An efficient electron transporting material, P-OXD (1,3-bis[(4-(4-diethylphosphoryl-butyl-phenyl))-1,3,4-oxidiazol-2-yl]phenylene), has been synthesized and thoroughly characterized. Due to its alcohol-soluble nature, P-OXD can be spin-coated atop the light emitting layer to form high quality film without dissolving the underlying layer. As a consequence, the double-layer blue electrophosphorescent device has been successfully fabricated, giving a peak luminous efficiency of 10.5 cd/A, and a maximum brightness of 8200 cd/m2 with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.33). The promising results indicate that P-OXD has a potential application in solution-processed multilayer polymer light-emitting diodes.
© 2011 OSA
Electrophosphorescent polymer light-emitting diodes (PhPLEDs) has been the subject of intense research nowadays owing to their theoretical 100% internal quantum efficiencies together with the low-cost solution process technologies for flexible, large-area displays and solid-state lighting . Here, low-work-function metals (Ca, Ba etc.) are often adopted as a cathode to enhance the electron injection/transporting. These metals are very sensitive to oxygen and moisture, and consequently detrimental quenching sites can be formed at the interface between the emitting layer (EML) and the cathode [2–4]. Therefore, the use of environmentally stable metal cathodes such as Al and Au has attracted much interest for practical applications. Unfortunately, their high work functions do not match the EML, which may lead to a charge injection barrier.
To overcome this hurdle, water- or alcohol-soluble cationic, zwitterionic and neutral conjugated polymeric surfactants have been developed as the electron injection/transporting materials, because their depositions on top of an organic-soluble EML do not result in interlayer mixing or removal of the underlying layer [5–11]. For example, using poly[9,9-bis(6’-(diethanolamino)hexyl)fluorene] (PFN-OH) as an electron-injection layer combined with Al cathode, Alex K. –Y. Jen et al. realized efficient PhPLEDs with a luminous efficiency as high as 14.2 cd/A for blue-light emission . In spite of the obtained promising device performance, these polymeric surfactants suffer from multidispersed molecular structures, batch to batch variances, and odious purification procedures. Therefore, it is highly desirable to design water- or alcohol-soluble small molecular surfactants [13–15].
Oxadiazole derivatives, such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7, Fig. 1 ), have been widely used as the electron transporting materials in PhPLEDs [16,17]. Among them, OXD-7 is especially favorable for blue and white PhPLEDs due to its higher triplet energy relative to PBD . In our previous studies, on the other hand, polyfluorene containing phosphonate groups at its side chain (PF-EP, Fig. 1) has been found to effectively facilitate the electron injection/transporting from the LiF/Al cathode to EML . Based on these two concerns, in this article, we design and synthesize a small molecular surfactant, 1,3-bis[(4-(4-diethylphosphoryl-butyl-phenyl))-1,3,4-oxidiazol-2-yl]phenylene (P-OXD) (Fig. 1), which comprises two phosphonate groups and a similar conjugated oxadiazole skeleton to OXD-7. For this design, several advantages can be speculated: 1) The conjugated oxadiazole backbone renders good hole-blocking and electron-transporting capability; 2) The phosphonate groups endow good solubility in alcohol-based solvents, and efficient electron injection from the LiF/Al cathode; 3) The non-conjugated linkage between the oxadiazole backbone and phosphonate fragments is beneficial to remain their relative independence. Our results demonstrate that, with P-OXD as the electron injection/transporting layer, the luminous efficiency as high as 10.5 cd/A can be achieved for the blue electrophosphorescent devices, which matches the PFN-OH-based devices well .
1H and 31P NMR spectra were obtained with a Bruker Avance 300 NMR spectrometer. Elemental analysis was performed using a Bio-Rad elemental analysis system. MALDI/TOF (Matrix assisted laser desorption ionization/Time-of-flight) mass spectra were performed on AXIMA CFR MS apparatus (COMPACT). UV-Vis absorption spectra were recorded by a Perkin-Elmer Lambda 35 UV/Vis spectrometer. PL spectra were recorded with a Perkin- Elmer LS50B spectrofluorometer. Phosphorescence spectra at 77 K were measured in toluene. The triplet energies were estimated as the maximum of the first vibronic mode (S0 ν = 0 ← T1 ν = 0) of the corresponding phosphorescence spectra at 77 K. Cyclic voltammetry experiments were performed on an EG&G 283 (Princeton Applied Research) potentiostat/galvanostat system. The measurements were carried out with a conventional three-electrode system consisting of a platinum working electrode, a platinum counter electrode, and an Ag/AgCl reference electrode. The supporting electrolyte was 0.1 M tetrabutylammonium perchlorate (n-Bu4NClO4). Ferrocene was used as a standard to calibrate the system. The atomic force microscopy (AFM) measurements were performed on SPA300HV with a SPI3800N controller, Seiko Instruments Industry, Co. Ltd.
All chemicals and reagents were used as received from commercial sources without further purification. Solvents for chemical synthesis were purified according to the standard procedures. All chemical reactions were carried out under an inert atmosphere. (4-bromo-butoxy)benzene 1 , isophthaloyl dichloride  and OXD-7  were prepared according to the literature procedures.
methyl 4-(4-phenoxy-butyl)benzoate (3): Under argon, a solution of (4-bromo-butoxy)benzene 1 (5.1 g, 22.0 mmol) in 120 ml THF was slowly added to the mixture of magnesium turnings (0.6 g, 26.4 mmol) and a catalyst amount of iodine. The reaction system was carefully heated to 70 °C, and kept at this temperature for 2h. Then the formed Grignard reagent was transferred dropwise to the solution of B(OCH3)3 (3.7 ml, 33.1 mmol) in 30 ml THF at −78 °C. After the addition, the mixture was slowly warmed to room temperature (r.t.), kept stirring overnight, quenched with 1M HCl, and then extracted with dichloromethane (DCM). The combined organic layers were washed with water and brine sequentially, and dried over anhydrous Na2SO4. After the solvent removal, the crude product 2 (3.7 g, 17.3 mmol), methyl 4-bromobenzoate (4.1 g, 19.1 mmol), Pd(dppf)Cl2·CH2Cl2 (1.4 g, 1.7 mmol), K2CO3 (7.2 g, 51.9 mmol) and Ag2O (10.0 g, 43.3 mmol) were added to 120 ml THF under argon, and the mixture was reacted at 70 °C for 16h. After cooling to r.t., the mixture was poured into water, and extracted with ethyl acetate. The combined organic layers were washed with diluted HCl, water and brine in turn. After removal of the solvent, the residue was purified by silica gel chromatography with petroleum/ethyl acetate = 50/1 as eluent. Total yield: 2.5 g (41%). 1H NMR (300 MHz, CDCl3, δ, ppm): 7.96 (d, J = 8.4 Hz, 2H), 7.30-7.25 (m, 4H), 6.94 (d, J = 7.5 Hz, 1H), 6.89 (d, J = 9.0 Hz, 2H), 3.97 (t, J = 5.7 Hz, 2H), 3.90 (s, 3H), 2.74 (t, J = 7.2 Hz, 2H), 1.83 (m, 4H).
1,3-bis[(4-(4-bromo-butyl-phenyl))-1,3,4-oxidiazol-2-yl]phenylene (7): The mixture of methyl 4-(4-phenoxy-butyl)benzoate 3 (2.6 g, 9.0 mmol) and 20 ml NH2NH2·H2O (50 wt.%) was heated at 120 °C for 24h. After cooling to r.t., the reaction mixture was poured into water, and filtered. The precipitate was collected, and recrystallized from ethanol to afford 4-(4-phenoxy-butyl)benzohydrazide 4. Then isophthaloyl dichloride (0.8 g, 4.0 mmol) dissolved in 10 ml NMP was added dropwise to the mixture of 4 (2.3 g, 8.0 mmol) and Et3N (0.8 g, 8.0 mmol) in 30 ml NMP at 35 °C. After the addition, the reaction was kept at this temperature for 5h, quenched with water, and then filtered. The residue 5 (1.8 g, 2.5 mmol) was washed with ethanol for several times, and dissolved in POCl3 (20 ml) at 140 °C. After reacted for 18h, the mixture was cooled to r.t., and poured into an ice/water mixture. The precipitate was collected by filtration, and washed with aqueous NaHCO3. After recrystallized from ethyl acetate, the obtained intermediate 6 (1.5 g, 2.3 mmol) was dissolved in 20 ml DCM at 0 °C, and BBr3 (2 ml, 18 mmol) in 20 ml DCM was added dropwise. After the addition, the mixture was allowed to warm to r.t., and reacted for another 24h. Water was added to quench the reaction, and the organic layer was separated. After the solvent being removed, the crude product was purified by silica gel chromatography with DCM/ethyl acetate = 20/1 as eluent. Yield: 0.6 g (22%). 1H NMR (300 MHz, CDCl3, δ, ppm): 8.78 (t, J = 1.5 Hz, 1H), 8.35 (dd, J = 7.8 and 1.8 Hz, 2H), 8.12 (d, J = 8.4 Hz, 4H), 7.74 (t, J = 7.8 Hz, 1H), 7.39 (d, J = 8.1 Hz, 4H), 3.46 (t, J = 6.0 Hz, 4H), 2.76 (t, J = 7.5 Hz, 4H), 1.99-1.80 (m, 8H).
1,3-bis[(4-(4-diethylphosphoryl-butyl-phenyl))-1,3,4-oxidiazol-2-yl]phenylene (P-OXD): A solution of 1,3-bis[(4-(4-bromo-butyl-phenyl))-1,3,4-oxidiazol-2-yl]phenylene 7 (0.6 g, 0.9 mmol) in P(OEt)3 (5 ml) was heated to 180 °C for 16 h. Then the excess P(OEt)3 was distilled in vacuum, and the final compound P-OXD was obtained after purification by silica gel chromatography using ethyl acetate as the eluent. Yield: 0.5 g (78%). 1H NMR (300 MHz, CDCl3, δ, ppm): 8.88 (t, J = 1.2 Hz, 1H), 8.35 (dd, J = 7.8 and 1.5 Hz, 2H), 8.11 (d, J = 8.1 Hz, 4H), 7.74 (t, J = 7.8 Hz, 1H), 7.38 (d, J = 8.1 Hz, 4H), 4.16-4.04 (m, 8H), 2.74 (t, J = 6.9 Hz, 4H), 1.85-1.63 (m, 12H), 1.32 (t, J = 7.2 Hz, 12H). 31P NMR (CDCl3, 295K, δ, ppm): 31.64. Anal. Calcd for C38H48N4O8P2: C, 60.79; H, 6.44; N, 7.46. Found: C, 60.34; H, 6.06; N, 7.33. MALDI-TOF (m/z): 751.3 [M+ + H].
2.3 Device fabrication and testing
To fabricate PhPLEDs, a 40-nm-thick poly(ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS, H. C. Starck) film was first deposited on the pre-cleaned indium tin oxide (ITO) glass substrates, and baked at 120 °C for 40 min. Then iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C 2]-picolinate (FIrpic) (10 wt.%) and P-OXD (30 wt.%) were doped into a poly(N-vinylcarbazole) (PVK) host in its chlorobenzene solution and 80 nm thick films were formed as the EML via spin-coating. Successively, the cathode Al (200 nm) and LiF/Al (1 nm/200 nm) were thermally evaporated on top of the EML for device B and device C, respectively, at a base pressure less than 10−6 Torr (1 Torr = 133.32 Pa) through a shadow mask with an array of 14 mm2 openings. For comparison, the control device A was also fabricated with OXD-7 as the electron transporting material instead of P-OXD. As for the double-layer device D, the chlorobenzene solution of 10 wt.% FIrpic doped into PVK was firstly spin-coated onto PEDOT:PSS as the EML, and annealed at 80 °C for 30 min to remove the residual solvent. Finally, a 10 nm thick P-OXD thin film was deposited on the top of the EML by spin-coating from its 3 mg/mL ethanol solution, followed by the thermal deposition of the LiF/Al cathode. The electroluminescence (EL) spectra, Commission Internationale de L’Eclairage (CIE) coordinates, current-voltage and brightness-voltage characteristics of the devices were measured with a Spectrascan PR650 spectrophotometer at the forward direction and a computer-controlled Keithley 2400 under ambient condition.
3. Results and discussion
3.1 Synthesis and characterization
The synthetic route of P-OXD is shown in Fig. 2 . From the word go, phenol was utilized as the protected group until the cyclization of 5 to afford the key precursor 6, which then was converted to its corresponding deprotected product 7 in the presence of BBr3. Finally, by refluxing 7 in triethyl phosphate, P-OXD was obtained in a yield of 78%. The structure of P-OXD was verified using 1H and 31P NMR spectroscopy, elemental analysis, and matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry. P-OXD exhibits good solubility in both nonpolar and polar solvents: toluene, chloroform, THF and alcohol etc. Noticeably, the alcohol-soluble nature can prevent interfacial mixing between the EML and the adjacent electron injection/transporting layer, which is a key challenge for fabricating multilayer PhPLEDs via a solution process . For comparison, OXD-7 was also synthesized according to the literature .
3.2 Photophysical and electrochemical properties
The photophysical properties of P-OXD and OXD-7 were firstly investigated, and their UV-Vis and photoluminescence (PL) spectra in solutions are displayed in Fig. 3 . As can be clearly seen, both the absorption and PL of P-OXD show almost the identical spectral features to those of OXD-7, except for a small red-shift of 4 nm for the emission maxima of P-OXD. The bathochromic shift can be attributed to the intermolecular interactions in P-OXD. In addition, their phosphorescent spectra at 77 K were also measured in toluene (Fig. 3). From the first vibronic peak, the triplet energy of P-OXD is estimated to be the same as OXD-7 (2.7 eV) . All these observations suggest that the incorporation of phosphonate groups via alkyl spacer does not change the optical properties of the oxadiazole backbone. Moreover, we note that, the triplet energy of P-OXD is higher than that of FIrpic (2.65 eV) . This means that P-OXD is suitable to be used as the electron transporting material for the FIrpic-based electrophosphorescent devices, for the triplet excitons can be effectively confined on FIrpic to avoid their loss.
Cyclic voltammetry (CV) was used to probe the electrochemical behavior of P-OXD and OXD-7. As presented in Fig. 4 , upon cathodic sweep in THF, they display two quasi-reversible reduction waves, which can be ascribed to the two oxadiazole units. According to their identical onsets of the first reduction potential (−2.27 eV) along with the absorption edge (3.79 eV), the lowest unoccupied and highest occupied molecular orbital (LUMO/HOMO) energy levels are calculated to be −2.53 and −6.32 eV for both P-OXD and OXD-7, respectively. Such low LUMO and HOMO energy levels indicate the efficient electron-transporting and hole-blocking capability of these oxadiazole derivatives. Additionally, the similarity of n-doping processes between P-OXD and OXD-7 further demonstrates the predominance of non-conjugated linkage, in agreement with the above discussion.
3.3 Electroluminescence properties
To explore the feasibility of using P-OXD as an electron transporting material, the single-layer devices B (ITO/PEDOT:PSS/PVK:P-OXD:FIrpic/Al) and C (ITO/PEDOT:PSS/PVK:P-OXD:FIrpic/LiF/Al) were firstly fabricated, where PVK and FIrpic acted as the host and triplet emitter, respectively. For comparison, the control device A (ITO/PEDOT:PSS/PVK:OXD-7:FIrpic/LiF/Al) was also prepared under the same condition. The device performance data are summarized in Table 1 , and the device characteristics are given in Fig. 5 . Consistent with the literature , the insertion of a thin layer LiF can further intensify the coordination interaction between the phosphonate groups and the cathode to lower the electron injection barrier. As a result, upon going from device B to C, the turn-on voltage reduces from 12.5 V to 8.8 V. Simultaneously, the maximum efficiency and brightness enhance from 0.9 cd/A and 750 cd/m2 to 6.5 cd/A and 4800 cd/m2, respectively. In addition, with respect to device A (2.3 cd/A, 2400 cd/m2), both the efficiency and brightness of device C are improved by about 2-3 times. These results prove that the introduction of phosphonate groups can favor the efficient electron injection/transporting from the LiF/Al cathode.
As mentioned above, P-OXD is also alcohol-soluble, which makes it possible to manufacture multilayer PhPLEDs via solution process. Therefore, bilayer device D was prepared with the configuration of ITO/PEDOT:PSS/PVK:FIrpic/P-OXD/LiF/Al. To our surprise, a peak luminous efficiency of 10.5 cd/A, a maximum brightness of 8200 cd/m2, and a turn-on voltage as low as 5.0 V have been realized (Fig. 5 and Table 1). At the same time, its EL spectrum solely exhibits the emission of FIrpic at around 472 nm with the CIE coordinates of (0.16, 0.33), and no emissions from PVK and P-OXD are observed (Fig. 5d). In comparison to device C, two more factors may contribute to the significant enhancement of the device performance. Firstly, high quality film of P-OXD atop of the EML can be prepared via spin-coating from its ethanol solution. As shown in the inset of Fig. 5d, the obtained film is very smooth with an average root-mean-square (RMS) surface roughness of 0.43 nm. This value is very close to the PFN-OH film (RMS = 0.53 nm) , suggesting that small molecules can also form good quality films by solution process. Secondly, originating from its deep HOMO energy level (−6.32 eV), the additional P-OXD layer can effectively block the hole, and thus prohibit the exciton quenching from the cathode [26–28]. It is worth noting that, P-OXD is among the few small molecular surfactants applied for the fabrication of solution-processed multilayer PLEDs [13–15], and its state-of-art performance is comparable to the polymeric surfactant, PFN-OH . Considering the fact that small molecules have well-defined structures, and are more easily purified as opposed to their polymeric counterparts, we believe, the investigation on water or alcohol-soluble small molecular surfactants will draw much more attention than ever in the future.
In conclusion, we have designed and synthesized an alcohol-soluble electron transporting material P-OXD functionalized with phosphonate groups, which can promote the efficient electron injection/transporting from the LiF/Al cathode. Meanwhile, a double-layer blue electrophosphorescent device has been successfully fabricated utilizing orthogonal solvent systems during the solution deposition, giving a promising efficiency as high as 10.5 cd/A. The comparable device performance suggests that small molecular surfactants are superior to the polymeric ones in terms of their monodispersed molecular structures as well as the ease of handling and purification.
The authors are grateful to the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CX07QZJC-24), the National Natural Science Foundation of China (Nos. 50828302 and 50803062), Science Fund for Creative Research Groups (No. 20921061), and 973 Project (2009CB623601) for financial support of this research.
References and links
1. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, and S. R. Forrest, “Highly efficient phosphorescent emission from organic electroluminescent devices,” Nature 395(6698), 151–154 (1998). [CrossRef]
2. I. D. Parker, Y. Cao, and C. Y. Yang, “Lifetime and degradation effects in polymer light-emitting diodes,” J. Appl. Phys. 85(4), 2441–2447 (1999). [CrossRef]
3. T.-W. Lee, M.-G. Kim, S. H. Park, S. Y. Kim, O. Kwon, T. Noh, J.-J. Park, T.-L. Choi, J. H. Park, and B. D. Chin, “Designing a stable cathode with multiple layers to improve the operational lifetime of polymer light-emitting diodes,” Adv. Funct. Mater. 19(12), 1863–1868 (2009). [CrossRef]
5. C. V. Hoven, A. Garcia, G. C. Bazan, and T.-Q. Nguyen, “Recent Applications of conjugated polyelectrolytes in optoelectronic devices,” Adv. Mater. (Deerfield Beach Fla.) 20(20), 3793–3810 (2008). [CrossRef]
6. H. Jiang, P. Taranekar, J. R. Reynolds, and K. S. Schanze, “Conjugated polyelectrolytes: synthesis, photophysics, and applications,” Angew. Chem. Int. Ed. Engl. 48(24), 4300–4316 (2009). [CrossRef] [PubMed]
7. F. Huang, H. Wu, and Y. Cao, “Water/alcohol soluble conjugated polymers as highly efficient electron transporting/injection layer in optoelectronic devices,” Chem. Soc. Rev. 39(7), 2500–2521 (2010). [CrossRef] [PubMed]
8. C. Zhong, C. Duan, F. Huang, H. Wu, and Y. Cao, “Materials and devices toward fully solution processable organic light-emitting diodes,” Chem. Mater. 23(3), 326–340 (2011). [CrossRef]
9. C. Duan, L. Wang, K. Zhang, X. Guan, and F. Huang, “Conjugated zwitterionic polyelectrolytes and their neutral precursor as electron injection layer for high-performance polymer light-emitting diodes,” Adv. Mater. (Deerfield Beach Fla.) 23(14), 1665–1669 (2011). [CrossRef] [PubMed]
10. J. Fang, B. H. Wallikewitz, F. Gao, G. Tu, C. Muller, G. Pace, R. H. Friend, and W. T. S. Huck, “Conjugated zwitterionic polyelectrolyte as the charge injection layer for high-performance polymer light-emitting diodes,” J. Am. Chem. Soc. 133(4), 683–685 (2011). [CrossRef] [PubMed]
11. X. Xu, B. Han, J. Chen, J. Peng, H. Wu, and Y. Cao, “2,7-Carbazole-1,4-phenylene Copolymers with polar side chains for cathode modifications in polymer light-emitting diodes,” Macromolecules 44(11), 4204–4212 (2011). [CrossRef]
12. F. Huang, Y.-H. Niu, Y. Zhang, J.-W. Ka, M. S. Liu, and A. K. Y. Jen, “A conjugated, neutral surfactant as electron-injection material for high-efficiency polymer light-emitting diodes,” Adv. Mater. (Deerfield Beach Fla.) 19(15), 2010–2014 (2007). [CrossRef]
13. X. Gong, S. Wang, D. Moses, G. C. Bazan, and A. J. Heeger, “Multilayer polymer light-emitting diodes: White-light emission with high efficiency,” Adv. Mater. (Deerfield Beach Fla.) 17(17), 2053–2058 (2005). [CrossRef]
14. R. Yang, Y. Xu, X.-D. Dang, T.-Q. Nguyen, Y. Cao, and G. C. Bazan, “Conjugated oligoelectrolyte electron transport/injection layers for organic optoelectronic devices,” J. Am. Chem. Soc. 130(11), 3282–3283 (2008). [CrossRef] [PubMed]
15. T. V. Pho, P. Zalar, A. Garcia, T.-Q. Nguyen, and F. Wudl, “Electron injection barrier reduction for organic light-emitting devices by quinacridone derivatives,” Chem. Commun. (Camb.) 46(43), 8210–8212 (2010). [CrossRef] [PubMed]
16. A. P. Kulkarni, C. J. Tonzola, A. Babel, and S. A. Jenekhe, “Electron transport materials for organic light-emitting diodes,” Chem. Mater. 16(23), 4556–4573 (2004). [CrossRef]
17. G. Hughes and M. R. Bryce, “Electron-transporting materials for organic electroluminescent and electrophosphorescent devices,” J. Mater. Chem. 15(1), 94–107 (2005). [CrossRef]
18. X. H. Yang, F. Jaiser, S. Klinger, and D. Neher, “Blue polymer electrophosphorescent devices with different electron-transporting oxadiazoles,” Appl. Phys. Lett. 88(2), 021107 (2006). [CrossRef]
19. B. Zhang, C. Qin, J. Ding, L. Chen, Z. Xie, Y. Cheng, and L. Wang, “High-performance all-polymer white-light-emitting diodes using polyfluorene containing phosphonate groups as an efficient electron-injection layer,” Adv. Funct. Mater. 20(17), 2951–2957 (2010). [CrossRef]
20. L. Chen, J. Ding, Y. Cheng, L. Wang, X. Jing, and F. Wang, “Twofold terminal post-functionalization of acetylacetone with hole- and electron-transporting fragments,” Tetrahedron Lett. 51(35), 4612–4616 (2010). [CrossRef]
21. T. Beissel, R. E. Powers, T. N. Parac, and K. N. Raymond, “Coordination number incommensurate cluster formation. 8. Dynamic isomerization of a supramolecular tetrahedral M4L6 cluster,” J. Am. Chem. Soc. 121(17), 4200–4206 (1999). [CrossRef]
22. C. S. Wang, G. Y. Jung, A. S. Batsanov, M. R. Bryce, and M. C. Petty, “New electron-transporting materials for light emitting diodes: 1,3,4-oxadiazole-pyridine and 1,3,4-oxadiazole-pyrimidine hybrids,” J. Mater. Chem. 12(2), 173–180 (2002). [CrossRef]
24. R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong, J. J. Brown, S. Garon, and M. E. Thompson, “Blue organic electrophosphorescence using exothermic host-guest energy transfer,” Appl. Phys. Lett. 82(15), 2422–2424 (2003). [CrossRef]
25. F. Huang, P.-I. Shih, C.-F. Shu, Y. Chi, and A. K. Y. Jen, “Highly efficient polymer white-light-emitting diodes based on lithium salts doped electron transporting layer,” Adv. Mater. (Deerfield Beach Fla.) 21(3), 361–365 (2009). [CrossRef]
26. V. E. Choong, M. G. Mason, C. W. Tang, and Y. G. Gao, “Investigation of the interface formation between calcium and tris-(8-hydroxy quinoline) aluminum,” Appl. Phys. Lett. 72(21), 2689–2691 (1998). [CrossRef]
27. Q. T. Le, L. Yan, Y. G. Gao, M. G. Mason, D. J. Giesen, and C. W. Tang, “Photoemission study of aluminum/tris-(8-hydroxyquinoline) aluminum and aluminum/LiF/tris-(8-hydroxyquinoline) aluminum interfaces,” J. Appl. Phys. 87(1), 375–379 (2000). [CrossRef]
28. M. Stoessel, G. Wittmann, J. Staudigel, F. Steuber, J. Blassing, W. Roth, H. Klausmann, W. Rogler, J. Simmerer, A. Winnacker, M. Inbasekaran, and E. P. Woo, “Cathode-induced luminescence quenching in polyfluorenes,” J. Appl. Phys. 87(9), 4467–4475 (2000). [CrossRef]