Localized Surface Plasmon Enhanced Organic Light-EmittingDiodes

Arunesh Kumar Yadav

Received: 22 January 2014 /Accepted: 31 March 2014# Springer Science+Business Media New York 2014

Abstract Current work demonstrates enhanced efficienciesin organic light-emitting diodes by using the localized surfaceplasmons originated from Au nanoclusters deposited usingthermal evaporation technique. The effect of localized surfaceplasmons on organic emitter was studied using UV–visabsorption spectra, steady state and time-resolvedphotoluminescence spectra. These studies have revealed thatthe optical properties like absorption, emission have beengreatly modified by the localized surface plasmon. Theseeffects were found to be dependent on the distance betweenthe emitter and Au layer. Further, efficiencies of the OLEDswere also found to be dependent on this distance.

Keywords Localized surface plasmon . Organiclight-emitting diode

Introduction

Organic light-emitting diodes (OLEDs) have emerged as apotential candidate for future lighting and displays [1–5]. In anOLED, when electron and hole meet each other in the emis-sive layer, they recombine to form a bound charge pair calledexciton. Device efficiency depends on the efficiency of radi-ative recombination after the formation of exciton. It is theratio of radiative recombination of excitons to the total numberof excitons formed in the emissive layer. This efficiency isdependent on two factors, the first factor is (i) ηST, which is theratio of singlet and triplet excitons contributing towards theradiative recombination and the second term is (ii) fluores-cence efficiency (ηPL) which is defined as the ration of radi-ative decay efficiency and the total decay efficiency.

Fluorescence efficiency or PL quantum yield depends uponthe radiative emission rate, which can be altered by changingthe excited state life time. If the excited state life time is higherfor a luminescent chromophore, there is a higher probability ofnonradiative decay. Therefore, the lower the excited state lifetime, the lower will be the probability of nonradiative decay[6]. Generally, the emissive organic molecules are divided intotwo subcategories fluorescent and phosphorescent emissivematerials [7, 8]. Phosphorescent emissive materials are pre-ferred over the fluorescent because in phosphorescent mate-rials, both triplet and singlet excitons can be exploited foremission while in case of fluorescent, only singlet excitonscan emit [9, 10]. However, phosphorescent emitters have theirexcited state life time quite long in comparison to the fluores-cent emitters. The excited state life time for fluorescent emit-ters is of the order of 0.1 nsec to 0.1 μsec, while for phospho-rescent emitters it is of the order of 1 μsec to 1 msec [9, 11].This is a disadvantage in case of phosphorescent emitters.

Recently, the coupling between excitons and SPs has beenactively studied for radiative emission rate enhancement insemiconducting devices [12–15]. SPs are the collective oscil-lations of free electrons in a metal at the interfaces between themetal and dielectric [16–18]. They are propagating waves atmetal/dielectric interface and coupling of light to SPs providea major loss channel. However, the energy confined in the SPmodes can be extracted as radiation through naturally formedsurface imperfections, such as nanostructures [19–21]. Theoverlap of local electromagnetic field of the excitons in theemissive layer and SPs results in the coupling effect, due towhich, effective energy transfer takes place between them,creating an alternate channel for emission [22–24]. Since thescattering of high momentum localized surface plasmons(LSPs) is much faster than the decay of excitons, couplingresults in the enhancement of radiation intensity [24].

In an exciton-SP system, there are two competitive pro-cesses: (i) radiation intensity enhancement due to LSPs and

A. K. Yadav (*)Physics Department, ITS Engineering College, Greater Noida, Indiae-mail: arunesh.yadav@gmail.com

PlasmonicsDOI 10.1007/s11468-014-9716-1

(ii) nonradiative losses due to metal. The efficiency of theinteraction between LSPs and excitons exponentially de-creases with increasing distance from the emissive layer tothe metal surface [25]. At the same time, nonradiativequenching of exciton at metal surface occurs when the dis-tance is very much smaller. Therefore, for practical applica-tions, an appropriate distance between the emissive materialand metal surface should be maintained to obtain radiationintensity enhancement. Additionally, the emission wavelengthof excitons should match with the absorption wavelength ofLSPs, which depends upon the size of nanostructures [26, 27].Therefore, the size of nanostructures must also be optimizedfor emission enhancement. In this work, we have utilized goldnanostructures to reduce the excited state life time offluorescent emitter.

Experimental

Au nanoclusters (NCs) were fabricated by thermal evapora-tion at a base pressure of 7×10−6 torr. OLEDs were fabricatedon indium tin oxide (ITO)-coated glass substrates having asheet resistance of 30 Ω/□. These substrates were patterned toremove the ITO other than from desired anode area of 3 mm×25 mm and cleaned before the fabrication of the device byusing deionised water, acetone, and isopropyl alcohol sequen-tially for 10 min each using an ultrasonic bath and residuals ofsolvents were removed in vacuum oven. Organic layers weredeposited onto these substrates under high vacuum (7×10−6 torr) at a deposition rate of 5 nm/min. The device struc-ture was ITO/TPD (60 nm)/Alq3 (40 nm)/LiF (0.5 nm)/Al(150 nm). Tris (8-hydroxyquinoline) aluminum (Alq3) (SigmaAldrich) and N,N,N′,N′-tetraphenyl benzidine (TPD) (SigmaAldrich) were used as the emissive and hole transportinglayers. Lithium fluoride (LiF)/aluminum (Al) and ITO hasbeen used as cathode and anode, respectively. The thicknessesused in these studies were the optimum thicknesses for max-imum efficiency. These thicknesses were obtained by varyingthe thickness of each layer (TPD and Alq3) in steps of 10 nmand bymeasuring the luminous efficiency of each device. Thiscareful variation of device structure has led to the optimumthicknesses for highest efficiency. The active area of eachdevice was 3 mm×3 mm.

UV–vis absorption spectrum was recorded using a home-made UV–vis spectrophotometer. The measurement setupconsists of a white light source (Xenon lamp), a mono-chromator, and a silicon photo detector. The mono-chromator was used to see the wavelength dependence ofabsorbed intensity. Photoluminescence spectra were studiedusing a LED of 370 nmwavelength and a spectrometer. Time-resolved PL spectrum has been measured using a 370-nmwavelength LED having pulse time of 300 psec. Hamamatsufluorescence system Quantaurus-Tau C11367-11 equipped

with LED and spectrometer was used for these measurements.The current density-voltage-luminescence (J-V-L) characteris-tics have been measured with a luminance meter (LMT-1009)interfaced with a keithley 2400 programmable current–volt-age digital source meter. All the measurements were carriedout at room temperature under ambient conditions.

Results and Discussion

Figure 1a shows the schematic device structure of the bilayerOLED with gold nanoclusters (NCs) proposed in this study,where the gold nanoclusters were inserted in between the holetransport layer. Gold nanoclusters were deposited by thermalevaporation at a base pressure of 7×10−6 torr with a depositionrate 0.2 nm/s. Figure 1b shows the SEM image of the Au NCsdeposited on a glass substrate. All the organic layers weredeposited at a rate of 5 nm/min and at a base pressure of 7×10−6 torr. Figure 2 shows the absorption spectrum of theemissive material Alq3 without and with Au NCs depositedon glass substrate. It can be seen from the figure that thesample consisting of AuNCs have enhancement of absorptionof Alq3, typically expected in case of localized surface plas-mon (LSP) effect. The figure demonstrates that the opticalproperties of Alq3 were modified efficiently by the presence ofAu NCs.

Glass

ITO

TPD

Alq3

LiF/Al

Au NCs

(a)

(b)Fig. 1 a Device structure of OLEDs. b SEM image of the Au NC layer

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Emission intensity can be enhanced by surface plasmoncoupling with exciton emission. For the emission intensityenhancement through the surface plasmon coupling, the dis-tance between the emissive layer and Au NCs is very impor-tant. In order to study the effect of distance on emissionintensity enhancement, we have prepared samples with AuNCs separated by a distance d from Alq3. We are planning toincorporate these NCs inside the OLED; therefore, we haveused TPD as the separator between the NCs and Alq3. Thethickness of TPDwas varied from 0–20 nm in steps of 2.5 nm.Figure 3 shows the plots of the intensity enhancement factoras a function of separation distance between Au NCs andemissive layer. It is evident from the figure that the emissionintensity has enhanced for the distances from 5–15 nm due toefficient exciton–LSP coupling. We have also measured thetime-resolved PL spectrum for all these samples and calculat-ed the excited state life time for each sample. The excited state

life time of the system without Au NCs was 15 ns. Figure 4shows the plot of excited state life time as a function of d.Here, we can observe that the excited state life time haschanged for the samples having the serration distances up to15 nm. Above this distance, the exciton–LSP coupling isweak and therefore, the effects cannot be observed. Thedecrease in excited state lifetime indicates towards an increasein the radiative rate of the emissive material.

Further, we have fabricated bilayer OLEDs with Au NCsincorporated between the hole transport layer at various posi-tions to study the effect of separation distance between theemissive material and Au NCs on the electroluminescence

0 5 10 15 20

0.0

0.4

0.8

1.2

1.6

Em

issi

on in

tens

ity e

nhan

cem

ent

Distance (nm)

Fig. 3 PL intensity enhancementfactor as a function of distancebetween the emitter and Au NCs

0 5 10 15 2010

11

12

13

14

15

16

Exc

ited

stat

e lif

e tim

e (n

s)

Distance (nm)

Fig. 4 Excited state life time of Alq3 as a function of distance betweenthe emitter and Au NCs

400 500 600 700

0.05

0.10

0.15

0.20

0.25

Abs

orpt

ion

(a.u

.)

Wavelength (nm)

Without Au NCs With Au NCs

Fig. 2 Absorption spectra of Alq3 without and with Au NCs

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intensity. We have fabricated four devices having separationdistances of 2.5, 5, 10, and 15 nm between emissive materialand Au NCs. Figure 5 shows the current density–lumines-cence (J-L) characteristics for the fabricated devices. Thefigure also shows the reference device without Au NCs. Itcan be seen that the device with 5 and 10 nm separationdistances have superior performance that the reference device.The device with 2.5-nm distances suffers from thenonradiative quenching by Au, while for the device with 15-nm distances, the distance is large enough for efficient LSPeffects. The current and power efficiencies have been summa-rized in Table 1. It is evident from the table that LSP effectleads to an almost 30 % enhancement in the efficiencies ofOLEDs.

Conclusions

In conclusion, we have demonstrated efficient bilayer OLEDsby using LSP effect originated by Au NCs deposited usingthermal evaporation technique. Absorption studies demon-strated that the optical properties of the emitting material were

modified by this LSP effect. Steady state and time-resolvedPL spectra were measured, and the effect of separation dis-tance was studied on the PL of emissive material. The emis-sion intensity was found to be increased for the sampleshaving separation distances between 5–15 nm. OLEDs werefabricated for utilizing the LSP effect, and the separationdistances of 5–10 nm were found to be optimum for theenhancement in the intensity of OLEDs.

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0 200 400 600 800 10000

1x103

2x103

3x103

4x103

5x103

Lum

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cenc

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Table 1 Current and power efficiencies of all the devices with Au NCs

Device Current efficiency (Cd/A) Power efficiency (lm/W)

Reference 2.3 1.5

2.5 nm 1.5 0.9

5 nm 3.1 1.8

10 nm 3.0 1.75

15 nm 2.4 1.55

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