A comprehensive analysis is given on the modifications of the exciton lifetime and internal quantum efficiency (int) for organic light-emitting devices (OLEDs). A linear relation is derived between the exciton lifetime and int, which is difficult to measure directly. The internal quantum efficiency can thus be estimated easily through the measurement of the exciton lifetime. The exciton lifetimes for OLEDs with weak or strong microcavity are studied experimentally and theoretically. The modification of the exciton lifetime is well explained through the microcavity effect and surface plasmon resonance. An excellent agreement between the experimental and theoretical results is achieved. © 2007 American Institute of Physics.published_or_final_versio

Chen, XW

Choy, WCH

He, S

Liang, CJ

Wai, PKA

Applied Physics Letters

Xue-Wen Chen

Wallace C. H. Choy

C. J. Liang

P. K. A. Wai

Sailing He

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Modifications of the exciton lifetime and internal quantum efficiency for organic light-emitting devices with a weak/strong microcavity

Chen, Xue-Wen

Choy, Wallace C. H.

Liang, C. J.

Wai, P. K. A.

He, Sailing,

Swepub

A comprehensive analysis is given on the modifications of the exciton lifetime and internal quantum efficiency (η[sub int]) for organic light-emitting devices (OLEDs). A linear relation is derived between the exciton lifetime and ηint, which is difficult to measure directly. The internal quantum efficiency can thus be estimated easily through the measurement of the exciton lifetime. The exciton lifetimes for OLEDs with weak or strong microcavity are studied experimentally and theoretically. The modification of the exciton lifetime is well explained through the microcavity effect and surface plasmon resonance. An excellent agreement between the experimental and theoretical results is achieved.Department of Electronic and Information EngineeringAuthor name used in this publication: P. K. A. Wa

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TitleModifications of the exciton lifetime and internal quantumefficiency for organic light-emitting devices with a weak/strongmicrocavityAuthor(s) Chen, XW; Choy, WCH; Liang, CJ; Wai, PKA; He, SCitation Applied Physics Letters, 2007, v. 91 n. 22Issued Date 2007URL http://hdl.handle.net/10722/57433Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Modifications of the exciton lifetime and internal quantum efficiency fororganic light-emitting devices with a weak/strong microcavityXue-Wen Chen, Wallace C. H. Choy,a and C. J. LiangDepartment of Electrical and Electronic Engineering, University of Hong Kong, Pokfulam Road, HongKongP. K. A. WaiPhotonics Research Centre, Department of Electronics and Information Engineering, The Hong KongPolytechnic University, Hung Hom, Kowloon, Hong KongSailing HeCentre for Optical and Electromagnetic Research and Joint Research Centre of Photonics of the RoyalInstitute of Technology (Sweden) and Zhejiang University, Zhejiang University, Zhijingang Campus,Hangzhou 310058, People’s Republic of ChinaReceived 24 September 2007; accepted 9 November 2007; published online 29 November 2007A comprehensive analysis is given on the modifications of the exciton lifetime and internal quantumefficiency int for organic light-emitting devices OLEDs. A linear relation is derived between theexciton lifetime and int, which is difficult to measure directly. The internal quantum efficiency canthus be estimated easily through the measurement of the exciton lifetime. The exciton lifetimes forOLEDs with weak or strong microcavity are studied experimentally and theoretically. Themodification of the exciton lifetime is well explained through the microcavity effect and surfaceplasmon resonance. An excellent agreement between the experimental and theoretical results isachieved. © 2007 American Institute of Physics. DOI: 10.1063/1.2819610Organic light-emitting devices OLEDs with variousdevice configurations have been the subject of intensive re-search due to their applications in display and lighting.1–4Considering the microcavity effect, OLEDs can be roughlycategorized into two types, i.e., weak microcavity OLEDsand strong microcavity OLEDs. Conventional bottom emit-ting OLEDs are weak microcavity devices, while OLEDswith distributed Bragg reflectors or two metallic electrodesare considered as strong microcavity devices. Light emissionproperties, including the internal quantum efficiency int,external quantum efficiency, exciton lifetime, and angulardependence, are distinct in the two types of OLEDs due tothe Purcell effect.5–8Exciton lifetimes of emitters in planar dielectric micro-cavity structures,9 near a partially reflecting surface,10,11 inweak microcavity OLEDs,12,13 and above metallic gratings14have been investigated either theoretically or experimentally.In this letter, a comprehensive analysis is given on the modi-fications of exciton lifetime for OLEDs with various devicestructures. A linear relation between the exciton lifetime andint will be derived, which means that int can be obtainedindirectly through the measurement of exciton lifetime. Inaddition, we investigate both experimentally and theoreti-cally the exciton lifetime for OLEDs with a weak or a strongmicrocavity.The theoretical analysis of exciton lifetime is based on aclassical approach where the emitter is considered as an elec-tric dipole running at a fixed current and with a randomorientation.6,15,16 As a consequence of Fermi’s golden rule,the radiative decay rate r at a wavelength  in an OLEDdevice is modified to15,16r = F · 0 , 1where 0 is the radiative decay rate in the infinite me-dium. Here, F, the so-called Purcell factor, is the normal-ized total emission power of the electric dipole with randomorientation normalized by the total power of the dipole inthe infinite medium. It should be noted that here, the emis-sion power coupled to the metal electrode is also included inF. We consider such decay channel as radiative decay sincethis part of emission can potentially be coupled out from thedevice through, for example, patterning the substrate.17 Inaddition, we typically assume that the nonradiative decayrate nr does not change by varying the thickness of thehole-transport layer HTL or the electron transport layerETL in the device. The initial quantum efficiency 0, int,and exciton lifetime OLED in the OLED are given by0 = 120dr120d + 2 − 1nr ,2int = 12F · 0dr12F · 0d + 2− 1nr , 3OLED = 0012F0d/120d + 1− 0 , 4where 1 ,2 is the wavelength range of the emission and 0aAuthor to whom correspondence should be addressed. FAX: 852 2559-8738. Electronic mail: chchoy@eee.hku.hk and wchchoy@hkucc.hku.uk.APPLIED PHYSICS LETTERS 91, 221112 20070003-6951/2007/9122/221112/3/$23.00 © 2007 American Institute of Physics91, 221112-1Downloaded 27 Apr 2009 to 147.8.17.95. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspis the exciton lifetime of the bulk emitting material. Equation4 can be written asOLED = 01 − int/1 − 0 . 5From Eq. 5, one observes that OLED linearly depends onint, which means that int can be determined from OLED. Asshown above, the Purcell factor F is a crucial quantity,which can be evaluated with the knowledge of the electricfield at the location of the dipole.6 The electric field can beefficiently calculated by integrating the plane wave compo-nent of the field along a proper integration path.16In the experiment, europium Eu organometallic com-plexes were used in the OLEDs as the emitters.18 As shownin Fig. 1, four sets of OLEDs, i.e., devices A, B, C, and D,were fabricated to study the exciton lifetime of Eu ion basedemitters in weak microcavity OLEDs devices A and Cand strong microcavity OLEDs devices B and D. TheEu complexes of Eua-thenoyltrifluoroacetonate3triphenylphosphine oxide2 EuTTA3TPPO2 andEudibenzoylmethanato3bathophenanthroline EuDBM3bath were used as the emitting layer in devices A and B anddevices C and D, respectively. The widely used material ofbis3-methylphenyl-diphenyl-benzidine was used as theHTL. UV emissive 1,3,5-benzinetriyl-tris1-phenyl-1-H-benzimidazole was used as the ETL. All the materials wereevaporated by thermal vacuum deposition at a pressure of10−6 Torr. The thickness of each layer is indicated in thedevice structure, where x denotes a variable thickness and tis the total thickness of the organic layers. The transient pho-toluminescence PL was excited by pulse laser at 337 nm.Figure 2 shows the measured transient PL intensities ofthe two devices, i.e., devices A and B with the same totalthickness of organic layers of 115 nm, as open circles andtriangles, respectively. The transient PL intensities with thelogarithmic scales are well fitted by linear functions shownas the solid lines, indicating an exponential decay of theintensities with time. From the linear fits, the exciton life-times of 0.441 and 0.325 ms are obtained for devices A andB, respectively. Simulations based on Eqs. 4 and 5 havebeen done for comparison with the experiment. The intrinsicemission spectrum of Eu complex, which is proportional to0, is shown in the inset of Fig. 2. The calculated Purcellfactors F of the two devices are also shown in the inset.According to Eq. 4 and the experimental results in Fig. 2,the initial internal quantum efficiency 0 and the lifetime 0for EuTTA3TPPO2 are determined to be 0.6 and 0.55 ms,respectively.From Eq. 5, we observe that the internal quantum ef-ficiency int depends linearly on the normalized excitonlifetime OLED /0. In Fig. 3, we plot the linear relation asdashed, solid, and dash-dotted lines for 0=0.8, 0.6, and 0.4,respectively. All the lines pass a fixed point OLED /0=0,int=1 and the slopes only depend on 0. Therefore, afterthe determination of 0 and 0, int of an OLED device canbe obtained by measuring the exciton lifetime, which ismuch easier as compared with the direct measurement. InFig. 3, the open circle and square are for devices A and Bt=115 nm studied in Fig. 2, respectively. Device B has alarger int than device A due to a larger Purcell effect indevice B.Figure 4a shows the experimental symbols and theo-retical lines exciton lifetimes of devices A and B as thefunctions of the total thickness of the organic layers t. Fromthe figure, we observe a good agreement between the simu-lated and experimental results. For device A, the lifetimeonly changes slightly when t varies since the weak cavityeffect has less influence on the spontaneous emission prop-erty. However, for device B strong microcavity OLEDs, theexciton lifetime changes significantly as t varies due to thestrong microcavity effect and the surface plasmon effect. Indevice B, when the emissive layer is close to the silver filmsmall t, the emission has a good coupling to the surfaceplasmon modes which is absent in device A. Thus, the exci-ton lifetime is significantly shortened at small t for device B.As t increases, the surface plasmon effect is quickly dimin-ished and microcavity effect gradually becomes pronounced.When the resonant condition of the microcavity is satisfied,the exciton lifetime of device B will reach a minimum,FIG. 1. Schematic diagrams of the devices A, B, C, and D.FIG. 2. The measured transient PL intensity of devices A open circles andB open triangles with the total thickness of organic layers of 115 nm. Theinset shows the Purcell factors of the two devices and the intrinsic spectrumof Eu complexes.FIG. 3. Internal quantum efficiency int varies as a function of the normal-ized exciton lifetime OLED /0 for various 0.221112-2 Chen et al. Appl. Phys. Lett. 91, 221112 2007Downloaded 27 Apr 2009 to 147.8.17.95. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspwhich indeed corresponds to the dip of the solid line at t=140 nm in Fig. 4a.Figure 4b shows the experimental symbols and theo-retical lines exciton lifetimes of devices C and D as func-tions of t. Here, 0 is recalibrated to be 0.33 ms forEuDBM3bath. Again, we observe a good agreement be-tween the experimental and theoretical results. One sees thatthe exciton lifetimes of all the devices drop significantly asthe distance from the LiF–Al cathode becomes short. This isdue to the excitation of the surface plasmon mode along theLiF–Al cathode which exists in both types of OLEDs. Mean-while, the microcavity effect is also pronounced for device Dand, thus, the exciton lifetimes of device D are shorter. Fordevice D, the simulated and experimental results agree wellin trend but have small discrepancy in absolute values. Thismay be due to some deviation in measuring the thickness ofthe silver film.In conclusion, a comprehensive analysis has been givenon the modifications of the exciton lifetime OLED and in-ternal quantum efficiency int in OLEDs. We have shownthat OLED has a linear relation with int. This means that int,which is important but difficult for a direct measurement, canbe estimated easily by measuring OLED. The modificationsof exciton lifetimes in various OLEDs have been well ex-plained through the microcavity effect and surface plasmoneffect.The authors want to acknowledge the support of UDFgrant, the strategic research grant in organic optoelectronicsof the University of Hong Kong, the grant 14300.324.01from the RGC of Hong Kong SAR, China, and a grant fromthe National Science Foundation of China 60688401. Oneof the authors P.K.A.W. wants to thank the support fromthe RGC of Hong Kong SAR, China Project No. PolyU5226/03E.1C. W. Tang and S. A. VanSyke, Appl. Phys. Lett. 51, 913 1987.2V. Bulovic, G. Gu, P. E. Burrows, S. R. Forrest, and M. E. Thompson,Nature London 380, 29 1996.3L. S. Liao, K. P. Klubek, and C. W. Tang, Appl. Phys. Lett. 84, 1672004.4Q. J. Sun, Y. F. Li, and Q. B. Pei, J. Disp. Technol. 3, 211 2007.5E. M. Purcell, Phys. Rev. 69, 681 1946.6W. Lukosz, Phys. Rev. B 22, 3030 1980.7K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A.Scherer, Nat. Mater. 3, 601 2004.8X. W. Chen, W. C. H. Choy, S. He, and P. C. Chui, J. Appl. Phys. 101,113107 2007.9G. Bjork, S. Machida, Y. Yamamoto, and K. Igeta, Phys. Rev. A 44, 6691991.10R. R. Chance, A. Prock, and R. Silbey, J. Chem. Phys. 60, 2744 1974.11H. Kuhn, J. Chem. Phys. 53, 101 1970.12M. Kauranen, Y. Van Rompaey, J. Makij, and A. Persoons, Phys. Rev.Lett. 80, 952 1998.13N. Tessler, Appl. Phys. Lett. 77, 1897 2000.14P. Andrew and W. L. Barnes, Phys. Rev. B 64, 125405 2001.15K. Neyts, J. Opt. Soc. Am. A 15, 962 1998.16X. W. Chen, W. C. H. Choy, and S. He, J. Disp. Technol. 3, 110 2007.17Y. J. Lee, S. H. Kim, J. Huh, G. H. Kim, and Y. H. Lee, Appl. Phys. Lett.82, 3779 2003.18C. J. Liang and W. C. H. Choy, Appl. Phys. Lett. 89, 251108 2006.FIG. 4. Exciton lifetime as a function of total thickness of organic layers fora EuTTA3TPPO2 based devices A and B and b EuDBM3 bath baseddevices C and D.221112-3 Chen et al. Appl. Phys. Lett. 91, 221112 2007Downloaded 27 Apr 2009 to 147.8.17.95. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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Modifications of the exciton lifetime and internal quantum efficiency for organic light-emitting devices with a weak/strong microcavity

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