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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 23 — Nov. 18, 2013
  • pp: 28040–28047
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Enhanced and balanced efficiency of white bi-directional organic light-emitting diodes

Jonghee Lee, Hyunsu Cho, Tae-Wook Koh, Changhun Yun, Simone Hofmann, Jae-Hyun Lee, Yong Hyun Kim, Björn Lüssem, Jeong-Ik Lee, Karl Leo, Malte C. Gather, and Seunghyup Yoo  »View Author Affiliations


Optics Express, Vol. 21, Issue 23, pp. 28040-28047 (2013)
http://dx.doi.org/10.1364/OE.21.028040


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Abstract

We report on the characteristics of enhanced and balanced white-light emission from bi-directional organic light-emitting diodes (BiOLEDs) enabled by the introduction of micro-cavity effects. The insertion of an additional metal layer between the indium tin oxide anode and the hole transporting layer results in similar light output of our BiOLEDs in both top and bottom direction and in reduced distortion of the electroluminescence spectrum. Furthermore, we find that by utilizing MC effects, the overall current efficiency can be improved by 26.2% compared to that of a conventional device.

© 2013 Optical Society of America

1. Introduction

White organic light-emitting diodes (OLEDs) are attracting widespread attention as next-generation low-cost and high-efficiency thin-film electroluminescent devices for both flat panel displays and lighting applications [1

1. 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]

3

3. S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan, and W. Huang, “Recent developments in top-emitting organic light-emitting diodes,” Adv. Mater. 22(46), 5227–5239 (2010). [CrossRef] [PubMed]

] due to recent rapid technical evolution. The uniqueness of these light-emitting devices can for example be demonstrated in transparent or bi-directional OLEDs (TOLEDs or BiOLEDs), where light is emitted from both the bottom and the top side of the device; this intrinsic bi-directional emission capability of OLEDs clearly distinguishes them from other light-emitting devices [4

4. H. W. Chang, J.-H. Lee, S. Hofmann, Y. H. Kim, L. Müller-Meskamp, B. Lüssem, C.-C. Wu, K. Leo, and M. C. Gather, “Nano-particle based scattering layers for optical efficiency enhancement of organic light-emitting diodes and organic solar cells,” J. Appl. Phys. 113(20), 204502 (2013). [CrossRef]

11

11. J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. H. Kim, B. Lüssem, and K. Leo, “Highly efficient bi-directional organic light-emitting diodes by strong micro-cavity effects,” Appl. Phys. Lett. 99(7), 073303 (2011). [CrossRef]

].

Here, we explore the characteristics of white BiOLEDs in which a 10-nm-thick Ag layer is added in between organic and ITO layers. We show that the added metal layer is thin enough not to cause a significant change in the spectral output of white BiOLEDs yet thick enough to enhance their efficiency and obtain balanced top/ bottom emissions. White BiOLEDs with a current efficiency (CE) enhancement of 26.2% are demonstrated, and the relative efficiency enhancement is shown to be relatively uniform across the whole visible spectrum.

2. Experiment

A series of white phosphorescent BiOLEDs based on p-i-n doped structures was fabricated with the following configuration: indium tin oxide (ITO) (90 nm)/inserted Ag layer (0 or 10 nm)/p-layer/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (10 nm)/emissive layer (EML, 20 nm)/ 2-(diphenylphosphoryl)spirofluorene (SPPO1) (10 nm)/n-layer/Ag (top cathode, 15 nm). The additional 10-nm-thick Ag layer deposited on top of the ITO anode was included to investigate the MC effect on the performance of the white BiOLEDs, as shown in Fig. 1
Fig. 1 Device structures of the bi-directional OLEDs tested in this study.
. As the p-type hole injection and transport layer, we used 30-nm-thick films of N,N,N,N’-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) doped with 4 wt % of the p-dopant 2,2'-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ, Novaled AG, Dresden). The n-type electron injection and transport layer was 30 nm of 4,7-diphenyl-1,10-phenanthroline (BPhen) doped with cesium (Cs). A 10-nm-thick film of TAPC and a 10-nm-thick film of SPPO1 were used as electron and hole blocking layers, respectively, to confine charge carriers and excitons within the EML. Two blue-emitting EMLs, one containing 7 wt% blue light-emitting iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C2’)picolinate (FIrpic) co-deposited with the hole-transport type host material 4,4’,4”-tri(N-carbazolyl)triphenylamine (TCTA, 5 nm) and another containing 10 wt% FIrpic co-deposited with the bipolar type host material 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy, 5 nm), were successively deposited [12

12. J. Lee, J. -I. Lee, J. -W. Lee, and H. Y. Chu, “Interlayer engineering with different host material properties in blue phosphorescent organic light-emitting diodes ” ETRI. J. 32, 32-38 (2011).

, 13

13. J.-I. Lee, J. Lee, J.-W. Lee, D.-H. Cho, J.-W. Shin, J.-H. Han, and H. Y. Chu, “Dependence of light-emitting characteristics of blue phosphorescent organic light-emitting diodes on electron injection and transport materials,” ETRI J. 34(5), 690–695 (2012). [CrossRef]

]. Two types of white BiOLEDs were fabricated: Device A was based on a 2-color white approach, a 1-nm-thick layer of TCTA doped with 6 wt% of the red emitter iridium (III) bis[2-methyldibenzo-(f,h)quinoxaline](acetylacetonate) (Ir(MDQ)2(acac)) was inserted between the two blue EMLs. Device B was a 3-color WOLED with a 1-nm-thick TCTA layer doped with 6 wt% Ir(MDQ)2(acac) and another 1-nm-thick TCTA layer doped with 6 wt% of the orange emitter iridium (III) bis(2-(9,9-dihexylfluorenyl)-1-pyridine) (acetylacetonate) (Ir(dhfpy)2(acac)). For all devices, a 50-nm-thick layer of TCTA was deposited as a dielectric capping layer on top of the cathode [9

9. J. Lee, S. Hofmann, M. Furno, M. Thomschke, Y. H. Kim, B. Lüssem, and K. Leo, “Influence of organic capping layers on the performance of transparent organic light-emitting diodes,” Opt. Lett. 36(8), 1443–1445 (2011). [CrossRef] [PubMed]

, 10

10. J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. H. Kim, B. Lüssem, and K. Leo, “Increased and balanced light emission of transparent organic light-emitting diodes by enhanced microcavity effects,” Opt. Lett. 36(15), 2931–2933 (2011). [CrossRef] [PubMed]

].

The current density-voltage-luminance (J-V-L) characteristics of the devices were measured with a source measure unit (Keithley 2400), and the spectral radiant intensity was determined by a calibrated spectroradiometer (Instrument Systems GmbH CAS140).

3. Results and discussions

It is noteworthy that the top/bottom balancing effect due to the insertion of the Ag layer in Device B2 is achieved without a significant compromise in the bottom emission, as shown in Fig. 3
Fig. 3 Current efficiency of bottom- and top-emission versus current density of (a) Devices A1 and A2 and (b) Devices B1 and B2.
and Table 1

Table 1. Performance parameters of the white BiOLEDs under study*

table-icon
View This Table
. In fact, the current efficiency (CE) for bottom emission decreases only slightly from 17.8 cd/A to 15.7 cd/A at J of 15 mA/cm2, when the Ag layer is added. The CE for top emission, on the other hand, increases significantly by a factor of 2.65 from 4.8 cd/A to 12.8 cd/A, resulting in an overall enhancement in the CE for total emission by more than 25%.

From the result of optical analysis shown in Fig. 4
Fig. 4 Calculated enhancement in multiple-beam resonance [gres(λ)] and two-beam interference [fTB(λ)] terms due to the additional Ag layer in the bottom electrode of the white BiOLEDs under study. Calculation was based on Fabry-Perot formalism in a simplified structure of front electrode/ organic layer/ rear electrode using Eqs. (1)-(3).
, we can attribute the observed enhancement to the multiple-beam cavity resonance enhancement as well as to constructive two-beam interference that can occur efficiently in both emission directions as the reflectance of the bottom Ag layer (anode side) is comparable to that of the top Ag layer (cathode side) [11

11. J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. H. Kim, B. Lüssem, and K. Leo, “Highly efficient bi-directional organic light-emitting diodes by strong micro-cavity effects,” Appl. Phys. Lett. 99(7), 073303 (2011). [CrossRef]

]. For top emission, the ratio of gres(λ) obtained for devices with the inserted Ag [ = gres(Ag)(λ) ] to that for devices without it [ = gres(no Ag)(λ) ] indicates that there is indeed a resonance-induced enhancement centered around λ of 550 nm with the peak enhancement ratio of 1.56. However, the two-beam interference effect appears to be more important by comparison; as can be seen in Fig. 4, the enhancement ratio for fTB(λ) ranges from 1.75 to 2.20 throughout the visible spectral range.

For bottom emission, on the other hand, the two-beam interference effect remains unchanged because there is no change in the reflectance from the top electrode. The multiple-beam cavity resonance enhancement given by the denominator in Eq. (2) is same as that in the top-emission case, but it is over shadowed by the reduced transmittance due to the Ag layer, which leads to a decrease in the ‘gres(Ag)/ gres(no Ag)’-ratio by a factor of Tbot(Ag)/Tbot(no Ag) (shown as a dashed gray line in Fig. 4) at a given wavelength. This reduced transmittance leads to a small net decrease in light output for the bottom direction. In addition, we observe that the bottom-to-top CE ratio (γCE = CE for bottom-emission / CE for top-emission) is strongly influenced by the inserted Ag layer. The γCE values of the reference Devices A1 and B1 without Ag are 3.89 and 3.67, respectively, implying imbalanced emissions in these devices, with the bottom emission being much stronger than the top emission. In contrast, the γCE values of Devices A2 and B2 with the Ag layer are 1.20 and 1.23, respectively. The introduction of the Ag layer on top of the ITO can thus achieve a good balance between the bottom emission and the top emission. As mentioned previously, spectrally balanced broadband white emission is of great importance for practical applications of white MC-OLEDs but achieving such characteristics has turned out to be very challenging, especially for unidirectional bottom- or top-emitting white MC-OLEDs.

For the BiOLEDs investigated here, the EL spectra shown in Fig. 5
Fig. 5 Electroluminescence spectra emitted in bottom and top-direction for (a) Devices A1 and A2 and (b) Devices B1 and B2 at a driving current density of 15 mA/cm2.
indicate that balanced broadband white emission is emitted in the top and bottom direction. Although introducing the additional Ag layer tends to increase MC effects, the EL spectra of Devices A2 and B2 do not exhibit significant distortions compared to the respective reference (Devices A1 and B1). As a result, the color rendering index (CRI), the Commission Internationale del'Eclairage (CIE) color coordinates, and the correlated color temperature (CCT), of the devices with additional Ag layer are comparable to the values for the reference devices. CRI, CIE and CCT are summarized in Table 1. For instance, for bottom emission, Device B2 exhibits a CRI of 74.8, CIE coordinates of (0.43, 0.44), and a CCT of 3404 K (at a current density of 15 mA/cm2). These values are near the values necessary to satisfy the Energy Star requirements for solid-state lighting applications [22

22. “ENERGY STAR ® Program Requirements for Solid State Lighting Luminaires Eligibility Criteria – Version 1.1” 1–23 (2008).

].

As can be seen in Fig. 6(a)
Fig. 6 (a) The overall enhancement ratio of the forward intensity I(λ) due to the additional Ag layer in the bottom electrode of white BiOLEDs: comparison between experimental data (Device A) and simulation results. (b)-(c) Simulated intensity for various thickness values of the bottom Ag layer ( = dAg(bott.)) for the thickness of the top Ag layer of 15 nm (b) and 30 nm (c). Shown in (b) and (c) are the values obtained for top-emission direction. Simulation in Fig. 6 was done using the full classical electromagnetic model considering dipole emitters embedded in a microcavity structure as described in Ref. 20 with the unity radiative quantum efficiency assumed.
, the simulation results obtained under a full classical electromagnetic formalism describing dipole emitters in a cavity structure [20

20. M. Furno, R. Meerheim, S. Hofmann, B. Lüssem, and K. Leo, “Efficiency and rate of spontaneous emission in organic electroluminescent devices,” Phys. Rev. B 85(11), 115205 (2012). [CrossRef]

, 21

21. W. Brütting, J. Frischeisen, T. D. Schmidt, B. J. Scholz, and C. Mayr, “Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling,” Phys. Status Solidi A 210(1), 44–65 (2013). [CrossRef]

, 23

23. M. Furno, T. C. Rosenow, M. C. Gather, B. Lüssem, and K. Leo, “Analysis of the external and internal quantum efficiency of multi-emitter, white organic emitting diodes,” Appl. Phys. Lett. 101(14), 143304 (2012). [CrossRef]

] show a good match to the experimental data obtained for Devices A, and it indicates that the intensity enhancement ratio between those with and without the bottom Ag layers follows a broad spectral envelope with the full-width half-maximum (FWHM) even comparable to the full visible spectral range, being consistent with the spectrally balanced enhancement in the proposed white BiOLEDs. (See Fig. 6(a)) The simulation results presented in Fig. 6(b) and 6(c) for top-emission direction further show that increasing the MC effect by making both of the Ag layers too thick indeed reduce the spectral width of the cavity-induced intensity enhancement, which may then cause undesirable side effects such as spectral distortion or reduced CRI. Nevertheless, the proposed white BiOLEDs are expected to exhibit spectrally balanced enhancement over the entire visible spectral range provided that the thickness of top and bottom Ag layers is maintained within the range of approximately 10 nm - 20 nm.

4. Conclusions

In summary, the performance of white-emitting BiOLEDs has been enhanced and the balance between bottom- and top-emission has been improved by controlled introduction of MC effects. We find that the overall current efficacy of BiOLEDs can be increased without significant distortion of the white EL spectra when the reflectance of the electrodes in the BiOLED is carefully managed.

Acknowledgments

The authors thank Novaled AG, Dresden for cooperation. This work was in part financed by the European Social Fund and the Free State of Saxony through the OrthoPhoto projectJ. Lee acknowledges the Alexander von Humboldt Foundation and the IT R&D program ofMSIP/KEIT (Grant No. 10041416, “The core technology development of light and space adaptable new mode display for energy savings on 7 inch and 2 W”). S. Yoo acknowledges a financial support by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2011-013-D00046 and CAFDC/Seunghyup Yoo/No. 2013042126).

References and links

1.

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]

2.

M. C. Gather, A. Köhnen, and K. Meerholz, “White organic light-emitting diodes,” Adv. Mater. 23(2), 233–248 (2011). [CrossRef] [PubMed]

3.

S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan, and W. Huang, “Recent developments in top-emitting organic light-emitting diodes,” Adv. Mater. 22(46), 5227–5239 (2010). [CrossRef] [PubMed]

4.

H. W. Chang, J.-H. Lee, S. Hofmann, Y. H. Kim, L. Müller-Meskamp, B. Lüssem, C.-C. Wu, K. Leo, and M. C. Gather, “Nano-particle based scattering layers for optical efficiency enhancement of organic light-emitting diodes and organic solar cells,” J. Appl. Phys. 113(20), 204502 (2013). [CrossRef]

5.

K. S. Yook, S. O. Jeon, C. W. Joo, and J. Y. Lee, “Transparent organic light emitting diodes using a multilayer oxide as a low resistance transparent cathode,” Appl. Phys. Lett. 93(1), 013301 (2008).

6.

H. Cho, J.-M. Choi, and S. Yoo, “Highly transparent organic light-emitting diodes with a metallic top electrode: the dual role of a Cs2CO3 layer,” Opt. Express 19(2), 1113–1121 (2011). [CrossRef] [PubMed]

7.

J.-H. Lee, S. Lee, J.-B. Kim, J. Jang, and J.-J. Kim, “A high performance transparent inverted organic light emitting diode with 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile as an organic buffer layer,” J. Mater. Chem. 22(30), 15262–15266 (2012). [CrossRef]

8.

J. W. Huh, J. Moon, J. W. Lee, D.-H. Cho, J.-W. Shin, J.-H. Han, J. Hwang, C. W. Joo, H. Y. Chu, and J.-I. Lee, “Directed emissive high efficient white transparent organic light emitting diodes with double layered capping layer,” Org. Electron. 13(8), 1386–1391 (2012). [CrossRef]

9.

J. Lee, S. Hofmann, M. Furno, M. Thomschke, Y. H. Kim, B. Lüssem, and K. Leo, “Influence of organic capping layers on the performance of transparent organic light-emitting diodes,” Opt. Lett. 36(8), 1443–1445 (2011). [CrossRef] [PubMed]

10.

J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. H. Kim, B. Lüssem, and K. Leo, “Increased and balanced light emission of transparent organic light-emitting diodes by enhanced microcavity effects,” Opt. Lett. 36(15), 2931–2933 (2011). [CrossRef] [PubMed]

11.

J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. H. Kim, B. Lüssem, and K. Leo, “Highly efficient bi-directional organic light-emitting diodes by strong micro-cavity effects,” Appl. Phys. Lett. 99(7), 073303 (2011). [CrossRef]

12.

J. Lee, J. -I. Lee, J. -W. Lee, and H. Y. Chu, “Interlayer engineering with different host material properties in blue phosphorescent organic light-emitting diodes ” ETRI. J. 32, 32-38 (2011).

13.

J.-I. Lee, J. Lee, J.-W. Lee, D.-H. Cho, J.-W. Shin, J.-H. Han, and H. Y. Chu, “Dependence of light-emitting characteristics of blue phosphorescent organic light-emitting diodes on electron injection and transport materials,” ETRI J. 34(5), 690–695 (2012). [CrossRef]

14.

S. Olthof, W. Tress, R. Meerheim, B. Lüssem, and K. Leo, “Photoelectron spectroscopy study of systematically varied doping concentrations in an organic semiconductor layer using a molecular p-dopant,” J. Appl. Phys. 106(10), 103711 (2009). [CrossRef]

15.

M. Agrawal, Y. Sun, S. R. Forrest, and P. Peumans, “Enhanced outcoupling from organic light-emitting diodes using aperiodic dielectric mirrors,” Appl. Phys. Lett. 90(24), 241112 (2007). [CrossRef]

16.

S. Hofmann, M. Thomschke, P. Freitag, M. Furno, B. Lüssem, and K. Leo, “Top-emitting organic light-emitting diodes: Influence of cavity design,” Appl. Phys. Lett. 97(25), 253308 (2010). [CrossRef]

17.

R. Meerheim, M. Furno, S. Hofmann, B. Lüssem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97(25), 253305 (2010). [CrossRef]

18.

T. Schwab, S. Schubert, S. Hofmann, M. Fröbel, M. Thomschke, L. Müller-Meskamp, K. Leo, and M. C. Gather, “Highly efficient color stable inverted white top-emitting OLEDs with ultra-thin wetting layer top electrodes,” Adv. Opt. Mater.in press., doi:. [CrossRef]

19.

C.-W. Chen, C.-L. Lin, and C.-C. Wu, “An effective cathode structure for inverted top-emitting organic light emitting devices,” Appl. Phys. Lett. 85(13), 2469–2471 (2004). [CrossRef]

20.

M. Furno, R. Meerheim, S. Hofmann, B. Lüssem, and K. Leo, “Efficiency and rate of spontaneous emission in organic electroluminescent devices,” Phys. Rev. B 85(11), 115205 (2012). [CrossRef]

21.

W. Brütting, J. Frischeisen, T. D. Schmidt, B. J. Scholz, and C. Mayr, “Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling,” Phys. Status Solidi A 210(1), 44–65 (2013). [CrossRef]

22.

“ENERGY STAR ® Program Requirements for Solid State Lighting Luminaires Eligibility Criteria – Version 1.1” 1–23 (2008).

23.

M. Furno, T. C. Rosenow, M. C. Gather, B. Lüssem, and K. Leo, “Analysis of the external and internal quantum efficiency of multi-emitter, white organic emitting diodes,” Appl. Phys. Lett. 101(14), 143304 (2012). [CrossRef]

OCIS Codes
(230.3670) Optical devices : Light-emitting diodes
(230.4170) Optical devices : Multilayers
(310.6845) Thin films : Thin film devices and applications

ToC Category:
Optical Devices

History
Original Manuscript: September 27, 2013
Revised Manuscript: October 28, 2013
Manuscript Accepted: October 29, 2013
Published: November 7, 2013

Virtual Issues
Vol. 9, Iss. 1 Virtual Journal for Biomedical Optics

Citation
Jonghee Lee, Hyunsu Cho, Tae-Wook Koh, Changhun Yun, Simone Hofmann, Jae-Hyun Lee, Yong Hyun Kim, Björn Lüssem, Jeong-Ik Lee, Karl Leo, Malte C. Gather, and Seunghyup Yoo, "Enhanced and balanced efficiency of white bi-directional organic light-emitting diodes," Opt. Express 21, 28040-28047 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-23-28040


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References

  1. 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]
  2. M. C. Gather, A. Köhnen, and K. Meerholz, “White organic light-emitting diodes,” Adv. Mater.23(2), 233–248 (2011). [CrossRef] [PubMed]
  3. S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan, and W. Huang, “Recent developments in top-emitting organic light-emitting diodes,” Adv. Mater.22(46), 5227–5239 (2010). [CrossRef] [PubMed]
  4. H. W. Chang, J.-H. Lee, S. Hofmann, Y. H. Kim, L. Müller-Meskamp, B. Lüssem, C.-C. Wu, K. Leo, and M. C. Gather, “Nano-particle based scattering layers for optical efficiency enhancement of organic light-emitting diodes and organic solar cells,” J. Appl. Phys.113(20), 204502 (2013). [CrossRef]
  5. K. S. Yook, S. O. Jeon, C. W. Joo, and J. Y. Lee, “Transparent organic light emitting diodes using a multilayer oxide as a low resistance transparent cathode,” Appl. Phys. Lett.93(1), 013301 (2008).
  6. H. Cho, J.-M. Choi, and S. Yoo, “Highly transparent organic light-emitting diodes with a metallic top electrode: the dual role of a Cs2CO3 layer,” Opt. Express19(2), 1113–1121 (2011). [CrossRef] [PubMed]
  7. J.-H. Lee, S. Lee, J.-B. Kim, J. Jang, and J.-J. Kim, “A high performance transparent inverted organic light emitting diode with 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile as an organic buffer layer,” J. Mater. Chem.22(30), 15262–15266 (2012). [CrossRef]
  8. J. W. Huh, J. Moon, J. W. Lee, D.-H. Cho, J.-W. Shin, J.-H. Han, J. Hwang, C. W. Joo, H. Y. Chu, and J.-I. Lee, “Directed emissive high efficient white transparent organic light emitting diodes with double layered capping layer,” Org. Electron.13(8), 1386–1391 (2012). [CrossRef]
  9. J. Lee, S. Hofmann, M. Furno, M. Thomschke, Y. H. Kim, B. Lüssem, and K. Leo, “Influence of organic capping layers on the performance of transparent organic light-emitting diodes,” Opt. Lett.36(8), 1443–1445 (2011). [CrossRef] [PubMed]
  10. J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. H. Kim, B. Lüssem, and K. Leo, “Increased and balanced light emission of transparent organic light-emitting diodes by enhanced microcavity effects,” Opt. Lett.36(15), 2931–2933 (2011). [CrossRef] [PubMed]
  11. J. Lee, S. Hofmann, M. Thomschke, M. Furno, Y. H. Kim, B. Lüssem, and K. Leo, “Highly efficient bi-directional organic light-emitting diodes by strong micro-cavity effects,” Appl. Phys. Lett.99(7), 073303 (2011). [CrossRef]
  12. J. Lee, J. -I. Lee, J. -W. Lee, and H. Y. Chu, “Interlayer engineering with different host material properties in blue phosphorescent organic light-emitting diodes” ETRI. J. 32, 32-38 (2011).
  13. J.-I. Lee, J. Lee, J.-W. Lee, D.-H. Cho, J.-W. Shin, J.-H. Han, and H. Y. Chu, “Dependence of light-emitting characteristics of blue phosphorescent organic light-emitting diodes on electron injection and transport materials,” ETRI J.34(5), 690–695 (2012). [CrossRef]
  14. S. Olthof, W. Tress, R. Meerheim, B. Lüssem, and K. Leo, “Photoelectron spectroscopy study of systematically varied doping concentrations in an organic semiconductor layer using a molecular p-dopant,” J. Appl. Phys.106(10), 103711 (2009). [CrossRef]
  15. M. Agrawal, Y. Sun, S. R. Forrest, and P. Peumans, “Enhanced outcoupling from organic light-emitting diodes using aperiodic dielectric mirrors,” Appl. Phys. Lett.90(24), 241112 (2007). [CrossRef]
  16. S. Hofmann, M. Thomschke, P. Freitag, M. Furno, B. Lüssem, and K. Leo, “Top-emitting organic light-emitting diodes: Influence of cavity design,” Appl. Phys. Lett.97(25), 253308 (2010). [CrossRef]
  17. R. Meerheim, M. Furno, S. Hofmann, B. Lüssem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett.97(25), 253305 (2010). [CrossRef]
  18. T. Schwab, S. Schubert, S. Hofmann, M. Fröbel, M. Thomschke, L. Müller-Meskamp, K. Leo, and M. C. Gather, “Highly efficient color stable inverted white top-emitting OLEDs with ultra-thin wetting layer top electrodes,” Adv. Opt. Mater.in press., doi:. [CrossRef]
  19. C.-W. Chen, C.-L. Lin, and C.-C. Wu, “An effective cathode structure for inverted top-emitting organic light emitting devices,” Appl. Phys. Lett.85(13), 2469–2471 (2004). [CrossRef]
  20. M. Furno, R. Meerheim, S. Hofmann, B. Lüssem, and K. Leo, “Efficiency and rate of spontaneous emission in organic electroluminescent devices,” Phys. Rev. B85(11), 115205 (2012). [CrossRef]
  21. W. Brütting, J. Frischeisen, T. D. Schmidt, B. J. Scholz, and C. Mayr, “Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling,” Phys. Status Solidi A210(1), 44–65 (2013). [CrossRef]
  22. “ENERGY STAR ® Program Requirements for Solid State Lighting Luminaires Eligibility Criteria – Version 1.1” 1–23 (2008).
  23. M. Furno, T. C. Rosenow, M. C. Gather, B. Lüssem, and K. Leo, “Analysis of the external and internal quantum efficiency of multi-emitter, white organic emitting diodes,” Appl. Phys. Lett.101(14), 143304 (2012). [CrossRef]

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