The magnetoelectroluminescence (MEL) of organic light emitting devices with a N, N′ -bis(l-naphthyl)- N, N′ -diphenyl- 1, l′ -biphentl- 4, 4′ -diamine:tris-(8-hydroxyquinoline) aluminum (NPB: Alq 3) mixed emission layer (EML) has been investigated. We find that MEL is maximized when the volume ratio of NPB of the mixed EML reaches 30% and the EML thickness is 40 nm. The features of MEL under various magnetic field strengths are insensitive to the change in EML thickness and mixing ratio. Meanwhile, MEL has a close relationship with the carrier mobility. We have conducted a theoretical study to further verify the relationship. Our experimental and theoretical results confirm that MEL can function as a tool to indicate the mobility. © 2010 American Institute of Physics.published_or_final_versio

Choy, WCH

Ding, BF

Ding, XM

Gao, XD

Hou, XY

Sun, ZY

Wang, ZJ

Wu, CQ

Yao, Y

Applied Physics Letters

B. F. Ding

Y. Yao

Z. Y. Sun

C. Q. Wu

X. D. Gao

Z. J. Wang

X. M. Ding

W. C. H. Choy

X. Y. Hou

Crossref

Magnetic field effects on the electroluminescence of organic light emitting devices: A tool to indicate the carrier mobility

HKU Scholars Hub

Title Magnetic field effects on the electroluminescence of organiclight emitting devices: A tool to indicate the carrier mobilityAuthor(s) Ding, BF; Yao, Y; Sun, ZY; Wu, CQ; Gao, XD; Wang, ZJ; Ding,XM; Choy, WCH; Hou, XYCitation Applied Physics Letters, 2010, v. 97 n. 16Issued Date 2010URL http://hdl.handle.net/10722/135129Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Magnetic field effects on the electroluminescence of organic light emittingdevices: A tool to indicate the carrier mobilityB. F. Ding, Y. Yao, Z. Y. Sun, C. Q. Wu, X. D. Gao et al.  Citation: Appl. Phys. Lett. 97, 163302 (2010); doi: 10.1063/1.3505343 View online: http://dx.doi.org/10.1063/1.3505343 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v97/i16 Published by the American Institute of Physics.  Related ArticlesControl of coherent acoustic phonon generation with external bias in InGaN/GaN multiple quantum wells Appl. Phys. Lett. 100, 101105 (2012) Small molecular phosphorescent organic light-emitting diodes using a spin-coated hole blocking layer Appl. Phys. Lett. 100, 083304 (2012) Small molecular phosphorescent organic light-emitting diodes using a spin-coated hole blocking layer APL: Org. Electron. Photonics 5, 53 (2012) AlGaN-based ultraviolet light-emitting diodes using fluorine-doped indium tin oxide electrodes Appl. Phys. Lett. 100, 081110 (2012) Temperature dependent efficiency droop in GaInN light-emitting diodes with different current densities Appl. Phys. Lett. 100, 081106 (2012)  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 07 Mar 2012 to 147.8.21.59. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsMagnetic field effects on the electroluminescence of organic light emittingdevices: A tool to indicate the carrier mobilityB. F. Ding,1,2 Y. Yao,2 Z. Y. Sun,2 C. Q. Wu,2 X. D. Gao,2 Z. J. Wang,2 X. M. Ding,2W. C. H. Choy,1,a and X. Y. Hou21Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road,Hong Kong2State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, People’s Republic of ChinaReceived 9 September 2010; accepted 3 October 2010; published online 21 October 2010The magnetoelectroluminescence MEL of organic light emitting devices with a N,N-bisl-naphthyl-N,N-diphenyl-1 , l-biphentl-4 ,4-diamine:tris-8-hydroxyquinoline aluminumNPB:Alq3 mixed emission layer EML has been investigated. We find that MEL is maximizedwhen the volume ratio of NPB of the mixed EML reaches 30% and the EML thickness is 40 nm.The features of MEL under various magnetic field strengths are insensitive to the change in EMLthickness and mixing ratio. Meanwhile, MEL has a close relationship with the carrier mobility. Wehave conducted a theoretical study to further verify the relationship. Our experimental andtheoretical results confirm that MEL can function as a tool to indicate the mobility. © 2010American Institute of Physics. doi:10.1063/1.3505343Magnetic field can moderately modify the electrolumi-nescence EL of organic light emitting devices OLEDs,namely, organic magnetoelectroluminescence MEL.1–5MEL is quantitatively expressed with EL /EL= ELB−EL0 /EL0, where ELB and EL0 are the EL inten-sity with and without external magnetic field B,respectively.6 MEL provides a reliable way to investigate thespin related exciton dynamics,6–10 the mechanism of thecarrier-to-photon conversion in organic semiconductors,8–12and even the operation mechanism of magnetic sensor andavian magnetic compass.13 Some methods have been usedto control and tune MEL, such as changing appliedvoltage4,5,12,14 and inserting insulating layer of LiF.5 How-ever, it is still not clear which factor governs MEL. It isbecause any change in the applied voltage will modify theelectric field and exciton density simultaneously, and all ofthem can cause change in MEL.In this letter, an OLED structure with the composite lightemitting layer EML is designed to tune the MEL. The MELis optimized by change the EML thickness and the volumeratio of N,N-bisl-naphthyl-N,N-diphenyl-1 , l-biphentl-4 ,4-diamine NPB of the EML. Our results show that MELis very sensitive and inversely related to the carrier mobilityof the organic semiconductors. The relationship between thecarrier mobility and the MEL is verified by the carrier hop-ping rate through our theoretical study of the MEL. The re-sults confirm that MEL can contribute to diagnose the carriermobility of an organic semiconductor.The device structure is shown in Fig. 1a. The fluores-cent EML with a thickness of 20 nm composes of tris-8-hydroxyquinoline aluminum Alq3 and NPB. The emittingarea is approximately 44 mm2. The 40-nm-thick NPB and50 nm-thick Alq3 form the hole-transport and electron-transport layers, respectively. ITO and Al/LiF are anode andcomposite cathode, respectively. By applying magnetic fieldto the OLEDs, the brightness of the EL will be increased.Figure 1b shows the EL /EL i.e., MEL of the OLEDswith eight different volume ratios of NPB varying from 0%to 100%. At low magnetic field i.e., 0–15 mT, the incre-ment of brightness is large, while at high magnetic field20 mT, the brightness almost saturates. The inset of Fig.1b shows that the normalized MELs of eight devices havethe same trend. The result indicates that the features of MELaAuthor to whom correspondence should be addressed. Electronic mail:chchoy@eee.hku.hk.FIG. 1. Color online a A schematic view of the device structure. b Theexperimental MEL of the eight OLEDs with different NPB concentrations inthe EML. All devices are driven at 500 A. Inset shows the normalizedMEL.APPLIED PHYSICS LETTERS 97, 163302 20100003-6951/2010/9716/163302/3/$30.00 © 2010 American Institute of Physics97, 163302-1Downloaded 07 Mar 2012 to 147.8.21.59. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsi.e., EL /EL with the strength of magnetic field are insen-sitive with the change in concentration ratio but not the truevalue of EL /EL.In order to intuitively show the concentration effectsand thus study the mobility effects of the mixed EML onthe value of EL /EL, the EL /EL of the eight devices at agiven magnetic field of 45 mT is plotted in Fig. 2a. Thedevices are driven at four different modes. Take the MELunder the constant driving current of 1000 A as an ex-ample, by varying the volume ratio of NPB from 0% to 30%,EL /EL increases from 2.9% to 5.3%. When the volumeratio is further increased from 30% to 100%, EL /EL de-creases from 5.3% to 2.7%. The optimized volume ratiotherefore is 30%. The MELs under other three driving modesincluding constant current 500 A, constant voltage 5.1 V,and constant initial brightness 200 cd /m2 all reveal that 30%is the optimized ratio for maximizing the MEL. More impor-tantly, the unique feature of the MEL with the concentrationof the mixed EML is merely from the concentration effectand not due to the driving conditions. The unique feature canbe used to diagnose the carrier mobility of the organic semi-conductor as will be discussed later.In fact, the MEL is can be further enhanced by changingthe thickness of the mixed EML. Figure 2b shows theOLEDs with six different EML thickness including 0, 6, 14,28, 40, and 80 nm. The device structure is ITO /NPB50−x /2 nm /NPB:Alq3x nm /Alq360−x /2 /LiF /Al. Thetotal thickness of the devices is constant and all volume ratioof NPB in the mixed EML is fixed at 30%. The normalizedEL /ELs of these OLEDs show the similar trend as that inthe inset of Fig. 2a. Consequently, the trend of MEL withthe strength of magnetic field is insensitive with the changein concentration and thickness of the EML.Concerning the magnitude of the EL /EL, when thethickness of EML increases from 0 to 40 nm, the magnitudeincreases in the whole range of the applied magnetic field.For the thickness from 40 to 80 nm, the enhancement withthickness of blended layer is almost saturated. As shown inFig. 2b, at the given magnetic field of 45 mT, EL /ELincreases from 2.5% to 6.3% when the thickness increasesfrom 0 to 40 nm. However, EL /EL only changes from6.3% to 6.5% when the thickness is further increased from40 to 80 nm. Consequently, with the increase in emissionregion, the EL /EL can be increased and optimized at about40 nm.Interestingly, we find that MEL is very sensitive to thecarrier mobility of the devices. When the electrical bias tothe OLED reduces, the whole EL /EL spectrum with themagnetic field reduces except the case of no magnetic field atwhich EL /EL=0 the plot is not shown. For a givenOLED, considering V=0dExdx, where V is the drivingvoltage, E is the electric field and d is the thickness of thedevice. When the voltage reduces, the electric field in theorganic structure reduces. Meanwhile, through the formulaof E=0expE, where E is the carrier mobilityat an applied field of E, 0 is the zero-field carrier mobilityand  is the Poole–Frenkel slope, we can see that when theelectric field reduces, the carrier mobility drops but theEL /EL i.e., MEL increases. This feature can be con-firmed by our results as shown in Fig. 2a. The results showthat the EL /EL is maximized when the volume ratio ofNPB is around 30%. We also plot the electron mobility of themixed NPB:Alq3 taken from Ref. 15. Remarkably, the peakof the EL /EL is almost at the trough of the measuredmobility.15,16In theory, we have introduced the Hubbard model17 todescribe the feature trend of the EL /EL. The Hamiltonian,including both the intermolecular transportation of electrons/holes and the interaction between electrons/holes and nuclearspins, is described as,H = H1 + H2, 1where H1 includes hopping and Coulomb interaction of car-riers asH1 = − i,j,ti,ih di, di, + tj,jecj, cj, + h . c .+ Ui,jni,↑h ni,↓h + ni,↑e ni,↓e  + V . 2In Eq. 2, dj, dj, and cj, cj, are the creation annihi-lation operators of hole and electron, respectively. The holeand electron have spin  which can be spin up ↑  or down↓  in the ith and jth molecule. i and j denote the neigh-bors of the ith and jth molecule, respectively, ti,ih tj,je  is thehopping rate of hole between the ith and ith moleculeselectron between jth and jth molecules with a unit of mi-croelectron volt. U is the Coulomb repulsive energy betweentwo holes or two electrons with different spins at the samemolecule. ni,h 	di, di, and ni,e 	cj, cj, are the corre-sponding creation and annihilation multiplied operators ofhole and electron, and V is the attractive interaction betweenhole and electron at the same molecule.FIG. 2. Color online a MEL vs the volume ratio of NPB of the mixedEML at a fixed magnetic field of 45 mT. Four different driving modesincluding constant current driving of 500 and 1000 A, constant voltagedriving of 5.1 V and constant initial brightness of 200 cd /m2. Black circledot denotes the experimental electron mobility of NPB:Alq3 mixed materi-als taken from Ref. 15. b MEL of the six devices with different EMLthicknesses. The OLEDs were driven at 1000 A.163302-2 Ding et al. Appl. Phys. Lett. 97, 163302 2010Downloaded 07 Mar 2012 to 147.8.21.59. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsH2 includes the effect of an external magnetic field andhyperfine interactions and can be expressed asH2 = gBi,jB ext + B hyp,i · S ih + B ext + B hyp,j · S je , 3where g=2.0 for organic materials,18 B is Bohr magnetron,B ext is the external magnetic field chosen to be along thez-direction, and B hyp,ij is the effective nuclear magneticfield of the ith jth molecule, S ih and S je are the classical spinvectors of the hole and electron in the ith and jth molecules,respectively. In this work, we mainly study the relationshipbetween the hopping rate of the carrier and MEL. Therefore,only the hopping rate is variable. Generally, hopping ratesti,ihand tj,jevary from molecule to molecule due to the dis-order arrangement of organic materials. Their values aretypically between 1 and 100 eV.15,19With the Hubbard model, the theoretical hopping rate ofelectron can be determined as shown in Fig. 3. It can be seenthat the change in the hopping rate of electron with the vol-ume ratio of NPB in the mixed EML is with the similar trendas that of the experimental electron mobility as shown in Fig.2a. The MEL is inverted related to the carrier hopping rateand mobility. Meanwhile, the maximum MEL can be ob-tained when the carrier mobility is at its minimum. This canprovide further evidence that MEL can be used as a tool todiagnose the carrier mobility of an organic material.In conclusion, we have presented how to use the com-posite EML layer compositing from NPB and Alq3 to tuneand control the MEL of organic semiconductor devices. Theoptimized MEL is achieved when the mixed EML has NPBvolume ratio of 30% and thickness of 40 nm. Our resultsshow that carrier mobility is a dominant factor to determinethe intensity of MEL. The trend of MEL is opposite to that ofthe mobility of organic semiconductors. Our finding offers away to tune MEL in OLEDs. In addition, we can use MEL asa tool to reflect the mobility of OLEDs.This work is supported by the UGC Grant Grant No.400327 of the University of Hong Kong, the grants GrantNos. HKU712108 and HKU712010 from the ResearchGrant Council of the HK Special Administrative Region,China, the National Natural Science Foundation of Chinaand Shanghai Science and Technology Commission GrantNo 08JC1402300, and the MST of China Grant No.2009CB929200. B. F. Ding acknowledges the support ofEngineering Postdoctoral Fellowship, HKU.1T. D. Nguyen, G. Hukic-Markosian, F. J. Wang, L. Wojcik, X. G. Li, E.Ehrenfreund, and Z. V. Vardeny, Nature Mater. 9, 345 2010.2P. Chen, Y. L. Lei, Q. L. Song, Y. Zhang, R. Liu, Q. M. Zhang, and Z. H.Xiong, Appl. Phys. Lett. 95, 213304 2009.3L. Y. Xin, C. N. Li, F. Li, S. Y. Liu, and B. Hu, Appl. Phys. Lett. 95,123306 2009.4F. J. Wang, H. Bassler, and Z. V. Vardeny, Phys. Rev. Lett. 101, 2368052008.5H. Odaka, Y. Okimoto, T. Yamada, H. Okamoto, M. Kawasaki, and Y.Tokura, Appl. Phys. Lett. 88, 123501 2006.6M. Wohlgenannt and Z. V. Vardeny, J. Phys.: Condens. Matter 15, R832003.7V. Dediu, L. E. Hueso, I. Bergenti, and C. Taliani, Nature Mater. 8, 7072009.8Z. H. Xiong, D. Wu, Z. V. Vardeny, and J. Shi, Nature London 427, 8212004.9B. F. Ding, Y. Q. Zhan, Z. Y. Sun, X. M. Ding, X. Y. Hou, Y. Z. Wu, I.Bergenti, and V. Dediu, Appl. Phys. Lett. 93, 183307 2008.10G. Li, J. Shinar, and G. E. Jabbour, Phys. Rev. B 71, 235211 2005.11Z. H. Xu, B. Hu, and J. Howe, J. Appl. Phys. 103, 043909 2008.12B. Hu and Y. Wu, Nature Mater. 6, 985 2007.13K. Maeda, K. B. Henbest, F. Cintolesi, I. Kuprov, C. T. Rodgers, P. A.Liddell, Devens Gust, C. R. Timmel, and P. J. Hore, Nature London 453,387 2008.14P. Desai, P. Shakya, T. Kreouzis, W. P. Gillin, N. A. Morley, and M. R. J.Gibbs, Phys. Rev. B 75, 094423 2007.15S. W. Liu, J. H. Lee, C. C. Lee, C. T. Chen, and J. K. Wang, Appl. Phys.Lett. 91, 142106 2007.16C. H. Hsiao, Y. H. Chen, C. T. Lin, C. C. Hsia, and J. H. Lee, Appl. Phys.Lett. 89, 163511 2006.17A. Auerbach, Interacting Electrons and Quantum Magnetism Springer,New York, 1998.18D. R. McCamey, H. A. Seipel, S.-Y. Paik, M. J. Walter, N. J. Borys, J. M.Lupton, and C. Boehme, Nature Mater. 7, 723 2008.19A. J. Drew, F. L. Pratt, J. Hoppler, L. Schulz, V. Malik-Kumar, N. A.Morley, P. Desai, P. Shakya, T. Kreouzis, W. P. Gillin, K. W. Kim, A.Dubroka, and R. Scheuermann, Phys. Rev. Lett. 100, 116601 2008.FIG. 3. The theoretical electron hopping rate of the OLEDs with NPB:Alq3mixed EML.163302-3 Ding et al. Appl. Phys. Lett. 97, 163302 2010Downloaded 07 Mar 2012 to 147.8.21.59. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

http://hub.hku.hk/bitstream/10722/135129/1/Content.pdf

Magnetic field effects on the electroluminescence of organic light emitting devices: A tool to indicate the carrier mobility

Abstract

Similar works

Full text

Available Versions

Crossref

HKU Scholars Hub