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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 16 — Aug. 2, 2010
  • pp: 16715–16721
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Estimation of the mean emission zone in phosphorescent organic light-emitting diodes with a thin emitting layer

Sei-Yong Kim, Dong-Seok Leem, and Jang-Joo Kim  »View Author Affiliations


Optics Express, Vol. 18, Issue 16, pp. 16715-16721 (2010)
http://dx.doi.org/10.1364/OE.18.016715


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Abstract

We presented an approach to estimate the emission zone (EZ) positions in high efficiency phosphorescent OLEDs with a thin emitting layer. Two devices with different distances between the emitting layer and the cathode (i.e. they are optically different), but exhibiting same current density-voltage characteristics (i.e. they are electrically the same) were used for this purpose. Mean EZ positions in the OLEDs were extracted from the comparison of the experimental luminous intensity ratio vs. the current density with the calculated intensity ratio vs. the EZ position. The validity of the approach was confirmed by the agreement between calculated and experimental spectral changes.

© 2010 OSA

1. Introduction

The location of the emission zone (EZ) significantly influences the performance of organic light emitting diodes (OLEDs), such as their emission spectrum, internal and outcoupling efficiencies and stability [1

1. C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Nearly 100% internal phosphorescence efficiency in an organic light emitting device,” J. Appl. Phys. 90(10), 5048 (2001). [CrossRef]

3

3. J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, and J.-J. Kim, “Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes,” Appl. Phys. Lett. 90(22), 223508 (2007). [CrossRef]

]. Therefore, the direct observation or estimation of the position of the EZ is of great importance in terms of realizing high efficiency and high stability OLEDs. Various approaches have been made to estimate the EZ up to now. These include the analysis of the polarization of the electroluminescence (EL) spectra emitted from emitting polymer chains aligned vertically and in parallel [4

4. J. Griiner, M. Remmers, and D. Neher, “Direct determination of the emission zone in a polymer light-emitting diode,” Adv. Mater. 9(12), 964–968 (1997). [CrossRef]

], and the comparison of the experimental and calculated EL spectra by assuming the distribution of EZ [5

5. W. M. V. Wan, N. C. Greenham, and R. H. Friend, “Interference effects in anisotropic optoelectronic devices,” J. Appl. Phys. 87(5), 2542 (2000). [CrossRef]

,6

6. L. A. Granlund, A. Pettersson, and O. Inganäs, “Determination of the emission zone in a single-layer polymer light-emitting diode through optical measurements,” J. Appl. Phys. 89(11), 5897 (2001). [CrossRef]

], or by a linear superposition of the calculated spectra at various positions inside the emitting polymer [7

7. M. C. Gather, M. Flämmich, N. Danz, D. Michaelis, and K. Meerholz, “Measuring the profile of the emission zone in polymeric organic light-emitting diodes,” Appl. Phys. Lett. 94(26), 263301 (2009). [CrossRef]

]. Most analyses were based on the principle of spectral change with the position of the emission zone in OLEDs by the interference effect and used polymeric LEDs with a thick emitting layer (EML) in the range of hundreds of nanometers where large EL spectral changes are observed due to the large shift of EZ.

Recent high efficiency and low driving voltage OLEDs adopt thin EMLs in the range of 10-30 nm by means of optimized material systems [1

1. C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Nearly 100% internal phosphorescence efficiency in an organic light emitting device,” J. Appl. Phys. 90(10), 5048 (2001). [CrossRef]

,8

8. D. Tanaka, H. Sasabe, Y. J. Li, S. J. Su, T. Takeda, and J. Kido, “Ultra High Efficiency Green Organic Light-Emitting Devices,” Jpn. J. Appl. Phys. 46(1), L10–L12 (2007). [CrossRef]

,9

9. S. J. Su, T. Chiba, T. Takeda, and J. Kido, “Pyridine-Containing Triphenylbenzene Derivatives with High Electron Mobility for Highly Efficient Phosphorescent OLEDs,” Adv. Mater. 20(11), 2125–2130 (2008). [CrossRef]

], p-i-n structures [10

10. M. Pfeiffer, S. R. Forrest, K. Leo, and M. E. Thompson, “Electrophosphorescent p-i-n Organic Light-Emitting Devices for Very-High-Efficiency Flat-Panel Displays,” Adv. Mater. 14(22), 1633–1636 (2002). [CrossRef]

12

12. D.-S. Leem, J.-H. Lee, J.-J. Kim, and J.-W. Kang, “Highly efficient tandem p-i-n organic light-emitting diodes adopting a low temperature evaporated rhenium oxide interconnecting layer,” Appl. Phys. Lett. 93(10), 103304 (2008). [CrossRef]

], and microcavity structures [13

13. H. J. Peng, X. L. Zhu, J. X. Sun, X. M. Yu, M. Wong, and H. S. Kwok, “Efficiency improvement of phosphorescent organic light-emitting diodes using semitransparent Ag as anode,” Appl. Phys. Lett. 88(3), 033509 (2006). [CrossRef]

]. Estimating EZs in these devices is still important to analyze and optimize the device performance. Unfortunately, however, determination of EZs based on the change of emission spectra is difficult in these devices since the shapes of the EL spectra are almost identical. The EZ shifts throughout the EML result in no significant difference in the interference effect due to the thin EML. Therefore, EZs have been determined experimentally in most cases by inserting sensing layers of a fluorescent dye at various positions in OLEDs [3

3. J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, and J.-J. Kim, “Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes,” Appl. Phys. Lett. 90(22), 223508 (2007). [CrossRef]

,14

14. C. W. Tang, S. A. VanSlyke, and C. H. Chen, “Electroluminescence of doped organic thin films,” J. Appl. Phys. 65(9), 3610 (1989). [CrossRef]

,15

15. E.-I. Aminaka, T. Tsutsui, and S. Saito, “Effect of layered structures on the location of emissive regions in organic electroluminescent devices,” J. Appl. Phys. 79(11), 8808 (1996). [CrossRef]

], but the additional layer modifies the charge transport within the device, resulting in an uncertainty of the EZs. Very recently, Young et al. [16

16. R. H. Young and M. E. Kondakova, “Optical Measurement of the Emission Zone in Organic Light-Emitting Diodes,” SID Int. Symp. Digest Tech. Papers 15, 1669–1672 (2009). [CrossRef]

] reported a method by using the linear superposition of the experimental EL spectra of the reference OLEDs with very thin EMLs located at different distances from the cathode to estimate the relevant EZ positions. However, this method still requires many experimental EL spectra of the reference OLEDs. Thus, a simple new simulation method without either significant electrical disturbance or vast experimental data is strongly required in order to determine the position of the EZs in OLEDs with a thin EML.

In this letter, we present an approach to estimate the EZ positions in high efficiency phosphorescent OLEDs with thin EMLs. We used two devices with different distances between the EML and the cathode (i.e. they are optically different) but exhibiting the same current density-voltage (J-V) characteristics (i.e. they are electrically the same). Therefore, the optical interference effect can be independently monitored from the electrical effect and considered as a main parameter for the difference in device performances. The mean EZ positions in the OLEDs were directly extracted from the comparison of the experimental luminous intensity ratio vs. the current density with the calculated intensity ratio vs. EZ position of the devices.

2. Experiment and brief description about theoretical approach

The OLEDs were fabricated on UV ozone-treated ITO substrates with the layer sequences of a 4 wt% rhenium oxide-doped N,N’-diphenyl-N,N’-bis(1,1’-biphenyl)-4,4’-diamine (NPB) p-hole transporting layer (p-HTL) (80 nm), undoped NPB HTL (20 nm), a double EML of 8 wt% Ir(ppy)3-doped 4-4’-N,N’-dicarbazolylbiphenyl (CBP) (10 nm) and 8wt % Ir(ppy)3-doped 4,7-diphenyl-1,10-phenanthroline (Bphen) (20 nm), undoped Bphen ETL (30 nm), 15wt % rubidium carbonate-doped Bphen n-ETL (15 or 25 nm), and an Al cathode. Further detailed device structures with a possible energy level diagram are depicted in the inset of Fig. 1
Fig. 1 The current efficiency-voltage-current density characteristics of the OLEDs with different n-ETL thicknesses. The inset shows the schematic energy level diagrams of the OLEDs used.
. The current density-voltage-luminance characteristics of the devices were measured by a Keithley 2400 semiconductor parameter analyzer and a Photo Research PR-650 spectrophotometer. All devices were encapsulated prior to the measurement. Classical electromagnetic theory has been applied for the optical modeling of the OLEDs [1

1. C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Nearly 100% internal phosphorescence efficiency in an organic light emitting device,” J. Appl. Phys. 90(10), 5048 (2001). [CrossRef]

,17

17. R. R. Chance, A. Prock, and R. Silbey, “Molecular Fluorescence and Energy Transfer Near Interface,” Adv. Chem. Phys. 37, 1–65 (1987). [CrossRef]

21

21. S. Y. Kim and J.-J. Kim, “Outcoupling efficiency of organic light emitting diodes and the effect of ITO thickness,” Org. Electron. 11(6), 1010–1015 (2010). [CrossRef]

]. Changes of the EL spectra and intensities with the position of the EZ within the devices were simulated under the assumptions of randomly oriented sheet dipoles in the layered structures. The position of the sheet dipole can be interpreted as the mean position of the emission zone. The assumption of the sheet dipoles will be discussed later. The intrinsic quantum yield of the emissive material (q) was varied in the calculation. Even though the photoluminescence quantum yield of Ir(ppy)3 in CBP is close to 100% [20

20. W.-I. Jeong, S.-Y. Kim, J.-J. Kim, and J.-W. Kang, “Thickness dependence of PL efficiency of organic thin films,” Chem. Phys. 355(1), 25–30 (2009). [CrossRef]

,22

22. Y. Kawamura, K. Goushi, J. Brooks, J. J. Brown, H. Sasabe, and C. Adachi, “100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films,” Appl. Phys. Lett. 86(7), 071104 (2005). [CrossRef]

], the q can be reduced at high current density [23

23. B. C. Krummacher, S. Nowy, J. Frischeisen, M. Klein, and W. Brütting, “Efficiency analysis of organic light-emitting diodes based on optical simulation,” Org. Electron. 10(3), 478–485 (2009). [CrossRef]

]. The optical constants of all organic layers for optical modeling were measured by spectroscopic ellipsometry (Korea Research Institute of Standards and Science).

3. Results and discussion

Figure 1 shows the luminous efficiency-current density-voltage (L-J-V) characteristics of the OLEDs with two different n-ETL thicknesses. The J-V characteristics of the devices are almost identical between the devices in spite of the different thicknesses of the n-ETL, indicating that carrier movements (charge injection and transport) are very similar in both OLEDs independent of the n-ETL thickness due to the negligible Ohmic loss for electron injection and transport in the layer by n-doping [24

24. D.-S. Leem, S.-Y. Kim, J.-J. Kim, M.-H. Chen, and C.-I. Wu, “Rubidium-Carbonate-Doped 4,7-Diphenyl-1,10-phenanthroline Electron Transporting Layer for High-Efficiency p-i-n Organic Light Emitting Diodes,” Electrochem. Solid-State Lett. 12(1), J8 (2009). [CrossRef]

] and thus, the exciton formation and the position of EZs within the EML must be the same in both devices. In contrast, the L-J characteristics are apparently different for the two OLEDs. At the low current density region, the device with the 25 nm-thick-n-ETL produces higher peak luminance efficiency than that of the device with the 15 nm-thick-n-ETL, whereas the efficiency is reversed in the high current density region. Since the electrical characteristics of the devices are identical, the difference in efficiencies between the devices originates from purely optical effects.

Figure 2
Fig. 2 Experimental (symbol) and calculated (line) EL spectra of OLEDs with the different n-ETL thicknesses at the current density of (a) 0.2 mA/cm2, (b) 2.46 mA/cm2, (c) 9.5 mA/cm2, and (d) 22 mA/cm2.
exhibits the EL spectra of the OLEDs with two different n-ETL thicknesses at four different current densities. The EL spectra of the devices are almost the same and do not change significantly with current densities, indicating that determination of the location of the EZ is not possible based on the variation of the emission spectra. However EL intensity ratio varies significantly with increasing current density as expected from the efficiency-current density plots of the OLEDs in Fig. 1. It is the intensity ratio that we are using to determine the mean position of the EZ. It is possible because the only difference between the two devices is the distance between the EZ and the cathode, due to the different ETL thicknesses.

The luminous intensities of the OLEDs are calculated for different EZ locations using classical electromagnetic theory. The relative luminous intensities and their intensity ratios are displayed in Fig. 3
Fig. 3 The calculated luminous intensity of the OLEDs with 25 nm and 15 nm-thick n-ETLs depending on the position of the emission zone when q = 1. The inset shows the relative ratios of the calculated luminous intensity (line) of two devices depending on the emission zone positions. For comparison, the relative ratios of the current efficiency (symbol) of two devices are also plotted. The values in the inset are the current densities corresponding to the relative ratios of current efficiencies of the two devices.
and in the inset of Fig. 3, respectively, when q = 1. The EZ positions of “0” and “30” in the figure correspond to the interface of the EML/ETL and the HTL/EML, respectively. If the EZ position is located below 16 nm, the calculated luminous intensity of the 25 nm-thick n-ETL device is higher than the 15 nm-thick n-ETL device, whereas the intensity is reversed for the EZ positions beyond 16 nm, similar to the L-J plots of the OLEDs in Fig. 1. The similarity between them is apparent in the intensity ratios shown in the inset of Fig. 3. The efficiency roll-off observed in Fig. 1 does not appear in the calculated luminous intensity in Fig. 3 because the annihilation of excitons was not considered in the calculation. By relating the experimentally obtained EL intensity ratio vs. the current density with the calculated EL intensity ratio vs. the EZ location, we can now extract the EZ location vs. the current density, allowing the estimation of the movement of the EZ in the OLEDs with the current density.

Figure 4
Fig. 4 The mean position of the emission zone of the OLEDs with applied current density at various internal quantum efficiencies (square: 1.0, circle: 0.75, and triangle: 0.65). (The lines are for a guide to the eye only).
shows the variation of the obtained mean location of the EZ with the current density in the OLEDs with 30 nm-thick EMLs for different q values. We repeated theprocedure displayed in Fig. 3 for different values of q. The ratio of current efficiency could be fitted only when q is greater than 0.65. When q is below 0.65, the average emission zone at high current density is positioned in the HTL, which is contradictory to the experimental results. The EL spectra show little emission from NPB even at the high current density of 30 mA/cm2 (not shown), indicating that the excitons are well confined in the EML even at high current densities. We believe this is due to our device geometry with the double EMLs and the p-i-n structure. The maximum deviation of the average emission zone is 3.3 nm at 30 mA/cm2 while q varies from 1.0 to 0.65. These results indicate that our approach is adequate with an accuracy of 3 nm.

The mean position of the EZ in the devices is located near the EML/ETL interface at low current densities and moves toward the HML/EML interface with increasing current density. The result is consistent with the experimental reports in a double EML OLED with a similar device structure [3

3. J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, and J.-J. Kim, “Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes,” Appl. Phys. Lett. 90(22), 223508 (2007). [CrossRef]

]. It is interesting to note that the EZ at low current density is located within the Ir(ppy)3 doped BPhen EML rather than at the 1st EML/2nd EML interface. Even with the energy barrier for hole injection from the CBP to the BPhen host, holes must be injected to the 2nd EML probably through the Ir(ppy)3 dopants even at low electric field [25

25. G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, and J. Salbeck, “High-efficiency and low-voltage p‐i‐n electrophosphorescent organic light-emitting diodes with double-emission layers,” Appl. Phys. Lett. 85(17), 3911 (2004). [CrossRef]

]. The EZ moves rather rapidly with increasing the current density in the range of 1-5 mA/cm2. Further increment of the current density shifts the EZ further toward the HTL/EML interface but with a rather slower rate.

The validity of our calculation on the profiles of the EZ in OLEDs with a thin EML can be obtained from the comparison of the calculated emission spectra with the experimental spectra shown in Fig. 2. Excellent agreement in the EL spectra and the relative intensities between the simulated and measured EL data are observed. Even though the EZ is located within the EML up to 10,000 cd/m2, it does not necessarily mean that all the excitons are confined in the EML. Excitons in real OLEDs must be distributed with a certain dispersion around an mean position rather than a delta function assumed in our calculation [3

3. J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, and J.-J. Kim, “Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes,” Appl. Phys. Lett. 90(22), 223508 (2007). [CrossRef]

,16

16. R. H. Young and M. E. Kondakova, “Optical Measurement of the Emission Zone in Organic Light-Emitting Diodes,” SID Int. Symp. Digest Tech. Papers 15, 1669–1672 (2009). [CrossRef]

]. Therefore the EZ position in Fig. 4 must be considered as the mean position in the distribution, even though the excellent agreement of the calculated spectra with the experimental spectra indicates that the distribution is narrow in these devices. The fact that the roll-off of the efficiency begins at a rather low current density of 1 mA/cm2 supports the narrow distribution of excitons in positions resulting in a high likelihood of triplet-triplet annihilation [3

3. J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, and J.-J. Kim, “Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes,” Appl. Phys. Lett. 90(22), 223508 (2007). [CrossRef]

]. A recent paper revealed that the width of the exciton recombination zone of a PhOLED is in the range of 5 nm [26

26. S. Reineke, G. Schwartz, K. Walzer, and K. Leo, “Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation,” Appl. Phys. Lett. 91(12), 123508 (2007). [CrossRef]

].

4. Conclusion

Acknowledgments

This research was supported by the Center for Nanoscale Mechatronics & Manufacturing, grant (2009K000069), from one of the 21st Century Frontier Research Programs, and a WCU program (R31-2008-000-10075-0) through National Research Foundation of the Ministry of Education, Science and Technology of Korea.

References and links

1.

C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Nearly 100% internal phosphorescence efficiency in an organic light emitting device,” J. Appl. Phys. 90(10), 5048 (2001). [CrossRef]

2.

Z. Wu, L. Wang, G. Lei, and Y. Qiu, “Investigation of the spectra of phosphorescent organic light emitting devices in relation to emission zone,” J. Appl. Phys. 97(10), 103105 (2005). [CrossRef]

3.

J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, and J.-J. Kim, “Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes,” Appl. Phys. Lett. 90(22), 223508 (2007). [CrossRef]

4.

J. Griiner, M. Remmers, and D. Neher, “Direct determination of the emission zone in a polymer light-emitting diode,” Adv. Mater. 9(12), 964–968 (1997). [CrossRef]

5.

W. M. V. Wan, N. C. Greenham, and R. H. Friend, “Interference effects in anisotropic optoelectronic devices,” J. Appl. Phys. 87(5), 2542 (2000). [CrossRef]

6.

L. A. Granlund, A. Pettersson, and O. Inganäs, “Determination of the emission zone in a single-layer polymer light-emitting diode through optical measurements,” J. Appl. Phys. 89(11), 5897 (2001). [CrossRef]

7.

M. C. Gather, M. Flämmich, N. Danz, D. Michaelis, and K. Meerholz, “Measuring the profile of the emission zone in polymeric organic light-emitting diodes,” Appl. Phys. Lett. 94(26), 263301 (2009). [CrossRef]

8.

D. Tanaka, H. Sasabe, Y. J. Li, S. J. Su, T. Takeda, and J. Kido, “Ultra High Efficiency Green Organic Light-Emitting Devices,” Jpn. J. Appl. Phys. 46(1), L10–L12 (2007). [CrossRef]

9.

S. J. Su, T. Chiba, T. Takeda, and J. Kido, “Pyridine-Containing Triphenylbenzene Derivatives with High Electron Mobility for Highly Efficient Phosphorescent OLEDs,” Adv. Mater. 20(11), 2125–2130 (2008). [CrossRef]

10.

M. Pfeiffer, S. R. Forrest, K. Leo, and M. E. Thompson, “Electrophosphorescent p-i-n Organic Light-Emitting Devices for Very-High-Efficiency Flat-Panel Displays,” Adv. Mater. 14(22), 1633–1636 (2002). [CrossRef]

11.

K. Walzer, B. Maennig, M. Pfeiffer, and K. Leo, “Highly efficient organic devices based on electrically doped transport layers,” Chem. Rev. 107(4), 1233–1271 (2007). [CrossRef] [PubMed]

12.

D.-S. Leem, J.-H. Lee, J.-J. Kim, and J.-W. Kang, “Highly efficient tandem p-i-n organic light-emitting diodes adopting a low temperature evaporated rhenium oxide interconnecting layer,” Appl. Phys. Lett. 93(10), 103304 (2008). [CrossRef]

13.

H. J. Peng, X. L. Zhu, J. X. Sun, X. M. Yu, M. Wong, and H. S. Kwok, “Efficiency improvement of phosphorescent organic light-emitting diodes using semitransparent Ag as anode,” Appl. Phys. Lett. 88(3), 033509 (2006). [CrossRef]

14.

C. W. Tang, S. A. VanSlyke, and C. H. Chen, “Electroluminescence of doped organic thin films,” J. Appl. Phys. 65(9), 3610 (1989). [CrossRef]

15.

E.-I. Aminaka, T. Tsutsui, and S. Saito, “Effect of layered structures on the location of emissive regions in organic electroluminescent devices,” J. Appl. Phys. 79(11), 8808 (1996). [CrossRef]

16.

R. H. Young and M. E. Kondakova, “Optical Measurement of the Emission Zone in Organic Light-Emitting Diodes,” SID Int. Symp. Digest Tech. Papers 15, 1669–1672 (2009). [CrossRef]

17.

R. R. Chance, A. Prock, and R. Silbey, “Molecular Fluorescence and Energy Transfer Near Interface,” Adv. Chem. Phys. 37, 1–65 (1987). [CrossRef]

18.

J. A. E. Wasey and W. L. Barnes, “Efficiency of spontaneous emission from planar microcavities,” J. Mod. Opt. 47, 725 (2000).

19.

C. L. Lin, H. C. Chang, K. C. Tien, and C. C. Wu, “Influences of resonant wavelengths on performances of microcavity organic light-emitting devices,” Appl. Phys. Lett. 90(7), 071111 (2007). [CrossRef]

20.

W.-I. Jeong, S.-Y. Kim, J.-J. Kim, and J.-W. Kang, “Thickness dependence of PL efficiency of organic thin films,” Chem. Phys. 355(1), 25–30 (2009). [CrossRef]

21.

S. Y. Kim and J.-J. Kim, “Outcoupling efficiency of organic light emitting diodes and the effect of ITO thickness,” Org. Electron. 11(6), 1010–1015 (2010). [CrossRef]

22.

Y. Kawamura, K. Goushi, J. Brooks, J. J. Brown, H. Sasabe, and C. Adachi, “100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films,” Appl. Phys. Lett. 86(7), 071104 (2005). [CrossRef]

23.

B. C. Krummacher, S. Nowy, J. Frischeisen, M. Klein, and W. Brütting, “Efficiency analysis of organic light-emitting diodes based on optical simulation,” Org. Electron. 10(3), 478–485 (2009). [CrossRef]

24.

D.-S. Leem, S.-Y. Kim, J.-J. Kim, M.-H. Chen, and C.-I. Wu, “Rubidium-Carbonate-Doped 4,7-Diphenyl-1,10-phenanthroline Electron Transporting Layer for High-Efficiency p-i-n Organic Light Emitting Diodes,” Electrochem. Solid-State Lett. 12(1), J8 (2009). [CrossRef]

25.

G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, and J. Salbeck, “High-efficiency and low-voltage p‐i‐n electrophosphorescent organic light-emitting diodes with double-emission layers,” Appl. Phys. Lett. 85(17), 3911 (2004). [CrossRef]

26.

S. Reineke, G. Schwartz, K. Walzer, and K. Leo, “Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation,” Appl. Phys. Lett. 91(12), 123508 (2007). [CrossRef]

OCIS Codes
(160.4890) Materials : Organic materials
(230.3670) Optical devices : Light-emitting diodes

ToC Category:
Optical Devices

History
Original Manuscript: May 19, 2010
Revised Manuscript: July 3, 2010
Manuscript Accepted: July 20, 2010
Published: July 23, 2010

Citation
Sei-Yong Kim, Dong-Seok Leem, and Jang-Joo Kim, "Estimation of the mean emission zone in phosphorescent organic light-emitting diodes with a thin emitting layer," Opt. Express 18, 16715-16721 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-16-16715


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References

  1. C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Nearly 100% internal phosphorescence efficiency in an organic light emitting device,” J. Appl. Phys. 90(10), 5048 (2001). [CrossRef]
  2. Z. Wu, L. Wang, G. Lei, and Y. Qiu, “Investigation of the spectra of phosphorescent organic light emitting devices in relation to emission zone,” J. Appl. Phys. 97(10), 103105 (2005). [CrossRef]
  3. J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, and J.-J. Kim, “Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes,” Appl. Phys. Lett. 90(22), 223508 (2007). [CrossRef]
  4. J. Griiner, M. Remmers, and D. Neher, “Direct determination of the emission zone in a polymer light-emitting diode,” Adv. Mater. 9(12), 964–968 (1997). [CrossRef]
  5. W. M. V. Wan, N. C. Greenham, and R. H. Friend, “Interference effects in anisotropic optoelectronic devices,” J. Appl. Phys. 87(5), 2542 (2000). [CrossRef]
  6. L. A. Granlund, A. Pettersson, and O. Inganäs, “Determination of the emission zone in a single-layer polymer light-emitting diode through optical measurements,” J. Appl. Phys. 89(11), 5897 (2001). [CrossRef]
  7. M. C. Gather, M. Flämmich, N. Danz, D. Michaelis, and K. Meerholz, “Measuring the profile of the emission zone in polymeric organic light-emitting diodes,” Appl. Phys. Lett. 94(26), 263301 (2009). [CrossRef]
  8. D. Tanaka, H. Sasabe, Y. J. Li, S. J. Su, T. Takeda, and J. Kido, “Ultra High Efficiency Green Organic Light-Emitting Devices,” Jpn. J. Appl. Phys. 46(1), L10–L12 (2007). [CrossRef]
  9. S. J. Su, T. Chiba, T. Takeda, and J. Kido, “Pyridine-Containing Triphenylbenzene Derivatives with High Electron Mobility for Highly Efficient Phosphorescent OLEDs,” Adv. Mater. 20(11), 2125–2130 (2008). [CrossRef]
  10. M. Pfeiffer, S. R. Forrest, K. Leo, and M. E. Thompson, “Electrophosphorescent p-i-n Organic Light-Emitting Devices for Very-High-Efficiency Flat-Panel Displays,” Adv. Mater. 14(22), 1633–1636 (2002). [CrossRef]
  11. K. Walzer, B. Maennig, M. Pfeiffer, and K. Leo, “Highly efficient organic devices based on electrically doped transport layers,” Chem. Rev. 107(4), 1233–1271 (2007). [CrossRef] [PubMed]
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