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Very Bright and Efficient Microcavity Top-Emitting Quantum Dot Light-Emitting Diodes with Ag Electrodes Guohong Liu, Xiang Zhou, and Shuming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03367 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 17, 2016

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ACS Applied Materials & Interfaces

Very Bright and Efficient Microcavity Top-Emitting Quantum Dot Light-Emitting Diodes with Ag Electrodes Guohong Liu1, 2, Xiang Zhou2, Shuming Chen1* 1

Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, 518055, P. R. China [email protected] 2

State Key Lab of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, P. R. China

Abstract The microcavity effect in top-emitting quantum dot light-emitting diodes (TQLEDs) is theoretically and experimentally investigated. By carefully optimizing the cavity length, the thickness of the top Ag electrode and the thickness of the capping layer, very bright and efficient TQLEDs with external quantum efficiency (EQE) of 12.5% are demonstrated. Strong dependence of luminance and efficiency on cavity length is observed, in good agreement with theoretical calculation. By setting the normal-direction resonant wavelength around the peak wavelength of the intrinsic emission, highest luminance of 112,000 cd/m2 (at a driving voltage of 7 V) and maximum current efficiency of 27.8 cd/A are achieved, representing a 12-fold and a 2.1-fold enhancement compared to 9,000 cd/m2 and 13.2 cd/A of the conventional bottom emitting devices, respectively. While highest EQE of 12.5% is obtained by setting the resonant wavelength 30 nm longer than the peak wavelength of the intrinsic emission. Benefit from the very narrow spectrum of QDs and the low absorption of silver electrodes, the potential of microcavity effect can be fully exploited in TQLEDs.

Keywords: Quantum Dot; Light-Emitting Diodes; Top-Emitting Structures; Microcavity Effect; Silver Electrodes 1 / 21

ACS Paragon Plus Environment


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1. Introduction Light-emitting diodes (LEDs) based on colloidal quantum dots (QDs) have attracted considerable interest in recent years owing to their attractive properties such as saturated tunable color emission, high luminescence quantum yield, inherent photophysical stability and simple fabrication process,1-5 which is a distinguishing technological advantage over conventional and organic LEDs (OLEDs). Rapid progresses in the performance of colloidal quantum dot light-emitting diodes (QLEDs) have been achieved by optimizing both the QD materials6-7 and the device architectures.8-16 For example, by engineering the functional layers such as charge transport and injection layers,8-12 electron blocking layers,13 energy transfer donors,14-15 highest external quantum efficiency (EQE) of 20% has been demonstrated recently,16 which further encourages the research interest in developing QLEDs for practical applications.

However, most reported QLEDs are based on bottom-emitting structures, while for display application, top-emitting architectures are more preferred, because light transmitting from the top contact greatly improves the aperture ratio of the displays and allows the fabrication of QLEDs on opaque or flexible substrates like Si or stainless steel.17-22 Moreover, the light-emitting efficiency can be significantly enhanced, because in top-emitting structures, the emission layers are sandwiched between a reflective bottom electrode and a semi-transparent top electrode; by optimizing the cavity length, light reflected from the top electrode and that from the bottom electrode interferes with each other constructively, and as a result, the emission is greatly sharpened and enhanced. Thus the top-emitting structures are widely used to improve the color saturation and enhance the efficiency of OLEDs.23-25 For example, the EQE of OLEDs can be enhanced by a factor of ~1.4 by incorporating top-emitting structure with double Ag electrodes, which was demonstrated both theoretically and experimentally,26-28 while the current efficiency can be enhanced by about 2 times based on different cavity design.28-29 2 / 21


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In case of QLEDs, the interference induced enhancement could be more significant if the device structure is fully optimized, since the emission spectra of QDs are much narrower than those of organic dyes, and thus more emission can interfere constructively and produce stronger emission ultimately. In addition, benefit from the very narrow emission spectra of QDs, the angular color shift, a typical issue of most top-emitting OLEDs, is negligible in top-emitting QLEDs (TQLEDs). Despite these advantages, TQLEDs are seldom reported. Only Yang et al. demonstrated a flexible TQLED with Al as bottom cathode and thin Ag as semitransparent top anode. Maximum luminance of 20,000 cd/m2 and EQE of 4.03% have been obtained.30 However, the microcavity effect was not systematically investigated and the structure was not fully optimized in terms of the reflective mirrors and the cavity length. Therefore, we here work on unlocking the full potential of microcavity structures on the performance of TQLEDs.

Microcavity structures may be applied by either lossless dielectric mirrors or metal mirrors. However, metal mirrors are more simple and easier to fabricate which render them more practical for real applications. For devices using two metal mirrors, mirrors with high-reflection and low-loss are essential for obtaining luminance enhancement.29 Among them, Ag is a perfect choice which is superior than Al or other metals since it has low absorption and high reflection. So it is expected that TQLEDs with double Ag electrodes could show much better performance than devices with Al electrodes as in the case of Yang’s report.30 The next step is to optimize the cavity geometries, which should be designed carefully and differently depending on the actual application.28 For microcavity devices with different emitters, the cavity length is the most important consideration which can significantly alter the optical properties of the devices. Besides, the thickness of the top electrode and the thickness of the capping layer are also very important for obtaining efficient devices.

In seeking high performance TQLEDs, the cavity length, the thickness of the top Ag electrode and the thickness of the capping layer are theoretically and 3 / 21



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experimentally optimized. Strong dependence of efficiency on cavity length was observed as expected, in good agreement with numerical simulations based on a rigorous electromagnetic model that we previously developed.31 By setting the normal-direction resonant wavelength around the peak wavelength of the intrinsic emission λPL, maximum luminance of 112,000 cd/m2 (at a driving voltage of 7 V) was obtained, which is the highest value ever reported and represents a 12-fold enhancement compared to 9,000 cd/m2 of the conventional bottom emitting devices. While highest EQE of 12.5% was obtained by setting the resonant wavelength 30 nm longer than λPL. The results and methods reported here can serve as feasible guidelines for designing efficient microcavity TQLEDs of all colors.

2.

Results and discussion Figure 1 (a) shows the structure of the TQLEDs and Figure 1 (b) displays the

energy band diagram of each functional layer. The TQLEDs under investigation consists of silver (Ag, 100 nm), ZnO nanocrystals, CdSe/CdS/ZnS core/shell/shell quantum dots (QDs, 25 nm), 4,4',4" -Tris(carbazol-9-yl)triphenylamine (TCTA, 30 nm),

N,N'-di(naphthalene-1-yl)-N,N'-

bis(phenyl)-benzidine

(NPB,

20

nm),

Dipyrazino [2,3-f:2',3'-h] quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN, 10 nm), and thin Ag. The structure can be optically treated as a Fabry-Perot (FP) cavity, in which the QD light-emitting layer is sandwiched between a bottom reflective mirror and a top semitransparent mirror. In the cavity, there exist two interference behaviors, namely wide-angle interference and multiple-beam interference. As shown in Figure 1 (c), wide-angle interference takes place between directly emitted light and reflected light from the bottom electrode that have the same wavevector, with the distance between emitter and reflective electrode d1 playing an important role. Multiple-beam interference takes place when the radiation is reflected back and forth between the two mirrors with the optical thickness of the total functional layers d1+d2 determining the resonant wavelength. The resonant conditions are determined by: 2 2 cos − = ∙ 2,

− 4 / 21


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2 2 ! " # cos − $ % ! & = ∙ 2,

'( − )

where n and ϕ are refractive index of the functional layers and phase shift of the metal electrodes, respectively. The resonant wavelength of the cavity can be rapidly determined once the cavity length d1+d2 and the distance from emitter to the reflective bottom electrode d1 are given. If the resonant wavelength equals to λPL of the emitter, then enhanced emission due to constructive interference can be obtained. Otherwise, the intrinsic emission of the emitter will be inhibited due to destructive interference. Therefore, the cavity length has to be carefully optimized so that it meets the resonant conditions.

Figure 1. (a) device structure and (b) energy band diagram of the TQLEDs, (c) schematic optical model of the TQLEDs. The light interaction in TQLEDs is governed by two mechanisms, i.e., wide-angle interference and multiple-beam interference. (d) simulated total emit intensity of the TQLEDs with different electrode 5 / 21



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structures.

To investigate the influence of the cavity length on the device performance, the total emit intensity of the devices as a function of cavity length is first simulated using a theoretical model that is based on a rigorous electromagnetic method. The details of the simulation are described in the supporting information. The thickness of the ZnO is varied because ZnO has a high electron mobility (~1.8×10-3 cm2V-1s-1)16 and thus the changing of thickness almost does not alter the electrical characteristics of the devices, while the organic layer on the anode side is set constant to be 60 nm. Several state-of-the-art device structures, such as TQLEDs with Ag 100 nm-Ag 25 nm electrodes, Al 100 nm-Ag 25 nm electrodes (as implemented in Yang’s work30) and bottom-emitting QLEDs with ITO-Ag 100 nm electrodes (as adopted by Dai16) are investigated. As shown in Figure 1 (d), the output intensity of the bottom-emitting QLEDs is slightly altered as ZnO thickness is varied, while the emit intensity of the TQLEDs is strongly modulated by the cavity. For example, the output intensity of the TQLEDs is highly sensitive to ZnO thickness, if the thickness of ZnO in the TQLED with Ag-Ag electrodes is lower than 30 nm or higher than 60 nm, the emit intensity is rapidly dropped far behind that of the bottom-emitting device. The strong modulation of the output as a function of the cavity length is a direct evidence of strong interference. Based on the simulation, several conclusions can be drawn. First, at appropriate cavity length, the output of the TQLEDs is significantly higher than that of the bottom-emitting devices and thus the cavity structure can be used to enhance the emission. Second, the cavity induced enhancement of TQLEDs with Ag-Ag electrodes is much greater than that of TQLEDs with Al-Ag electrodes. This is reasonable since Ag has a relatively lower absorption and higher reflection than Al at the wavelength of 630 nm (which is the λPL of the QDs we used) as is shown in Figure S3. Third, the cavity induced enhancement is higher if the intrinsic emission spectrum is narrower. In this regard, the cavity structure is more suitable for QDs due to their narrow emission spectrum. 6 / 21


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Inspired by the simulation, TQLEDs with Ag-Ag electrodes were fabricated. The influence of cavity length on device performance is first investigated, followed by examining the effect of top Ag thickness on device characteristics, and finally, the impact of capping layer on device efficiency is evaluated.

2.1 The influence of cavity length on device performance To investigate the influence of cavity length on device performance, devices with structure of glass/Ag (100 nm)/ZnO x/QDs (25 nm)/TCTA (30 nm)/NPB (20 nm)/HATCN (10 nm)/Ag (22 nm) was fabricated, with the thickness of ZnO varied from 35 nm, 45 nm to 56 nm, which is corresponded to the normal direction resonant wavelength of 632 nm, 660 nm and 700 nm, respectively. The normal direction resonant wavelength of the TQLEDs with 35 nm ZnO is equal to the λPL of QDs, and thus the electroluminescent (EL) spectra of the ZnO 35-devices are very similar with that of the bottom-emitting devices, as is shown in Figure 2 (a). Because the intrinsic emission is strongly resonant in the cavity, the full width at half maximum (FWHM) in the normal direction of the spectra is narrowed from 36 nm to 26 nm, which further enhances the color saturation of the emission. For devices with 45 nm ZnO, the emission peak is red shifted to 642 nm. Further increasing ZnO thickness to 56 nm, the emission peak is shifted back to 636 nm accompanied with a shoulder peak at 720 nm. It is noted that the emission peak is not always equal to the resonant wavelength of the cavity, since the emission pattern is controlled not only by the microcavity modification but also by the spectral shape of the intrinsic emission.

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Figure 2. (a) EL spectra, (b) J-V-L, (c) current efficiency-J and (d) EQE-J characteristics of the devices with different thickness of ZnO. Benefit from the very narrow spectra of QDs and strong cavity modulation, the current efficiency of TQLEDs with 35 nm ZnO is enhanced by 2.1 times and the EQE of TQLEDs with 45 nm ZnO is improved by a factor of 2.

Figure 2 (b) shows the current density (J)-voltage (V)-luminance (L) characteristics of the devices. Due to the high electron mobility of ZnO, TQLEDs with different thickness of ZnO exhibit almost identical J-V characteristics. At a certain voltage, the TQLEDs exhibit significantly higher current density than the bottom-emitting devices. This is because the resistance of 25 nm Ag (~1 Ω/□) is significantly lower than that of the ITO (20 Ω/□). In addition, the work function of Ag (4.3 eV) is lower than that of ITO (4.7 eV), and thus electron injection from Ag to ZnO is more efficient than from ITO. All devices exhibit similar turn on voltage of about 3 V. However, the TQLEDs exhibit remarkably higher forward luminance than the bottom-emitting devices. For example, at a driving voltage of 7 V, TQLEDs with 8 / 21


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35 nm ZnO exhibit a luminance of 112,000 cd/m2, which is the highest value ever reported and represents a 12-fold enhancement compared to 9,000 cd/m2 of the bottom-emitting devices. Such significant improvement is mainly attributed to two reasons: optically, the emission is strongly resonant in the cavity since the cavity length satisfies the resonant conditions, and as a result of constructive interference, the forward emission is greatly enhanced; electrically, the injected current is remarkably higher due to efficient electron injection and small resistance of the Ag electrodes. To rule out the electrical effects and solely display the influence of the cavity on the performance of devices, the current efficiency was measured and shown in Figure 2 (c). The TQLEDs with 35 nm ZnO exhibit a maximum efficiency of 27.8 cd/A, which is 2.1-fold higher than 13.2 cd/A of the bottom-emitting devices. However, for devices with thicker ZnO, the current efficiency rapidly dropped down and is lower than that of the bottom-emitting devices. This is because for thicker ZnO, the cavity resonant wavelength is longer than the λPL of QDs, and thus the intrinsic emission of the emitter is greatly inhibited, resulting in a low luminous efficiency. Interestingly, though TQLEDs with 45 nm ZnO exhibit relatively low current efficiency, it showed a highest EQE of 11.7%, which is about 2.0-fold higher than 6.0% of the bottom-emitting devices, while devices with highest current efficiency only exhibit a 1.7-fold enhancement of EQE, as shown in Figure 2 (d). Since the EQE takes the emission intensity of all angles into account, while the current efficiency only considers the forward emission, the discrepancy between EQE and current efficiency thus implies that the angular emission pattern is different for TQLEDs with different thickness of ZnO.

9 / 21



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Figure 3. (a) angular emission pattern of the devices with different thickness of ZnO. (b) EQE and simulated emit intensity as a function of ZnO thickness. Angular EL spectra of devices with 35 nm ZnO (c) and 45 nm ZnO (d).

To verify our assertion, the angular emission pattern of the devices was measured and shown in Figure 3 (a). The Lambertian factor which is defined as ΣI(θ)/I(00) is used to characterize the angular emission of the devices, where ΣI(θ) is the total emit intensity and I(00) is the normal direction emit intensity of the devices. Standard Lambertian emission exhibits constant luminance at any angles with a Lambertian factor of π, while TQLEDs with 35 nm ZnO exhibit a small Lambertian factor of 0.55π, indicating that the emission is strongly directed toward the surface normal. This is because the intrinsic emission is largely resonant in the normal direction. At off-axis directions, the resonant wavelength λθ=λ0cosθ becomes shorter than λPL of the QDs, and thus the emission is greatly suppressed, and as a result, the emission is forced to escape from the normal direction. Due to redistribution of the emission 10 / 21


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direction, the devices exhibit highest forward luminance and highest current efficiency enhancement, as shown in Figure 2 (c). For TQLEDs with 45 nm ZnO, the intrinsic emission is resonant at an off-axis angle and thus the emission pattern shows the highest intensity at an off-axis angle with a large Lambertian factor of 1.50π.The Lambertian factor is further increased to 3.57π by increasing the thickness of ZnO to 56 nm. Because the emission is directed towards an off-axis direction, the devices exhibit relatively lower forward luminance and current efficiency, as is shown in Figure 2 (c). However, in terms of EQE, which takes the intensity of all angles into account, all TQLEDs exhibit 1.7-2.0 times enhancement. Devices with 45 nm ZnO shows the highest value of 11.7%. This is because the total intensity is controlled by both the cavity modification and the intrinsic emission. By setting the normal direction resonant wavelength 30 nm longer than λPL, emission spectrum at different viewing angle can largely overlap with the intrinsic spectrum (Figure 3 (d)), and thus the devices can have higher total emission intensity. To rule out the experimental errors and uncertainties, over 92 devices fabricated from 6 independent experiments with different thickness of ZnO were characterized. As shown in Figure 3 (b), the distribution of average EQE agrees fairly with the simulation results and the maximum EQE is indeed obtained at a ZnO thickness of about 45 nm. Figure 4 (c) and (d) show the EL spectra of the devices viewed at different angles. For all TQLEDs, the spectra are blue-shifted as the viewing angle is increased, which is a typical issue in microcavity devices. However, compared with the microcavity OLEDs, the color shift caused by the variation of spectra is negligible due to the narrow spectrum of the QDs. The key characteristics data of the device performance (extracted from figure 2 which are from one representative batch of devices) are summarized in Table 1. To conclude, in TQLEDs, highest luminance and luminous efficiency enhancement can be obtained by setting the resonant wavelength equal to the λPL of QDs, while highest EQE improvement can be achieved by setting the resonant wavelength 30 nm longer than λPL of the QDs.

Table 1. performance of the devices with different thickness of ZnO 11 / 21



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ZnO

EQEmax

CEmax

PEmax

L @ 7V 2

ELpeak

0o FWHM

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0o CIE

80o CIE

Lambertian

(X, Y)

(X, Y)

factor

(nm)

(%)

(cd/A)

(lm/W)

(cd/cm )

(nm)

Bottom

6.0

13.2

5.4

9,000

630

36

(0.69, 0.31)

(0.69, 0.31)

0.71π

35

10.1

27.8

10.8

112,000

632

26

(0.70, 0.30)

(0.66, 0.34)

0.55π

45

11.7

9.1

12.3

39,000

642

35

(0.70, 0.30)

(0.66, 0.34)

1.50π

56

10.0

3.3

10.1

13,000

636

42

(0.70, 0.30)

(0.67, 0.33)

3.57π

(nm)

2.2 The effect of top Ag thickness on device characteristics The top Ag electrode should have appropriate thickness so that its reflection is high enough to induce strong interference. Meanwhile, it should have high transmission to allow light transmission. There is trade-off between reflection and transmission, and thus the thickness of the top Ag electrode should be carefully optimized. Figure 4 (a) shows the J-V-L characteristics of the TQLEDs with difference thickness of the top Ag electrodes. The thickness of ZnO is fixed at 45 nm so that the devices can exhibit the highest EQE. At a certain voltage, the current density and luminance increase as the thickness of the Ag electrode is increased, mainly because of reduced resistance of the Ag electrode. As shown in Figure 4 (b), TQLEDs with 25 nm Ag electrode exhibit an EQE of 11.9%, which is the highest value compared to 10.8% and 9.8% for the devices with 16 nm and 30 nm Ag electrode, respectively. As shown in Figure 4 (d), for thin Ag electrodes, though the transmission is high, the reflection is too low to induce strong interference, and thus the cavity induced enhancement is weak. For thick Ag electrodes, though the reflection is high enough to achieve substantial microcavity induced enhancement, the transmission is too low and thus most of the emission cannot escape from the devices. In this case, best trade-off between reflection and transmission is obtained when the thickness of the Ag electrode is optimized to be 25 nm. Due to strong microcavity modulation effect, TQLEDs with thicker Ag electrode exhibit relatively narrower emission spectra, as displayed in Figure 4 (c). Theoretically, the linewidth of the cavity resonance can be expressed as:32

FWHM cavity =

λ 2 1 − R1R2 × 2 L π 4 R1R2

12 / 21


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Where λ denotes the resonant wavelength, L = ∑ ni d i is the effective cavity length, i

R1 and R2 are the reflectivity of bottom and top electrode, respectively. So, with higher R2, the linewidth of the cavity resonance become narrower. For example, the FWHM of the devices is remarkable reduced from 41 nm to 21 nm when the thickness of the top Ag electrode is increased from 16 nm to 30 nm. The key characteristics data of the device performance are summarized in Table 2 (extracted from figure 4 which are from one representative batch of devices).

Figure 4. (a) J-V-L, (b) EQE-J characteristics and (c) EL spectra of the TQLEDs with different thickness of top Ag electrode. (d) reflection, transmission and absorption of the Ag electrode as a function of thickness.

Table 2. Performance of devices with different thickness of top Ag electrode. Ag

EQEPeak

CEPeak

PEPeak

L@7V

ELpeak

0o FWHM

0o CIE

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80o CIE

Lambertian


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(nm)

(%)

(cd/A)

(lm/W)

(cd/m2)

(nm)

(nm)

(X, Y)

(X, Y)

factor

16

10.8

7.5

11.2

26,000

636

41

(0.70, 0.30)

(0.66, 0.34)

1.79π

25

11.9

9.5

12.7

40,000

636

25

(0.71, 0.29)

(0.66, 0.34)

1.37π

30

9.8

10.2

11.0

47,000

638

21

(0.71, 0.29)

(0.66, 0.34)

1.23π

2.3 The impact of capping layer on device efficiency In TOLEDs, the outcoupling efficiency can be remarkably enhanced by capping an index matching layer on top of the Ag electrode.33 To examine the effect of capping layer on device performance, TQLEDs with or without 60 nm NPB capping layer were fabricated. As shown in Figure 5 (a), both devices exhibit identical J-V characteristics; however, devices with capping layer exhibit substantially higher luminance and efficiency. As shown in Figure 5 (b), maximum EQE of 12.5% is obtained, which is 8.7% higher than 11.5% for the devices without the capping layer. Such improvement is mainly due to the change of optical properties of the Ag electrode. To investigate the effect of the capping layer on the optical properties of the electrode, the transmission and reflection at wavelength of 630 nm of the Ag 25 nm/NPB x nm is calculated and shown in Figure S4. With 60 nm NPB capping layer, the reflection of the top electrode is reduced from 67.7% to 60.7%, meanwhile the transmission is improved from 25.6% to 33.0%. It should be noted that, highest transmission can be acquired at a capping layer thickness of 55 nm. However, maximum EQE is obtained at a capping layer thickness of 60 nm. In other words, maximum efficiency enhancement does not occur at highest transmission of the electrode but is determined by the interplay between the reflection and the transmission, which is governed by the thickness of the capping layer. Figure 5 (b) inset shows the photos of the devices. Very bright and efficient emission at a voltage of 5 V can be observed.

Figure 5 (c), (d) shows the EL spectra and the angular emission pattern of the TQLEDs, respectively. Due to enhancement of transmission of the Ag electrode, devices with capping layer exhibit similar spectra with that of the bottom-emitting devices. In addition, the angular emission pattern is more close to the Lambertian 14 / 21


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pattern. The key characteristics data of the device performance are summarized in Table 3 (extracted from figure 5 which are from one representative batch of devices). To conclude, the optical properties of the top Ag electrode can be significantly altered by controlling the thickness of the capping layer, which further modifies the interference effects, tunes the spectral characteristics and improves the light outcoupling efficiency of the TQLEDs.

Figure 5. (a) J-V-L, (b) EQE-J characteristics, (c) EL spectra and (d) angular emission pattern of the TQLEDs with or without the capping layer. Inset of (b) shows the photos of the TQLEDs. Very bright and efficient emission can be observed at a voltage of 5 V.

Table 3. Performance of devices with or without the 60 nm NPB capping layer. Capping layer

EQEPeak (%)

CEPeak (cd/A)

PEPeak

ELpeak

0o FWHM

0o CIE

80o CIE

Lambertian

(cd/m )

(nm)

(nm)

(X, Y)

(X, Y)

factor

L @ 7V

(lm/W)

2

without

11.5

10.6

12.7

47,000

638

26

(0.70, 0.30)

(0.66, 0.34)

1.37π

with

12.5

18.0

14.1

79,000

632

32

(0.69, 0.31)

(0.66, 0.34)

1.05π

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3. Summary In conclusion, very bright and efficient TQLEDs have been demonstrated by using Ag as anode and cathode. The device efficiency is highly dependent on the cavity length, which is in good agreement with numerical simulations. By setting the resonant wavelength equal to λPL of the QDs, highest luminance of 112,000 cd/m2 has been obtained, which represents a 12-fold improvement compared to 9,000 cd/m2 of the bottom-emitting devices. While by setting the resonant wavelength 30 nm longer than λPL, highest EQE of 11.7% has been achieved, which is about 2.0-fold higher than 6.0% of the bottom-emitting devices. In addition, the thickness of the top Ag electrode and the thickness of the capping layer also play important roles in determining the device performance. By carefully balancing the trade-off between transmission and reflection, maximum EQE of 12.5% has been obtained using an Ag 25 nm/NPB 60 nm structure. Benefit from the very narrow spectrum of QDs, the microcavity induced efficiency enhancement is more significant in TQLEDs than in TOLEDs; also, the issue of angular color shift caused by the microcavity effect is negligible in TQLEDs. The results and methods presented here can serve as feasible guidelines for designing efficient microcavity TQLEDs of all colors.

4. Experimental Section Synthesis of Colloidal ZnO nanocrystals. ZnO nanoparticles were synthesized by hydrolysis and condensation of zinc acetate by potassium hydroxide using a Zn2+:OH －

ratio of 1:1.9 similar to Beek et al.34 Zinc acetate (0.839 g, 4.6 mmol) was dissolved

in methanol (42 mL), then deionized water (125 µL) was added into the solution. After heating the solution to 60 °C under magnetic stirring, a methanol solution (23 mL) of potassium hydroxide (0.49 g, 8.8 mmol) was slowly added to the zinc acetate solution over 10-15 min. The mixture was stirred at 60°C for 150 min. Then the ZnO nanocrystals were precipitated by centrifugation (2500 rpm; 10 min) and washed twice with methanol. Finally, the precipitate was redispersed in butanol at a concentration of 20 mg/ml. 16 / 21


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Device Fabrication. The TQLEDs consists of glass/Ag (100 nm/ZnO x/QDs (25 nm)/TCTA (30 nm)/NPB (20 nm)/HATCN (10 nm)/Ag y. Dipyrazino [2,3-f:2',3'-h] quinoxaline-2,3,6,7,10,11-hexacarbonitrile N,N'-di(naphthalene-1-yl)-N,N'-

(HATCN),

bis(phenyl)-benzidine

-Tris(carbazol-9-yl)triphenylamine (TCTA)

(NPB),

and

4,4',4"

were organic layers acting as hole

injection layer, hole transport layer, hole transport and electron blocking layer, respectively. To fabricate the devices, a 100 nm thick Ag film patterned by a shadow mask was first deposited on blank glass substrate in a high vacuum chamber with a base pressure of 6×10-4 Pa. Then the samples were transferred to a N2-filled glove box for spin-coating ZnO and QDs. ZnO nanoparticles were spin-coated onto the Ag-coated glass substrates at 500-2500 rpm, followed by drying at 130 °C for 10 min. Commercially available CdSe/CdS/ZnS QDs with a peak photoluminescence (PL) wavelength of 630 nm were dissolved in chlorobenzene (20 mg/mL) and then spin-coated at 1500 rpm, followed by annealing at 100 °C for 5 min. Then the samples were transferred back to the vacuum chamber to deposit the subsequent organic layers in sequence. Characterization. The thickness of ZnO and QD layers were determined by Dektak Step Profiler. The input EL spectra of QDLEDs for EQE measurements were collected by an integration sphere and a fiber optic spectrometer (Ocean Optics USB2000). The angle-dependent EL spectra were measured using a Keithley 2614 programmable source meter, a rotating stage, and a PR670 spectrometer. Measurement of EQE was achieved by a method recommended by SR Forrest8, 35 in which a large area PIN-25D silicon photodiode was placed in contact with the devices. The active area (2 mm×2 mm) of the device under test is much smaller than that of the photodiode area (613 mm2). Light emitted from the TQLED was absorbed by the photodiode which has a known responsivity and was measured as photodiode current (calibrated by the integrated emission spectrum of the device being tested), while the driving current through the devices was monitored simultaneously by a dual-channel Keithley 2614B source measure unit. The experimental set up is shown in Figure S5. These two quantities were then used to calculate the EQE (as photons per electron) by 17 / 21



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converting the photodiode current to emitted photons and the driving current to electrons. The photon flux can be converted into luminous flux by means of the eye-sensitivity (or standard luminosity) function V(λ). Then the luminance can be determined as the luminous flux divided by the emission area and the emission solid angle of the devices. The emission solid angle is determined by measuring the angular emission pattern of the devices using a rotating stage and a PR670 spectrometer. For bottom-emitting devices, the emission solid angle is roughly equal to π, while for top-emitting devices, depending on the structure, the emission solid angle can be smaller or larger than π. The luminance acquired from this method was calibrated by PR670.

Supporting Information Supporting Information is available from the internet or from the author. The details of the theoretical model, the calculated reflectance, absorbance of 100 nm Ag and 100 nm Al film in the visible wavelength range, the calculated reflectance, transmission and absorbance of Ag 25 nm/NPB x nm at 630 nm, the QLED characterization system. This information is available free of charge via the Internet at ?

ACKNOWLEDGMENTS This work was supported by the Basic Research Program of Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20140417105742713)， the National Natural Science Foundation of China (61405089), the Guangdong Natural Science Fund for Distinguished Young Scholar (2016A030306017) and the Guangdong Special Support Program for Young Talent Scholar (2014TQ01X015). Reference 1.

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The table of contents entry Efficient top-emitting red quantum dot light-emitting diodes (TQLEDs) with maximum external quantum efficiency (EQE) of 12.49% are demonstrated, which is 2.08 times higher than 6% of the reference devices.

Keyword Quantum Dot; Light-Emitting Diodes; Top-Emitting Structures; Microcavity Effect; Silver Electrodes

Guohong Liu, Xiang Zhou, Xiaowei Sun, Shuming Chen* Very Bright and Efficient Microcavity Top-Emitting Quantum Dot Light-Emitting Diodes with Ag Electrodes ToC figure

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Fig1 1431x1172mm (96 x 96 DPI)


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Fig2 1543x1134mm (96 x 96 DPI)



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Fig3 1549x1098mm (96 x 96 DPI)


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Fig4 1545x1096mm (96 x 96 DPI)



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Fig5 1587x1126mm (96 x 96 DPI)


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Very Bright and Efficient Microcavity Top-Emitting ... - ACS Publications

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