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Abstract
Transparent quantum-dot light-emitting diodes (Tr-QLEDs) with an inverted architecture has been developed. The inverted Tr-QLEDs are designed for integrating with thin-film transistors (TFTs) circuit easily. The 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) is employed as a hole injection layer (HIL) as well as a buffer layer in the inverted Tr-QLEDs. An optimized HAT-CN as dual-functional modified layer facilitates charge injection balance and meanwhile reduces the plasma damage caused by sputtering process. High performance device with a peak current efficiency (CE) and maximum external quantum efficiency (EQE) of 14.7 cd/A and 11.3% was obtained, wherein the EQE is the highest record for Tr-QLEDs. The transmittance of the Tr-QLEDs at 550 nm reached up to 78%. These Tr-QLEDs possess potential for the next-generation transparent displays applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of transparent electronics, there have been an increasing interest on variety of transparent and translucent optoelectronic devices in the visible range. Especially for transparent conducting films and transparent light-emitting diodes (LEDs), significant advancements have been made in the past two decades [1,2]. Due to the potential applications of transparent optoelectronic films and devices in the consumer electronics, new energy and automotive industries, this area has been attracting enormous attentions from both of academics and industries. Transparent quantum dots light-emitting diodes (Tr-QLEDs) have become the potential candidate for the next-generation transparent displays, augmented reality (AR), and wearable electronics [3–8], because of their unique characteristics such as tunable narrow linewidth emission, high electroluminescence (EL) efficiency, high color saturation, and solution process fabrication [9–13]. Great progress has been made in the development of QLEDs during the past 30 years, but Tr-QLEDs still suffer from low performance, imbalanced carriers injection, energy band mismatching of the top transparent electrode, and plasma sputtering damage to the electron transport layer (ETL) or the hole transport layer (HTL) [14–17].
In order to address the aforementioned issues, great efforts have been implemented in recent years [8,18,19]. Wang et al. reported that ZnO nanocrystals with a thickness of 82 nm were employed as the buffer layer and the ETL, and they suggested that this thicker ZnO layer can reduce the plasma sputtering damage, thus improving the EL properties of transparent device [19]. Moreover, ZnO nanoparticles were used as ETL in the Tr-QLEDs with ultrathin Al2O3 buffer layer, which can balance electron/hole injection into quantum dots (QDs) and suppress nonradiative recombination processes [8].
In addition to optimizing the ETL and buffer layer in the Tr-QLEDs, the inverted structure also was used to develop high-performance device. An inverted QLEDs can be directly connected to the drain electrode of n-channel metal oxide thin-film transistors (TFTs) backplanes, which have been widely used in the large area and high-resolution displays, and it is a very promising device architecture for display application [20]. Moreover, the HTL using organic small molecules are deposited on the QD layer by thermally evaporation to avoid damaging the QD layer in the inverted structure [21].
In 2014, thin silver film was used as an anode deposited by thermal evaporation in the inverted QLEDs [22]. An 18-nm-thick Ag top electrode was optimized to achieve a maximum transparency of approximately 45% in the semi-transparent inverted QLEDs. Kim and co-workers reported an inverted hybrid Tr-QLEDs with inorganic ZrO2 nanoparticles as the ETL [23]. The two-step sputtering process of indium zinc oxide (IZO) top electrode was applied to these inverted devices. This study realized Tr-QLEDs with transmittance of more than 74% at 550 nm, however, the luminescence of and the current efficiency of the device were 200 cd/m2 and 0.47 cd/A, respectively, which was much lower than that of the conventional structure device. A recent reported inverted Tr-QLEDs based on a composite AgNWs/AZO (Ag nanowires and aluminum-doped zinc oxide) transparent electrode showed the current efficiency of 15.33 cd/A and the transmittance of 75.66% at 530 nm [24]. However, the PVK dissolved in toluene or chlorobenzene was used as HTL in the inverted device, which can damage the underlying QDs layer and lead to low EL efficiency [24,25]. Therefore, performance of inverted Tr-QLEDs have been far from satisfactory.
In this work, ITO as the top electrode of inverted Tr-QLEDs is deposited by sputtering process at room temperature. An optimized 30 nm HAT-CN is used as the hole injection layer (HIL) as well as a buffer layer reducing the plasma damage caused by sputtering process to realize high performance Tr-QLEDs with a transmittance of 78% at 550 nm. The external quantum efficiency (EQE) of inverted Tr-QLEDs is 11.3%, and the current efficiency (CE) is 14.7 cd/A. These results demonstrate that our inverted Tr-QLEDs have competitive performance in comparison with other Tr-QLEDs based on conventional architecture, which suggests that this damage-free sputtering process will not degrade the EL properties of Tr-QLEDs with the HAT-CN as dual-functional modified layer.
2. Experimental
2.1 Materials
The red CdSe/ZnS quantum dots used were purchased from Suzhou Xingshuo Nanotech Co., Ltd, and ZnMgO nanoparticles were from Guangdong Poly Opto-Electronics Co., Ltd. The organic and inorganic semiconductors including the 4,4′-bis(carbazole-9-yl)biphenyl (CBP), MoO3, and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) were from Luminescence Technology Corp. All reagents were used as received without further experimental purification.
2.2 Device fabrication
ITO glass substrate was ultrasonic treated in acetone, isopropyl alcohol and deionized water for 15 minutes, and then treated in an oven at 100℃ for 75 minutes. The treated substrate was transferred to a nitrogen-filled glove box (O2<0.1 ppm and H2O<1 ppm). ZnMgO nanoparticles (ethanol, 20 mg/mL) and CdSe/ZnS QDs (n-octane, 15 mg/mL) were deposited layer by layer on the substrate by spin coating. Rotate and coat ZnMgO at 2000 rpm for 45 seconds. The ZnMgO nanoparticles and QDs layers were baked at 120℃ for 10 minutes and 100℃ for 5 minutes, respectively. Then, the 4,4′-bis(carbazole-9-yl)biphenyl (CBP), MoO3, and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) layers were deposited by thermal evaporation system at the deposition rates of 1-2 Å/s, 0.02-0.04 Å/s, and 0.08-0.15 Å/s, respectively. Finally, top electrode ITO was deposited by magnetron sputtering system at a working pressure of 0.67 Pa, a power of 50 W, an Ar flow of 20 sccm and a deposition time of 20 minutes.
2.3 Characterization
The thicknesses of the solution processed films were measured using a Bruker Stylus Profiler. The transmittance spectra were recorded by the Perkinemer Lamda 950 spectrophotometer. The current density-voltage-luminance curves were acquired by Ocean Optics system equipped with a Keithley 2450 source meter, an integrating sphere, and an Ocean Optics QE65000 spectrometer. All measurements were performed in air at room temperature.
3. Results and discussion
To study the effect of buffer layer on the EL characteristics of device, inverted Tr-QLEDs with the following structure ITO/ZnMgO/CdSe/ZnS QDs/CBP/MoO3/HAT-CN/ITO were prepared (Fig. 1(a)). ZnMgO, CdSe/ZnS QDs, CBP, MoO3 and HAT-CN are used as EIL and ETL, emission layer (EML), HTL, HIL and buffer layer, respectively. In this inverted architecture, it is worth mentioning that HAT-CN acts as both HIL and buffer layer in this device. Figure 1(b) shows the energy level diagram of QLEDs [26]. The HIL and buffer layer of HAT-CN used in the device has the following advantages: (i) the previous results show that the work function of MoO3 matches with the energy level of the highest occupied molecular orbital (HOMO) of HAT-CN, which can realize the suppression of space charge accumulation in the quantum dot-emitting region due to the improved charge balance [26], (ii) the thicker buffer layer can reduce the plasma damage during ITO deposition process [19].
Fig. 1. (a) Schematic diagram of the transparent inverted QLEDs. (b) An energy-band diagram of the materials and the device configuration.
According to the J-V characteristics shown in Fig. 2(a), the current density of the device gradually decreases with the increase of HAT-CN thickness, indicating that the increase of HAT-CN thickness reduces the hole injection through the anode ITO and increases the series resistance of the device. In addition, we also notice that the 15 nm HAT-CN has a significant high density current at low voltage, which indicates that the thinner buffer layer may be more vulnerable to plasma sputtering damage, thus resulting in a leakage current path [19]. The details of EQE as a function of current density of the inverted Tr-QLEDs with different HAT-CN thickness have been shown in Fig. 2(b)-(d).
Fig. 2. (a) Current density-voltage (J-V) characteristics and EQE of the inverted Tr-QLEDs with different HAT-CN thickness at (b) 15 nm, (c) 30 nm and (d) 45 nm.
Figure 3(a) shows the maximum EQE of Tr-QLEDs with different HAT-CN thicknesses. The total EQE is the sum of bottom and top EQE. For example, as shown in Fig. 3(a), our inverted Tr-QLEDs with 30 nm-thickness-HAT-CN exhibit a EQE of 6.1% for bottom emission, 5.2% for top emission and 11.3% for total emission. Figure 3(b) indicates that the bottom and top EL spectra of inverted Tr-QLED are perfectly coincident and their EL peak is at 628 nm. Transmittance spectra of the bottom (ITO/ZnMgO) and top (ITO/HAT-CN/MoO3/CBP) unit in the inverted Tr-QLEDs is shown in the Fig. 3(c), and the bottom and top transmittance at 628 nm is 87.2% and 81.1%, respectively. The EL range of our Tr-QLEDs is from 570 nm to 680 nm in the Fig. 3(b), and average transmittance of the bottom (ITO/ZnMgO) and top (ITO/HAT-CN/MoO3/CBP) unit in this wavelength region is 86.9% and 81.2%. Therefore, we propose that the transmittance difference between the bottom (ITO/ZnMgO) and top (ITO/HAT-CN/MoO3/CBP) unit in the EL region is a possible reason for the different EQE between bottom and top emission in our inverted Tr-QLEDs as shown in the Fig. 3(a). Moreover, we also can find that the current density decreased, especially the leakage current is eliminated at low voltage, while the EQE increased from 10.7% to 11.3% when thickness of HAT-CN increased from 15 nm to 30 nm (see Fig. 2(a) and 3(a)), which suggests that the thicker buffer layer is more robust to resist the plasma damage and thus it can inhibit leakage current. Nevertheless, the EQE is significantly reduced to 9.3% when the thickness of HAT-CN reaches 45 nm, implying that the more thicker buffer layer could block the hole injection and result in imbalanced charge injection.
Fig. 3. (a) Maximum EQE of the inverted Tr-QLEDs with different HAT-CN thickness. (c) EL spectra of Tr-QLEDs. (d) Transmittance spectra of the bottom (ITO/ZnMgO) and top (ITO/HAT-CN/MoO3/CBP) unit in the inverted Tr-QLEDs.
In addition, we also investigate the morphology of the HAT-CN layers with different thicknesses as shown in the Fig. 4. The root-mean-square (RMS) roughness of the HAT-CN layers with different thicknesses deposited on ITO/ZnMgO/QD/CBP/MoO3 is 2.322 nm, 2.426 nm and 3.900 nm, respectively. The HAT-CN layer with low RMS roughness facilitates the deposited process of high-performance ITO top electrodes.
Fig. 4. Typical AFM images of the HAT-CN layers with different thicknesses in the inverted Tr-QLED: (a) 15 nm, (b) 30 nm and (c) 45 nm.
Figure 5 shows the current density-luminance-voltage (J-L-V) characteristics, current efficiency, EQE and CIE 1931 x-y chromaticity diagram of inverted fully Tr-QLEDs with 30 nm-thickness-HAT-CN buffer layer. Figure 5 exhibits that the maximum luminance of the device is 21200 cd/m2 at 12 V, the peak current efficiency is 14.7 cd/A, and the maximum EQE is 11.3%. Our inverted fully Tr-QLEDs shows the saturated color at (CIEx, CIEy)=(0.67, 0.32) in Fig. 5(d) for both top and bottom emission, and there is no color shift at both emission directions. According to the comparison of Table 1, the EQE of our inverted device is the highest reported so far for Tr-QLEDs . This result further demonstrates that the performance of our inverted Tr-QLEDs is competent to Tr-QLEDs based on conventional architecture, promoting the practical applications of inverted Tr-QLEDs in industry by integrating with TFT circuits. For this inverted Tr-QLEDs, the bottom emission is as bright and efficient as the top emission, because the carriers transporting and capability of light extraction between the bottom and top ITO electrodes are almost the same [8,19]. Nevertheless, other inverted Tr-QLEDs were very different bottom and top emission for luminance and efficiency of device due to the different bottom and top transparent electrodes with different optical-electrical properties [16,22]. The above results show that our ITO electrode and device fabrication process is stable and reliable, and suitable for the inverted structure.
Fig. 5. Performance of the inverted Tr-QLEDs with 30 nm-thickness-HAT-CN buffer layer: (a) luminance-voltage (L-V) characteristics, (b) current efficiency, (c) EQE as a function of current density and (d) CIE coordinates of top and bottom emission.
Table 1. Comparison of our device with reported Tr-QLEDs.
Figure 6(a) displays the transmission spectrum of a fully transparent device. An average transmittance of inverted transparent QLEDs is 75% in the visible region, in particular, it is 78% at 550 nm. The transmittance of transparent device at 550 nm is lower than that of the bottom (ITO/ZnMgO) and top (ITO/HAT-CN/MoO3/CBP)unit in the inverted Tr-QLEDs as shown in the Fig. 3(c), which originates from absorbance of QD film in the device. The illustration of Fig. 6(a) shows two photos of Tr-QLEDs turned off and on under the room light. Due to the high transparency of devices, the logo on the back of the devices can be clearly seen. Figure 6(b) is a photo of Tr-QLEDs when it turned on in dark environment. By the mirror, we can observe both the top and bottom emission at the same time. There is a very bright and uniform top and bottom emission in the Tr-QLEDs. Furthermore, the device lifetime is 0.5 h at 9550 cd/m2 as shown in Fig. 6(c), which corresponds to the device lifetime of 466.5 h at 100 cd/m2 (lifetime × I01.5=const.) [8].
Fig. 6. (a) Transmittance spectra of device. Inset: these devices turn off and on. (b) A photo of Tr-QLEDs turn on in the front of a mirror. (c) Lifetime of inverted Tr-QLEDs.
4. Conclusions
In summary, inverted Tr-QLEDs using HAT-CN as a HIL and buffer dual-functional layer have been developed. A non-transparent inverted device has the following advantages: (i) organic small molecules as HTL are deposited by thermal evaporation thereby avoiding acids PEDOT:PSS from etching the ITO transparent electrode, (ii) it can be directly connected to the drain of N-channel metal oxide TFTs backplane, which can simplify the circuit design and process, (iii) multilayer HTL and HIL can be thermally evaporated continuously without any damage to the underlying layer. In our inverted Tr-QLEDs, an optimized 30 nm-thickness HAT-CN as dual-functional modified layer can facilitate charge injection balance and reduce the plasma sputtering damage from ITO deposition at the same time. The inverted Tr-QLEDs exhibited a high transmittance of 78% at 550 nm, a current efficiency of 14.7 cd/A, and a high EQE of 11.3%. This study indicates that these inverted Tr-QLEDs have the potential for use in energy-saving building, AR, wallpaper display and medical imaging in the future.
Funding
National Key Research and Development Program of China administrated by the Ministry of Science and Technology of China (2016YFB0401702, 2017YFE0120400); National Natural Science Foundation of China (61674074, 61704170, 61875082); Natural Science Foundation of Guangdong Province (2017B030306010); Guangdong University Key Laboratory for Advanced Quantum Dot Displays and Lighting (2017KSYS007); Shenzhen Peacock Team Project (KQTD2016030111203005); Development and Reform Commission of Shenzhen Project ([2017]1395).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. H.-J. Seok, J.-H. Lee, J.-H. Park, S.-H. Lim, and H.-K. Kim, “Transparent Conducting Electrodes for Quantum Dots Light Emitting Diodes,” Isr. J. Chem. 59(8), 729–746 (2019). [CrossRef]
2. L. Liu, K. Cao, S. Chen, and W. Huang, “Toward See-Through Optoelectronics: Transparent Light-Emitting Diodes and Solar Cells,” Adv. Opt. Mater. 8(22), 2001122 (2020). [CrossRef]
3. E. Downing, L. Hesselink, J. Ralston, and R. Macfarlane, “A three-color, solid-state, three-dimensional display,” Science 273(5279), 1185–1189 (1996). [CrossRef]
4. H.-S. Kim, E. Brueckner, J. Song, Y. Li, S. Kim, C. Lu, J. Sulkin, K. Choquette, Y. Huang, R. G. Nuzzo, and J. A. Rogers, “Unusual strategies for using indium gallium nitride grown on silicon (111) for solid-state lighting,” Proc. Natl. Acad. Sci. U. S. A. 108(25), 10072–10077 (2011). [CrossRef]
5. C. Wang, D. Hwang, Z. Yu, K. Takei, J. Park, T. Chen, B. Ma, and A. Javey, “User-interactive electronic skin for instantaneous pressure visualization,” Nat. Mater. 12(10), 899–904 (2013). [CrossRef]
6. M. K. Choi, I. Park, D. C. Kim, E. Joh, O. K. Park, J. Kim, M. Kim, C. Choi, J. Yang, K. W. Cho, J.-H. Hwang, J.-M. Nam, T. Hyeon, J. H. Kim, and D.-H. Kim, “Thermally Controlled, Patterned Graphene Transfer Printing for Transparent and Wearable Electronic/Optoelectronic System,” Adv. Funct. Mater. 25(46), 7109–7118 (2015). [CrossRef]
7. X. Dai, Y. Deng, X. Peng, and Y. Jin, “Quantum-Dot Light-Emitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization,” Adv. Mater. 29(14), 1607022 (2017). [CrossRef]
8. M. K. Choi, J. Yang, D. C. Kim, Z. Dai, J. Kim, H. Seung, V. S. Kale, S. J. Sung, C. R. Park, N. Lu, T. Hyeon, and D.-H. Kim, “Extremely Vivid, Highly Transparent, and Ultrathin Quantum Dot Light-Emitting Diodes,” Adv. Mater. 30(1), 1703279 (2018). [CrossRef]
9. P. Shen, F. Cao, H. Wang, B. Wei, F. Wang, X. W. Sun, and X. Yang, “Solution-Processed Double-Junction Quantum-Dot Light-Emitting Diodes with an EQE of Over 40%,” ACS Appl. Mater. Interfaces 11(1), 1065–1070 (2019). [CrossRef]
10. H. Shen, Q. Gao, Y. Zhang, Y. Lin, Q. Lin, Z. Li, L. Chen, Z. Zeng, X. Li, Y. Jia, S. Wang, Z. Du, L. S. Li, and Z. Zhang, “Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency,” Nat. Photonics 13(3), 192–197 (2019). [CrossRef]
11. X. Qu, N. Zhang, R. Cai, B. Kang, S. Chen, B. Xu, K. Wang, and X. W. Sun, “Improving blue quantum dot light-emitting diodes by a lithium fluoride interfacial layer,” Appl. Phys. Lett. 114(7), 071101 (2019). [CrossRef]
12. L. Wang, J. Lin, Y. Hu, X. Guo, Y. Lv, Z. Tang, J. Zhao, Y. Fan, N. Zhang, Y. Wang, and X. Liu, “Blue Quantum Dot Light-Emitting Diodes with High Electroluminescent Efficiency,” ACS Appl. Mater. Interfaces 9(44), 38755–38760 (2017). [CrossRef]
13. P. Jing, W. Ji, Q. Zeng, D. Li, S. Qu, J. Wang, and D. Zhang, “Vacuum-free transparent quantum dot light-emitting diodes with silver nanowire cathode,” Sci. Rep. 5(1), 12499 (2015). [CrossRef]
14. M. Chrzanowski, M. Banski, and A. Podhorodecki, “Semi-transparent quantum-dot light-emitting diode with ultrathin Au electrode embedded in solution-processed coatings,” Mater. Lett. 257, 126709 (2019). [CrossRef]
15. Z. Li, “High performance of quantum dots light-emitting diodes based on double transparent electrodes,” Microelectron. Int. 35(4), 215–219 (2018). [CrossRef]
16. L. Yao, X. Fang, W. Gu, W. Zhai, Y. Wan, X. Xie, W. Xu, X. Pi, G. Ran, and G. Qin, “Fully Transparent Quantum Dot Light-Emitting Diode with a Laminated Top Graphene Anode,” ACS Appl. Mater. Interfaces 9(28), 24005–24010 (2017). [CrossRef]
17. J.-T. Seo, J. Han, T. Lim, K.-H. Lee, J. Hwang, H. Yang, and S. Ju, “Fully Transparent Quantum Dot Light-Emitting Diode Integrated with Graphene Anode and Cathode,” ACS Nano 8(12), 12476–12482 (2014). [CrossRef]
18. H. Zhang and S. Chen, “An ZnMgO: PVP inorganic- organic hybrid electron transport layer: towards efficient bottom- emission and transparent quantum dot light- emitting diodes,” J. Mater. Chem. C 7(8), 2291–2298 (2019). [CrossRef]
19. W. Wang, H. Peng, and S. Chen, “Highly transparent quantum-dot light-emitting diodes with sputtered indium-tin-oxide electrodes,” J. Mater. Chem. C 4(9), 1838–1841 (2016). [CrossRef]
20. A. Castan, H.-M. Kim, and J. Jang, “All-Solution-Processed Inverted Quantum-Dot Light-Emitting Diodes,” ACS Appl. Mater. Interfaces 6(4), 2508–2515 (2014). [CrossRef]
21. J. Pan, C. Wei, L. Wang, J. Zhuang, Q. Huang, W. Su, Z. Cui, A. Nathan, W. Lei, and J. Chen, “Boosting the efficiency of inverted quantum dot light-emitting diodes by balancing charge densities and suppressing exciton quenching through band alignment,” Nanoscale 10(2), 592–602 (2018). [CrossRef]
22. H.-M. Kim, A. R. B. M. Yusoff, T.-W. Kim, Y.-G. Seol, H.-P. Kim, and J. Jang, “Semi-transparent quantum-dot light emitting diodes with an inverted structure,” J. Mater. Chem. C 2(12), 2259–2265 (2014). [CrossRef]
23. H. Y. Kim, Y. J. Park, J. Kim, C. J. Han, J. Lee, Y. Kim, T. Greco, C. Ippen, A. Wedel, B.-K. Ju, and M. S. Oh, “Transparent InP Quantum Dot Light-Emitting Diodes with ZrO2 Electron Transport Layer and Indium Zinc Oxide Top Electrode,” Adv. Funct. Mater. 26(20), 3454–3461 (2016). [CrossRef]
24. X. Jiang, Z. Song, G. Liu, Y. Ma, A. Wang, Y. Guo, and Z. Du, “AgNWs/AZO composite electrode for transparent inverted ZnCdSeS/ZnS quantum dot light-emitting diodes,” Nanotechnology 31(5), 055201 (2020). [CrossRef]
25. L. Chen, M.-H. Lee, Y. Wang, Y. S. Lau, A. A. Syed, and F. Zhu, “Interface dipole for remarkable efficiency enhancement in all-solution-processable transparent inverted quantum dot light-emitting diodes,” J. Mater. Chem. C 6(10), 2596–2603 (2018). [CrossRef]
26. L. Wang, Y. Lv, J. Lin, Y. Fan, J. Zhao, Y. Wang, and X. Liu, “High-efficiency inverted quantum dot light-emitting diodes with enhanced hole injection,” Nanoscale 9(20), 6748–6754 (2017). [CrossRef]
