This study presents a substantial enhancement in electroluminescence achieved by depositing Ag nanoparticles on an ITO-coated glass substrate (Ag/ITO) for approximately 10-s to form novel window materials for use in polymer light-emitting diodes (PLEDs). The PLEDs discussed herein are single-layer devices based on a poly[9,9-dioctylfluorene-co-benzothiadiazole] (F8BT) emissive layer. In addition to its low cost, this novel fabrication method can effectively increase the charge transport properties of the active layer to meet the high performance requirements of PLEDs. Due to the increased conductivity and work function of the Ag/ITO substrate, the electroluminescence intensity was increased by nearly 3.3-fold compared with that of the same PLED with a bare ITO substrate.
© 2013 Optical Society of America
Polymer light-emitting diodes (PLEDs) that are based on conjugated polymers have attracted much attention in recent years due to their extreme thinness, high peak brightness, high dark room contrast, lower power consumption, superb viewing ability, and fast response time for use in low-cost optoelectronic devices. The typical structure of a PLED is metal (cathode)/polymer film/hole transport layer (HTL)/transparent conducting oxide (TCO) film (anode)/glass substrate, and its quantum efficiency depends on efficient dual carrier injection and transportation to balance the electrons and holes. TCO films are typically manufactured from indium tin oxide (ITO). ITO has been extensively used as an anode in PLEDs because it has excellent visible transmittance, low electrical resistivity, and a relatively high work function (WF). To further improve the luminescence efficiency of PLEDs, several studies [1–7] have strived to increase the conductivity and the WF of ITO anodes by coating an additional hole transport layer. However, the introduction of Ag nanoparticles between the HTL layer and the ITO layer has not been investigated in detail with regard to their effect on the performance of PLEDs.
In this work, we report on the characteristics of a Ag nanoparticle-dispersed ITO anode for enhancing hole injection in phosphorescent PLEDs. The Ag nanoparticles were fabricated using a low-power sputtering method at room temperature. Atomic force microscopy (AFM), conducting atomic force microscopy (CAFM), and scanning surface potential microscopy (SSPM) were employed to elucidate the impact of the Ag nanoparticles on the surface microstructure and electrical properties of the ITO film. Finally, a polymer material was spin-coated onto the Ag nanoparticle-dispersed ITO substrate, followed by deposition onto the metal electrodes to form the proposed PLED structure [Fig. 1(a)]. A spectral measurement system was used to observe any enhancements in the optical gain during electroluminescence. The experimental results showed that the novel PLED device is highly practical and that the simple and low-cost fabrication methods make it commercially feasible.
The samples studied in this work were prepared by a direct current (DC) sputtering system, which has already been described in detail elsewhere [7,8]. Ag films were grown on ITO-coated glass substrates (Merck Display Technologies) with a DC power of 20 W under a constant argon background pressure of 300 mTorr at room temperature. The AFM mass thickness calibration was performed by masking half of a glass substrate and depositing Ag onto the other half. The AFM tip repeatedly scanned the boundary, and the average height was calculated. When the Ag deposition process time was set to 10-s, the Ag was appeared in the form of nanoparticles on the glass surface. The thicknesses of the Ag films (as listed in Table 1) measured by AFM were 0.97, 1.99, 2.98, and 3.98 nm at deposition process times of 20-, 30-, 40-, and 50-s, respectively. ITO-coated glass substrates with a film thickness of 0.2 μm and a sheet resistance of approximately 15 Ω/square were used. Prior to Ag deposition, the substrates were cleaned in a detergent solution (Merck Extran). Then, the substrates were immersed sequentially in a heated ultrasonic bath of deionized (DI) water, isopropyl alcohol, and ethanol for 15-min each, followed by a rinse in DI water. The transmittance spectra of the all of the Ag-dispersed ITO (Ag/ITO) samples were obtained using a Hitachi U-3410 spectrometer (Tokyo, Japan). The optical transmittance of the 10- and 20-s Ag-dispersed ITO substrates shown in Fig. 1(b) was higher than 80% in the green bands.
The local electrical properties of the Ag/ITO substrates were analyzed under ambient conditions using a commercial AFM (Dimension 3100, Veeco Instruments, Santa Barbara, CA). Rectangular Si tips (PPP-EFM, Nanosensors, Switzerland) with a spring constant of 2.50 N/m and a resonant frequency of 75 kHz were used for the AFM measurements. The contact force was held at approximately 60 nN, as determined from a force-distance plot. The tips were pre-coated with a Cr layer and were subsequently coated with a 20 nm PtIr film via ion sputtering. The CAFM and SSPM procedures employed in our studies have been previously described [9–13]. The chemical states of the films were identified by X-ray photoelectron spectroscopy (XPS) using a VG Scientific ESCALAB 250 spherical sector analyzer with a monochromatic AlKα radiation source (1500 eV). The elemental composition was determined by calculating the relative peak areas at specific binding energies.
The proposed structure of the PLED in our experiments is shown in Fig. 1(a). A PEDOT:PSS (poly[3,4-ethylenedioxythiophene]:poly[styrenesulfonate]) film used as the HTL was spin-coated to form a 40-nm-thick film on a Ag/ITO substrate, which was dried in a vacuum at 80 °C for 12-h. F8BT (poly[9,9-dioctylfluorene-co-benzothiadiazole]), which was used as the light-emitting layer, was spin-coated from a toluene solution to form a 50-nm-thick film on top of the PEDOT:PSS layer. Finally, a 1-nm-thick lithium fluoride (LiF) film interfacial layer and a 60-nm-thick Al cathode were selectively deposited through a shadow mask to fabricate the hole array patterns, which had an area of 2 mm × 2 mm, to define the light-emitting region. A sample without Ag nanoparticles was also prepared in the same manner as the standard PLED device to quantitatively evaluate the electroluminescence (EL) enhancement. The current density-voltage (J-V) and luminance-voltage (L-V) characteristics of the devices were both measured using a source meter (Keithley-2400) and a luminance meter (LS-100). All of the current density-voltage-luminance (J-V-L) measurements were performed at room temperature and in an ambient atmosphere without any protective coatings. To elucidate the gain of the quantum efficiency, the EL spectra were characterized by an optical spectrum analyzer (Ando-6315).
3. Results and discussion
To investigate variations in the composition of the Ag/ITO substrates prepared with different Ag deposition process times, we performed elemental analyses of the Ag/ITO surfaces using XPS. The Ag concentration [shown in Table 1] is determined by calculating the peak areas of the binding energy at 364 eV. The calculated concentrations (in atomic percentages) for the Ag/ITO samples deposited with Ag deposition process times of 10-, 20-, 30-, 40-, and 50-s are 15.1%, 31.4%, 47.8%, 54.3%, and 61.9%, respectively.
For a bias of + 10 mV applied to the samples during scanning, topography (left) and current (right) images of the bare ITO and Ag/ITO samples for various Ag deposition process times are shown in Fig. 2. AFM imaging reveals changes in the surface morphology as the Ag deposition process time increased. Table 1 lists the variations in the root-mean-square (RMS) roughness values obtained from the topography images in Fig. 2. The topography image of the bare ITO substrate shown in Fig. 2(a) exhibits a typical crystalline structure with rectangular- and square-shaped grains. As shown in the topography images in Figs. 2(b) and 2(c), when the Ag deposition process time is set to 10- and 20-s, the Ag was evenly distributed in the form of nanoparticles on the ITO surface, and the average size of the nanoparticles was 30 and 50 nm, respectively. As the Ag deposition process time increases to 30-50-s [see topography images in Figs. 2(d)-2(f)], the Ag concentration is high enough to form a uniform layer of Ag nanoparticles, with surface roughness values lower than those obtained for Ag deposition process times of 10- and 20-s.
In the current images, both the conducting and insulator regions are present on the surfaces, and the conducting region is characterized by its brightness. According to the local current-voltage (I-V) measurements, when the tip is located at a point where the current exceeded 10 nA, the measured I-V data follow the relationships expected for Ohmic or Schottky contacts, where the turn-on voltage is less than 1 V, and these surface regions are referred to as conducting regions. However, in the regions in which the current is less than 10 nA, the Fowler–Nordheim electron tunneling mechanism is responsible for the detected current, where the turn-on voltage is more than 5 V, and these regions are referred to as insulator regions. The coverages of the conducting regions in the Ag/ITO samples, as listed in Table 1, are higher than that of the bare ITO substrate. In addition, the coverage of the conducting regions on the Ag/ITO surface increases as the Ag deposition process time increases. To investigate this phenomenon, XPS measurements were performed to evaluate the oxygen vacancies on the Ag/ITO surface.
Figure 3(a) shows the XPS spectrum of O 1s for the bare ITO and the Ag/ITO substrates. In comparison with the bare ITO substrate, the peak shifted slightly toward lower binding energies after the deposition of Ag. The typical O 1s peak on the Ag/ITO surface can be consistently fitted using three nearly Gaussian curves [14–16]. The component on the low binding energy side of the O 1s spectrum at 527.3 ± 0.08 eV [shown in Fig. 3(b)] is attributed to the In2O3-like oxygen corresponding to O2- ions in the tetrahedral interstices of the face-centered cubic ln3+ lattice. The high binding energy component located at 529.8 ± 0.06 eV [shown in Fig. 3(d)] has been reported to correspond to the presence of loosely bound oxygen on the surface of the Ag/ITO film belonging to specific species of adsorbed H2O or O2−. The intermediate binding energy component centered at 528.6 ± 0.05 eV [shown in Fig. 3(c)] is associated with O2− ions in oxygen-deficient regions within the matrix of In2O3. Therefore, the changes in the intensity of this component may be partially due to variations in the concentration of oxygen vacancies.
As shown in Fig. 3(c), the Ag deposited on ITO for approximately 10-s increases the oxygen vacancies by approximately 18%. However, when the Ag deposition process time is set at 20-, 30-, 40-, and 50-s, the oxygen vacancies in the Ag/ITO samples are lower than the oxygen vacancies on the bare ITO substrate. As the Ag deposition process time increases, the oxygen vacancies in the Ag/ITO samples decreases, which is primarily due to the Ag being distributed on the ITO surface in the form of nanoparticles [see topography image in Fig. 2(b)] when the Ag deposition process time is set to approximately 10-s. The Ag nanoparticles capture some of the oxygen atoms on the ITO surface to form Ag2O nanoparticles that enhance the production of oxygen vacancies on the ITO surface. The conductivity of the Ag/ITO surface during this time is determined by the oxygen vacancies. However, when the Ag deposition process time gradually increases from 20- to 50-s, the Ag coverage ratio on the ITO surface gradually increases, and the Ag free electrons become the main source of conductivity on the Ag/ITO surface. This result arises because the conductivity of the Ag/ITO surface increases significantly as the number of oxygen vacancies on the Ag/ITO surface rapidly decreases due to the increased Ag deposition process time.
The contact potential difference (CPD) distribution on the Ag/ITO surface is captured by SSPM in the “lift mode” at a lift scan height (tip-sample separation) of 30 nm with a 5-V AC voltage applied to the tip. The measurement is performed concurrently with the topography scan in tapping mode using a Cr/PtIr-coated tip. The mean WFs of the bare ITO and the Ag/ITO samples, calculated from the CPD values , are listed in Table 1. According to the analysis results, when the Ag deposition process time is set to approximately 10-s, the mean WF of the Ag/ITO surface is greater than that of the bare ITO surface because the majority of the Ag nanoparticles on the ITO surface form Ag2O nanoparticles. The WF of Ag2O is 5.20 eV, which is higher than that of bare ITO at 4.82 eV, and thus enhances the mean WF of the Ag/ITO surface. When the Ag deposition process time is longer than 30-s, as indicated in the topography images [see Figs. 2(d)-2(f)], the density of the Ag nanoparticles on the Ag/ITO surface can form a thin film state. Then, the mean WF of the Ag/ITO surface is determined by the Ag film. The WF of Ag is 4.20 eV, which is lower than that of bare ITO (i.e., 4.82 eV). Therefore, the mean WF of the Ag/ITO surface decreases as the Ag deposition process time increases. It should be noted that when Ag is deposited on ITO for approximately 20-s, the distribution density of the Ag2O nanoparticles on the ITO surface is equivalent to the density of the Ag nanoparticles. In this work, the mean WF of the Ag/ITO surface is similar to the mean WF of the bare ITO surface.
To further understand the functions of the Ag/ITO substrate, the hole-only devices were fabricated with the structures of Au/F8BT/PEDOT:PSS/ITO and Au/F8BT/PEDOT:PSS/Ag (various deposition process times)/ITO. The J-V characteristics are depicted in Fig. 4. It is revealed that with the 10-s Ag-dispersed ITO, hole-only device shows much higher hole current and lower turn-on voltage than others. The hole-only device with 10-s Ag-dispersed ITO anode exhibits a good improvement because the injection barrier for holes taken as the difference between the WF of PEDOT:PSS (5.0 eV) and the WF of the 10-s Ag-dispersed ITO (4.92 eV) is smaller than that of other hole-only devices. This result is in agreement with the data of SSPM measurement.
Figure 5 compares the performance of the Ag-dispersed PLEDs with various Ag deposition process times. Figure 5(a) shows that as the Ag deposition process time increases, the luminance continuously decreases and undergoes a sharp decrease for the 30-s Ag-dispersed PLED. In comparison with the normal PLED, both the turn-on voltage (at 1 cd/m2) and the operation voltage (at 100 cd/m2) decrease from 5.0 V and 6.5 V to 4.7 V and 6.0 V, respectively, and the device peak luminance continuously increases up to 2500 cd/m2 for the 10-s Ag-dispersed PLED. The resulting current efficiencies in Fig. 5(b) show a much higher EL efficiency for the 10- and 20-s Ag-dispersed PLED compared with the normal PLED. The PLED with 10-s Ag-dispersed ITO anode exhibits a high performance due to the reduction of the hole injection barrier for holes between the HTL layer and TCO layer. In addition, the coverage of the conducting regions of the 10-s Ag-dispersed ITO is higher than that of the bare ITO substrate [see Table 1]. The decrease in the hole-injection barrier and increase in the conductivity result in an improved EL efficiency for the PLEDs. Figure 5(c) shows the luminance enhancement as a function of the applied voltage for the Ag-dispersed PLEDs. In particular, the 10- and 20-s Ag-dispersed PLEDs exhibit the highest luminance enhancement of 122% and 90% at 8.2 and 7.7 V, respectively. Figure 5(d) shows the room-temperature EL spectra recorded at 8.2 V and normalized by the peak intensities for all PLEDs. All of the spectra coincide in terms of the peak positions and shapes but have obviously distinct intensities at a wavelength of 540 nm. This result strongly indicates that the origin of the trap states [17–19] is not due to the presence of the Ag/ITO substrate but is due to a relatively high density of natural impurities in the polymer thin films. For the EL intensity at 540 nm, the 10-s Ag-dispersed PLED is nearly 3.3-fold higher than that of the PLED with a bare ITO anode.
This paper proposes a PLED structure consisting of Al/LiF/F8BT/PEDOT:PSS/Ag nanoparticles/ITO. The CAFM measurement showed that the Ag deposited on ITO at approximately 10-s increases the oxygen vacancies, resulting in an increase in the coverage of the conducting regions by 5.9%. In addition, due to the Ag2O nanoparticles on the Ag/ITO surface, the mean WF measured by SSPM increases from 4.82 eV to 4.92 eV, which can effectively improve the hole injection efficiency for PLEDs. When the optimal Ag-deposited ITO substrate is used as the anode material for a green PLED, the EL intensity increases by 330% compared with a standard green PLED with a bare ITO substrate. The prepared device offers a new design scheme for optimizing carrier injection and recombination, which may be advantageous for various organic semiconductor-based devices.
The authors gratefully acknowledge financial support from the National Science Council of Taiwan under grant No. NSC 100-2112-M-415-001-MY3.
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