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Synthetic Metals 160 (2010) 756–761

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

hite electroluminescence from stacked organic light emitting diode

riyanka Tyagi, Ritu Srivastava ∗, Arunandan Kumar, Virendra Kumar Rai1, Rakhi Grover, M.N. Kamalasanan

enter for Organic Electronics, Polymeric and Soft Materials Section, National Physical Laboratory (Council of Scientific and Industrial Research), Dr. K.S. Krishnan Road,ew Delhi 110012, India

r t i c l e i n f o

rticle history:eceived 13 September 2009eceived in revised form7 December 2009

a b s t r a c t

Electroluminescent zinc complex [(2-(2-hydroxyphenyl)benzoxazole)(2-methyl-8-hydoxyquinoline)]zinc [Zn(hpb)mq] has been synthesized. It has been used with a blue emitting zinc complex Zn(hpb)2, tofabricate stacked organic light emitting diode (OLED). Thickness of layers of these materials has been opti-mized to achieve white light emission. The maximum luminescence of the device was 8390 Cd/m2 at 14 V.

ccepted 14 January 2010vailable online 18 February 2010

eywords:lectroluminescenceOLED

Commission Internationale de l’Eclairage coordinates of the device, with 40 nm thickness of Zn(hpb)2 and15 nm thickness of Zn(hpb)mq, were (0.29, 0.38) at 7 V and were well within the white region. A model hasbeen presented for simulating the electroluminescence (EL) spectrum of stacked OLED based on Gaussiandisorder model using Monte-Carlo technique. From the model the energetic disorders, highest ocuppiedmolecular orbital and lowest unoccupied molecular orbital positions of different emissive layers and the

estim

onte-Carlo simulationaussian DOS

excimer layer have been

. Introduction

White organic light emitting diodes (WOLEDs) have been widelynvestigated due to their potential application in making full colorLEDs to be used as next generation light source for general light-

ng [1–3]. There are reports on white OLEDs using single emissiveayers and stacked emissive layers. However for the conventionalingle unit OLED the peak efficiency is achieved at low current den-ity and hence low brightness remains a drawback for solid stateighting. In contrast stacked OLED with N emissive units is showing

times luminous efficiency as compared to that of conventionalingle unit device [4–7]. In this type of devices it is easy to achieveigh efficiency at high luminance because the driving current den-ity is N times smaller than that of the single unit device for the sameuminance. This feature makes stacked OLED highly attractive forolid state lighting application.

Zinc complexes are very useful because of their good color

unability properties, high photoluminescence quantum efficiencynd good thermal stability. Further these zinc complexes areasy to synthesize and have broad spectral features. Zinc com-lexes having wide EL spectrum have been previously used to

∗ Corresponding author. Tel.: +91 11 45608596.E-mail addresses: ritu@mail.nplindia.ernet.in, ritu@mail.nplindia.org

R. Srivastava).1 Present address: Organometallic Chemistry Laboratory, RIKEN [The Institute of

hysical and Chemical Research], 2-1, Hirosawa, Wako-shi, Saitama-ken 351-0198,apan.

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.01.016

ated.© 2010 Elsevier B.V. All rights reserved.

fabricate WOLEDs [7]. We have synthesized a zinc complex [(2-(2-hydroxyphenyl)benzoxazole)(2-methyl-8-hydoxyquinoline)] zinc[Zn(hpb)mq] having a wide EL spectrum with a peak in yellow-ish region. We have used stacked layers of two zinc complexesZn(hpb)mq and Zn(hpb)2 for the fabrication of WOLED.

There is a lack of theoretical explanation in terms of electro-luminescence in OLEDs. Monte-Carlo simulation technique [8–14]is a powerful technique to model carrier transport in organicsemiconductor. We have developed a model based on Monte-Carlo simulation technique to model the disordered semiconductor(assuming Gaussian density of states) to generate the electrolu-minescence spectrum of stacked OLED (SOLED) for white lightemission. This model has been used to explain the behavior (disor-der and energy levels of the two materials) of stacked layers usedin WOLED.

2. Experimental

2.1. Material synthesis

The synthesis scheme for Zn(hpb)2 and Zn(hpb)mq is shownin Fig. 1. Zn(hpb)2 was synthesized by the reaction of zincacetate and hydroxyphenyl benzoxazole by the method reported

elsewhere [16–18]. Zn(hpb)mq was synthesized in a 100 ml round-bottomed flask, 2-(2-hydroxyphenyl) benzoxazole (HPB) (0.844 g)(Sigma–Aldrich) was dissolved in 40 ml of anhydrous ethanol at70 ◦C under a nitrogen atmosphere. The solution was stirred for 2 h,after which 2-methyl-8-hydoxyquinoline(mq) (0.636 g) was dis-

P. Tyagi et al. / Synthetic Metals 160 (2010) 756–761 757

F droxyh tate.

ssdmolose(t

2

gtdhdfwuTq((e(2((a

ig. 1. Chemical reaction for the formation of Scheme 1: (1) Zn(hpb)2 from 2-(2-hyydroxyphenyl) benzoxazole (HPB), 2-methyl-8-hydoxyquinoline(mq) and zinc ace

olved in 40 ml of anhydrous ethanol and added drop wise whiletirring continued for 2 h, after which solution of zinc acetate dihy-rate (0.876 g) in water (6 ml) was added drop wise to the reactionixture while stirring continued. After 2 h of stirring a precipitate

f the complex was separated, which was filtered and recrystal-ized from a mixture of acetone and ethanol and dried in a vacuumven. The synthesized material was further purified by vacuumublimation and characterized using different techniques (reportedlsewhere [15]). Photoluminescence was studied using a FluorologJobin Yvon – Horiba, model-3-11) spectrofluorometer at roomemperature.

.2. Device fabrication

The OLEDs were fabricated on indium–tin–oxide (ITO) coatedlass substrates having a sheet resistance of 20 �/� and ahickness of 120 nm which were patterned and cleaned usingeionised water, acetone, trichloroethylene and isopropyl alco-ol sequentially for 20 min each using an ultrasonic bath andried in vacuum oven. Prior to organic film deposition ITO sur-ace was treated with oxygen plasma for 5 min to increase itsork function. Organic layers were deposited onto glass substratesnder high vacuum (4 × 10−6 torr) at a deposition rate of 0.4 Å/s.hickness of the deposited layers were measured in situ by auartz crystal thickness monitor. The device structure was ITO120 nm)/�-NPD (30 nm)/EML (xnm)/BCP (6 nm)/Alq3 (28 nm)/LiF1 nm)/Al (150 nm). Five OLEDs were fabricated with differ-nt EML combinations Zn(hpb)2 (35 nm) (device A), Zn(hpb)mq

35 nm) (device B) and Zn(hpb)2 (40 nm)/Zn(hpb)mq (15, 17.5,0 nm) (devices C–E). Tris (8-hydroxyquinoline) aluminum (Alq3)Sigma–Aldrich) and N,N′-di-[(1-naphthalenyl)-N,N′-diphenyl]-1-1′-biphenyl)-4,4′-diamine (�-NPD) (Sigma–Aldrich) were useds the electron and hole transporting layers. 2,9 Dimethyl 4,7

phenyl) benzoxazole (HPB) and zinc acetate, Scheme 2: (2) Zn(hpb)mq from 2-(2-

diphenyl 1,10 phenanthrolene (BCP) (Sigma–Aldrich) which has ahigh ionization potential (6.5 eV) has been used as hole blockinglayer and lithium floride (LiF)/aluminum (Al) and ITO have beenused as cathode and anode respectively. The size of each pixel was5 mm × 5 mm. The spectrum has been measured with a high resolu-tion spectrometer (ocean optics HR-2000 CG UV-NIR). The currentdensity–voltage–luminescence (J–V–L) characteristics have beenmeasured with a luminance meter (LMT-1009) interfaced with akeithley 2400 programmable current–voltage digital source meter.All the measurements were carried out at room temperature underambient conditions.

2.3. Simulation technique

The electroluminescence (EL) spectrum in an OLED is generatedby the recombination of a positive charge carrier with a negativecharge carrier in the emitting layer. The emitted photon will haveenergy equal to the difference of energies of negative and positivecharge carriers. Inside the organic semiconductor HOMO and LUMOare distributed with a Gaussian density of states (DOS) [g(E)] witha variance � and centered around EHOMO and ELUMO,

g(E) = N√2��

exp

(− E2

�2

),

ELUMO and EHOMO are the energies of LUMO and HOMO with vari-ances �LUMO, �HOMO for which the DOS for energy is maximum.

In the Gaussian DOS charge carrier profile also has a Gaussian

shape. When a set of charge carriers move inside this Gaussian DOS,their Gaussian profile will be centered �2

LUMO/KT below to ELUMOfor electrons and �2

HOMO/KT above to EHOMO for holes. This is due torelaxation of carriers [8–10,12]. The relaxation time is very low ascompared to the recombination time as well as hoping time so that

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trum of Zn(hpb)2 (device A) shows a broad peak in bluish greenregion and the EL of Zn(hpb)mq (device B) shows a peak in yellow-ish green region. There is a significant overlap in the EL spectrumof Zn(hpb)2 and Zn(hpb)mq and hence combining their EL spec-trum covers almost the entire visible spectrum due to which their

58 P. Tyagi et al. / Syntheti

he carriers can be assumed to be moving in a Gaussian profile cen-ered at energy ELUMO − (�2

LUMO/KT) (for negative carriers). Due tohis reason, in organic semiconductors the wavelength correspond-ng to maximum luminescence generally corresponds to an energymaller than the energy difference of ELUMO and EHOMO, where theOS for energy is maximum, e.g. for Alq3 ELUMO − EHOMO = 2.7 eVandeak luminescence of EL comes at 520 nm (∼2.3 eV).

Movement of the charge carriers in organic semiconductorss analogous to the random walk in a rough energy landscape

ith superimposed long range coulomb potential. Study based ononte-Carlo simulation [8,10–13] has been conducted to model

harge carrier recombination inside the organic semiconductor. Inhe present study a test sample with lattice sites 70 × 70 × X (X ishickness of emissive layer divided by lattice constant a) has beenenerated by computer. Two sets of random energies (one for LUMOnd one for HOMO) taken from a Gaussian distribution of varianceLUMO and �HOMO has been assigned to the sites. To avoid the sur-

ace effects periodic boundary conditions were applied in x and yirections. To simulate electroluminescence 10,000 electrons and0,000 holes has been started from Z = 1a plane and Z = Xa plane andhey were allowed to hop under the action of an electric field F act-ng along the z-direction. Electrons are moving in the energy sitesentered around LUMO and holes allowed to move in the energyites centered around HOMO. Carrier is allowed to hop within aube consisting of 7 × 7 × 7 sites. The jump rate has been assumedf the Miller–Abraham form, i.e., the product of a prefactor, a waveunction overlap factor and a Boltzmann factor containing the dif-erence in site energies including field term and the energies ofOMO and LUMO. For jumps to lower energy states the Boltzmann

actor was set equal to 1:

ij = �0 exp

(−��Rij

a

){exp(−εj − εi

KT); εj > εi

1; εj < εi

ncorporating the electrostatic potential generated by a field in theite energies εi, and εj. Here �Rij = |Ri − Rj| and � is overlap factoretween two lattice sites which has been taken to be equal to 5/a.

The probability that a carrier jumps from a site i to any site jithin a cube consisting of 7 × 7 × 7 sites is

ij = �ij∑i /= j�ij

random number xR from a uniform distribution is chosen and thispecifies to which site the particle jumps because each site is givenlength in random number space according to Pij. The time for the

ump is determined from

ij = xei

⎡⎣∑

i /= l

�il

⎤⎦

−1

here xei is taken from another exponential distribution of randomumbers.

After every hop the carrier is allowed to relax. Hops of carriersre time dependent, i.e., if one carrier takes longer time for a jumphan the other, jumps for that carrier have not been made (whilehe other carrier is moving) until the time becomes the same foroth the carriers. The computation was terminated when a neg-tive carrier and a positive carrier reach very near to each otherless than 2 nm) where they recombine and a photon of the energyqual to the difference of the two energies is generated. By counting

he number of photons for different wavelengths, an electrolu-

inescence spectrum is generated for the device. This techniqueas been extended for two-layer structure. For two-layer struc-ure in the z-direction, Z1 layer of sites are assigned for the firstayer and (X–Z1) layer of sites for the second layer. For each layer,

ls 160 (2010) 756–761

two different sets of energy have been assigned from a Gaussiandistribution of variances (�1HOMO, �1LUMO, �2HOMO, �2LUMO) cen-tered around energies E1HOMO, E1LUMO, E2HOMO, E2LUMO respectively.Negative charge carriers are started from Z = 1a plane and positivecharge carriers are started from Z = Xa plane and by using the sametechnique suggested for the single layer, the EL spectrum can begenerated. Further to include the effect of other states, e.g. statesdue to excimer/exciplex formation or due to interface between twomaterials, a separate layer having a different DOS for energy hasbeen assumed. In the z-direction three layers having Z1, Z2, Z3 layerof sites respectively for first emissive layer, extra layer and secondemissive layer having different sets of three energy sites, assignedfrom a Gaussian distribution of variances (�1HOMO, �1LUMO, �2HOMO,�2LUMO, �3HOMO, �3LUMO) centered around energies E1HOMO, E1LUMO,E2HOMO, E2LUMO, E3HOMO, E3LUMO respectively have been taken andsimilar to the double layer case the EL spectrum for this structurehas also been generated.

3. Results and discussions

PL spectrum of Zn(hpb)2 and Zn(hpb)mq were measured withfilm deposited on quartz substrates and shown in Fig. 2. The PLspectrum of Zn(hpb)2 shows broad features with blue emission at478 nm and Zn(hpb)mq shows yellowish green emission at 522 nmat excitation wavelength of 368 nm. The PL spectrum of the twomaterials covers almost the entire visible spectrum from blue tored wavelengths and hence stacked layers of these materials hasbeen used to fabricate WOLED.

Fig. 3 shows the schematic energy level diagram of device struc-ture used in this study. HOMO and LUMO values of Zn(hpb)2 andZn(hpb)mq have been taken from the references [7,15]. Since theHOMO of Zn(hpb)2 lies below the HOMO of Zn(hpb)mq and LUMOof Zn(hpb)mq lies above the LUMO of Zn(hpb)2, holes can be eas-ily transferred from HOMO of Zn(hpb)2 to HOMO of Zn(hpb)mqand electrons from the LUMO of Zn(hpb)mq to LUMO of Zn(hpb)2.Therefore layer of Zn(hpb)mq has been placed adjacent to the layerof Zn(hpb)2 so that carrier transport has not been impeded by theinterface barrier if operated under forward bias (ITO as anode).

Fig. 4 shows the EL spectra of devices (A)–(E). The EL spec-

Fig. 2. Normalized photoluminescence spectrum of Zn(hpb)2 and Zn(hpb)mq.

P. Tyagi et al. / Synthetic Metals 160 (2010) 756–761 759

Fr

sdZadwv

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apats

FZ2

ig. 3. Schematic energy level diagram for the stacked layer structure used to fab-icate WOLED. The values are in eV.

tacked layer gives white light. In this way three stacked layerevices (devices C–E) have been fabricated using Zn(hpb)2 andn(hpb)mq. Thickness of Zn(hpb)2 has been kept constant (40 nm)nd the thickness of Zn(hpb)mq has been varied (15, 17.5, 20 nm) inevices C–E respectively to observe the color shift. In all the devicese have observed broad EL spectrum covering almost the entire

isible region.In the stacked layer holes are injected from ITO side and elec-

rons are injected from Al. Broad EL spectrum has been observedn all the three devices. As the thickness of Zn(hpb)mq layer isncreasing the dominant wavelength shifts from bluish region toellowish region. From the EL spectrum of device C, three peaksave been observed at wavelengths 450, 485 and 550 nm. The peakt 450 and 485 nm may be due to the recombination of electronsnd holes in Zn(hpb)2 layer and the peak at 550 nm may be dueo the recombination in Zn(hpb)mq layer. The peak at 485 nm isue to the excimer formation in Zn(hpb)2 [17,18]. Therefore the ELpectrum of stacked layer is due to the overlap of the EL spectrumf two layers: Zn(hpb)2 and Zn(hpb)mq.

The experimental EL spectrum for device B [pure Zn(hpb)mq]

nd theoretically simulated EL spectrum for this device with fittingarameters EHOMO = 5.45 eV and ELUMO = 2.7 eV and �HOMO = 90 meVnd �LUMO = 100 meV are shown in Fig. 5. This shows a good fit inhe theoretical and experimentally observed EL spectrum. A broadpectrum for device A [pure Zn(hpb)2] has been observed experi-

ig. 4. Electroluminescence spectrum of device A [pure Zn(hpb)2], device B [puren(hpb)mq], devices C–E [stacked layers of Zn(hpb)2 (40 nm) with 15, 17.5 and0 nm thick Zn(hpb)mq].

Fig. 5. Normalized experimental and simulated EL spectrum for device B [pureZn(hpb)mq].

mentally. This has been accounted by incorporating extra energystates in the form of excimer states. In the simulation, these extrastates have been assigned different values of HOMO/LUMO energiesand variance and the technique has been extended for two-layerstructures.

For two-layer structure, in the z-direction Z1 layer of sites wereassigned for the first layer and (X–Z1) layer of sites for the secondlayer. For both the layers two different sets of HOMO and LUMOenergies have been assigned from a Gaussian distribution of vari-ances (�1HOMO, �1LUMO, �2HOMO, �2LUMO) centered around energiesE1HOMO, E1LUMO, E2HOMO, E2LUMO respectively and these layers ofsites were placed randomly. Negative charge carriers were startedfrom Z = 1a plane and positive charge carriers are started from theZ = Xa plane and the EL spectrum was generated as discussed forthe single layer device.

The fit of simulated EL spectrum for two-layer structure and theexperimentally observed EL spectrum for device A is shown in Fig. 6.A good fit has been obtained for the following set of parameters:

E1HOMO = 6.4 eV, E1LUMO = 2.8 eV, E2HOMO = 6.0 eV, E2LUMO = 2.8 eV,�1HOMO = 110 meV, �1LUMO = 100 meV, �2HOMO = 115 meV,�2LUMO = 100 meV, Z1 = 20, Z2 = 15.

Fig. 6. Normalized experimental EL spectrum for device A [pure Zn(hpb)2] andsimulated EL spectrum assuming two-layer structure.

7 c Metals 160 (2010) 756–761

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60 P. Tyagi et al. / Syntheti

In stacked layer devices C–E, the thickness of Zn(hpb)mq (15,7.5, 20 nm) has been varied and the thickness of Zn(hpb)2 has beenept constant (40 nm). From the EL spectrum, it has been observedhat as the thickness of Zn(hpb)mq layer is increased, the peak dueo Zn(hpb)2 (at 450 nm) has been suppressed in comparison to peakn(hpb)mq layer. For 20 nm thickness of Zn(hpb)mq layer the peakue to Zn(hpb)mq has become much dominant as compared to theeak due to Zn(hpb)2. This may be due to the shifting of recombina-ion zone in the stacked layers, as observed by Vestweber et al. [19].hifting of recombination zone can be explained by the followingrguments: (1) organic semiconductors are generally p-type semi-onductors, (2) it is generally agreed that the majority carriers in theystem under study are holes, injected from ITO, of which a certainraction recombines with injected electrons, (3) injected electronsas been assumed to be the minority carriers inside the system,nd (4) mobility of holes inside the organic semiconductor is veryigh in comparison to the mobility of electrons. Therefore a uni-

orm sea of holes can be assumed in the stacked layers and the flowf electrons is deciding the recombination zone. As the thickness ofn(hpb)mq increases electrons are recombining in the Zn(hpb)mqayer and at the Zn(hpb)2/Zn(hpb)mq interface. For further valida-ion of these qualitative arguments, simulation technique has beensed.

The values of HOMO and LUMO, obtained from the simulationor device A [pure Zn(hpb)2] and device B [pure Zn(hpb)mq], arelmost matching with the real values. After applying on the singleayer device, Monte Carlo simulation technique has been used toimulate the EL spectrum of the devices C–E and simulated spectrare given in Fig. 7. For comparison the experimental EL spectrumf the devices is also shown in the same figure. The simulation haseen performed on a system composed of three layers of differentaussian DOS having different variances (�1HOMO, �1LUMO, �2HOMO,2LUMO, �3HOMO, �3LUMO) and centered around energies E1HOMO,1LUMO, E2HOMO, E2LUMO, E3HOMO, E3LUMO respectively for Zn(hpb)2nd Zn(hpb)mq. Here two energy states have been assigned ton(hpb)2 to account for the �–�* levels as well as excimer states.he fitting parameters are given in Table 1. For simplicity vari-nces of LUMO energy sites of three layers have been kept same100 meV).

In simulation, we could generate the EL spectrum of the stackedayer with various thicknesses of Zn(hpb)mq without varying thehickness of Zn(hpb)2 layer and the results were in agreement withhe experiments. The direct impact of this thickness variation haseen observed in EL spectrum. As the thickness of Zn(hpb)mq layeras been increased, the color dominance of Zn(hpb)mq layer is

ncreasing, which is supporting the shifting of recombination zone.n this way simulation can be used to optimize the thickness oftacked layer for the fabrication of white OLED using different mate-ials.

Fig. 8 shows the current density–voltage–luminescence (J–V–L)haracteristics of stacked layer devices with 40 nm thick Zn(hpb)2nd 15 nm thick Zn(hpb)mq. The J–V characteristics show nonlin-ar nature. The turn on voltage (corresponding to a brightness of

Cd/m2) is 5 V. The operating voltage for a brightness of 100 Cd/m2

as 8.5 V. The maximum brightness of the device was 8390 Cd/m2

t a current density of 518 mA/cm2. The onset voltages, CIE coor-inates, and maximum luminescence of devices are shown in the

Fig. 7. (a–c) Electroluminescence spectrum experimentally observed for stackedlayer devices C–E and EL spectrum generated from simulation.

able 1idth (in units of lattice constant) of two emissive layers and excimer states (Z1, Z2, Z3) and variances (�1HOMO, �1LUMO, �2HOMO, �2LUMO, �3HOMO, �3LUMO) and E1HOMO, E1LUMO,

2HOMO, E2LUMO, E3HOMO, E3LUMO for the three layers estimated from the simulation.

Z1 Z2 Z3 �1HOMO, �1LUMO �2HOMO, �2LUMO �3HOMO, �3LUMO E1HOMO, E2HOMO, E3HOMO (eV) E1LUMO, E2LUMO, E3LUMO (eV)

Device C 29 13 13 110 meV, 100 meV 115 meV, 100 meV 90 meV, 100 meV 6.4, 6.0, 5.45 2.8, 2.8, 2.7Device D 29 13 16 112 meV, 100 meV 120 meV, 100 meV 85 meV, 100 meV 6.4, 6.0, 5.45 2.8, 2.8, 2.7Device E 29 14 17 108 meV, 100 meV 118 meV, 100 meV 84 meV, 100 meV 6.4, 6.0, 5.45 2.8, 2.8, 2.7

P. Tyagi et al. / Synthetic Meta

Fig. 8. Current density–luminescence–voltage characteristics of device C [stackedlayer of Zn(hpb)2 (40 nm)/Zn(hpb)mq (15 nm)].

Table 2Onset voltage, maximum luminescence and CIE coordinates for devices C–E.

Onsetvoltage

Max. luminescence CIEx CIEy

twDw

4

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[[

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[Solid Films 515 (2007) 5075–5078.

Device C 5 V 8390 Cd/m2 (at 14 V) 0.29 (at 7 V) 0.38 (at 7 V)Device D 5 V 6909 Cd/m2 (at 12 V) 0.31 (at 7 V) 0.42 (at 7 V)Device E 5 V 7391 Cd/m2 (at 11 V) 0.31 (at 8 V) 0.45 (at 8 V)

abular form (Table 2). CIE coordinates of all the devices were wellithin the white region. All devices have low turn on voltage (5 V).evice C has been found to have the optimum CIE coordinates forhite light emission (0.29, 0.38).

. Conclusion

Zinc complex Zn(hpb)mq, showing an electroluminescenceeak at 550 nm, has been synthesized and used with another zincomplex Zn(hpb)2 (peaks at 450 and 485 nm) to fabricate stackedLED for white light emission. The maximum luminescence of390 Cd/m2 has been achieved at 14 V with 40 nm thickness of

[

[[

ls 160 (2010) 756–761 761

Zn(hpb)2 and 15 nm thickness of Zn(hpb)mq. Commission Interna-tionale de l’Eclairage (CIE) coordinates of the device are (0.29, 0.38)at 7 V and are well within the white region. Further a model hasbeen presented for simulating white light emission from stackedOLED based on Monte-Carlo simulation technique from which theenergetic disorders and HOMO, LUMO positions of different emis-sive layer and excimer states have been estimated.

Acknowledgements

Authors are thankful to Director, National Physical Laboratory,New Delhi for continuous encouragement. The authors also wouldlike to thank Dr. S.S. Bawa and Dr. S.K. Dhawan for suggestionsand discussion. The authors also gratefully recognize the finan-cial support from the Department of Science and Technology (DST)and Council of Scientific and Industrial Research (CSIR), New Delhi,India.

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