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Highly flexible light emitting diodes based on a quantum dots-polymer composite emitting layer
Vacuum 163 (2019) 282–286
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Highly flexible light emitting diodes based on a quantum dots-polymer composite emitting layer
T
Lu Xue1, Yang Liu1, Fushan Li∗, Kai Sun, Wei Chen, Kaiyu Yang, Hailong Hu, Jintang Lin, Huipeng Chen, Zunxian Yang, Tailiang Guo Institute of Optoelectronic Technology, Fuzhou University, Fuzhou, 350116, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flexible PMMA Quantum dots Charge transport
In this work, we report highly flexible QLEDs based on a quantum dots-polymer composite emitting layer. Conventional quantum dot films exhibit cracks after rough bending. Herein, polymethyl methacrylate (PMMA) was doped into quantum dot solution, acting as binder for neighboring quantum dots. In addition, PMMA can be serving as an electron blocking layer due to its insulating property, and then balancing the charge recombination and improving device performance. The flexible QLEDs have excellent mechanical properties with an ultimate bend radius of 1.5 mm, and they show no significant performance degradation after 500 times bending with radius of 4 mm. The flexible QLEDs exhibit high performance with maximum luminance of 14880 cd/m2 and maximum current efficiency of 21.3 cd/A.
1. Introduction Colloidal quantum dots (QD) are expected to be an excellent luminescent materials because of their unique quantum confinement effect. The quantum confinement effect is manifested in that people can realize the spectral adjustability according to actual needs by changing the size distribution of quantum dots and the composition of quantum dots [1–6]. In addition, QDs have important advantages in the display and lighting fields, because of their high color purity of QD [maximum full width at half maximum (FWHM), close to 10 nm], low-cost solution processability, high quantum yield (close to 100%), and excellent light stability, which make quantum dot light-emitting diodes (QLEDs) a candidate in the next generation displays [7–9]. And these features can be well compatible with flexible substrates, enabling flexible displays, collapsible displays, stretchable displays, and wearable displays [10–12]. Since the first light-emitting diode using CdSe quantum dots was reported in 1994 [13], the device performance have achieved rapid progress [14]. Although significant improvements have been obtained in device performance, these are based on rigid substrates, such as common glass substrates, which make limitations in the smart, foldable and wearable displays. Flexible QLEDs with light weight, thinner thickness, and compatibility to roll-to-roll manufacturing methods, should be developed for the next generation displays [15–17]. Recently, Zhao et al. [19] reported highly flexible light-emitting
devices by utilizing silver nanowire based polymer electrodes as anodes, and CH3NH3PbBr3 quantum dots as the emissive layer. The flexible device showed no discernible drop in performance after 1000 times bending with radius of 4 mm. Shen et al. [20] reported the fabrication of an efficient flexible white QLED with mixed red, green and blue QDs as emitters by an all-solution process. The current efficiency can reach 10.5 cd/A and the brightness can reach 3554 cd/m2. Even though some works regarding the improvement of QLED device performance have been reported, there are still some key problems that should be solved, such as low stability, sharp decrease in current efficiency at high current densities, and low mechanical properties [18]. In this work, we fabricated QLEDs with high flexibility and strong mechanical properties on the polyethylene terephthalate (PET) substrates coated with indium tin oxide (ITO) by using solution process [21]. Unlike our previous work concerning the reduction of electron transport capability by doping polymer in ZnO nanoparticles, herein, insulating polymethylmethacrylate (PMMA) was introduced into the quantum dot solution with composite solvent in order to alleviate the loss resulting from slits, even the microcracks, that would emerge when the functional layers were repeatedly bended with a small curvature radius. Moreover, PMMA can serve as an electron blocking layer because of its insulating property, leading to the balance of the charge transport, and improving performance of flexible devices. ZnO nanoparticles (ZnO NPs) are generally well represented as
∗
Corresponding author. E-mail address: [email protected] (F. Li). 1 These authors contribute equally to this work. https://doi.org/10.1016/j.vacuum.2019.02.033 Received 11 January 2019; Received in revised form 30 January 2019; Accepted 19 February 2019 Available online 20 February 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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electron transport layers in QLED devices. Based on the previous work [22], we synthesized ZnO nanoparticles using a method that was suitable for our work and slightly modified. Firstly, 0.59 g of zinc acetate dihydrate was dissolved in 25 mL methanol solution, and 0.3 g of KOH was dissolved in 13 mL methanol solution, and then both were placed in a water bath heating apparatus, heated at 62 °C and vigorously stirred until completely dissolve. Then, the KOH solution was dropped into the zinc acetate dihydrate solution at a constant rate using a plastic dropper. Because the droplet velocity affected the particle size of the asgenerated ZnO nanoparticles, it is important to control the dropping speed. Next, after heating at 63 °C under constant temperature and magnetic stirring for 2.5 h, the solution became a turbid white ZnO nanoparticle liquid. Finally, the solution was taken to static settlement at room temperature and the supernatant was separated, the precipitate was washed twice with methanol, and the clean precipitate was obtained by centrifugation at 1500 rpm using a centrifuge, and the precipitate was dispersed in 14 ml of n-Butanol and 1 ml of chloroform to get the concentration of 6 mg/ml ZnO nanoparticle solution. The QD solution was dispersed in n-Octane solution at the concentration of 15 mg/ml, and an appropriate amount of PMMA solution which was dissolved in chloroform at 4 mg/ml was added into former to obtain quantum dots-polymer composite solution with a suitable gradient ratio. The flexible QLEDs device structure was composed of PET/ITO/poly (ethylenedioxythiophene): polystyrene sulphonate (PEDOT: PSS)/poly (9,9-dioctylfluorene-co-N-(4-Butylphenyl)diphenylamine) (TFB)/ ZnCdSeS QD (green)/zinc oxide (ZnO)/Ag, wherein PEDOT:PSS was used as a hole injection layer, TFB was used as a hole transport layer, QD were used as a light-emitting layer, and ZnO was used as an electron transport layer. The flexible PET substrate was purchased from Csgholding Co. Ltd. in Shenzhen. The thickness of the PET substrate was 125 μm, and the surface of the substrate was covered with an ITO film with the thickness of 350 Å. The sheet resistance of the ITO film was 150 Ω/sq. First of all, the PET film was cleaned by acetone, ethanol, and deionized water for 10 min, respectively, and dried in an oven at 60 °C. Before spin coating PEDOT: PSS, the substrate was subjected to oxygen plasma to increase the hydrophilicity of the PET/ITO substrate. Then, PEDOT:PSS was spin-coated at 3000 rpm for 40 s and heat-annealed on a heating table at 120 °C for 15 min. The sample was then transferred to a glove box, and TFB (8 mg/ml, toluene) was spin-coated at 3000 rpm for 40 s. It was baked at 120 °C for 15 min. The QDs was spin-coated at 2000 rpm for 40 s, and baked on a hot plate at 60 °C for 20 min. The ZnO NPs solution as the electron transport layer was spincoated at 1700 rpm for 40 s and baked at 60 °C for about 20 min. Finally, an Ag cathode with a thickness of 100 nm was deposited by using thermal evaporation at a vacuum of 2.2 × 10−3 Pa or less. The electroluminescence (EL) spectra of the QLEDs were measured with a fiber optic spectrometer (Ocean Optics USB2000+). The surface morphology of the film was characterized by using an atomic force microscope (AFM, Bruker Multimode 8), by which the surface roughness was obtained as well. The luminance intensity of the device was obtained with a Topcon SR-3A spectroradiometer. The current-voltage (IeV) characteristics were obtained with a semiconductor characterization system (Keithley 4200-SCS). All samples were working under atmospheric conditions and all test were performed without encapsulation. The schematic device structure and energy band diagram of the flexible QLEDs are shown in Fig. 1(a) and (b), respectively. Here, the flexible QLED devices were fabricated with a multilayer structure PET/ ITO/PEDOT:PSS(30 nm)/TFB(30 nm)/QD (20 nm)/ZnO NPs (30 nm)/ Ag(100 nm). The film quality, energy level and the matching between functional layers would significantly affect the device performance. In Fig. 1(b), it is indicated that TFB utilized its highest occupied molecular orbital (HOMO) to achieve hole injection. Although effective holes were injected into the EML, there is a clear difference in energy levels at the interface between the HTL and the EML. On the other hand, the ZnO
Fig. 1. (a) Schematic of the device structure for the flexible QLEDs. (b) Energy band diagram of the flexible QLEDs.
NPs were used as ETL due to their high efficient electron mobility, which lead to the imbalance charge injection in the EML and rapid degradation of device performance [23–25]. In order to obtain balanced electron and hole injection in EML, we introduce insulating polymer PMMA into QD solution. Poly-TPD was widely used as hole transport layer material with a HOMO of −5.1 eV. With the presence of PMMA in EML, the holes can be better confined in the EML [26,27]. When the doped polymer reached a suitable amount, due to its good insulation, it can act as an electron blocking layer. Therefore, it can achieve a balance of charge transport, increase the probability of radiative recombination, and increase the brightness of devices. As a result, current efficiency and external quantum efficiency were enhanced, achieving the aim of optimizing the performance of devices [28,29]. In order to introduce PMMA into EML, we used the method of composite solvent. QDs and PMMA were dissolved in a hybrid solved composing n-octane and chloroform. The PMMA (dissolved in chloroform) was doped into the QD solution (dissolved in n-octane) according to the doping specific gravity of 0 wt%, 0.8 wt%, 1.3 wt%, 1.8 wt%, 2.5 wt%, 3.3 wt%, respectively. As shown in Fig. 2, AFM observation was performed for the films based on QDs/PMMA nanocomposite with the PMMA doping ratio of 0 wt%, 1.3 wt%, 2.5 wt%, and 3.3 wt%, respectively. The corresponding root-mean-square surface roughness (RMS) were 1.84 nm, 1.94 nm, 1.42 nm, and 1.16 nm, respectively. It can be seen that with the increase of PMMA doping ratio, the film RMS increases as well. When the PMMA doping ratio is 1.3 wt%, the film roughness reaches a maximum of 1.94 nm, and then the film RMS shows a downward trend. When a polymer is blended with spherical semiconductor quantum dots, the long-chain polymer forms depressed area and bulgy area on the surface, resulting in the increase of the surface roughness. However, with the increase of the amount of polymer, the extra long-chain polymer fills the gap between the spherical quantum dots, eventually leading to a smooth surface. Due to the insulating nature of the polymer, it is impossible to further increase the amount of PMMA with the aim of reducing surface roughness. The EL spectra of the QLED devices are shown in Fig. 3(a), with a full width at half maximum (FWHM) of about 22.07 nm and a luminescence peak at 531 nm, which was at the saturated green peak position. The inset is the optical images of a glass-based device and a PETbased device, respectively. The flexible QLED was operated in bent state with a luminous area of 2 × 3 mm2. Fig. 3(b) showed the current density–voltage-luminance (J–V-L) characteristics for the devices based on glass substrate with six different doping ratios. It is indicated that with the increase of proportion of doped PMMA, the device performance was improved significantly. At the doping ratio of 2.5 wt%, the current efficiency reached the maximum of 28.95 cd/A, and EQE reached the maximum of 6.7%, which is twice as the device without doping. Meanwhile, the device obtained the maximum brightness of 325752 cd/m2. Based on this, it was confirmed that doping PMMA into EML indeed obtained significant improvement in device performance, and the optimal doping ratio was 2.5 wt%. Moreover, we successfully fabricated the flexible QLED devices on PET substrate, whose J-V-L characteristics 283
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Fig. 2. (a)–(d) AFM images of QD films with 0 wt%, 1.3 wt%, 2.5 wt%, and 3.3 wt% PMMA dopant ratio, respectively.
with PMMA doping showed longer lifetime than those without PMMA. Apart from good optoelectronic performance, high flexibility and strong mechanical properties are also desirable for flexible QLEDs. As shown in Fig. 5(a), at a constant voltage of 4.5 V, the device was
were shown in Fig. 4(a). In comparison with the device without PMMA doping, the flexible QLEDs doping with PMMA exhibited enhanced device performance with maximum luminance of 14880 cd/m2, maximum current efficiency of 21.3 cd/A. In addition, the flexible QLEDs
Fig. 3. (a) EL spectrum of the green flexible QLED. Inset are the optical image of the glass-based device and PET-based device. (b) Current density–voltage-luminance (J–V-L) characteristics; (c) current efficiency-luminance characteristics; (d) external quantum efficiency (EQE)-current density characteristics of the flexible QLED devices with various PMMA contents. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 284
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Fig. 4. (a) Current density–voltage-luminance (J–V-L) characteristics of different flexible QLED devices. (b) Current efficiency-current density characteristics. (c) Stability test of the flexible QLEDs, L/L0 (L, instantaneous device luminance; L0, initial device luminance)-time characteristics.
Fig. 5. (a) Luminance of flexible QLEDs as a function of bending radius, and inset image is the working flexible QLEDs with a curvature radius of 1.5 mm, 4 mm, 6 mm, 8 mm, 10 mm, respectively. (b) L/L0 (L, instantaneous device luminance; L0, initial device luminance) as a function of bending cycles with a curvature radius of 4 mm.
doping PMMA into QD solution, the long-chain polymer acted as a binder during the bending process, slowing down the performance loss of the functional layers, protecting the QDs from directly contacting with moisture and oxygen, and improving the flexibility and mechanical properties of the devices. In conclusion, we fabricated flexible QLED devices with strong mechanical properties by doping long-chain polymer PMMA to QD emission layer. Based on the quantum dot-polymer composite emitting layer, the efficiency loss of the functional layers for flexible QLEDs was reduced after repeated bending at a small curvature radius. Herein, PMMA not only played a role as the binder for neighboring quantum dot, but also acted as an electron blocking layer to balance charge transport and improve the device performance. The flexible QLEDs have an ultimate bending radius of 1.5 mm, and after 500 times bending with the curvature radius of 4 mm, the device performance showed no significant degradation. It is indicated that this strategy holds promising for potential applications in next generation flexible lighting and display field.
subjected to a bending test with different curvature radius. And the curvature radius were set at 1.5 mm, 4 mm, 6 mm, 8 mm, and 10 mm, respectively. The flexible devices could steadily work give bright green emission under various bending conditions, even with the extremely small curvature radius of 1.5 mm. Fig. 5(b) showed the L/L0 (L, instantaneous device luminance; L0, initial device luminance) as a function of bending cycles under the curvature radius of 4 mm. It can be observed that in comparison with the device without PMMA doping, the flexible QLED doping with 2.5 wt% PMMA can still maintain L/L0 of 67% after 500 times repeated bending at a constant voltage of 4.5 V. And the degradation speed was significantly smaller than that of the device without doping. The PET substrate itself had high water oxygen permeability, moreover, PEDOT:PSS as HIL was an organic material whose solvent was water and it also had acidity and hygroscopicity. In addition, when the device was repeatedly bent with a small curvature radius, the functional layers of QLEDs had serious loss in device performance due to the presence of gaps or even slightly cracks, which resulted the unbalanced charge transport [30,31]. However, after 285
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China (Grants U1605244), the National Key Research and Development Program of China (Grant 2016YFB0401600) and the Natural Science Foundation of Fujian Province (2016J01296). References [1] Y. Fu, D. Kim, H. Moon, H. Yang, H. Chae, Hexamethyldisilazane-mediated, fullsolution-processed inverted quantum dot-light-emitting diodes, J. Mater. Chem. C 5 (3) (2017) 522–526. [2] W. Ji, T. Wang, B. Zhu, H. Zhang, R. Wang, D. Zhang, L. Chen, Q. Yang, H. Zhang, Highly efficient flexible quantum-dot light emitting diodes with an ITO/Ag/ITO cathode, J. Mater. Chem. C 5 (18) (2017) 4543–4548. [3] J. Pan, J. Chen, Q. Huang, Q. Khan, X. Liu, Z. Tao, W. Lei, F. Xu, Z. Zhang, Flexible quantum dot light emitting diodes based on ZnO nanoparticles, RSC Adv. 5 (100) (2015) 82192–82198. [4] C.-J. Chiang, C. Winscom, S. Bull, et al., Mechanical modeling of flexible OLED devices, Org. Electron. 10 (7) (2009) 1268–1274. [5] Y. Liu, F. Li, Z. Xu, et al., Efficient all-solution processed quantum dot light emitting diodes based on inkjet printing technique, ACS Appl. Mater. Interfaces 9 (30) (2017) 25506–25512. [6] K. Yang, F. Li, C.P. Veeramalai, et al., A facile synthesis of CH3NH3PbBr3 perovskite quantum dots and their application in flexible nonvolatile memory, Appl. Phys. Lett. 110 (8) (2017) 083102. [7] B.S. Mashford, M. Stevenson, Z. Popovic, et al., High-efficiency quantum-dot lightemitting devices with enhanced charge injection, Nat. Photon. 7 (5) (2013) 407–412. [8] Y. Yang, Y. Zheng, W. Cao, et al., High-efficiency light-emitting devices based on quantum dots with tailored nanostructures, Nat. Photon. 9 (4) (2015) 259–266. [9] R. Yu, F. Yin, X. Huang, W. Ji, Molding hemispherical microlens arrays on flexible substrates for highly efficient inverted quantum dot light emitting diodes, J. Mater. Chem. C 5 (27) (2017) 6682–6687. [10] H. Chen, J. He, R. Lanzafame, I. Stadler, H.E. Hamidi, H. Liu, J. Celli, M.R. Hamblin, Y. Huang, E. Oakley, Quantum dot light emitting devices for photomedical applications, J. Soc. Inf. Disp. 25 (3) (2017) 177–184. [11] M.K. Choi, J. Yang, K. Kang, D.C. Kim, C. Choi, C. Park, S.J. Kim, S.I. Chae, T.H. Kim, J.H. Kim, Wearable red–green–blue quantum dot light-emitting diode array using high-resolution intaglio transfer printing, Nat. Commun. 6 (2015) 7149. [12] J. Kim, H.J. Shim, J. Yang, M.K. Choi, D.C. Kim, J. Kim, T. Hyeon, D.H. Kim, Ultrathin quantum dot display integrated with wearable electronics, Adv. Mater. 29 (38) (2017) 1700217. [13] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer, Nature 370 (6488) (1994) 354. [14] X. Dai, Z. Zhang, Y. Jin, Y. Niu, H. Cao, X. Liang, L. Chen, J. Wang, X. Peng, Solution-processed, high-performance light-emitting diodes based on quantum dots, Nature 515 (7525) (2014) 96. [15] M.K. Choi, J. Yang, T. Hyeon, D.-H. Kim, Flexible quantum dot light-emitting diodes for next-generation displays, npj Flexible Electron. 2 (1) (2018) 10.
286
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Highly flexible light emitting diodes based on a quantum dots-polymer composite emitting layer
T
Lu Xue1, Yang Liu1, Fushan Li∗, Kai Sun, Wei Chen, Kaiyu Yang, Hailong Hu, Jintang Lin, Huipeng Chen, Zunxian Yang, Tailiang Guo Institute of Optoelectronic Technology, Fuzhou University, Fuzhou, 350116, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flexible PMMA Quantum dots Charge transport
In this work, we report highly flexible QLEDs based on a quantum dots-polymer composite emitting layer. Conventional quantum dot films exhibit cracks after rough bending. Herein, polymethyl methacrylate (PMMA) was doped into quantum dot solution, acting as binder for neighboring quantum dots. In addition, PMMA can be serving as an electron blocking layer due to its insulating property, and then balancing the charge recombination and improving device performance. The flexible QLEDs have excellent mechanical properties with an ultimate bend radius of 1.5 mm, and they show no significant performance degradation after 500 times bending with radius of 4 mm. The flexible QLEDs exhibit high performance with maximum luminance of 14880 cd/m2 and maximum current efficiency of 21.3 cd/A.
1. Introduction Colloidal quantum dots (QD) are expected to be an excellent luminescent materials because of their unique quantum confinement effect. The quantum confinement effect is manifested in that people can realize the spectral adjustability according to actual needs by changing the size distribution of quantum dots and the composition of quantum dots [1–6]. In addition, QDs have important advantages in the display and lighting fields, because of their high color purity of QD [maximum full width at half maximum (FWHM), close to 10 nm], low-cost solution processability, high quantum yield (close to 100%), and excellent light stability, which make quantum dot light-emitting diodes (QLEDs) a candidate in the next generation displays [7–9]. And these features can be well compatible with flexible substrates, enabling flexible displays, collapsible displays, stretchable displays, and wearable displays [10–12]. Since the first light-emitting diode using CdSe quantum dots was reported in 1994 [13], the device performance have achieved rapid progress [14]. Although significant improvements have been obtained in device performance, these are based on rigid substrates, such as common glass substrates, which make limitations in the smart, foldable and wearable displays. Flexible QLEDs with light weight, thinner thickness, and compatibility to roll-to-roll manufacturing methods, should be developed for the next generation displays [15–17]. Recently, Zhao et al. [19] reported highly flexible light-emitting
devices by utilizing silver nanowire based polymer electrodes as anodes, and CH3NH3PbBr3 quantum dots as the emissive layer. The flexible device showed no discernible drop in performance after 1000 times bending with radius of 4 mm. Shen et al. [20] reported the fabrication of an efficient flexible white QLED with mixed red, green and blue QDs as emitters by an all-solution process. The current efficiency can reach 10.5 cd/A and the brightness can reach 3554 cd/m2. Even though some works regarding the improvement of QLED device performance have been reported, there are still some key problems that should be solved, such as low stability, sharp decrease in current efficiency at high current densities, and low mechanical properties [18]. In this work, we fabricated QLEDs with high flexibility and strong mechanical properties on the polyethylene terephthalate (PET) substrates coated with indium tin oxide (ITO) by using solution process [21]. Unlike our previous work concerning the reduction of electron transport capability by doping polymer in ZnO nanoparticles, herein, insulating polymethylmethacrylate (PMMA) was introduced into the quantum dot solution with composite solvent in order to alleviate the loss resulting from slits, even the microcracks, that would emerge when the functional layers were repeatedly bended with a small curvature radius. Moreover, PMMA can serve as an electron blocking layer because of its insulating property, leading to the balance of the charge transport, and improving performance of flexible devices. ZnO nanoparticles (ZnO NPs) are generally well represented as
∗
Corresponding author. E-mail address: [email protected] (F. Li). 1 These authors contribute equally to this work. https://doi.org/10.1016/j.vacuum.2019.02.033 Received 11 January 2019; Received in revised form 30 January 2019; Accepted 19 February 2019 Available online 20 February 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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electron transport layers in QLED devices. Based on the previous work [22], we synthesized ZnO nanoparticles using a method that was suitable for our work and slightly modified. Firstly, 0.59 g of zinc acetate dihydrate was dissolved in 25 mL methanol solution, and 0.3 g of KOH was dissolved in 13 mL methanol solution, and then both were placed in a water bath heating apparatus, heated at 62 °C and vigorously stirred until completely dissolve. Then, the KOH solution was dropped into the zinc acetate dihydrate solution at a constant rate using a plastic dropper. Because the droplet velocity affected the particle size of the asgenerated ZnO nanoparticles, it is important to control the dropping speed. Next, after heating at 63 °C under constant temperature and magnetic stirring for 2.5 h, the solution became a turbid white ZnO nanoparticle liquid. Finally, the solution was taken to static settlement at room temperature and the supernatant was separated, the precipitate was washed twice with methanol, and the clean precipitate was obtained by centrifugation at 1500 rpm using a centrifuge, and the precipitate was dispersed in 14 ml of n-Butanol and 1 ml of chloroform to get the concentration of 6 mg/ml ZnO nanoparticle solution. The QD solution was dispersed in n-Octane solution at the concentration of 15 mg/ml, and an appropriate amount of PMMA solution which was dissolved in chloroform at 4 mg/ml was added into former to obtain quantum dots-polymer composite solution with a suitable gradient ratio. The flexible QLEDs device structure was composed of PET/ITO/poly (ethylenedioxythiophene): polystyrene sulphonate (PEDOT: PSS)/poly (9,9-dioctylfluorene-co-N-(4-Butylphenyl)diphenylamine) (TFB)/ ZnCdSeS QD (green)/zinc oxide (ZnO)/Ag, wherein PEDOT:PSS was used as a hole injection layer, TFB was used as a hole transport layer, QD were used as a light-emitting layer, and ZnO was used as an electron transport layer. The flexible PET substrate was purchased from Csgholding Co. Ltd. in Shenzhen. The thickness of the PET substrate was 125 μm, and the surface of the substrate was covered with an ITO film with the thickness of 350 Å. The sheet resistance of the ITO film was 150 Ω/sq. First of all, the PET film was cleaned by acetone, ethanol, and deionized water for 10 min, respectively, and dried in an oven at 60 °C. Before spin coating PEDOT: PSS, the substrate was subjected to oxygen plasma to increase the hydrophilicity of the PET/ITO substrate. Then, PEDOT:PSS was spin-coated at 3000 rpm for 40 s and heat-annealed on a heating table at 120 °C for 15 min. The sample was then transferred to a glove box, and TFB (8 mg/ml, toluene) was spin-coated at 3000 rpm for 40 s. It was baked at 120 °C for 15 min. The QDs was spin-coated at 2000 rpm for 40 s, and baked on a hot plate at 60 °C for 20 min. The ZnO NPs solution as the electron transport layer was spincoated at 1700 rpm for 40 s and baked at 60 °C for about 20 min. Finally, an Ag cathode with a thickness of 100 nm was deposited by using thermal evaporation at a vacuum of 2.2 × 10−3 Pa or less. The electroluminescence (EL) spectra of the QLEDs were measured with a fiber optic spectrometer (Ocean Optics USB2000+). The surface morphology of the film was characterized by using an atomic force microscope (AFM, Bruker Multimode 8), by which the surface roughness was obtained as well. The luminance intensity of the device was obtained with a Topcon SR-3A spectroradiometer. The current-voltage (IeV) characteristics were obtained with a semiconductor characterization system (Keithley 4200-SCS). All samples were working under atmospheric conditions and all test were performed without encapsulation. The schematic device structure and energy band diagram of the flexible QLEDs are shown in Fig. 1(a) and (b), respectively. Here, the flexible QLED devices were fabricated with a multilayer structure PET/ ITO/PEDOT:PSS(30 nm)/TFB(30 nm)/QD (20 nm)/ZnO NPs (30 nm)/ Ag(100 nm). The film quality, energy level and the matching between functional layers would significantly affect the device performance. In Fig. 1(b), it is indicated that TFB utilized its highest occupied molecular orbital (HOMO) to achieve hole injection. Although effective holes were injected into the EML, there is a clear difference in energy levels at the interface between the HTL and the EML. On the other hand, the ZnO
Fig. 1. (a) Schematic of the device structure for the flexible QLEDs. (b) Energy band diagram of the flexible QLEDs.
NPs were used as ETL due to their high efficient electron mobility, which lead to the imbalance charge injection in the EML and rapid degradation of device performance [23–25]. In order to obtain balanced electron and hole injection in EML, we introduce insulating polymer PMMA into QD solution. Poly-TPD was widely used as hole transport layer material with a HOMO of −5.1 eV. With the presence of PMMA in EML, the holes can be better confined in the EML [26,27]. When the doped polymer reached a suitable amount, due to its good insulation, it can act as an electron blocking layer. Therefore, it can achieve a balance of charge transport, increase the probability of radiative recombination, and increase the brightness of devices. As a result, current efficiency and external quantum efficiency were enhanced, achieving the aim of optimizing the performance of devices [28,29]. In order to introduce PMMA into EML, we used the method of composite solvent. QDs and PMMA were dissolved in a hybrid solved composing n-octane and chloroform. The PMMA (dissolved in chloroform) was doped into the QD solution (dissolved in n-octane) according to the doping specific gravity of 0 wt%, 0.8 wt%, 1.3 wt%, 1.8 wt%, 2.5 wt%, 3.3 wt%, respectively. As shown in Fig. 2, AFM observation was performed for the films based on QDs/PMMA nanocomposite with the PMMA doping ratio of 0 wt%, 1.3 wt%, 2.5 wt%, and 3.3 wt%, respectively. The corresponding root-mean-square surface roughness (RMS) were 1.84 nm, 1.94 nm, 1.42 nm, and 1.16 nm, respectively. It can be seen that with the increase of PMMA doping ratio, the film RMS increases as well. When the PMMA doping ratio is 1.3 wt%, the film roughness reaches a maximum of 1.94 nm, and then the film RMS shows a downward trend. When a polymer is blended with spherical semiconductor quantum dots, the long-chain polymer forms depressed area and bulgy area on the surface, resulting in the increase of the surface roughness. However, with the increase of the amount of polymer, the extra long-chain polymer fills the gap between the spherical quantum dots, eventually leading to a smooth surface. Due to the insulating nature of the polymer, it is impossible to further increase the amount of PMMA with the aim of reducing surface roughness. The EL spectra of the QLED devices are shown in Fig. 3(a), with a full width at half maximum (FWHM) of about 22.07 nm and a luminescence peak at 531 nm, which was at the saturated green peak position. The inset is the optical images of a glass-based device and a PETbased device, respectively. The flexible QLED was operated in bent state with a luminous area of 2 × 3 mm2. Fig. 3(b) showed the current density–voltage-luminance (J–V-L) characteristics for the devices based on glass substrate with six different doping ratios. It is indicated that with the increase of proportion of doped PMMA, the device performance was improved significantly. At the doping ratio of 2.5 wt%, the current efficiency reached the maximum of 28.95 cd/A, and EQE reached the maximum of 6.7%, which is twice as the device without doping. Meanwhile, the device obtained the maximum brightness of 325752 cd/m2. Based on this, it was confirmed that doping PMMA into EML indeed obtained significant improvement in device performance, and the optimal doping ratio was 2.5 wt%. Moreover, we successfully fabricated the flexible QLED devices on PET substrate, whose J-V-L characteristics 283
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Fig. 2. (a)–(d) AFM images of QD films with 0 wt%, 1.3 wt%, 2.5 wt%, and 3.3 wt% PMMA dopant ratio, respectively.
with PMMA doping showed longer lifetime than those without PMMA. Apart from good optoelectronic performance, high flexibility and strong mechanical properties are also desirable for flexible QLEDs. As shown in Fig. 5(a), at a constant voltage of 4.5 V, the device was
were shown in Fig. 4(a). In comparison with the device without PMMA doping, the flexible QLEDs doping with PMMA exhibited enhanced device performance with maximum luminance of 14880 cd/m2, maximum current efficiency of 21.3 cd/A. In addition, the flexible QLEDs
Fig. 3. (a) EL spectrum of the green flexible QLED. Inset are the optical image of the glass-based device and PET-based device. (b) Current density–voltage-luminance (J–V-L) characteristics; (c) current efficiency-luminance characteristics; (d) external quantum efficiency (EQE)-current density characteristics of the flexible QLED devices with various PMMA contents. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 284
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Fig. 4. (a) Current density–voltage-luminance (J–V-L) characteristics of different flexible QLED devices. (b) Current efficiency-current density characteristics. (c) Stability test of the flexible QLEDs, L/L0 (L, instantaneous device luminance; L0, initial device luminance)-time characteristics.
Fig. 5. (a) Luminance of flexible QLEDs as a function of bending radius, and inset image is the working flexible QLEDs with a curvature radius of 1.5 mm, 4 mm, 6 mm, 8 mm, 10 mm, respectively. (b) L/L0 (L, instantaneous device luminance; L0, initial device luminance) as a function of bending cycles with a curvature radius of 4 mm.
doping PMMA into QD solution, the long-chain polymer acted as a binder during the bending process, slowing down the performance loss of the functional layers, protecting the QDs from directly contacting with moisture and oxygen, and improving the flexibility and mechanical properties of the devices. In conclusion, we fabricated flexible QLED devices with strong mechanical properties by doping long-chain polymer PMMA to QD emission layer. Based on the quantum dot-polymer composite emitting layer, the efficiency loss of the functional layers for flexible QLEDs was reduced after repeated bending at a small curvature radius. Herein, PMMA not only played a role as the binder for neighboring quantum dot, but also acted as an electron blocking layer to balance charge transport and improve the device performance. The flexible QLEDs have an ultimate bending radius of 1.5 mm, and after 500 times bending with the curvature radius of 4 mm, the device performance showed no significant degradation. It is indicated that this strategy holds promising for potential applications in next generation flexible lighting and display field.
subjected to a bending test with different curvature radius. And the curvature radius were set at 1.5 mm, 4 mm, 6 mm, 8 mm, and 10 mm, respectively. The flexible devices could steadily work give bright green emission under various bending conditions, even with the extremely small curvature radius of 1.5 mm. Fig. 5(b) showed the L/L0 (L, instantaneous device luminance; L0, initial device luminance) as a function of bending cycles under the curvature radius of 4 mm. It can be observed that in comparison with the device without PMMA doping, the flexible QLED doping with 2.5 wt% PMMA can still maintain L/L0 of 67% after 500 times repeated bending at a constant voltage of 4.5 V. And the degradation speed was significantly smaller than that of the device without doping. The PET substrate itself had high water oxygen permeability, moreover, PEDOT:PSS as HIL was an organic material whose solvent was water and it also had acidity and hygroscopicity. In addition, when the device was repeatedly bent with a small curvature radius, the functional layers of QLEDs had serious loss in device performance due to the presence of gaps or even slightly cracks, which resulted the unbalanced charge transport [30,31]. However, after 285
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Acknowledgments
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