OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 2 — Jan. 16, 2012
  • pp: 1816–1824
« Show journal navigation

Polarizer-free, high-contrast inverted top-emitting organic light emitting diodes: effect of the electrode structure

Hyunsu Cho and Seunghyup Yoo  »View Author Affiliations


Optics Express, Vol. 20, Issue 2, pp. 1816-1824 (2012)
http://dx.doi.org/10.1364/OE.20.001816


View Full Text Article

Acrobat PDF (1466 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Circular polarizer (CP)-free, high-contrast inverted top-emitting organic light-emitting diodes (ITOLEDs) are demonstrated by using a semitransparent Ag electrode capped with a single dielectric layer as a top anode and Cs2CO3 (1.5 nm)/Al (1.5 nm)/Cr (100nm) as a partially absorbing bottom cathode. Low luminous reflectance is achieved by combining the broadband absorption of Cr, the weak but inherent cavity structure, and the optimal thickness of the capping layer yielding a high transmittance of a top electrode. With the optimized organic capping layer, contrast-enhanced ITOLEDs exhibit a luminous reflectance as low as 3.6% with a large thickness margin. Their luminous efficiency is shown to be comparable to or even higher than that of CP-based conventional OLEDs.

© 2012 OSA

1. Introduction

There has been a great deal of interest in organic light-emitting diodes (OLEDs) due to their various advantages in displays and lighting [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]

] When used for displays, OLEDs must be easily recognizable under ambient illumination. However, this objective is challenging for native OLED geometry due to the presence of reflective metal electrodes. Under conditions of bright illumination, the reflected light can be comparable to the light emitted from OLEDs and the resultant level of contrast can become poor. This problem can be circumvented by attaching a circular polarizer (CP), which consists of a quarter-wave plate and a linear polarizer, to the opposite side of a substrate. This method is very effective in suppressing the ambient light reflection but also incurs additional production costs. In addition, the relatively thick CP could impose a limitation on the making highly flexible nature of OLEDs. Hence there have been numerous studies devoted to the development of CP-free high contrast ratio (CR) OLEDs [2

2. A. N. Krasnov, “High-contrast organic light-emitting diodes on flexible substrates,” Appl. Phys. Lett. 80(20), 3853–3855 (2002). [CrossRef]

7

7. C. Py, D. Poitras, C.-C. Kuo, and H. Fukutani, “High-contrast organic light emitting diodes with a partially absorbing anode,” Opt. Lett. 33(10), 1126–1128 (2008). [CrossRef] [PubMed]

].

2. Experiment

Figure 1(a)
Fig. 1 (a) The device structure of the proposed circular polarizer (CP)-free high contrast ratio (CR) ITOLED. The number in parenthesis indicates the thickness of the corresponding layer given in nm. (b) The schematic diagram for the simplified multilayer geometry used for thin-film optic calculation. The letters stand for the Fresnel coefficients and electric fields relevant to the calculation done in this work.
shows the structure of the proposed CP-free high-CR ITOLED. It has a 100-nm-thick layer of Cr as a partially absorbing bottom electrode (cathode) and a 15-nm-thick layer of Ag as a semitransparent top electrode (anode). For efficient carrier injection from the Cr electrode, we inserted very thin layers of Al (1.5nm) and Cs2CO3 (1.5nm) between the Cr layer and a layer of tris(8-hydroxy-quinolinato) aluminum (Alq3) (50nm), which is used as an emission/ electron transporting layer (EML/ ETL) [9

9. J. Huang, Z. Xu, and Y. Yang, “Low-work-function surface formed by solution-processed and thermally deposited nanoscale layers of cesium carbonate,” Adv. Funct. Mater. 17(12), 1966–1973 (2007). [CrossRef]

]. Likewise, a layer of WO3 (5nm) is inserted for efficient hole injection between Ag and a hole transport layer of N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) (50nm). A layer of ZnS (35nm) or Alq3 (60nm) is deposited on top of the Ag electrode as a capping layer. A combination of a WO3 layer, Ag, and a dielectric capping layer may be viewed as an alternative type of a dielectric-metal-dielectric multilayer transparent electrode; it works as an effective anode for the bottom electrode geometry [10

10. H. Cho, C. Yun, J.-W. Park, and S. Yoo, “Highly flexible organic light-emitting diodes based on ZnS/ Ag/ WO3 multilayer transparent electrodes,” Org. Electron. 10(6), 1163–1169 (2009). [CrossRef]

] and the top electrode geometry [11

11. C. Yun, H. Cho, H. Kang, Y. Lee, Y. Park, and S. Yoo, “Electron injection via pentacene thin films for efficient inverted organic light-emitting diodes,” Appl. Phys. Lett. 95(5), 053301 (2009). [CrossRef]

]. Cr is deposited by sputtering on a pre-cleaned glass substrate; all other depositions are made by thermal evaporation in a vacuum chamber (HS-1100, Digital Optics & Vacuum). A source-measure unit (Keithley 2400) and a calibrated photodiode were used to measure the current density-voltage-luminance (J-V-L) characteristics in a N2-filled glove box. A UV-VIS spectrometer (SV2100, K-MAC) was used to measure the reflectance in air.

Optical analysis was based on transfer matrix formalism (for the reflectance, transmittance, and phase) [12

12. L. A. A. Pettersson, L. S. Roman, and O. Inganäs, “Modeling photocurrent action spectra of photovoltaic devices based on organic thin films,” J. Appl. Phys. 86(1), 487–496 (1999). [CrossRef]

] and on a microcavity model (for the light output from OLEDs) [13

13. H. Cho, C. Yun, and S. Yoo, “Multilayer transparent electrode for organic light-emitting diodes: tuning its optical characteristics,” Opt. Express 18(4), 3404–3414 (2010). [CrossRef] [PubMed]

,14

14. D. G. Deppe, C. Lei, C. C. Lin, and D. L. Huffaker, “Spontaneous emission for planar microstructures,” J. Mod. Opt. 41(2), 325–344 (1994). [CrossRef]

]. The optical constants used for the calculation were borrowed from the literature or measured by means of spectroscopic ellipsometry [13

13. H. Cho, C. Yun, and S. Yoo, “Multilayer transparent electrode for organic light-emitting diodes: tuning its optical characteristics,” Opt. Express 18(4), 3404–3414 (2010). [CrossRef] [PubMed]

]. As the figure of merit for the CR performance, we used luminous reflectance, RL(λ), which is defined as follows [15

15. D. Poitras, C.-C. Kuo, and C. Py, “Design of high-contrast OLEDs with microcavity effect,” Opt. Express 16(11), 8003–8015 (2008). [CrossRef] [PubMed]

]:
RL=V(λ)S(λ)ROLED(λ)dλV(λ)S(λ)dλ
(1)
where S(λ) is the spectrum of a standard ambient light source (We used the CIE standard light source D65), V(λ) is the photopic response of the human eye sensitivity, and ROLED(λ) is the reflectance spectrum of an OLED under test [15

15. D. Poitras, C.-C. Kuo, and C. Py, “Design of high-contrast OLEDs with microcavity effect,” Opt. Express 16(11), 8003–8015 (2008). [CrossRef] [PubMed]

].

3. Results and discussions

3.1 The optimal optical structure of the proposed high-CR OLEDs

Figures 2(a)
Fig. 2 (a) Optical constants (n, k) of the Cr and (b) calculated and measured reflectance spectra of the Cr layer for light incident from air (solid line) or from organic layer (dashed line).
and 2(b) present the measured optical constants (n, k) and reflectance spectra Rbot(λ) of the Cr layer under study. The simulated curve (solid line in Fig. 2(b)) for Rbot(λ), obtained with those optical constants, show quantitative agreement with the experimental data. As desired, Rbot(λ) is confirmed to be relatively low throughout the visible spectrum. At λ of 520nm, for example, R(λ) is 34.5% when the light is incident from air to the Cr layer. This value is translated into Rbot(λ) of approximately 18.9% when the light is incident from the organic layer (n = 1.74) to the Cr layer (as indicated by the dashed line in Fig. 2(b)).

With the Cr layer used as a bottom electrode in the proposed top-emitting OLED geometry, we calculated the reflectance, ROLED (in %), and the light emission, IOLED (in arbitrary units), from the total OLED structure as a function of the capping layer thickness (dcap) and λ for the capping layers of ZnS and Alq3. (See each column in Figs. 3(a)
Fig. 3 Calculated reflectance ROLED (in %) and the light emission IOLED (in arbitrary units) from the total OLED structure as a function of the capping layer thickness (dcap) and wavelength (λ) for capping layers of (a) ZnS and (b) Alq3.
and 3(b)) Note that for both types of capping layers, there is an optimum dcap which can result in a relatively low ROLED in most of the visible spectral region. In such an optimum case, RL for devices with ZnS and Alq3 turns out to be as small as 3.9% and 2.3%, respectively.

Figure 4(a)
Fig. 4 (a) Experimental and simulated reflectance spectra from the total OLED structure with capping layers of: Alq3, ZnS, and nothing. (b) Trend of maximum IOLED vs. Ttop for the proposed ITOLEDs and regular top-emitting OLEDs. Δφ rt is set as 2 (m: integer) in (b).
shows the experimental reflectance spectra obtained for dcap of 35nm (ZnS) and 60nm (Alq3), both of which are close to their respective optimum dcap. RL in these devices turned out to be 7.3% (ZnS) and 3.4% (Alq3). These relatively low values, which are comparable to or lower than those of typical CP-based OLEDs (~4%) [5

5. C.-J. Yang, C.-C. Lin, C.-C. Wu, Y.-H. Yeh, C.-C. Cheng, Y.-H. Kuo, and T.-H. Chen, “High-contrast top-emitting organic light-emitting devices for active-matrix displays,” Appl. Phys. Lett. 87(14), 143507 (2005). [CrossRef]

], confirm that the effectiveness of the proposed simple, single-capping-layer geometry in obtaining a high-CR without a CP.

In addition to its effectiveness for low RL, Fig. 3 further reveals that (i) the optimum condition for RL almost coincides with that for the maximum light output for both types of OLEDs; and that (ii) the ranges of dcap and λ that correspond to the low ROLED are much wider for devices with an Alq3 capping layer than for devices with a ZnS capping layer. The same behavior occurs also for the range of dcap that corresponds to a high IOLED near the emission wavelength (~520 nm). In fact, IOLED varies only slightly (that is, less than 20% from the peak) when dcap varies from 0nm to 100nm in case of the Alq3 capping. Note that such insensitiveness is rarely observed in conventional top-emitting microcavity-based OLEDs; the IOLED of the latter strongly depends on dcap [16

16. H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. Rieß, “Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling,” Appl. Phys. Lett. 82(3), 466–468 (2003). [CrossRef]

].

Several observations can be made from the above Eqs. (3) and (4). Firstly, ROLED is reduced to zero by the destructive interference when Rtop(+) = Rbot(+)Ttop2 (that is, the intensity balance between E1 and E2) and Δφ refl = (2m + 1)π where m is the integer number (that is, phase reversal between E1 and E2). Secondly, fFP2 and IOLED are monotonically increasing functions of Ttop and reach their maximum level when the round-trip phase difference Δφ rt = 2 (phase matching) in most cases as shown in Fig. 4(b). This is because the present OLED structure has a very limited cavity resonance effect. Note that this trend is in contrast with the case of typical top-emitting OLEDs, IOLED of which peaks at a relatively small Ttop (a large Rtop) as can be seen from the gray, short-dashed curve in Fig. 4(b) [17

17. C.-L. Lin, H. W. Lin, and C.-C. Wu, “Examining microcavity organic light-emitting devices having two metal mirrors,” Appl. Phys. Lett. 87(2), 021101 (2005). [CrossRef]

].

For optimization, one should look for the conditions that simultaneously lead to a low ROLED and high IOLED. It has to be kept in mind that the conditions for a low ROLED should be fulfilled as much as possible throughout the visible spectrum whereas the conditions for high IOLED could be fulfilled only around the emission wavelength. Figure 5
Fig. 5 Calculated ROLED, Rtop, Ttop2Rbot(+), and Δϕrefl to monitor the intensity and phase relationships between the reflected components E1 and E2 in devices under study for the cases of (a) no capping, (b) ZnS-capping, and (c) Alq3-capping. Inset: Calculated absorption within the capping layer.
presents each parameter of the devices under study near the optimum condition shown in Fig. 3. One can see that the optimized device structure indeed fulfills the design guidelines outlined above. Let us take an example of ITOLEDs with a 60-nm-thick Alq3 capping layer. First, it can be seen that Δφrt (≈-0.05π) is close to zero near λEML so that light emission from these ITOLEDs is enhanced. Under this condition, Δφrefl (≈0.96π) becomes close to π due to the inherent complementary phase relation between the Δφrefl and Δφrt of the metal-dielectric-metal microcavities [15

15. D. Poitras, C.-C. Kuo, and C. Py, “Design of high-contrast OLEDs with microcavity effect,” Opt. Express 16(11), 8003–8015 (2008). [CrossRef] [PubMed]

], automatically fulfilling the low ROLED at least near λEML. As for balancing of the intensity between the major reflected components, the optimal transmittance of the top electrode may be found by solving Rbot(+)Ttop2 = Rtop(+) = 1- Atop(+)-Ttop for Ttop. With Rbot(+) of (20 ± 5)% and At(+) of 15%, the optimal Ttop is estimated to be (74 μ 2)%. This value is quite close to what occurs near λEML (Table 1

Table 1. Optical components of the simplified device diagram with different capping layers*

table-icon
View This Table
). In fact, such a high transmittance is close to the maximum Ttop that can be accessed in the given top electrode structure upon variation of dcap. This behavior, together with the aforementioned phase relation and the fact that IOLED increases with Ttop, explains why ROLED and IOLED share a common optimum for dcap in the proposed ITOLED devices.

In the spectral region that is relatively far from λEML, a low ROLED can be attained by means of several different mechanisms. Since the strict phase condition for a low ROLED is not fulfilled in this spectral region, the intensity of the major reflected light components can become a more important factor. For example, a low ROLED may be attained by having a low Rtop( + ); in this case, the overall intensity of E1, which is generally the most significant reflected component, can be kept low regardless of the interference effect. Likewise, a low ROLED may be attained if the intensity is balanced at more than one wavelength. ZnS- or Alq3-capped devices not only have a low Rtop(+) in general but also have multiple wavelengths where intensity values of E1 and E2 are balanced. In the ZnS-capped devices, the intensity balance is achieved at two different wavelengths to result in ‘W-shaped’ ROLED spectrum shown in Fig. 5(b). In the Alq3-capped devices, the intensity balance is fulfilled over the spectral “band” of 510 nm to 580 nm also to result in low ROLED throughout the visible spectrum. On the other hand, the device without a capping layer suffers from a relatively high Rtop(+) and, consequently, a relatively large ROLED. The high level of Rtop(+) ( = low Ttop) in the device without a capping layer makes it hard to find a wavelength that can fulfill the intensity balance condition, further aggravating the situation. (See Fig. 5(a)).

The fact that the Alq3-capped devices meet the intensity balance conditions over the relatively broad spectral band results partly from the low spectral variation of Rtop(+). Note that this is consistent with the insensitiveness of their ROLED and IOLED to λ and dcap. This is because the optical properties are determined not by the physical thickness but by the optical thickness. Since the refractive index of Alq3 (1.7 ~1.8) is smaller than that of ZnS (2.3 ~2.5), the change in the optical thickness of the Alq3 layer is smaller than that of the ZnS layer for the same change in physical thickness. Such insensitiveness is expected to be highly beneficial in a real manufacturing because a certain target RL can be easily met with a wide margin for dcap.

Another feature that Alq3 capping layers can offer is the selective absorption in the blue-UV region (Inset in Fig. 5(b)). The 60-nm-thick Alq3 capping layer absorbs approximately 40% of the photons with λ of 400 nm. This effectively suppresses Rtop(+) and ROLED in the UV-blue spectral range, further helping the OLEDs with an Alq3 capping layer produce RL as low as 3.6% with the improved spectral neutrality. It should be noted that such absorption can be disadvantageous in case of blue OLEDs. Nevertheless, it should not be too much of concern because there are numerous materials that are transparent in the blue spectral region and have refractive indices similar to that of Alq3.

3.2 Device performance of the ITOLEDs: electron injection from the proposed bottom electrode

Electron injection from a bottom cathode is a major limiting factor in inverted OLEDs. Thus, finding a suitable combination of an electrode and injection layer has been the key step in developing sustainable inverted OLEDs. Thomschke et al., for example, inserted a layer of 4,7-diphenyl-1,10-phenanthroline (Bphen) doped with Cs and Ag in between the Ag bottom electrode and a Cs-doped Bphen layer to reduce the operating voltage and improve the carrier balance in an inverted top-emitting geometry [18

18. M. Thomschke, S. Hofmann, S. Olthof, M. Anderson, H. Kleemann, M. Schober, B. Lüssem, and K. Leo, “Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing,” Appl. Phys. Lett. 98(8), 083304 (2011). [CrossRef]

]. Similarly, Chen, Lin, and Wu inserted an ultrathin trilayer of Alq3/LiF/Al in between the Ag bottom electrode and an Alq3 layer [19

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]

]. Recently, it was found that a thermally evaporated ultrathin layer of Cs2CO3, which works as an electron injection layer (EIL) when used with an Al top electrode, can work as an EIL with a bottom cathode as well [20

20. C. Yun, H. Cho, T.-W. Koh, J.-H. Kim, J. W. Kim, Y. Park, and S. Yoo, “Doping-free inverted top-emitting organic light-emitting diodes with high power efficiency and near-ideal emission characteristics,” IEEE Trans. Electron. Dev. 59(1), 159–166 (2012). [CrossRef]

]. However, the bottom cathode in that study was also with an Al layer, so it may not be directly applicable to the present case, which requires a partially absorbing electrode such as Cr (this work) or Mo [5

5. C.-J. Yang, C.-C. Lin, C.-C. Wu, Y.-H. Yeh, C.-C. Cheng, Y.-H. Kuo, and T.-H. Chen, “High-contrast top-emitting organic light-emitting devices for active-matrix displays,” Appl. Phys. Lett. 87(14), 143507 (2005). [CrossRef]

].

Figure 7
Fig. 7 (a) J-V-L and (b) L-J characteristics of the proposed ITOLEDs with the capping layer of nothing, ZnS, or Alq3. Inset in (b): The photograph of the Alq3-capped ITOLED under operation.
shows the current density-voltage-luminance (J-V-L) and J-L characteristics of the OLEDs under study. The photograph of the Alq3-capped device is also shown in its inset. With the bottom cathode configuration of Cr (100nm)/ Al (1.5nm)/ Cs2CO3 (1.5nm), the proposed ITOLED devices exhibit a turn-on voltage (=the onset voltage for luminance of 1 cd/m2) as low as 3.5V, which is comparable to the typical turn-on voltage of conventional NPB/ Alq3-based OLEDs (≈2.5V) [10

10. H. Cho, C. Yun, J.-W. Park, and S. Yoo, “Highly flexible organic light-emitting diodes based on ZnS/ Ag/ WO3 multilayer transparent electrodes,” Org. Electron. 10(6), 1163–1169 (2009). [CrossRef]

]. Moreover, the device with the 60-nm-thick Alq3 capping layer exhibits a current efficacy of about 2.2 cd/A, which is twice as high as that of devices without capping layer and comparable to or slightly higher than that of conventional bottom-emitting OLEDs based on NPB/Alq3 with a CP [5

5. C.-J. Yang, C.-C. Lin, C.-C. Wu, Y.-H. Yeh, C.-C. Cheng, Y.-H. Kuo, and T.-H. Chen, “High-contrast top-emitting organic light-emitting devices for active-matrix displays,” Appl. Phys. Lett. 87(14), 143507 (2005). [CrossRef]

]. Although it is unlikely to obtain a large enhancement of luminous efficiency in the proposed ITOLED devices due to the low Rbot( + ), which tends to induce a relatively low cavity enhancement effect, a part of the light emitted toward the bottom electrode still survives the reflection at the Cr electrode. This surviving light consequently contributes to the overall external light emission to some degree and leaves room for improvement in luminous efficiency. Such improvement may be realized if one can increase Rbot while maintaining RL low.

4. Conclusions

In summary, we investigated circular-polarizer (CP)-free high-CR inverted top-emitting top-emitting OLEDs (ITOLEDs). To produce them with a relatively simple structure, we examined whether a top-emitting geometry with a single dielectric capping layer would be sufficient for this purpose. Our results confirm that a properly chosen capping layer and a partially absorbing bottom electrode structure are an effective solution for CP-free high-CR OLEDs. A capping layer of Alq3, the refractive index of which is high but not too high, provides low reflectance throughout the visible spectral range with a relatively low dependence on its thickness and the wavelength of the light. Together with the absorption in the UV-blue spectral region, the proposed ITOLEDs with the optimized Alq3 capping layer exhibit a low luminous reflectance of 3.6% with improved spectral neutrality. The proposed bottom electrode, which consists of a Cr layer covered with thin Al/ Cs2CO3 layers, effectively injects electrons to Alq3 layers and consequently produces a low turn-on voltage. Despite the relatively low microcavity effect, the optimized devices have a luminous efficacy that is comparable to or slightly higher than that of conventional bottom-emitting OLEDs with CP. Given the simplicity of the structure and the benefits of ITOLEDs, the study shown here provides important information for producing cost-effective high-CR OLEDs that are compatible with emerging backplane technologies.

Acknowledgment

This work was supported by the RFID R&D program of MKE/KEIT (10035225, Development of core technology for high performance AMOLED on plastic).

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.

A. N. Krasnov, “High-contrast organic light-emitting diodes on flexible substrates,” Appl. Phys. Lett. 80(20), 3853–3855 (2002). [CrossRef]

3.

H. Aziz, Y.-F. Liew, H. M. Grandin, and Z. D. Popovic, “Reduced reflectance cathode for organic light-emitting devices using metalorganic mixtures,” Appl. Phys. Lett. 83(1), 186–188 (2003). [CrossRef]

4.

W. Ji, L. Zhang, Z. Tianyu, W. Xie, and H. Zhang, “High-contrast and high-efficiency microcavity top-emitting white organic light-emitting devices,” Org. Electron. 11(2), 202–206 (2010). [CrossRef]

5.

C.-J. Yang, C.-C. Lin, C.-C. Wu, Y.-H. Yeh, C.-C. Cheng, Y.-H. Kuo, and T.-H. Chen, “High-contrast top-emitting organic light-emitting devices for active-matrix displays,” Appl. Phys. Lett. 87(14), 143507 (2005). [CrossRef]

6.

S. M. Chen, Y. B. Yuan, J. R. Lian, and X. Zhou, “High-efficiency and high-contrast phosphorescent top-emitting organic light-emitting devices with p-type Si anodes,” Opt. Express 15(22), 14644–14649 (2007). [CrossRef] [PubMed]

7.

C. Py, D. Poitras, C.-C. Kuo, and H. Fukutani, “High-contrast organic light emitting diodes with a partially absorbing anode,” Opt. Lett. 33(10), 1126–1128 (2008). [CrossRef] [PubMed]

8.

C.-C. Wu, C.-W. Chen, C.-L. Lin, and C.-J. Yang, “Advanced organic light-emitting devices for enhancing display performances,” J. Display Technol. 1(2), 248–266 (2005). [CrossRef]

9.

J. Huang, Z. Xu, and Y. Yang, “Low-work-function surface formed by solution-processed and thermally deposited nanoscale layers of cesium carbonate,” Adv. Funct. Mater. 17(12), 1966–1973 (2007). [CrossRef]

10.

H. Cho, C. Yun, J.-W. Park, and S. Yoo, “Highly flexible organic light-emitting diodes based on ZnS/ Ag/ WO3 multilayer transparent electrodes,” Org. Electron. 10(6), 1163–1169 (2009). [CrossRef]

11.

C. Yun, H. Cho, H. Kang, Y. Lee, Y. Park, and S. Yoo, “Electron injection via pentacene thin films for efficient inverted organic light-emitting diodes,” Appl. Phys. Lett. 95(5), 053301 (2009). [CrossRef]

12.

L. A. A. Pettersson, L. S. Roman, and O. Inganäs, “Modeling photocurrent action spectra of photovoltaic devices based on organic thin films,” J. Appl. Phys. 86(1), 487–496 (1999). [CrossRef]

13.

H. Cho, C. Yun, and S. Yoo, “Multilayer transparent electrode for organic light-emitting diodes: tuning its optical characteristics,” Opt. Express 18(4), 3404–3414 (2010). [CrossRef] [PubMed]

14.

D. G. Deppe, C. Lei, C. C. Lin, and D. L. Huffaker, “Spontaneous emission for planar microstructures,” J. Mod. Opt. 41(2), 325–344 (1994). [CrossRef]

15.

D. Poitras, C.-C. Kuo, and C. Py, “Design of high-contrast OLEDs with microcavity effect,” Opt. Express 16(11), 8003–8015 (2008). [CrossRef] [PubMed]

16.

H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. Rieß, “Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling,” Appl. Phys. Lett. 82(3), 466–468 (2003). [CrossRef]

17.

C.-L. Lin, H. W. Lin, and C.-C. Wu, “Examining microcavity organic light-emitting devices having two metal mirrors,” Appl. Phys. Lett. 87(2), 021101 (2005). [CrossRef]

18.

M. Thomschke, S. Hofmann, S. Olthof, M. Anderson, H. Kleemann, M. Schober, B. Lüssem, and K. Leo, “Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing,” Appl. Phys. Lett. 98(8), 083304 (2011). [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.

C. Yun, H. Cho, T.-W. Koh, J.-H. Kim, J. W. Kim, Y. Park, and S. Yoo, “Doping-free inverted top-emitting organic light-emitting diodes with high power efficiency and near-ideal emission characteristics,” IEEE Trans. Electron. Dev. 59(1), 159–166 (2012). [CrossRef]

OCIS Codes
(230.3670) Optical devices : Light-emitting diodes
(230.4170) Optical devices : Multilayers
(310.6860) Thin films : Thin films, optical properties
(310.6845) Thin films : Thin film devices and applications

ToC Category:
Optical Devices

History
Original Manuscript: October 27, 2011
Revised Manuscript: January 2, 2012
Manuscript Accepted: January 3, 2012
Published: January 12, 2012

Citation
Hyunsu Cho and Seunghyup Yoo, "Polarizer-free, high-contrast inverted top-emitting organic light emitting diodes: effect of the electrode structure," Opt. Express 20, 1816-1824 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-2-1816


Sort:  Author  |  Year  |  Journal  |  Reset  

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. A. N. Krasnov, “High-contrast organic light-emitting diodes on flexible substrates,” Appl. Phys. Lett.80(20), 3853–3855 (2002). [CrossRef]
  3. H. Aziz, Y.-F. Liew, H. M. Grandin, and Z. D. Popovic, “Reduced reflectance cathode for organic light-emitting devices using metalorganic mixtures,” Appl. Phys. Lett.83(1), 186–188 (2003). [CrossRef]
  4. W. Ji, L. Zhang, Z. Tianyu, W. Xie, and H. Zhang, “High-contrast and high-efficiency microcavity top-emitting white organic light-emitting devices,” Org. Electron.11(2), 202–206 (2010). [CrossRef]
  5. C.-J. Yang, C.-C. Lin, C.-C. Wu, Y.-H. Yeh, C.-C. Cheng, Y.-H. Kuo, and T.-H. Chen, “High-contrast top-emitting organic light-emitting devices for active-matrix displays,” Appl. Phys. Lett.87(14), 143507 (2005). [CrossRef]
  6. S. M. Chen, Y. B. Yuan, J. R. Lian, and X. Zhou, “High-efficiency and high-contrast phosphorescent top-emitting organic light-emitting devices with p-type Si anodes,” Opt. Express15(22), 14644–14649 (2007). [CrossRef] [PubMed]
  7. C. Py, D. Poitras, C.-C. Kuo, and H. Fukutani, “High-contrast organic light emitting diodes with a partially absorbing anode,” Opt. Lett.33(10), 1126–1128 (2008). [CrossRef] [PubMed]
  8. C.-C. Wu, C.-W. Chen, C.-L. Lin, and C.-J. Yang, “Advanced organic light-emitting devices for enhancing display performances,” J. Display Technol.1(2), 248–266 (2005). [CrossRef]
  9. J. Huang, Z. Xu, and Y. Yang, “Low-work-function surface formed by solution-processed and thermally deposited nanoscale layers of cesium carbonate,” Adv. Funct. Mater.17(12), 1966–1973 (2007). [CrossRef]
  10. H. Cho, C. Yun, J.-W. Park, and S. Yoo, “Highly flexible organic light-emitting diodes based on ZnS/ Ag/ WO3 multilayer transparent electrodes,” Org. Electron.10(6), 1163–1169 (2009). [CrossRef]
  11. C. Yun, H. Cho, H. Kang, Y. Lee, Y. Park, and S. Yoo, “Electron injection via pentacene thin films for efficient inverted organic light-emitting diodes,” Appl. Phys. Lett.95(5), 053301 (2009). [CrossRef]
  12. L. A. A. Pettersson, L. S. Roman, and O. Inganäs, “Modeling photocurrent action spectra of photovoltaic devices based on organic thin films,” J. Appl. Phys.86(1), 487–496 (1999). [CrossRef]
  13. H. Cho, C. Yun, and S. Yoo, “Multilayer transparent electrode for organic light-emitting diodes: tuning its optical characteristics,” Opt. Express18(4), 3404–3414 (2010). [CrossRef] [PubMed]
  14. D. G. Deppe, C. Lei, C. C. Lin, and D. L. Huffaker, “Spontaneous emission for planar microstructures,” J. Mod. Opt.41(2), 325–344 (1994). [CrossRef]
  15. D. Poitras, C.-C. Kuo, and C. Py, “Design of high-contrast OLEDs with microcavity effect,” Opt. Express16(11), 8003–8015 (2008). [CrossRef] [PubMed]
  16. H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. Rieß, “Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling,” Appl. Phys. Lett.82(3), 466–468 (2003). [CrossRef]
  17. C.-L. Lin, H. W. Lin, and C.-C. Wu, “Examining microcavity organic light-emitting devices having two metal mirrors,” Appl. Phys. Lett.87(2), 021101 (2005). [CrossRef]
  18. M. Thomschke, S. Hofmann, S. Olthof, M. Anderson, H. Kleemann, M. Schober, B. Lüssem, and K. Leo, “Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing,” Appl. Phys. Lett.98(8), 083304 (2011). [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. C. Yun, H. Cho, T.-W. Koh, J.-H. Kim, J. W. Kim, Y. Park, and S. Yoo, “Doping-free inverted top-emitting organic light-emitting diodes with high power efficiency and near-ideal emission characteristics,” IEEE Trans. Electron. Dev.59(1), 159–166 (2012). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited