An ultra-thin NaF film was thermally deposited between ITO and NPB as the buffer layer and then treated with the ultraviolet (UV) ozone, in the fabrication of organic light emitting diodes (ITO/NaF/NPB/Alq3/LiF/Al) to study its effect on hole-injection properties. The treatment drastically transforms the role of NaF film from hole-blocking to hole-injecting. This transformation is elucidated using hole-only devices, energy band measurement, surface energy, surface polarity, and X-ray photoelectron spectra. With the optimal thickness (3 nm) of the UV-ozone-treated NaF layer, the device performance is significantly improved, with a turn-on voltage, maximum luminance, and maximum current efficiency of 2.5 V, 15700 cd/m2, and 4.9 cd/A, respectively. Results show that NaF film is not only a hole-blocking layer, but also a promising hole-injecting layer after UV-ozone treatment.
©2010 Optical Society of America
Multilayer organic light-emitting diodes (OLEDs) have been considered as promising candidates for light-weight, fast-response, full-color displays ever since Tang and Van Slyke reported the first efficient OLED [1–3]. A lot of effort had been put into improving the performance of OLEDs and understanding their underlying mechanisms. Many studies have demonstrated that the interface between electrode and organic layer plays a crucial role in the efficiency of an OLED [4,5]. For example, various treatments, such as O2 plasma and UV-ozone, have been applied to indium tin oxide (ITO) to improve device performance [6–8]. So the an ultra-thin buffer layer has been inserted between ITO and the organic layer to enhance the efficiency of hole-injection or reduce it to balance the concentration between holes and electrons. The insertion of a metal-oxide such as MoOx, ZnO, or Fe3O4 has attracted a lot of attention due to its capability to lower the hole-injection barrier to increase efficiency [9–11]. Metal fluorides such as NaF have been adopted as the hole-blocking layer to achieve a good balance between holes and electrons . However, each attempted buffer material reported plays just one role in effecting the charges injection, i.e. increasing or decreasing the hole-injection. In this study, a hole-blocking NaF buffer layer that can be drastically transformed into an efficient hole-injecting layer using UV-ozone treatment is demonstrated. With the optimal thickness of the UV-ozone treated NaF thin layer, the performance of OLEDs can be greatly enhanced. The mechanisms of performance enhancement are discussed using the results of work function, surface energy, surface polarity, X-ray photoelectron spectra, and hole-only devices. The treatment of NaF thin layer using UV-ozone increases work function, surface energy and, surface polarity, which lead to the improved hole-injection.
2. Experimental details
Prior to film deposition, the ITO glass substrate was cleaned sequentially with a neutraler reiniger/deionized water (1:3 volume) mixture, deionized water, isopropanol and ethanol, followed by UV-ozone treatment (Jetlight UVO-42) for 20 min. The substrate was then covered with a NaF layer using the conventional vacuum thermal evaporation and treated with additional UV-ozone for 20 min. The hole-transporting α-naphthylphenylbiphenyl diamine (NPB) layer, green-emitting tris-8-quinolinolato-aluminum (Alq3) layers, LiF, and an aluminum cathode were deposited to obtain organic light emitting devices with a structure of ITO/NaF(0~3 nm)/NPB(40 nm)/Alq3(40 nm)/LiF(1 nm)/Al(100 nm). The thickness of each layer was controlled by a calibrated quartz crystal oscillator. The deposition rate was 0.1 Å/s for inorganic layers, about 1 Å/s for the organic layer and 4 Å/s for the Al metal cathode. The active area of the devices was 2 mm × 2 mm. The current density-voltage (J-V) and luminance-voltage (L-V) characteristics were measured with a source meter (Keithely-2400) and a luminance meter (LS-100). The UPS (ultraviolet photoelectron spectroscopy) and high resolution XPS (X-ray photoelectron spectroscopy) measurements were performed in air with Riken Keiki AC-2 and Kratos Axis Ultra DLD with a monochromatic Al Kα (1486.6 eV) source, respectively. The contact angle of NaF thin films was measured using the Sessile drop technique with a Contact Angle Goniometer (MagicDrop, USA) under ambient conditions. De-ionized water (H2O) and methylene iodide (CH2I2) were used as the test liquids. The surface energy was estimated from the measured contact angles using the following geometric-mean expression13-15:Table 1 shows the surface tension components of water and CH2I2, which represent polar and nonpolar solvents, respectively .
3. Results and discussion
Figure 1 shows the current density-voltage and luminance-voltage characteristics of the OLEDs with a pristine or a UV-ozone treated NaF layer deposited between the ITO substrate and hole-transporting NPB layer. The deposition of a pristine NaF layer on ITO resulted in an increased threshold voltage, which can be attributed to its hole-blocking effect . The threshold voltage is significantly decreased when the NaF layer was treated with UV-ozone for 20 min. As shown in Fig. 1(a) the current density increased significantly when NaF film thickness was increased (from 1 nm to 3 nm) under a constant operating voltage, indicating highly efficient hole-injection of the UV-ozone-treated NaF thin film. Figure 1(b) shows the luminance-voltage relationship of the devices. Similarly, the turn-on voltage (the bias required to produce a measurable luminance of 1 cd/m2) increased from 3.3 V for the device without a NaF layer to higher than 4.8 V for devices with a 1 nm UV-ozone-treated NaF layer or a pristine NaF layer. The turn-on voltage decreased drastically to 2.5 V and 2.8 V when the thickness of the treated NaF layer was increased to 2 nm and 3 nm, respectively, and returned to about 3.5 V at 4 nm (not shown). To make the J-V and L-V plots understood clearly, we show thickness up to optimal values of 3 nm. The optimal thickness for UV-ozone treated NaF layer is about 3 nm for hole-injecting applications. The operation voltage corresponding to 100 cd/m2 was about 6 V for the OLED device without a NaF layer, whereas it is higher than 7.5 V for those with a pristine NaF layer (2 nm and 3 nm). However, the operation voltage significantly decreased to about 4 V for the device with a 3-nm-thick UV-ozone-treated NaF layer. Moreover, its brightness reached 15700 cd/m2 at 8.5 V, which is superior to 5290 cd/m2 for the device without a NaF layer and a few hundred for those with a pristine NaF layer. Current efficiency increased to maximum 4.9cd/A, as compared to 3.5cd/A for standard device. This is attributable to the enhanced hole-injection efficiency due to the UV-ozone-treated NaF layer. The performance of the device with a standard structure (ITO/NPB/Alq3/LiF/Al) was comparable to that obtained by other groups [16,17]. Table 2 compares optoelectronic performance of our NaF-based OLED device with ZnO- and Li-doped ZnO (LZO)-based devices reported previously [10,20]. The performance of NaF-based OLED device is better than the other two devices, i.e., with lower turn-on voltage and higher current efficiency. The Fig. 1(c) show the current density-voltage characteristics of the hole-only devices with a structure of ITO/with or without NaF/NPB(80 nm)/Au(5 nm)/Au(100 nm). No electroluminescence (EL) was detected for this device due to the negligibly small number of electrons injected from the cathode. This confirms the nature of hole-injection in the OLEDs mentioned above. Under a given operating voltage, the UV-ozone-treated NaF layer (3 nm) obviously increased the current density when compared with that of an untreated one. The device without a NaF layer had the smallest current density. The results indicate that UV-ozone treatment can transform the effect of the NaF layer from hole-blocking to hole-injecting. This dual property of NaF thin layer makes it a versatile material for the fabrication of organic light emitting devices.
Figure 2 (a) shows the relationship between photoelectron emission and incident photon energy for NaF layers deposited on an ITO substrate, pristine and after UV-ozone treatment, as measured by a photoelectron spectrometer (AC-2). The onset of photoelectron emission, defined as the work function, is determined by the intersection of two solid lines. According to our previous study10, the work function of bare ITO is 4.78 eV. It shifts to 4.98 eV and 5.09 eV for the pristine and UV-ozone treated NaF layers, respectively, showing an increase of 0.2 eV and 0.31 eV in incident photon energy relative to bare ITO substrate, respectively. The increase in work function results in lowered Fermi level as shown in the energy band diagram [Fig. 2(b)]. Accordingly, the energy barrier for hole-injection from the anode (UV-ozone treated NaF) is significantly reduced due to its Fermi level being close to the highest occupied molecular orbital (HOMO) of hole-transporting NPB layer. This leads to enhanced hole-injection. However, the hole-blocking property of the pristine NaF layer contradicts the slightly increased work function (4.98 eV) obtained in the UPS measurement. This discrepancy is discussed in a later section.
The effect of surface UV-ozone or O2 plasma treatment and their mechanism have been mentioned in several studies [13–15,18,19]. UV-ozone treatment is a photo cleaning technique to remove trace organic contaminants on surface. The treatment can refill the oxygen vacancy of oxide to increase its work function and to change its surface energy. For non-oxide materials such as silver and copper the treatment can oxidize them to form as metal oxides. However, the studies focused mainly on surface property of neat ITO after surface treatment, barely on those of buffer layer deposited on ITO. In this study, ITO substrates deposited with NaF film were treated with UV-Ozone to investigate their surface property changes. The changes in surface energy and composition of NaF deposited ITO will be discussed in the following sections.
Table 3 summarizes the contact angles of bare ITO substrate, and pristine NaF (3 nm) and UV-ozone treated NaF layers (3 nm) deposited on an ITO substrate, measured using the Sessile drop technique with water and methylene iodide as test liquids. The UV-ozone treatment of NaF layer clearly reduced the contact angles of both test liquids from 55.3° and 36.8° to 14.6° and 21.5°, respectively. In contrast, the pristine NaF layer increased the contact angles to 68.8° and 42° for water and methylene iodide, respectively. This indicates that the deposition of the NaF layer lowers the polarity of the ITO substrate; its polarity, however, can be greatly increased to higher than that of a bare ITO substrate by treating with UV-ozone. Surface energy and surface polarity () can be readily calculated using Eq. (1) and the measured contact angles. These two parameters depend strongly on the physical properties of the surface. As shown in Fig. 3 , the surface energy and polarity of bare ITO substrate are 51.14 mJ/m2 and 0.35, respectively; they decrease to 42.51 mJ/m2 and 0.25 after the substrate is covered with a NaF layer. However, they significantly increase to the highest surface energy of 72.22 mJ/m2 and polarity of 0.56 by after UV-ozone treatment. These results demonstrate that covering an ITO surface with UV-ozone-treated NaF thin layer increases surface energy, surface polarity, and the work function, which in turn improves the device performance. Without UV-ozone treatment, only the work function of the ITO is increased, which leads to inferior device performance. The increase in surface energy and polarity of the NaF layer after UV-ozone treatment may have resulted from its oxidation during the treatment. Thus the treated NaF layer contacts intimately with the hole-transporting NPB layer to facilitate hole-injection. Our results demonstrate that the work function is not the exclusive factor in determining the hole-injection efficiency. Surface energy and polarity play dominating role as well. All three parameters (work function, surface energy, and surface polarity) should be increased simultaneously to enhance device performance.
The enhanced mechanism of the work function and surface energy of NaF film after UV-ozone treatment can be investigated using high resolution X-ray photoelectron spectroscopy (XPS). Figure 4 shows the Na 1s X-ray photoelectron spectra of NaF films before and after UV-ozone treatment. The pristine NaF film shows two peaks which correspond to the binding energies of Na 1s core electrons in Na-F and Na-O bonds, respectively. After the UV-ozone treatment, the maximum peak shifts. This shift of binding energy indicates that most of the Na-F bond transforms to Na-O bond after the UV-ozone treatment. Accordingly, the surface becomes enriched in Na2O due to oxidation during treatment. Furthermore, the change of surface composition caused by oxidization from UV-ozone treatment explains why the work function and surface energy change after UV-zone treatment.
Two absolutely different properties of ultra-thin NaF film as the anode buffer layer in OLEDs (ITO/NaF/NPB/Alq3/LiF/Al) were demonstrated. Depositing a NaF layer on an ITO anode decreased the hole-injection efficiency and made it a hole-blocking layer. However, treating the NaF layer with UV-ozone transformed it to a hole-injecting layer that enhanced the hole-injection efficiency significantly. With the optimal thickness of the UV-ozone-treated NaF layer (3 nm), the turn-on voltage and operating voltage of the devices at 100 cd/m2 decreased to 2.5 V and 4 V, respectively, compared to those of the device without NaF layer (3.3 V, 6 V) . The maximum luminance and maximum current efficiency reached 15700 cd/m2 and 4.9cd/A at 8.5 V, respectively. On the contrary, the devices with a pristine NaF layer had increased turn-on and operating voltages at 100 cd/m2 of 4.8 V and 7.5 V, respectively. To explain this interesting transformation, the changes in work function, surface energy, and surface polarity after the UV-ozone treatment were measured. The treatment led to an increased work function (5.09 eV), surface energy (72.22 mJ/m2) and surface polarity (0.56) relative to those of bare ITO (4.78 eV, 51.14 mJ/m2, and 0.35). Although depositing untreated NaF film slightly increased the work function, it greatly decreased the surface energy and polarity, degrading the device performance. The XPS spectra results suggest that the UV-ozone treatment greatly increased the composition of Na-O bond due to oxidation. The results demonstrate that the UV-ozone treatment of NaF film effectively improves device performance, making it applicable for the fabrication of efficient OLEDs.
This work was supported by the National Science Council of Taiwan under grants NSC 97-ET-7-006-005-ET and NSC-98-2112-M-415-001-MY3. The authors thank the Center for Micro/Nano Technology Research, National Cheng Kung University, Taiwan, for providing a Single-Slide Mask Aligner.
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