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The Optical Effects of Capping Layers
on the Performance of Transparent Organic
Light-Emitting Diodes
Volume 4, Number 1, February 2012
Jin Woo Huh
Jaehyun Moon
Joo Won Lee
Doo-Hee Cho
Jin-Wook Shin
Jun-Han Han
Joohyun Hwang
Chul Woong Joo
Jeong-Ik Lee
Hye Yong Chu
DOI: 10.1109/JPHOT.2011.2176478
1943-0655/$26.00 ©2011 IEEE
The Optical Effects of Capping Layers
on the Performance of Transparent
Organic Light-Emitting Diodes
Jin Woo Huh, Jaehyun Moon, Joo Won Lee, Doo-Hee Cho, Jin-Wook Shin,
Jun-Han Han, Joohyun Hwang, Chul Woong Joo,
Jeong-Ik Lee, and Hye Yong Chu
OLED Lighting Research Team, Electronics and Telecommunications Research Institute,
Daejeon 305-700, Korea
DOI: 10.1109/JPHOT.2011.2176478
1943-0655/$26.00 Ó2011 IEEE
Manuscript received September 29, 2011; revised November 3, 2011; accepted November 8, 2011.
Date of publication November 18, 2011; date of current version December 27, 2011. This work was
supported in part by the IT R&D Program of MKE/KEIT under Grant KI002068 (Development of
Eco-Emotional OLED Flat-Panel Lighting). Corresponding author: J.-I. Lee (e-mail: jiklee@etri.re.kr).
Abstract: In transparent organic light-emitting diodes (TOLEDs), the asymmetry in the
optical paths causes difference between the bottom and top emitting lights, both in
emissions and spectral distributions. Capping layers (CLs) can be used as an optical
functional to enhance the emissions and adjust the spectral distributions. Here, we report on
the optical effects of an organic CL, 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), on
the characteristics of TOLEDs Both simulated and experimental results are presented. We
demonstrate the possibility of improving the total emission and achieving spectral matching
of bottom and top emissions by varying the CL thickness. The optical effects of CLs have
been interpreted from interference perspectives. Finally, we have presented a guideline that
is practically useful in designing high-performance TOLEDs with CLs.
Index Terms: Transparent organic light-emitting diodes, capping layer, refractive-index,
bottom to top ratio, spectra matching, high efficiency.
1. Introduction
Transparent organic light-emitting diodes (TOLEDs) have received growing interest for their wide
application in see-through displays [1], [2] and lightings which can be integrated into architectural
windows, automobile windshields, aesthetic light sources and so on. However, to realize practical
TOLEDs a couple of technical problems have to be resolved. First, the total efficiency of TOLEDs
should be improved to be comparable to that of the conventional bottom emissive OLEDs.
Currently, the top light-emitting efficiency is lower about 30% than its counterpart. Second, the
transmittance of the top emitting part has to be improved. Third, it is desirable to obtain identical
spectra from the bottom and top emissions and control the ratio of bottom and top emissions.
Optical engineering can be employed as a mean of enhancing and tailoring the emission
properties of OLEDs [3]–[7]. Key characteristics such as external quantum efficiency (EQE) and
spectral distribution can be effectively modulated by inducing the desired optical interference effects
through optical designs. Hung et al. [2] reported an efficient top-emitting OLED with organic capping
layer (CL) and achieved maximum efficiency when the transparency of the top contact was the
highest. Riel et al. [8] have shown an efficient top-emitting OLED using an inorganic CL above the top
contact. Both works strongly suggest that transmittance of the top contact is playing an important role
Vol. 4, No. 1, February 2012 Page 39
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
in improving the light-emitting efficiency. However, there are only a few works on CLs in TOLEDs [9],
[10]. Systematic studies, which combine both experimental and theoretical investigations, are rare.
In this work, as an effort to improve and tailor the emission characteristics of TOLEDs, an organic
dielectric layer, 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), was added on top metal
cathode in single color and white TOLED as a CL. Both in experiments and simulations, the
thickness of the CL layer was varied to investigate the thickness effect of the CL on the devices. As
a result, solely due to the optical effect, enhanced total EQE and identical bi-directional spectra
were achieved in our TOLEDs. Optical interference theory was invoked to interpret our results. We
have deduced useful guidelines for designing high-performance TOLEDs with CLs.
2. Experimental
The TOLED structures of orange and white are shown in Fig. 1. The organic material of each layer is
listed in Table 1. In both cases, we have used glass support. Starting from the bottom part, the
orange has a stacking sequence of indium tin oxide (ITO) (70 nm)/TAPC (55 nm)/DCzPPy:
Bt2Iracac(10%) (20 nm)/BmPyPB (55 nm)/LiF(1 nm)/Al(1.5 nm)/Ag(15 nm)/CL(TAPC, 0, 20, 40, 60,
80, 100, 120, 140 nm). In the case of white TOLED the stacking is ITO (70 nm)/TAPC (55 nm)/TCTA:
Firpic (7%) (5 nm)/TCTA:Ir(2-phq)3 (9%) (1 nm)/DCzPPy:Firpic (10%) (5 nm)/BmPyPB (55 nm)/
LiF(1 nm)/Al(1.5 nm)/Ag(15 nm)/CL(TAPC, 0, 20, 40, 80, 100, 120, 140 nm). The fabrication
processes have been described in our previous works [11]. Briefly, we delineate the process. Prior to
the deposition ITO was treated by oxygen plasma to remove any residual contaminates. The base
pressure of all deposition processes were below 6:66 105Pa ð5107torrÞ. Using an
automated vacuum facility, all samples were transferred to designated deposition chambers without
breaking vacuum. The active area ð22mm
2Þof the devices was defined by the cathode shadow
mask. The fabricated OLEDs were transferred directly from vacuum into an inert environment glove-
box, where they were encapsulated using a UV-curable epoxy, and a glass cap with a moisture
getter. The electroluminescence spectrum was measured using a Minolta CS-1000. The current–
voltage (I–V) and luminescence–voltage (L–V) characteristics were measured with a current/
voltage source/measure unit (Keithley 238) and a Minolta CS-100. Transmittance of the glass cap-
encapsulated TOLED was measured using a UV–visible spectrophotometer (U-3501, Hitachi).
Simulations were performed using an OLED optical simulator, SimOLED [12]–[14] to optimize the
characteristics of TOLEDs as a function of the thickness of the TAPC. Briefly, the program has inputs
of refractive index and thickness of every layer. The program employs thin film optics and the dipole
Fig. 1. Cross-sectional structures of TOLED in (a) orange and (b) white devices.
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
Vol. 4, No. 1, February 2012 Page 40
emission model to calculate the optical and spectral characteristic of OLEDs. In order to obtain
realistic simulation results, we used all measured optical constants (n, k) of organic materials, which
were obtained using an ellipsometer (M-2000D, J. A. Woollam Co.).
3. Results and Discussion
EL emission characteristics were investigated with experimental and simulated results as a function
of thickness of the dielectric CL, TAPC. Based on preliminary optical simulation results, the
thickness of CL layer was chosen to vary in a range of 0–140 nm, both in simulations and
experiments.
Fig. 2(a) shows the simulated radiances of orange and white TOLED with different CL
thicknesses. Bottom, top and total radiances are plotted. Both devices show similar radiance
dependence on the CL thickness. Radiance has either maximum or minimum at a specific CL
thickness. However, the radiance dependency of top and bottom direction on the CL thickness is
different. In case of bottom-side, the radiance decreases gradually as the CL thickness increases.
The radiance reaches its minimum close to 60 nm CL. From the minimum point, it increases again
as the thickness increases. On the contrary, in top-side variation is opposite and the maximum is
observed at 60 nm. In both cases, the highest total radiances were obtained in the thickness range
of 120–140 nm and the lowest was observed near 60 nm. It is noticeable that the radiance of the
TOLED is a strong function of the CL thickness with distinct thicknesses corresponding to maximum
or minimum radiance values. Apparently, the overall behaviors of total and the bottom radiances
show close resemblance, while the top radiances are almost opposite with much lower values.
In order to verify the effect of the CL thickness on real devices, TOLEDs were fabricated and their
EL emission characteristics were measured. EQEs at 10 mA/cm2were plotted in Fig. 2(b).
Remarkable similarity between the simulated and measured can be observed in the EQE
dependency on the CL thickness. We found that CL had a distinct influence on the efficiency in
emission characteristics of TOLEDs. To be specific, with the introduction of CL in the orange
TOLED, the top-side efficiency increased, while opposite happened in the bottom-side efficiency. In
top-side case the EQE increased from 1.90% to 3.06% as the thickness was increased from 0 nm to
60 nm. In bottom-side case the EQE decreased from 11.55% to 10.10% as the thickness was
increased from 0 nm to 60 nm. This corresponds to emission increment by 1.16% in the top-side
and decrement in the bottom-side by 1.45%. Following up, the maximum total EQE from the both-
sides was 14% at CL of 120 nm. Without CL the total EQE is 13.16%. These changes in the
emission properties at both-sides led changes in emission ratio of bottom to top (B/T) of TOLEDs.
The B/T at CL 0 nm and 60 nm are 3.30 and 6.08, respectively. Similar behavior was found in white
TOLED as well. In the white TOLED, the maximum emission of top-side was obtained at 40 nm.
TABLE 1
List of organic materials in orange and white TOLEDs
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
Vol. 4, No. 1, February 2012 Page 41
Extrapolation predicts maximum at 40–60 nm. The maximum total EQE was 12.65% at 100 nm,
while without TAPC the EQE was 11.52%. It was observed that B/T varied from 1.66 to 4.25 upon
CL increase. From these results, we found that emission from each side can be controlled by
adjusting the CL thickness, leading to tunable B/T and enhanced total EQE.
The electrical properties of the orange TOLEDs are shown in Fig. 3(a) and (b). It can be readily
noticed that the TOLEDs with different CL thicknesses have almost identical current density ðJÞ-
voltageðVÞcharacteristics. In particular, the turn-on voltages are almost same. In other words, the
J–Vplots demonstrate that the turn-on voltage is not a function of the CL thickness. Thus, referring
to the results of Figs. 2 and 3, we draw a conclusion that the CL effects on the optical properties are
induced not electrically but optically. This result agrees well with previous reports [15].
By conceptually dividing our TOLED structure into two parts of OLED and CL, we seek to
understand our results. We consider normally incident light. The illustration relevant to this
approach is presented in Fig. 4. In our TOLEDs, maximum bottom-side emission was achieved at a
CL thickness of 140 nm in simulated radiance. The integral bottom emission is due to the
contribution from the light wave ðDEOÞwhich travels toward the bottom side directly from the light-
emitting layer, and IERand the IIER, which have been reflected from the interface I and interface II.
Interface I and interface II refer to OLED/CL and CL/Air interfaces. In order to have maximum
bottom emission, the IERand the IIERmust have phase difference that gives constructive
interference. Invoking theory of thin layers, the interference mode has a direct dependency on the
Fig. 2. (a) Simulated radiance and (b) experimental EQE in orange and white TOLED with thickness of
CL. (Photographs show emission at a driving voltage of 8 V in orange (left) and white (right) TOLEDs.)
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
Vol. 4, No. 1, February 2012 Page 42
dielectric thickness. Therefore, we infer that the CL thickness 140 nm is a dielectric film thickness
which gives a phase difference between IERand IIERvery close to in-phase condition. In case of
top-side, the maximum top emission was achieved at a CL thickness of 60 nm. Transmittance of
dielectrics through which the IETtravels is the point of consideration. The II ETcan be understood as
the measurable top emission. To have maximum emission, it is desirable to have a dielectric film in
which the IETcan easily transmit. CL thickness of 60 nm has the highest maximum top emission,
and hence, may correspond to the thickness of highest transmittance. The maximum total bi-
direction emission was obtained at the point of maximum bottom-side emission rather than that of
maximum top-side emission. Referring to Fig. 2, this is because the bottom-side emission is
dominantly higher than that of the top emission. The light component ðUEOÞmay be understood as
the sum of two components, the reflected component ðIERÞat organics/CL interface and the
traveling component ðIETÞ. Because the total energy must be conserved the energy of IETmust be
lower than that of UEO. Assuming identical intensity of DEOand UEO, therefore, the top emission
cannot be the stronger than the bottom emission, even the IERand the IIERare interfering in a totally
destructive way. Furthermore, the gain in the top emission due to the presence of CL is less than
the loss in the bottom at 60 nm. In a simplified interference model of thin films [16], the phase
difference ðÞis given as 4dn= [17]. d, n and are the film thickness, refractive index and
wavelength, respectively. Taking ¼565 nm as the main peak and n ¼1:66, one obtains s of 0.71
and 1.65for 140 nm and 60 nm, which correspond to maximum and minimum bottom emissions,
respectively. The deviations from ideal s(and 2) for constructive and destructive values are
thought to have their origin in the structure and the non-monochromatic light as follows. First, the thin
metal film of Ag layer, which is positioned beneath the CL layer, can cause phase shift of the traveling
wave [18], [19]. Second, in real devices the emitted light is not strictly monochromatic. The emitted light
is a composite of various wavelengths. Thus various wavelengths and their relative intensities must be
Fig. 4. Optical components in TOLEDs with a CL (see the text for abbreviations).
Fig. 3. Current density–voltage (J–V) of TOLED in (a) bottom-side and (b) top-side as a function of CL
thickness.
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
Vol. 4, No. 1, February 2012 Page 43
considered to appropriately calculate the . Analytical calculations of the exact as function of the d
remain as a future work.
Fig. 5 shows the measured and simulated transmittance as a function of the CL thickness. Also
the simulated reflectance is shown. In the measurement we took the thin film (ITO/organic/LiF/Al/
Ag) at the orange peak position of Bt2Iracac, which is ¼565 nm. The variation trends of the
transmittances obtained from simulation and experiments are in good accordance. The
transmittance without a CL is only 47%. But, as expected in the emission results of Fig. 2,
maximum transmittance, 70%, was obtained at 60 nm CL where the maximum top emission
occurred. The reflectance at 60 nm shows a minimum with 11%. By introducing a CL layer of 60 nm
thickness, transmission enhancements of 49% and 40% were obtained in orange and white
devices, respectively. This result clearly shows that the top-side emission is strongly affected by the
enhancement in transmittance in top-contact due to an optical interference effect with CL thickness,
and demonstrates how the thickness of the CL in optically engineering TOLEDs is important.
Based on above interpretation of Figs. 4 and 5, we deduce some important guidelines, which are
practically useful in the CL designing of TOLEDs. First, in order to enhance the bottom emission,
the CL thickness must be designed to give a constructive interference condition between two
waves, which have been reflected from interfaces of the dielectrics. Second, the top emission can
be enhanced by choosing a dielectric film thickness of high transmittance. Third, the total bi-
directional emission is dominated by the bottom emission. The enhanced total emission originates
from the increased bottom emission, which is due to the interference effect. However, because
constructive interference condition does not necessarily coincide with the high transmittance
condition, enhancements both in total and top emission come out as a difficult engineering task to
achieve. In our TOLED structure, the total emission can only be achieved at the expense of low
transmission.
In this section we seek to analyze the effect of the CL on the EL spectra. Fig. 6 show the 565 nm
peak-normalized EL spectra, and the I605=I565 as a function of the CL thickness. I605 and I565 are the
spectral intensities at 605 nm and 565 nm, respectively. EL spectra in orange devices have
representative two peaks, main and shoulder peaks, positioned at 565 nm and 605 nm,
respectively. Variation in CL brought forth change in the shape of the EL spectra. While the two
peak positions remain unchanged, the peak intensity ratio ðI605=I565 Þshows a strong dependence
on the CL thickness. Up to a CL thickness of 40 nm, the bottom EL intensity of 605 nm [Fig. 6(a)]
gradually increases as the CL thickness increases. Then the intensity decreases again with thicker-
thickness. It is noticeable that the intensity corresponding to CL of 60 nm is almost identical to that
Fig. 5. Experimental and simulated transmittance as a function of thickness of CL. Simulated
reflectance also is displayed (photograph of TOLED located above the institutional logo).
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
Vol. 4, No. 1, February 2012 Page 44
of 0 nm. Such trend can be clearly seen in the inset. Fig. 6(b) shows the plot of the measured
intensities (upper) and simulated radiances (lower) of both wavelengths as a function of CL
thickness. In the plot, the measured intensities have been collected from the spectra in Fig. 6(a) and
have been normalized with respect to the highest values of 565 nm and 605 nm. The variations in
the measured and simulated values show same dependency on CL thickness. The intensities and
radiances of 565 nm and 605 nm decrease till the CL thickness reaches 60 nm. Up to this thickness
the intensities and radiances of 605 nm are higher than those of 565 nm. At CL thickness 60 nm,
turn-around occurs in the intensities and radiances of 565 nm and 605 nm. In the CL thickness
range of 60–140 nm, the intensities and radiances of 565 nm are higher. This is accordance with the
inset of Fig. 6(a). Such dependency has been inferred based on optical interference. The
interference, as can be deduced from Fig. 2, has periodicity which is wavelength dependent. To be
Fig. 6. 565 nm peak-normalized EL spectra of (a) bottom- and (c) top-side in orange TOLEDs, and plots
of the measured intensities (upper) and simulated radiances (lower) of both 605 nm and 565 nm
wavelengths as a function of CL thickness in (b) bottom- and (d) top-side. (Inset of (a) and (c) show peak
intensity ratio of 605 nm to 565 nm ðI605=I565 Þin bottom- and top-side, respectively.)
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
Vol. 4, No. 1, February 2012 Page 45
specific, the condition for constructive interference predicts longer periodicities for longer
wavelengths. Based on this, we interpret the turn-around as the difference in interference
periodicity. Now, we pay our attention to the top emission [Fig. 6(c) and (d)]. As can be noticed in
Fig. 6(c), the EL intensity of 605 nm peak value gradually increases till the thickness becomes 80 nm
and reaches its maximum at 80 nm. The intensities of devices with CL thicker than 80 nm do not vary
significantly. As have been explained in the previous section (Fig. 4), the top emission is mainly
governed by the transmittance of the CL. In order to explore this feature, we have plotted the
measured intensities (upper) and simulated radiances (lower) of both wavelengths as a function of
CL thickness [Fig. 6(d)]. The measured intensities of both wavelength show very similar variation
trend with having their maxima at 60 nm. Presumable due to the dominant effect of transmittance on
top emission, compared to the bottom emission case, the interference effect is not strongly
pronounced. The simulated radiances show similar trend. The simulated radiances show much
obvious symmetry around 60 nm. If the interference effect is dominating the emission, light with
different wavelength must show wavelength dependency in interference periodicity. However, the
difference in periodicity of 565 nm and 605 nm is very small. Thus, we draw a conclusion that the
interference effect in top emission is rather weak.
Assuming no significant change in the width of main spectral peaks, the peak intensity ratio
ðI605=I565 Þis very useful in comparing spectra obtained from devices of different CL thicknesses. For
instance, by comparing the ratio, one can easily find that the bottom emission spectra of CL 60 nm
and CL 0 nm are identical. The corresponding color coordinates of CL 60 nm and CL 0 nm are (0.526,
0.4716) and (0.526, 0.4717), respectively. In addressing the issue of matching the spectra of bottom
and top, we have used the I605=I565 as the criteria. The bottom and top spectra were most similar at a
CL thickness range of 100–120 nm. The bottom and top spectra with CL 120 nm are shown in Fig. 7.
The extracted color coordinates of the bottom and top spectra are (0.5189, 0.4786) and (0.5092,
0.4880), respectively. However, these spectra are not identical to those of CL 0 nm, which has a
color coordinate of (0.526, 0.4717) and (0.5021, 0.4949), respectively. Due to the difference in the
optical paths, the spectra of the bottom and top spectra in TOLEDs are unavoidably different. Our
investigations show that the bottom and top spectra of TOLEDs can be adjusted by varying the
CL thickness. In our work, the spectra matching of bottom and top is not obtained at the CL
thickness which corresponds to maximum top emission but at the CL thickness which correspond
to maximum total emission.
So far, we have focused on the analyses on the orange color. From our analyses, it is evident that
interference is the key in understanding the emission characteristics of TOLEDs equipped with CLs.
Although certain degree of complexity is expected, interference can be effectively used to interpret
the results observed in white TOLEDs with CLs. Also, with the use of CLs, spectra matching can be
achieved in white TOLED by adjusting the peak intensity ratio of dominating peaks.
Fig. 7. Bottom and top spectra with CL thickness of 120 nm.
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
Vol. 4, No. 1, February 2012 Page 46
4. Conclusion
We have suggested the use of an organic dielectric CL of TAPC on metal cathode in TOLED to
fabricate TOLEDs with improved characteristics. Technical parameters of concerns were efficiency,
tuning of B/T ratio- and spectral matching. Simulations and experiments were accompanied to
investigate the optical effects of the CL on the EL emission characteristics of orange and white
TOLEDs. The CL has distinct and different effect on the bottom and top emissions. In bottom
emission, maximum efficiency was obtained at CL 120 nm, which is interpreted as a result of
constructive interference effect. In top emission, the maximum efficiency was obtained at CL 60 nm,
which corresponds to the thickness of highest transmittance. The bottom emission is dominantly
higher than that of top. As a result, maximum total emission is obtained when the bottom emission is
maximum. By varying the CL thickness, it was possible to tune the B/T ratios in ranges of 3.30–6.08
and 1.66–4.25 in orange and white TOLEDs, respectively. By comparing the ratios of main spectral
EL peaks, it was possible to achieve identical bi-directional spectra. The corresponding thickness
was observed to coincide with the thickness of maximum total emission. Based on our results, we
have presented useful guidelines for optically designing high-performance TOLEDs with CLs. We
expect organic CL to emerge as an effective yet simple tool in improving and modulating the
TOLED performances.
References
[1] V. Bulvoic, G. Gu, P. E. Burrow, M. E. Thomson, and S. R. Forrest, BTransparent light-emitting devices,[Nature, vol. 380,
no. 6569, p. 29, Mar. 1996.
[2] L. S. Hung, C. W. Tang, M. G. Mason, P. Raychaudhuri, and J. Madathil, BApplication of an ultrathin LiF/Al bilayer in
organic surface-emitting diodes,[Appl. Phys. Lett., vol. 78, no. 4, pp. 544–546, Jan. 2001.
[3] S. K. So, W. K. Choi, L. M. Leung, and K. Neyts, BInterference effects in bilayer organic light-emitting diodes,[Appl.
Phys. Lett., vol. 74, no. 14, pp. 1939–1941, Apr. 1999.
[4] Q. Huang, K. Walzer, M. Pfeiffer, V. Lyssenko, G. He, and K. Leo, BHighly efficient top emitting organic light-emitting
diodes with organic outcoupling enhancement layers,[Appl. Phys. Lett., vol. 88, no. 11, pp. 113 515-1–113 515-3,
Mar. 1999.
[5] Y. Fukuda, T. Watanabe, T. Wakimoto, S. Miyaguchi, and M. Tsuchida, BCarrier transport properties of organic
materials for EL device operation,[Synth. Met., vol. 111/112, pp. 331–333, Jun. 2000.
[6] T. A. Beierlein, H.-P. Ott, H. Hofmann, H. Riel, B. Ruhstaller, B. Crone, S. Karg, and W. RieQ,BCombinatorial device
fabrication and optimization of multilayer organic LEDs,[in Proc. SPIE, 2002, vol. 4464, pp. 178–186.
[7] H. Riel, S. Karg, T. Beierlein, W. RieQ;, and K. Neyts, BTuning the emission characteristics of top-emitting organic light-
emitting devices by means of a dielectric capping layer: An experimental and theoretical study,[J. Appl. Phys., vol. 94,
no. 8, pp. 5290–5296, Oct. 2003.
[8] J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, S. Liu, and K. Leo, BLow-voltage organic electroluminescent devices
using pin structures,[Appl. Phys. Lett., vol. 80, no. 1, pp. 139–141, Jan. 2002.
[9] B. J. Chen, X. W. Sun, and S. C. Tan, BTransparent organic light-emitting devices with LiF/Mg:Ag cathode,[Opt. Exp.,
vol. 13, no. 3, pp. 937–941, Feb. 2005.
[10] H. Cho, J.-M. Choi, and S. Yoo, BHighly transparent organic light-emitting diodes with a metallic top electrode: The dual
role of a Cs2CO3layer,[Opt. Exp., vol. 19, no. 2, pp. 1113–1121, Jan. 2011.
[11] J. Lee, J.-I. Lee, J.-W. Lee, and H. Y. Chu, BInterlayer engineering with different host material properties in blue
phosphorescent organic light-emitting diodes,[ETRI J., vol. 33, no. 1, pp. 32–38, Feb. 2011.
[12] Sim4tec GmbH. [Online]. Available: www.sim4tec.com
[13] J. Staudigel, M. Stoessel, F. Steuber, and J. Simmere, BA quantitative numerical model of multilayer vapor-deposited
organic light emitting diodes,[J. Appl. Phys., vol. 86, no. 7, pp. 3895–3910, Oct. 1999.
[14] R. Meerheim, R. Nitsche, and K. Leo, BHigh-efficiency monochrome organic light emitting diodes employing enhanced
microcavities,[Appl. Phys. Lett., vol. 93, no. 4, pp. 043310-1–043310-3, Jul. 2008.
[15] H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. RieQ,BPhosphorescent top-emitting organic light-emitting devices
with improved light outcoupling,[Appl. Phys. Lett., vol. 82, no. 3, pp. 466–468, Jan. 2003.
[16] O. S. Heavens, BOptical properties of thin films,[Rep. Progr. Phys., vol. 23, pp. 1–65, 1960.
[17] F. A. Jenkins and H. E. White, Fundamentals of Optics, 4th., New York: McGraw-Hill, 1986.
[18] N. D. Goldina, BCalculation of the reflection coefficient of metal-dielectric structures in frustrated total internal reflection,[
Optoelectron. Instrum. Data Process., vol. 45, no. 6, pp. 571–575, Dec. 2009.
[19] J. M. Bennett, BPrecise method for measuring the absolute phase change on reflection,[J. Opt. Soc. Amer., vol. 54,
no. 5, pp. 612–624, May 1964.
IEEE Photonics Journal Optical Effects of CLs on TOLEDs
Vol. 4, No. 1, February 2012 Page 47
