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Abstract
Quantum-dot light-emitting diodes (QLEDs) are promising components for next-generation displays and related applications. However, their performance is critically limited by inherent hole-injection barrier caused by deep highest-occupied molecular orbital levels of quantum dots. Herein, we present an effective method for enhancing the performance of QLEDs by incorporating a monomer (TCTA or mCP) into hole-transport layers (HTL). The impact of different monomer concentrations on the characteristics of QLEDs were investigated. The results indicate that sufficient monomer concentrations improve the current efficiency and power efficiency. The increased hole current using monomer-mixed HTL suggests that our method holds considerable potential for high-performance QLEDs.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Quantum dots (QDs) constitute a semiconductor material with a bandgap and can implement both photoluminescence and electroluminescence. In particular, quantum-dot light-emitting diodes (QLEDs) leverage QD electroluminescence and are the primary component of next-generation displays given their high efficiency, color purity, color tunability, and other outstanding features [1–4]. To fully exploit the superior characteristics of QLEDs, the key factor to be considered is the charge balance between injected holes and electrons. However, QDs generally have a deep highest-occupied molecular orbital (HOMO) level, which results in significant charge imbalance between rich electrons and poor holes. The charge imbalance increases non-radiative recombination, reducing the device performance in terms of light efficiency and lifespan [5–7].
Two approaches have been proposed to solve the charge imbalance in QLEDs. The first approach is inhibiting the excess electrons being injected. The second approach is strengthening hole injection. Studies on inhibiting excess electrons injected into the emissive layer (EML) have focused on changing the composition of the electron transport layer (ETL) [8–11], treating the ETL surface [12,13], and inserting an interlayer as an injection barrier into the surface of the EML and ETL [14–16]. However, suppression of the electrons being injected could reduce the current density and increase the driving voltage, which would in turn degrade the performance of the device.
Strengthening hole injection has been extensively studied, with the most widely known method being the application of an inverted structure [17]. It entails coating the ETL/EML using a solution process and subsequently depositing the monomer for organic LEDs onto the EML via thermal evaporation to complete the device. Furthermore, studies have been conducted to enhance the injection properties by cross-linking HTL materials [18] or by creating a multilayer structure that involves the hole injection layer (HIL) and HTL to control the hole injection barrier [19–21]. Alternatively, hole injection has been enhanced by controlling the HTL properties using various additives such as Lewis acid [22], Lithium-TFSI [23], 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) [24,25], 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP) [26,27], and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) [28] within the HTL. In particular, the wherein additives were mixed and applied to the HTL have shown that the hole injection into the EML is strengthened and that the device characteristics are improved. However, it is difficult to accurately analyze and determine whether the improvement in device characteristics is because of the change in the hole mobility of the HTL or because of the change in the injection barrier from HTL to EML.
In this study, we combined two different monomers, namely, 4,4’,4''-tris(N-carbazolyl)-triphenylamine (TCTA) and 1,3-Di-9-carbazolylvenzene (mCP), with poly(N-vinylcarbazole) (PVK) to enhance the device characteristics by strengthening the hole injection. Subsequently, the combinations were applied to the HTL, and the devices were evaluated. To enhance the hole injection into the EML, we considered both the application of the HTL with high hole mobility and the reduction of the injection barrier owing to the formation of a gradual energy level. As the monomer was deposited via a solution process rather than thermal deposition, the impacts of the material on the process, such as solubility or glass transition temperature, were considered. Moreover, the effects of related phenomena on the device properties were also analyzed. When TCTA and mCP were mixed with PVK and applied as HTLs, the charge balance of the device improved with the strengthened hole injection. Consequently, the current and power efficiency was improved by 24% and 52%, respectively.
2. Experimental section
2.1 Fabrication of QLEDs
The structure of the QLED reference device used in the experiment is shown in Fig. 1(a). An indium tin oxide (ITO)-patterned glass substrate was used to fabricate the device. The substrate was sequentially cleaned using a sonicator with acetone/isopropyl alcohol/deionized water for 15 min before fabrication and then dried for approximately 1 h in an oven at 100 °C. Subsequently, thermal treatment was performed for 1 h in an air atmosphere inside a furnace at 350 °C. Upon applying ultraviolet/ozone surface treatment for 5 min to obtain a uniform deposition, the substrate was placed in a glove box in an Ar atmosphere to fabricate the device with the structure shown in Fig. 1(a). Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, AI4083, HERAEUS) was used as a HIL by spin coating at 2000rpm for 60 s before annealing at 120 °C for 30 min. PVK (2 mg/ml in chloroform, Sigma Aldrich Co.) was used as an HTL by coating at 4000 rpm for 30 s and annealing at 100 °C for 30 min. To deposit the HTL mixed with a monomer, TCTA (Tokyo Chemical Ind. Co., LTD) and mCP (Tokyo Chemical Ind. Co., LTD) were dissolved in chloroform at a concentration of 2 mg/mL, and a solution mixed with 2 mg/mL of PVK in volume ratio was used. The EML of the QD (20 mg/ml in octane, CdSe/ZnS, quantum yield ∼ 90%, ZEUS) was coated at 2000rpm for 60 s and then annealed at 80 °C for 30 min. ZnO (40 mg/ml in Butanol, N-13, Avantama) was used as an ETL by coating at 4000 rpm for 30 s and then annealing at 80 °C for 30 min. The phase and morphology of the nanoparticles of ZnO used in this experiment were analyzed by using an X-ray diffractometer and CS-TEM. The related data are presented in Fig. S1 in Supplement 1. Once spin coating was complete, the device was transferred to a vacuum chamber to form a cathode with LiF (1 nm)/Al (80 nm) in a 10−7 Torr atmosphere. The fabricated QLED devices were then encapsulated using ultraviolet-curable resin and cavity glass to prevent degradation owing to moisture and oxygen. Finally, the device characteristics were evaluated.
Fig. 1. (a) Device structure of multi-layered QLED. (b) Cross-sectional TEM image of the device. (c) Molecular structure of the monomers of TCTA and mCP. (d) Energy level diagram for various layers of the QLEDs.
2.2 Characterization of QLEDs
We used transmission electron microscopy (Cs-TEM, JEM-ARM200F, JEOL Ltd) to verify the cross-section of the fabricated devices and morphologies of the nanoparticles. Furthermore, the absorption and emission spectra were obtained using ultraviolet–visible spectroscopy (Lambda35, Perkin Elmer) and photoluminescence equipment (Photoluminescence, FlouTime300, PicoQuant) to determine the optical properties of the QD emitter. The electro-optical properties were measured using a spectrometer (CS-2000, Konica Minolta) and a digital multimeter (Keithley 2000, Keithly), and a source measurement unit (Keithley 237, Keithley) for voltage sweeping. The surface roughness was observed using an atomic force microscope (XE-150, PSIA), and ultraviolet photoelectron spectroscopy (AXIS SUPRA, Kratos, U.K.) measurements were acquired for energy level analysis. An X-ray diffractometer (D8 Advance, Bruker) was utilized to conduct a phase analysis of the inorganic material.
3. Results and discussions
A red QD with an oleic acid ligand and CdSe/ZnS core-shell structure was used in the experiment. The absorption and photoluminescence (PL) emission spectra are illustrated in Fig. 2(a). A CdSe/ZnS QD has a wide absorption spectrum in the visible region. The peak wavelength of PL is located at 620 nm, and the full width at half maximum has a highly narrow spectrum at approximately 32 nm. As shown in the TEM image in Fig. 2(b), the average size of the CdSe/ZnS QDs was determined to be 8 nm, and it was confirmed that they were spherically shaped with a uniform size distribution.
The structure of the QLED used in the experiment and a transmission electron microscopy image are shown in Figs. 1(a) and 1(b), respectively. QLEDs fabricated by the all-solution process had the structure of ITO/PEDOT:PSS (20 nm)/PVK or PVK:monomer (25 nm)/QDs (20 nm)/ZnO (35 nm)/LiF (1 nm)/Al (80 nm). The hole was injected through the ITO anode, and the electron was inserted into the device through the Al cathode. PEDOT:PSS was used as the HIL, PVK or PVK:monomer as the HTL, QD as the EML, and ZnO as the ETL. The molecular structures of TCTA and mCP mixed with PVK, which constitute the HTL to enhance hole injection, are shown in Fig. 1(c), and the flat-band energy level diagram including TCTA and mCP is shown in Fig. 1(d). The HOMO and lowest unoccupied molecular orbital (LUMO) reported in other studies were used to characterize materials [24,25,29]. The hole mobility of PVK used as HTL was 2.5 × 10−6 cm2/V·s [24], which is much slower than that of ZnO (10−3–10−4 cm2/V·s) [25] and disadvantageous for both injection and transport. As TCTA has a shallower HOMO than PVK, it cannot reduce the injection barrier from the HTL to the EML. However, because the material has a very high hole mobility of 4 × 10−4 cm2/V·s [30], it was expected to deliver a hole quickly to the EML. Moreover, mCP, whose HOMO is deeper than that of PVK, can reduce the injection barrier from the HTL to the EML. Hence, mCP was expected to contribute to the improvement of both injection and transport because it has a higher mobility (3.4 × 10−5 cm2/V·s) than that of PVK [31].
To investigate the effect of HTL mixed with TCTA on the device properties, devices were fabricated by mixing TCTA with PVK in concentrations of 10%–40%. Figure 3 shows the performance of the fabricated devices. Figure 3(a) shows that as the mixing ratio of the TCTA increased, the current density in the applied voltage tended to increase along with the luminance. To further clarify the luminescence characteristics in the low-voltage region, the luminance versus voltage in the light turn-on region (∼100 cd/m2) is plotted on the inset graph of Fig. 3(a). It can be confirmed that as the concentration of the TCTA increased, a shift occurred in the direction of a decrease in light turn-on voltage. An increase in current density and decrease in turn-on voltage indicate that hole injection enhanced with the TCTA mixture. The highest improvement in efficiency was obtained for the device mixed with 20% TCTA. The current efficiency for the mixture was 6.56 cd/A, which was 24% higher than that of the PVK-only device (5.29 cd/A). The external quantum efficiency was found to be 4.99%, which represents a 23% improvement compared to the reference, 4.04%. Moreover, when TCTA was mixed, the current, power, and external quantum efficiencies increased. For the 30% TCTA mixture, the current efficiency decreased compared with the 20% TCTA mixture. As hole injection further improved, the effect of reducing the driving voltage increased the power efficiency by 52% compared with the PVK-only device. The detailed properties of the device per condition are summarized in Table 1.
Fig. 3. Electroluminescence (EL) performance of the QLEDs with PVK or PVK:TCTA HTLs. (a) Current density-voltage-luminance (J-V-L) characteristics, inset shows voltage-luminance (V-L) in the range under 100 cd/m2. (b) Current efficiency-current density characteristics. (c) Power efficiency-current density characteristics. (d) External quantum efficiency-current density characteristics.
Table 1. Performance Summary of QLEDs with various HTLsa
Figure 4 shows the electroluminescence spectrum of the PVK:TCTA 20% device by voltage. The peak wavelength of electroluminescence was located at 626 nm, and no changes in the spectrum occurred even when the driving voltage was changed. Moreover, the absence of other emission peaks indicates that excitons were suitably confined inside the QD EML and smoothly recombined. When compared with the PL spectrum in Fig. 2(a), the central wavelength redshifted by approximately 6 nm due to the quantum confined Stark effect in the QD layer [32].
Fig. 4. EL spectra of QLED with PVK:TCTA 20%, inset shows a light-emitting image of the device. (scale bar : 1 mm)
The electroluminescence of the device with PVK:mCP HTL is shown in Fig. 5. Similar to the experiment conducted with the TCTA mixture, by increasing the mixing ratio of mCP, the current density and luminance increased at an applied voltage. The PVK:mCP device exhibits the highest performance for the 30% mCP mixture. Compared with the PVK-only device, the current efficiency was improved by 20% (from 5.38 cd/A to 6.49 cd/A), the power efficiency was improved by 47% (from 3.01 lm/W to 4.43 lm/W) and the external quantum efficiency was improved by 20% (from 4.13% to 4.96%).
Fig. 5. Electroluminescence (EL) performance of the QLEDs with PVK or PVK:mCP HTLs. (a) Current density-voltage-luminance (J-V-L) characteristics, inset shows voltage-luminance (V-L) in the range under 100 cd/m2. (b) Current efficiency-current density characteristics. (c) Power efficiency-current density characteristics. (d) External quantum efficiency-current density characteristics.
The surface roughness was measured with an atomic force microscope to check for changes in the film quality when each of the two monomers was applied to the HTL. The contact interface properties between two layers are critical for determining the properties of a multilayer film device. Suitable interface properties are essential for QLEDs fabricated via spin coating [21,33]. Figure 6 shows the measurement results of surface roughness in the HTL. When roughness root mean square (Rq) of the reference layer (with PVK only) was 0.901 nm, the surface roughness of the PVK:TCTA HTL was 0.668 nm, and the surface roughness of the PVK:mCP layer was 1.268 nm. All of these values indicate that a sufficiently flat surface was obtained when different HTLs were coated via the spin coating process. Nevertheless, the difference in the roughness of thin films may be attributed to the glass transition temperature (Tg) of each monomer [34]. For the TCTA, Tg is extremely high at 155 °C, and the surface is smoothened even if it is mixed with PVK and spin coated before annealing at 100 °C. On the other hand, mCP appears to increase the surface roughness as crystallization occurs in part of the annealing process because Tg is very low at 64 °C.
Fig. 6. AFM images and surface roughness values of different HTLs (a) PVK only. (b) PVK:TCTA 20%. (c) PVK:mCP 20%.
A hole-only device (HOD) was fabricated to observe the behavior of the hole injected into the EML and identify the underlying reason for the improvement in the device properties by doping a monomer into the PVK HTL. The hole-only device had the structure of ITO/PEDOT:PSS (20 nm)/HTLs (25 nm)/EML (20 nm)/MoO3 (1 nm)/Al (50 nm). The structure was used to compare different properties, including hole transport in the HTL and injection in the EML. Figures 7(a) and 7(b) show the hole-only devices of PVK:TCTA and PVK:mCP according to the mixing concentration. An increased current density with the TCTA or mCP concentration in the mixture with PVK can be clearly identified in both devices. Hence, hole injection in the EML was strengthened. This is also consistent with the current–voltage properties of the QLED device, as shown in Figs. 3(a) and 5(a).
Fig. 7. Current density versus voltage characteristics of the hole only devices (HODs) with different HTLs for (a) PVK:TCTA and (b) PVK:mCP.
In addition, to quantitatively compare the hole transport ability owing to the monomer mixture, we extracted mobility from the J-V curve of the HODs. The following equation was utilized to extract mobility (µ).
In the above equation, J refers to the current density in space charge limited current (SCLC) region, and V represents the applied voltage. ɛ0 and ɛr denote the vacuum dielectric constant (8.85 × 10−14 C/V·cm) and the relative dielectric constant, respectively. In this study, an ɛr value of 3 was applied for the organic material. [28] Additionally, d represents the distance between the anode and cathode. Based on the J-V curve of the hole-only device, the calculated mobility values were 8.32 × 10−7 cm2/V·s for the PVK only device, 1.44 × 10−6 cm2/V·s for the PVK:TCTA 20% device, and 1.12 × 10−6 cm2/V·s for the PVK:mCP 20% device. It was observed that the mobility tended to increase as the mixing ratio of TCTA or mCP increased, and detailed results are presented in Fig. S2 in Supplement 1. Consequently, the current density in the HOD increased more significantly for the device with TCTA compared to the device with mCP which can be attributed to the difference in hole mobility between TCTA and mCP.
An energy level analysis was conducted to determine the effect of mixing TCTA and mCP on the energy level of the HTL. Ultraviolet photoelectron spectroscopy (UPS) was performed by fabricating three samples of PVK, PVK:TCTA 20%, and PVK:mCP 20%. The secondary electron cut-off and valence band onset regions are illustrated in Figs. 8(a) and 8(b), respectively. The HOMO, or valence band maximum (VBM), was calculated based on the incident photon energy (21.2 eV), which can be expressed as follows [35]:
Based on the measurement results, the HOMO (VBM) was calculated to be 5.93 eV for PVK, 5.87 eV for PVK:TCTA 20% and 5.95 eV for PVK:mCP 20%. Hence, when TCTA was mixed with PVK, the energy level was upshifted and the injection barrier to the EML increased. In contrast, for the mCP mixture, the energy level was downshifted, reducing the injection barrier to the EML and consequently changing the direction of smoothing hole injection. Hole injection into the EML affected both the injection barrier of the HTL/EML interface and also the hole mobility in the HTL. Therefore, despite the increased injection barrier by mixing TCTA, the rapid hole mobility further strengthened the hole injection of the device containing TCTA compared with the device containing mCP.
Fig. 8. UPS spectra of (a) the secondary electron cutoff region and (b) the valence-band edge region for PVK only, PVK:TCTA 20% and PVK:mCP 20%, respectively.
4. Conclusion
In this study, we propose a high-efficiency, low-power-driving QLED device structure by mixing two different monomers, TCTA and mCP, with PVK. When a TCTA-mixed HTL was applied to the device, the HTL hole mobility increased and EML hole transport accelerated, which improved the current efficiency and power efficiency by 24% and 52%, respectively. Similarly, using an mCP-mixed HTL, improvements were observed in current efficiency and power efficiency by 20% and 47%, respectively, owing to energy level shifting by the reduction in the injection barrier and increase in hole transport. The proposed fabrication method can enhance the device properties by simply mixing monomers with PVK. Hence, we expect this method to be a simple yet effective method applicable to the development and fabrication of QLED devices for application in mobile devices with high efficiency and low power consumption.
Funding
Ministry of Science and ICT, South Korea (2020M3D1A2101801).
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.
Supplemental document
See Supplement 1 for supporting content.
References
1. C. R. Kagan, E. Lifshitz, E. H. Sargent, and D. V. Talapin, “Building devices from colloidal quantum dots,” Science 353, aac5523 (2016). [CrossRef]
2. 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]
3. Y. E. Panfil, M. Oded, and U. Banin, “Colloidal Quantum Nanostructures: Emerging Materials for Display Applications,” Angew. Chem. Int. Ed.Wiley57(16), 4274–4295 (2018). [CrossRef]
4. J. R. Manders, L. Qian, A. Titov, J. Hyvonen, J. Tokarz-Scott, K. P. Acharya, Y. Yang, W. Cao, Y. Zheng, J. Xue, and P. H. Holloway, “High efficiency and ultra-wide color gamut quantum dot LEDs for next generation displays: Quantum dot LEDs for next generation displays,” J. Soc. Inf. Disp. 23(11), 523–528 (2015). [CrossRef]
5. B. S. Mashford, M. Stevenson, Z. Popovic, C. Hamilton, Z. Zhou, C. Breen, J. Steckel, V. Bulovic, M. Bawendi, S. Coe-Sullivan, and P. T. Kazlas, “High-efficiency quantum-dot light-emitting devices with enhanced charge injection,” Nat. Photonics 7(5), 407–412 (2013). [CrossRef]
6. W. K. Bae, Y.-S. Park, J. Lim, D. Lee, L. A. Padilha, H. McDaniel, I. Robel, C. Lee, J. M. Pietryga, and V. I. Klimov, “Controlling the influence of Auger recombination on the performance of quantum-dot light-emitting diodes,” Nat. Commun. 4(1), 2661 (2013). [CrossRef]
7. W.-C. Chao, T.-H. Chiang, Y.-C. Liu, Z.-X. Huang, C.-C. Liao, C.-H. Chu, C.-H. Wang, H.-W. Tseng, W.-Y. Hung, and P.-T. Chou, “High efficiency green InP quantum dot light-emitting diodes by balancing electron and hole mobility,” Commun. Mater. 2(1), 96 (2021). [CrossRef]
8. A. Alexandrov, M. Zvaigzne, D. Lypenko, I. Nabiev, and P. Samokhvalov, “Al-, Ga-, Mg-, or Li-doped zinc oxide nanoparticles as electron transport layers for quantum dot light-emitting diodes,” Sci. Rep. 10(1), 7496 (2020). [CrossRef]
9. B. Kim, D. Lee, B. Hwang, Y. Eun, M.-Y. Ha, and C. K. Kim, “High Performance Top-Emission Quantum Dot Light-Emitting Diodes with Mg-Doped ZnO Nanoparticles Used as an Electron Transport Layer,” J. Nanosci. Nanotechnol. 21(7), 3747–3752 (2021). [CrossRef]
10. J. Jing, L. Lin, K. Yang, H. Hu, T. Guo, and F. Li, “Highly efficient inverted quantum dot light-emitting diodes employing sol-gel derived Li-doped ZnO as electron transport layer,” Org. Electron. 103, 106466 (2022). [CrossRef]
11. N. N. Mude, H. I. Yang, T. T. Thuy, and J. H. Kwon, “Performance enhancement by sol-gel processed Ni-doped ZnO layer in InP-based quantum dot light-emitting diodes,” Org. Electron. 112, 106696 (2023). [CrossRef]
12. D. S. Chung, T. Davidson-Hall, G. Cotella, Q. Lyu, P. Chun, and H. Aziz, “Significant Lifetime Enhancement in QLEDs by Reducing Interfacial Charge Accumulation via Fluorine Incorporation in the ZnO Electron Transport Layer,” Nanomicro Lett. 14(1), 212 (2022). [CrossRef]
13. S.-Y. Yoon, Y.-J. Lee, H. Yang, D.-Y. Jo, H.-M. Kim, Y. Kim, S. M. Park, S. Park, and H. Yang, “Performance Enhancement of InP Quantum Dot Light-Emitting Diodes via a Surface-Functionalized ZnMgO Electron Transport Layer,” ACS Energy Lett. 7(7), 2247–2255 (2022). [CrossRef]
14. K. Ding, H. Chen, L. Fan, B. Wang, Z. Huang, S. Zhuang, B. Hu, and L. Wang, “Polyethylenimine Insulativity-Dominant Charge-Injection Balance for Highly Efficient Inverted Quantum Dot Light-Emitting Diodes,” ACS Appl. Mater. Interfaces 9(23), 20231–20238 (2017). [CrossRef]
15. N. N. Mude, S. J. Kim, R. Lampande, and J. H. Kwon, “An efficient organic and inorganic hybrid interlayer for high performance inverted red cadmium-free quantum dot light-emitting diodes,” Nanoscale Adv. 4(3), 904–910 (2022). [CrossRef]
16. J. Yun, J. Kim, H.-K. Jang, K. J. Lee, J. H. Seo, B. J. Jung, G. Kim, and J. Kwak, “Controlling charge balance using non-conjugated polymer interlayer in quantum dot light-emitting diodes,” Org. Electron. 50, 82–86 (2017). [CrossRef]
17. J. Kwak, W. K. Bae, D. Lee, I. Park, J. Lim, M. Park, H. Cho, H. Woo, D. Y. Yoon, K. Char, S. Lee, and C. Lee, “Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting Diodes Using an Inverted Device Structure,” Nano Lett. 12(5), 2362–2366 (2012). [CrossRef]
18. W. Sun, L. Xie, X. Guo, W. Su, and Q. Zhang, “Photocross-Linkable Hole Transport Materials for Inkjet-Printed High-Efficient Quantum Dot Light-Emitting Diodes,” ACS Appl. Mater. Interfaces 12(52), 58369–58377 (2020). [CrossRef]
19. J. Chen, D. Song, S. Zhao, B. Qiao, W. Zheng, and Z. Xu, “Highly efficient all-solution processed blue quantum dot light-emitting diodes based on balanced charge injection achieved by double hole transport layers,” Org. Electron. 94, 106169 (2021). [CrossRef]
20. J. H. Hwang, J. Kim, B. J. Kim, M. Park, Y. W. Kwon, M. An, D. Y. Shin, J. M. Jeon, J. Y. Kim, W. Lee, J. Lim, and D. Lee, “Hole injection of quantum dot light-emitting diodes facilitated by multilayered hole transport layer,” Appl. Surf. Sci. 558, 149944 (2021). [CrossRef]
21. H. Chen, K. Ding, L. Fan, W. Liu, R. Zhang, S. Xiang, Q. Zhang, and L. Wang, “All-Solution-Processed Quantum Dot Light Emitting Diodes Based on Double Hole Transport Layers by Hot Spin-Coating with Highly Efficient and Low Turn-On Voltage,” ACS Appl. Mater. Interfaces 10(34), 29076–29082 (2018). [CrossRef]
22. M. Yang, Q. Zhang, W. Zhang, Z. Hao, Y. Qin, X. Hai, F. Li, D. Zhou, and Y. Zhang, “Enhanced hole transport by doping of a lewis acid to Poly(9-vinylcarbazole) for high efficient quantum dot light-emitting diodes,” Org. Electron. 85, 105875 (2020). [CrossRef]
23. Y.-L. Shi, F. Liang, Y. Hu, X.-D. Wang, Z.-K. Wang, and L.-S. Liao, “High-efficiency quantum dot light-emitting diodes employing lithium salt doped poly(9-vinlycarbazole) as a hole-transporting layer,” J. Mater. Chem. C 5(22), 5372–5377 (2017). [CrossRef]
24. J. Pan, J. Chen, Q. Huang, L. Wang, and W. Lei, “A highly efficient quantum dot light emitting diode via improving the carrier balance by modulating the hole transport,” RSC Adv. 7(69), 43366–43372 (2017). [CrossRef]
25. S.-K. Kim, H. Yang, and Y.-S. Kim, “Control of carrier injection and transport in quantum dot light emitting diodes (QLEDs) via modulating Schottky injection barrier and carrier mobility,” J. Appl. Phys. 126(18), 185702 (2019). [CrossRef]
26. Y. Fang, P. Bai, J. Li, B. Xiao, Y. Wang, and Y. Wang, “Highly Efficient Red Quantum Dot Light-Emitting Diodes by Balancing Charge Injection and Transport,” ACS Appl. Mater. Interfaces 14(18), 21263–21269 (2022). [CrossRef]
27. Q. Huang, J. Pan, Y. Zhang, J. Chen, Z. Tao, C. He, K. Zhou, Y. Tu, and W. Lei, “High-performance quantum dot light-emitting diodes with hybrid hole transport layer via doping engineering,” Opt. Express 24(23), 25955 (2016). [CrossRef]
28. X. Lin, X. Wu, J. Zheng, H. Rui, Z. Zhang, Y. Hua, and S. Yin, “Enhanced Performance of Green Perovskite Quantum Dots Light-Emitting Diode Based on Co-Doped Polymers as Hole Transport Layer,” IEEE Electron Device Lett. 40(9), 1479–1482 (2019). [CrossRef]
29. B. S. Kim and J. Y. Lee, “Engineering of Mixed Host for High External Quantum Efficiency above 25% in Green Thermally Activated Delayed Fluorescence Device,” Adv. Funct. Mater. 24(25), 3970–3977 (2014). [CrossRef]
30. S. Noh, C. K. Suman, Y. Hong, and C. Lee, “Carrier conduction mechanism for phosphorescent material doped organic semiconductor,” J. Appl. Phys. 105(3), 033709 (2009). [CrossRef]
31. Y. Tao, C. Yang, and J. Qin, “Organic host materials for phosphorescent organic light-emitting diodes,” Chem. Soc. Rev. 40(5), 2943 (2011). [CrossRef]
32. V. Wood, M. J. Panzer, J.-M. Caruge, J. E. Halpert, M. G. Bawendi, and V. Bulović, “Air-Stable Operation of Transparent, Colloidal Quantum Dot Based LEDs with a Unipolar Device Architecture,” Nano Lett. 10(1), 24–29 (2010). [CrossRef]
33. 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]
34. Y. Tsuchiya, N. Nakamura, S. Kakumachi, K. Kusuhara, C.-Y. Chan, and C. Adachi, “A convenient method to estimate the glass transition temperature of small organic semiconductor materials,” Chem. Commun. 58(80), 11292–11295 (2022). [CrossRef]
35. J.-H. Kim, C.-Y. Han, K.-H. Lee, K.-S. An, W. Song, J. Kim, M. S. Oh, Y. R. Do, and H. Yang, “Performance Improvement of Quantum Dot-Light-Emitting Diodes Enabled by an Alloyed ZnMgO Nanoparticle Electron Transport Layer,” Chem. Mater. 27(1), 197–204 (2015). [CrossRef]
