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Addition of Lithium 8-Quinolate into Polyethylenimine Electron-Injection Layer in OLEDs: Not Only Reducing Driving Voltage but Also Improving Device Lifetime Takayuki Chiba, Yong-Jin Pu, Takahumi Ide, Satoru Ohisa, Hitoshi Fukuda, Tatsuya Hikichi, Dai Takashima, Tatsuya Takahashi, So Kawata, and Junji Kido ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017
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ACS Applied Materials & Interfaces
Addition of Lithium 8-Quinolate into Polyethylenimine Electron-Injection Layer in OLEDs: Not Only Reducing Driving Voltage but Also Improving Device Lifetime
Takayuki Chiba, Yong-Jin Pu*, Takahumi Ide, Satoru Ohisa, Hitoshi Fukuda, Tatsuya Hikichi, Dai Takashima, Tatsuya Takahashi, So Kawata, Junji Kido*
Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
E-mail:
[email protected],
[email protected] Tel & Fax: +81-238-26-3595
Abstract Solution-processed electron injection layers (EILs) comprising lithium 8-quinolate (Liq) and polyethylenimine ethoxylated (PEIE) are highly effective for enhancing electron injection from ZnO to organic layers and improving device lifetime in organic light-emitting devices (OLEDs). Doping of Liq into PEIE further reduces the work function of zinc oxide (ZnO) by enhancing dipole formation. The intermolecular interaction between Liq and PEIE was elucidated by UV-vis absorption measurement and quantum chemical calculation. The OLEDs with ZnO covered with PEIE:Liq mixture exhibited lower driving voltage than that of the device without Liq. Furthermore, as doping concentration of Liq into PEIE increased, the device lifetime and voltage stability during constant current operation was successively improved.
Keywords: zinc oxide, lithium phenolate complex, polyethylenimine, electron injection, organic light-emitting device, long lifetime
1. Introduction Electron injection layers (EILs) have an important role in organic light-emitting devices (OLEDs) to facilitate electron injection from a cathode into several functional organic layers such as the electron transporting layer (ETL) or emitting layer (EML).1 Energy-level alignment between the cathode and the functional organic layer for electron injection has generally been achieved using EILs containing alkali metals,2-4 alkali metal halides,5-7 and alkali metal carbonates.8-10 However, these alkali metal compounds are unstable in air owing to their high reactivity with oxygen and moisture, resulting in degradation of the device properties after long-term operation. The alkali metal compounds also normally cannot be used for solution processing because of their poor ACS Paragon Plus Environment
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solubility and instability. Solution-processed n-type semiconducting zinc oxide (ZnO) has recently been reported as an effective electron injection layer in OLEDs and electron collection layer in organic photovoltaics (OPVs).11-15 Generally, thin-film of ZnO shows high electron transporting properties, transparency, and stability, and can be easily deposited by various solution processes such as sol-gel11 or nanoparticle methods13. The electron injection barrier between the work function (WF) of ZnO (~ 4 eV) and the lowest unoccupied molecular orbital (LUMO) level of electron transporting materials or emitting materials (2–3 eV) is large. Therefore, several approaches have been explored to improve electron injection from the ZnO into the organic layer by surface modification of the ZnO with cesium carbonate (Cs2CO3),16-20 barium hydroxide (Ba(OH)2),21 ionic liquid molecules,22 self-assembled dipole monolayers,23 and cationic polymers.24 These surface modifications cause the formation of a strong dipole moment on the ZnO, which reduces the electron injection barrier remarkably between the WF of ZnO and the LUMO level of the organic layers. In particular, nonionic and nonconjugated polyethylenimine (PEI) and polyethylenimine ethoxylated (PEIE) have also been widely used as an effective EIL.25-35 The PEI and PEIE comprise an aliphatic ethylenimine main chain and amine or hydroxyl side chain group (Figure 1a). The ZnO covered with PEI or PEIE has been reprted to exhibit a smaller WF of 3.3–3.6 eV (ZnO/PEIE) and 2.5–3.4 eV (ZnO/PEI) compared with 4.4 eV of the pure ZnO and 3.6 eV of the ZnO with Cs2CO3.29 Similarly, amine-based small molecules have also been demonstrated as a surface modification material to ZnO for inverted OLEDs.36-37 As the number of amine groups in the molecule increases from two to six, the WF of ZnO coated with the amine-based small-molecules effectively decreases owing to the interfacial dipole formation on the ZnO. These surface modification materials for ZnO have recently been used to improve electron injection in inverted OLEDs. Fukagawa et al. reported the long-term storage stability of inverted OLEDs with ZnO/PEI bilayer as an EIL.38 The device with ZnO/PEI exhibited no dark spot after 250 days storage, while the regular configuration of OLEDs without ZnO/PEI exhibited many dark spots after only 15 days storage. However, the operational lifetime, which differs from the storage stability, of such inverted OLEDs with ZnO/PEI was still shorter than that of regular OLEDs without the ZnO/PEI EIL. The operational lifetime of the devices with PEI and PEIE has not been sufficiently studied to date. We previously reported that the alkali metal organic complex, lithium 8-quinolate (Liq), served as an efficient solution-processed EIL (Figure 1a).39 Liq showed high solubility in polar alcohol solvents, and can be spin-coated from solution to form amorphous films that are more stable to oxidation and less hygroscopic compared with alkali metal inorganic salts such as Cs2CO3. Dispersion of Liq into organic molecules or polymers was also reported to improve performance of the EIL. Poly(vinylphenylpyridine)s are an effective polymer binder to Liq, showing superior electron ACS Paragon Plus Environment
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injection properties even with relatively large thickness of 10 nm. Doping of Liq into small molecule ETL was also reported to improve operational device lifetime of the devices.40-41 Here we report an effective electron injection property and long-term operational lifetime of the OLEDs with the solution-processed EILs comprising Liq-doped PEIE/ZnO bilayer. The Liq doping effect on WF shift, UV-vis absorption, thermal properties, and device performance was investigated. As the doping concentration of Liq into PEIE increased, the WF of PEIE/ZnO further decreased, compared with that of PEIE/ZnO without Liq. The OLEDs with PEIE:Liq/ZnO showed lower turn-on voltage and longer operational lifetime, as the doping concentration of Liq increased, than those of the device without Liq; this resulted from the reduced electron injection barrier between ZnO and the adjacent organic layer and improved rigidity of PEIE by intermolecular interaction with Liq.
2. Results and Discussion The effect of Liq doping into PEIE on the WF shift of ZnO was investigated. ZnO nanoparticles were synthesized from zinc acetate precursor with potassium hydroxide in methanol. The average nanoparticle size was found to be approximately 5 nm, determined from transmission electron microscopy (TEM). The optical energy gap (Eg) of the ZnO nanoparticles was found to be 3.5 eV from the absorption edge. The ZnO nanoparticles were spin-coated from 2-ethoxyethanol dispersion prepared to be 10 mg ml–1 onto ITO substrate. The Liq-doped PEIE solutions were prepared at concentration of 5 mg ml–1 in 2-ethoxyethanol, and spin-coated onto ZnO layer to form a 10-nm-thick film. Ultraviolet photoelectron spectroscopy (UPS) was performed on PEIE:Liq/ZnO bilayers with various Liq doping concentrations of 0, 10, 30, 50, 70, 90, and 100 wt%, as shown in Figure 1b. The WF of ZnO film with PEIE was 3.6 eV, shifted from 4.2 eV of the ZnO film without PEIE. This shift was almost the same as for the previous report.29 PEIE forms large dipole moment with ZnO, resulting in large WF shift at the ZnO layer and reduction of the electron injection barrier between the ZnO and adjacent ETL or EML. On the other hand, covering only with a thin Liq layer without PEIE (i.e. 100 wt% of Liq) onto ZnO without PEIE produced only small WF shift of 0.2 eV. However, doping of Liq into PEIE further decreased the WF of the ZnO with PEIE. The WF of ZnO covered with PEIE:Liq mixture gradually shifted from 3.6 eV without Liq to 3.3 eV with 50 wt% Liq, as shown in Figure 1c. The effect of Liq doping into PEIE on the WF shift of ZnO became saturated at 50wt% concentration of Liq. In the high doping concentration region of Liq, the WF decreased to 3.4 eV at 70wt% and 3.5 eV at 90 wt% Liq. To understand the WF shift of ZnO by the PEIE:Liq mixture, we estimated the dipole moment of Liq and the complex of Liq with diethylamine or triethylamine, which represents a partial structure ACS Paragon Plus Environment
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of PEIE, by density functional theory (DFT) calculation using Gaussian 09 with B3LYP functional and 6-311+G(d,p) basis set. The dipole moment of Liq itself was found to be 4.75 D. On the other hand, when diethylamine or triethylamine were placed close to the Li atom of Liq and their geometry was optimized, the dipole moment of the complex of Liq with diethylamine or triethylamine increased to 6.42 D or 6.09 D, respectively (Figure 2). These results suggest that doped Liq forms a complex with alkylamine PEIE and enhances the dipole moment of the PEIE/ZnO bilayer, resulting in further WF shift of PEIE/ZnO. The UV-vis absorption spectroscopy of PEIE:Liq mixture film with various mixture ratios was performed to investigate molecular interaction between PEIE and Liq as shown in Figure 3. PEIE itself is transparent and has no absorption in the UV-vis region. The strong absorption at 260 nm is derived from the π-π* transition of the 8-quinolinol ligand of Liq, and the two weak absorptions at 340-380 nm are derived from the n-π* transition with a partially forbidden nature. The strong peak at 257 nm of Liq with 10 wt% PEIE was bathochromically shifted to 262 nm as the ratio of PEIE increased to 50 wt%. The weak peak at 364 nm was also bathochromically shifted to 376 nm as the ratio of PEIE increased from 10 wt% to 50 wt%. The peak intensity simultaneously decreased compared with the normalized intensity of the π-π* transition peak of Liq, showing the isosbestic point. These results demonstrate the intermolecular interaction between Liq and PEIE. On the other hand, the intensity and wavelength of another weak peak at 337 nm were almost independent of the concentration of PEIE. To understand these different dependences of the three absorption peaks of Liq on the interaction with PEIE, time dependent (TD)-DFT calculations were performed. The calculation gave three singlet transitions of Liq with relatively large oscillator strength (f) (Table S1): S0 S8 transition with f = 0.4261 mainly ascribed to the transition from HOMO-2 to LUMO, S0 S3 transition with f = 0.0467 mainly ascribed to the transition from HOMO to LUMO+2, and S0 S1 transition with f = 0.0267 mainly ascribed to the transition from HOMO to LUMO. Therefore, the strong absorption peak at 257 nm is ascribed to the S8 transition from HOMO-2 to LUMO because of its largest oscillator strength. The other weak two peaks are ascribed to the partially forbidden S3 and S1 transition from HOMO to LUMO+2 and from HOMO to LUMO, respectively. The molecular orbitals are shown in Figure 4. Among the molecular orbitals involving these three transitions of S1, S3, and S8, only LUMO has the orbital density between Li and N atoms that can form intramolecular coordination, and the other molecular orbitals do not have the wave function on the Li atom. In the intermolecular interaction between PEIE and Liq, a nitrogen atom having a lone electron pair in PEIE is the electron donor and the Li atom with the cationic nature of Liq is the electron acceptor. Therefore, the absorption peaks derived from the transitions S1 and S8 involving LUMO are affected by the concentration of PEIE, and the absorption peak derived from the ACS Paragon Plus Environment
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transition S3 not involving LUMO is independent of the concentration of PEIE. To investigate the WF shift of ZnO by covering with PEIE:Liq mixture, the OLEDs with following configuration were fabricated; ITO (130 nm)/ZnO (10 nm)/PEIE:Liq (10 nm)/ tris(8-hydroxyquinoline)aluminum
(Alq3)
(20
bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum nm)/4,4''-bis(N-carbazolyl)-1,1''-biphenyl
(CBP):8wt%
(BAlq)
nm)/ (10
tris(2-phenylpyridinato)iridium(III)
(Ir(ppy)3) (30 nm)/4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPD) (40 nm)/MoO3 (10 nm)/Al (100 nm) (Figure 5a). The ZnO and PEIE:Liq layers were spin-coated from solution, and the other subsequent functional layers were deposited by vacuum evaporation to eliminate any additional effects on the PEIE:Liq layer from the solution process. The schematic energy diagram of the OLEDs with different Liq concentrations is shown in Figure 5b. Electroluminescence spectra of the devices with various Liq concentrations were identical to emission from Ir(ppy)3 (Figure S1). Current density-voltage (J-V) and luminance-voltage (L-V) characteristics of the devices are shown
in Figures 6a and 6b. The efficiencies and driving voltages of the devices are summarized in Figure S2 and Table S2. The control device with only PEIE (without Liq) exhibited a turn-on voltage of 4.2 V at 1 cd m–2 and driving voltages of 4.8 and 5.3 V at 100 and 1000 cd m–2, respectively. However, the turn-on voltage of the devices with the PEIE:Liq mixture gradually decreased from 3.8 to 3.5 V as the Liq concentration was increased from 10 to 50 wt%, owing to reduction of the electron injection barrier between ZnO and Alq3. Conversely, at high Liq concentration (50, 70 and 90 wt%), the driving voltage increased. These results are consistent with the WF shifts of ZnO-covered PEIE:Liq observed by UPS measurement, and clearly demonstrated that the addition of a small amount of Liq into PEIE further enhances electron injection from ZnO:PEIE into the organic layers. The current efficiency and external quantum efficiency were improved by the Liq doping in the current density region up to 5 mA cm–2, compared with the device with only PEIE. In the higher current density region than 5 mA cm–2, the high Liq concentration (50, 70 and 90 wt%) rather decreased especially power efficiency. This is probably corresponding to the recovery of the driving voltage of the device with PEIE only in the higher voltage region than 5 V (Figure 6a). Reorientation of PEIE might be induced by high electric field, but further detailed investigation is necessary. The device efficiencies are lower than those of state-of-the-art Ir(ppy)3-based OLEDs42 because of the use of commercially available general compounds for HTL, EML, and ETL such as NPD, CBP, BAlq and Alq3, respectively. However, the efficiencies are comparable to those of the previously reported other devices in which same compounds were used for HTL, EML and ETL, but evaporated Liq was used for EIL.43 Therefore, the use of solution-processed PEIE:Liq for EIL is not primary factor of the low efficiency of the devices. ACS Paragon Plus Environment
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Figure 6c and 6d show the luminescence and voltage shift during the lifetime test of the devices with ZnO covered PEIE:Liq (0, 10, 30, 50, 70, or 90 wt%), measured at room temperature of 25 ˚C and with constant current density operation of 7 mA cm–2 for an initial luminance of approximately 1000 cd m–2. The control device with only PEIE (without Liq) showed shorter operational lifetime of LT50 = 135 h and higher operational voltage shifts (∆V) of 2 V than those of the devices with Liq. On the other hand, the lifetime of the devices with PEIE:Liq improved greatly as the Liq concentration increased, with LT50 = 340 h for 10wt% Liq, 535 h for 30wt% Liq, 700 h for 50wt% Liq, 790 h for 70 wt% Liq, and 1000 h for 90wt% Liq. The voltage shifts were also significantly suppressed in the devices compared with the control device without Liq. These results demonstrated that the addition of Liq into PEIE is highly effective at improving device lifetime, as well as improving the electron injection properties. The longer lifetime was obtained with the highest concentration of Liq with PEIE; however, the effect of Liq addition on the improvement of electron injection became saturated at around 50wt% doping. We also fabricated the devices with only ZnO (without PEIE:Liq) and ZnO/100wt%Liq (without PEIE) to demonstrate the effect of PEIE:Liq as shown in Figure S3. The devices with only ZnO and ZnO/Liq exhibited a higher driving voltage due to large electron injection barrier between WF of ZnO (4.2 eV) and LUMO level of Alq3 (3.1 eV). However, the devices with only ZnO and ZnO/Liq showed longer lifetime compared to the device with PEIE:Liq. These results strongly demonstrated that PEIE causes short device lifetime. The glass transition temperature (Tg) of PEIE was measured by differential scanning calorimetry (DSC) to investigate the relationship between the operational lifetime of the device and the thermal properties of PEIE as shown in Figure 7. PEIE is usually viscous at room temperature owing to its flexible aliphatic and branched structure, and its Tg was observed at –24 ˚C. On the other hand, Tg of the the PEIE:Liq mixture gradually increased from –24 ˚C to –16 ˚C as the Liq concentration increased to 10 wt%, and in the higher concentration of Liq than 10wt%, Tg was not changed. This increased rigidity of PEIE is probably resulted from the intermolecular interaction between Liq and PEIE suppressing the degree of freedom and mobility of PEIE molecules. However, this effect is limited up to 10wt% concentration of Liq. Therefore, improved thermal property of PEIE is probably a part of the reasons for improved stability of the devices. Further investigation will be necessary, especially from the viewpoint of electrical property, in the future.
3. Conclusion We demonstrated that addition of Liq into PEIE as an EIL is highly effective at enhancing enhance electron injection from ZnO into organic layers, and improves the device lifetime of OLEDs. The intermolecular interaction between Liq and PEIE was demonstrated by the continuous changed of UV absorption of Liq with various PEIE concentrations, and this interaction increased ACS Paragon Plus Environment
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the dipole moment of PEIE formed on the ZnO layer. The WF of ZnO was gradually decreased as Liq concentration in PEIE increased to 50wt%. The driving voltage of the OLEDs with PEIE:Liq as the EIL also gradually decreased as Liq concentration increased to 50wt%, because of the reduced electron injection barrier between ZnO and Alq3. Furthermore, the addition of Liq into PEIE simultaneously improved the device lifetime and voltage shift for constant current operation, owing to the improved rigidity of PEIE by intermolecular interaction with Liq. This combination of Liq and PEIE as EIL is not only a promising technique for simultaneous achieving low driving voltage and long-term operational lifetime of the OLEDs, but would also be useful for other organic electronics devices such as organic photovoltaics and organic transistors involving electron injection from the electrode.
4. Experimental Section Materials. The aqueous solution (37%, w v–1) of polyethylenimine (Mw, 110000), 80% ethoxylated, was purchased from Sigma-Aldrich. The dispersion of ZnO nanoparticles in 2-ethoxyethanol was prepared according to previously reported. NPD, CBP, Ir(ppy)3, BAlq, Alq3, and Liq were purchased from e-Ray Optoelectronics Technology Co., Ltd., and used as received.
Characterization. The work functions were determined by ultraviolet photoelectron spectroscopy (UPS, Thermo Fisher Scientific Theta probe) under high vacuum condition of ~10-6 Pa. UV-bis absorption spectra were measured using a Shimadzu UV-3150. Differential scanning calorimetry was performed using a Perkin-Elmer Diamond DSC Pyris instrument under nitrogen atmosphere at a heating rate of 10 ˚C min–1. OLEDs fabrication. Indium tin oxide (ITO) substrates (sheet resistance 15 Ω sq–1) were first cleaned by deionized water with ultrasonic spin cleaning and then by UV–ozone treatment for 10 min. After the UV–ozone treatment, spin coating and annealing processes of ZnO and PEIE:Liq were performed in a nitrogen-filled glove box. ZnO nanoparticles (10 mg mL–1 dispersion in 2-ethoxyethanol) were spin coated onto the ITO substrate at slope for 2.5 s and then at 4000 rpm for 30 s. Annealing at 100 ˚C for 10 min resulted in a 10 nm thick layer. Aqueous solution of PEIE was diluted with 2-ethoxyethanol. For fabrication of electron injection layer with PEIE and Liq were dissolved in 2-ethoxyethnol with total concentration of 5 mg mL–1. PEIE:Liq films were spin coated onto ZnO and annealed at 100 ˚C for 10 min, resulting in an approximately 10 nm. Other functional layers of Alq3, BAlq, CBP, Ir(ppy)3, NPD, MoO3 and Al anode were deposited by thermal evaporated under high vacuum (~10−5 Pa). Active area of the device was 2 mm2. The devices were characterized after encapsulation using epoxy glue and a glass cover. Electroluminescence spectra ACS Paragon Plus Environment
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were recorded using Hamamatsu PMA-11 photonic multichannel analyzer. The current-density– voltage and luminance–voltage characteristics were measured using a Keithley source measure unit 2400 and a Minolta CS200 luminance meter, respectively. The operational lifetime of the devices was measured using System Engineers EAS-203 at a constant current density of 7 mA cm–2.
Acknowledgments The authors would like to thank the Strategic Promotion of Innovative R&D Program, Japan Regional Innovation Strategy Program by Excellence, and The Center of Innovation Program of the Japan Science and Technology Agency (JST). The authors would like also thanks the “Grant-in Aid for Scientific Research A Grant Number 15H02203” of the Japan Society for the Promotion of Science (JSPS). Y.-J. Pu. thanks the PRESTO (Sakigake), JST for support.
Supporting Information Electroluminescence spectra, device characteristics, device lifetime, and orbital transition analysis.
References 1.
Tang, C. W.; Vanslyke, S. A., Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915.
2.
Kido, J.; Matsumoto, T., Bright Organic Electroluminescent Devices Having a Metal-Doped Electron-Injecting Layer. Appl. Phys. Lett. 1998, 73, 2866-2868.
3.
Pfeiffer, M.; Forrest, S. R.; Leo, K.; Thompson, M. E., Electrophosphorescent P-I-N Organic Light-Emitting Devices for Very-High-Efficiency Flat-Panel Displays. Adv. Mater. 2002, 14, 1633-1636.
4.
He, G. F.; Pfeiffer, M.; Leo, K.; Hofmann, M.; Birnstock, J.; Pudzich, R.; Salbeck, J., High-Efficiency and Low-Voltage P-I-N Electrophosphorescent Organic Light-Emitting Diodes with Double-Emission Layers. Appl. Phys. Lett. 2004, 85, 3911-3913.
5.
Hung, L. S.; Tang, C. W.; Mason, M. G., Enhanced Electron Injection in Organic Electroluminescence Devices Using an Al/Lif Electrode. Appl. Phys. Lett. 1997, 70, 152-154.
6.
Lee, J.; Park, Y.; Kim, D. Y.; Chu, H. Y.; Lee, H.; Do, L. M., High Efficiency Organic Light-Emitting Devices with Al/Naf Cathode. Appl. Phys. Lett. 2003, 82, 173-175.
7.
Guan, X.; Zhang, K.; Huang, F.; Bazan, G. C.; Cao, Y., Amino N-Oxide Functionalized Conjugated Polymers and Their Amino-Functionalized Precursors: New Cathode Interlayers for High-Performance Optoelectronic Devices. Adv. Funct. Mater. 2012, 22, 2846-2854.
8.
Wu, C. I.; Lin, C. T.; Chen, Y. H.; Chen, M. H.; Lu, Y. J.; Wu, C. C., Electronic Structures and Electron-Injection Mechanisms of Cesium-Carbonate-Incorporated Cathode Structures for Organic Light-Emitting Devices. Appl. Phys. Lett. 2006, 88, 152104. ACS Paragon Plus Environment
Page 8 of 15
Page 9 of 15
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
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9.
Huang, J. S.; Li, G.; Wu, E.; Xu, Q. F.; Yang, Y., Achieving High-Efficiency Polymer White-Light-Emitting Devices. Adv. Mater. 2006, 18, 114-117.
10.
Lee, S.; Shin, H.; Kim, J. J., High-Efficiency Orange and Tandem White Organic Light-Emitting Diodes Using Phosphorescent Dyes with Horizontally Oriented Emitting Dipoles. Adv. Mater. 2014, 26, 5864-5868.
11.
Bolink, H. J.; Coronado, E.; Repetto, D.; Sessolo, M., Air Stable Hybrid Organic-Inorganic Light Emitting Diodes Using Zno as the Cathode. Appl. Phys. Lett. 2007, 91, 223501.
12.
Kabra, D.; Song, M. H.; Wenger, B.; Friend, R. H.; Snaith, H. J., High Efficiency Composite Metal Oxide-Polymer Electroluminescent Devices: A Morphological and Material Based Investigation. Adv. Mater. 2008, 20, 3447-3452.
13.
Qian, L.; Zheng, Y.; Choudhury, K. R.; Bera, D.; So, F.; Xue, J. G.; Holloway, P. H., Electroluminescence from Light-Emitting Polymer/Zno Nanoparticle Heterojunctions at Sub-Bandgap Voltages. Nano Today 2010, 5, 384-389.
14.
Youn, H.; Yang, M., Solution Processed Polymer Light-Emitting Diodes Utilizing a Zno/Organic Ionic Interlayer with Al Cathode. Appl. Phys. Lett. 2010, 97, 243302.
15.
Chiba, T.; Pu, Y. J.; Sasabe, H.; Kido, J.; Yang, Y., Solution-Processed Organic Light-Emitting Devices with Two Polymer Light-Emitting Units Connected in Series by a Charge-Generation Layer. J. Mater. Chem. 2012, 22, 22769-22773.
16.
Bolink,
H.
J.;
Brine,
H.;
Coronado,
E.;
Sessolo,
M.,
Phosphorescent
Hybrid
Organic-Inorganic Light-Emitting Diodes. Adv. Mater. 2010, 22, 2198-2201. 17.
Kabra, D.; Lu, L. P.; Song, M. H.; Snaith, H. J.; Friend, R. H., Efficient Single-Layer Polymer Light-Emitting Diodes. Adv. Mater. 2010, 22, 3194-3198.
18.
Vaynzof, Y.; Kabra, D.; Chua, L. L.; Friend, R. H., Improved Electron Injection in Poly(9,9 '-Dioctylfluorene)-Co-Benzothiodiazole Via Cesium Carbonate by Means of Coannealing. Appl. Phys. Lett. 2011, 98, 113306.
19.
Park, J. S.; Lee, J. M.; Hwang, S. K.; Lee, S. H.; Lee, H. J.; Lee, B. R.; Park, H. I.; Kim, J. S.; Yoo, S.; Song, M. H.; Kim, S. O., A Zno/N-Doped Carbon Nanotube Nanocomposite Charge Transport Layer for High Performance Optoelectronics. J. Mater. Chem. 2012, 22, 12695-12700.
20.
Chiba, T.; Pu, Y.-J.; Hirasawa, M.; Masuhara, A.; Sasabe, H.; Kido, J., Solution-Processed Inorganic-Organic Hybrid Electron Injection Layer for Polymer Light-Emitting Devices. ACS Appl. Mater. Interfaces 2012, 4, 6104-6108.
21.
Lu, L. P.; Kabra, D.; Friend, R. H., Barium Hydroxide as an Interlayer between Zinc Oxide and a Luminescent Conjugated Polymer for Light-Emitting Diodes. Adv. Funct. Mater. 2012, 22, 4165-4171.
22.
Lee, B. R.; Choi, H.; SunPark, J.; Lee, H. J.; Kim, S. O.; Kim, J. Y.; Song, M. H., Surface ACS Paragon Plus Environment
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
Modification of Metal Oxide Using Ionic Liquid Molecules in Hybrid Organic-Inorganic Optoelectronic Devices. J. Mater. Chem. 2011, 21, 2051-2053. 23.
Park, J. S.; Lee, B. R.; Lee, J. M.; Kim, J. S.; Kim, S. O.; Song, M. H., Efficient Hybrid Organic-Inorganic Light Emitting Diodes with Self-Assembled Dipole Molecule Deposited Metal Oxides. Appl. Phys. Lett. 2010, 96, 243306.
24.
Park, J. S.; Lee, B. R.; Jeong, E.; Lee, H. J.; Lee, J. M.; Kim, J. S.; Kim, J. Y.; Woo, H. Y.; Kim, S. O.; Song, M. H., High Performance Polymer Light-Emitting Diodes with N-Type Metal Oxide/Conjugated Polyelectrolyte Hybrid Charge Transport Layers. Appl. Phys. Lett. 2011, 99, 163305.
25.
Xiong, T.; Wang, F. X.; Qiao, X. F.; Ma, D. G., A Soluble Nonionic Surfactant as Electron Injection Material for High-Efficiency Inverted Bottom-Emission Organic Light Emitting Diodes. Appl. Phys. Lett. 2008, 93, 123310.
26.
Zhou, Y. H.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.; Barlow, S.; Graham, S.; Bredas, J. L.; Marder, S. R.; Kahn, A.; Kippelen, B., A Universal Method to Produce Low-Work Function Electrodes for Organic Electronics. Science 2012, 336, 327-332.
27.
Chen, J. S.; Shi, C. S.; Fu, Q.; Zhao, F. C.; Hu, Y.; Feng, Y. L.; Ma, D. G., Solution-Processable Small Molecules as Efficient Universal Bipolar Host for Blue, Green and Red Phosphorescent Inverted Oleds. J. Mater. Chem. 2012, 22, 5164-5170.
28.
Stolz, S.; Scherer, M.; Mankel, E.; Lovrincic, R.; Schinke, J.; Kowalsky, W.; Jaegermann, W.; Lemmer, U.; Mechau, N.; Hernandez-Sosa, G., Investigation of Solution-Processed Ultrathin Electron Injection Layers for Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2014, 6, 6616-6622.
29.
Kim, Y. H.; Han, T. H.; Cho, H.; Min, S. Y.; Lee, C. L.; Lee, T. W., Polyethylene Imine as an Ideal Interlayer for Highly Efficient Inverted Polymer Light-Emitting Diodes. Adv. Funct. Mater. 2014, 24, 3808-3814.
30.
Hofle, S.; Schienle, A.; Bruns, M.; Lemmer, U.; Colsmann, A., Enhanced Electron Injection into Inverted Polymer Light-Emitting Diodes by Combined Solution-Processed Zinc Oxide/Polyethylenimine Inter Layers. Adv. Mater. 2014, 26, 2750-2754.
31.
Hofle, S.; Schienle, A.; Bernhard, C.; Bruns, M.; Lemmer, U.; Colsmann, A., Solution Processed, White Emitting Tandem Organic Light-Emitting Diodes with Inverted Device Architecture. Adv. Mater. 2014, 26, 5155-5159.
32.
Hofle, S.; Bernhard, C.; Bruns, M.; Kubel, C.; Scherer, T.; Lemmer, U.; Colsmann, A., Charge Generation Layers for Solution Processed Tandem Organic Light Emitting Diodes with Regular Device Architecture. ACS Appl. Mater. Interfaces 2015, 7, 8132-8173. ACS Paragon Plus Environment
Page 10 of 15
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33.
Pu, Y.-J.; Chiba, T.; Ideta, K.; Takahashi, S.; Aizawa, N.; Hikichi, T.; Kido, J., Fabrication of Organic Light-Emitting Devices Comprising Stacked Light-Emitting Units by Solution-Based Processes. Adv. Mater. 2015, 27, 1327-1332.
34.
Chiba, T.; Pu, Y.-J.; Kido, J., Solution-Processed White Phosphorescent Tandem Organic Light-Emitting Devices. Adv. Mater. 2015, 27, 4681-4687.
35.
Fan, C. J.; Lei, Y.; Liu, Z.; Wang, R. X.; Lei, Y. L.; Li, G. Q.; Xiong, Z. H.; Yang, X. H., High-Efficiency Phosphorescent Hybrid Organic-Inorganic Light-Emitting Diodes Using a Solution-Processed Small-Molecule Emissive Layer. ACS Appl. Mater. Interfaces 2015, 7, 20769-20778.
36.
Lee, B. R.; Jung, E. D.; Park, J. S.; Nam, Y. S.; Min, S. H.; Kim, B. S.; Lee, K. M.; Jeong, J. R.; Friend, R. H.; Kim, J. S.; Kim, S. O.; Song, M. H., Highly Efficient Inverted Polymer Light-Emitting Diodes Using Surface Modifications of Zno Layer. Nat. Commun. 2014, 5, 4840.
37.
Lee, B. R.; Lee, S.; Park, J. H.; Jung, E. D.; Yu, J. C.; Nam, Y. S.; Heo, J.; Kim, J. Y.; Kim, B. S.; Song, M. H., Amine-Based Interfacial Molecules for Inverted Polymer-Based Optoelectronic Devices. Adv. Mater. 2015, 27, 3553-3559.
38. Fukagawa, H.; Morii, K.; Hasegawa, M.; Arimoto, Y.; Kamada, T.; Shimizu, T.; Yamamoto, T., Highly Efficient and Air-Stable Inverted Organic Light-Emitting Diode Composed of Inert Materials. Appl. Phys. Express 2014, 7, 082104. 39.
Chiba, T.; Pu, Y. J.; Takahashi, S.; Sasabe, H.; Kido, J., Lithium Phenolate Complexes with a Pyridine-Containing Polymer for Solution-Processable Electron Injection Layers in Pleds. Adv. Funct. Mater. 2014, 24, 6038-6045.
40.
Kwon, J. H.; Lee, J. Y., High Efficiency and Long Lifetime in Organic Light-Emitting Diodes Using Bilayer Electron Injection Structure. Synth. Met. 2009, 159, 1292-1294.
41.
Tsang, D. P. K.; Adachi, C., Operational Stability Enhancement in Organic Light-Emitting Diodes with Ultrathin Liq Interlayers. Sci. Rep. 2016, 6, 22463.
42.
Su. S.-J.; Chiba. T.; Takeda. T, Kido. J. Pyridine-Containing Triphenylbenzene Derivatives with High Electron Mobility for Highly Efficient Phosphorescent OLEDs. Adv. Mater. 2008, 20, 2125-2130.
43.
Ohisa, S.; Kagami, S.; Pu. Y.-J.; Chiba, T,; Kido. J. A Solution-Processed Heteropoly Acid Containing MoO3 Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices. ACS Appl. Mater. Interfaces 2016, 8, 20946-20954.
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a)
b)
c)
Figure 1. a) Chemical structures of PEIE and Liq. b) The secondary electron cut-off region of UPS measurement with various Liq concentration (wt%) in PEIE on ZnO. c) The work function of ZnO covered with PEIE:Liq mixture.
Figure 2. Dipole moment of Liq and Liq:alkylamine complexes estimated by DFT calculation. a) Liq, b) Liq:diethylamine, and c) Liq:triethylamine.
Figure 3. UV-vis absorption spectra of PEIE:Liq film.
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Figure 4. Molecular orbitals of Liq. a)
b)
Figure 5. a) Device structure and b) energy diagram of the device.
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a)
b)
c)
d)
Figure 6. a) Current density-voltage (J-V) characteristics, b) luminance-voltage (L-V) characteristics, c) relative luminance–driving time, and d) driving voltage–driving time. In the lifetime measurement, the constant current density was 7 mA cm–2, and the initial luminance was 1176 cd m–2 for the PEIE only, 1038 cd m–2 for the 10wt% Liq, 1108 cd m–2 for the 30wt% Liq, 1063 cd m–2 for the 50wt% Liq, 1133cd m–2 for the 70wt% Liq, and 1060cd m–2 for the 90wt% Liq.
Figure 7. DSC curves of PEIE with various Liq concentration.
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