Blue Tandem Quantum-Dot Light-Emitting Diodes

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Efficient Red/Green/Blue Tandem Quantum-Dot Light-Emitting Diodes with External Quantum Efficiency Exceeding 21% Heng Zhang, Shuming Chen, and Xiao Wei Sun ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07867 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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Efficient Red/Green/Blue Tandem Quantum-Dot Light-Emitting Diodes with External Quantum Efficiency Exceeding 21% Heng Zhang, Shuming Chen*, Xiao Wei Sun Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, 518055, P. R. China [email protected]

Abstract Highly efficient tandem quantum-dot light-emitting diodes (QLEDs) are developed by using an inter-connecting layer (ICL) with the structure of ZnMgO/Al/HATCN/MoO3. The developed ICL exhibit high transparency, efficient charge generation/injection capability as well as high robustness to resist the solvent damage during deposition of the upper functional layers. With the proposed ICL, full color (red/green/blue, R/G/B) tandem QLEDs are demonstrated with extremely high current efficiency (CE) and external quantum efficiency (EQE): 17.9 cd/A and 21.4% for B-QLEDs, 121.5 cd/A and 27.6% for G-QLEDs, 41.5 cd/A and 23.1% for R-QLEDs. To the best of our knowledge, these are the highest values ever reported. In addition, the EQE of R-, Gand B-QLEDs are all exceeds 21%. The high efficiency can be well maintained over a wide range of luminance from 102 to 104 cd/m2. For example, even at a high brightness of 20000 cd/m2, the EQE of R-, G- and B-QLEDs can still sustain its 96%, 99% and 78% maximum value, respectively. The demonstrated full-color tandem QLEDs, with extremely high efficiency, long operational lifetime, low efficiency roll-off and high color purity, would be ideal candidates to bring QLEDs into the next generation full-color displays and the solid-state lighting market. Keywords: quantum-dot; light-emitting diodes; tandem structure; inter-connecting layer; high efficiency

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The optical and chemical properties of colloidal quantum dots (QDs), which include tunable emission color, narrow spectra linewidth, high material stability and simple solution processability, make them very attractive for use in light-emitting diodes (LEDs) for next-generation displays and lighting application.1-7 Since the first introduction of quantum-dot LEDs (QLEDs), great progresses have been made over the past few years due to the rapid development of QD materials and device architectures.1,2 However, even though the materials and the devices have been improved greatly, the performances such as the efficiency and the lifetime of QLEDs are still lower than those of their competitors: organic LEDs (OLEDs). For example, external quantum efficiencies (EQE) > 20% have been routinely reported in OLEDs area,8-10 whereas it is still rarely reported in QLEDs field. Particularly, the efficiency and the lifetime of blue QLEDs are significantly lower than those of their OLEDs counterpart, which are a big concern for their future commercialization.6,11,12 One of the promising ways to simultaneously achieve high current efficiency (CE) and long operational lifetime is to employ the tandem structures. In tandem devices, two or more electroluminescent (EL) units are serially connected via a transparent inter-connecting layer (ICL). Since the same current passes two or more EL units, the CE/EQE of tandem LEDs are two or more times higher than those of the single devices.13,14 The higher CE means that at a certain luminance, the tandem devices can be operated at a much lower current density, which is beneficial for their operational lifetime. Therefore, tandem structures have been widely adopted in various optoelectronic devices like OLEDs or solar cells.13-17 However, there are few reports on tandem QLEDs due to the challenges of developing: (1) efficient and robust ICL and (2) multiple solution-deposition processes.2,18,19 We recently reported the inverted tandem QLEDs with a CE of 71 cd/A and a EQE of 16.76% by using PEDOT:PSS/ZnMgO

as

the

ICL.2

Though

the

solution-processed

PEDOT:PSS/ZnMgO is a good ICL for the tandem devices, the inverted structure is somehow problematic. For one thing, in inverted structures, the QD emitting layer is easily damaged by the solvent during deposition of the upper functional layers, 2 / 21


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thereby resulting in a relatively low CE (EQE) of 71 cd/A (16.76%).2 For another, for display

application,

the

inverted

devices

can

only

work

with

n-type

thin-film-transistors (TFTs), which are unstable especial the metal-oxide TFTs.20,21 Therefore, to further improve the performance and to compatible with p-type TFTs, it is necessary to develop tandem QLEDs with conventional non-inverted structures. QLEDs with conventional structures can be fabricated by all-solution-process without sacrificing efficiency, and thus have been widely reported.1,5 To serially connect two QLEDs, an ICL that can efficiently generate/inject charge carriers is one of the key challenges. However, there are no reported ICLs that can be used in non-inverted tandem QLEDs. Previously, we demonstrated that PEDOT:PSS/ZnMgO is an ideal ICL for inverted devices,2 and thus, one may wonder whether the reversed film sequence ZnMgO/PEDOT:PSS is a good ICL for non-inverted tandem QLEDs. To answer this question, very recently, we demonstrated white tandem QLEDs by using ZnMgO/PEDOT:PSS as ICL, but the device efficiency is lower than the theoretical value, indicating that ZnMgO/PEDOT:PSS is a problematic ICL for the non-inverted QLEDs.19 This is because it is difficult to deposit the aqueous PEDOT:PSS on the ZnMgO, which is hydrophobic. As shown in Figure S1 (a)-(c), the nonuniform distribution of the PEDOT:PSS can be observed, which is mainly due to the relatively large contact angle (57.5°) of the PEDOT:PSS on ZnMgO. Though the contact angle can be further reduced by adding IPA into PEDOT:PSS, the deposited film still exhibit very poor uniformity and quality, as shown in Figure S1 (g)-(l). On the contrary, one can easily deposit the ZnMgO on top of PEDOT:PSS, as shown in Figure S1 (d)-(f), and thus PEDOT:PSS/ZnMgO is a good ICL for inverted devices whereas the reverse ZnMgO/PEDOT:PSS cannot work as ICL for non-inverted QLEDs.

In this work, we specifically develop an efficient ICL that can be used for non-inverted tandem QLEDs. From aforementioned discussion, the commonly used PEDOT:PSS cannot serve as p-type material for the ICL. Therefore, herein we adopt 3 / 21


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the evaporated small molecule HATCN and metal oxide MoO3 as the substitute to the PEDOT:PSS. Moreover, an ultra-thin Al layer is inserted between the n-type and the p-type materials to further enhance the electrical performance of the ICL. The resultant ICL with structure of ZnMgO/Al/HATCN/MoO3 exhibit high transparency, efficient charge generation/injection capability as well as high robustness to resist the solvent damage during deposition of the upper functional layers. With the proposed ICL, full color (red/green/blue, R/G/B) tandem QLEDs are demonstrated with extremely high CE (EQE): 17.9 cd/A (21.4%) for B-QLEDs, 121.5 cd/A (27.6%) for G-QLEDs, and 41.5 cd/A (23.1%) for R-QLEDs. To the best of our knowledge, these are the highest values ever reported. In addition, this is also the first report that the EQE of R-, G- and B-QLEDs all exceeds 21%. The demonstrated full-color tandem QLEDs, with extremely high efficiency, low efficiency roll-off and high color purity, would be ideal candidates to bring QLEDs into the next generation full-color displays and the solid-state lighting market.

Results and discussion Figure 1 (a) shows the device structures of the single and the tandem QLEDs. For tandem devices, two QLEDs are serially connected via the transparent ICL with the structure of ZnMgO/Al/HATCN/MoO3. All layers except the Al, the HATCN and the MoO3 were fabricated by spin-casting. To protect the HATCN from dissolving by the solvents of the upper functional layers, a 12 nm MoO3 was deposited on top of HATCN. To investigate whether all functional layers still survive after multiple solution-processing, the cross-sectional transmission electron microscopic (TEM) images of the tandem QLEDs were taken. As shown in Figure 1(b), all functional layers including the ultra-thin (~3 nm) Al can be clearly observed, which confirms the existence of all layers even after treatment by various solvents for many times. Figure 1(c) shows the energy band diagrams of the tandem QLEDs. Because of the well-aligned energy levels, the proposed ICL is expected to efficiently generate and inject carriers to the bottom and the top QLEDs under forward bias. 4 / 21


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Figure 1. (a) Schematic device structure of bottom single QLEDs, top single QLEDs and tandem QLEDs. (b) Cross-sectional TEM image of the tandem QLEDs. (c) Energy band diagram of the tandem QLEDs.2,4,22,23 An ICL based on ZnMgO/Al/HATCN/MoO3 is used to connect the two individual QLEDs.

The performance of the tandem devices is largely affected by the electrical and the optical properties of the ICL. We propose four ICL with structures (i) ZnMgO/MoO3, (ii)

ZnMgO/HATCN/MoO3,

(iii)

ZnMgO/Al/MoO3

and

(iv)

ZnMgO/Al/HATCH/MoO3, where ZnMgO nanoparticles were chosen as etectron transport material due to its high electorn mobility and matched conduction band level with that of QDs,2 and MoO3 was chosen as hole injection material because of its high work function.7 To evaluate the charge generation capability of the ICL, ICL-only devices

with

structures

(i)

ITO/ZnMgO/MoO3/PVK/Al,

(ii)

ITO/ZnMgO/HATCN/MoO3/PVK/Al, (iii) ITO/ZnMgO/Al/MoO3/PVK/Al and (iv) ITO/ZnMgO/Al/HATCN/MoO3/PVK/Al were fabricated, where ITO, Al work as anode and cathode, respectively. Figure 2 (a) displays the current density-voltage (J-V) characteristics of the ICL-only devices. Under forward bias, charge carriers are generated from the ICL and ultimately are extracted by the electrodes, whereas under reverse bias, charge carriers are injected from the electrodes and finally are quenched by the ICL. Under forward bias, charge carriers cannot be efficiently injected from the 5 / 21


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electrodes due to the high injection barrier, thus the forward current solely reflects the charge generation capability of the ICL. It is obvious that device (iv) exhibits the largest forward current, indicating that with ZnMgO/Al/HATCN/MoO3, the charge generation and transportation is the most efficient. The ultrathin Al and the HATCN play an important role in enhancing the charge generation and transportation. As shown in Figure 1 (c), the charge carriers are generated at the interface of MoO3/PVK, where electrons from the HOMO of PVK can efficiently transfer to the conduction band of MoO3 due to their aligned energy level. Under forward bias, the electrons can further transport to the LUMO of HATCN and Al, and finally inject from Al to ZnMgO. The ultrathin Al and the HATCN work as energy ladders so that the generated electrons can stepwisely transport from the MoO3 to the ZnMgO. In addition, the holes can be also efficiently injected from MoO3 to PVK by continuously extracting the electrons from the HOMO of PVK and subsequently transporting the electrons to the ZnMgO. To evaluate whether the ICL can work normally, tandem QLEDs were fabricated and their J-V-L characteristics are displayed in Figure 2 (b) and Figure S2. As expected, devices with ZnMgO/Al/HATCN/MoO3 ICL exhibit the largest forward current, the lowest turn on voltage and the highest efficiency. Without Al or HATCN, the injection of the charge carriers is difficult due to the high injection barrier, and thus leads to a high turn on voltage and a low efficiency. For example, for devices with ZnMgO/MoO3 ICL, the turn on voltage for 1 cd/m2 is 21 V, whereas by inserting ultrathin Al between ZnMgO and MoO3, the turn on voltage is reduced to 13 V and by further using HTACN, the turn on voltage is effectively decreased to 6 V. Through comparison, we identify that ZnMgO/Al/HATCN/MoO3 is the most efficient ICL for the tandem QLEDs. The thickness of the ultrathin Al can largely affect the electrical and the optical properties of the ICL. The J-V characteristics and the transmittance spectra of the ICL-only devices with Al thickness varied from 1 to 5 nm are shown in Figure 2 (c) and (d), respectively. With only 1 nm Al, the current density is greatly improved due 6 / 21


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to the enhanced electron injection by using the ultrathin Al as the energy ladder. The current density is further increased by increasing the thickness of Al. This is because the resistivity of the ultrathin Al is influenced by its thickness.24 It has been found that the ultrathin metal film consist of discontinuous islands. By increasing the thickness, the islands are coalesced leading to the formation of continuous film and thus results in a reduced resistance.24,25 However, the transmittance of the ICL is significantly decreased as the increase of the Al thickness. The transmittance of the ICL also has great impact on the performance of the tandem devices. Therefore, to make a good balance, the optimal thickness of the Al is chosen as 3 nm, which can guarantee a moderate charge generation/injection as well as a high transmittance of ~80%. To examine whether the ultrathin Al or the small molecular HATCN layer is damaged by the solvent (chlorobenzene, CB) of the PVK, the transmittance and the absorbance spectra of the ICL treated by CB were measured. As shown in Figure 2 (e), after CB rinsing, the intensities of the transmittance and the absorbance of the ICL are identical with those of the ICL before rinsing, indicating that the ultrathin Al and the HATCN remain intact. Since the evaporated HATCN can be easily damaged by the solvents, the intact HATCN shown here thereby indicates that the MoO3 can effectively protect the HATCN from damaging by the solvents. For tandem QLEDs, one of the challenges is that the bottom QD layer is easily damaged during deposition of the upper layers, because it is subjected by various solvent treatments for many times.2,18,19 Fortunately, our robust ICL can effectively protect the bottom QD layer from damaging by the solvents. As shown in Figure 2 (f), before and after CB rinsing, the photoluminescence (PL) intensities of the QD layer coated with ICL are nearly identical, indicating that the ICL can effectively resist the penetrating of the solvents and thus protect the QD layer from damaging by the solvents.

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ZnMgO/MoO3

10 Current density (mA/cm2)

ZnMgO/HATCN/MoO3 ZnMgO/Al/MoO3

101

ZnMgO/Al/HATCN/MoO3

100 10-1 10

-2

10

-3

10

(c)

4

103

ZnMgO/MoO3

3

ZnMgO/HATCN/MoO3 ZnMgO/Al/MoO3

2

ZnMgO/AL/HATCN/MoO3

101 10

0

10-1 10

10-4 10-5 -18 -15 -12 -9 -6 -3

(d)

0

3

6

9

10-3

12 15 18

100

80

80

60

40 Al 1nm Al 3nm Al 5nm

450

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650

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8

10

700

750

800

12

14

16

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20

22

60 Before CB Rinse After CB Rinse

0.2

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Wavelength (nm)

400

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650

100 10-1 10

-2

10

-3

10-4 -8

-4

(f)

0

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Voltage (V)

1.2

0.4

0.3

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10-5 -20 -16 -12

24

Voltage (V)

100

0 400

4

(e)

Voltage (V)

20

-2

Al 1nm Al 3nm Al 5nm

102

Absorbance (a.u.) Normalized PL Intensity (a.u.)

102

Transmittance (%)

Current density (mA/cm2)

103


(b) 10

(a)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 700

Before CB Rinse After CB Rinse

1.0 0.8 0.6 0.4 0.2 0.0 500

520

540

560

580

Wavelength (nm)

Wavelength (nm)

Figure 2. (a) J–V characteristics of the ICL-only devices with different ICL, (b) J–V characteristics

of

the

tandem

devices

with

different

ICL:

ZnMgO/MoO3,

ZnMgO/HATCN/MoO3, ZnMgO/Al/MoO3 and ZnMgO/Al/HATCN/MoO3, (c) J-V characteristics of the ICL-only devices, ICL: ZnMgO/Al/HATCN/MoO3 with Al thickness varied from 1 to 5 nm, (d) Transmittance spectra of the ICL: ZnMgO/Al/HATCN/MoO3 with Al thickness varied from 1 to 5 nm, (e) Transmittance and absorbance spectra of the ICL (ZnMgO/Al/HATCN/MoO3) before and after CB rinsing, (f) PL spectra of the G-QDs covered with ICL (ZnMgO/Al/HATCN/MoO3) before and after CB rinsing.

The excellent electrical and optical properties of the ZnMgO/Al/HATCN/MoO3 ICL prompt us to further explore its application in tandem QLEDs. Full color tandem QLEDs were fabricated with structure glass /ITO /PEDOT:PSS (45 nm) /PVK (30 nm) /QDs (R/G/B) /ZnMgO (40 nm) /Al (3 nm) /HATCN (15 nm) /MoO3 (12 nm) /PVK (30 nm) /QDs (R/G/B) /ZnMgO (40 nm) /Al (100 nm). For comparison, the bottom single QLED with structure glass /ITO /PEDOT:PSS (45 nm) /PVK (30 nm) /QDs (R/G/B) / ZnMgO (40 nm) /Al (100 nm) and the top single QLED with structure glass/ITO/HATCN (15 nm) /MoO3 (12 nm) /PVK (30 nm) /QDs (R/G/B) /ZnMgO (40 nm) /Al (100 nm) were also fabricated. The J-V, L-V and CE-J-EQE characteristics of 8 / 21


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the devices are shown in Figure 3, respectively. The J-V characteristics of the devices with the current density plotted in log scale are also shown in Figure S3. The top QLEDs exhibit relatively poor performances such as higher driving voltage and lower efficiency than those of the bottom QLEDs. For example, the top G-QLEDs exhibit a turn on voltage of 4 V and a maximum CE of 53.7 cd/A, which is relatively poorer than 3.4 V and 63.3 cd/A of the bottom G-QLEDs. This is because the top QLEDs use HATCN/MoO3 as the hole injection layer (HIL), resulting in a relatively inefficient hole injection as compared with the bottom QLEDs, which adopt the PEDOT:PSS as the HIL. Though both PEDOT:PSS and MoO3 have similar work function, the conductivity of PEDOT:PSS is higher than that of MoO3, and thus the hole injection of the bottom devices is more efficient than that of the top devices. For the tandem QLEDs, the efficiencies (and the driving voltage) are nearly equal to the sum of those of the bottom and the top-QLEDs, indicating that the ICL can effectively connect the bottom and the top QLEDs. More specifically, the ICL can efficiently generate charge carriers and inject them to the bottom and the top-QLEDs. Thanks to the efficient connection, very impressive CE and EQE are achieved with 17.9 cd/A and 21.4% for B-QLEDs, 121.5 cd/A and 27.6% for G-QLEDs, 41.5 cd/A and 23.1% for R-QLEDs, which, to the best of our knowledge, are the highest values ever reported. Especially, the EQE of B-QLEDs is significantly higher than those reported in literatures,12,25 which exceeds 20%. The power efficiency (PE)–J characteristics of all devices are depicted in Figure S4, and the detailed device performances are summarized in Table 1. As listed in Table 1, the efficiencies of the tandem devices are even higher than the sum of those of the bottom and the top QLEDs, indicating that the charge injection from the ICL is even more efficient than that from the electrodes. The higher EQE indicates that the devices may have a longer operational lifetime, since at a certain brightness, the tandem devices can be operated at a much smaller current density. We have measured the lifetime of the fabricated QLEDs. The devices were simply encapsulated by a glass cover and a UV epoxy in a N2 glove box. As shown in Figure S5, the lifetime of the tandem devices is significantly longer than that of the single 9 / 21


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devices. For example, when driven at a constant current of 0.3 mA under ambient conditions, the half-lifetime, T50 of the green bottom QLEDs and the green top QLEDs are 50.3 h and 28.5 h at an initial luminance (L0) of 2200 cd/m2 and 1916 cd/m2, respectively. However, the T50 of green tandem QLEDs is longer than 220 h at a higher initial luminance of 3911 cd/m2. Moreover, after the initial fast decay, the tandem devices exhibit positive aging effect, i.e., the luminance is increased over time. This is because the efficiency of QLEDs encapsulated with a UV-curable acidic resin will increase during the storage time.26 The detailed mechanisms are still under investigated. By using the relation L0nT = constant and assuming an acceleration factor of n=1.5,1,4 the T50 of the tandem QLEDs at 100 cd/m2 is predicted to be over 53808 h, which is significantly longer than 5190 h (T50) and 2390 h (T50) for the bottom QLEDs and the top QLEDs, respectively.

B-QLED

20

103 102 101

Voltage (V)

G-QLED

20

(g) 100

4

6

2

104 103 102

Bottom Device Top Device Tandem Device

101

Luminance (cd/m2)

R-QLED

20

-8

-4

0

4

8

12

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40

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80

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6

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18

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(i) 45

103 102

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120

(h)

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8

8 10 12 14 16 18 20 22 24 26

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-5 -10 -15 100


Figure 3. J–V characteristics, L–V characteristics and CE-J-EQE characteristics of the: (a) (b) (c) blue QLEDs, (d) (e) (f) Green QLEDs, (g) (h) (i) red QLEDs.

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ACS Nano

Table 1. The device performances of R-, G- and B-QLEDs. EQE (%)

CE (cd/A) Color

Device

Von (V)

250/2500/ 25000 2

(cd/m )

Blue

Green

Red

PE (lm/W)

Max

250/2500/ 25000 2

(cd/m )

Max

250/2500/ 25000 (cd/m2)

Max

Bottom

4.4

4.9/7.1/6.8

7.7

6.1/8.8/8.5

9.6

2.6/3.3/2.3

3.3

Top

4.7

5.9/7.6/6.0

7.6

7.3/9.4/7.4

9.4

2.5/2.3/1.1

2.6

Tandem

8.8

17.6/17.5/12.0

17.9

21.1/21.0/14.3

21.4

3.9/3.0/1.4

4.2

Bottom

3.4

21.88/52.5/61.27

63.3

5.0/12.0/14.0

14.5

29.4/33.0/30.5

33.6

Top

4.0

30.0/46.4/53.3

53.7

6.7/10.7/12.3

12.4

14.2/17.0/13.7

17.1

Tandem

6.1

93.1/117.9/120.5

121.5

21.2/26.8/27.4

27.6

30.5/29.9/23.4

31.8

Bottom

2.4

8.7/17.9/23.0

23.5

5.5/11.2/14.5

14.8

8.1/12.3/10.8

12.4

Top

3.9

8.7/13.6/15.8

15.8

5.4/8.4/9.8

9.8

3.8/3.9/3.0

4.1

Tandem

5.6

37.9/41.2/39.6

41.5

21.1/23.0/22.0

23.1

9.5/7.4/5.0

11.1

To examine whether such high performances are repeatable, over 24 G-QLEDs from different batches were fabricated and their CE are depicted in Figure 4 (a). These devices exhibit a mean CE of 108 cd/A with a low standard deviation of 5.8, indicating the performances are highly repeatable. The corresponding histogram of the EQE of these devices is shown in Figure S6. Figure 4 (b) shows the normalized EL spectra of all devices. The spectra of the tandem devices are identical with those of the bottom and the top QLEDs. The devices exhibit a central wavelength of 474 nm, 534 nm and 622 nm, with a full width at half maximum (FWHM) of 28 nm, 29 nm and 27 nm for the B-, G- and R-QLEDs, respectively. Due to their narrow FWHM, the CIE coordinates are almost located at the edge of the color chart, indicating their emission colors are highly saturated, as shown in Figure 4 (c). The EL spectra of the tandem devices viewed at different angles were also investigated. As shown in Figure S7, the spectra are almost unchanged when viewed at different angles. This is because our ICL is highly transparent, which helps to suppress the microcavity effect. Figure 4 (d) displays the photos of the tandem B-, G- and R-QLEDs operated at a current density of ~100 mA/cm2. Very bright and uniform emission can be observed. The 11 / 21


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demonstrated tandem devices with high brightness over 26800, 115500 and 65900 cd/m2 for B-, G- and R-QLEDs, respectively, can also find a wide variety application in the lighting area.

Figure 4. (a) Histogram of maximum CE of 24 tandem G-QLEDs from different batches. (b) Normalized EL spectra of the R-, G- and B-QLEDs. (c) CIE coordinates of R-, G- and B-QLEDs. (d) The photos of the tandem R-, G- and B-QLEDs operated at a current density of 100 mA/cm2.

Figure 5 shows the evolution of the QLEDs efficiency since 1994.1,3-6,8,23,27-56 To the best of our knowledge, the EQE of our developed full color tandem QLEDs are the highest. Moreover, this is also the first report that the EQE of R-, G- and B-QLEDs all exceeds 21%. Furthermore, the efficiency of our tandem QLEDs only slightly rolls off, and can still maintain a very high value even operated at a high luminance. For example, even at a high brightness of 20000 cd/m2, the CE/EQE of R-, G- and B-QLEDs can still sustain its 96%, 99% and 78% maximum values, respectively. The demonstrated full color tandem QLED with high efficiency, high 12 / 21


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color saturation and low efficiency roll-off, would be a strong competitor for OLED for next generation display application.

30

25

Blue Tandem QLED in this work Green Tandem QLED in this work Red Tandem QLED in this work

20

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

10

Blue single QLED in literature Green single QLED in literature Red single QLED in literature Blue single QLED in this work Green single QLED in this work Red single QLED in this work

5

0 1992

1996

2000

2004

2008

2012

Work 2016 This 2020

Year

Figure 5. Evolution of the EQE of R-, G- and B-QLEDs. 1,3-6,8,23,27-56 Most data are based on Cd-based QLEDs. The EQE of our tandem R-, G- and B-QLEDs all exceeds 21%.

Conclusions In summary, highly efficient non-inverted tandem QLEDs have been developed by using an ICL based on ZnMgO/Al/HATCN/MoO3. The ultrathin Al and HATCN work as energy ladders for enhancing the charge transportation of the ICL. The ICL is also highly robust, which can effectively protect the bottom QD layer from damaging by the solvents. With the proposed ICL, the tandem QLEDs exhibit extremely high CE (EQE): 17.9 cd/A (21.4%) for B-QLEDs, 121.5 cd/A (27.6%) for G-QLEDs and 41.5 cd/A (23.1%) for R-QLEDs, which, to the best of our knowledge, are the highest value ever reported. This work demonstrates that it is a good strategy to boost the performances particularly the CE and EQE of QLEDs by using our developed ICL. The demonstrated full-color tandem QLEDs, with extremely high efficiency, long operational lifetime, low efficiency roll-off and high color purity, would be ideal 13 / 21


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candidates to bring QLEDs into the next generation full-color displays and the solid-state lighting market.

Experimental Section Device Structures: Non-inverted tandem QLEDs with structure of galss /ITO /PEDOT:PSS (45 nm) /PVK (30 nm) /QDs (R/G/B) /ZnMgO (40 nm) /Al (3 nm) /HATCN (15 nm) / MoO3 (12 nm) /PVK (30 nm) /QDs (R/G/B) /ZnMgO (40 nm) /Al (100 nm) were fabricated. For comparison, the bottom single QLED with structure glass /ITO /PEDOT:PSS (45 nm) /PVK (30 nm) /QDs (R/G/B) / ZnMgO (40 nm) /Al (100 nm) and the top single QLED with structure glass /ITO /HATCN (15 nm) /MoO3 (12 nm) /PVK (30 nm) /QDs (R/G/B) /ZnMgO (40 nm) /Al (100 nm) were also fabricated.

Abbreviations:

PEDOT:PSS

(Poly(3,

4-ethylenedioxythiophene)/polystyrene sulfonate), PVK (poly(9-vinlycarbazole)), HATCN (dipyrazino [2,3-f:2′,3′-h] quinoxaline-2,3,6,7,10,11-hexacarbonitrile).

Device Fabrication: The ITO substrates (sheet resistance = 25 Ω sq −1 ) were first cleaned in ultrasonic detergent for 30 min and sprayed by deionized water, followed by soaking in ultrasonic deionized water for 15 min and baking in oven for 30 min. Then they were cleaned with a UV-ozone cleaner for 30 min prior to spin-casting processes. The spin-casting processes were performed in air. After UV-ozone treatment, the PEDOT:PSS (Clevios AI 4083) hole injection layer were spin-coated at 2000 rpm and baked at 130°C for 15 min in air, sequentially. Then the PVK (10 mg/ml in chlorobenzene) hole transport layer was coated at 4000 rpm for 40 s and annealed at 110 °C for 10 min. For the different color (R/G/B) QLEDs fabrication, the 15 mg/ml R-QD (CdZnSe/ZnS/OT, core and shell ~12.2 nm , obtained from Mesolight Inc., QY~85%), 10 mg/ml G-QD (CdZnSeS/ZnS/oleic acid, core and shell ~11.6 nm, obtained from Mesolight Inc., QY~85%) and 10 mg/ml B-QDs (CdZnSeS/ZnS/OT, core and shell ~10.5 nm, obtained from Mesolight Inc., QY ~85%) dissoved in hexane solution were spin-coated at 3500 rpm, 3000 rpm and 1500 rpm for 40 s, respectively, followed by baking at 100 °C for 5 min. The commercially 14 / 21


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obtained ZnMgO (purchased from Guangdong Poly Optoelectronics Co., Ltd.) nanoparticles were spin coated on QDs layer at 2500 rpm from a 30 mg/ml ethanol solution and baked at 90 °C for 10 min. The TEM image of the ZnMgO nanoparticles are shown in our previous work (reference 57) and the dc conductivity of the ZnMgO is ~ 10-7 S/cm.58 After that, the samples were transferred to a high-vacuum evaporation chamber to sequentially deposit Al (3 nm), HATCN (15 nm) and MoO3 (12 nm) at a base pressure of 5 × 10−4 Pa. The subsequent solution-processed functional layers including PVK, QDs (red, green and blue) and ZnMgO were all sequentially deposited with the same fabrication conditions as before except the annealing conditions, which were fixed at 90 °C for 5 min. Finally, a 100 nm Al cathode was evaporated in a high-vacuum evaporation chamber at a base pressure of 5 × 10−4 Pa.

Device Characterizations: The cross-sectional TEM images of the tandem QLEDs were characterized via high resolution (HR)-TEM (FE-HR-TEM, JEOL-2100F), and the samples were sliced using a focused ion beam (FIB) system (Dual-Beam FIB, SEIKO SMI3050SE). The thicknesses of all the solution-processed films were measured by a Bruker DektakXT Stylus Profiler. The evaporation rates and the thicknesses of HATCN, MoO3 and Al were in-situ monitored by a quartz crystal microbalance. The measurement results were further calibrated by a Bruker DektakXT Stylus Profiler. The thickness of all functional layers can also be estimeted from the TEM image. Transmittances of the ICL were measured using a 150 mm integrating sphere coupled with a Cary 5000 UV−VIS−NIR spectrophotometer (Agilent Technologies). The angular dependent EL spectra were measured using a Keithley 2614B programmable source meter, a rotating stage and a PR670 spectrometer. The active area of the devices was 2 × 2 mm2. The EQE was measured by using a method recommended by Stephen R. Forrest.3,59,60 The setup of the system and

measurement

steps

can

be

referred

to

ref.

60.

The

current

density−voltage−luminance (J−V−L) characteristics of the devices were measured by a dual-channel Keithley 2614B source measure unit and a calibriated PIN-25D silicon 15 / 21


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photodiode.

Acknowledgements This work was supported by the National Natural Science Foundation of China (61775090), the Guangdong Natural Science Funds for Distinguished Young Scholars (2016A030306017), the Guangdong Special Funds for Science and Technology Development (2017A050506001), the Basic Research Program of Science, Technology

and

Innovation

(JCYJ20170307105259290),

the

Commission National

Key

of

Shenzhen

R&D

Program

Municipality of

China

(2016YFB0401702), and the Shenzhen Peacock Plan (KQTD2015071710313656).

Supporting Information Supporting Information Available: Contact angle and microscopic images of “PEDOT:PSS droplet on ZnMgO” and “ZnMgO droplet on PEDOT:PSS”, device characteristics of the QLEDs with different ICLs, current density (in log scale)–voltage and power efficiency-current density characteristics of the devices, the lifetime data of the devices, histogram of maximum EQE of green tandem QLEDs, and angular EL spectra of the tandem QLEDs.This material is available free of charge on the ACS Publications website at DOI: XXX.

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Blue Tandem Quantum-Dot Light-Emitting Diodes

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