Vanadium-doped indium tin oxide as hole-injection layer in organic light-emitting devices

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Vanadium-doped indium tin oxide as hole-injection layer in organic light-emitting

devices

T.-H. Chen, Y. Liou, T. J. Wu, and J. Y. Chen

Citation: Applied Physics Letters 87, 243510 (2005); doi: 10.1063/1.2137892

View online: http://dx.doi.org/10.1063/1.2137892

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/87/24?ver=pdfcov Published by the AIP Publishing

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Vanadium-doped indium tin oxide as hole-injection layer

in organic light-emitting devices

T.-H. Chen

ULVAC Taiwan Inc., Taipei, Taiwan and Display Institute, National Chiao Tung University, Hsinchu, Taiwan

Y. Lioua兲

Institute of Physics, Academia Sinica, Taipei, Taiwan

T. J. Wu and J. Y. Chen

ULVAC Taiwan Inc., Taipei, Taiwan

共Received 13 July 2005; accepted 4 October 2005; published online 8 December 2005兲

Organic light-emitting devices were fabricated by using vanadium-doped indium tin oxide共ITO兲 as the hole-injection layers between the hole transport layer, N,N

⬘

-dia共1-napthl兲-N,N

⬘

-diphenyl benzidine and the ITO anode. The vanadium-doped ITO layer was 15-nm thick with three different vanadium concentrations 共6, 10.5, and 12.5 mol %兲. Three different resistivities 共10, 500, and 10 000⍀ cm兲 and work functions 共5, 5.2, and 5.4 eV兲 were obtained. The device with 6 mol % V-doped ITO layer possessing the least resistivity 共10 ⍀ cm兲 and work function 共5 eV兲 has the lowest turn-on voltage 共below 3 V兲, the lowest operating voltage 共below 7 V兲, the highest luminance 共1000 cd/m2_{below 7 V}_{兲, and the highest power efficiency 共⬎5 lm/W at 10 mA/cm}2_兲

关DOI:10.1063/1.2137892兴

Organic light-emitting devices 共OLEDs兲 with high brightness, high efficiency, full color, and low operating volt-age have already been recognized as one of the future major flat panel displays. Highly transparent and conducting in-dium tin oxide 共ITO兲 films have been the most commonly used anode on optoelectronic devices for a long time. Recent applications of ITO films as the anode for OLEDs have been widely reported.1–5A typical OLED consists of an ITO anode followed by a hole transport layer 共HTL兲, a light emissive layer共LEL兲, an electron transport layer 共ETL兲, and a metal cathode. Two organic materials, a typical HTL material, N,N

⬘

-bis-共1-naphthyl兲-N,N

⬘

-diphenyl1-1,1-biphenyl 1-4, 4

⬘

-diamine 共NPB兲, and the green LEL and ETL material, tris共8-hydroxyquinoline兲 aluminum 共Alq3兲, are commonly

used in this structure. Charge injection from electrodes to organic materials plays an important role in the device per-formance. Usually, the mobility of the hole in the HTL is much higher than that of the electron in the ETL used in OLEDs. Reducing the number of holes in the HTL or en-hancing electron injection in the ETL may help to improve electron-hole current balance in OLEDs. To modify the two electrodes is usually a common way to balance the current in OLEDs. The hole injection at the anode can be adjusted by different surface treatments as well as by inserting a buffer layer between the ITO and the HTL共Refs. 6–10兲. Oxygen plasma treatment of the ITO anode is the most common way to improve the device performance. In this way, both the contact resistance and the work function of the ITO anode have been increased due to the reduction of the oxygen de-ficiency near the surface.11,12It is well known that a layer of the copper phthalocyanine共CuPc兲 on the ITO anode can eas-ily form a compact and smooth surface and dramatically

en-hance the device stability and performance.13,14 The CuPc layer may enhance the hole injection by reducing the effec-tive barrier between the ITO and the HTL but it may also decrease the hole-injection efficiency leading to a better bal-ance of the charge’s 共holes and electrons兲 injection and recombination.15Various high work-function metals共Pt, Pd, etc.兲, insulating materials 共Teflon, SiON兲, and oxides 共SiO2,

transition-metal oxides, etc.兲 used as buffer layers on the ITO anode have demonstrated their capabilities of lowering the turn-on voltage, and enhancing the luminance efficiency and lifetime of OLEDs 共Refs. 16–19兲. The improvement of the device performance was mostly attributed to the hole-injection enhancement which resulted from the energy bar-rier reduction between the ITO anode and the HTL by in-creasing the anode work function. For thin 共⬃2 nm兲 oxide layers, the tunneling model was proposed and the barrier reduction was attributed to the band-bending mechanism. However, the hole may accumulate at the interface between the ITO anode and the oxide layer due to the non-Ohmic contact and these positive charges may suppress the hole injection from the ITO anode to the HTL. From our previous study, we have shown that the balance of the energy barriers from both sides of the hole-injection layer is also important to improve the device performance.20 However, the balance between the hole injection and the hole suppression has not yet been clarified.

In this study, three different concentrations of vanadium were dope into the ITO as the buffer layer. Most of the doped vanadium in the ITO layer has been transformed into stable vanadium oxides共VOx, VO2, or V2O5兲, which is confirmed

by the electron spectroscopy for chemical analysis 共ESCA兲 method 共the binding energy of 2P_3/2 photoelectrons has a broadened peak about 516– 517 eV兲. The uniformly doped VOxin the ITO layer has reduced the oxygen deficiency and increased the resistivity. The lifetime of the device with the

a兲_{Author to whom correspondence should be addressed; electronic mail:}

[email protected]

APPLIED PHYSICS LETTERS 87, 243510共2005兲

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V-doped ITO layer has also been extended to about twice longer than that of the original device with pure ITO. Three different work functions and resistivies have been obtained. When compared with the low resistivity共⬃2⫻10−4_{⍀ cm兲}

of the undoped ITO anode, the high concentration 共12%兲 V-doped ITO film has a large resistivity共⬃10 000 ⍀ cm兲 at room temperature, which makes it similar to an insulating oxide layer. The V-doped ITO layers with different work functions have different influences on the hole injection be-cause the energy barriers between the ITO anode and the HTL have been changed. However, with the addition of a 15-nm-thick doping layer, there was no significant change 共⬍2%兲 on the transparency of the whole ITO film 共155-nm thick兲. A green coumarin derivative, 10- 共2-benzothiazolyl兲-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo关l兴pyrano关6,7,8–ij兴 quinolizin-11-one 共C-545T兲, was doped in the light emissive layer 共Alq3兲 due to its

good thermal stability and electroluminescence 共EL兲 performance.21 To compare and understand the significance of the device performance between the doped and undoped emissive layer, readers have to check with other previously published reports.22,23

ITO films about 140-nm thick with V-doped ITO layers 共15 nm兲 were deposited on glass substrates by using a RF 共for ITO兲 and dc 共for V兲 cosputtering system. The ITO target is composed of In₂O₃: SnO₂共9:1兲, and the V target is 99.9%

pure. The base pressure in the system was approximately 1.0⫻10−6_{Torr. The system pressure was about 5 mTorr}

during the film deposition. By controlling the gas flow rate, a gas mixture of about 8% of oxygen in argon was achieved. The substrate temperature was kept at 300 ° C. To control the V contents in ITO, we changed the sputtering rate of each target. The concentrations and the uniformity of the V in ITO have been determined by both the Rutherford backscattering spectroscopy 共RBS兲 and the secondary-ion-mass spectrom-etry 共SIMS兲 depth profile. We have observed a constant V concentration in the doped ITO layer共15 nm兲 and a sharply decayed V profile共⬍20 nm兲 in the undoped ITO film. The work function of the V-doped ITO was determined by the commonly used ultraviolet photoelectron spectroscopy 共UPS兲 method. The transmittance of the V-doped ITO 共150 Å兲/ITO 共1400 Å兲 film was about 90% 共at 550 nm兲. After the ITO 共140 nm兲 film and the V-doped ITO layer 共15 nm兲 are deposited, all organic layers, NPB 共40 nm兲 and Alq3 共40 nm兲共doped with C545T兲, were deposited

consecu-tively in vacuum by thermal evaporation with a base pressure of less than 10−7_{Torr. Before and after the sample was}

trans-ferred into the vacuum chamber, standard cleaning proce-dures 共ultrasonic agitation in acetone and oxygen plasma sputtering兲 were applied. The influence of the oxygen plasma sputter cleaning on the work function of the V-doped ITO has been minimized by doing it quickly共5 min兲 and at a low power 共100 W兲. For comparison, we have used a standard sample with a 15-nm-thick layer of CuPc 共work function = 5.2 eV兲 on top of the ITO film 共140 nm兲 without the V-doped ITO layer by thermal evaporation during the device fabrication. The active area of the device was about 0.1 cm2_.

These devices were completed with encapsulation in a dry argon glove box. The EL emission spectra and current-voltage-luminance characteristics were measured with a di-ode array rapid scan system using a Photo Research PR650 spectrophotometer and a computer-controlled dc source.

We have chosen three different concentrations共6, 10.5, and 12.5 mol %兲 of V 共V1, V2, V3, respectively兲 to dope ITO films as buffer layers between the ITO anode and the HTL共NPB兲 to fabricate the EL devices. Since the vanadium metal is oxidized quickly in the film, we expected that the more the vanadium doped, the more the vanadium oxide formed. The resistivity at room temperature of these three different V-doped ITO layers increased from 2⫻10−4_{⍀ cm}

FIG. 1. The logarithmic plots of the current density-voltage curves of de-vices with V-doped ITO layers and the standard device.

FIG. 2. The logarithmic plots of the luminance-voltage curves of devices with V-doped ITO layers and the standard device.

FIG. 3. The power efficiency–current density curves of devices with V-doped ITO layers and the standard device.

243510-2 Chen et al. Appl. Phys. Lett. 87, 243510共2005兲

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of undoped ITO to higher resistivities, 10 共V1兲, 500 共V2兲, and 10 000 共V3兲 ⍀ cm, respectively, due to the vanadium oxide formation. Since the resistivity–temperature curves have shown the semiconductor characteristics of the V-doped ITO layer, the resistivity was mainly determined by the car-rier concentration. The work functions of these V-doped ITO layers were also increased from 4.7 eV of the undoped ITO anode to 5共V1兲, 5.2 共V2兲, and 5.4 eV 共V3兲, respectively. The increase of the resistivity and the work function have re-sulted in the lowering of the carrier concentration and the energy barrier for the hole injection. The current density– voltage curves clearly show that the device with the V1-doped ITO layer has the lowest turn-on共⬍3 V兲 and operat-ing共⬍7 V兲 voltages, as shown in Figs. 1. The devices with V2- or V3-doped ITO layers have similar characteristics as the standard sample with CuPc buffer layer, because both the V2- and V3-doped ITO layers have relatively high resistivi-ties and work functions as CuPc. This similarity was also observed in the luminance-current density curves, as shown in Figs. 2. The device with the V1-doped ITO layer has the EL luminance of 1000 cd/ m2 _{at an operating voltage below}

7 V. From the power efficiency–current density curves, as shown in Fig. 3, the device with the V1-doped ITO layer has the power efficiency over 5 lm/ W at a current density of 10 mA/ cm2_{. We attributed the best performance of the}

de-vice with the V1-doped ITO layer to its relatively smaller

resistivity and work function. If the energy barrier reduction was important to enhance the device performance, the device with the V3-doped ITO layer should have the best perfor-mance because it has the highest work function 共5.4 eV兲. However, because the device with the V3-doped ITO layer has performed poorly, the hole injection must have been sup-pressed by the hole accumulation. If the balance of the en-ergy barriers from both sides of the hole-injection layer is important, the best choice must be the device with the V2-doped ITO layer, which has a work function 共5.2 eV兲 just between the work function of the ITO 共4.7 eV兲 anode and that of the HTL共NPB, 5.7 eV兲. Actually, the device with the V2-doped ITO layer performed well in the luminance versus current density and the current efficiency versus current den-sity, as shown in Figs. 4共a兲 and 4共b兲.

In summary, we have demonstrated that the performance of OLEDs has been improved by inserting a V-doped ITO layer as the hole-injection layer. The resistivity and the work function of the V-doped ITO layer were dependent on the vanadium concentrations. With a small work function共5 eV兲 and the lowest resistivity共10 ⍀ cm兲 of the 6 mol % V-doped ITO layer, the device has achieved a turn-on voltage below 3 V, an operating voltage below 7 V共at 10 mA/cm2_{兲, a}

lu-minance of 1000 cd/ m2below 7 V, and a power efficiency over 5 lm/ W at 10 mA/ cm2_{. The balance between the}

car-rier concentration and the energy barcar-rier for hole injection was important for the best device performance.

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FIG. 4.共a兲 The luminance–current density curves of devices with V-doped ITO layers and the standard device.共b兲 The current efficiency-current den-sity curves of devices with V-doped ITO layers and the standard device.

243510-3 Chen et al. Appl. Phys. Lett. 87, 243510共2005兲