Organic Electronics 13 (2012) 2959–2966

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Organic Electronics

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Effect of a critical percolation threshold in purified short carbonnanotube-polymer/ZnS:Cu,Cl composite on electroluminescence

Jin-Young Kim a, Hoonbae Kim a, Shang Hyeun Park b,⇑, Taewon Jeong b, Min Jong Bae b,Yong Churl Kim b, Intaek Han b, Donggeun Jung a, SeGi Yu c,⇑a Department of Physics, BK 21 Physics Research Division, Sungkyunkwan University, Suwon 440-746, Republic of Koreab Electronic Materials Lab., SAIT Samsung Electronics, Yongin 446-712, Republic of Koreac Department of Physics, Hankuk University of Foreign Studies, Yongin 449-791, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 March 2012Received in revised form 14 June 2012Accepted 11 August 2012Available online 5 September 2012

Keywords:Carbon nanotubePercolation thresholdField enhancementElectroluminescence

1566-1199/$ - see front matter � 2012 Elsevier B.Vhttp://dx.doi.org/10.1016/j.orgel.2012.08.021

⇑ Corresponding authors. Tel.: +82 31 280 8167 (S4398 (S. Yu).

E-mail addresses: sh100.park@samsung.com (S.Hs.ac.kr (S. Yu).

AC driven inorganic electroluminescence (EL) of the carbon nanotube (CNT)-polymer/phos-phor composite was fabricated to investigate the effect of the critical percolation thresholdby CNT concentration on EL performance. In order to control the appropriate CNT conditionin EL device, CNTs were shortened by cryogenic crushing and purified by thermal treat-ment. Among various CNT concentrations in the composite film, the critical percolationthreshold can be found to be 0.0925 wt.% by fitting conductivity data of the composite film.Near the critical percolation threshold of a CNT concentration, the EL performances of thecomposite EL were greatly increased compared to the reference EL. The tunneling barrierthickness at the ZnS–CuxS contact could become thin to induce more charge carrier tunnel-ing into ZnS host lattice by the local field enhancement of CNTs, resulting in increased elec-tron–hole recombination to produce more light emission.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Over the past few decades, the electrically percolativepolymer composite with conducting fillers has been widelystudied, because their electrical properties exhibit the sig-nificant change at the critical percolation threshold of theconducting filler concentration. These percolative compos-ite is often applied to the dielectric film, the electroactivematerial, and solar cell due to the flexibility and the uniqueelectric property [1–3]. In the percolative polymer basedcomposite, nanomaterials, such as carbon blacks, metalparticles, carbon fibers, and carbon nanotubes (CNTs) werewidely used as conducting fillers [4–7]. Among theseconducting fillers, CNTs have been considered to be oneof excellent conducting fillers due to their unique

. All rights reserved.

.H. Park), +82 31 330

. Park), segiyu@huf-

characteristics, such as highly mechanical strength andchemically stable ability [8,9]. In addition, field emissioncharacteristics of CNTs have been investigated in manyareas, such as field emission displays and vacuum flatlamps [10,11].

Recently, considerable research effort has been pouredinto future potential flat panel displays. Among these can-didates, alternating current (AC) driven thick-film electro-luminescence (EL) devices have attracted interest due totheir simple fabrication process, low cost, and mass pro-duction [12–14]. Since AC EL produces the light emissionunder a high electric field, a dielectric layer is needed toprevent catastrophic dielectric breakdown in the device,where the EL structure is generally consisted of a top elec-trode, an emitting layer, a dielectric layer, and a bottomelectrode [15,16]. The practical applications of AC thick-film EL devices are currently confined to backlighting ofcellular phones or mood lighting, due mainly to the insuf-ficient luminance and efficiency of EL devices compared toother types of flat panel displays [13,14]. Lots of efforts

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have been poured into developments of the EL device, suchas improving the crystallinity of the phosphor, changingthe device structure, and adopting the organic dye[14,17,18]. These efforts are still insufficient to solve theabove problems. In the case of CNT application to EL usinga composite emitting layer, high performance of the EL de-vice would be expected by field enhancement of CNTs nearthe critical percolation threshold of CNT concentration. Theadoption of nanomaterials, which lead to recent intensiveresearch activities, may suggest the new direction to ob-tain large improvement of EL performance.

In this paper, an AC driven powder EL device with acomposite emitting layer composed of single-walled CNT(SWCNT)-polymer/ZnS:Cu,Cl was fabricated to investigatethe effect of a critical percolation threshold of SWCNTson electroluminescence. In order to utilize the field emis-sion effect of SWCNTs efficiently, SWCNTs were shortenedby the cryogenic crushing method and purified by thermaltreatment at the atmospheric environment. To find thepercolation threshold of SWCNTs, the conductivities ofthe composite emitting layer were measured by the con-centration of SWCNTs. The composite EL devices exhibitedhigh EL performance near the critical percolation thresholdof SWCNTs. Purified short SWCNTs in the composite emit-ting layer could enhance the electric field applied to thephosphor particles due to local field enhancement. Thishigh electric field could increase the charge carrier tunnel-ing into ZnS host lattice, resulting in increased electron–hole recombination.

2. Experimental

Fig. 1 shows the schematic structure and SEM image ofthe EL device using the SWCNT-polymer/phosphor com-posite, and the inset is the photo images of the light emis-sion for the composite EL device. In order to minimize theconducting networks originated from the percolation pathsby CNTs, SWCNTs (CNI Co., CE601B) were shortened by thecryogenic crushing method [19], where the length of theSWCNT was reduced from a few micrometer to submicron.Shortened SWCNTs were treated by thermal annealing un-der the atmosphere environment at 430 �C for 30 min toreduce defects of SWCNTs. In order to compare the EL per-

Fig. 1. The schematic structure and SEM image of the EL device using the SWCNTEL device.

formances of the composite EL device, three kinds of the ELdevices, i.e., devices with non-treated short SWCNTs, de-vices with thermally purified short SWCNTs, and deviceswithout SWCNTs, were fabricated.

Dielectric pastes consisted of barium titanate (BaTiO3)powder particles with the perovskite structure (SamsungFine Chemicals, SBT03) and cyanoethyl pullulan (Shin-EtsuChemicals, CR-S) as an organic binder, were prepared forall EL devices, where the weight ratio of the BaTiO3 toCR-S binder was varied from 1 to 1.5. In the case of thecomposite EL device with thermal-treated SWCNTs, theemitting paste was prepared as follows, as shown inFig. 2(a). Thermal-treated SWCNTs of varying SWCNT con-centration, i.e., 0.03, 0.06, 0.09, 0.13, and 0.16 wt.%, weredispersed in 10 g dimethyl formamide (DMF) organic sol-vent, containing 1% sodium dodecylbenzene sulfonate asa surfactant by a tip sonicator (Branson, Sonifier450) for2 h. 18 wt.% CR-S powder particles were dissolved in theSWCNT–DMF solution, which was stirred for 4 h. 70 wt.%ZnS:Cu,Cl phosphor powder particles with 505 nm of thegreen emission (Sylvania, GG44) were mixed with theCR-S/SWCNT binder, which was stirred for 1 day. For thecase of EL devices, i.e., devices with non-treated SWCNTsand devices without SWCNTs, the emitting pastes wereprepared at the same procedure and materials with theemitting paste of the thermal-treated SWCNTs compositeEL device. Three kinds of the EL devices were fabricatedas follows. 200-nm-thick aluminum as a bottom electrode,was deposited on the glass substrate by DC sputtering. Thedielectric layer was deposited on the bottom electrode byscreen-printing and then dried at 130 �C for 30 min. Theemitting layer was deposited on the dielectric layer byscreen-printing and dried at 130 �C for 30 min. Finally, in-dium-tin-oxide (ITO) as a transparent top electrode wasdeposited on the emitting layer by DC sputtering.

The EL characteristics of the EL devices were examinedby applying a sinusoidal AC voltage between the two elec-trodes of the devices using combination of a function gen-erator (Agilent, 33250A) and an amplifier (Trek, P0610B-K)at the frequency ranges of 0.1–20 kHz. An oscilloscope(Hewlett–Packard 400D) was then connected to the deviceto monitor an actual value of the applied voltage. All ACreadings in this paper mean the peak-to-peak values. Theabsolute luminance was measured using a luminance col-

-polymer/phosphor composite. Inset is the light emission of the composite

Fig. 2. (a) The process of fabricating the three phase composite, i.e., SWCNT-polymer/phosphor. (b) Raman spectra of short SWCNTs before and after thethermal treatment. Insets are digital camera images of SWNCT-polymer composite. (c) Raman spectra of the emitting layer with and without SWCNTs.

J.-Y. Kim et al. / Organic Electronics 13 (2012) 2959–2966 2961

orimeter (Topcon, BM-7). The field emission of the short-ened SWCNTs on the ITO-coated glass was measured in avacuum chamber under 1 � 10�7 Torr using Keithley 248and 6517 measurement units. The gap between the anodeand cathode plates was maintained to be 400 lm with aglass spacer. An ITO plate coated with a green phosphorwas used as the anode.

3. Results and discussion

All the thickness for the EL devices was maintained tobe 26 lm, where the thickness of the dielectric layer wastuned to be 8 lm, as shown in Fig. 1. The shortenedSWCNTs by the cryogenic crushing method exhibit rela-tively low sidewall damages because SWCNTs werecrushed on the rigid condition by liquid nitrogen [19].However, the intensity of D-band (1320 cm�1) in Ramanspectra for the shortened SWCNT is increased due to thedefects of CNTs during the crushing, as shown inFig. 2(b). In order to reduce the defect state of the CNTs,shortened SWCNTs were fired at 430 �C for 30 min in theatmosphere. D-band at 1320 cm�1 for the thermal treat-ment of the short SWCNTs was decreased (see Fig. 2(b)),leading to improved the properties of the field emission.It was caused by fact that amorphous carbon particlesand disordered CNT debris, which were formed duringthe crushing SWCNTs, would be easily reacted with theoxygen, resulting in removal of them and restoring robust

SWCNTs with high concentration [20]. For this reason,when short SWCNTs with and without thermal purificationtreatment were mixed in a polymer matrix, the compositewith thermal treated short SWCNTs showed the lightercolor compared to composite without thermal treatmentdespite of the same SWCNT concentration, as shown inthe inset of Fig. 2(b). In order to confirm the existence ofSWCNTs in the emitting layer, Raman characteristics wereinvestigated, as shown in Fig. 2(c). D-band and G-bandaround 1320 and 1580 cm�1 only appeared from theSWCNT-polymer/phosphor composite emitting layer. Fur-thermore, the sharp radial breathing mode (RBM), whichis proportional to the nanotube diameter [21], wasfounded in the SWCNT-polymer/phosphor compositeemitting layer at the 220 cm�1, while the RBM of an EL de-vice with no SWCNTs was not appeared. The peak at350 cm�1 in Raman spectra is known to be the longitudinaloptical (LO) mode in both the hexagonal and cubic ZnS[22], which was observed both in the EL devices with andwithout SWCNTs. Therefore, we can easily notice thatSWCNTs were well mixed with the phosphor powders inan emitting layer.

In the CNT and polymer composite, the sharp potentialchange around CNTs by the work function difference be-tween the CNT and polymer could provide a considerablehigh electric field, which was reported in Ref. [23]. How-ever, it is difficult to measure directly the local electric fieldof SWCNTs in the composite emitting layer because it isdifficult to consider the effect of the polymer and phosphor

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surrounding SWCNTs. For this reason, in order to verify theeffect of SWCNT condition on a local electric field, indirectmeasurement was utilized here. That is, field emissioncharacteristics of shortened SWCNTs coated on the ITO-coated glass were measured, where the properties ofSWCNTs, i.e., concentration and length, were similar tothose of the injected SWCNTs in the composite EL device.Although the intensity of field emission of SWCNTs in anemitting layer is different from the SWCNTs on the ITO-coated glass, the tendency of field enhancement ofSWCNTs could be deduced by comparing the two cases.Fig. 3(a) shows the schematic structure as a diode configu-ration for the measurement of the field emission proper-ties. Fig. 3(b) is the field emission plot for the shortSWCNT layer on the ITO-coated glass, and the insets aretheir corresponding Fowler–Nordheim (F–N) plot [24,25]to verify the data reliability of field emission and their dig-ital images during field emission. The turn-on electricfields of the SWCNTs with and without thermal treatmentwere 1.2 and 1.5 V/lm, respectively, at a current density of1 lA/cm2. The emission current of the SWCNT with ther-mal treatment was higher than that without thermal treat-ment. It was caused by the CNT defects reduced. In order tocalculate the field enhancement factor (b = [cm�1]), the I–Vgraph was fitted by the F–N plot. The F–N plot can be de-picted by the formula, i.e., log(I/V2) = a + b/V (a is the inter-cept, a ¼ log ð1:4� 10�6ab2=/Þ þ 4:26=

ffiffiffiffi

/p

; and b is theslope, b = �2.82 � 107/3/2/b), where a and / are the emit-ting area and the work function of CNTs, respectively[25,26]. The slope was obtained from the F–N plot and bcan be calculated by the above formula with the workfunction of 5 eV [27,28]. The values of b were found to be1.3 � 105 cm�1 for the thermal-treated SWCNTs and6.5 � 104 cm�1 for the non-treated ones. The b value isbasically determined by the geometry of CNTs, since it isalso described as h/r, where h and r are a length and adiameter of CNTs, respectively [29]. High b value of CNTs

Fig. 3. (a) The schematic structure as a diode configuration for the measuremenSWCNT layer on the ITO-coated glass and the insets are their corresponding Fowfield emission properties, such as an emission current density, a turn-on electric fipurification.

can induce the huge electron emission [30,31] due to thehigh electric field around CNTs. The characteristics of fieldemission for the short SWCNTs with and without thermaltreatment were summarized as shown in Fig. 3(c). Basedon the equation, E ¼ �bE0, where �b is a dimensionless fieldenhancement factor ð�b ¼ b� dÞ and E0 (=V/d, V is and ap-plied voltage and d is a distance between a cathode andan anode) is a macroscopic field, the local electric field, E,is generally increased in proportional to �b. Therefore, puri-fied SWCNTs in an emitting layer with the high b (or �b) va-lue would enhance the electric field, leading to moreelectron tunneling and acceleration into the phosphorlayer. The detail mechanism will be discussed later.

Fig. 4(a) shows the digital images of purified shortSWCNTs dispersed in the polymer matrix, using the so-dium dodecylbenzenesulfonate (NaDDBS) surfactant. Asshown in Fig. 4(a), images of the top row are suspensionsafter the sonication. NaDDBS is well-known as a commonsurfactant for dispersing CNTs due to its amphiphilic prop-erty [32,33]. Dispersion mechanism by the surfactant is asfollowing. Hydrophobic tails of NaDDBS molecules couldinteract with the hydrophobic surface of SWCNTs andhydrophilic head face outward [32,33]. SWCNT-polymercomposites showed the long-term stability of the SWCNTsuspensions even after three weeks, as shown in the bot-tom row of Fig. 4(a). In order to investigate the critical con-centration of SWCNTs inducing high EL performance, theconductivity (r) of the three phase composite films com-posed of the SWCNTs, the polymer, and the phosphor, withvarying SWCNT concentration, was measured (seeFig. 4(b)). Over the SWCNT concentration of 0.09 wt.%,the conductivity of the composite film was rapidly in-creased, which could be explained by the percolation the-ory [7,34–37]. When the SWCNT concentration in thephosphor-polymer matrix is increased, percolation pathscould be formed via the SWCNT network. In the threephase composite film, the SWCNT concentration could

t of the field emission properties. (b) The field emission plot for the shortler–Nordheim plot and images of the light emission. (c) Summary of theeld, and field enhancement factor of the short SWCNT before and after the

Fig. 4. (a) Digital camera images of various purified short SWCNT concentrations dispersed in a polymer matrix including the NaDDBS surfactant. Top rowimages are suspensions immediately after the sonication of SWCNT-polymer composite and bottom row images are suspensions after 3 weeks. (b)Conductivity of the SWCNT-polymer/phosphor composite as a function of various SWCNT concentrations. Inset is fitting data of the conductivity by log–logplot. (c) The EL intensities of the composite EL devices as a function of various SWCNTs concentrations (150 V and 400 Hz).

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control the conductivity of the film. Near the critical con-centration, the percolation theory is given as follow[7,37]. r / ðfc � f Þ�s0 for f < fc where fc is the percolationthreshold, f is the weight percent of nanotubes in the com-posite, and s0 is the critical exponent in the insulating re-gion [7]. In order to determine the percolation thresholdand the critical exponent, experimental data of Fig. 4(b).was fitted. In the case of the best result for fitting r data,fc and s0 were found to be 0.0925 wt.% and 0.998, respec-tively, as shown in the inset of Fig. 4(b). To investigatethe effect of the purified SWCNT concentration on EL per-formance, the EL devices were fabricated using three phasecomposite film at the same SWCNT concentration of theabove results. Fig. 4(c) shows the EL intensity of the EL de-vices as a function of the SWCNT concentration. The ELintensity of EL devices was measured at the applied voltageof 150 V and the frequency of 400 Hz. The EL device withthe SWCNT concentration of 0.09 wt.% exhibited the bestEL intensity among devices. The SWCNT concentration of0.09 wt.% is similar to the calculated percolation threshold(0.0925 wt.%). In the composite EL device, the SWCNT con-centration near the percolation threshold as a critical pointcould induce highly local field enhancement without theconducting network, resulting in increased the EL inten-sity. Over the SWCNT concentration of 0.12 wt.%, the ELintensity of the EL devices was substantially decreased.This is caused by the fact that the percolation paths bySWCNT networks in the EL device can interrupt the suffi-cient electric field applied to the phosphor, resulting in

the poor EL performance. The detail mechanism bySWCNTs will be discussed later.

In order to investigate the effect of SWCNT conditionson the EL performance, the luminance and current densityfor three kinds of EL devices, i.e., the composite EL deviceswith and without thermal treatment, and the EL devicewithout SWCNTs (referred to the reference EL device),were measured and calculated the efficiency based onthe equation, g ¼ p � L=J � V (lm/W) where L is the lumi-nance, J is the current density, and V is the applied voltage[13,14], as shown in Fig. 5(a) and (b). 0.09 wt.% SWCNTconcentration, which is near the critical percolationthreshold, was applied to another set of a composite emit-ting layer. The luminance of composite EL devices with andwithout the thermal treated SWCNTs was 218 and 189 cd/m2, respectively, at the driving voltage of 150 V and thefrequency of 400 Hz, while that of the reference EL devicewas the lowest, i.e., 139 cd/m2, as shown in Fig. 5(a). Thecomposite EL device without the thermal treatment was36% brighter than the reference EL device. However, thecomposite EL device with the thermal treatment exhibited57% increase in the luminance compared with the refer-ence EL device. The phenomenon of increased luminancefor the composite EL devices can be interpreted by the localfield enhancement of SWCNTs as following. In theZnS:Cu,Cl EL phosphor, CuxS needles were generallyformed due to exceeding Cu concentration during the syn-thesis of phosphor powder over the temperature of1100 �C. These CuxS needles could form the ZnS–CuxS–

Fig. 5. (a) The luminance and efficiency of the EL devices as a function of the driving voltage. (b) The current density of the EL devices as a function of thedriving voltage. 0.09 wt.% SWCNTs were used for the composite EL devices regardless of the thermal purification treatment. Schematic diagram of the ELmechanism for the EL device of (c) before and (d) after the inserting SWCNT. (e) The luminance of the EL devices as a function of the field enhancementfactor. (f) EL intensity of the EL devices as a function of EL spectra.

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ZnS hetero-junction in the phosphor [13,14]. When theelectric field of 106 V/m was applied to the composite ELdevice, electrons and holes tunneling into ZnS host latticeare occurred at the each end of CuxS conducting needles[38], respectively, as shown in Fig. 5(c). Under the consid-eration of CNT Fermi level and its band bending, the tun-neling barrier thickness could become thin to inducemore charge carrier tunneling into ZnS host lattice by thelocal field enhancement of SWCNTs at the same electricfield compared to the reference EL device, as shown inFigs. 5(c) and (d). Holes are trapped at the Cu recombina-tion centers and electrons are trapped at the Cl donor sites.At the reversal phase of the electric field, the more emittedelectrons by field enhancement of SWCNTs can be recom-bined with the trapped holes to produce more light emis-sions, as shown in Fig. 5(d). The composite EL devicewith thermal treated SWCNTs showed higher luminancethan that without thermal treatment. It implies that the ef-fect of field enhancement of purified short SWCNTs on ELperformances is more effective than non-treated shortSWCNTs. As a result, this result from EL performancesagrees well with the above results of field emission ofFig. 3, which is summarized in Fig. 5(e). Fig. 5(b) showsthe current densities of the EL devices as a function ofthe applied voltage. The current densities of the compositeEL devices with and without thermal treatment (0.438 and0.444 mA/cm2, respectively) are lower than that of the ref-erence EL device (0.572 mA/cm2). The composite EL de-vices exhibited 23% improvement in the average currentdensity compared to the reference one. The isolation ofthe short SWCNTs in an emitting layer could increase thecapacitance of the device due to the space charge polariza-tion. If SWCNTs cannot form conducting networks fromeach other in the SWCNT-polymer/phosphor compositeemitting layer, electrons were trapped at the interface be-tween the SWCNT and the polymer (or phosphor) [2,39].

Trapped electrons could induce the space charge polariza-tion when an alternating current electric field applied to acomposite EL device. High value of the capacitance com-pared to the reference EL device could influence theimpedance of the EL device, because of its AC operatingcondition. Therefore, although the emitting layer of thecomposite EL device has conducting fillers, the currentdensity could be lower than that of the reference EL device.

In order to support the above experimental results,impedances of EL devices with and without thermally puri-fied SWCNTs were calculated from measured LCR data (LCRmeter) based on the impedance equation, Z = [R2 + (XL -� XC)2]1/2, where R is a resistance, XL (=2pfL) is an inductivereactance, and XC (=1/2pfC) is a conductive reactance. Thecomposite EL device exhibited 5% increase in the imped-ance compared to the reference device, as shown inFig. 6(a). That is, the decrease in the current density ofthe composite EL device for calculation value is 5% basedon the equation, I = V/Z. However, this decrease in the cur-rent density is lower than the measurement value (23%).This may be caused by LCR measurement conditions,where the operating conditions at the frequency of400 Hz and the voltage of 1 V. LCR measurement unit hasa limited voltage range from �30 to +30 V for applyingvoltage. On the other hand, the current density of the com-posite EL device was measured under the voltage of 150 V.Although the ratio of decrease in the current density wasdifferent, the tendency of decreased current density forthe composite EL device could be matched between twooperating voltages, such as 1 and 150 V for measured LCRvalues. The efficiencies of the composite EL devices withand without thermal treatment were 1.20 and 1.02 lm/W,respectively, at the 150 V and 400 Hz, while that of the ref-erence EL device was 0.60 lm/W (See Fig. 5(a)). Theimprovements in the efficiencies of the composite EL de-vices with and without thermal treatment were 100%

Fig. 6. (a) Calculated impedance from measured L, C, and R values of the reference and composite EL devices with purified SWCNTs at the voltage of 1 V andthe frequency of 400 Hz. (b) An applied electric field to phosphor layer (EP) as a function of the dielectric constant of an emitting layer (ep) and a dielectriclayer (ei).

Fig. 7. The luminance of three kinds of EL devices, i.e., the reference EL device, composite EL device with thermal-treated SWCNTs, and composite EL devicewith non-treated SWCNTs, as a function of the frequency at the fixed voltage of 150 V.

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and 70%, respectively, compared with the reference device.The efficiencies of EL devices with and without the thermaltreatment were greatly enhanced due to the high lumi-nance and low current. In general, the composite filmexhibited large dielectric constant near the percolationthreshold of the conducting filler concentration. A lot of re-searches about the large increment of the dielectric con-stant for the composite film were reported [1,4,6,7]. Inorder to investigate the effect of increasing dielectric con-stant of emitting and dielectric layers on an electric field,we calculated an electric field applied to the phosphor(EP) at the varying dielectric constants of emitting anddielectric layers, as shown in Fig. 6(b). Basically, the rela-tionship between the dielectric constant and the thicknessof component layers in the EL device can be expressed bythe equation of an electric field applied to the phosphor,Ep = eiV/(eidp + epdi), where ei (ep) is the dielectric constantof a dielectric (phosphor) layer, V is an applied voltage tothe EL device, and di (dp) is a thickness of the dielectric(phosphor) layer [40,41]. Ep was gradually decreased (in-creased) with increasing the dielectric constant of theemitting layer (the dielectric layer) (see Fig. 6(b)).

Dielectric constants of the raw and composite emitting lay-ers (46 and 92) in this experiment were applied to calculat-ing Ep at V of 150 V and ei of 80. The Ep values of the rawand composite emitting layers can be found to be6.6 � 106 and 5.5 � 106 V/m, respectively. The increase inthe dielectric constant of the composite emitting layer nearthe percolation threshold cannot affect improvement inthe EL performance directly, but rather decreasing electricfield applied to phosphor (17%). As a result, SWCNTs in theemitting layer could increase a capacitance by spacecharge polarization and decrease the applied field to phos-phor. However, the local field enhancement formed aroundSWCNTs could enhance the applied field to phosphor,resulting in increased charge carrier tunneling into ZnShost lattice.

In order to investigate the EL performance of compositedepending on the frequency, the luminance and EL spectraof the EL devices were measured at the various frequenciesand fixed driving voltage of 150 V, as shown in Fig. 7.0.09 wt.% SWCNT concentration, which is near the criticalpercolation threshold, was applied to a composite emittinglayer. The luminance was gradually increased along with

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increasing frequency for all of EL devices. The luminance ofthe composite EL with and without thermal treatment was4460 and 3430 cd/m2, respectively, at the frequency of20 kHz and the voltage of 150 V, while the luminance ofthe reference EL was 2840 cd/m2. The composite EL withthermal treated SWCNTs exhibited 57% increase in theluminance compared to the reference EL. In general, theEL intensity is affected by the polarity of applied voltage,because a high EL intensity is occurred at the invertedpolarity due to its mechanism [13,14]. In addition, theemission color changed from green to blue over the fre-quency 10 kHz. It was caused by the fact that a green emis-sion (520 nm) transition occurred from Cl on a S site (ClS)to Cu on a Zn site (CuZn) under the frequency of 10 kHz,while a blue emission (440 nm) transition occurred fromClS to interstitial Cu (Cui) over the frequency of 10 kHz[14,42]. The composite EL device with thermal treatedSWCNTs showed the highest EL intensity due to the fieldenhancement by SWCNTs at the high frequency of20 kHz and the voltage of 150 V. In general, the EL spectraof ZnS:Cu,Cl phosphors depend on the operation frequencyand Cu concentration [42,43]. However, there were no dif-ferences in the change of EL spectra between the referenceand the composite EL devices. It could imply that SWCNTscan just assist the enhancement of an electric field appliedto phosphors and cannot affect the luminescent transitionmechanism.

4. Conclusion

In summary, we have fabricated AC driven powder ELdevices using a SWCNT-polymer/ZnS:Cu,Cl compositeemitting layer to investigate the effect of a critical percola-tion threshold on EL performances. In order to optimizeSWCNT conditions in the composite emitting layer, shortSWCNTs were prepared by the cryogenic crushing methodand then purified by the thermal treatment at ambientatmosphere. After the above treatments, SWCNTs wereshortened to less than the sub-micron range and SWCNTdefects, which were generated during CNT crushing, weredecreased. Among varying SWCNT concentrations, a criti-cal percolation threshold (0.0925 wt.%) was found by fit-ting the conductivity data. Near the percolation thresholdof the SWCNT concentration in a composite emitting layer,high performance, such as 100% (57%) increase in the effi-ciency (luminance) of the EL device was obtained. An elec-tric field applied to phosphors could be enhanced by fieldenhancement of SWCNTs, leading to increased charge car-rier tunneling into ZnS host lattice. This increase in thetunneling charge carrier could induce the more electron–hole recombination, resulting in increased luminance. Inaddition, higher impedance enabled a low current intothe device, which could contribute to increased efficiency.The composite emitting layer could advance the EL deviceas a potential candidate in the future displays.

Acknowledgement

This work was supported in part by the National Re-search Foundation of Korea [NRF 2010-0011894].

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