Hybrid light-emitting diodes based on flexible sheets of mass-produced ZnO nanowires

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Nanotechnology 20 (2009) 445203 (6pp) doi:10.1088/0957-4484/20/44/445203

Hybrid light-emitting diodes based onflexible sheets of mass-produced ZnOnanowiresJinzhang Liu, Y H Ahn, Ji-Yong Park, Ken Ha Koh andSoonil Lee1

Division of Energy Systems Research, Ajou University, Suwon 443-749, Korea

E-mail: soonil@ajou.ac.kr

Received 16 June 2009, in final form 24 August 2009Published 7 October 2009Online at stacks.iop.org/Nano/20/445203

AbstractWe report the production of free-standing thin sheets made up of mass-produced ZnOnanowires and the application of these nanowire sheets for the fabrication of ZnO/organichybrid light-emitting diodes in the manner of assembly. Different p-type organicsemiconductors are used to form heterojunctions with the ZnO nanowire film.Electroluminescence measurements of the devices show UV and visible emissions. Identicalstrong red emission is observed independent of the organic semiconductor materials used in thiswork. The visible emissions corresponding to the electron transition between defect levelswithin the energy bandgap of ZnO are discussed.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

One-dimensional (1D) nanostructures have taken center stagein research and development in many science and engineeringfields. A large variety of 1D inorganic nanostructures, such asmetal oxides (ZnO, SnO2, In2O3) [1, 2], nitrides (GaN) [3],sulfides (ZnS [4], CdS [5]) and phosphides (InP [6]), havebeen grown on a large scale by the vapor-phase growthmethod. However, application of these randomly oriented 1Dsemiconductor nanocrystals has not gained much attention, inspite of their high yields, compared to that of their counterpartswhich show aligned growth on planar substrates.

ZnO has a wide bandgap of 3.37 eV and a largeexciton binding energy of 60 meV, making ZnO suitablefor room-temperature light-emitting diodes (LEDs). It isknown that ZnO is naturally n-type, and the preparation ofpure and stable p-type ZnO is still a challenge. There-fore, a number of different approaches are developed tofabricate ZnO-based hybrid LEDs by using some other p-type semiconductors, either inorganic or organic. ZnO–organic hybrid LEDs have shown UV and visible elec-troluminescence (EL) emissions in previous reports [7–9].Typically, vertically aligned ZnO nanowire (NW) arrays were

1 Author to whom any correspondence should be addressed.

used to make contact with organic semiconductors, such aspoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PE-DOT:PSS) [7, 8] and N,N ′-di(naphtha-2yl)-N,N ′-diphenyl-benzidine (NPB) [9]. Elaborate processes to make theinsulating polymer infiltrate the vertical NW arrays, to exposetips of the ZnO nanowires by using a lithography technique,and, subsequently, to deposit organic semiconductors to formheterojunctions at the ZnO-NW tips are necessary to fabricatethe devices. UV–visible EL emission takes place only at theinterfaces between the semiconducting polymer and the ZnO-NW tips.

Currently, flexible optoelectronic devices are the mostexciting trend. Analogous to the robust free-standingfilms of carbon nanotubes that were made by a filtrationmethod [10], it is anticipated that the mass-produced ZnONWs can be processed to be flexible network films. Inthis paper, we demonstrate a simple approach to fabricatefree-standing thin sheets of ZnO nanowires. We usedthese ZnO-NW sheets to make ZnO/organic hybrid LEDs.P-type organic semiconductors, N,N ′-diphenyl-N,N ′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), poly(9-vinylcarbazole) (PVK), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl(CBP) and a blend of PVK and CBP were used in this study.Electroluminescence (EL) properties of the hybrid devices arepresented.

0957-4484/09/445203+06$30.00 © 2009 IOP Publishing Ltd Printed in the UK1

Nanotechnology 20 (2009) 445203 J Liu et al

Figure 1. (a) A photograph of the cotton-like product consisting of ZnO nanowires. (b) A photograph of a free-standing film made up of themass-produced ZnO nanowires. (c) Schematic of the process to fabricate a ZnO/organic hybrid LED using the nanowire film.

2. Experimental details

The ZnO NWs were synthesized within a horizontal quartztube furnace (inner diameter 5 cm) at atmospheric pressurewithout using any catalyst. Mixtures of ZnO and graphitepowders (2–3 g), a weight ratio of 1:1, were heated to 1100–1200 ◦C and the vaporized growth species were transportedby a gas flow that consisted of 1000 sccm N2 and 30 sccmO2. Cotton-like white products were deposited in the low-temperature region (between ∼200 ◦C and room temperature).Typical growth time was 30 min. Figure 1(a) showsa photograph of the product that consists of ZnO NWs.Thin sheets of the ZnO NWs were fabricated by a simplefiltration method. First, a ZnO-NW suspension solution, at aconcentration of 1 mg ml−1, was prepared by ultrasonicallydispersing the nanowires in isopropanol. Second, the ZnO-NW suspension was vacuum-filtered through a porous anodealuminum oxide (AAO) membrane, a diameter of 4.3 cm andpore size of 200 nm, purchased from Whatman Co. Third,a network film of ZnO NWs on an AAO membrane wasdried in air at 100 ◦C for 1 h. Finally, a thin sheet ofZnO NWs was detached off the membrane filter. A typicalfree-standing sheet of ZnO NWs, mass 3.5 mg, is shown infigure 1(b). It is interesting to note that the thin ZnO-NWsheet is flexible and translucent, and that the paper-like sheetcan be cut into any size and shape, using a blade, to beused for hybrid LED fabrication. Figure 1(c) illustrates thefabrication process and structure of a ZnO-NW-sheet/organichybrid LED. Firstly, a hole-injection layer, PEDOT:PSS, wasspin-coated onto a patterned ITO–glass substrate and driedin N2 atmosphere. Onto this layer the other hole-transportorganic layer was spin-coated from solution (10 mg ml−1

in chloroform) at 2000 rpm. Next, we placed a piece ofZnO-NW sheet onto the organic layer and pressed it downusing a roller. In the roller-pressing process the pressure wasmechanically controlled, resulting in even adhesion of a ZnO-NW sheet to an organic layer. Finally, after evaporating Al to

form a cathode onto the ZnO nanowire sheet, the device wasencapsulated by a glass cap in dry N2 atmosphere. TPD, PVK,CBP and a blend of PVK and CBP (weight ratio 1:2) wererespectively used to form heterojunctions with the ZnO-NWfilms. Photoluminescence (PL) and EL characterizations wereperformed at room temperature.

3. Results and discussion

Figures 2(a) and (b) show the field emission electron scanningmicroscopy (FE-SEM) images of the ZnO-NW sheet infigure 1(b). The ZnO NWs, which are a few tens of nmin thickness, are interlaced with each other to form a felt-like morphology. It has to be emphasized that no adhesiveadditive was used to make ZnO-NW sheets with a paper-likeappearance, and that we were able to control the thickness ofZnO-NW sheets by controlling the total amount of ZnO NWsin dispersion solutions that we vacuum-filtered. The thicknessof the ZnO-NW sheet shown in figure 2(b) is measured as∼2.6 µm so that the density of the ZnO-NW sheet is estimatedto be about 1 g cm−3, which is almost six times smallerthan that of a ZnO bulk crystal (5.61 g cm−3). The x-raydiffraction (XRD) pattern of the nanowire sheet is shown infigure 2(c). The fact that we are able to index all the peaksaccording to the wurtzite ZnO structure indicates good crystalquality of ZnO nanowires. However, we cannot rule out theexistence of point defects that can influence optical propertiesof ZnO nanowires based on this XRD result. In particular,because the ZnO nanowires were synthesized under an oxygen-rich condition and started to nucleate and grow before a reactorreached a stable growth temperature, it is very likely thatthe ZnO nanowires are nonstoichiometric with many nativedefects, such as zinc vacancies (VZn), oxygen interstitials(Oi), antisite oxygen (OZn) and oxygen vacancies (VO) [11].We exclude zinc interstitials because of the comparativelyhigh formation energy of such a defect under oxygen-richconditions. Figure 3 shows the cross-sectional FE-SEM image

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Figure 2. Tilt view (a) and cross-sectional view (b) FE-SEM images of a typical ZnO nanowire film. (c) XRD pattern of the ZnO nanowirefilm.

Figure 3. Cross-sectional view FE-SEM image of typical organiclayer on ITO substrate.

of a typical substrate, ITO/PEDOT:PSS/TPD, that was used forattaching the ZnO-NW film. The thickness of the PEDOT layeris about 60 nm. Normally, the organic layers that we spin-cast onto the PEDOT layer are about 60–80 nm in thickness.Because the ZnO-NW film is flexible and the nanowires are inparallel with the substrate surface, the organic layer is unlikelyto be locally penetrated by single nanowires during the roll-pressing process, as can be testified by the current–voltage (I –V ) measurements.

Figure 4(a) shows the current–voltage (I –V ) curves of twodevices, TPD/ZnO and PVK/ZnO. Good rectifying behaviors

of the heterojunctions were observed. PL spectra of the ZnO-NW film, TPD and PVK are shown in figure 4(c). TheZnO-NW film exhibits a UV peak at 380 nm, known as thenear-band-edge (NBE) emission, and a broad visible emissionband covering purple, blue and green, due to the nativedefects of ZnO. TPD and PVK show PL emissions peakingat 428 nm and 415 nm, respectively. In figures 4(b) and (d)the EL spectra of the TPD/ZnO and PVK/ZnO devices areplotted, corresponding to the anode biases of 20 V and 16 V,respectively. For the EL spectra of TPD/ZnO heterojunction,the characteristic UV emission of ZnO emerged, accompaniedby a broad visible emission band peaking at 630 nm. Thedominance of the red emission at 630 nm was enhancedwith increasing anode bias. Meanwhile, a suppressed greenemission at ∼520 nm, typically due to the defects of ZnO, canbe found from the spectra of the TPD/ZnO device. The UVemission failed to emerge in the EL spectra of the PVK/ZnOdevice, and a low and broad emission band at ∼420 nm thatcan be assigned to the emission of PVK appeared. However,similar to the TPD/ZnO structure, the PVK/ZnO device alsoexhibits a dominant red emission at 630 nm.

Among the available p-type organic semiconductors, CBP,a small-molecule material, has the deepest HOMO (highestoccupied molecular orbit) level that is expected to moreefficiently inject holes into the valence band of ZnO. However,CBP is recognized to have bipolar transport character. Weblended CBP with a unipolar transporter PVK to modifythe hole transporting property and improve the film quality.

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Figure 4. Comparison of the measurement results of two devices: TPD/ZnO and PVK/ZnO. (a) I–V characteristics. (b) and (d) EL spectra ofthe two devices at different anode biases. (c) PL spectra of the materials (ZnO NWs, TPD and PVK films).

Figure 5(a) depicts the I –V curves of two devices, CBP/ZnOand CBP + PVK/ZnO, showing that the pure CBP has highercarrier mobility than the blend of CBP and PVK. In fact,the two spin-coated organic films are equivalent in thickness.The EL spectra of the two devices, measured at forwardbiases of 18 V and 15 V, are compared in figures 5(b)and (d), respectively. Interestingly, the red emission at 630 nmappeared as well. Besides, the CBP/ZnO device exhibited ablue peak at 435 nm, whereas the PVK + CBP/ZnO deviceshowed a UV peak at 390 nm. For the CBP/ZnO device,the characteristic UV emission of ZnO at ∼380 nm appearedfaintly at low anode bias, but it was unstable and vanished withincreasing bias. On the other hand, the blue emission at 435 nmwas enhanced so as to surpass the red emission peak at higherbias. The PL spectra in figure 5(c) with similar shapes are fromthe spin-coated films of PVK + CBP and pure CBP on glass.The peak at 377 nm is the characteristic emission of CBP.

Now we discuss the EL emissions of the four typesof organic/ZnO LEDs by using the energy band diagram infigure 6(a). These devices show an identical red emissionaround 630 nm (1.97 eV), regardless of the different HOMOlevels of p-type organic semiconductors used in this work,indicating that the exciplexes between ZnO and organicsemiconductors can be ruled out, causing this emission.The energy bandgap of ZnO is 3.37 eV, within which theenergy positions of various defects had been calculated orexperimentally measured. Generally, VO and Zni are donorsand VZn, Oi and OZn are acceptors. The V+

O that is frequentlyreferred to as the origin of the green emission from ZnO has the

energy level located about 2.45 eV (506 nm) above the valence-band maximum [12, 13]. Assuming that the red emission isdue to the electron transition from the V+

O level to an acceptorlevel, this acceptor level is deduced to be 0.48 eV above thevalence-band top. Among the aforementioned acceptor levels,it appears that the V2−

Zn level, which was calculated to be0.51 eV above the valence-band top [14], is closest to thisrequirement. Therefore, we conclude that the red emissioncentered at 630 nm originates from the electron transition ofV+

O → V2−Zn . The oxygen interstitial O−

i has a level 0.66 eVabove the valence-band maximum [11]. The V+

O → O−i

transition may result in an emission band at 693 nm (1.79 eV),which accidentally corresponds to the right shoulder of thered emission band in the EL spectrum of the TPD/ZnO devicemeasured at lower anode bias (figure 4(d)).

The energy of the blue emission at 435 nm (2.85 eV) fromthe CBP/ZnO device (figures 5(b) and (d)) is too large to beinvolved in exciplex emission according to the energy banddiagram in figure 6(a). It has been reported that degradationof a CBP solid film could result in a low energy (LE) bandemission at ∼438 nm [15]. However, normally this peak wouldnot appear alone. Other LE peaks, such as at 413 and 560 nm,could be concomitants. Coincidently, a blue emission of ZnOat ∼430 nm (2.88 eV) is predicted from the transition of theconduction band to V2−

Zn , which could be responsible for theEL peak at 435 nm from the CBP/ZnO device (figures 5(b)and (d)). It is worth mentioning that V−

Zn, a deep acceptor witha calculated level of 0.18 eV above the maximum of valenceband [16], can cause an emission at ∼390 nm, which matchesthe UV peak of the CBP + PVK/ZnO device. Nevertheless the

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Figure 5. Comparison of the measurement results of CBP/ZnO and CBP + PVK/ZnO devices. (a) I–V characteristics. (b) and (d) EL spectraof the two devices at different anode biases. (c) PL spectra of the spin-coated films of CBP and blend of CBP and PVK.

CBP host can have an EL peak at ∼400 nm [17], making theUV emission of the CBP + PVK/ZnO device intriguing.

The barriers from the conduction band of ZnO to LUMOlevels of these organic semiconductors are in the range of 1.5–2.0 eV, and the gaps between the valence band and HOMOlevels are 1.6–2.2 eV. Such high barriers normally cause holeand electron accumulation and energy band bending of ZnOat the organic/ZnO interface when the device is biased. Asan example, figure 6(b) shows the energy band diagram ofthe biased TPD/ZnO heterostructure. The carrier mobility oforganic semiconductor can be a factor influencing the bendingdegree of ZnO energy bands. The hole mobilities of TPD andCBP are very close and of the order of 10−3 cm2 V−1 s−1 [18],much higher than that of PVK (10−6 cm2 V−1 s−1). Infigures 4(b) and (d), the NBE emission of ZnO appearsfrom the TPD/ZnO device rather than the PVK/ZnO device.Previous reports on the EL of PVK/ZnO structure showed therelatively strong emission of PVK at either 450 or 550 nm(excitons between different carbazole molecules) [19, 20]. Infigures 5(b) and (d) the EL emission related to PVK excitonsis faint compared with the red peak, indicating that most ofthe electrons contribute to the transition V+

O → V2−Zn . The

fact that the ZnO NBE emission is reluctant to emerge in thisPVK/ZnO device can be attributed to the poor hole mobilityof PVK. In comparison, the higher hole mobility of TPD

results in more holes accumulated at the interface, and high-degree upward bending of ZnO energy bands. The elevatedvalence band renders the possibility of receiving holes from theHOMO of TPD, giving rise to the NBE emission in additionto other visible emissions (figure 6(b)). On the other hand,the comparatively low bending degree of the ZnO bands at thePVK/ZnO interface maintains a large barrier for transferringholes from the PVK HOMO to the valence band of ZnO, onlyfavoring the transitions between defects. As mentioned aboveCBP has a bipolar transport property, as its electron mobilityis relatively large (10−4 cm2 V−1 s−1). This is a disadvantageof reducing the barrier via band bending for injecting holesinto the valence band of ZnO. Most probably the excitons ofCBP would be produced in the CBP/ZnO device, giving riseto the singlet-related emission at ∼400 nm. However, thisenergy could be degraded via Forster energy transfer to ZnO,causing the blue emission at 435 nm related to Ec → V2−

Znthat avoids the nonradiative process Ec → VO. The blendof CBP and PVK has properties differing from pure CBP,such as in transporting holes and blocking excitons, as canbe seen in figure 5(a) that the CBP + PVK blend exhibitshigher carrier transport and better rectifying behavior thanCBP when heterostructured with ZnO. It is discernible thatthe broad UV band peaking at 390 nm in the EL spectraof the CBP + PVK/ZnO device is a composition of multiple

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Figure 6. (a) Energy band diagram of the organic/ZnO heterojunctions accounting for the EL emissions from ZnO. (b) Energy band diagramof the TPD/ZnO heterostructure LED under a positive anode bias, showing charge accumulation and band bending at the interface.

emissions through UV to blue. The transition Ec → V−Zn

is unlikely to take a main role since it is not found fromthe other three devices. Previously, the EL emission ofZnO at 393 nm, attributed to a bound exciton, was observedfrom the PEDOT/ZnO-nanorod and TPD/ZnO-nanoparticledevices [7, 21]. Such a peak of ZnO has a half-maximumwidth of 20–40 nm, sharper than the EL peak of CBP. Thepeak position at 390 nm is blueshifted to the usual EL peak at∼400 nm of the CBP host. Presumably, the UV emission ofZnO and near-UV emission of CBP coexist when the EL of theCBP + PVK/ZnO device occurs.

4. Conclusion

In summary, we made a free-standing thin sheet of ZnOnanowires that were mass-produced by a vapor-phase synthesismethod at atmospheric pressure, and used this ZnO nanowiresheet to fabricate ZnO/organic hybrid LEDs. We simplyattached a piece of flexible ZnO-NW sheet to a p-typeorganic semiconductor layer by a roller-press process. Fourtypes of structures, TPD/ZnO, PVK/ZnO, CBP/ZnO andCBP + PVK/ZnO, are presented. Due to the abundant nativedefects, the characteristic UV emission of ZnO was weak orfailed to appear for the devices of TPD/ZnO, PVK/ZnO andCBP/ZnO. The structure of CBP + PVK/ZnO favors the UVemission, but emissions related to the deep acceptor levelof Zn− in ZnO and excitons of CBP are incorporated. Anotable red emission at 630 nm is commonly observed in theEL spectra of the devices, which is attributed to the electrontransition between defect levels of ZnO, V+

O → V2−Zn . A blue

emission originated from the transition of the conduction bandto V2−

Zn is enhanced for the CBP/ZnO structure. Our workshows a new way to use the mass-produced ZnO nanowiresthrough the fabrication of paper-like sheets and the simpleassembly of hybrid LEDs. The fabrication approach of NWfilm is applicable to a large variety of other semiconductingnanowires, which can be synthesized in large scale, to be usedto fabricate diverse devices such as LEDs, displays, solar cellsand sensors, etc.

Acknowledgment

This work was supported by the Ministry of Science andTechnology through the Nanoscopia Center of Excellence atAjou University, and the Korea Research Foundation (grantNo. KRF-2007-412-J04003).

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