Chan, J

Chan, WK

Cheah, KW

Cheung, CH

Chui, PC

Djurišić, AB

Kwong, CY

Liu, ZT

Rakić, AD

Shore, JD

Tam, HL

Applied Physics Letters

C. H. Cheung

A. B. Djurišić

C. Y. Kwong

H. L. Tam

K. W. Cheah

Z. T. Liu

W. K. Chan

P. C. Chui

J. Chan

A. D. Rakić

Crossref

Response to “Comment on ‘Change of the emission spectra in organic light-emitting diodes by layer thickness modification’” [Appl. Phys. Lett. 86, 186101 (2005)]

HKU Scholars Hub

Comment on "change of the emission spectra in organic light-emitting diodes by layer thickness modification" [Appl. Phys. Lett. 85, 2944 (2004)]

Joel D. Shore

Comment on “Change of the emission spectra in organic light-emitting diodes by layer thickness modification” [Appl. Phys. Lett. 85, 2944 (2004)]

Djurisic, AB

Rakic, AD

Title Response to “Comment on ‘Change of the emission spectra inorganic light-emitting diodes by layer thickness modification’”Author(s) Cheung, CH; Djurisic, AB; Kwong, CY; Tam, HL; Cheah, KW; Liu,ZT; Chan, WK; Chui, PC; Chan, J; Rakic, ADCitation Applied Physics Letters, 2005, v. 86 n. 18, article no. 186102Issued Date 2005URL http://hdl.handle.net/10722/209296Rights Creative Commons: Attribution 3.0 Hong Kong LicenseResponse to “Comment on ‘Change of the emission spectra in organiclight-emitting diodes by layer thickness modification’”Appl. Phys. Lett. 86, 186101 2005C. H. Cheung and A. B. Djurišiæa!Department of Physics, University of Hong Kong, Pokfulam Road, Hong KongC. Y. KwongDepartment of Electrical and Electronic Engineering, University of Hong Kong, Pokfulam Road,Hong KongH. L. Tam and K. W. CheahDepartment of Physics, Hong Kong Baptist University, Kowloon Tong, Hong KongZ. T. LiuDepartment of Physics, University of Hong Kong, Pokfulam Road, Hong KongW. K. ChanDepartment of Chemistry, University of Hong Kong, Pokfulam Road, Hong KongP. C. ChuiDepartment of Electrical and Electronic Engineering, University of Hong Kong, Pokfulam Road,Hong KongJ. Chan and A. D. RakiæSchool of Information Technology and Electrical Engineering, The University of Queensland,Brisbane Qld4072, AustraliasReceived 4 January 2005; accepted 11 April 2005; published online 29 April 2005dfDOI: 10.1063/1.1925322gIn our recent paper,1 we presented the study of the emis-sion spectra of tris-s8-hydroxyquinolined aluminum sAlqdbased organic light-emitting diodes sOLEDsd as a function oforganic layer thickness. Both calculations and experimentalresults were presented. The discrepancy between the calcu-lated and measured emission spectra was noted, and possiblereasons were discussed. Finally, it was concluded that furtherstudy is needed to conclusively establish whether any otherphenomena in addition to simple interference play a role inthe obtained results. In his recent comment,2 Shore claimedthat our experimental data can be entirely explained bysimple interference phenomena, and presented calculationswhich qualitatively “reproduce” basic behavior of our de-vices.However, the calculations in our work fFig. 1sbdg1 alsoshow similar behavior as those presented in the comment byShore sFig. 2d.2 Since Shore2 changed only the Alq layerthickness, while we considered devices with different N,N8-disnaphthalene-1-yld-N,N8-diphenylbenzidine sNPBd andAlq thicknesses, direct comparison can only be made for thedevice with 65 nm NPB and 139 nm Alq. It is obvious fromFig. 1sbd in our letter1 and Fig. 2 in the recent comment2 thatboth calculations show essentially the same features. Yet,quantitative agreement between the experimental data andthe calculated results is lacking. Since the thicknesses of or-ganic layers were verified by step profiler and spectroscopicellipsometry after deposition, thickness errors in the fabri-cated devices are unlikely. Another possible reason for thediscrepancy, as correctly identified by Shore,2 is differentemission region thickness. Figure 1 illustrates the influenceof the assumed emission region thickness on the calculatedelectroluminescence sELd spectra. Further studies with con-fined emitting layers of known thickness are in progress toexperimentally establish the influence of the emitting layerthickness.We would also like to point out that, regardless of theassumed emission layer thickness, the calculated spectra al-ways show two peaks, while some of the experimental spec-tra showed clear shoulders in addition to two peaks. Com-parison between the calculated and the experimental spectrafor the 65/153 s65 nm NPB and 153 nm Alqd device isshown in Fig. 2sad. It can be observed that the calculatedspectrum, exhibiting a two-peak structure, does not describethe experimental spectrum well. The calculation for the sameadElectronic mail: dalek@hkusua.hku.hkFIG. 1. Influence of the emitting layer thickness. The spectra have beennormalized and shifted for clarity.APPLIED PHYSICS LETTERS 86, 186102 s2005d0003-6951/2005/86~18!/186102/2/$22.50 © 2005 American Institute of Physics86, 186102-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  147.8.230.88On: Mon, 20 Apr 2015 06:40:15device, shown in Fig. 3 in the comment by Shore,2 shows thesame two-peak structure, although with a different ratio ofthe peak intensities which is likely due to the emission re-gion width of 30 nm, instead of more commonly used20 nm.3It should also be noted that the claim by Shore2 of theNPB thickness variation being less important disagrees withboth experimental and calculated results fFig. 1b in Ref. 1gin spite of the similar optical functions of NPB and Alqabove ,450 nm, since the distance of the light emittinglayer, i.e., NPB/Alq interface, from the cathode plays a sig-nificant role in the devices with the same total thickness.This is also the case for the photoluminescence sPLd spectra,as shown in Fig. 2sbd, which is in disagreement with theclaim by Shore2 that any interference effects are cancelled inthe PL. Therefore, it is still necessary to improve the agree-ment between the experimental results and calculated emis-sion spectra swhich is the case both for our calculations andcalculations by Shore2 which exhibit essentially the samefeaturesd, and further experimental and simulation studies arein progress to positively identify the causes of discrepanciesbetween calculated and experimental spectra.1C. H. Cheung, A. B. Djurišiæ, C. Y. Kwong, H. L. Tam, K. W. Cheah, Z.T. Liu, W. K. Chan, P. C. Chui, J. Chan, and A. D. Rakiæ, Appl. Phys. Lett.85, 2944 s2004d.2J. D. Shore, Appl. Phys. Lett. 86, 186101 s2005d, preceding paper.3A. Dodabalapur, L. J. Rothberg, and T. M. Miller, Appl. Phys. Lett. 65,2308 s1994d.FIG. 2. sad Comparison between the experimental and the calculated ELspectra for the device with 65 nm NPB and 153 nm Alq. Arrows indicatepositions of the Gaussian peak fit. sbd PL spectra in a device with totalorganic thickness of 204 nm for different positions of NPB/Alq interface.PL spectrum from 204 nm Alq film is also shown.186102-2 Cheung et al. Appl. Phys. Lett. 86, 186102 ~2005! This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  147.8.230.88On: Mon, 20 Apr 2015 06:40:15

Response to “Comment on ‘Change of the emission spectra in organic light-emitting diodes by layer thickness modification’”

Cheung, C. H.

Djurisic, A. B.

Kwong, C. Y.

Tam, H. L.

Cheah, K. W.

Liu, Z. T.

Chan, W. K.

Chui, P. C.

Chan, J.

Rakic, A. D.

University of Queensland eSpace

Response to "Comment on 'Change of the emission spectra in organic light-emitting diodes by layer thickness modification'"

Djurisic, A.B.

Kwong, C.Y.

Tam, H.L.

Cheah, K.W.

Liu, Z.T.

Chan, W.K.

Chui, P.C.

Chan, J. C.

Rakic, A.

Comment on “Change of the emission spectra in organic light-emitting diodes by layer
thickness modification” [Appl. Phys. Lett.85, 2944 (2004)]
Joel D. Shore 
 
Citation: Applied Physics Letters 86, 186101 (2005); doi: 10.1063/1.1925321 
View online: http://dx.doi.org/10.1063/1.1925321 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/86/18?ver=pdfcov 
Published by the AIP Publishing 
 
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 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  130.102.82.98 On: Thu, 06 Oct 2016
06:49:10
Comment on “Change of the emission spectra in organic light-emitting
diodes by layer thickness modification” Appl. Phys. Lett. 85, 2944 2004
Joel D. Shorea!
Research and Development Laboratories, Eastman Kodak Company, Rochester, New York 14650-2142
sReceived 12 November 2004; accepted 21 March 2005; published online 29 April 2005d
fDOI: 10.1063/1.1925321g
In a recent letter, Cheung et al.1 report “unexpected be-
havior” in the electroluminescence sELd and photolumines-
cence sPLd spectra of a simple organic light-emitting diode
sOLEDd device using a tris-s8-hydroxyquinolined aluminum
sAlq3d layer as the emitter. They express surprise at their
results and hypothesize various exotic mechanisms. Here, we
demonstrate that all of the observed phenomena can be ex-
plained simply by optical interference effects, which can be
qualitatively reproduced by optical modeling of the device.
When light emission occurs in a layered device, the
spectrum can be dramatically modified by optical interfer-
ence. For a conventional OLED structure, the most important
interference effect occurs between light emitted directly out
of the device through the glass substrate and that reflected off
of the metallic cathode. Smaller interference contributions
occur from reflections at other interfaces, particularly the in-
terface between the indium tin oxide sITOd anode and the
glass substrate. The resulting emission spectrum can be ex-
pressed as the product of the “innate” spectrum of the emitter
and the optical response of the device. Figure 1 demonstrates
how the double-peaked emission structure, seen in Cheung et
al.,1 can emerge when the peak of the innate emission spec-
trum and a minimum in the optical response nearly coincide.
The optical model used here is described in Ref. 2; however,
the minimum seen in the optical response can be understood
quite simply as being due to the destructive interference of
the light reflected off of the cathode with that emitted di-
rectly out of the device.
In Fig. 2, we vary the thickness of the Alq3 to study the
basic trends seen in Fig. 1sad of Cheung et al.1 We have not
attempted to also vary the N ,N8-disnaphthalene-1-yld-
N ,N8-diphenylbenzidine sNPBd thickness sas Cheung et al.1
didd because this has a smaller effect on the behavior. In
addition, it would be necessary to know the ITO thickness,
and any variation in it, in order to accurately account for
effects of NPB thickness variation, and Cheung et al.1 did
not specify the ITO thickness that they used. The important
point is that the basic trends and features seen in Cheung et
al.1 with varying Alq3 thickness are well reproduced by the
optical model.3
In Fig. 3, we consider the angular dependence of a de-
vice and reproduce behavior similar to that seen in Fig. 2 of
Cheung et al.1 They express surprise that what is observed is
not simply a small blueshift of the peak. However, this fails
to recognize that, while the optical response curve of a de-
vice does tend to shift in the blue direction, the EL emission,
being the product of this curve with the innate emission
spectrum, can exhibit more complicated behavior. For ex-
ample, it is clear that if the optical response curve shown in
Fig. 1 were to shift in the blue direction, the resulting EL
spectrum would show a decrease in the height of the blue
peak and an increase in the height of the red peak.
We can also reproduce the basic behavior that Cheung et
al.1 see in Fig. 3sbd, where they show that the angular depen-
adElectronic mail: joel.shore@kodak.com
FIG. 1. Normalized on-axis spectra for a device having the structure
glass/40 nm ITO/65 nm NPB/146 nm Alq3/70 nm Ag, as predicted by op-
tical modeling. Emission is assumed to occur uniformly from a 30 nm re-
gion of Alq3 adjacent to the NPB. The EL spectrum is given by the product
of the innate spectrum of the Alq3 and the optical response of the device.
FIG. 2. Normalized on-axis EL spectra for the same structure as in Fig. 1
but with various Alq3 layer thicknesses. Spectra are vertically offset for
clarity.
APPLIED PHYSICS LETTERS 86, 186101 s2005d
0003-6951/2005/86~18!/186101/2/$22.50 © 2005 American Institute of Physics86, 186101-1 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  130.102.82.98 On: Thu, 06 Oct 2016
06:49:10
dence of the PL spectrum of the device sat an excitation
wavelength that is expected to excite mainly the Alq3d is
considerably different and less complex than what is seen for
the EL spectrum of the same device. The important differ-
ence in the two spectra, from an optical point of view, is that
the EL spectrum results from emission in the Alq3 occurring
primarily within a short distance of the Alq3-NPB interface,
whereas the PL spectrum results from emission throughout
the entire Alq3 layer. This broad spatial distribution of the PL
emission results in a substantial cancellation of any interfer-
ence effects.
Finally, while Cheung et al.1 are correct in that the ap-
propriate tuning of the layer thicknesses can give near-white
emission fe.g., we predict CIE 1931 chromaticity coordinates
of s0.36, 0.35d can be obtainedg, our modeling also predicts
that the resulting efficiency will be very low, with the on-axis
radiance being only about 13% that of a device structure
tuned for maximum efficiency swhich gives a yellowish
green emissiond. This is because the double-peak structure is
obtained by creating destructive interference of the light near
the peak wavelength of the Alq3 emitter. As a result of this
low efficiency, the practical utility of such a device is very
limited.
In conclusion, we have shown that all of the qualitative
behaviors seen in Cheung et al.1 can be attributed to simple
optical interference phenomena within the device. It, there-
fore, seems unnecessary to appeal to more exotic mecha-
nisms.
1C. H. Cheung, A. B. Djurišiæ, C. Y. Kwong, H. L. Tam, K. W. Cheah, Z.
T. Liu, W. K. Chan, P. C. Chui, J. Chan, and A. D. Rakiæ, Appl. Phys. Lett.
85, 2944 s2004d.
2K. B. Kahen, Appl. Phys. Lett. 78, 1649 s2001d; O. H. Crawford, J. Chem.
Phys. 89, 6017 s1988d.
3The small quantitative discrepancies at which various spectral features are
seen could be due to errors in measurement of the Alq3 thickness used in
the experiment or to our assumption about the region over which the
emission is occurring or our approximation of keeping the NPB thickness
fixed.
FIG. 3. Normalized EL spectra at various angles for the same structure as in
Fig. 1 but with 153 nm of Alq3. Spectra are vertically offset for clarity.
186101-2 Joel D. Shore Appl. Phys. Lett. 86, 186101 ~2005!
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06:49:10


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