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Phosphorescent organic light emitting diode efficiency enhancement using functionalized silver nanoparticlesFeng Liu and Jean-Michel NunziCitation: Appl. Phys. Lett. 99, 123302 (2011); doi: 10.1063/1.3640892View online: /10.1063/1.3640892View Table of Contents: /resource/1/APPLAB/v99/i12Published by the American Institute of Physics.Related ArticlesIncreased efficiency of light-emitting diodes incorporating anodes functionalized with fluorinated azobenzene monolayers and a green-emitting polyfluorene derivativeAPL: Org. Electron. Photonics 5, 230 (2012)Increased efficiency of light-emitting diodes incorporating anodes functionalized with fluorinated azobenzene monolayers and a green-emitting polyfluorene derivativeAppl. Phys. Lett. 101, 153306 (2012)An earth-isolated optically coupled wideband high voltage probe powered by ambient lightRev. Sci. 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Redistribution subject to AIP license or copyright; see /about/rights_and_permissionsPhosphorescent organic light emitting diode efficiency enhancement using functionalized silver nanoparticlesFeng Liu1and Jean-Michel Nunzi1,2,a)1Department of Chemistry,Queen’s University,90Bader Lane,Kingston,Ontario K7L3N6,Canada2Department of Physics,Queen’s University,90Bader Lane,Kingston Ontario K7L3N6,Canada(Received22June2011;accepted27August2011;published online19September2011)Silica coated silver nanoparticles(NPs)were incorporated into the emissive layer of aphosphorescent organic light emitting diode(PHOLED).As a result,the luminescence efficiencyof the PHOLED was significantly improved under low charge carrier injection level.The silverNPs facilitate the exciton formation event and are responsible for the PHOLED performanceimprovement.In contrast,the incorporation of uncoated bare silver NPs greatly suppressesluminescence of the PHOLED due to metal NPs induced luminescence and exciton quenching.Thesilica shell coated on Ag NPs surface plays a dual role in enhancing the PHOLED luminescenceefficiency:as an optical spacer,it avoids luminescence quenching of the dye caused by Ag NPs;asan insulator,it prevents exciton recombination at the Ag NPs surface.Therefore,a silica shell withthickness13nm or above is essential to enhance the PHOLED performance.V C2011AmericanInstitute of Physics.[doi:10.1063/1.3640892]Phosphorescent organic light emitting diode(PHOLED) is a type of organic light emitting diode(OLED)with high luminescence efficiency in which the electroluminescence (EL)originates from the phosphorescence of an organome-tallic complex.The transition between triplet and singlet states is efficiently allowed owing to the spin-orbit coupling promoted by the metal orbitals.1The recombination of triplet excitons in principle makes the internal quantum efficiency of PHOLED up to four-times larger than that offluorescent OLED.2PHOLED was thus attracting significant research and technological development in the past decade.Diverse methods so far were developed to optimize PHOLED performance:the addition of an electron transport material in the emissive layer greatly balanced the hole/elec-tron ratio and thus improved PHOLED efficiency;3the use of an additional hole blocking layer also improved PHOLED performance;4a metal cathode with lower work function could also favor higher performance.5As an alternative method,the surface plasmon resonance of noble metal nano-particles(NPs)was recently explored for the enhancement of OLED performance.6,7Although the application of surface plasmons to surface enhanced Raman spectroscopy(SERS), non-linear optics and metal enhancedfluorescence(MEF) were well recognized,8–10there is few research dedicated to surface plasmon enhanced electroluminescence.In this work,we report on the optimization of the per-formance of a PHOLED via metal NPs surface plasmon. Silica coated silver NPs were inserted into the emissive layer of PHOLED,silica shell thickness dependent PHOLED lu-minescence efficiency improvement was observed.The role of the silica coating layer is interpreted and the underlying enhancement mechanism is revealed.Silver NPs were obtained by the traditional sodium ci-trate reduction method,11large particles were removed by centrifugation at500rpm.Silver NPs with different silica shell thickness were prepared in the way reported by Liu et al.:1250mL silver NPs aqueous solution was mixed with 200mL ethanol and p H of the solution was adjusted to10by adding6.5ml30wt.%ammonia.1mL tetraethoxysilane (TEOS)ethanol solution(10mM)was subsequently added to the silver NPs solution every30min.In total,X mL (X¼2,5,10,and30)TEOS ethanol solution was added to obtain different silica layer thickness.The Ag-SiO2core-shell NPs and bare silver NPs obtained from50mL initial silver NPs aqueous solution werefinally dispersed into8mL chrolobenzene.PHOLED was fabricated with the structure shown in Fig.1.PEDOT:PSS was spin coated on pre-cleaned ITO glass and baked in a vacuum oven at110 C for30min.The emissive layer consisting of68.4wt.%polyvinylcarbazole (PVK),29.4wt.%(2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4–oxadiazole(PBD),and2.2wt.%tris(2-phenylpyri-dine)iridium(Ir(ppy)3)in chlorobenzene was subsequently spin coated on top of the PEDOT:PSS.6nm Bathocuproine (BCP)and30nm8-Hydroxyquinoline Aluminum(Alq3) were subsequently evaporated,followed by200nm alumi-num under10À6mbar vacuum.Bare Ag NPs or silica coated Ag NPs were incorporated into the OLED devices bymixing FIG.1.(Color online)Configuration of PHOLED doped with Ag-SiO2 core-shell NPs.a)Author to whom correspondence should be addressed.Electronic mail:nunzijm@queensu.ca.0003-6951/2011/99(12)/123302/3/$30.00V C2011American Institute of Physics99,123302-1APPLIED PHYSICS LETTERS99,123302(2011)400l L Ag-SiO 2NPs or bare Ag NPs chlorobenzene suspen-sion with 400l L emissive layer solution.The blend was spin coated on top of the PEDOT layer after 10-min ultra-sonication.An Hitachi-7000operated at 75kV was used for trans-mission electron microscopy (TEM),a Keithley (4200-SCS)for recording the current-voltage characteristics,EL intensity was measured using a calibrated silicon photodiode (S2281series,Hamamatsu Photonics).Fig.2reveals the microstructure of silica coated silver NPs from the dark field TEM images of Ag NPs grown in so-lution with different TEOS concentration.We see two differ-ent contrast regions in all TEM images,the bright white one assigned to the Ag NPs core with its heavy atomic weight,and the dim halo assigned to the SiO 2shell.It shows that shell thickness was readily tuned by adjusting the TEOS amount for polycondensation.We obtained tunable silica shell thicknesses of 7nm,13nm,27nm,and 60nm,respec-tively.We recorded the UV-Vis spectra of silica coated Ag NPs,the surface plasmon resonance peak red shifts on cover-ing the Ag NPs with the silica shell,owing to the refractive index change of the surrounding medium,in accordance with surface plasmon theory.13That further confirms forma-tion of the Ag-SiO 2core-shell nanostructure.The current density-voltage characteristics of PHOLEDs incorporating Ag-SiO 2NPs and bare Ag NPs are given in Fig.3.The current density of the PHOLED with bare silver NPs as dopant in the emissive layer is remarkably increased and the turn-on voltage of the device is thus lowered signifi-cantly,which indicates that bare silver NP doping improves the charge transport characteristics of the emissive layer.In the present PHOLED configuration,holes were reported to be trapped by Ir(ppy)3favoring exciton recombination on the phosphorescent dye,14as was reported earlier in dye doped PVK based OLEDs.15A minority electron current is believed to be the limiting step for effective exciton formation and recombination,which is responsible for the high driving voltage in the PVK-Ir(ppy)3PHOLED.5Therefore,an elec-tron transport molecule such as PBD is added into theemissive layer to increase the electron current and device performance.In our case,it is more likely that bare silver NPs increase the electron conductivity in the emissive layer due to the settling of an additional conductive pathway through silver island bridges.In contrast,the current density of PHOLEDs incorporating Ag-SiO 2NPs does not show obvious difference from that of the reference diode.This is ascribed to the insulating nature of the silica shell which does not offer a shortcut for electrons to cross the emissive layer and does not improve the conductivity.Fig.4(a)shows the luminescence efficiency of PHO-LEDs doped with silver NPs of different silica shell thick-ness.The luminescence efficiency of bare silver NP doped PHOLEDs is greatly suppressed compared to the reference diode.According to the J-V characteristic however,bare Ag NPs improve electron conductivity and thus the hole/electron balance,which in principle could have improved the lumi-nescence efficiency.This experimental contradiction wit-nesses the competition with other mechanisms.It was reported that bare silver NPs can cause charge carrier recom-bination at their surface,16which leads to exciton quenching and the inhibition of device performances.Moreover,according to the MEF theory,17bare Ag NPs may also quench Ir(ppy)3luminescence when the phosphorescent dye is in the vicinity of the Ag NPs.In consequence,we see that the quenching effect from bare Ag NPs dominates although bare NPs effectively lowered the PHOLED driving voltage.According to previous work on metal enhanced fluores-cence,an optical spacer is critical to avoid luminescence quenching.18Therefore,as shown in Fig.4(a),the lumines-cence efficiency of PHOLEDs with Ag-SiO 2NPs exhibits re-markable improvement in the low current density region.In the high current density region,the maximum luminescence efficiency of all the PHOLEDs becomes identical,except for the 7nm silica shell one.Since the quantum efficiency of Ir(ppy)3is close to 100%,19the limited effect of Ag-SiO 2NPs on improving the maximum luminescence efficiency of Ir(ppy)3PHOLED is not a surprise.The reduced maximum luminescence efficiency obtained by using Ag-SiO 2NPs with 7nm shell thickness suggests that surfaceplasmonFIG.2.TEM images of Ag-SiO 2core-shell NPs with different shell thick-ness,(a)2mL,(b)5mL,(c)10mL,and (d)30mL TEOS (10mM)are added for shellgrowth.FIG.3.(Color online)J-V characteristics of PHOLEDs doped with Ag-SiO 2NPs of different silica shell thickness.Particle concentration was kept the same.quenched luminescence is still present and a 7nm silica shell is an insufficient spacer to avoid luminescence quenching,in agreement with literature reports.17As the shell thickness is increased to 13nm or above,no suppression on the maxi-mum luminescence efficiency is observed.Meanwhile,exci-ton recombination on the surface of silica coated Ag NPs is also eliminated by the insulating silica coating layer.Luminescence efficiency is enhanced by NPs doping in the low current density region,we attribute it to enhanced exciton formation probability by the metal particles.In the low field regime,holes are trapped by the dye molecules,Error:Reference source not found the electron density is low and so is the recombination probability which is proportional to the product of electron and hole densities in the Langevin theory.However,local electric field at Ag NPs may enhance the cross section for electron-hole capture and in conse-quence,the recombination probability in the Langevin model.Our finding is that Ag-SiO 2NPs with 13nm shell offer maximum enhancement to the PHOLED luminescence efficiency,which corresponds to an optimized tradeoff between charge recombination and exciton quenching.Whereas in the high field regime (the high current density region),the exciton formation probability saturates even in the absence of NPs due to the high electron density in the emissive layer.In that case,NPs cannot improve PHOLED efficiency anymore.Fig.4(b)presents the variation of PHOLED lumines-cence efficiency with brightness.Although PHOLED lumi-nescence efficiency is not enhanced under high brightness >30000cd/m 2,we find that the PHOLED luminescence effi-ciency is improved by a factor of 3at a moderate brightness of 200cd/m 2which is a typical display industry standard brightness,using Ag-SiO 2NPs with 13nm shell.As known to us,the Joule heating inside OLEDs is part of the device degradation factors due to facilitated crystallization of amor-phous organic materials.Therefore,the improvement of PHOLED luminescence efficiency by incorporating Ag-SiO 2NPs will not only cut down power consumption but also reduce unwanted joule heating in the device,prolonging de-vice working lifespan,which is important for practical dis-play applications.As a conclusion,silica coated Ag NPs were synthesized and used into PHOLED successfully.Introducing bare silver NPs into PHOLED reduces device performance due to unwanted exciton recombination at NPs as well as metal induced luminescence quenching.Ag-SiO 2core-shell struc-ture,however,improves the luminescence efficiency of PHOLED.The silica shell serves as a spacer to avoid lumi-nescence quenching and an insulator to shunt charge recom-bination.A 13nm or above shell thickness is needed to avoid luminescence quenching.Silver NPs notably enhance the PHOLED luminescence efficiency under low current injection level by favoring exciton formation;however that is not valid anymore under high current density since exciton formation probability saturates.Research was supported by the Natural Sciences and Engineering Research Council,the Canada Research Chair Program on Chiral Photonics and Queen’s University.1R.C.Evans,P.Douglas,and C.J.Winscom,Coord.Chem.Rev.250,2093(2006).2H.Yersin,Top.Curr.Chem.241,1(2004).3F.C.Chen,Y.Yang,M.E.Thompson,and J.Kido,Appl.Phys.Lett.80,2308(2002).4A.Nakamura,T.Tada,M.Mizukami,and S.Yagyu,Appl.Phys.Lett.84,130(2004).5X.Gong,M.Robinson,J.C.Ostrowski,D.Moses,G.C.Bazan,and A.J.Heeger,Adv.Mater.14,581(2002).6F.Liu,B.S.Rao,and J.M.Nunzi,Org.Electron.12,1279(2011).7A.Fujiki,T.Uemura,N.Zettsu,M.Akai-Kasaya,A.Saito,and Y.Kuwa-hara,Appl.Phys.Lett.96,043307(2010).8D.S.Wang and M.Kerker,Phys.Rev.B 24,1777(1981).9J.M.Nunzi and D.Ricard,Appl.Phys.B 35,209(1984).10S.Mackowski,S.Wormke,A.J.Maier,T.H.P.Brotosudarmo,H.Ho ¨aru-tyunyan,A.Hartschuh,orov,H.Scheer,and C.Bra ¨uchle,Nano.Lett.8,558(2008).11P.C.Lee and D.J.Meisel,Phys.Chem.86,3391(1982).12S.Liu,Z.Zhang,and M.Han,Anal.Chem.77,2595(2005).13W.A.Murray and W.L.Barnes,Adv.Mater.19,3771(2007).14K.Mu ¨llen and U.Scherf,Organic Light-Emitting Devices (Wiley,Wein-heim,2006).15E.Gautier-Thianche,C.Sentein,A.Lorin,C.Denis,P.Raimond,and J.M.Nunzi,J.Appl.Phys.83,4236(1998).16S.Pan and L.J.Rothberg,Proc.SPIE 6641,664109(2007).17kowicz,Y.Shen,S.D’Auria,J.Malicka,J.Fang,Z.Gryczynski,and I.Gryczynski,Anal.Biochem.301,261(2002).18F.Liu,G.Aldea,and J.M.Nunzi,J.Lumin.130,56(2010).19K.Goushi,Y.Kawamura,H.Sasabe,and C.Adachi,Jpn.J.Appl.Phys.43,L937(2004).FIG.4.(Color online)Luminescence efficiency vs.current density (a)and brightness (b)of PHOLEDs doped with Ag-SiO 2NPs of different silica shell thickness.Particle concentration was kept the same.。