Organic Electronics 11 (2010) 1555–1560

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Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Letter

Highly efficient blue organic light-emitting devices with indium-freetransparent anode on flexible substrates

Liang Wang, James S. Swensen, Evgueni Polikarpov, Dean W. Matson, Charles C. Bonham,Wendy Bennett, Daniel J. Gaspar, Asanga B. Padmaperuma *

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, United States

a r t i c l e i n f o

Article history:Received 5 April 2010Received in revised form 23 June 2010Accepted 25 June 2010Available online 6 July 2010

Keywords:Blue OLEDPhosphorescentTransparent conductive oxideFlexible substrate

1566-1199/$ - see front matter � 2010 Elsevier B.Vdoi:10.1016/j.orgel.2010.06.018

* Corresponding author.E-mail address: asanga.padmaperuma@pnl.gov (

a b s t r a c t

Indium-free transparent conducting oxides (TCOs) may provide a lower cost solution forthe transparent anode in large area displays and solid-state lighting. Low temperaturedeposition processes are essential for manufacturing compatible with flexible substratematerials. We report herein a near room temperature sputtering process for generatingan indium-free TCO coating on a flexible substrate. Specifically, we deposited gallium-doped zinc oxide (GZO) uniformly over a 1200 diameter area at nominally room temperatureon polyethylene terephthalate (PET), without any noticeable damage to the PET substrate.The GZO films exhibit excellent physical, optical and electrical properties: roughness�7 nm, transmittance >85% and resistivity �10�3 ohm cm. Phosphorescent blue organiclight-emitting devices (OLEDs) were fabricated on these substrates with comparable per-formance (16% external quantum efficiency and 33 lm/W power efficiency at 1 mA/cm2)to that of devices fabricated on glass with GZO or indium tin oxide (ITO) as the anodes.These results demonstrate the utility of using GZO instead of the higher cost materialITO on PET for flexible displays and solid-state lighting.

� 2010 Elsevier B.V. All rights reserved.

Organic light-emitting devices (OLEDs) based on smallmolecules and conjugated polymers have gained greatinterest due to their potential to provide low-cost andhigh-efficiency solutions for applications such as large areadisplays and solid-state lighting. Significant developmentefforts have focused on electronic circuitry and light-emit-ting devices on flexible substrates driven by increasingconsumer demand for inexpensive, light-weight and porta-ble devices [1,2]. Polymers such as polyethylene tere-phthalate (PET) and polyethylene naphthalate (PEN) arepromising materials for use as flexible substrates, havingmany advantageous properties including transparency,light weight, flexibility, chemical resistance and low coeffi-cients of thermal expansion [3]. In addition, their compat-ibility with low cost/high volume manufacturing methodssuch as solution-based processes and roll-to-roll coating

. All rights reserved.

A.B. Padmaperuma).

have the potential to increase throughput, reduce produc-tion costs, and ultimately allow for wide market imple-mentation of plastic-based optoelectronics devices.

The near exclusive use of indium tin oxide (ITO) as thetransparent conductive anode in OLEDs has been identifiedas one of the key challenges for broad implementation ofOLEDs in display and lighting applications. Negative char-acteristics of indium-based materials in OLED applicationsinclude the scarcity, and thus high cost of indium, issueswith spikes or asperities in the ITO film (i.e., high Z-max,even with low RMS roughness) as well as a tendency for in-dium to migrate into the organic layers of the OLED [4]. Anumber of alternative metal oxides and doped metal oxi-des have been evaluated for use as TCO materials in lieuof ITO with varying degrees of success [5,6]. Recently therehave been attempts to apply coatings of alternative TCOmaterials for use as electrodes on flexible substrates [7–11]. Implementation of effective indium-free TCOs for useon flexible substrates is a key developmental step in the

1556 L. Wang et al. / Organic Electronics 11 (2010) 1555–1560

application of flexible low-cost, large-area solid-statelighting. However, to the best of our knowledge, there havebeen no published demonstrations of a high-efficiencyblue phosphorescent OLED (the most challenging compo-nent for white OLED lighting applications) using an alter-native anode material on a flexible substrate.

In this letter, we report the use of a scalable coatingtechnique to deposit high-quality Ga-doped ZnO (GZO)films on flexible substrates at near room temperature.While we recognize that gallium is also a scarce, relativelyexpensive material, the total amount of gallium used in theGZO films is very small (5 at.%), whereas ITO contains>90 at.% indium. Hence, these GZO anodes may be a costeffective replacement for ITO. Using these GZO/PET sub-strates, we demonstrate blue phosphorescent OLEDs withhigh quantum and power efficiencies at a practical bright-ness level suitable for general lighting.

Radio frequency (RF) magnetron sputtering of GZOfilms was performed on glass and flexible PET substratessimultaneously in a 0.9 m box vacuum chamber having abase pressure of 4 � 10�7 Torr. A 15 cm diameter sputter-ing target containing 97.5% ZnO/2.5% Ga2O3 (at.%) with99.99% purity (Cerac Inc., Milwaukee, WI) was used asthe source. Substrates were mounted on one of three30.5 cm diameter stainless steel pallets suspended fromthe chamber ceiling which allows double planetary rota-tion to improve coating uniformity. The plane of rotationof the substrate holders was 16.5 cm above the sputteringcathode. Prior to coating, the substrates were cleanedin situ with an ion beam generated by a source mountedon the chamber floor. Sputtering parameters includinggas pressure and gas composition were evaluated for theproduction of high quality GZO films. 4% hydrogen in Arsputtering gas at a pressure of 1 to 3 mTorr was found tobe optimal for this system in this geometry. With theapplication of the double planetary rotation during deposi-tion, the average variation of film thickness over the30.5 cm pallet area was �4%. The glass transition temper-ature of PET (150 �C) limits the maximum processing tem-perature for PET substrates [3]. During deposition, nosupplementary heating was applied to the substrates andtheir temperature was up to 60 �C due to the energeticsputtering conditions. No damage was observed on PETsubstrates after deposition. Selected samples of GZO/PETfilms were annealed after deposition to investigate its ef-fect on GZO film roughness, conductivity, and transparency

Table 1Comparison of commercial ITO/glass to GZO films fabricated on glass and flexibleannealing temperature was 110 �C for both annealing environments. Key OLED pOLEDs built on these anode materials.

Fabrication condition Resistivity(ohm cm)

Carrierconcentration(cm�3)

Mobility(cm2/Vs)

Thick(Å)

ITO/glass 1.42 � 10�4 5.47 � 1021 8.05 1400GZO/glass (as deposited) 1.66 � 10�3 3.62 � 1021 1.04 1550GZO/PET (as deposited) 3.03 � 10�3 5.88 � 1021 0.351 1550GZO/PET

(annealed in 4% H2/Ar)2.20 � 10�3 5.59 � 1021 0.508 1550

GZO/PET(annealed in low vacuum)

2.19 � 10�3 5.73 � 1021 0.498 1550

as compared to the as-deposited films. Specifically(Table 1), annealing was performed at 110 �C under twodifferent conditions: (a) 10 h in an air ambient at reducedpressure (253 Torr) and (b) 150 min in the atmosphere of4% H2/Ar (4% H2/Ar flux at 25 standard cm3/minute (sccm)to maintain the system pressure at 1.3 mTorr on top of thehigh vacuum background of 4 � 10�7 Torr). After theseannealing processes no damage was noticeable on PET sub-strates. As seen in Table 1, annealing improves the conduc-tivity of the GZO film on PET mainly by enhancement ofmobility. However, this improvement did not lead to anobservable difference between the performances of OLEDsbuilt on the annealed GZO/PET and those built on the as-deposited GZO/PET.

GZO and ITO substrates not subject to UV ozone treat-ment were characterized using a series of methods. Thecommercial ITO/glass substrates were purchased from Col-orado Concept Coatings LLC, having a 1400 Å thickness andnominal sheet resistance of 15 X/h for the ITO layer. Themorphology of the as-deposited GZO films on PET sub-strates was characterized by atomic force microscope(See supporting information in the supplemental), with astatistical grain height of 7.4 nm and a Zmax of 18.5 nm,comparable to commercial ITO on glass (grain height of4.5 nm and a Zmax of 13.8 nm). The electrical propertiesof these materials without UV ozone treatment were deter-mined using conductive atomic force microscope (c-AFM),Hall measurement and four-point probe measurements.The overall scale of c-AFM images in Fig. 1 shows thatthe as-deposited GZO film on PET conducts less current un-der the same bias (500 mV) compared to the commercialITO on glass used as a control sample. This is explainedby the difference in texture seen in Fig. 1, which suggeststhat compared to the control sample, the GZO film onPET has more uniformly distributed conductive domains,but those domains are much smaller in size. Carrier mobil-ities were measured with Hall measurement at room tem-perature using the Van der Pauw technique on acommercial Hall system from MMR Technologies. Thethickness of the films was measured separately with a stepprofilometer (Tencor Instruments, Alpha Step 200 model).It was found that the carrier mobilities in the GZO filmswere one order of magnitude lower than that in commer-cial ITOs (see Table 1). The higher grain boundary densityin the GZO films, and thus lower mobility due to increaseddisorder at grain boundaries, translates into higher resis-

PET substrates with different post-deposition processing environments. Theerformance metrics are also reported at a current density of 1 mA/cm2 for

ness Opticaltransmittanceat 475 nm

OLEDoperatingvoltage (V)

EQE(%)

gp

(lm W�1)CIEcoordinates

89.2% 4.35 ± 0.03 16.7 ± 0.5 32.1 ± 0.9 (0.15, 0.38)88.0% 4.05 ± 0.02 15.2 ± 0.1 31.6 ± 0.3 (0.14, 0.39)85.2% 4.21 ± 0.02 16.6 ± 0.2 33.1 ± 0.3 (0.14, 0.38)86.5% 4.06 ± 0.02 16.3 ± 0.3 33.7 ± 0.4 (0.13, 0.35)

86.3% 3.99 ± 0.01 16.0 ± 0.6 33.7 ± 1.2 (0.14, 0.37)

Fig. 1. Conductive AFM (c-AFM) images of films of ITO on glass (left) and GZO on PET (right), where the ITO/glass was obtained commercially and the GZOfilm was sputtered onto PET in 4% H2/Ar at low temperature without annealing. Images were taken under a DC bias of 500 mV. No UV ozone treatment wasperformed prior to the c-AFM experiment.

L. Wang et al. / Organic Electronics 11 (2010) 1555–1560 1557

tivity. This was confirmed by four-point probe measure-ments showing an order of magnitude higher resistivityfor the GZO films than that of the ITO deposited on glass(Table 1).

The optical properties of the GZO films on PET wereexamined using UV–vis-NIR spectrometry and comparedto the corresponding properties of ITO and GZO on glass.As seen in Fig. 2, the GZO films deposited on PET and glass,including the samples annealed under different conditions,all exhibited transmittance spectra close to that of thecommercial ITO on glass. The transmittance curves forGZO on PET mainly reflect the optical properties of thedeposited GZO film as is evident when comparing theGZO/PET transmittance spectra to the GZO/glass transmit-tance spectrum. However, the transmittance curves of all

Fig. 2. Optical transmittance spectra for commercial ITO/glass, neat PET, GZO/glaannealed under low vacuum (253 Torr). The inset – electroluminescent spectraFIrpic/500 Å PO15/10 Å LiF/1000 Å Al. The oscillations in the spectra of GZO/PETsupplementary information).

the GZO films on PET featured small oscillations as a func-tion of wavelength showing larger periods of oscillation atlonger wavelengths. This behavior can be attributed to thehard coating on the other side (opposite to the side withGZO film) of the GZO/PET substrates (PET with hardcoating on one side were purchased from Tekra Inc.) whichcaused a noticeable interference effect. A detailed discus-sion of this phenomenon can be found in the supplementalinformation.

We utilized GZO films deposited on both glass and PETsubstrates to fabricate phosphorescent blue OLEDs withthe device architecture: TCO/350 Å TAPC/150 Å HM-A1:5%FIrpic/500 Å PO15/10 Å LiF/1000 Å Al. Here the 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) serves asthe hole-transport layer (HTL) [12]. The emissive layer

ss, GZO/PET as deposited, GZO/PET annealed under 4%H2/Ar, and GZO/PETfor OLEDs with the structure of TCO anode/350 Å TAPC/150 Å HM-A1:5%films are due to an interference effect involving the PET substrate (see the

1558 L. Wang et al. / Organic Electronics 11 (2010) 1555–1560

(EML) is composed of the host material 4-(diphenylphos-phoryl)-N,N-diphenylaniline (HM-A1) [13] dopedwith phosphorescent emitter iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C20]picolinate (FIrpic). Thehole-blocking/electron-transporting layer (ETL) is 2,8-bis(diphenylphosphoryl)dibenzothiophene (PO15) [14].A thin lithium fluoride (LiF) layer facilitates electron injec-tion from the aluminum (Al) cathode. GZO/PET substrateswere cleaned in three steps: in a dilute Tergitol solution,deionized water, and 2-propanol with sonication. As con-trols, TCO films (GZO and ITO) on glass substrates werecleaned by sonication in a sequential series of solvents: a di-lute Tergitol solution, deionized water, trichloroethylene,acetone, and 2-propanol. These substrates were then driedwith flowing nitrogen. As a final step before device fabrica-tion, all the glass and PET substrates were treated with UVozone (UVO-Cleaner, Jelight Company, Inc.) at 15 mW/cm2

for 15 min. Organic layers were sequentially deposited ontothe substrates by thermal sublimation from tantalum boatsin a high vacuum chamber with base pressure below3 � 10�7 Torr. LiF/Al cathodes were defined by thermaldeposition through a shadow mask with 1 mm diameter cir-cular openings. Quartz crystal oscillators were used to mon-itor the thicknesses of the films, which were calibrated exsitu using ellipsometry or profilometry.

Key OLED performance metrics at a current density of1 mA/cm2 are summarized in Table 1. All the OLEDsemploying a GZO anode, whether on a rigid or flexible sub-strate, manifested performance comparable to OLEDs pro-duced using an ITO anode. OLEDs built on PET substratesexhibit a low operating voltage (�4 V), high external quan-tum efficiency (EQE) (16%), and high power efficiency (gp)(33 lm/W). Fig. 3a is a typical plot of EQE and gp as a func-tion of luminance. It can be seen throughout the practicalbrightness range for general lighting (1000–4000 cd/m2)that the OLEDs fabricated using GZO anode on PET per-formed nearly the same as the OLEDs fabricated usingcommercial ITO anode on glass. The voltage dependenceof current density and luminescence of these devices areshown in Fig. 3b. The OLED devices built on GZO/PET andGZO/glass show charge transport behavior similar to ITO/glass devices, clearly demonstrating a transition from theintrinsic bulk transport regime (at low voltages, freecharges are thermally generated in organic layers) to thetrap filling transport regime (at high voltages, free chargesinjected from electrodes are filling traps in organic layers)[15]. Ohmic type J–V curve is observed in the intrinsic bulktransport regime for all three types of devices, indicatingthat none of the three TCO anodes is limiting charge trans-port by hole injection. GZO/glass devices exhibit a higherleakage current at low voltages as opposed to GZO/PETand ITO/glass devices, which is possibly attributable to agreater asperity on the surface of GZO/glass substrates.All three types of devices show similar turn-on voltagearound 3 V. This low turn-on voltage contributes to theachieved high power efficiency as shown in Table 1. It isnoteworthy that in yielding high-performance devices,the GZO/PET anode material fully processed at near roomtemperature performs as well as those films with post-deposition annealing (Table 1), thus indicating that theannealing step may be eliminated. This would allow a

reduction in processing cost and time, and accommodatethe low temperature requirements for certain devices suchas top-emitting OLEDs. Additionally, as shown in the insetof Fig. 2, the electroluminescence (EL) spectra for all of thedevices produced on various substrates appear very simi-lar. Small oscillations can be observed in the EL spectrafor OLEDs fabricated on GZO/PET substrates due to theinterference effect previously mentioned. Consistentlywith the comparable EL spectra, similar Commission inter-nationale de l’éclairage (CIE) coordinates were obtained(shown in Table 1) from operating OLEDs on the variousTCO substrates. All these facts demonstrate that GZO/PETis a potential substitute for ITO/glass.

Based on the transmittance curves (Fig. 2), we calcu-lated the average refractive index (n) and extinction coeffi-cient (k) over visible spectrum for the materials used inthis work: n(ITO) = 1.683, k(ITO) = 0.033; n(GZO) = 1.834,k(GZO) = 0.064. Since the refractive index of GZO is higherthan ITO, more light will be trapped in the GZO layer of theOLED device and thus potentially reduce outcoupling effi-ciency so that the GZO/glass devices exhibit a slightly low-er EQE than ITO/glass devices (Table 1). We also note thatin spite of similar transmittance around the FIrpic emissionpeaks, GZO/PET substrates give slightly higher EQE thanGZO/glass substrates for the same OLED device stack. Weattribute this increase of EQE to the enhanced light outcou-pling through a combination of two factors. First, morelight escapes the GZO layer into the PET due to its higherrefractive index (1.58) compared to glass (1.50) [16]. Sec-ond, due to total internal reflection above the critical angleat the air-substrate interface, only a fraction of the lightcan escape the substrate [17] and get collected by thephoto detector. Given the certain experimental setting offinite distance from a sample’s bottom to the surface ofphoto detector, the smaller thickness of the PET substrate(0.19 mm) compared to the glass substrate (1 mm) trans-lates into shorter distance from light emission to the pho-todetector and thereby allows for less loss in the lightcollection. A detailed optical model is needed to quantita-tively analyze the outcoupling factors in comparing de-vices built on GZO/PET and GZO/glass substrates.Together, these two effects are responsible for the slightlyhigher EQE of OLEDs based on GZO/PET versus GZO/glass.Interestingly, the higher resistivity of the GZO versus ITOand the as-deposited versus annealed GZO as the anodedoes not affect the OLED performance for these small pixeldevices (diameter = 1 mm, 7.854 � 10�3 cm2). For theOLEDs tested in this work, the lateral conductance (GZOfilm sheet resistance �100 X/h) across the anode is farhigher than that of vertical transport through the devicestack which thereby dominates the overall conductanceof an OLED. The same organic layers deposited on differentTCO anode materials (GZO or ITO) exhibited similar chargetransport behavior (as seen from J–V characteristics inFig. 3b) and operating voltage (shown in Table 1), indicat-ing that hole injection from different TCO anodes into thesame HTL is not a limiting factor for charge transport inthese devices. We attribute the similar injection propertiesfor the different TCO anodes (GZO or ITO) to the modifica-tion of anode surface work function through UV ozonetreatment [18]. When the pixel size is small, the higher

Fig. 3. (a) The luminescence dependence of EQE and power efficiency and (b) the voltage dependence of current density and luminescence for OLEDs withthe structure: TCO anode/350 Å TAPC/150 Å HM-A1:5% FIrpic/500 Å PO15/10 Å LiF/1000 Å Al. Here ITO/glass was commercially purchased and GZO/glassand GZO/PET were as deposited with no post annealing. All the TCO anode substrates were treated with UV ozone before organic deposition.

L. Wang et al. / Organic Electronics 11 (2010) 1555–1560 1559

resistivity of the GZO films will not significantly influencedevice performance. For applications where larger pixelsizes are involved (>1 cm2) such as lighting panels for so-lid-state lighting purposes, the resistivity of the GZO filmsneeds to be decreased by at least an order of magnitude tobe at levels near that of commercial ITO in order to operatethe large-area devices at low and uniform voltages andavoid Joule heating. As is seen in Table 1, the annealingprocess for GZO/PET substrates served to decrease theresistivity of the GZO films by 28%. Unfortunately, this isstill an order of magnitude higher than the resistivity ofcommercial ITOs. Hence different strategies need to be ap-plied such as inserting a thin, nearly transparent silverlayer into the GZO film [19] or placing metal grids on the

GZO surface to help distribute electrical current better[20].

In summary, indium-free GZO films have been uni-formly deposited on flexible substrates over a large areausing a low temperature sputtering process. Highly effi-cient phosphorescent blue OLED devices were built onthe GZO films, which functioned as the anode. The perfor-mance of these devices compared favorably with that ofthe same device stack built on rigid (glass) substrates usinga commercial ITO anode. This result demonstrates that in-dium-free TCOs on flexible substrates can serve as a low-cost and flexible substitute for OLED device manufacturing.Another interesting application of sputtered TCO films isthe top contact electrode in a top-emitting, inverted OLED.

1560 L. Wang et al. / Organic Electronics 11 (2010) 1555–1560

Based on our results, the as-deposited GZO, even without apost-deposition anneal, should be able to function as a topelectrode in a top emitting inverted OLED for small areapixels. The greater challenge in using the GZO depositionprocess to form a top electrode in this device architecturewould be to modify the process in order to reduce and/oreliminate damage to the underlying organic layer duringthe GZO deposition [21].

Acknowledgements

The authors thank Mark Gross for helpful discussionson the optical measurement of substrates. This projectwas funded by the Solid-State Lighting Program withinthe Building Technologies Program (BT; managed by theNational Energy Technology Laboratory/NETL) of the En-ergy Efficiency and Renewable Energy Division of the USDepartment of Energy, award No. M6642866. A portionof the research described in this paper was performed inthe Environmental Molecular Sciences Laboratory, a na-tional scientific user facility sponsored by the Departmentof Energy’s Office of Biological and Environmental Researchand located at Pacific Northwest National Laboratory. Paci-fic Northwest National Laboratory (PNNL) is operated byBattelle Memorial Institute for the US Department of En-ergy (DOE) under Contract DE-AC06-76RLO 1830.

Appendix A. Supplementary material

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.orgel.2010.06.018.

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