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We fabricated a flexible bottom-emitting white organic light-emitting diode (BEWOLED) with a structure of PET/Ni/Ag/Ni (3/6/3 nm)/ NPB (50 nm)/mCP (10 nm)/7% FIrpic:mCP (10 nm)/3% Ir(pq)2acac:TPBi (5 nm)/7% FIrpic:TPBi (5 nm)/TPBi (10 nm)/Liq (2 nm)/ Al (100 nm). To improve the performance of the BEWOLED, a multilayered metal stack anode of Ni/Ag/Ni treated with oxygen plasma for 60 sec was introduced into the OLED devices. The Ni/Ag/Ni anode effectively enhanced the probability of hole-electron recombination due to an efficient hole injection into and charge balance in an emitting layer. By comparing with a reference WOLED using ITO on glass, it is verified that the flexible BEWOLED showed a similar or better electroluminescence (EL) performance.
©2013 Optical Society of America
1. Introduction
White organic light-emitting diodes (WOLEDs) have attracted an enormous degree of attention and have already proved a promising, renewable, and low-cost process. In particular, due to the nature of the organic materials in fields of the application on flexible electronics, OLEDs are one of the most suitable candidates compared to the other emitting devices [1–5]. However, a growing demand for flexible OLEDs for lighting applications or smart phone displays is limited because of the structural defects of indium tin oxide (ITO) generated from its high temperature fabrication process, cracking, and low bendable property. Moreover, a limited supply of indium material will be definitely crucial challenges for the commercialization of ITO-based OLED technology on flexible substrate [6–9].
Therefore, recently, transition metal oxides, such as molybdenum, vanadium, nickel, and tungsten oxide (MoO3, V2O5, NiO, WO3), or Al-doped ZnO (AZO) placed between an anode electrode and a hole transporting layer (HTL) could serve as an effective hole injection layer (HIL) in OLEDs [10–14]. One approach is to use NiO or thin Ni film, a very attractive p-type transparent conductive oxide that has a higher work function (5.4 eV) than ITO [15,16]. This higher work function results in a more effective hole injection into HTL than is generally formed from materials such as N,N'-bis-(1-naphthalenyl)-N,N'-bis-phenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) and N,N'-Di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD).
In this work, a multilayered metal stack of semi-transparent nickel/silver/nickel (Ni/Ag/Ni) with an oxygen-plasma-treated surface was used as the anode of bottom-emitting WOLED (BEWOLED) on flexible substrate. The current density, luminance, luminous efficiency, Internationale de L’Eclairage (CIEx,y) coordinates, and electroluminescence (EL) spectra of flexible BEWOLED was characterized by comparison to conventional BEWOLED with ITO anode on rigid glass substrate.
2. Experimental procedure
First, Ni/Ag/Ni (3/6/3 nm) thin metal stacked anode for BEWOLED was formed on flexible polyethylene terephthalate (PET) film. The anode is evaporated using the thermal evaporation method at a rate of 1Å/s through a shadow mask. To investigate an improvement of hole injection in the BEWOLED, ITO and Ni/Ag/Ni as anode were treated with the same oxygen plasma of 125 W for 60 sec at 2 × 10−2 Torr. Then, all organic layers were sequentially deposited onto the surface of the plasma-treated ITO or Ni/Ag/Ni anode at high vacuum (~5 × 10−7 Torr). 50-nm-thick NPB was used as HTL, as well as 45-nm-thick N,N'-dicarbazolyl-3,5-benzene (mCP) for hole transport. We employed iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2] picolinate (FIrpic) and bis[2-phenyl-1-quinolone]iridium acetylacetonate (Ir(pq)2acac) as a blue and red phosphorescent emitter in WOLED, respectively. 1,3,5-tris[N-phenylbenzimidazole-2-yl]benzene (TPBi) and a 2-nm-thick lithium quinolate (Liq) were also selected as electron transporting layer (ETL) and electron injection layer (EIL). Finally, an Al (100 nm) cathode was deposited at a rate of 10 Å/s for complete BEWOLED with active emitting area of 3 × 3 mm2.
All measurements were carried out at room temperature under ambient conditions. The optical and electrical properties of BEWOLEDs, such as the current density, luminance, EL efficiencies, and CIEx,y coordinates, were measured by using both a Keithley 2400 source meter and a CS-1000A spectroradiometer. The transmittance spectra of conventional glass, ITO-coated glass, Ni/Ag/Ni-coated glass, and Ni/Ag/Ni-coated PET were examined by a Shimadzu UV-2450 spectrophotometer. The work function of nickel was measured by AP-2001 scanning Kelvin probe microscope (SKPM). The resistivity and sheet resistance of Ni/Ag/Ni thin metal stack were measured using the AIT CMT-SR2000N four-point probe system. Surface of top nickel layer was oxidized with a PFG 300 RF radio frequency generators and oxygen-plasma-treated nickel surface was analyzed with a Thermo Scientific K-Alpha X-ray photoelectron spectrometer (XPS).
3. Results and discussion
The fabricated BEWOLEDs had two configurations of substrate/anode as i) glass/ITO (180 nm) and ii) PET/(Ni/Ag/Ni) (3/6/3 nm), and had the identical part as NPB (50 nm)/mCP (10 nm)/7% FIrpic:mCP (10 nm)/3% Ir(pq)2acac:TPBi (5 nm)/7% FIrpic:TPBi (5 nm)/TPBi (10 nm)/Liq (2 nm)/Al (100 nm) as shown in Fig. 1(a). In this device structure, the oxygen-plasma-treated Ni/Ag/Ni anode plays an important role because a thin NiO film forms during the oxidation of the Ni surface by oxygen plasma. Generally, hole-injection efficiency depends strongly on the work function of the anode in the OLED structure. Effective hole injection from the anode into the NPB HTL was achieved because the work function (5.4 eV) of an oxidized Ni anode is similar to the highest occupied molecular orbital (HOMO) level (5.5 eV) of the NPB layer [17]. Therefore, it was expected experimentally that a total 12-nm-thick Ni/Ag/Ni anode treated with oxygen plasma enhances efficient hole injection in the fabricated BEWOLEDs. Figure 1(b) shows the energy band diagram of the BEWOLEDs; the thickness of Ni/Ag/Ni was optimized through various combinations of Ag and Ni on Ni/Ag/Ni film.
From Figs. 2(a) and 2(b), it was verified that optical transparencies of the Ni/Ag/Ni (3/6/2 nm) and the Ni/Ag/Ni (3/4/3 nm) layer were higher than that of the Ni/Ag/Ni (3/6/3 nm) layer. However, sheet resistance of the Ni/Ag/Ni (3/6/2 nm) layer (14.6 Ω/sq) and the Ni/Ag/Ni (3/4/3 nm) layer (34.8 Ω/sq) layers was also higher than that of the Ni/Ag/Ni (3/6/3 nm) layer (12.1 Ω/sq) without oxygen-plasma treatment. After oxygen plasma treatment, sheet resistance increased slightly, about 1~2 Ω/sq, because NiO formed at a surface of the Ni/Ag/Ni metal stack. Balance of electrical and optical characteristics such as current density, luminance, EL efficiencies, and transmittance, is significant to achieve reliable OLEDs for a light source. Considering their sheet resistances and transparencies for BEWOLED’s overall performance, we selected the Ni/Ag/Ni (3/6/3 nm) metal stack as optimized anode. The details on both electrical/optical properties of Ni/Ag/Ni (3/6/3 nm) metal stack for different oxygen plasma treatment time and the OLED device performances can be found elsewhere [18].
Fig. 2 Transmittance spectra on Ni/Ag/Ni metal stack according to various thickness of (a) Ag and (b) Ni.
Figure 3(a) shows the transmittance and the reflectance spectra of semitransparent Ni/Ag/Ni (3/6/3 nm) metal stack and ITO anodes affected by oxygen-plasma treatment. 180-nm-thick ITO film has a transparency of ~81% at a wavelength of 500 nm, while the total 12-nm-thick Ni/Ag/Ni (3/6/3 nm) thin film shows a transparency of ~44% at the same wavelength. Transmittance of the Ni/Ag/Ni metal stack with oxygen-plasma treatment was slightly increased than that without plasma treatment. From the results, it is supposed that oxygen or oxygen ions react with Ni atoms and diffuse into Ni surface. After that, NiO film with a little more transparent property was formed on top of the Ni/Ag/Ni metal stack. However, we observed that transmittance change of the total 12-nm-thick Ni/Ag/Ni stack was negligible in case of increasing oxygen-plasma treatment time from 0 to 240 sec. The transmittance of NiO film on glass substrate increases slightly up to a maximum of 42.5% at a wavelength of 500 nm on increasing oxygen-plasma time as described in a previous study [18]. Maximum reflectance of Ni/Ag/Ni anode with oxygen-plasma treatment for 60 sec is 18.8% at wavelengths near 500 nm, whereas that of ITO anode has only 14.7%. Therefore, the micro-cavity effect will be potentially stronger in the OLED with semitransparent Ni/Ag/Ni anode than in a conventional OLED with ITO anode.
Fig. 3 (a) Transmittance and reflectance spectra of the 12-nm-thick Ni/Ag/Ni (3/6/3 nm) metal stack and 180-nm-thick ITO anode layer. (b) The optical transmission spectrum of the 3-nm-thick oxygen-plasma-treated NiO film. The inset shows (αhν)2 versus hν plot of the plasma treated NiO film.
To grasp an optical property of the NiO film, Ni metal was deposited onto a quartz substrate by thermal evaporation, and then 3-nm-thick Ni film was treated with an oxygen-plasma process. Oxygen-treated Ni film exhibits 71.7% of transmittance in the visible spectral region between 400 and 800 nm as shown in Fig. 3(b). The band gap energy (Eg) of the fabricated NiO film can be calculated from the (αhν)2 versus hν plot by assuming (αhν)2∝(hν - Eg), where α is the absorption coefficient and hν is the photon energy [19,20]. The absorption coefficient is determined by the following relation: α = [2.303 × log(1/T)]/d, where T is the transmittance and d is the film thickness. From the inset of Fig. 3(b), the bandgap energy of the oxygen-plasma-treated Ni film is determined to be 3.65 eV by extrapolating to (αhν)2 = 0, which is nearly consistent with the reported value in the literature [21].
Qualitative and quantitative surface analyses using ion-etched depth profiling of XPS were also carried out for thin Ni films with oxygen-plasma treatment. Figure 4 shows the XPS spectra of the Ni 2p peaks measured from the Ni/Ag/Ni film with oxygen-plasma treatment. It was found that the peak positions for the NiO film are located mostly at the binding energy regime of 853.9, 855.7, 860.9 and 880 eV, which agrees with the oxidized Ni state shown in the published data, indicating that the Ni-O chemical bonding state becomes more dominated by the Ni-O phase generated under oxygen-plasma treatment for 60 sec [22–24]. It was also verified that the thin NiO film is a fraction of top Ni layer, because binding energy peaks of NiO film were reduced and those of Ni film at 852.6 and 869.3 eV were increased when etching time was increased [24].
Fig. 4 XPS spectra of Ni 2p measured from NiO film with different durations by etching time at ion-etched depth profiling of XPS.
Figure 5(a) shows plots of current density and luminance versus voltage of BEWOLEDs with the ITO and Ni/Ag/Ni anode with maximum current densities of 165.9 mA/cm2 (at 9.5 V) and 83.5 mA/cm2 (at 8.5 V), almost identical to each other. Moreover, the luminance versus voltage characteristics of two BEWOLEDs had maximum luminance of 7554 cd/m2 at 9.5 V and 8323 cd/m2 at 8.5 V, and turn-on voltages of 3.77 V and 3.81 V at 1 cd/m2, respectively. These results reveal that more efficient hole injection from the anode into NPB HTL may be achieved when the work function of an oxidized Ni anode (5.4 eV) is similar to the HOMO energy level (5.5 eV) of the NPB layer, and that hole injection from anode to HTL strongly depends on the work function of the Ni/Ag/Ni anode in a flexible BEWOLED.
Fig. 5 (a) Luminance and current density as a function of applied voltage, and (b) luminous efficiency as a function of the luminance of BEWOLEDs with Ni/Ag/Ni metal stack and ITO anode layers formed by oxygen-plasma treatment for 60 sec. The inset shows quantum efficiency versus luminance characteristics of BEWOLEDs.
Figure 5(b) shows plots of luminous efficiency (LE) versus luminance (L) characteristics of BEWOLEDs with the ITO and Ni/Ag/Ni anode. The BEWOLEDs exhibited maximum LEs of 12.06 and 13.13 cd/A, and maximum quantum efficiencies (QEs) of 6.55% and 5.85%, respectively. These devices also showed LEs of 10.18 and 12.9 cd/A, and QEs of 5.50% and 5.74% at 20 A/cm2, respectively. In various aspects of EL performance, the flexible BEWOLED using Ni/Ag/Ni anode showed a less luminous and quantum efficiency roll-off than those of the BEWOLED with ITO anode as shown in Fig. 5(b) and the inset of that figure.
Even though the transparency of the semitransparent Ni/Ag/Ni multilayered metal stack anode was lower than that of the ITO, these results indicate that the flexible BEWOLED with Ni/Ag/Ni anode effectively enhanced the recombination probability of the holes and electrons due to improved charge balance in EML. The difference in LE and QE roll-off values between 100 and 7000 cd/m2 are 51.08% and 51.99% for BEWOLED with ITO anode. Those of the BEWOLED with Ni/Ag/Ni anode are also 26.33% and 25.94% at the same conditions, respectively. It is considered that these results, as improved efficiency roll-off of BEWOLED with Ni/Ag/Ni metal stack, are due to micro-cavity effect.
Figure 6 shows EL spectra of BEWOLEDs including ITO and Ni/Ag/Ni anode treated with oxygen plasma for 60 sec. Both EL spectra were normalized to compare the relative change of blue and red emission peaks. A strong red emission of both devices appeared around 600 nm due to a phosphorescent red Ir(pq)2acac emitter, and BEOLED with Ni/Ag/Ni anode has a higher-intensity blue emission peak at 497 nm than that of BEOLED with ITO anode. These different EL spectra patterns of the two BEOLEDs could be presumed to be the optical effect through the micro-cavity of Ni/Ag/Ni anode working as a strong mirror rather than a shift of recombination zone according to a different hole injection capability through ITO and Ni/Ag/Ni anode. This is because the maximum reflectance of ITO and Ni/Ag/Ni anode is about 14.7% and 18.8% at wavelengths near 500 nm. Therefore, the intensity of the blue emission peak will be potentially stronger in the device with Ni/Ag/Ni anode than in that with ITO anode by micro-cavity effect at 497 nm. The CIEx,y coordinates of the BEWOLEDs with ITO and Ni/Ag/Ni anode at 1000 cd/m2 were (0.42, 0.35) and (0.38, 0.39), respectively. Various EL performances on BEWOLEDs with ITO and Ni/Ag/Ni anode are listed in Table 1.
Table 1. EL Performance of Fabricated BEWOLEDs
Figure 7 describes some change at the CIEx,y coordinates of BEWOLEDs with ITO and Ni/Ag/Ni anode at increasing viewing angles from 0° to 60° with 15° step. The CIEx,y coordinates of ITO and Ni/Ag/Ni devices changed slightly from (0.42, 0.35) and (0.38, 0.39) at 0° to (0.44, 0.37) and (0.34, 0.35) at 60°. These results are not caused by the shifting of the recombination zone but by the micro-cavity effect of the multilayered metal stack anode. Micro-cavity effect of OLED typically exhibits significant blue shift with the viewing angle changes in the CIEx,y coordinates [25–27] and the angular dependence of flexible BEWOLED with Ni/Ag/Ni anode are slightly weaker than usual.
Fig. 7 The CIEx,y coordinates of BEWOLEDs with Ni/Ag/Ni metal stack and ITO anode at different viewing angles.
4. Conclusions
In summary, we successfully fabricated a flexible bottom-emitting white OLED consisting of PET/Ni/Ag/Ni (3/6/3 nm)/NPB (50 nm)/mCP (10 nm)/7% FIrpic:mCP (10 nm)/3% Ir(pq)2acac:TPBi (5 nm)/7% FIrpic:TPBi (5 nm)/TPBi (10 nm)/Liq (2 nm)/Al (100 nm). NiO film formed by oxygen-plasma treatment on Ni/Ag/Ni metal stack anode enhanced a hole injection from anode to a hole transport layer in the BEWOLED and more effectively increased a probability of hole-electron recombination due to improving the balance of charge carriers between anode/HTL and ETL. The flexible BEWOLED was efficiently modified by the oxygen-plasma-treated anode without using an additional organic or inorganic hole injection layer or ITO electrode on plastic substrate. Consequently, the flexible BEWOLED showed a maximum value for a luminous efficiency of 13.13 cd/A, a quantum efficiency of 5.85%, and CIEx,y coordinates of (0.38,0.39) at 1000 cd/m2, respectively. The luminous and quantum efficiency roll-off values of the BEWOLED were 26.33% and 25.94% between 100 and 7000 cd/m2, which had improved values due to micro-cavity effect than ITO-based conventional BEWOLED.
Acknowledgments
Authors would like to thank Dr. Gun Woo Hyung for helpful discussion. This work was supported by the ERC program (No. 20100009882) of the National Research Foundation (NRF) of Korea grant funded by the Korea Ministry of Education, Science and Technology (MEST) and the Ministry of Knowledge Economy (MKE) of Korea, under the Information Technology Research Center (ITRC)/Convergence Information Technology Research Center (CITRC) support program (NIPA-2012-H0301-12-4013) supervised by the National IT Industry Promotion Agency (NIPA).
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