JOURNAL OF APPLIED PHYSICS 107, 043103 共2010兲
Highly efficient blue organic light emitting device using indium-free transparent anode Ga:ZnO with scalability for large area coating Liang Wang,1 Dean W. Matson,1 Evgueni Polikarpov,1 James S. Swensen,1 Charles C. Bonham,1 Lelia Cosimbescu,1 Joseph J. Berry,2 David S. Ginley,2 Daniel J. Gaspar,1 and Asanga B. Padmaperuma1,a兲 1
Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA 2 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado 80401, USA
共Received 28 September 2009; accepted 30 November 2009; published online 19 February 2010兲 Organic light emitting devices have been achieved with an indium-free transparent anode, Ga doped ZnO 共GZO兲. A large area coating technique was used 共RF magnetron sputtering兲 to deposit the GZO films onto glass. The respective organic light emitting devices exhibited an operational voltage of 3.7 V, an external quantum efficiency of 17%, and a power efficiency of 39 lm/W at a current density of 1 mA/ cm2. These parameters are well within acceptable standards for blue OLEDs to generate a white light with high enough brightness for general lighting applications. It is expected that high-efficiency, long-lifetime, large area, and cost-effective white OLEDs can be made with these indium-free anode materials. © 2010 American Institute of Physics. 关doi:10.1063/1.3282526兴 I. INTRODUCTION
The availability of economically produced and environmentally stable transparent conductive oxide 共TCO兲 coatings is critical for the development of a variety of electronic devices requiring transparent electrodes. Such devices include liquid crystal displays organic light emitting devices 共OLEDs兲,1,2 solar cells,3,4 low-emissivity and electrochromic windows,5,6 and electrically heated windows.7,8 The materials fulfilling these requirements are usually wide band gap inorganic TCOs such as indium tin oxide and fluorine doped tin oxide.9 Tin-doped indium oxide 共ITO兲 has traditionally been used for optoelectronic TCO applications because of its low resistivity, high work function, and transparency over 80% in the visible. Due to the high cost of indium, problems with processing ITO films, and the tendency of indium to migrate into the device, there has been an increased research interest in developing indium-free TCO materials. A number of alternative metal oxides and doped oxides have been evaluated as TCO materials with varying degrees of success.10,11 Among these alternatives to ITO, gallium-doped zinc oxide2,12 共GZO兲 and aluminum-doped zinc oxide13,14 共AZO兲 have drawn particular attention. These materials have demonstrated resistivities and transparencies approaching those of the best ITO, along with low toxicity and much lower materials cost 共1/5 to 1/10 that of ITO兲. Although AZO is attractive as a TCO electrode material, GZO provides the advantage of having a greater resistance to oxidation as a result of gallium’s greater electronegativity compared to aluminum and a broader range of doping concentrations.2,15,16 Gallium doped 共at low concentrations兲 in ZnO acts as an electron donor with a shallow donor level located in the band gap. This leads to GZO films with room temperature conductivities better than 10−4 ⍀ cm and optical transparencies higher than 80% in visible spectrum.17–19 It is noteworthy a兲
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that the total amount of gallium 共with a similar natural abundance to indium兲 is very small in GZO, whereas ITO contains 90% indium. This fact makes GZO an economic alternative to ITO. Various deposition techniques may be used to produce GZO thin films suitable for TCO applications. Included among these are pulsed laser deposition 共PLD兲, RF and DC magnetron sputtering, and vacuum arc plasma evaporation. While suitable for the demonstration of GZO as a viable TCO material, some of these methods are difficult to scale up to a commercial process. PLD has been used to demonstrate proof of concept films with very high performance2 but it is difficult to produce these results over large areas. The degree of variation across the films produced by small-scale methods makes it difficult to produce uniformly coated substrates more than a few cm2 in size.20 Therefore a deposition process capable of generating uniform GZO films having high electrical and optical quality over a large area is a key requirement for scale-up from laboratory research to manufacturing.21 One of the most viable potential methods for scale-up of ZnO-based TCOs is sputtering, where coating uniformity over large areas can be controlled by cathode size and shape as well as by application of substrate motion relative to the sputtering source. To date, scale-up efforts have focused on AZO. The quality of the scaled-up AZO films is well below that of commercially available ITO. Much less work has been done on GZO scale-up. So far, only small diameter 共ⱕ10 cm兲 targets and short throw distances 共ⱕ10 cm兲 to the substrates. We report here the scaled-up deposition of uniform GZO films onto substrates larger than 100 cm2 using RF magnetron sputtering. Films have been obtained with conductivities of 共0.5– 2.1兲 ⫻ 10−3 ⍀ cm and transparencies from 80% to 90% in the visible. Further increases in substrate size should be possible as we have demonstrated ⫾1% optical thickness uniformity on 1 m2 glass substrates with a variety of oxide, nitride, and metal coatings using DC magnetron sputtering capabilities.22
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This method, upon successful demonstration for TCO coatings, offers a key route for market penetration of indium-free TCOs for applications such as large area displays, general lighting, and solar panels. A key opportunity for GZO TCOs is to serve as the anode in an OLED for either display or lighting purposes. Current efforts are underway around the world to develop high-efficiency OLEDs to play a significant role in improving energy efficiency. White OLEDs have been recognized for their potential to provide low-cost, high-efficiency solidstate lighting which will replace incandescent and fluorescent technologies.23,24 Currently, the efficiency of white OLEDs at the research and development stage is comparable to conventional halogen lamps. For instance, in 2007, Konica-Minolta25 and Novaled AG26 reported white OLED power efficiencies of 64 lm W−1 with 10 000 h lifetime and 38 lm W−1 with 100 000 h lifetime, respectively, both at initial brightness of 1000 cd m−2. These results included the use of light outcoupling techniques to improve efficiency. Since the luminescence of a single organic molecule does not span the entire visible spectrum, white light emission has to be achieved by modifying the spectral characteristics of OLEDs, either through downconversion of blue OLEDs by phosphors or through color mixing by multiple emitters.27 Both of these methods require highly efficient and stable blue emission—a key challenge in the development of white OLEDs. It is therefore clear that TCOs meant to be used in a white OLED must exhibit excellent transparency throughout the entire visible spectrum. Although GZO deposited by a PLD process has recently been demonstrated as the anode in a green OLED with low efficiency,2 there have been few reports of high-efficiency OLEDs produced using indium-free TCOs as anodes. This is particularly evident in the case of blue OLEDs, which are required for the production of large area white light panels. In this paper, we report the fabrication and characterization of highly efficient OLED devices with blue emission originating from the phosphor iridium共III兲 bis共4,6共difluorophenyl兲-pyridinato-N , C2⬘兲 picolinate 共FIrpic兲 fabricated on GZO films that were sputtered uniformly over a large area 共30.5 cm diameter兲. These OLEDs achieved an average performance very close to that of control samples produced on ITO-coated glass. Best-case operational parameters of these OLED devices included an operating voltage of 3.7 V, an external quantum efficiency 共EQE兲 of 17%, a power efficiency 共p兲 of 39 lm W−1, and a luminance of 459 cd m⫺2 at a current density of 1 mA cm−2. To the best of our knowledge, this is the first example of high power efficiency blue OLEDs fabricated using an indium-free GZO anode at brightness levels suitable for general lighting. Moreover, this performance was achieved without outcoupling enhancement; it is expected that with proper light outcoupling techniques, blue OLEDs produced on sputtered GZO anodes can be made ⬎80% more efficient.28–31 Furthermore, due to the higher sensitivity of the eye to white light as compared to blue light, 1.8 times improvement in the power efficiency is seen when white light is produced using downconversion of blue OLEDs.32 We expect that these enhancements should permit the demonstration of a white OLED with
⬎100 lm W−1 at 1000 cd m−2. Thus, these results demonstrate the potential for white OLEDs using a GZO anode to approach power efficiencies comparable to those of industrial white OLEDs produced using ITO anodes. II. EXPERIMENTAL
There are characteristics beyond just conductivity and transparency that determine whether a TCO film can be used in an OLED. These include surface roughness, stability, work function, and surface electronic structure. The Ga doping level was selected after evaluation of film characteristics using a combinatorial PLD study.2,33,34 In general, dopant levels on the order of 2%–5% Ga2O3 in ZnO have been shown to be optimal for producing low resistivity GZO coatings. Note that this is a broader range than AZO which can begin to show Al segregation above 2% Al.21 Several experimental parameters, both during sputtering and in postdeposition treatments, have been reported in the literature to affect the quality and uniformity of the resulting GZO films. These include substrate temperature during deposition,33–36 postdeposition annealing conditions,37–40 argon sputtering gas pressure,41,42 cathode power and/or deposition rate,42 oxygen partial pressure,35 hydrogen partial pressure,36 cathode field strength,24 background water partial pressure,35 sputtering gas species,43 and incidence angle of sputtered material to the substrate.34,44 In this work, we report on our exploration of a number of these parameters and their influence on the GZO characteristics important for OLED performance. We report various GZO deposition conditions that we explored to optimize film quality and ultimately device performance of OLEDs based on these materials. Our typical sputtering setup consisted of glass substrates 共2.5⫻ 7.5 cm2兲 mounted on three pallets of 30.5 cm diameter, operated with double planetary rotation to enhance thin film uniformity. We obtained our best results using RF magnetron sputtering from a 15 cm diameter target with a 16.5 cm throw distance to the substrates. Among tens of sputtered GZO samples with thickness ranging from 300 to 5000 Å, the average variation in film thickness over the 30.5 cm pallet area was 4.4%. For a given thickness, there was no appreciable variation in the optical or electrical properties over the entire area of our GZO films. A. GZO substrate preparation
RF magnetron sputtering of GZO films was performed in a 0.9 m box vacuum chamber with a base pressure of 4 ⫻ 10−7 Torr. A 15 cm diameter sputtering target having a composition of 97.5% ZnO/2.5% Ga2O3 共at. %兲 with 99.99% purity 共Cerac, Inc., Milwaukee, WI兲 was used as the source. Glass substrates were mounted on one of three 30.5 cm diameter stainless steel pallets suspended from the chamber ceiling. Double planetary rotation was used during deposition to enhance the uniformity of thin film deposition. The plane of rotation of the substrate holders was 16.5 cm above the sputtering cathode. The substrates were cleaned in situ with ion beam sputtering and were not actively heated during deposition 共other than minor heating of 45– 60 ° C from the sputtering source兲. GZO films were prepared by sputtering in
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an atmosphere of Ar 共inert ambient兲 or 4% H2 in Ar 共reducing ambient兲 at a pressure of 3 mTorr. Annealing was performed in rough vacuum 共0.1 Torr兲 or high vacuum of 3.5 ⫻ 10−5 Torr at temperatures ranging from 200 to 300 ° C for 1–10 h. Both as-deposited and annealed GZO films exhibit strong adhesion to the glass substrate, with no delamination noted during tape-peeling tests. B. GZO substrate characterization
X-ray diffraction 共XRD兲 analysis was performed in -2 scan mode at room temperature on a Philips Xpert x-ray diffractometer with a Cu K␣ source at 1.54 Å. The XRD spectra show that the predominant 共002兲 peak shifts slightly to a smaller value of 2 after annealing, indicating a slight lattice expansion in the 共002兲 direction, probably due to the full inclusion of gallium into the ZnO lattice. It is noteworthy that upon annealing, the 共002兲 peak is also slightly narrowed due to an increase in crystallite size of the Ga-doped ZnO films by the heat treatment. A combination of resistivity and Hall measurements was performed with a commercial Hall system from MMR Technologies using the van der Pauw technique. The measurement was performed at room temperature by varying the magnetic field from 2000 to 4000 G in a direction perpendicular to the sample plane. Contacts were made by mechanically pressing four probes onto the sample surface. During the entire measurement, the temperature drift of the sample due to Joule heating was within 2 ° C as measured by the instrument in situ sensor, with a negligible effect on the resistivity and Hall measurements. The sheet resistance was measured in a van der Pauw configuration and calculated by the system’s built-in program applying the van der Pauw equation. The sheet carrier density was determined by measuring the Hall voltage under a certain magnetic field, which in combination with the measured sheet resistance gives the carrier mobility in the film. For the purpose of calculating resistivity and charge density, the thickness of the film was measured separately with a step profilometer scanning over a line of 400 m with a stylus force of 18 mg 共Tencor Instruments, Alpha Step 200 model兲. C. OLED fabrication and measurement
TCO substrates 共GZO films or reference commercial ITO, both on glass兲 were cleaned by sonication in a sequential series of solvents, including a dilute Tergitol solution, de-ionized water, trichloroethane, acetone, and 2-propanol. The substrates were then dried with flowing nitrogen. As a final cleaning step before device fabrication, the substrates were treated with UV ozone 共UVO-Cleaner, Jelight Co., Inc.兲 at 15 mW/ cm2 for 15 min. The substrates were then loaded into a nitrogen glove box 共⬍1 ppm H2O , ⬍ 1.5 ppm O2兲 coupled to a multichamber vacuum deposition system. Organic layers were sequentially deposited onto the TCO-coated substrates by thermal evaporation from tantalum boats in a high vacuum chamber with a base pressure below 3 ⫻ 10−7 Torr. Cathodes were defined by thermally depositing a 1 nm thick layer of LiF immediately followed by a 100 nm thick layer of Al through
a shadow mask with 1 mm diameter circular openings. Quartz crystal oscillators were used to monitor the thicknesses of the films, which were calibrated ex situ using ellipsometry. The deposited stack of organic and metal layers is depicted as TCO/300 Å 1,1-bis关共di-4tolylamino兲phenyl兴cyclohexane 共TAPC兲/50 Å 4 , 4⬘ , 4⬙tris共carbazol-9-yl兲triphenylamine 共TCTA兲/150 Å 5% FIrpic: 4-共diphenylphosphoryl兲-N , N-diphenylaniline 共HMA1兲/500 Å 2,8-bis共diphenylphosphoryl兲dibenzothiophene 共PO15兲/10 Å LiF/1000 Å Al. Here the TAPC and TCTA constitute the hole-transport layer 共HTL兲. The emissive layer 共EML兲 is composed of host HM-A1 doped with a blue phosphor FIrpic. The hole blocking/electron-transporting layer is comprised of PO15. Optical and electrical characteristics of the devices were determined in air, with electrical contact made via a tungsten probe tip on the TCO anode and a 0.002 in. diameter gold wire directly probing the Al cathode. Current-voltage characteristics were measured with an Agilent Technologies 4155B semiconductor parameter analyzer. The light output was detected using a 1 cm2 Si photodetector placed behind the OLED, and the device brightness was directly measured using a Newport multifunction optical meter. No corrections were made for light wave guided in the organic thin films or the substrate. Electroluminescence 共EL兲 spectra were recorded with an Acton Research Corporation 共ARC兲 SpectrumMM charge coupled device detector on an ARC Spectra 150 dual grating monochromator. III. RESULTS AND DISCUSSION A. Microstructure study
Various deposition and annealing conditions were evaluated in an attempt to optimize the quality of the GZO films for OLED performance. The films were prepared by sputtering both in pure argon 共Ar; inert ambient兲 and 4% H2 in Ar 共reducing ambient兲 at a pressure of 3 mTorr. Different temperatures, atmospheres, and durations of annealing the sputtered GZO films were explored in order to investigate their influence on the film’s quality in terms of adhesion, conductivity, and transparency. Characterization of the films was performed using a series of techniques including XRD, scanning electron microscopy 共SEM兲, atomic force microscopy 共AFM兲, optical transmittance, and Hall measurement. Selected samples with typical characteristics were then chosen for further studies in a blue OLED structure, which led to a choice of an optimal GZO recipe. GZO substrates made with this recipe on different days yielded repeatable results for both film characteristics and OLED performance. As shown in Fig. 1, XRD data of GZO films prepared by RF magnetron sputtering in Ar gas before and after 1 h annealing at 200 ° C in rough vacuum showed no evidence of Ga2O3 segregation. This observation is consistent with other reports of PLDdeposited GZO films.2,12 This GZO film was also characterized with Kelvin probe force microscopy and exhibited a work function of 4.66 eV in agreement with data for the 1010 prism face of ZnO45 and close to that for ITO 共4.7 eV兲.46 SEM 关Figs. 2共a兲 and 2共b兲兴 of GZO films both before and after annealing indicates that the films are polycrystalline with fairly small grain sizes 共in the order of 50 nm兲.
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FIG. 1. 共Color online兲 XRD spectrum of the 共200兲 peak for a Ga 共2.5 at. %兲:ZnO film deposited on glass by RF magnetron sputtering in Ar before 共upper curve兲 and after annealing 共lower curve兲 at 200 ° C for 1 h.
Cross-section SEM 关Fig. 2共c兲兴 showed the thickness uniformity of the GZO film. AFM 关Fig. 2共d兲兴 showed that the film roughness was ⬃3 nm. The GZO film sputtered under 4% H2 / Ar and annealed in high vacuum for 3 h gives the lowest roughness ⬃1 nm 共data not shown兲. B. The optical and electrical properties of GZO substrates
The optical transmission was measured at room temperature in the wavelength range 400–750 nm for as-deposited and annealed GZO samples using a Perkin Elmer Lambda 12 ultraviolet-visible spectrometer 共Fig. 3兲. The annealing processes for the respective GZO films shown in Fig. 3 were conducted at 3.5⫻ 10−5 Torr and 300 ° C for 3 h. The as-
FIG. 2. 共Color online兲 SEM and AFM demonstrating the microstructure of a typical GZO film prepared in the same way as the film in Fig. 1. The grain structure is seen in the SE micrographs of the film 共a兲 as deposited and 共b兲 after annealing. SEM cross section of the film after annealing is shown in 共c兲, while 共d兲 shows an AFM image after annealing.
J. Appl. Phys. 107, 043103 共2010兲
FIG. 3. 共Color online兲 The optical transmittance spectra recorded for GZO samples deposited at room temperature under Ar or 4% H2 / Ar and annealed in high vacuum 共3.5⫻ 10−5 Torr兲 at 300 ° C for different lengths of time. 共Inset: A photo taken to show the transparency of a GZO film deposited on glass.兲
deposited GZO films sputtered in 4% H2 in Ar 共reducing ambient兲 exhibited transmittance 共⬎80%兲 similar to that of a commercial ITO reference sample 共supplied by Colorado Concept Coatings兲 within the entire visible and near infrared 共NIR兲 range. For the GZO films sputtered in a reducing ambient, the optical transmission was slightly improved after 3 h of annealing, whereas longer annealing times did not improve transmission further. In contrast, GZO films sputtered in Ar showed a significant improvement in transmission upon 3 h of annealing by an average of 20% in transmittance over the entire spectrum, achieving a level close to that of commercial ITO. We attribute these differences in transmission improvement to the varying amounts of excess oxygen in the TCO films under different deposition ambient and annealing treatments.47 We characterized the charge transport properties of a series of GZO films with a commercial Hall measurement system using the van der Pauw technique at room temperature prior to OLED device fabrication. The results indicate that these films are predominantly electron transporting with a high carrier concentration of ⬎1021 cm−3, consistent with values previously reported for GZO films.2,48 Table I summarizes the carrier concentration, mobility, and resistivity of GZO films prepared under various conditions, along with a commercial ITO film 共15 ⍀ / 䊐兲 as a reference. The mobile electron concentration in GZO films is similar to the reference ITO film, while the electron transport mobility is significantly lower, resulting in higher resistivity in our GZO films. GZO films deposited in 4% H2 / Ar exhibited higher carrier concentration and higher mobility with much less improvement, resulting from postdeposition annealing than the films sputtered in Ar. It is well known that electrical conduction in pure ZnO is dominated by electrons generated from the oxygen vacancies 共VO兲 and zinc interstitial atoms 共Zni兲.49 Ga is a substitutional dopant in the ZnO lattice with a donor depth of 30 meV below the bottom of ZnO conduction band.21,47,50 When hydrogen is present during the deposition of GZO, it reacts with excess oxygen and releases one free
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TABLE I. Summary of electrical properties for GZO films prepared under different deposition gases and with different annealing durations, along with corresponding blue OLED performance at a luminance of 800 cd m−2. Commercial ITO is used as a reference. 共OLED device structure is TCO/300 Å TAPC/50 Å TCTA/150 Å 5% FIrpic: HM-A1/500 Å PO15/10 Å LiF/1000 Å Al.兲
Fabrication condition
Resistivity 共⍀ cm兲
300 K mobility 共cm2 V−1 s−1兲
Carrier concentration 共cm−3兲
Optical transmittance at 475 nm
OLED operating voltage 共V兲
EQE 共%兲
共lm W−1兲
ITO GZO, Ar sputter, GZO, Ar sputter, GZO, 4% H2 / Ar GZO, 4% H2 / Ar GZO, 4% H2 / Ar
1.42⫻ 10−4 2.05⫻ 10−3 4.77⫻ 10−4 1.41⫻ 10−3 1.14⫻ 10−3 2.09⫻ 10−3
16.97 1.79 4.45 2.18 3.02 3.05
2.58⫻ 1021 1.70⫻ 1021 2.94⫻ 1021 2.03⫻ 1021 1.81⫻ 1021 9.79⫻ 1020
89.2% 55.9% 77.6% 89.1% 91.3% 90.8%
4.35 5.97 5.04 4.62 4.51 4.57
15.7 8.2 12.9 14.1 15.7 14.1
30.2 11.5 20.4 25.6 29.2 25.9
unannealed 3 h annealeda sputter, unannealed sputter, 3 h annealeda sputter, 10 h annealeda
p
CIE coordinates 共0.13, 共0.13, 共0.14, 共0.14, 共0.13, 共0.13,
0.35兲 0.30兲 0.30兲 0.36兲 0.35兲 0.36兲
a
Annealed under a vacuum of 3.5⫻ 10−5 Torr at 300 ° C.
plane charge transport, whereas in OLEDs the out-of-plane charge transport is important. In contrast, the optical transparency of our GZO substrates exhibits a strong influence on the OLED performance at the FIrpic emission wavelength, as seen in Table I. The GZO samples deposited in 4% H2 / Ar display a much higher transparency in the blue-green range than those sputtered in Ar, which leads to higher EQE and thereby higher power efficiency when incorporated in a blue OLED structure. Differences in the transmission spectrum of various GZO substrates lead to differences in the measured EL spectrum, as shown in Table I by the calculated International Commission on Illumination 共CIE兲 coordinate of color space chromaticity. Compared to those sputtered in Ar, blue OLEDs made on GZO substrates sputtered in 4% H2 / Ar show CIE coordinates fairly close to those of ITO. Specifically, GZO exposed to a 3 h anneal in high vacuum yields CIE coordinates exactly the same as those of ITO control. Furthermore, as shown in Table I, postdeposition annealing in high vacuum for 3 h improves the optical transparency of the GZO substrates sputtered in 4% H2 / Ar while prolonged annealing 共10 h兲 leads to no further improvement. This observation might be attributed to the removal of defects 共acting as color centers兲 from the film during the annealing process. Given their excellent optical transmission similar to that of the commercial ITO reference sample along the entire visible-NIR range, these GZO substrates should be suitable for use as the transparent anode in a white OLED for general lighting. We selected the best GZO substrates 共deposited in a reducing ambient and annealed for 3 h兲 for further OLED studies using several different device structures incorporating different EMLs and HILs.
electron which contributes to a higher electron density in the film.47 Similarly, annealing in high vacuum will remove excess oxygen, yielding additional free electrons. These two processes also improve the mobility within the films by decreasing electron scattering by excess oxygen. As seen in Table I, in general the resistivity of our GZO films is significantly higher than that of the reference ITO film, which is mainly due to the poor mobility in these GZO films rather than the carrier density 共comparable to that in the reference ITO兲. The film microstructure, as shown in Figs. 2共a兲, 2共b兲, and 2共d兲, contributes to the low mobility in the GZO films. The very small grains 共⬃50 nm兲 and resulting large number of grain boundaries in the sputtered GZO films lead to low mobility due to increased disorder at grain boundaries. However, the lower mobility in the GZO anode does not significantly affect the OLED performance because of the small size of the OLEDs tested in this work. The lateral conductance 共the sheet resistance of our GZO films is ⬃50 ⍀ / 䊐兲 across the anode is far greater than that of the vertical transport through the OLED stack. The charge transport bottleneck in an OLED stack is due to low charge mobility in the organic layers as well as charge buildup at layer interfaces. The work function of the anode 共as previously mentioned, the work function of our GZO films is very close to that of commercial ITO兲 along with the HOMO of the HTL determines the hole injection barrier. This barrier for hole injection into the OLED can also play a determining role in the magnitude of the operation voltage, as seen from Table II for devices with different hole injection layers 共HILs兲. Another reason could be that the mobility/ conductivity of the GZO film alone measured by Hall measurements is not enough to fully explain the role of GZO in blue OLEDs, as these measurements characterize the in-
TABLE II. Summary of OLED performance at 800 cd m−2, using ITO and GZO deposited in 4% H2 / Ar as anode, with different HILs and EMLs/hosts. Values are the average of duplicate devices. 共OLED structure: TCO/HIL/300 Å TAPC/50 Å TCTA/150 Å FIrpic: host/500 Å PO15/10 Å LiF/1000 Å Al.兲 TCO HIL Host Operating voltage 共V兲 EQE 共%兲 p 共lm/ W1兲
ITO
GZO
ITO
CuPC
GZO
ITO
HAT-CN
CuPC
mCP 5.3 17.0 27.0
5.0 14.8 24.0
4.2 17.5 34.8
4.1 16.4 33.4
GZO
5.5 15.0 23.0
ITO
GZO
HAT-CN HM-A1 5.0 4.0 3.9 15.1 16.6 15.6 25.4 34.6 33.2
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FIG. 4. 共Color online兲 Electroluminescent spectra for OLEDs with a stacking structure of TCO/300 Å TAPC/50 Å TCTA/150 Å 5% FIrpic: HM-A1/ 500 Å PO15/10 Å LiF/1000 Å Al 共solid line, ITO substrate; dashed line, GZO substrate兲. 共Inset: Color space chromaticity diagram shows CIE coordinates for OLEDs listed in Tables I and II.兲
C. Highly efficient blue OLED using GZO substrate as anode
We focused our blue OLED fabrication and testing on novel ambipolar host and hole blocking molecules recently developed in our laboratories, which have been carefully designed to improve device charge balance.51 Our standard OLED device architecture was TCO/HTL/EML/ETL/ cathode. After fabrication, we measured the EL spectrum for each OLED. As shown in Fig. 4, the EL spectra are nearly identical for devices using ITO or GZO as an anode. Furthermore, the color deviation of the EL spectrum is fairly small for all the OLED devices depicted in Tables I and II, as seen from the color space chromaticity diagram in the inset of Fig. 4, except for OLEDs using GZO deposited in Ar. These OLEDs use different TCOs for the anode as well as varying device structures, but all use the same phosphorescent dopant, FIrpic. The optoelectronic performance of these OLEDs 共operated at 800 cd m−2 brightness兲 is summarized in Tables I and II 共each value represents the average of multiple devices兲. As indicated by the data in Table I, the best results were obtained for OLED devices fabricated on GZO sputtered in 4% H2 / Ar with a 3 h annealing period which exhibited excellent operating voltages and efficiencies. These results are comparable to those measured for the reference ITO samples and superior to those of GZO samples prepared under different deposition and annealing conditions. In order to explore further difference between our best GZO and commercial ITO performance, we fabricated OLEDs using two different hosts: HM-A1 or 3 , 5⬘-N , N⬘-dicarbazole-benzene 共mCP兲, both doped with FIrpic. Additionally a thin layer 共100 Å兲 of copper phthalocyanine 共CuPc兲 or 1,4,5,8,9,11hexaazatriphenylenehexacarbonitrile 共HAT-CN兲 was introduced as a HIL between the TCO and HTL. Devices using HAT-CN as a HIL exhibited improved charge injection due to the deep lowest unoccupied molecular orbital 共LUMO兲 level of HAT-CN and commensurate OLED performance improvements.52 The LUMO of HAT-CN is very close to its Fermi level and deeper 共⫺6.1 eV兲 than the highest occupied molecular orbital 共HOMO兲 of most HTL materials. This en-
FIG. 5. 共Color online兲 Device characterization for the OLEDs with the structure TCO/100 Å HAT-CN/300 Å TAPC/50 Å TCTA/150 Å 5% FIrpic: HM-A1/500 Å PO15/10 Å LiF/1000 Å Al. 共Inset: Device structure.兲 共a兲 Current density and luminance as a function of operating voltage. 共b兲 EQE and power efficiency as a function of luminance. TCOs compared are commercial ITO 共solid squares兲 and GZO 共solid circles兲. The GZO was deposited in 4% H2 / Ar and annealed in high vacuum at 300 ° C for 3 h. Solid symbols indicate the left y-axis, while hollow symbols indicate the right y-axis.
ergy level alignment allows the electrons from the HTL HOMO to be easily transferred to the HAT-CN LUMO. Meanwhile the HAT-CN LUMO is close to its Fermi level which aligns to the Fermi level of TCO and thereby the hole injection from TCO 共actually electron transfer from the HIL to TCO兲 is also enhanced. Both of these behaviors lead to facilitation of hole injection from the TCO to the HTL.52 Experiments using different EML hosts and HILs are summarized in Table II for OLED performance at 800 cd m−2 brightness where each value represents the average of multiple devices. The data in Table II suggest that across a series of device architectures, using GZO instead of ITO as an anode will slightly lower the operating voltage while maintaining the EQE. We achieved our best blue OLED performance using a GZO anode, HM-A1 as the host for the EML, and HAT-CN as the HIL. These devices had an operating voltage of 3.7 V, EQE of 17% EQE, and p of 39 lm W−1 at a current density of 1 mA cm−2. In Fig. 5, the current density and luminance versus voltage, along with EQE and power efficiency versus luminance, are shown for this device structure and a comparable device using commercial ITO as the anode. At operating voltages greater than 4 V, the current density and luminance of the GZO device are better than that
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of the ITO device, while the EQE of the GZO and ITO devices remains almost the same throughout the measured luminance range up to 30 000 cd m−2—an important feature for general lighting.53 Overall, GZO devices have better power efficiencies than the respective ITO devices at a luminance beyond 800 cd m−2.
IV. CONCLUSIONS
In summary, a cost-effective alternative to ITO is a critical need identified for commercialization of white OLED solid-state lighting for general illumination. The high cost of substrates, including the cost of ITO, is a limiting factor in the commercial production of general lighting OLEDs. Increased demand for ITO and indium for commerical applications, including photovoltaic productions, displays manufacturing, and solid-state lighting has the potential to dramatically increase the rate of indium utilization and the resulting cost of ITO. Furthermore, the diffusion of indium out of ITO into the organic stack may degrade the lifetime of the OLED. An indium-free material such as GZO offers a promising alternative to ITO, especially with the demonstration of highly efficient phosphorescent blue OLEDs using a variety of device structures. The device characteristics reported herein 共operating voltage of ⬍4 V and power efficiency of ⬎35 lm W−1 at a luminance of 800 cd m−2兲 show promise for use in general lighting applications, where a brightness of ⬃5000 cd m−2 at 4.9 V with a power efficiency of 26 lm W−1 will provide the desired small panel footprint without unacceptable glare for practical indoor lighting.53 This performance is comparable to that of the state-of-the-art blue OLEDs produced using an ITO anode. The mobility in the GZO films can be further improved by adjusting the deposition/annealing conditions, which will consequently enhance both electrical and optical properties in the film. A recent study reported a substantial improvement in mobility and conductivity in the GZO film through this strategy.20 The measured optical transmittance, similar to that of commercial ITO substrates throughout the entire visible-NIR range, indicates that these GZO substrates are suitable for use as the transparent anode in white OLEDs. Coupled with downconversion or color-mixing technologies and proper light outcoupling techniques, GZO substrates offer promise in achieving the efficiency targets of the current U.S. DOE lighting program.54 Very important for production, the GZO films were deposited with a RF magnetron sputtering process which is capable of scaling up to large area manufacturing 共⬎m2兲. Finally, as indicated in Table I, in a reducing ambient, our RF magnetron sputtering process produces GZO films of high electrical and optical quality with the substrate at room temperature, Fairly high device performance has been demonstrated on the GZO films without postdeposition annealing. Room temperature processing is crucial if flexible substrates are to be used. GZO deposited on flexible substrates could find wide application in displays, general lighting, decorative lighting, and solar panels. There is considerable interest in producing OLEDs on polymeric
substrates where their flexibility can be utilized for mass production through high volume processes such as roll-to-roll manufacturing. ACKNOWLEDGMENTS
The authors thank Wendy D. Bennett for helpful discussions on the uniformity of thin film deposition by magnetron sputtering. This project was funded by the Solid Sate Lighting Program within the Building Technologies Program 共BT兲 managed by the National Energy Technology Laboratory 共NETL兲 of the Energy Efficiency and Renewable Energy Division of the U.S. Department of Energy Award No. M6642866. A portion of the research described in this paper was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory. Pacific Northwest National Laboratory 共PNNL兲 is operated by the Battelle Memorial Institute for the U.S. Department of Energy 共DOE兲 under Contract No. DEAC06-76RLO 1830. T. Minami, Thin Solid Films 516, 1314 共2008兲. J. J. Berry, D. S. Ginley, and P. E. Burrows, Appl. Phys. Lett. 92, 193304 共2008兲. 3 Y. Nagoya, B. Sang, Y. Fujiwara, K. Kushiya, and O. Yamase, Sol. Energy Mater. Sol. Cells 75, 163 共2003兲. 4 B. Sang, K. Kushiya, D. Okumura, and O. Yamase, Sol. Energy Mater. Sol. Cells 67, 237 共2001兲. 5 C. G. Granqvist, Solid State Ionics 53–56, 479 共1992兲. 6 E. S. Lee and D. L. DiBartolomeo, Sol. Energy Mater. Sol. Cells 71, 465 共2002兲. 7 A. Mitsui and K. Sato, Vacuum 74, 747 共2004兲. 8 J. H. Kim, B. D. Ahn, C. H. Kim, K. A. Jeon, H. S. Kang, and S. Y. Lee, Thin Solid Films 516, 1330 共2008兲. 9 D. S. Ginley and C. Bright, MRS Bull. 25, 8 共2000兲. 10 T. Minami, Thin Solid Films 516, 5822 共2008兲. 11 T. Minami, Semicond. Sci. Technol. 20, S35 共2005兲. 12 S.-M. Park, T. Ikegami, and K. Ebihara, Thin Solid Films 513, 90 共2006兲. 13 Y. Tomita, C. May, M. Toerker, J. Amelung, M. Eritt, F. Loeffler, C. Luber, and K. Leo, Appl. Phys. Lett. 91, 063510 共2007兲. 14 J. Meyer, P. Görrn, S. Hamwi, H.-H. Johannes, T. Riedl, and W. Kowalsky, Appl. Phys. Lett. 93, 073308 共2008兲. 15 H. Kim, C. M. Gilmore, J. S. Horwitz, A. Pique, H. Murata, G. P. Kushto, R. Schlaf, Z. H. Kafafi, and D. B. Chrisey, Appl. Phys. Lett. 76, 259 共2000兲. 16 K. Yim, H. W. Kim, and C. Lee, Mater. Sci. Technol. 23, 108 共2007兲. 17 M. Hiramatsu, K. Imaeda, N. Horio, and M. Nawata, J. Vac. Sci. Technol. A 16, 669 共1998兲. 18 B. Meyer, J. Sann, D. Hofmann, C. Neumann, and A. Zeuner, Semicond. Sci. Technol. 20, S62 共2005兲. 19 V. Bhosle, A. Tiwari, and J. Narayan, J. Appl. Phys. 100, 033713 共2006兲. 20 C. W. Gorrie, J. J. Berry, and D. S. Ginley, “Effect of Deposition Distance and Temperature on Electrical, Optical and Structural Properties of RFSputtered ZnO:Ga,” Thin Solid Films 共submitted兲. 21 K. Ellmer, A. Klein, and B. Rech, Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells, Springer Series in Materials Science, 104, 1st ed. 共Springer-Verlag, Berlin Heidelberg, 2008兲. 22 W. T. Pawlewicz, J. H. King, D. C. Stewart, P. M. Martin, I. B. Mann, and W. D. Bennett, Proceedings of the Topical Meeting on High Power Laser Optical Components, Boulder, CO, October 1987 共unpublished兲. 23 B. W. D’Andrade and S. R. Forrest, Adv. Mater. 共Weinheim, Ger.兲 16, 1585 共2004兲. 24 X. Yang, D. C. Müller, D. Neher, and K. Meerholz, Adv. Mater. 共Weinheim, Ger.兲 18, 948 共2006兲. 25 T. Nakayama, K. Hiyama, K. Furukawa, and H. Ohtani, Konica Minolta Technology Report 5, 115 共2008兲. http://www.konicaminolta.jp/about/ research/technology_report/2008/pdf/treatise_006.pdf. 1 2
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