very wide color gamut which can be larger than 100% NTSC.25-26 However, the efficiency of QLEDs is still lower than that of OLEDs, though steady and rapid progresses have been made in recent years.27 The advantage of QLEDs is a perfect complement to the weakness of OLEDs. Thus, to simultaneously realize high color saturation and high efficiency, we herein propose a hybrid tandem structure which integrates both OLED and QLED in a single device architecture. The proposed structure combines the advantages while compensates the weaknesses of QLED and OLED, thus achieving win-win results. It should be noted that, most of the previous work focused on developing tandem OLEDs that are comprised by two organic electroluminescent (EL) units, where the first EL unit and the second EL unit are basically the same with an identical light 2 / 17


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emitting layer.28-30 The present work is the first demonstration of hybrid tandem LEDs, which simultaneously exhibit high efficiency and saturated color emission. To realize the hybrid tandem LEDs, a new inter-connecting layer (ICL) based on ZnMgO/Al/HATCN is used to connect the green QLED and the green phosphorescent OLED. The demonstrated hybrid tandem devices exhibit maximum current efficiency (CE) and external quantum efficiency (EQE) of 96.28 cd/A and 25.90%, respectively; meanwhile, the FWHM of the emission spectra is remarkably decreased from 68 nm to 44 nm. With the proposed hybrid tandem structure, the color gamut of the displays can be effectively increased from 81% to 100% NTSC. The results indicate that the advantages of different LED technologies can be combined in a hybrid tandem structure. 2. Results and discussion Figure 1 shows the schematic device structure and energy band diagram of the functional layers. The hybrid tandem LED comprises a gradient alloyed CdZnSeS/ZnS-based green QLED and a Ir(ppy)2(acac)-based green phosphorescent OLED. We chose green color to optimize, because human eyes are most sensitive to the green among all colors, and therefore the color gamut of displays can be effectively improved by only enhancing the saturation of the green emission. Both QLED and OLED are connected in series via an ICL with a structure of ZnMgO/Al/HATCN. N-type ZnMgO nanoparticles are adopted as the electron injection and transport materials because of their high electron mobility as well as matched conduction band level with that of QD, while p-type HATCN is used as the hole injection material because of its deep lowest unoccupied molecular orbital (LUMO) level, which can effectively inject the holes to adjacent hole transport layer. To help electron transport more smoothly, an ultrathin Al layer (~1 nm) is further inserted between the n-type ZnMgO and the p-type HATCH. With such configuration, a good PN junction is obtained which can effectively generate charge carriers under reverse bias. 3 / 17



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Figure 1. (a) Schematic device structure of QLED EL unit, OLED EL unit and hybrid tandem LED. (b) Energy band diagram of the hybrid tandem LED. A new ICL based on ZnMgO/Al/HATCN is used to connect the green QLED and the green phosphorescent OLED. (The energy level of ZnMgO was measured by ultraviolet photoelectron spectroscopy and is shown in supporting information Figure S1. The Femi level and valence band maximum are estimated from the secondary electron cut-off region and the valence band edge region, respectively. )

Figure 2 (a) shows the charge generation process of the ICL under reverse bias. 4 / 17


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Because the LUMO level of HATCN is perfectly matched with the highest occupied molecular orbital (HOMO) level of NPB, electrons can transport from the HOMO level of NPB to the LUMO level of HATCN effectively, and as a result, holes and electrons are generated in NPB and HATCN, respectively. The generated charge carriers are then drifted to the electrodes by the reverse electric field. When the PN junction is forward biased, as shown in Figure 2 (b), holes and electrons are injected from the electrodes and subsequently recombined in the ICL. To investigate the charge generation capability of the proposed ICL, we fabricated the ICL-only device with a structure of ITO/ZnMgO (40 nm)/Al (1 nm)/HATCN (10 nm)/NPB (50 nm)/Al (100 nm). As shown in Figure 2 (c), the current of the ICL-only device increases rapidly as the driving voltage increases. The forward current is contributed by the charge carriers that are generated by the ICL, while the reverse current is originated from the charge carriers that are injected from the electrodes, as schematically depicted in inset of Figure 2 (c). It is obvious that the forward current is substantially larger (nearly four orders of magnitude) than the reverse current, which indicates that the proposed ICL can efficiently generate the charge carriers and the charge generation process is more efficient than the charge injection process. This is reasonable since charge carriers can be easily generated at the interface of HATCN/NPB and subsequently transported to the electrodes, while charge injection from electrodes is relatively difficult because of the large injection barrier. The proposed ICL also exhibits high transparency of ~90% at a wavelength of 500–800 nm, as shown in Figure 2 (d), which is beneficial for light out-coupling.

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Figure 2. (a) PN junction under reverse bias, charge carriers are generated by the ICL. (b) PN junction under forward bias, charge carriers are injected and subsequently recombined in the ICL. (c) Current-voltage characteristics of the ICL-only device. Inset: Equivalent circuit of the ICL-Only devices under forward and reverse bias. (d) Transmittance of the ICL and reflectance of the bottom QLED unit.

Both efficient charge generation capability and high transparency of the proposed ICL encourage us to explore its application in hybrid tandem LEDs. Bottom QLED EL unit with structure glass/ITO/PEDOT:PSS (40 nm)/PVK (30 nm)/G-QD (25 nm)/ZnMgO (40 nm)/Al (100 nm) was fabricated by solution processing, while top OLED EL unit with structure glass/ITO/HATCN (10 nm)/NPB (50 nm)/CBP:10% 6 / 17


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Ir(ppy)2(acac) (25 nm)/TPBi (50 nm)/LiF (1 nm)/Al(100 nm) was fabricated by vacuum evaporation. Hybrid tandem LED was realized by stacking the OLED unit on top of QLED unit; the configuration is glass/ITO/PEDOT:PSS (40 nm)/PVK (30 nm)/QD (25 nm)/ZnMgO (40 nm)/Al (1 nm)/HATCN (10 nm)/NPB (50 nm)/CBP:10% Ir(ppy)2(acac) (25 nm)/TPBi (50 nm)/LiF (1 nm)/Al (100 nm). Figure 3 (a) and (b) compare the current density-voltage (J-V) and luminance-voltage (L-V) characteristics of the single EL unit with those of the tandem LED, respectively. At a certain current density, the hybrid tandem LED exhibits substantially larger driving voltage than that of the single EL unit. This is reasonable since the tandem LED is comprised by two EL units that are connected in series, and thus the driving voltage of tandem LED theoretically equals to the sum of that of individual EL unit. At a certain current, the luminance of tandem LED also equals to the sum of that of single EL unit, because the same current can pass all EL units and generate multiple photons. For example, at a driving current of 10 mA/cm2, which corresponds to a luminance (voltage) of 2923 cd/m2 (7 V) and 6654 cd/m2 (6.6 V) for the QLED and the OLED unit, respectively, the luminance (voltage) of the tandem LED is 9648 cd/m2 (13.6 V), which is exactly the sum of that of individual EL unit. The results indicate that the proposed ICL can efficiently generate charge carriers for the EL units, and because of efficient injection and transportation of the generated carriers, the voltage losses at the ICL is negligible. Figure 3 (c) and (d) shows the CE-J and EQE-J characteristics of the devices. The tandem LED exhibits a very high CE (EQE) of 96.28 cd/A (25.9%), which is significantly higher than 25.55 cd/A (7.81%) of the QLED and 69.31 cd/A (19.04%) of the OLED. Also, the efficiency roll-off at high brightness, a typical issue in phosphorescent OLED 31-32, is remarkably alleviated. For example, at a high brightness of 10000 cd/m2, the efficiency of tandem LED still maintains 97.48% of its peak value and only slightly rolls-off from 96.28 cd/A (25.9%) to 93.85 cd/A (25.24%), while for the OLED, the efficiency rapidly decreases from 69.31 cd/A (19.04%) to 57.59 cd/A (15.79%). The reduced roll-off can mainly be attributed to the high stability of the QLED, which can maintain high 7 / 17



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efficiency at high brightness, as shown in Figure 3 (c). Besides, the introduction of tandem structure does not increase the power consumption of the device because of efficient injection and transportation of the generated carriers. Figure S2 (Supporting Information) shows the power efficiency (PE)-J characteristics of each EL unit. The tandem LED exhibits the same PE value with that of OLED at a high current density. The key performance data of the devices are summarized in Table 1.

Figure 3. Performance of green QLED EL unit, OLED EL unit, and hybrid tandem LED. (a) J-V characteristics, (b) L-V characteristics, (c) CE– J characteristics and (d) EQE– J characteristics.

Table 1．The device performance of green QLED EL unit, OLED EL unit and tandem LED. Device

V (V)

CE [cd A-1]

PE [lm W-1]

102/103/104 cd/m2

102/103/104 cd/m2

Max

102/103/104 cd/m2

Max

102/103/104 cd/m2

Max

EQE (%)

Von

QLED EL unit

4.3

5.9/6.8/7.9

8.58/16.86/23.37

25.55

2.62/5.16/7.15

7.81

4.48/7.79/9.17

7.86

OLED EL unit

2.9

3.9/5.0/7.2

64.15/67.72/57.49

69.31

17.62/18.60/15.79

19.04

47.96/42.53/25.07

64.01

Tandem LED

6.8

9.1/11.0/13.8

87.86/92.69/93.85

96.28

23.63/24.92/25.24

25.90

27.05/26.46/21.35

24.78

Abbreviation: Turn on Voltage (Von). The relevant statistical data of the device performance are shown in Supporting Information Figure S3. 8 / 17


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Figure 4. (a) Normalized EL spectra of the tandem and the individual EL unit, (b) EL spectra of the devices and the predicted EL spectra of the tandem LED, (c) Normalized EL spectra of the tandem device at different viewing angles, (d) Normalized EL spectra of the tandem device at different driving voltage.

The demonstrated hybrid tandem LED not only shows high efficiency, but also exhibits saturated color emission. The high efficiency is mainly contributed by the OLED unit, while the enhanced color saturation is originated from the QLED unit. Figure 4 (a) shows the normalized EL spectra of the devices. All devices show similar EL peak at ~525 nm. The QLED exhibits very pure color with a FWHM of only 24 nm, which is significantly smaller than 68 nm of the OLED. To improve the color saturation of OLED without losing the efficiency, the QLED is introduced and stacked with the OLED. In such hybrid tandem structure, the LED exhibits a relatively narrow FWHM of 44 nm, which is remarkably smaller than that of OLED and represents a 35% improvement. The spectrum of tandem LED can be predicted by simply summing the 9 / 17



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spectra of both EL units. Figure 4 (b) shows the predicted spectrum and the spectra of the EL units. The emission intensity is scaled so that it is proportional to the efficiency of the devices. At a driving current of 100 mA/cm2, the OLED and QLED contribute ~62% and ~38% photons to the total emission, respectively. Due to the contribution of the QD emission, the predicted spectrum is remarkably narrowed than that of the OLED, as shown in Figure S4. The predicted spectrum is slightly wider than the measured one, indicating that there are other mechanisms that modify the spectrum of the tandem LED. To find out the reasons, we examined the angular emission spectra of the tandem LED. As shown in Figure 4 (c), when the viewing angle is increased, the spectra are blue shifted, which is a typical phenomenon in microcavity LED. According

to

Figure

2

(d),

the

glass/ITO/PEDOT:PSS/PVK/QD/ZnMgO/Al/HATCN/NPB

film

exhibits

a

stack moderate

reflectance of 20-30% at wavelength of 400-500 nm. Such reflectance can induce microcavity effect for the top OLED unit and thus can narrow the emission spectrum of the tandem LED. Figure 4 (d) shows the EL spectra of tandem LED at different driving voltage. The FWHM of the spectra is gradually reduced as driving voltage increases. This is because at high voltage, the efficiency of OLED drops-off rapidly while that of QLED can still maintain its value. As the contribution of QLED emission increases, the color saturation of the tandem LED is gradually enhanced.

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NTSC OLED, 81%NTSC QLED, 117%NTSC Tandem, 100%NTSC

Figure 5. Color gamut of NTSC standard, and displays based on OLED, QLED and tandem LED in a CIE 1931 chromaticity diagram.

By introducing the green QLED unit, the tandem LED exhibits substantially improved color saturation, which can effectively improve the color gamut of the displays. Figure 5 shows the color gamut of displays based on OLED, QLED and tandem LED. The CIE coordinates of blue and red are fixed and adopted the NTSC standards. Only the green color is optimized since human eyes are most sensitive to the green, and thus a little improvement of green saturation can result in a significant enhancement of color gamut. The QLED display exhibits very high color gamut of 117% NTSC, however, its efficiency is still lower than that of state-of-the-art OLED. The CIE coordinates of the tandem LED is (0.209, 0.704), which is very close to (0.21, 0.71) of NTSC standards. By using the proposed tandem LED, the color gamut of the displays is 100% NTSC, which is remarkably higher than 81% NTSC of the OLED displays and represent a 22.2% improvement. This results demonstrate that the combination of QLED and OLED in a hybrid tandem structure not only improves the efficiency, but also increases the color gamut of displays effectively.

3. Conclusions In summary, hybrid tandem LEDs have been demonstrated by connecting the 11 / 17



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QLED and OLED in series using a new ICL of ZnMgO/Al/HATCN. The demonstrated hybrid tandem LEDs feature high efficiency and high color saturation simultaneously, which exhibit a maximum CE and EQE of 96.28 cd/A and 25.90%, respectively, and a relatively narrow FWHM of 44 nm. Also, efficiency roll-off at high brightness has been greatly alleviated. The high efficiency is mainly contributed by the OLED unit, while the enhanced color saturation is originated from the QLED unit. With the proposed hybrid tandem LED, the color gamut of the displays can be effectively increased from 81% to 100% NTSC. Our study indicates that the advantages of different LED technologies can be combined in a hybrid tandem structure while the weaknesses can be compensated by the strengths of each subunit.

4. Experimental Section OLED, QLED and hybrid LED with structures of glass/ITO/PEDOT:PSS (40 nm)/PVK (30 nm)/G-QD (25 nm)/ZnMgO (40 nm)/Al (100 nm), glass/ITO/HATCN (10 nm)/NPB (50 nm)/CBP:10% Ir(ppy)2(acac) (25 nm)/TPBi (50 nm)/LiF (1 nm)/Al (100 nm) and glass/ITO/PEDOT:PSS (40 nm)/PVK (30 nm)/QD (25 nm)/ZnMgO (40 nm)/Al (1 nm)/HATCN (10 nm)/NPB (50 nm)/CBP:10% Ir(ppy)2(acac) (25 nm)/TPBi (50 nm)/LiF (1 nm)/Al (100 nm), respectively, were fabricated (abbreviation: PEDOT:PSS:

poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate),

poly(9-vinlycarbazole),

HATCN:

1,

11-hexaazatriphenylene-hexacarbonitrile,

4, NPB:

5,

8,

PVK: 9,

N,N′-biphenyl-N,

N′-bis(1-naphthyl)-benzidine, CBP: 4,4'- bis(9-carbazolyl)biphenyl, Ir(ppy)2(acac): bis(2-phenylpyridine)iridium(III)

acetylacetonate,

TPBi:

1,

3,

5-tris

(N-phenylbenzimiazole-2-yl) benzene). The device structures are schematically shown in Figure 1 (a). To fabricated the devices, the glass substrates coated with ITO (Rs≈13 Ω/□) were first cleaned by soaking in an ultrasonic detergent for 30 min, followed by spraying in deionized water for 30 min. Then, the cleaned ITO was treated by UV-ozone for 10 min. After UV-ozone treatment, the PEDOT:PSS (Clevios P VP AI 4083) hole 12 / 17


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injection layer was casted at 3000 rpm onto the ITO substrate, followed by annealing at 150 °C for 20 min in air. Then the samples were transferred to a nitrogen-filled glove box for depositing other functional layers. The PVK (10 mg/ml in chlorobenzene) hole transport layer was deposited (4000 rpm, 45 s) and subsequently annealed (110 °C, 10 min) in the glove box. The commercial obtained G-QD (CdZnSeS/ZnS/oleic acid) light emitting layer was spin-coated from a 5 mg/ml hexane solution at 2000 rpm for 45 s, followed by baking at 100°C for 5 min. After that, ZnMgO nanoparticles dissolved in butanol solvent with concentration of 20 mg/mL were spin-casted at 2000 rpm, followed by baking at 80 °C for 15 min. And then the samples were transferred to a custom high-vacuum evaporation chamber to deposit the top OLED unit. The Al (1 nm), HATCN hole injection layer, NPB hole transport layer, CBP:10% Ir(ppy)2(acac) light-emitting layer, TPBi electron transport layer, LiF/Al cathode were deposited in sequence at a base pressure of 5 ×10−4 Pa. The thicknesses of the vacuum deposited layers were monitored by quartz crystal microbalance and were calibrated by Dektak XT profilometer. Transmittance and reflectance of the ICL were measured using a 150 mm integrating sphere in a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent Technologies). The EL spectra of devices were measured by fiber optic spectrometer (Ocean Optics USB2000). The EQE was measured by using a method recommended by SR Forrest 33-34. The set up of the system and measurement steps can be referred to Ref. 35. The current density-voltage-luminance (J-V-L) characteristics of the devices were measured by a dual-channel Keithley 2014B source measure unit and a PIN-25D silicon photodiode. The electroluminescent (EL) spectra of the devices were taken in normal direction by a spectrometer (Ocean Optics USB 2000). The effective device area was 2×2 mm2. All measurements were conducted at room temperature in air without encapsulation.

Supporting Information Supporting Information is available from the internet or from the author. The PE-J characteristics of green QLED EL unit, OLED EL unit, and tandem EL unit; 13 / 17



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The relevant statistical data of the devices performances. This information is available free of charge via the Internet at ?

ACKNOWLEDGMENTS This work was supported by the Guangdong Natural Science Funds for Distinguished Young Scholars (2016A030306017), the National Natural Science Foundation of China (61405089), the Guangdong Special Support Program for Young Talent Scholars (2014TQ01X015) and the National Key Research and Development Program of China (2016YFB0401702).

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ToC figure

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Improved Efficiency and Enhanced Color Quality of Light-Emitting

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