Enhancing the Performance of Quantum-Dot Light-Emitting Diodes by

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Functional Inorganic Materials and Devices

Enhancing the Performance of Quantum-Dot LightEmitting Diodes by Post-Metallization Annealing Qiang Su, Heng Zhang, Yizhe Sun, Xiao Wei Sun, and Shuming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08470 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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ACS Applied Materials & Interfaces

Enhancing the Performance of Quantum-Dot Light-Emitting Diodes by Post-Metallization Annealing Qiang Su, Heng Zhang, Yizhe Sun, Xiao Wei Sun, Shuming Chen* Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, 518055, P. R. China [email protected]

Abstract The effect of post-annealing on the device characteristics is systematically investigated. The external quantum efficiency (EQE) of blue quantum-dot light-emitting diodes (QLEDs) is significantly improved from 5.22% to 9.81% after post-annealing. Similar results are obtained in green and red QLEDs, whose EQEs are enhanced from 11.47% and 13.60% to 15.57% and 16.59%, respectively. The annealed devices also exhibit larger current density. The origin of efficiency improvement is thoroughly investigated. Our finding indicates that post-annealing promotes the interfacial reaction of Al and ZnMgO, and consequently leads to the metallization of AlZnMgO contact and the formation of AlOx interlayer. Due to the metallization of AlZnMgO, the contact resistance is effectively reduced and thus the electron injection is enhanced. On the other hand, the formation of AlOx interlayer can effectively suppress the quenching of excitons by metal electrode. Due to enhancement of electron injection and suppression of exciton quenching, the annealed blue, green and red QLEDs exhibit a 1.9-, a 1.3- and a 1.2-fold efficiency improvement, respectively. We envision the results offer a simple yet effective method to enhance the charge injection and the efficiency of QLED devices, which would promote the practical application of QLEDs.

Keywords: quantum-dots; light-emitting diodes; post-annealing; interlayer; exciton quenching

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ACS Applied Materials & Interfaces 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

1. Introduction Considerable research efforts have been devoted to colloidal quantum-dot (QD) light-emitting diodes (QLEDs) recently because of their attractive features such as tunable emission color, high color purity, excellent flexibility, simple and low-cost fabrication, which make them promising candidates for next generation display.1-5 With the rapid development of QD synthesizing and device engineering, the external quantum efficiency (EQE) and the stability of QLEDs are substantially improved in the past few years. For example, the EQEs and the lifetime of red and green QLEDs are over 20.5%, 100000 h (predicted half-lifetime, T50) @ 100 cd/m2 and 21%, 76000 h (T50) @ 100 cd/m2,6-8 respectively, which are very close to those of organic (O) LEDs and approach the entry level of industrial requirement. However, the performance especially the stability of the blue QLEDs9,10 is significantly lower than those of red and green QLEDs and is still far below the industrial requirement, which limits the application of QLEDs in full color display. The poor performance of blue QLEDs is in part due to inefficient charge injection induced by the wide bandgap of blue QDs.1,11 In addition, the blue excitonic energy, which is relatively large, can efficiently dissipate through various nonradiative channels such as inter-QD resonant energy transfer, Auger recombination and/or quenched by interfacial defects.12 To improve the performance of blue QLEDs, the charge injection and transport have been enhanced by replacing the insulated oleic acid ligands with shorter 1-octanethil ligands,13 or modifying the surface of ZnO using PEIE, which lowers the work function of ZnO and thus decreases the electron injection barrier.14 To suppress the quenching of excitons, QDs with thick shell and gradient alloyed structure have been adopted, which effectively suppress nonradiative Auger recombination and inter-QD resonant energy transfer.9,15 The quenching of exciton can further be suppressed by inserting a thin insulated layer (e.g., PMMA, PEI) between QDs and ZnO electron transport layer.16,17 All these device engineering efforts are aim at improving the charge injection and/or reducing the exciton quenching in blue QLEDs. 2


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In this work, we develop a simple possessing method, i.e., post-metallization annealing, to simultaneously enhance the charge injection and reduce the exciton quenching. The resultant annealed blue QLEDs exhibit significantly improved EQE of 9.81%, which is 1.9-fold higher than 5.22% of the un-annealed devices. Post-metallization annealing is widely adopted in semiconductor industry to reduce the contact resistance of metal/semiconductor.18-20 For example, in standard CMOS (complementary metal oxide semiconductor) process, metal deposition onto silicon is followed by annealing to form silicide (i.e., a metal silicon compound), which yields an alloyed AlSi interface with ohmic contact characteristics.18 Inspired by the benefits brought by post-metallization annealing, we tend to reduce the contact resistance and thus enhance the charge injection in QLEDs using the same method. We find that alloyed contact can similarly be obtained by post-annealing the QLED devices. Our results indicate that post-annealing promotes the interfacial reaction of Al and ZnMgO, and consequently leads to the metallization of AlZnMgO contact and the formation of AlOx interlayer. Due to the metallization of AlZnMgO, the contact resistance is effectively reduced and thus the electron injection is enhanced. On the other hand, the formation of AlOx interlayer can effectively suppress the quenching of excitons by metal electrode. The proposed method is also applicable to red and green QLEDs, whose EQEs are enhanced from 11.47% and 13.60% to 15.57% and 16.59%, respectively. Our results offer a simple yet effective method to enhance the charge injection and the efficiency of QLED devices, which would promote the practical application of QLEDs.

2. Results and Discussion

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Figure 1. (a) Schematic device structure and (b) energy level alignment of the QLEDs. (c) schematic illustration of the post-annealing process.

QLEDs

with

structure

glass/ITO/PEDOT:PSS/PVK/QDs/ZnMgO/Al

were

fabricated, followed by post-annealing in a hot plate at 100-160 °C for 20 minutes. The schematic device structure, energy level alignment and post-annealing process are shown in Figure 1. The effect of post-annealing on the device performance is systematically

investigated.

Figure

2

(a)-(f)

show

the

current

density-voltage-luminance (J-V-L) and the EQE-J characteristics of the devices, which were post-annealed at different temperature. It is clear that after post-annealing, the devices exhibit significantly larger current density, higher luminance, and more importantly, higher efficiency. The improvement is strongly related with the annealing temperature. For example, at an optimal annealing temperature of 120 °C, the EQE of blue QLEDs is significantly improved from 5.22% to 9.81%, whereas it is decreased to 8.34% when the temperature is increased to 140 °C. It is possible that at high temperature, the organic ligands of QDs are damaged, leading to the reduction of efficiency, while at low temperature, the alloying process is not fully activated, thus resulting in a low current density and a low EQE, as shown in Figure 2 (a) and (b). Similar results are also obtained in red and green QLEDs. For example, the red and green devices exhibit high EQEs of 13.60% and 16.59% when annealed at optimal 4


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temperature of 140 °C and 100 °C, respectively, which are 1.3- and 1.2-fold higher than 11.47% and 15.57% of the un-annealed devices. It is noted that the EQE improvement of red and green devices is not as significant as that of the blue devices, which might imply that the improvement mechanisms are rather different, as will be discussed later. The detailed device performance data are summarized in Table 1. To investigate whether such improvement is repeatable, the EQEs of over 48 blue QLEDs from different fabrication batches were measured. As statisticed in Figure 2 (g), most annealed devices exhibit a 1.8~2.0-fold EQE improvement, indicating the high reproducibility of our method. Figure 2 (h) and (i) show the electroluminescence (EL) spectra and the photos of the annealed devices, which are identical to those of the un-annealed devices, indicating the QDs remain intact after post-annealing.

Figure 2. J-V-L and EQE-J characteristics of (a) (b) blue QLEDs, (c) (d) green QLEDs, (e) (f) red QLEDs post-annealed at different temperature. (g) quantity statistics of maximum EQE of blue QLEDs without and with post-annealing. (h) EL spectra and (i) photographs of the red/green/blue monochromatic QLEDs.

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Table 1. The performance data of blue, green and red QLEDs with and without post-annealing

Post

CE a

EQE b

PE c

EQE

(cd/m2)

(cd/A)

(%)

(lm/W)

Enhancement

@ 12 (V)

max

max

max

Factor

Von(V)

Devices annealing

Luminance

2

(1 cd/cm )

Blue

without

4.7

19250

4.30

5.22

1.63

QLEDs

120 ℃

4.7

29750

7.80

9.81

3.28

Green

without

3.7

447066

50.43

11.47

21.43

QLEDs

140 ℃

3.7

576211

68.55

15.57

26.39

Red

without

2.8

185500

22.12

13.60

8.81

QLEDs

100 ℃

2.5

237100

27.04

16.59

12.21

~ 85 %

~ 30 %

~ 20 %

a

current efficiency; b external quantum efficiency; c power efficiency

The performance of QLEDs is effectively improved by simply post-annealing the devices, which is rather useful because it neither increases the fabrication complexity nor alters the structure of devices. During annealing, the thin film morphologies and the interfacial contacts such as PVK/QD, QD/ZnMgO or Al/ZnMgO could be altered, which might be responsible for the efficiency improvement. To identify which contact plays a major role, post-annealing was performed before or after Al cathode deposition. As shown in Figure 3 (a), if annealing is performed prior to the deposition of Al cathode, the EQE of the device is similar with that of the un-annealed device, indicating that post-annealing does not alter the thin film morphologies as well as the contacts of PVK/QD and QD/ZnMgO. However, if annealing is performed after the Al deposition, the EQE is significantly improved form 4.8% to 8.3%. This implies that the Al/ZnMgO contact is effectively modified by post-annealing, and such modification accounts for the performance improvement. To investigate whether annealing ambience affects the performance, the devices were annealed in air or in N2-filled glove box. As shown in Figure 3 (b), the devices exhibit similar EQE, indicating that annealing ambience does not play a major role in modifying the 6


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Al/ZnMgO contact.

Figure 3. EQE-J characteristics of blue QLEDs post-annealed (a) before or after Al deposition and (b) in air or in nitrogen ambience.

How does post-annealing modify the Al/ZnMgO contact? Previous studies disclosed that post-annealing induces the interdiffusion and promotes the interaction of metal and semiconductor, and as a result, leads to the metallization of metal/semiconductor contact and thereby reduces the contact resistance.18-20 To characterize the Al/ZnMgO contact, x-ray photoelectron spectroscopy (XPS) was performed. To probe the interface, the Al was carefully removed by sputter etching and the etching depth was monitored in situ by analyzing the amount of elements, as schematically shown in Figure 4 (a). Figure 4 (b) shows the O 1s spectra of ZnMgO, which are Gaussion fitted by three curves centered at 529.7 ± 0.1 eV (O-I), 531.1 ± 0.1 eV (O-II) and 531.7 ± 0.1 eV (O-III), corresponding to the oxygen in oxide lattices (O-I), oxygen vacancies (O-II) and oxygen bonds in the hydroxide (O-III), respectively.21 It is clear that after post-annealing, the O-I state is effectively reduced from 70.14% to 44.84%, while the O-II and the O-III states are significantly increased. This implies that the oxygen is out-diffused from the ZnMgO, leading to the accumulation of oxygen vacancies at the surface of ZnMgO. Since oxygen vacancies are regarded as donors in ZnO, this generates a heavily doped region near the ZnMgO surface18-20,22 and thus enhances the conductivity of ZnMgO. The out-diffused oxygen subsequently reacts with the Al and thereby leads to the formation of interfacial AlOx, 7



as can be verified from the Al 2p spectra shown in Figure 4 (c). The Al 2p spectra consist of two peaks, which are centered at 72.6 ± 0.1eV and 75.3 ± 0.1eV, corresponding to Al and amorphous AlOx, respectively.23 It is noted that there already exists a thin AlOx layer for the sample without post-annealing, which is reasonable due to the strong reaction between Al and O in ZnMgO layer. Such interfacial reaction can further be effectively promoted by post-annealing, as can be verified by a higher AlOx intensity. This behavior is consistent with the fact that the enthalpy of formation ° ° for Al2O3 ∆G 1492 kJ/mol is much smaller than that for ZnO ∆G

324 kJ/mol.24

Figure 4. (a) Schematic sample structure used for XPS characterization. XPS spectra of (b) O 1s scan and (c) Al 2p scan at the interface of Al/ZnMgO. (d) schematic front view and corss section view of the test structure for TLM characterization. (e) TLM data of samples with and without post-annealing. (f) J-V characteristics of electron-only devices with and without post-annealing.

The interdiffusion and interfacial reaction of Al and oxygen yield an AlZnMgO alloyed contact that would significantly reduce the contact resistance. To verify this assertion, the contact resistance was measured by using transmission line measurement (TLM) method, which is commonly used to characterize the 8


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metal/semiconductor contact resistance of transistors and other electronic devices.25 The configuration of the test structure is schematically shown in Figure 4 (d). To enhance the current laterally spreading between two Al contacts, the ZnMgO was deposited onto the ITO. The width of the semiconductor under tested is W=0.5 mm and the length L is varied from 4 to 16 mm. The total resistance between two Al contacts is

2 + 2 + ……………………………… (1) where is the resistance of Al, is the contact resistance of Al/ZnMgO, and is the resistance of bilayer ZnMgO/ITO. The resistivity of the metal is so low that

>> , and thus can be ignored. In addition,

□ ………………………………………….. (2) where □ is the sheet resistance of the bilayer ZnMgO/ITO. The total resistance can then be rewritten as:

!"□

# + 2 ……………………………………. (3)

We can see that is a function of L. By measuring several of resistors with different length L, we can plot the curve of ~L, as displayed in Figure 4 (e). In the limit of L=0, the would be just twice the contact resistance, and can be obtained from the graph by extrapolating back to L=0. The curves shown in Figure 4 (e) are fitted by linear functions of 0.99L + 0.026 kΩ and 0.99L + 0.05 kΩ , corresponding to the samples with and without post-annealing, respectively. By setting L=0, the contact resistance can be obtained, which is effectively decreased from 0.05 to 0.026 kΩ after post-annealing. In addition, according to equation (3), the sheet resistance of ZnMgO/ITO can also be obtained from the slope of the curves. Both samples exhibit the same reasonable sheet resistance of 0.99

+, --

× 0.5 mm ≈

0.5 kΩ/□, indicating that post-annealing does not affect the resistance of ZnMgO/ITO. The reduction of the contact resistance and the generation of the heavily doped ZnMgO surface contribute to the improvement of electron injection, and thus the post-annealed devices exhibit significantly larger current density, as shown in Figure 2 (a)-(e). This can further be verified by measuring the electron current in 9



electron-only device. As shown in Figure 4 (f), the post-annealed device indeed exhibit larger electron current, while the hole current (shown in supporting information Figure S1) remains almost the same. It should be noted that, in blue QLEDs, the carrier injection is relatively inefficient due to the wide bandgap of the blue QDs,11,26 and therefore, by enhancing the electron injection, the efficiency of the blue QLEDs can be significantly improved. However, in other color QLEDs especially the red QLEDs, the electron injection is relatively efficient,6,27 and thus the enhancement of electron injection might not improve the efficiency of red and green QLEDs. The observed EQE improvement in red and green QLEDs therefore implies that there are other mechanisms responsible for the improvement. Besides carrier injection, the performance of devices is largely affected by the photoluminescence (PL) of QDs, and this prompts us to further probe the exciton recombination in QDs. The exciton recombination is probed by using PL and time resolved (TR) PL spectroscopy. To ensure the accuracy of the measurement, the PL intensity was collected using an integrating sphere and the excitation/detection conditions were kept the same for different measurements. As shown in Figure 5 (a) and supporting information Figure S2, for the sample without Al, the PL intensity and TRPL decay of the post-annealed sample are identical to those of the un-annealed sample, indicating that post-annealing does not affect the exciton recombination in QD/ZnMgO. However, if Al cathode is introduced, the post-annealed sample exhibits significantly higher PL intensity and slower TRPL decay, as shown in Figure 5 (b) and (c), respectively, indicating that Al plays a major role in enhancing the PL efficiency of QDs. The TRPL decay curves were fitted with a triexponential function, and the average exciton lifetime is estimated by the relationship 012 34 04 + 3 0 + 35 05 with the parameters of time components (06 ) and corresponding weights (36 ). The fitting results are summarized in supporting information Table S1. After post-annealing, the average exciton lifetime of the blue, green and red samples are increased from 3.67, 4.87 and 9.13 ns to 4.10, 5.23 and 10.74 ns, respectively. The lengthened exciton lifetime and the enhancement of PL intensity indicate that the exciton quenching is effectively suppressed, which is likely due to the formation of 10


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interfacial AlOx. As schematically illustrated in Figure 5 (d), the defect states originated from oxygen vacancies in ZnMgO could effectively trap holes from QDs and electrons from metal electrode; these defect states are regarded as effective charge recombination centers,21 which quench the excitons. In other words, the electrons from Al cathode can be effectively captured by the defect states of ZnMgO. Due to the well aligned energy levels, the trapped electrons can further transfer to the valence band of QDs and thus quench the excitons. The formation of interfacial AlOx, as promoted by post-annealing can effectively block the pathway of electron trapping, and consequently suppress the exciton quenching, as briefly illustrated in Figure 5 (d). To conclude, the efficiency improvement of red and green QLEDs is mainly due to the suppression of exciton quenching, while both enhancement of electron injection and suppression of exciton quenching contribute to the improvement of blue QLEDs. This also explains why blue QLEDs exhibit the highest efficiency improvement of 1.9-fold, as compared to 1.3- and 1.2-fold of the green and the red devices, respectively.

11



Figure 5. PL spectra of (a) quartz/QD/ZnMgO and (b) quartz/QD/ZnMgO/Al with and without post-annealing, (c) TRPL decay curves of quartz/QD/ZnMgO/Al with and without post-annealing, (d) schematic illustration of exciton quenching process.

It is worth to mention that, the Al could react with oxygen from ZnMgO even without post-annealing. However, this native reaction is very slow and usually take several days to form a thin AlOx. Due to the gradual formation of AlOx and the progressive reduction of contact resistance, the performances of the QLED devices are increased with time, as shown in supporting information Figure S3. This phenomenon is termed as positive-aging.28 More details about positive-aging and the exact origin will be discussed in other report. Fortunately, the positive-aging can be alleviated by post-annealing the devices, as shown in supporting information Figure S3. This is because the formation of alloyed contact and AlOx interlayer can rapidly be obtained after post-annealing, and thus the devices can achieve the highest efficiency soon after annealing.

3. Conclusions In conclusion, post-metallization annealing has been proposed as a simple and effective method to enhance the performance of QLED devices. The effect of post-annealing on device characteristics is systematically investigated. The results indicated that post-annealing promotes the interfacial reaction of Al and ZnMgO, and consequently leads to the metallization of AlZnMgO contact and the formation of AlOx interlayer. Due to the metallization of AlZnMgO, the contact resistance is effectively reduced and thus the electron injection is enhanced. On the other hand, the formation of AlOx interlayer can effectively suppress the quenching of excitons. Due to enhancement of electron injection and suppression of exciton quenching, the annealed blue, green and red QLEDs exhibit a 1.9-, a 1.3- and a 1.2-fold efficiency improvement, respectively. The positive-aging effect can also be alleviated by post-annealing. The proposed method, which neither increases the fabrication complexity nor alters the structure of devices, can effectively enhance the charge 12


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injection and the efficiency of QLED devices, which would promote the practical application of QLEDs.

4. Experimental Section Device Fabrication: ITO (indium-tin-oxide) with sheet resistance = 25 Ω sq −1 was used as anode. The ITO-coated glass substrates were thoroughly cleaned by scrubbing, followed by ultrasonic cleaning in detergent for 30 min, soaking in deionized water for 10 min and baking for 30 min with an oven. Prior to the deposition of functional layers, the ITO-coated glass substrates were treated by a UV-ozone cleaner for 30 min. The spin-coating processes were carried out in ambient atmosphere. And then, the hole injection layer PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) was spin-casted at 3000 rpm and baked at 130°C for 20 min. Then the hole transport layer PVK (poly(9-vinlycarbazole), 10 mg/ml in chlorobenzene) was spin-casted at 4000 rpm for 45 s and baked at 120 °C for 20 min. And after that, the Red-QDs (15 mg/ml in octane solution, CdxZn1-xSe@ZnS/OT), Green-QDs (10 mg/ml in octane solution, CdxZn1-xSeyS1-y@ZnS/oleic acid) and Blue-QDs (10 mg/ml in octane solution, CdxZn1-xSeyS1-y@ZnS/OT) layers were spin-casted at 3000 rpm, 3000 rpm and 2500 rpm for 45 s, respectively. All QDs are commercially available from Mesolight Inc. After that, the commercially available Zn0.85Mg0.15O nanoparticles were spin-casted on QDs at 3000 rpm from a 20 mg/ml ethanol solution and baked at 100 °C for 10 min. Last, Al cathode with a thickness of 100 nm was evaporated in a high-vacuum thermal evaporator under the pressure of less than 5 × 10−4 Pa.

Device Characterization: The thicknesses of the films were measured by a Stylus Profiler (Bruker DektakXT). The PL and TRPL spectra were measured with Edinburgh

instruments

FS5

Fluorescence

Spectrometer.

J-V-L

(current

density−voltage−luminance) characteristics were characterized by a programmable source meter (Keithley 2614B) and a PR670 spectrometer.

13



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Supporting Information The hole-current of hole-only devices with and without post-annealing. The TRPL and fitting results of sample: Quartz/QDs/ZnMgO with and without post-annealing. The J-V-L and CE-J characteristics tested at different time period of greed QLEDs with and without post-annealing.

Acknowledgements Q. S. and H. Z. contributed equally to this work. This work was supported by the National Natural Science Foundation of China (grant no. 61775090), the Guangdong Natural

Science

Funds

for

Distinguished

Young

Scholars

(grant

no.

2016A030306017), the Guangdong Special Funds for Science and Technology Development (grant no. 2017A050506001), the Basic Research Program of Science, Technology and Innovation Commission of Shenzhen Municipality (grant no. JCYJ20170307105259290),

the

Shenzhen

Peacock

Plan

(grant

no.

KQTD2015071710313656) and the National Key R&D Program of China (grant no. 2016YFB0401702).

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Toc Figure

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Enhancing the Performance of Quantum-Dot Light-Emitting Diodes by

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