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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 2, Iss. 5 — May. 1, 2012
  • pp: 663–670
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Electrical and optical properties of blue organic light-emitting devices fabricated utilizing color conversion CdSe and CdSe/ZnS quantum dots embedded in a poly(N-vibyl carbazole) hole transport layer

Young Pyo Jeon, Sung June Park, and Tae Whan Kim  »View Author Affiliations


Optical Materials Express, Vol. 2, Issue 5, pp. 663-670 (2012)
http://dx.doi.org/10.1364/OME.2.000663


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Abstract

Blue organic light-emitting devices (OLEDs) with color conversion quantum dots (QDs) embedded in a poly(N-vinyl carbazole) (PVK) hole transport layer (HTL) were fabricated. The absorbance and the photoluminescence spectra for the CdSe and the CdSe/ZnS QDs showed dominant exciton peaks. Current densities as functions of the voltage showed enhanced hole trapping and a decreased hole current in the OLEDs containing CdSe and CdSe/ZnS QDs embedded in a HTL. The phenomena were intensified due to the existence of the ZnS shell. The luminance-voltage curve and the electroluminescence spectra showed that the brightness of the blue OLEDs fabricated utilizing the HTL based on CdSe and CdSe/ZnS QDs embedded in a PVK layer reached over 3,000 cd/m2 and that the dominant exciton peak was shifted to longer wavelength.

© 2012 OSA

1. Introduction

Rapid advancements in the growth technology for organic layers have made possible the fabrication of high-efficiency organic light-emitting devices (OLEDs) for applications in full color displays [1

1. Y. Zhang, M. Slootsky, and S. R. Forrest, “Enhanced efficiency in high-brightness fluorescent organic light emitting diodes through triplet management,” Appl. Phys. Lett. 99(22), 223303 (2011). [CrossRef]

3

3. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009). [CrossRef] [PubMed]

]. OLEDs have emerged as excellent candidates for potential applications in mobile displays, flat panel displays and lighting sources with excellent advantages of low-power consumption, fast response time, thin layer, high contrast ratio and low-cost production. Because, however, organic materials with low durability in the presence of moisture and oxidation are used in the fabrication of OLEDs, the stability of the OLEDs is low due to their degradation [4

4. Y. C. Han, C. Jang, K. J. Kim, K. C. Choi, K. H. Jung, and B.-S. Bae, “The encapsulation of an organic light-emitting diode using organic-inorganic hybrid materials and MgO,” Org. Electron. 12(4), 609–613 (2011). [CrossRef]

,5

5. T. D. Schmidt, A. Buchschuster, M. Holm, S. Nowy, J. A. Weber, and W. Brutting, “Degradation effect on the magnetoresistance in organic light emitting diodes,” Synth. Met. 161(7-8), 637–641 (2011). [CrossRef]

]. Blue OLEDs still have inherent problems of low efficiency, poor color purity and short lifetime in comparison with red or green OLEDs [6

6. J. Wan, C. J. Zheng, M. K. Fung, X. K. Liu, C. S. Lee, and X. H. Zhang, “Multifunctional electron-transporting indolizine derivatives for highly efficient blue fluorescence, orange phosphorescence host and two-color based white OLEDs,” J. Mater. Chem. 22(10), 4502–4510 (2012). [CrossRef]

]. Alternative fabrication technologies for fabricating OLEDs utilizing quantum dots (QDs) have been suggested to achieve high efficiency, high stability, high durability and low cost [7

7. L. Qian, Y. Zheng, J. H. Xue, and P. H. Holloway, “Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures,” Nat. Photonics 5(9), 543–548 (2011). [CrossRef]

9

9. S. O. Jeon, K. S. Yook, and J. Y. Lee, “Efficiency improvement of polymer light-emitting diodes using a quantum dot interlayer between a hole transport layer and an emitting layer,” Synth. Met. 160(1-2), 39–41 (2010). [CrossRef]

], but the efficiencies of OLEDs fabricated utilizing QDs with low dispersion and low carrier injection due to band-gap alignment with a supporting layer are still low. Even though some studies on the fabrication and the device characteristics of blue OLEDs utilizing QDs embedded in a supporting layer have been performed [10

10. K. S. Lee, D. U. Lee, D. C. Choo, T. W. Kim, E. D. Ryu, S. W. Kim, and J. S. Lim, “Organic light-emitting devices fabricated utilizing core/shell CdSe/ZnS quantum dots embedded in a polyvinylcarbazole,” J. Mater. Sci. 46(5), 1239–1243 (2011). [CrossRef]

12

12. S. O. Jeon, K. S. Yook, and J. Y. Lee, “Bistability and improved hole injection in organic bistable light-emitting diodes using a quantum dot embedded hole transport layer,” Synth. Met. 160(11-12), 1216–1218 (2010). [CrossRef]

], very few studies concerning the electrical and the optical properties of OLEDs with color conversion QDs embedded in an organic layer have been reported.

This paper reports data for the electrical and the optical properties of blue OLEDs fabricated utilizing color conversion CdSe and CdSe/ZnS QDs embedded in a poly(N-vinyl carbazole) (PVK) HTL. Current density-voltage-luminance and electroluminescence (EL) measurements were performed to investigate electrical and optical properties of the blue OLEDs with color conversion CdSe and CdSe/ZnS QDs embedded in a PVK HTL.

2. Experimental details

The blue OLEDs with color conversion CdSe and CdSe/ZnS QDs embedded in a PVK HTL used in this study were fabricated on indium-tin-oxide (ITO) layers coated on glass substrates. The sheet resistivity and the thickness of the ITO layer coated on the glass substrates used in this study were 15 ohm/square and 100 nm, respectively. The ITO substrates were cleaned in trichloroethylene, acetone, and methanol at 60°C for 15 min by using an ultrasonic cleaner and were rinsed in de-ionized water thoroughly. After the chemically cleaned ITO substrates had been dried by using N2 gas with a purity of 99.9999%, the surfaces of the ITO substrates were treated with an ultraviolet-ozone cleaner for 10 min at room temperature and a system pressure of 1 atm. After the prepared ITO substrates had been introduced into the evaporation chamber through a glove box in a high-purity N2 atmosphere, the organic layers and the metal layer were deposited on the ITO substrates at a substrate temperature of 27°C and a system pressure of 9.5 × 10−7 Torr. The deposition rates of the organic layers and the metal layer were approximately 1 and 1.5 Å/s, respectively, which were controlled by using a quartz crystal deposition rate/thickness monitor (Inficon, SQM-160). The poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) purchased from Sigma-Aldrich was spin-coated on the ITO-coated glass substrates and then annealed at 150°C for 10 min. The solutions of CdSe and CdSeZnS were synthesized in toluene at concentrations of 10 and 15 mg/ml, respectively. The QD solution and PVK solution blended with 0.7 wt% chlorobenzene were stirred at a 2:1 ratio for 12 h. The compound solutions were spin-coated on the PEDOT:PSS layer and then annealed at 150°C for 10 min. The solution process was performed in a glove box under a nitrogen atmosphere.

The two kind of OLEDs with color conversion QDs embedded in the PVK HTL used in this study were deposited on ITO substrates and consisted of the following structures from the top: an Al cathode electrode (200 nm)/a lithium quinolate (Liq) electron injection layer (EIL) (0.5 nm)/a 4,7-diphenyl-1,10-phenanthroline (BPhen) electron transport layer (30 nm)/a 1,4-bis(2,2-diphenylvinyl) biphenyl (DPVBi) (30 nm) emitting layer (EML)/a N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (NPB) (15 nm) spacer layer/color conversion CdSe or CdSe/ZnS QDs embedded a PVK HTL/a PEDOT:PSS hole injection layer (HIL) (40 nm)/an ITO anode (150 nm)/a glass substrate. The NPB spacer layer was used to enhance radiative energy transfer from the blue EML to the color conversion QDs [13

13. R. Carminati, J.-J. Greffet, C. Henkel, and J. M. Vigoureux, “Radiative and non-radiative decay of a single molecule close to a metallic nanoparticle,” Opt. Commun. 261(2), 368–375 (2006). [CrossRef]

,14

14. T. Schwab, M. Thomschke, S. Hofmann, M. Furno, K. Leo, and B. Lussem, “Efficiency enhancement of top-emitting organic light-emitting diodes using conversion dyes,” J. Appl. Phys. 110(8), 083118 (2011). [CrossRef]

]. The OLEDs with HTLs containing CdSe and CdSe/ZnS QDs are denoted by devices I and II, respectively.

Schematic diagrams of the OLED structures and their corresponding energy bands are shown in Figs. 1(a)
Fig. 1 Schematic diagrams of the (a) blue organic light-emitting device (OLED) structure and (b) corresponding energy bands of the OLEDs containing CdSe quantum dots (QDs) or CdSe/ZnS QDs.
and 1(b), respectively [15

15. W. K. Bae, J. Kwak, J. W. Park, K. H. Char, C. H. Lee, and S. Lee, “Highly efficient green-light-emitting diodes based on CdSe@ZnS quantum dots with a chemical-composition gradient,” Adv. Mater. 21(17), 1690–1694 (2009). [CrossRef]

17

17. P. Jing, X. Yuan, W. Ji, M. Ikezawa, X. Liu, L. Zhang, J. Zhao, and Y. Masumoto, “Efficient energy transfer from hole transporting materials to CdSe-core CdS/ZnCdS/ZnS-multishell quantum dots in type II aligned blend films,” Appl. Phys. Lett. 99(9), 093106 (2011). [CrossRef]

]. A desiccant material was used to absorb the residual moisture and oxygen in the encapsulated device. The blue OLEDs were encapsulated by using a covering glass and an adhesive epoxy sealant. The current-voltage characteristics were measured on a programmable electrometer with built-in current and voltage measurement units (M6100, McScience). The brightness was measured by using a brightness meter, and the EL spectrum was measured by using a luminescence spectrometer (CS-1000, Minolta).

3. Results and discussion

The luminance efficiencies as functions of the current density for devices I and II are shown in Fig. 4
Fig. 4 Luminance efficiencies as functions of the current density for devices I and II. Filled circles and opened circles represent devices I and II, respectively.
. The luminance efficiencies of devices I and II increased rapidly with increasing current density to approximately 100 mA/cm2, after which they remained fairly constant at values of 1.10 and 0.14 cd/A, respectively. The lower efficiency of device I in comparison with that of device II originated from the decrease of the current density and the brightness due to the strong carrier trapping and optical absorption forces resulting from the existence of the ZnS shell.

4. Summary and conclusions

The electrical and the optical properties of OELDs fabricated with a PVK HTL containing CdSe and CdSe/ZnS QDs were measured. PL spectra for the CdSe and the CdSe/ZnS QDs showed dominant exciton peaks. Current densities as functions of the voltage showed that the enhanced hole trapping and decreased hole current in the OLEDs could be attributed to the existence of the ZnS shell. The brightness of the OLEDs with CdSe/ZnS QDs was much lower than that of OLEDs with CdSe QDs because CdSe/ZnS QDs had strong carrier trapping and optical absorption force than CdSe QDs resulting from the ZnS shell. EL spectra for blue OLEDs containing CdSe and CdSe/ZnS QDs showed that the differences in the peak positions and the FWHMs were color conversion effects resulting from the insertions of the CdSe and the CdSe/ZnS QDs in the HTL. Even though the brightness of device II was lower than that of device I, the color conversion efficiency of the device II was higher than that of device I demonstrated by slight red-shifted EL emission spectra. The red-shift magnitude of the PL peaks for OLEDs dependent on the host materials and the type of the QDs affected the device performance. These results can help improve understanding of blue OLEDs based on color conversion QDs embedded in a HTL.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0018877).

References and links

1.

Y. Zhang, M. Slootsky, and S. R. Forrest, “Enhanced efficiency in high-brightness fluorescent organic light emitting diodes through triplet management,” Appl. Phys. Lett. 99(22), 223303 (2011). [CrossRef]

2.

H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe, G. Wagenblast, N. Langer, O. Molt, E. Fuchs, C. Lennartz, and J. Kido, “High-efficiency blue and white organic light-emitting devices incorporating a blue iridium carbene complex,” Adv. Mater. 22(44), 5003–5007 (2010). [CrossRef] [PubMed]

3.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009). [CrossRef] [PubMed]

4.

Y. C. Han, C. Jang, K. J. Kim, K. C. Choi, K. H. Jung, and B.-S. Bae, “The encapsulation of an organic light-emitting diode using organic-inorganic hybrid materials and MgO,” Org. Electron. 12(4), 609–613 (2011). [CrossRef]

5.

T. D. Schmidt, A. Buchschuster, M. Holm, S. Nowy, J. A. Weber, and W. Brutting, “Degradation effect on the magnetoresistance in organic light emitting diodes,” Synth. Met. 161(7-8), 637–641 (2011). [CrossRef]

6.

J. Wan, C. J. Zheng, M. K. Fung, X. K. Liu, C. S. Lee, and X. H. Zhang, “Multifunctional electron-transporting indolizine derivatives for highly efficient blue fluorescence, orange phosphorescence host and two-color based white OLEDs,” J. Mater. Chem. 22(10), 4502–4510 (2012). [CrossRef]

7.

L. Qian, Y. Zheng, J. H. Xue, and P. H. Holloway, “Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures,” Nat. Photonics 5(9), 543–548 (2011). [CrossRef]

8.

Y. Q. Zhang and X. A. Cao, “Electroluminescence of green CdSe/ZnS quantum dots enhanced by harvesting excitons from phosphorescent molecules,” Appl. Phys. Lett. 97(25), 253115 (2010). [CrossRef]

9.

S. O. Jeon, K. S. Yook, and J. Y. Lee, “Efficiency improvement of polymer light-emitting diodes using a quantum dot interlayer between a hole transport layer and an emitting layer,” Synth. Met. 160(1-2), 39–41 (2010). [CrossRef]

10.

K. S. Lee, D. U. Lee, D. C. Choo, T. W. Kim, E. D. Ryu, S. W. Kim, and J. S. Lim, “Organic light-emitting devices fabricated utilizing core/shell CdSe/ZnS quantum dots embedded in a polyvinylcarbazole,” J. Mater. Sci. 46(5), 1239–1243 (2011). [CrossRef]

11.

A. Uddin and C. C. Teo, “Differential capacitance of hybrid organic/inorganic CdSe/ZnS quantum dots light-emitting device,” Appl. Phys., A Mater. Sci. Process. 105(1), 39–43 (2011). [CrossRef]

12.

S. O. Jeon, K. S. Yook, and J. Y. Lee, “Bistability and improved hole injection in organic bistable light-emitting diodes using a quantum dot embedded hole transport layer,” Synth. Met. 160(11-12), 1216–1218 (2010). [CrossRef]

13.

R. Carminati, J.-J. Greffet, C. Henkel, and J. M. Vigoureux, “Radiative and non-radiative decay of a single molecule close to a metallic nanoparticle,” Opt. Commun. 261(2), 368–375 (2006). [CrossRef]

14.

T. Schwab, M. Thomschke, S. Hofmann, M. Furno, K. Leo, and B. Lussem, “Efficiency enhancement of top-emitting organic light-emitting diodes using conversion dyes,” J. Appl. Phys. 110(8), 083118 (2011). [CrossRef]

15.

W. K. Bae, J. Kwak, J. W. Park, K. H. Char, C. H. Lee, and S. Lee, “Highly efficient green-light-emitting diodes based on CdSe@ZnS quantum dots with a chemical-composition gradient,” Adv. Mater. 21(17), 1690–1694 (2009). [CrossRef]

16.

W. Ki Bae, J. H. Kwak, J. H. Lim, D. G. Lee, M. Ki Nam, K. H. Char, C. H. Lee, and S. H. Lee, “Deep blue light-emitting diodes based on Cd1-xZnx S @ ZnS quantum dots,” Nanotechnology 20(7), 075202 (2009). [CrossRef] [PubMed]

17.

P. Jing, X. Yuan, W. Ji, M. Ikezawa, X. Liu, L. Zhang, J. Zhao, and Y. Masumoto, “Efficient energy transfer from hole transporting materials to CdSe-core CdS/ZnCdS/ZnS-multishell quantum dots in type II aligned blend films,” Appl. Phys. Lett. 99(9), 093106 (2011). [CrossRef]

18.

F. Li, T. Guo, and T. W. Kim, “Charge trapping in hybrid electroluminescence device containing CdSe/ZnS quantum dots embedded in a conducting poly(N-vinylcarbozole) layer,” Appl. Phys. Lett. 97(6), 062104 (2010). [CrossRef]

19.

B. H. Zhu, H. C. Zhang, Z. Y. Zhang, Y. P. Cui, and J. Y. Zhang, “Effect of shell thickness on two-photon absorption and refraction of colloidal CdSe/CdS core/shell nanocrystals,” Appl. Phys. Lett. 99(23), 231903 (2011). [CrossRef]

20.

I. W. Wu, P. S. Wang, W. H. Tseng, J. H. Chang, and C. I. Wu, “Correlations of impedance-voltage characteristics and carrier mobility in organic light emitting diodes,” Org. Electron. 13(1), 13–17 (2012). [CrossRef]

21.

C. T. Sun, I. H. Chan, P. C. Kao, and S. Y. Chu, “Electron injection and transport mechanisms of an electron transport layer in OLEDs,” J. Electrochem. Soc. 158(12), H1284–H1288 (2011). [CrossRef]

OCIS Codes
(230.0230) Optical devices : Optical devices
(230.5170) Optical devices : Photodiodes
(230.5590) Optical devices : Quantum-well, -wire and -dot devices

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: February 16, 2012
Revised Manuscript: April 5, 2012
Manuscript Accepted: April 6, 2012
Published: April 20, 2012

Virtual Issues
Quantum Dots for Photonic Applications (2012) Optical Materials Express

Citation
Young Pyo Jeon, Sung June Park, and Tae Whan Kim, "Electrical and optical properties of blue organic light-emitting devices fabricated utilizing color conversion CdSe and CdSe/ZnS quantum dots embedded in a poly(N-vibyl carbazole) hole transport layer," Opt. Mater. Express 2, 663-670 (2012)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-2-5-663


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References

  1. Y. Zhang, M. Slootsky, and S. R. Forrest, “Enhanced efficiency in high-brightness fluorescent organic light emitting diodes through triplet management,” Appl. Phys. Lett.99(22), 223303 (2011). [CrossRef]
  2. H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe, G. Wagenblast, N. Langer, O. Molt, E. Fuchs, C. Lennartz, and J. Kido, “High-efficiency blue and white organic light-emitting devices incorporating a blue iridium carbene complex,” Adv. Mater.22(44), 5003–5007 (2010). [CrossRef] [PubMed]
  3. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature459(7244), 234–238 (2009). [CrossRef] [PubMed]
  4. Y. C. Han, C. Jang, K. J. Kim, K. C. Choi, K. H. Jung, and B.-S. Bae, “The encapsulation of an organic light-emitting diode using organic-inorganic hybrid materials and MgO,” Org. Electron.12(4), 609–613 (2011). [CrossRef]
  5. T. D. Schmidt, A. Buchschuster, M. Holm, S. Nowy, J. A. Weber, and W. Brutting, “Degradation effect on the magnetoresistance in organic light emitting diodes,” Synth. Met.161(7-8), 637–641 (2011). [CrossRef]
  6. J. Wan, C. J. Zheng, M. K. Fung, X. K. Liu, C. S. Lee, and X. H. Zhang, “Multifunctional electron-transporting indolizine derivatives for highly efficient blue fluorescence, orange phosphorescence host and two-color based white OLEDs,” J. Mater. Chem.22(10), 4502–4510 (2012). [CrossRef]
  7. L. Qian, Y. Zheng, J. H. Xue, and P. H. Holloway, “Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures,” Nat. Photonics5(9), 543–548 (2011). [CrossRef]
  8. Y. Q. Zhang and X. A. Cao, “Electroluminescence of green CdSe/ZnS quantum dots enhanced by harvesting excitons from phosphorescent molecules,” Appl. Phys. Lett.97(25), 253115 (2010). [CrossRef]
  9. S. O. Jeon, K. S. Yook, and J. Y. Lee, “Efficiency improvement of polymer light-emitting diodes using a quantum dot interlayer between a hole transport layer and an emitting layer,” Synth. Met.160(1-2), 39–41 (2010). [CrossRef]
  10. K. S. Lee, D. U. Lee, D. C. Choo, T. W. Kim, E. D. Ryu, S. W. Kim, and J. S. Lim, “Organic light-emitting devices fabricated utilizing core/shell CdSe/ZnS quantum dots embedded in a polyvinylcarbazole,” J. Mater. Sci.46(5), 1239–1243 (2011). [CrossRef]
  11. A. Uddin and C. C. Teo, “Differential capacitance of hybrid organic/inorganic CdSe/ZnS quantum dots light-emitting device,” Appl. Phys., A Mater. Sci. Process.105(1), 39–43 (2011). [CrossRef]
  12. S. O. Jeon, K. S. Yook, and J. Y. Lee, “Bistability and improved hole injection in organic bistable light-emitting diodes using a quantum dot embedded hole transport layer,” Synth. Met.160(11-12), 1216–1218 (2010). [CrossRef]
  13. R. Carminati, J.-J. Greffet, C. Henkel, and J. M. Vigoureux, “Radiative and non-radiative decay of a single molecule close to a metallic nanoparticle,” Opt. Commun.261(2), 368–375 (2006). [CrossRef]
  14. T. Schwab, M. Thomschke, S. Hofmann, M. Furno, K. Leo, and B. Lussem, “Efficiency enhancement of top-emitting organic light-emitting diodes using conversion dyes,” J. Appl. Phys.110(8), 083118 (2011). [CrossRef]
  15. W. K. Bae, J. Kwak, J. W. Park, K. H. Char, C. H. Lee, and S. Lee, “Highly efficient green-light-emitting diodes based on CdSe@ZnS quantum dots with a chemical-composition gradient,” Adv. Mater.21(17), 1690–1694 (2009). [CrossRef]
  16. W. Ki Bae, J. H. Kwak, J. H. Lim, D. G. Lee, M. Ki Nam, K. H. Char, C. H. Lee, and S. H. Lee, “Deep blue light-emitting diodes based on Cd1-xZnx S @ ZnS quantum dots,” Nanotechnology20(7), 075202 (2009). [CrossRef] [PubMed]
  17. P. Jing, X. Yuan, W. Ji, M. Ikezawa, X. Liu, L. Zhang, J. Zhao, and Y. Masumoto, “Efficient energy transfer from hole transporting materials to CdSe-core CdS/ZnCdS/ZnS-multishell quantum dots in type II aligned blend films,” Appl. Phys. Lett.99(9), 093106 (2011). [CrossRef]
  18. F. Li, T. Guo, and T. W. Kim, “Charge trapping in hybrid electroluminescence device containing CdSe/ZnS quantum dots embedded in a conducting poly(N-vinylcarbozole) layer,” Appl. Phys. Lett.97(6), 062104 (2010). [CrossRef]
  19. B. H. Zhu, H. C. Zhang, Z. Y. Zhang, Y. P. Cui, and J. Y. Zhang, “Effect of shell thickness on two-photon absorption and refraction of colloidal CdSe/CdS core/shell nanocrystals,” Appl. Phys. Lett.99(23), 231903 (2011). [CrossRef]
  20. I. W. Wu, P. S. Wang, W. H. Tseng, J. H. Chang, and C. I. Wu, “Correlations of impedance-voltage characteristics and carrier mobility in organic light emitting diodes,” Org. Electron.13(1), 13–17 (2012). [CrossRef]
  21. C. T. Sun, I. H. Chan, P. C. Kao, and S. Y. Chu, “Electron injection and transport mechanisms of an electron transport layer in OLEDs,” J. Electrochem. Soc.158(12), H1284–H1288 (2011). [CrossRef]

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