Low Threshold Voltage and Carrier Injection Properties of Inverted

Sep 24, 2009 - Hiroshi Yanagi*, Ki-Beom Kim, Ikue Koizumi, Maiko Kikuchi, Hidenori .... Hidenori Hiramatsu , Toshio Kamiya , Tetsuya Tohei , Eiji Iken...
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J. Phys. Chem. C 2009, 113, 18379–18384

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Low Threshold Voltage and Carrier Injection Properties of Inverted Organic Light-Emitting Diodes with [Ca24Al28O64]4+(4e-) Cathode and Cu2-xSe Anode Hiroshi Yanagi,*,†,| Ki-Beom Kim,† Ikue Koizumi,† Maiko Kikuchi,† Hidenori Hiramatsu,‡ Masashi Miyakawa,§ Toshio Kamiya,†,‡ Masahiro Hirano,‡,§ and Hideo Hosono†,‡,§ Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, ERATO-SORST, Japan Science and Technology Agency in Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, and Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: July 7, 2009; ReVised Manuscript ReceiVed: September 2, 2009

Carrier injection properties including threshold voltages of inverted top-emission organic light-emitting diodes (ITOLED) were improved by applying room temperature stable electride [Ca24Al28O64]4+(4e-) (C12A7:e-), which has a low work function of ∼2.4 eV, and a p-type degenerated semiconductor Cu2-xSe to bottom cathode and top anode buffer layers, respectively. The formation of a low-barrier electron injection contact between C12A7:e- and tris(8-hydroxyqunoline)aluminum (Alq3) is demonstrated by the current-voltage characteristics of electron-only devices, as well as by photoelectron spectroscopy. The threshold voltage of the ITOLED is reduced by changing the bottom cathode from Al to C12A7:e- from 9 to 7.6 V at 10 mA cm-2. A 5 nm thick Cu2-xSe top anode buffer layer, deposited at room temperature, reduced the threshold voltage further to ∼2 V. The luminance efficiency of ITOLED with a Cu2-xSe buffer layer is nearly twice as large as that without the buffer layer. We emphasize that developing new electrode materials is an effective means to improve the performance of not only OLED but also other new optoelectronic devices. Introduction Despite recent progress, the lifetime and performance of organic light-emitting diodes (OLEDs) needs further improvement. One of the most serious issues is that of carrier injection from the metal electrodes to the organic layers. This is because metal/organic interfaces have large band discontinuities with barrier heights >1 eV, which leads to high operating voltage and subsequent degradation.1 Improving electron injection from the cathode to the organic layer has been more challenging than hole injection because the lowest unoccupied molecular orbitals (LUMOs) of organic electron transport materials (ETM) (2.6-3.3 eV)2 are much shallower than the work functions (φWF) of typical metal electrodes such as Al (4.0-4.3 eV).3,4 From this standpoint, room temperature (RT) stable electride (electride is a crystal in which electrons serve as anions5), [Ca24Al28O64]4+(4e-) (C12A7:e-), proves promising because it has an extremely low φWF of 2.4 eV and high chemical stability.6,7 This small value originates from the very wide band gap of the host material [Ca24Al28O64]4+(2O2-) (>6 eV). Our previous photoelectron spectroscopy (PES) study suggested that C12A7:e- should form a very low electron injection barrier (EEIB), ∼0.6 eV, at the interface with a representative ETM, tris-(8-hydroxyqunoline)aluminum (Alq3).8 However, the low EEIB observed by PES does not guarantee an improvement in electron injection properties because electron transport at the interface is affected not only * To whom correspondence should be addressed. E-mail: hyanagi@ yamanashi.ac.jp. † Materials and Structures Laboratory, Tokyo Institute of Technology. ‡ ERATO-SORST, Japan Science and Technology Agency in Frontier Research Center, Tokyo Institute of Technology. § Frontier Research Center, Tokyo Institute of Technology. | Current address: Interdisciplinary Graduate School of Medical & Engineering, University of Yamanashi, 4-4-37 Takeda, Kofu, Yamanashi 400-8510, Japan.

by EEIB but also by electron transmittance through the interface. Therefore, it is necessary to examine the electron injection properties of an actual device. To use C12A7:e- in OLEDs, inverted top-emitting OLED (ITOLED) structures that employ a bottom cathode and a top transparent anode are preferred. This is because high-temperature processes are required to prepare C12A7:e- films and it is difficult to use it for a top cathode.9 Further, ITOLED structures are expected to provide efficient active-matrix OLEDs because a bottom cathode structure can produce superior electron transfer properties from the drain of an n-channel TFT, such as amorphous silicon and oxide TFTs.10-13 In addition, a top transparent anode structure has the advantage of integrating opaque and large bottom TFTs by maintaining a high aperture ratio. However, ITOLED fabrication involves several technical issues.12,13 One is the damage caused by ion bombardment to the organic layers during the sputtering deposition of the top transparent anode (usually made of indium-tin oxide (ITO)).14 Furthermore, the fabrication sequence of ITOLED results in poor carrier injection efficiency at the anode/organic interface due to lack of surface oxidation processes such as O2 plasma15 and UV-ozone,16 which are required for an ITO anode to increase the φWF at its surface, and reduce the hole injection barriers (EHIB) consequently. By contrast, an oxidation treatment is not applicable when ITO is deposited on an organic layer as a top anode. To solve this issue, Cu2-xSe can serve as an anode buffer layer. Cu2-xSe films deposited at RT exhibit high electrical conductivity (4.5 × 103 cm-1) and φWF of ∼4.6 eV,17 which is larger than that of unoxidized ITOs (3.9-4.4 eV).18-20 In this study, we report that a C12A7:e- cathode reduces threshold voltages in actual devices, including C12A7:e-/Alq3/ Al electron-only devices and ITOLEDs. A C12A7:e- cathode combined with a Cu2-xSe anode buffer layer produces high-

10.1021/jp906386q CCC: $40.75  2009 American Chemical Society Published on Web 09/24/2009

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efficiency ITOLED. Carrier injection mechanisms are also discussed based on the electrical characteristics of these devices.

Yanagi et al. TABLE 1: Structures of OLED Devices Using Conventional ITO Anode layer structure

Experimental Methods Thin films of polycrystalline (p-) C12A7:e- (200 nm thick) were prepared on single-crystalline MgO (100) substrates using pulsed laser deposition (PLD) and a subsequent reduction treatment.9 First, an amorphous C12A7 (a-C12A7) layer was deposited on MgO(100) substrate by PLD (at RT, oxygen pressure (PO2) of ∼1 × 10-3 Pa) followed by thermal annealing at 1100 °C in air to achieve crystallization. Further, the crystallized p-C12A7 film was reintroduced into the PLD chamber and an additional a-C12A7 layer was deposited on it at 700 °C under a pressure below 10-3 Pa. This treatment converted the insulating p-C12A7 layer to a conducting p-C12A7: e- layer with conductivity up to 800 S cm-1 (PLD reduction treatment). The surface a-C12A7 layer, which was formed on the C12A7:e- thin film after the PLD reduction treatment, was removed by chemical mechanical polishing and the thickness of the C12A7:e- film was reduced to 150 nm. The C12A7:efilms on MgO(100) substrates were transferred to a combined system of two preparation chambers (for plasma treatment and vacuum annealing), two evaporation chambers (for Alq3, NPB, CuPc, Al, and LiF), and an ultraviolet photoelectron spectroscopy (UPS) chamber connected under vacuum. Prior to the deposition of organic layers, the C12A7:e- film surfaces were cleaned with He plasma (radio frequency (rf) plasma in a pure He gas at 0.6 Pa for 30 s and RF power of 50 W) and subsequent vacuum annealing (300 °C, 30 min, ∼1 × 10-5 Pa).8 To fabricate the OLED devices, ITO and Cu2-xSe were deposited on organic layers as top anodes. This is because ITO is a widely used conventional anode and Cu2-xSe forms the lower EHIB with NPB and CuPc as against that in ITO.17 The depositions were conducted at RT by a PLD deposition system that was not connected to the combined system under vacuum (below 10-5 Pa) using a KrF excimer laser (COMPex 102 made by Lambda Physik, with a wavelength of 248 nm) as an excitation source. Detailed deposition conditions of Cu2-xSe are described elsewhere.17 The energy level alignments at interfaces between inorganic electrodes and organic semiconductors were measured by in situ UPS (∼5 × 10-8 Pa) (Omicron, Germany). Electron-only devices were fabricated to examine the electron injection properties of C12A7:e- cathode with actual devices. The EEIB at the C12A7:e-/Alq3 interface was measured during fabrication of the electron-only devices so as to confirm that a small EEIB was obtained in the actual devices. The electrononly devices, composed of an MgO/C12A7:e- (150 nm)/Alq3 (150 nm)/Al (100 nm) structure, were fabricated by thermal evaporation of the Alq3 and Al layers on the C12A7: e- layer. For comparison, other electron-only devices composed of glass/ Al (100 nm)/LiF (0.5 nm)/Alq3 (150 nm)/Al (100 nm) and glass/ Al (100 nm)/Alq3 (150 nm)/Al (100 nm) were also fabricated. The deposition rates of Alq3 and Al were controlled at 0.05 and 0.02 nm s-1, respectively, with a precalibrated quartz oscillator under a pressure of ∼2 × 10-6 Pa at RT. To conduct a preliminary experiment prior to ITOLED fabrication using C12A7:e- and Cu2-xSe, OLED devices using conventional ITO anodes were fabricated to examine the effect of ITO surface oxidation. Normal stacking OLED devices (composed of glass/oxidized ITO/CuPc/NPB/Alq3/Al (N-ITOAl)) and inverted stacking OLED devices (composed of glass/ Al/Alq3/NPB/CuPc/unoxidized ITO (I-Al-ITO)) were fabricated as shown in Table 1. ITO surface oxidation in the N-ITO-Al devices was carried out with an oxygen plasma (PO2 ≈ 0.6 Pa, 60 s, 50 W) in the preparation chamber of the combined system.

N-ITO-Al I-Al-ITO

glass/oxidized ITO (100 nm)/CuPc (20 nm)/NPB (20 nm)/Alq3 (60 nm)/Al (100 nm) glass/Al (100 nm)/Alq3 (60 nm)/NPB (20 nm)/ CuPc (20 nm)/unoxidized ITO (100 nm)

TABLE 2: Structures of ITOLED Devices layer structure glass/Al (100 nm)/Alq3 (60 nm)/NPB (20 nm)/CuPc (20 nm)/ITO (100 nm) CA-ITO MgO/C12A7:e- (150 nm)/Alq3 (60 nm)/NPB (20 nm)/ CuPc (20 nm)/ITO (100 nm) Al-Cu-ITO glass/Al (100 nm)/Alq3 (60 nm)/NPB (20 nm)/CuPc (50 nm)/Cu2-xSe (5 nm)/ITO (100 nm) CA-Cu-ITO MgO/C12A7:e- (150 nm)/Alq3 (60 nm)/NPB(20 nm)/ CuPc (50 nm)/Cu2-xSe (5 nm)/ITO (100 nm)

Al-ITO

ITOLED devices were fabricated to examine the potential of the C12A7:e- cathode and the Cu2-xSe anode in an actual OLED device. Four different structures, summarized in Table 2 (where CA and Cu denote C12A7:e- and Cu2-xSe, respectively), were fabricated to separate the effects of the C12A7:ecathode and the Cu2-xSe anode. Since the band gap of Cu2-xSe is 1.2 eV21 and is not completely transparent in the visible region, a thin Cu2-xSe layer (5 nm thick) with 69% transmittance at 550 nm was used as a buffer layer between the CuPc and ITO layers. The ITO layers were deposited by PLD. To minimize the ion bombardment damage to the organic layers during ITO deposition, the first 40 nm thick layer was deposited at a slow rate of ∼0.02 nm s-1. Then the deposition rate was increased to ∼0.1 nm s-1 for the last 60 nm for saving process time. Current density-voltage (J-V) characteristics were measured in a vacuum chamber at RT without breaking vacuum, using a Keithley 236 source measurement unit. The luminance-voltage (L-V) characteristics of the OLED devices were measured using a LS-110 luminance meter (Minolta, Japan) placed in a drybox filled with argon. To measure very low current accurately, a Keithley 6517A picoammeter was also used for some cases. Results and Discussion Figure 1a shows the cutoff region (left) and the valence band spectra (right) of a C12A7:e- cathode layer and a 5 nm thick Alq3 film deposited on the C12A7:e- layer. Here, EEIB is defined as the energy difference between the Fermi level (EF) of the cathode and the LUMO* (the LUMO level after accepting an electron) of Alq3. The highest occupied molecular orbital (HOMO) level of Alq3 with respect to EF was determined by extrapolating the lower binding energy tail of the shallowest energy peak to the baseline (shown by broken lines and a red arrow in the inset of Figure 1a). Finally the LUMO* level was determined by subtracting the transport gap (reported to be Et ) 4.6 eV for Alq322) from the observed HOMO level (EF EHOMO ) 3.8 eV). Figure 1b gives a schematic illustration of the energy level diagram of the interface built from these values. The energy level diagram provides an EEIB value of 0.8 eV, which is much smaller than that of the best conventional cathode structure Al/LiF (0.5 nm)/Alq3 interface (1.4 eV)23 and is consistent with our previous report (EEIB ) 0.6 eV).8,24 Figure 2a compares the J-V characteristics of the electrononly devices with different electron injection structures: (i) C12A7:e-/Alq3/Al (denoted C12A7:e-), (ii) Al/LiF/Alq3/Al (Al/

Properties of Inverted Organic Light-Emitting Diodes

Figure 1. (a) Cutoff (left) and valence band spectra (right) of C12A7: e- and Alq3 thin film (5 nm) deposited on it. The binding energy is measured from EF of C12A7:e-. (b) Energy level diagram of the C12A7:e-/Alq3 interface along with that of the Alq3/Al interface. The LUMO* of Alq3 is evaluated from the Et (transport gap) and the HOMO level. EEIB is determined as the energy difference between the EF of the C12A7:e- and the LUMO* of the Alq3 layer. The hole injection barriers from C12A7:e- to Alq3 and from Al to Alq3 are very large, 3.8 and 2.9 eV, respectively, and they block hole transport in C12A7: e-/Alq3/Al electron-only devices.

LiF (0.5 nm)), and (iii) Al/Alq3/Al (Al). In these device structures, an Al anode inhibits the hole injection into Alq3 due to a large EHIB of ∼2.9 eV at the interface between Al and Alq3 as shown in Figure 1b. The C12A7:e- cathode device showed a lower threshold voltage than the Al/LiF and the Al cathode devices. To investigate the electron transport mechanism, log J-V1/2 plots are shown in Figure 2b. All the plots show straight lines, suggesting that electron injection is limited by thermionic emission over a Schottky barrier, which follows the relation, log J ) J0 + CV1/2 (J0 and C are constants). The C values (1.19 ( 0.05) are similar for all devices and agree roughly with the theoretical value, C ) (e/4πε)1/2/kBT(d1/2) ) 1.88, estimated by using the reported relative dielectric constant (εr) of 4.0.25 This supports the conclusion that Schottky barriers formed at the cathode/Alq3 interfaces control the electron injection even for the improved C12A7:e- cathode device. Figure 2b also shows that the C12A7:e- cathode device operates at the lowest voltage and exhibits the highest current density among these devices because it has the largest J0 value. The J0 values are related to the Schottky barrier height (SBH, φSB) at the cathode/Alq3 interface through the relation J0 ) A*T2 exp(-φSB/kBT) (A* is the effective Richardson constant), which yields φSB values of 0.90, 0.91, and 0.93 eV for the C12A7:e-, Al/LiF, and Al

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Figure 2. (a) J-V characteristics of electron-only devices fabricated with three different cathodes: C12A7:e-, Al/LiF, and Al. Among them, the device with the C12A7:e- cathode had the lowest threshold voltage. (b) J versus V1/2 plots. The solid lines show linear approximation in the J-V1/2 plots, and the Schottky barrier heights were estimated from the slopes.

cathodes, respectively. These values do not agree quantitatively with the observed EEIB values, but the order of the φSB values is consistent with that of the observed EEIB values, demonstrating that C12A7:e- works as an efficient electron injection electrode with better properties than the conventional cathodes. Next, we fabricated OLED devices to examine the potential of the C12A7:e- cathode and the Cu2-xSe anode in actual OLED devices. We first measured the energy level alignments of the constituent electrodes and organic semiconductors by in situ UPS, as summarized in Figure 3a. The EHIB between CuPc and oxidized ITO was as small as ∼0.4 eV, but increased to 0.7-1.0 eV when surface oxidation was not applied to the ITO.26,27 The different EHIB values result in largely different operation voltages of actual devices as shown in Figure 3b, which compares an oxidized ITO anode with the normal structure (N-ITO-Al) and an unoxidized ITO anode with the inverted structure (I-Al-ITO), where the ITO was deposited on organic layers. This shows that the threshold voltage of the oxidized ITO device (N-ITOAl) was as low as ∼5 V, but increased to ∼9 V when the ITO surface was not oxidized. Figure 3a also shows that Cu2-xSe has a small EHIB of ∼0.5 eV, which is comparable to that of oxidized ITO, although the Cu2-xSe layer was not subjected to a special surface treatment. These results suggest that Cu2-xSe can reduce the operating voltage in inverted structure devices such as the I-Al-ITO and ITOLED device examined below. Figure 3c compares the effects of Al and C12A7:e- cathodes on the J-V and L-V character-

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Figure 4. (a) Luminance efficiency of the ITOLED devices shown in panels c and d of Figure 3. Luminance efficiency is improved at large J when Cu2-xSe buffer layers are used. (b) Double-logarithm plot of the CA-Cu-ITO device. Carrier transport in this device is classified into three regions. In the low-field region of 0.1-1.5 V, J increases linearly with V. In the intermediate region, J increases rapidly with an exponent factor of m + 1 ) 18. Above 2.5 V, J increases with the slope of 2.2, which is close to the theoretical value of space charge limited current, 2.0.

Figure 3. (a) Energy level alignments in the ITOLED devices. The energy levels were determined by UPS measurements and a reported value.26,27 (b) J-V characteristics of OLED devices using conventional ITO anodes listed in Table 1. J-V characteristics of (O) a normal stacked OLED using an oxidized ITO anode (N-ITO-Al) and (b) an inverted stacked OLED using an unoxidized ITO anode (I-Al-ITO). The threshold voltage (at J ) 10 mA cm-2) of an unoxidized ITO device is 9.1 V, which is much higher than that of the oxidized ITO device (5.5 V). (c) J-V-L plots of the ITOLED devices using different cathodes: Al (denoted Al) and C12A7:e- (denoted CA). Open symbols represent the J-V characteristics and closed symbols represent the L-V characteristics. The CA-ITO device (C12A7:e- cathode) has a lower threshold voltage than the Al-ITO device (Al cathode). (d) Effects of the Cu2-xSe buffer layer; Cu2-xSe buffer layers reduce the threshold voltages to ∼2 V and improve the current densities.

istics of ITOLED devices with conventional CuPc/ITO anode structures. The C12A7:e- cathode device (CA-ITO) exhibits superior characteristics compared to the conventional Al cathode device (Al-ITO): e.g., current density of 60 mA cm-2 is obtained at 8.5 V for CA-ITO, but 10 V was necessary to obtain the same current density for Al-ITO. This demonstrates that the C12A7:e- cathode has a lower EEIB than the Al cathode in the actual OLED device. However, the threshold voltage (here, determined as the voltage at J ) 10 mA cm-2) remained high (7.6 V) even for the CA-ITO device. We speculate that the large EHIB of the CuPc/unoxidized ITO interface limits the hole injection. Therefore, a 5 nm thick Cu2-xSe buffer layer was inserted between the CuPc and the ITO layers, which completely altered the J-V characteristics of the devices as shown in Figure 3d. The injection currents in the ITO anode devices (Al-ITO, CA-ITO in Figure 3c) started increasing at 7-8 V, while those in the Cu2-xSe buffer devices (Al-Cu-ITO, CA-Cu-ITO in Figure 3d) started increasing at 2 V; the currents of the latter devices increased linearly with voltage and provided larger currents than the Al-ITO/CA-ITO devices. This suggests that a very low EHIB is formed at the anode interfaces due to the Cu2-xSe buffer layers.28,29 Although current injection begins at the low voltages (∼2 V), light emission starts at higher voltages of 6-7 V. Figure 4a shows the luminance efficiencies of the ITOLED devices in panels c and d of Figure 3 as a function of J. When the conventional CuPc/ITO anode structure is used, the luminance efficiencies do not change throughout the measured J region, but remain relatively low. On the other hand, the luminance efficiencies of the Cu2-xSe buffer devices are low at low applied voltages (i.e., low J), and increase with J. These results imply that a large fraction of the injected electrons or

Properties of Inverted Organic Light-Emitting Diodes holes do not contribute to light emission at low applied voltages. One possible reason is that the hole injection is improved drastically by the reduced EHIB formed by the Cu2-xSe buffer layer, but the electron injection is still controlled by the remaining EEIB ≈ 0.8 eV even for the improved C12A7:ecathode. This causes an unbalanced hole and electron injection, especially at low applied voltages, where only holes are injected and the hole current contributes to energy dissipation without light emission. However, at higher voltages, light emission is improved as more electrons are injected, and consequently the luminance efficiency increases monotonously with increasing applied voltage for the Cu2-xSe buffer devices. The saturating behavior observed for the CA-Cu-ITO device would imply that the best balance of the hole and electron injections would be achieved at a little bit larger current density. The maximum luminance efficiency reaches 3.9 cd A-1 at 115 mA cm-2 in the CA-Cu-ITO device by using the C12A7:e- cathode, which is nearly twice of that of the Al-Cu-ITO device with an Al cathode and the Cu2-xSe/ITO anode. This enhancement is attributed to the better balanced charge injection realized by employing the C12A7:e- cathode. Figure 4b shows a double logarithm plot of the CA-Cu-ITO device. This plot has three regions defined by the slope. The lowest voltage region shows a linear J-V behavior and is interpreted as an ohmic injection. The current in the highest voltage region (>2.5 V) is proportional to V2.2 and follows a space charge limited current (SCLC) model. In the intermediate region, the current steeply increases with increasing V and appears to follow the relation J ∝ V.18 This behavior would be explained by trapped charge limited current (TCLC).30 We also examined other models to explain the intermediate region because this device has a multihetrojunction diode structure. We found that two different models, (i) the simple homo pn junction model, which is expressed by J ) J0exp(-eV/nkBT) (J0 is a prefactor and n the ideality factor), and (ii) the Schottky model used for the analysis of Figure 2b, which is expressed by log J ) J0 + CV1/2, reproduced well the J-V characteristic in the intermediate region. However, the n value in the model (i) was ∼390 and too large to be explained by a simple pn junction (the theoretical value can vary from a diffusion current limit n ) 1 to a recombination current limit n ) 2). The C value obtained by model ii was also very large, ∼28. This value can be explained if the external voltage is applied only in a few tens of nanometers thick layer, but we observed such a large band bending neither at the anode nor the cathode interface. Therefore, we should conclude these two diode models cannot explain the steep J-V characteristic in the intermediate region. Concluding Remarks We proposed two new electrode materials, C12A7:e- and Cu2-xSe, to improve the performance of inverted top-emission OLED devices. C12A7:e- forms a very low electron injection barrier with EEIB ) 0.6-0.8 eV, which was confirmed by both UPS and actual electron-only devices. Moreover, C12A7:e- improved electron injection properties in ITOLED devices, compared to those with an Al cathode. It was also found that a 5 nm thick Cu2-xSe buffer layer reduced the threshold voltages of ITOLED to ∼2 V, providing a larger current flow, which improved the luminance efficiency at high applied voltages. Although Cu2-xSe has a small band gap and is not transparent, it can form on organic layers at RT. The present study reveals that only a 5 nm thick layer is sufficient to obtain improved hole injection, and is compatible with the conventional OLED

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18383 structures and processes. Since ITOLED devices using a C12A7: e- cathode require high process temperatures and MgO singlecrystal substrates, it would be difficult to apply C12A7:edirectly to the actual OLED devices. We expect that a lowtemperature process to fabricate C12A7:e- films, e.g., by hydrothermal synthesis and laser crystallization/reduction, will be developed, but it still remains a challenging issue. The present approach demonstrates that development of a new material is an efficient and inevitable method for improving the performance of new optoelectronic devices. We developed a low work function electrode, C12A7:e-, by doping high-density electrons to very wide band gap oxide C12A7, where the subnanometer-sized cages in the crystal structure and the free oxygen ions encaged in them are utilized to achieve high-density electron doping. A similar approach in exploring materials will develop a low work function electrode that can be formed at a low temperature on organic devices. Acknowledgment. This work was partially supported by JSPS Grant-in-Aid for Scientific Research (S) 21226015 and funding for Element Science and Technology Project, MEXT. References and Notes (1) Hung, L. S.; Chen, C. H. Mater. Sci. Eng. 2002, R 39, 143. (2) Matsushima, T.; Goushi, K.; Adachi, C. Chem. Phys. Lett. 2007, 435, 327. (3) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605. (4) Seki, K.; Ishii, H. J. Electron Spectrosc. Relat. Phenom. 1998, 8891, 821. (5) Dye, J. L. Science 2003, 301, 607. (6) Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. Science 2003, 301, 626. (7) Toda, Y.; Yanagi, H.; Ikenaga, E.; Kim, J. J.; Kobata, M.; Ueda, S.; Kamiya, T.; Hirano, M.; Kobayashi, K.; Hosono, H. AdV. Mater. 2007, 19, 3564. (8) Kim, K. B.; Kikuchi, M.; Miyakawa, M.; Yanagi, H.; Kamiya, T.; Hirano, M.; Hosono, H. J. Phys. Chem. C 2007, 111, 8403. (9) Miyakawa, M.; Kamiya, T.; Hirano, M.; Hosono, H. Appl. Phys. Lett. 2007, 90, 182105. (10) Chen, C. W.; Lin, C. L.; Wu, C. C. Appl. Phys. Lett. 2004, 85, 2469. (11) Bulovic, V.; Tian, P.; Burrows, P. E.; Gokhale, M. R.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 1997, 70, 2954. (12) Dobbertin, T.; Kroeger, M.; Heithecker, D.; Schneider, D.; Metzdorf, D.; Neuner, H.; Becker, E.; Johannes, H.-H.; Kowalsky, W. Appl. Phys. Lett. 2003, 82, 2. (13) Chu, T. Y.; Chen, J. F.; Chen, S. Y.; Chen, C. J.; Chen, C. H. Appl. Phys. Lett. 2006, 89, 053503. (14) Vaufrey, D.; Khalifa, M. B.; Tardy, J.; Ghica, C.; Blanchin, M. G.; Sandu, C.; Roger, J. A. Semicond. Sci. Technol. 2003, 18, 4. (15) Lee, K. H.; Jang, H. W.; Kim, K. B.; Tak, Y. H.; Lee, J. L. J. Appl. Phys. 2004, 95, 586. (16) Kim, S. Y.; Lee, J. L.; Kim, K. B.; Tak, Y. H. J. Appl. Phys. 2004, 95, 2560. (17) Hiramatsu, H.; Koizumi, I.; Kim, K. B.; Yanagi, H.; Kamiya, T.; Hirano, M.; Matsunami, N.; Hosono, H.J. Appl. Phys. 2008, 104113723. (18) Chkoda, L.; Heske, C.; Sokolowski, M.; Umbach, E.; Steuber, F.; Staudigel, J.; Sto¨βel, M.; Simmerer, J. Synth. Met. 2000, 111-112, 315. (19) Nuesch, F.; Rothberg, L. J.; Forsythe, E. W.; Le, Q. T.; Gao, Y. Appl. Phys. Lett. 1999, 74, 880. (20) Park, Y.; Choong, V.; Gao, Y.; Hsieh, B. R.; Tang, C. W. Appl. Phys. Lett. 1996, 68, 2699. (21) Padam, G. K. Thin Solid Films 1987, 150, L89. (22) Hill, I. G.; Kahn, A.; Soos, Z. G.; Pascal, R. A., Jr. Chem. Phys. Lett. 2000, 327, 181. (23) Yokoyama, T.; Yoshimura, D.; Ito, E.; Ishii, H.; Ouchi, Y.; Seki, K. Jpn. J. Appl. Phys. 2003, 42, 3666. (24) This value is 0.2 eV larger than that in our previous report due to the different surface treatment conditions of the C12A7:e- thin films. The vacuum annealing temperature was decreased from 500 to 300 °C. (25) Berleb, S.; Bru¨tting, W. Phys. ReV. Lett. 2002, 89, 286601.

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(26) Lee, S. T.; Wang, Y. M.; Hou, X. Y.; Tang, C. W. Appl. Phys. Lett. 1999, 74, 670. (27) Hill, I. G.; Kahn, A. J. Appl. Phys. 1999, 86, 2116. (28) It should be noted that similar very small threshold voltages have been observed at a LaCuOSe:Mg/NPB interface. Thus, Cu2-xSe and LaCuOSe:Mg should have common mechanisms for the small threshold voltages.

Yanagi et al. (29) Yanagi, H.; Kikuchi, M.; Kim, K. B.; Hiramatsu, H.; Kamiya, T.; Hirano, M.; Hosono, H. Org. Electron. 2008, 9, 890. (30) Yamamoto, H.; Kasajima, H.; Yokoyama, W.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 083502.

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