The electroluminescence (EL) and photoluminescence properties of organic light-emitting diodes (LED) were analyzed using layer thickness modification. Investigations show that the EL spectra exhibited significant variation with the increase of the viewing angle. It was found that multiple peak emission could be achieved for a certain range of thickness values. It was concluded that near white emission with CIE coordinates (0.32, 0.43) could be obtained for optimized device thickness.published_or_final_versio

Chan, J

Chan, WK

Cheah, KW

Cheung, CH

Chui, PC

Djurišić, AB

Kwong, CY

Liu, ZT

Rakić, AD

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

Change of the emission spectra in organic light-emitting diodes by layer thickness modification

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Title Change of the emission spectra in organic light-emitting diodesby layer thickness modificationAuthor(s) Cheung, CH; Djuriši, AB; Kwong, CY; Tam, HL; Cheah, KW; Liu,ZT; Chan, WK; Chui, PC; Chan, J; Raki, ADCitation Applied Physics Letters, 2004, v. 85 n. 14, p. 2944-2946Issued Date 2004URL http://hdl.handle.net/10722/42363Rights Creative Commons: Attribution 3.0 Hong Kong LicenseAPPLIED PHYSICS LETTERS VOLUME 85, NUMBER 14 4 OCTOBER 2004Change of the emission spectra in organic light-emitting diodesby layer thickness modificationC. H. Cheung and A. B. Djurišiæa)Department of Physics, University of Hong Kong, Pokfulam Road, Hong KongC. Y. KwongDepartment of Electrical & 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 & 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 Qld 4072, Australia(Received 14 April 2004; accepted 12 August 2004)Electroluminescence and photoluminescence of organic light-emitting diodes consisting of anindium tin oxide anode, N,N8-di(naphthalene-1-yl)-N,N8-diphenyl-benzidine as a hole transportlayer, tris (8-hydroxyquinoline) aluminum as emitting layer, and an Ag cathode were measured fordifferent layer thickness values. It was found that, for a certain range of thickness values, multiplepeak emission can be achieved. In addition, the emission spectra were dependent on the viewingangle. For the optimized thickness values, normal incidence chromaticity coordinates achieved were0.32 and 0.43. Possible explanations for observed unexpected behavior are discussed. © 2004American Institute of Physics. [DOI: 10.1063/1.1802386]Tris (8-hydroxyquinoline) aluminum sAlq3d is a materialthat is very commonly used in organic light-emitting diodes(OLEDs), either as an electron transport or emitting layer,having a broad green emission.1,2 N,N8-di(naphthalene-1-yl)-N,N8-diphenylbenzidine (NPB) is a commonly used holetransport material, which can also be used for realization ofblue light-emitting devices.3,4 OLEDs based on NPB and/orAlq3 have been extensively studied. While someinterference5 and weak microcavity6,7 effects have been pre-viously reported, it is generally considered that a simpleNPB/Alq3 device would give a single peak emission thatwould exhibit small, if any, blueshift with the viewing angle.The multiple peak emission can be achieved from a singleemitting layer using microcavity structures.8,9 However, amicrocavity OLED typically exhibits a significant blueshiftwith the viewing angle.9–11In this work, we demonstrate that a multipeak emissioncan be achieved from a conventional, two-layer OLED con-sisting of an indium tin oxide (ITO) anode, a layer of NPB asa hole transport layer, a layer of Alq3 as electron transport/emitting layer, and a top silver cathode s,70 nmd. The NPBand Alq3 (from H. W. Sands) were purified by sublimationbefore device fabrication, and microcavity OLEDs were fab-ricated by evaporation in a high vacuum s,10−6 Torrd. Thea)Electronic mail: dalek@hkusua.hku.hk0003-6951/2004/85(14)/2944/3/$22.00 2944Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to Alayer thickness during deposition was given by a quartzthickness monitor, and verified after deposition by a stepprofiler. A HeCd laser (325 nm) and Xe lamp with a mono-chromator (420 nm) were used as excitation sources for pho-toluminescence (PL), while for electroluminescence (EL) aKeithley 2400 source meter was used to bias the devices.The spectra were recorded using fiberoptic spectrometerPDA-512-USB (Control Development Inc.).Figure 1 shows the normal incidence EL spectra for dif-ferent NPB/Alq3 thickness values. It can be observed thatthe emission spectrum is strongly dependent on the devicethickness, as well as the position of the NPB/Alq3 interfacefor the same total device thickness. In some cases, up tothree peaks can be observed (devices with 59/125 and59/139 NPB/Alq3 thickness in nanometers). For large de-vice thickness, the emission spectrum becomes very broad.Two-peak emission as a consequence of wide-angle interfer-ence was reported by So et al.5 The important differencebetween the results presented here and their work is that ourdevices contain no additional SiO2 layer and that we ob-served the multiple peak emissions for a much wider rangeof thickness values. Existing theory on the emission spectraof the microcavity devices8,9 can be easily modified to simu-late the spectra of an ordinary OLED by treating the ITOlayer as a mirror. While the simulated spectra demonstratethat it is indeed possible to obtain two-peak emission forsuitably chosen NPB and Alq3 thicknesses, the agreement is© 2004 American Institute of PhysicsIP license or copyright, see http://apl.aip.org/apl/copyright.jspAppl. Phys. Lett., Vol. 85, No. 14, 4 October 2004 Cheung et al. 2945not equally good for all the devices. In addition, we observedsignificant differences between the EL and PL spectra, indi-cating that simple interference phenomena are likely not asufficient explanation for the observed experimental results.In order to investigate the origin of the observed featuresin more detail, we have measured the EL spectra at differentviewing angles. A behavior significantly different from ex-pected small blueshift was found. Figure 2 shows the ELspectra of the 65/153 device for different viewing angles.The inset shows the chromaticity coordinates. At 0° viewingangle, the device exhibits blue-white emission, while atlarger viewing angles the device exhibits yellow-white emis-sion. The surface-normal emission shows two peaks, oneblue-green and one orange. The peaks do not show a signifi-cant shift with the viewing angle. However, the ratio of thepeak intensities significantly changes with the viewing angle.At smaller angles the higher energy peak is the dominantone, while the opposite is the case at larger angles. Such abehavior is unexpected, considering that the previous worksdemonstrated that a noncavity OLED would show no signifi-cant change in the emission spectra with the viewing angle.Another unexpected feature is the difference between the an-gular dependence of the EL and PL spectra, especially whenthe wavelength is chosen in such a manner that only Alq3 isFIG. 1. (a) EL spectra of OLEDs for different thicknesses of NPB and Alq3layers. (b) EL spectra of OLEDs for different positions of NPB/Alq3 inter-face. Solid line denotes experimental data, while dashed line denotes calcu-lated spectra. All the spectra were measured at 0° viewing angle, and thelayer thickness is given in nanometers.excited. Some difference between the EL and PL spectra isDownloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to Aexpected due to the different positions of the emission regionfor electrical and optical excitation. However, the interfer-ence phenomena as suggested by So et al.,5 do not providesufficient explanation of the measured spectra. It should alsobe noted that the EL spectra do not show significant depen-dence on the driving voltage, which would be expected ifthere were significant shifts in the location of the recombi-nation zone.The obtained PL spectra for different viewing angles areshown in Fig. 3(a) (excitation wavelength 325 nm) and 3(b)(excitation wavelength 420 nm). For 325 nm excitation, wecan observe the spectrum which strongly resembles a super-position of a strong NPB and weak Alq3 PL. When we excitethe structure with a 420 nm light, we can see a small redshiftof the PL peak upon increasing the viewing angle. While thePL spectra of NPB/Alq3 film on a quartz substrate show PLfrom both NPB and Alq3 similar to the PL of the OLEDdevice, the intensity ratios of the two peaks are different (PLfrom Alq3 is stronger from the film). For 420 nm excitationof the NPB/Alq3 film, no significant shift with the viewingangle can be observed, and the shoulder at ,460 nm, whichcan be observed in the PL of an OLED, is absent in the PL ofthe film. PL spectra at both wavelengths are significantlydifferent from the EL spectra. It is known that the differentEL and PL spectra can be obtained from the blends of or-ganic materials due to formation of exciplex complexes.12,13However, an exciplex emission should not exhibit any sig-nificant variation with the viewing angle. Interference effectshypothesis does not provide sufficient explanation for thedifferences between the EL and PL spectra and their behaviorwith respect to the viewing angle, although it is possible thatthe agreement between the calculated and experimental datacould be improved by better estimate of the emission regionposition and width, as well as different emitting dipole den-sity distribution in the case of optical excitation.In order to explain the origin of the unusual behaviorobserved, we have fitted the EL spectra shown in Fig. 2 withthe sum of three Gaussian peaks for each of the viewingangles. The peak positions for different viewing angles areFIG. 2. EL spectra of a NPB s65 nmd /Alq3 s153 nmd OLED for differentviewing angles. The dash-dot lines indicate peak positions as obtained in thefitting process. The position of the PL from an Alq3 film is also shown(dashed line). The inset shows chromaticity coordinates corresponding to theEL spectra.also shown in Fig. 2. The position of the PL from the Alq3IP license or copyright, see http://apl.aip.org/apl/copyright.jsp2946 Appl. Phys. Lett., Vol. 85, No. 14, 4 October 2004 Cheung et al.film is also indicated. The PL peak from NPB is,2.75 eVs,450 nmd. It can be observed that the positionsof the two lower wavelength (higher energy) features do notchange significantly with the viewing angle, and the weakred emission shows a blueshift with the increasing viewingangle. The shift of the chromaticity coordinates is due mainlyto the decreasing intensity of the blue-green emission. It wasproposed that the PL of Alq3 originates from three differentlevels.14,15 The proposed energies of these transitions were2.45, 2.30, and 2.04 eV (506, 539, and 608 nm),14 or 2.62,2.60, and 2.34 eV (473, 477, and 530 nm).15 The peak posi-tions obtained in this work are closer to those proposed byCurry and Gillin,14 although our peak positions do not ex-actly match those proposed in their work.It should be noted that in both previous reports14,15 theenergy levels were deduced by fitting, while here, two emis-sion peaks are clearly resolved. Moreover, the variation inthe relative peak intensity with the viewing angle can beclearly observed (at smaller angles the ,495 nm peak isdominant, at larger angles the ,570 nm peak is dominant).There are two possible causes for this: interference phenom-ena or some kind of an energy transfer between two or three(weak feature between 1.9 and 2.0 eV) different levels. Ineither case, observed phenomena are related to the weak mi-crocavity effects in the device, since multiple peaks are notobserved in the emission from the edge of the substrate. Wehave already shown that the theoretical simulations do notshow good agreement with the emission spectra for all de-FIG. 3. (a) PL of a NPB s65 nmd /Alq3 s153 nmd OLED for differentviewing angles at 325 nm excitation; and (b) PL of aNPB s65 nmd /Alq3 s153 nmd OLED for different viewing angles at 420 nmexcitation.Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to Avices. Increased discrepancy between the calculated and ex-perimental spectra for larger device thickness can also beobserved in other reported works in the literature.5,7 An al-ternative explanation could possibly be associated with thepolariton emission due to coupling between the photon andthe exciton mode. Polariton emission has, up to date, beendemonstrated only in PL from organic microcavities contain-ing relatively narrow emitting material.16 It requires largeoscillator strength, and matching of the widths of the photonand exciton modes. The estimated widths of the photon modes,100 nmd and Alq3 absorption peak s,65 nmd are similar.Device thickness, which is larger than usual in OLEDs, mayaccount for the fact that this effect has been previously un-observed. The change of the peak intensities with the view-ing angle and thus the longitudinal wave vector bears simi-larity to polariton emission. Since the phenomenon is moreclearly demonstrated in EL than PL, either the electric fieldor the excitation of different energy levels plays a significantrole. However, further study is needed to conclusivelyestablish the mechanisms responsible for the observedphenomena.To summarize, we have fabricated Alq3-based OLEDswith different device thickness values. For a range of de-vices, multiple peak emissions were observed, and the ELspectra exhibited significant variation with the increase ofthe viewing angle. With increasing viewing angle, energyexchange between the two peaks can be clearly resolved inthe EL spectra, which has some similarities with the behaviorof polariton PL. For optimized device thickness, near whiteemission with CIE coordinates (0.32, 0.43) can be obtained.This work is supported by The Research Grants Councilof the Hong Kong Special Administrative Region (ProjectNo. HKU 7056/02E and HKBU 2051/02P). The authorswould like to thank Dr. J. Gao for the use of step profiler toverify the device thickness.1X. Zhou, M. Pfeiffer, J. S. Huang, J. Blochwitz-Nimoth, D. S. Qin, A.Werner, J. Drechsel, B. Maening, and K. Leo, Appl. Phys. Lett. 81, 922,(2002).2Z. D. Popovic and H. Aziz, IEEE J. Sel. Top. Quantum Electron. 8, 362(2002).3Y. Kijima, N. Asai, and S-I. Tamura, Jpn. J. Appl. Phys., Part 1 38, 5274(1999).4T. Tsuji, S. Naka, H. Okada, and H. Onnagawa, Appl. Phys. Lett. 81, 3329(2002).5S. K. So, W. K. Choi, L. M. Leung, and K. Neyts, Appl. Phys. Lett. 74,1939 (1999).6P. E. Burrows, V. Khalfin, G. Gu, and S. R. Forrest, Appl. Phys. Lett. 73,435 (1998).7V. Buloviæ, V. B. Khalfin, G. Gu, P. E. Burrows, D. Z. Garbuzov, and S. R.Forrest, Phys. Rev. B 58, 3730 (1998).8T. Shiga, H. Fujikawa, and Y. Taga, J. Appl. Phys. 93, 19 (2003).9A. Dodabalapur, L. J. Rothberg, R. H. Jordan, T. M. Miller, R. E. Slusher,and J. M. Phillips, J. Appl. Phys. 80, 6954 (1996).10S. Dirr, S. Wiese, H.-H. Johanns, D. Ammermann, A. Böhler, W. Grahn,and W. Kowalsky, Synth. Met. 91, 53 (1997).11R. H. Jordan, L. J. Rothberg, A. Dodabalapur, and R. E. Slusher, Appl.Phys. Lett. 69, 1997 (1996).12J. S. Kim, B. W. Seo, and H. B. Gu, Synth. Met. 132, 285 (2003).13M. Mazzeo, D. Pisignano, L. Favaretto, G. Sotgiu, G. Barbarella, R.Cingolani, and G. Gigli, Synth. Met. 139, 675 (2003).14R. J. Curry and W. P. Gillin, J. Appl. Phys. 88, 781 (2000).15W. Humbs, H. Zhang, and M. Glasbeek, Chem. Phys. 254, 319 (2000).16R. F. Oulton, N. Takada, J. Koe, P. N. Stavrinou, and D. D. C. Bradley,Semicond. Sci. Technol. 18, S419 (2003).IP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Change of the emission spectra in organic light-emitting diodes by layer thickness modification

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