Open Access
Add to BibSonomyAbstract
A precisely controlled metallic nanomesh was fabricated by using nanosphere lithography to pattern the silver thin film to form hexagonal nanohole arrays with excellent uniformity, high conductivity and good transparency. An Alq3 based OLED, with the silver nanomesh electrode of high ðll factor of 70.2% demonstrated a considerable luminous efðciency of 4.8 cd/A, which is 60.9% higher than the referenced device with ITO anode. The periodical nanohole array not only increased the transparency but also helped extracting surface plasmonic wave in organic layers. By attaching the microlens array to further extract the trapped light in substrate, the extraction efficiency enhancement of device with nanomesh anode was 73.8% higher than 50.2% of the referenced device with ITO anode. And the overall current efficiency of device with nanomesh anode was 87.7% higher than traditional ITO based device.
©2013 Optical Society of America
1. Introduction
Organic light-emitting diode (OLED) is of particular importance because of its various applications in flat panel display and solid-state lighting. It takes advantages of excellent electroluminescence performances, good flexibility and convenient fabrication [1, 2]. Indium tin oxide (ITO) has been the most widely employed material to form the transparent electrode in OLEDs due to its excellent conductivity and high transparency. But due to indium scarcity and resulting high cost, another effective techniques for ITO alternative have become intense investigation for applications as transparent electrodes. In recent years, many researchers have proposed different kinds of transparent conductive electrodes, such as conducting oxides, carbon nanotubes and graphene [3–9]. These materials show the potentials to replace the ITO electrodes. On the other hand, metallic thin films have high conductivity but lack of transparency. Very recently, researchers patterned the metal thin film to form periodic nanohole array to increase its transmittance and preserve good conductivity [10, 11]. This kind of nanoporous metal electrode has drawn a lot of attention for its high transparency and conductivity. The nanoporous metal electrode also shows good ductility which has great potential for flexible optoelectronic devices. For light-emitting devices, this kind of patterned electrode has the additional advantage to couple out light trapped in the high-index organic layers. In general, only a small fraction of photons generated inside can escape from device, because of the total internal reflection (TIR) at the substrate surface and wave-guiding effect in the high index organic layers [12–14]. Lower than one third of the generated photons can escape from emitting devices. One third of these photons are guided in the glass substrate and the others are trapped in the organic layers [14]. Many techniques had been proposed to couple light out of the high-index guided mode, such as the fabrication of the micro/nanostructures into devices or insertion of low index material [15–18]. But the extracted light from organic layers also suffers from the TIR between glass substrate and air. A more luminous enhancement would be achieved after applying light-coupling structures on the substrate to enhance both internal and external outcoupling efficiency [19]. In this paper, we fabricated a metallic nanomesh as the transparent anode of OLED using nanosphere lithography. In addition, a microlens array was attached onto the proposed device to further extract the trapped photons in the device substrate. By combining the patterned electrode and the microlens array, the extraction efficiency enhancement was increased 73.8% and the overall current efficiency was 87.7% higher than traditional ITO based device.
2. Silver nanomesh by using nanosphere lithography
The fabrication of OLED with metallic nanomesh electrode can be divided into two major parts, including metallic nanomesh patterning by nanosphere lithography and conventional organic layers evaporation. The schematic diagram of fabrication process is shown in Fig. 1 . In the part of nanosphere lithography, a monolayer of polystyrene (PS) nanospheres was formed on the glass substrate as a mask of hexagonal array. In this fabrication process, commercial PS nanospheres were dispersed in a solution of methanol and surfactant Triton X-100 and spin-coated onto a clean flat glass substrate. By controlling the concentration of nanosphere solution and rotation speed of the spin coater, the nanospheres were self-assembled to form a monolayer with closed-packed arrangement.
The period of the hexagonal array was 590 nm which is determined by the diameter of PS spheres. Then oxygen reactive-ion etching (RIE) was used to etch the edge of the PS nanospheres and reduce their size in order to adjust the fill-factor. The fill-factor is defined by the ratio of the area of nanohole array divided by the total area. The corresponding SEM images of nanospheres under different etching condition are shown in Figs. 2(a) -2(c). Figure 2(d) shows the diameter of nanosphere and the corresponding fill-factor as a function etching time. The fill-factor can be tuned from 0.7 to 0.5 by controlling the RIE etching time. Subsequently, electron beam evaporation is used to deposit metals on the substrate to fill the gap with silver thin film. The sample was then put into the dichloromethane (CH2Cl2) solution in an ultrasonic cleaner. After lift-off process, the PS nanospheres were removed. The silver remained on the substrate and formed a periodic silver nanomesh.
Fig. 2 SEM images of close-packed PS nanospheres with different etching conditions: (a) RIE power of 60W for 5 minutes (fill-factor: 0.70), (b) RIE power of 60W for 5 minutes and 30W for 3 minutes (fill-factor: 0.58), (c) RIE power of 60W for 5 minutes and 30W for 6 minutes (fill-factor: 0.55), and (d) diameter and fill-factor of PS nanosphere as a function of etching time.
The SEM images of the silver nanomesh with different fill-factor were shown in Figs. 3(a) -3(c). The patterned thin film was further treated with an annealing process at 200°C for 3 hours under ultra-high vacuum condition (<10−7 torr) in order to reform more compact metal grains. The annealing treatment is crucial to the reduction of background defects and decrease of the grain boundary scattering resistivity [10, 20]. The transmittance spectra of the silver nanomesh with different fill-factor and Ag thickness were shown in Figs. 3(d) and 3(e). Higher fill-factor and thinner film thickness generally result in a higher transmittance. However, the sheet resistance is also increased with the transmittance. The relation between average visible transmittance and sheet resistance is shown in Fig. 3(f). There is a trade-off between the optical transmittance and electrical resistance of the silver nanomesh. In this work, we used the silver nanomesh with film thickness of 15 nm and a fill-factor of 70.2% as our OLED transparent anode, which has an average transmittance of 74.2% and a sheet resistance of 30.67Ω/□.
Fig. 3 SEM images of silver nanomesh by using nanospheres with different etching conditions: (a) RIE power of 60W for 5 minutes (fill-factor: 0.70), (b) RIE power of 60W for 5 minutes and 30W for 3 minutes (fill-factor: 0.58), (c) RIE power of 60W for 5 minutes and 30W for 6 minutes (fill-factor: 0.55). Transmittance of (d) 15 nm and (e) 10 nm Ag thin film. (f) Relation between sheet resistance and average visible transmittance.
3. Light extraction improvement of OLED with silver nanomesh anode
The OLED devices are prepared on the silver nanomesh with surface treatment by using self-assembled monolayers (SAMs) of binary mixtures of n-decanethiol and the fluorinated analogue [21]. The SAM layer was not only used as the protection layer to smooth the patterned surface but also increased the work function of silver thin film (from 4.68 eV to 5.6 eV). After the surface treatment, organic layers were deposited in ultra-high vacuum (around 5 × 10−6 torr) thermal evaporator. Hole-transport layer (NPB, 60 nm) and electron-transport layer (Alq3, 80 nm) were evaporated sequentially on the surface-modified anode. LiF and Al were set as the opaque cathode of the device. After evaporation, the devices were encapsulated with glass in glove box chamber. For comparison, we also fabricated OLEDs with conventional ITO anode, planar silver thin film.
Figure 4(a) shows the J-V curves and the optical images of the bottom emission for those devices with different kinds of anodes. Due to the better energy level alignment between the work function of the anode and the HOMO level of the NPB, the turn-on voltages for SAM-coated devices was lower than the ITO one. Figure 4(b) shows the luminous efficiency versus the current density. The saturated current efficiency was 2.86 cd/A (ITO anode), 4.65 cd/A (planar silver anode), and 5.4 cd/A (silver nanomesh anode), respectively. After applying microlens, the luminous efficiency was 4.31 cd/A (ITO anode), 5.12 cd/A (planar silver anode), and 8.08 cd/A (silver nanomesh anode), respectively. The luminous enhancement of device with nanomesh anode attached with microlens array was 73.8% higher than 50.2% of the referenced device with ITO anode. Combined with microlens arrays on the substrate surface, the overall current efficiency of nanomesh anode was 87.7% higher than traditional ITO based device. To verify the mechanisms for the efficiency enhancement, we measured the luminous spectra of the devices with different anodes. Figures 4(c) and 4(d) show luminous spectra with and without the microlens array attachment. The strong microcavity effect of the device with planar silver anode narrows the emission spectrum. It increases photons directly escaped from emitting devices. Therefore, the current efficiency of planar silver anode is better than ITO anode. After fabricating nanohole array on the silver anode using nanosphere lithography, the emission spectrum is broadened and the current efficiency is increased. It indicates that cavity effect is reduced and light extraction from trapped photons is increased.
Fig. 4 (a) I-V curves (inset: optical images of OLEDs with different kinds of anodes), (b) current efficiency versus current density, (c) luminous spectra without microlens array film attachment, and (d) luminous spectra with microlens array film attachment.
To confirm the light extraction enhancement, we measured the emission light at different view angles. Figure 5(a) shows SEM images of the square array of the hemispherical polymer microlens, where the diameter and gap are 10 μm and 2 μm, respectively. The testing devices were attached with microlens array film by index matching oil. This microlens array acts as the backlight enhancement film (BEF) which can efficiently extract the substrate guiding wave. The microlens array is made of polymethylmethacrylate (PMMA) with a refractive index of 1.49. Figure 5(b) shows luminous intensity of devices at different viewing angles with a constant current injection (60 mA/cm2). When we used the silver nanomesh as a transparent anode, the angular emission pattern was broadened. The angular FWHM of radiation pattern was 63.8° (ITO anode), 56.8° (planar silver anode), and 74.4° (silver nanomesh anode), respectively. The luminance of ITO and unpatterned silver film drops as increasing view angles, but there is a curve peak for patterned silver thin film. In a typical OLED, generally less than one third of the generated photons can escape from emitting devices. One third of these photons are guided in the glass substrate and the others are trapped in the organic layers [22] as shown in Fig. 6(a) . When periodic patterns are made in the metallic anode, those trapped photons are coupled out from glass to air (glass substrate mode) and from organic layers to glass as indicated in Fig. 6(b).
Fig. 5 (a) SEM images of microlens array, (b) angle-dependent luminance of OLED, (c) angle-dependent luminance of OLED attached with microlens array film, and (d) luminous enhancement ratio at different viewing angle (injection current: 60 mA/cm2)
Fig. 6 Light extraction of trapped photons in glass substrate (ray I) and organic layers (ray II) of bottom emission OLED. (a) Planar transparent anode, (b) patterned transparent anode, and (c) patterned transparent anode with microlens array attachment.
The coupling of light can be understood by using the grating coupling equation. For the light trapped in the substrate (ray I), the diffraction angle, θb in the glass substrate is calculated by
Where m is diffraction order, kglass is the propagation constant in the glass. In the case of nb = 1.52 (BK7 glass), m = 1, λ0 = 550 nm and periodic silver structures (Λ = 590 nm), θb is 22.75°. In the air, the refraction angle, θ, is 36° as calculated by the Snell's law, sin(θ) = 1.52*sin(θb). This angle is in good agreement with the curve peak for the patterned silver anode as seen in Fig. 5(b). In organic layers, the interface between metallic electrodes and organic materials forms surface plasmonic waveguide. For the surface plasmonic mode, the coupling equation is written asWhere εm is the permittivity of metal, nf is the refractive index of organic materials. In the case of nf = 1.8 and εm = −10, the diffraction angle in the glass substrate is 55.8°. This angle is larger than the critical angle (43.14°) of the glass/air interface. Therefore, photons coupled from the organic layers are still confined in the glass substrate. By attaching the microlens on the device surface, those photons can directly emit into the air without the TIR confinement as depicted in Fig. 6(c). It results in a higher luminescence and the curve peak is occurred at a larger viewing angle as seen in Fig. 5(c). Figure 5(d) shows the luminous enhancement ratio as a function of viewing angle for different anodes with the microlens array. Compared to the ITO anode, the angular spectrum of OLED with flat silver anode is narrowed due to the cavity effect. More photons directly emit into the air without TIR confinement. Therefore, light extraction enhancement by the microlens array is smaller than the ITO-based device. For patterned silver anode, the 590-nm-period Ag nanomesh couples surface plasmonic mode into glass substrate mode. More light is extracted by the microlens array due to the increase of trapped photons in the glass substrate.4. Conclusions
In summary, an Alq3-based OLED using the silver nanomesh anode with high transparency and good conductivity was demonstrated. Compared to the traditional ITO-based device, the periodic nanohole array of the silver nanomesh effectively enhances the extraction efficiency of the OLEDs. The patterned metallic anode plays two roles for the efficiency enhancement of the device. First, it helps coupling light out of the glass substrate. Second, it leads to the redistribution of surface plasmon mode to substrate mode. After applying the microlens onto the device, the trapped substrate mode is further coupled out. Therefore, a higher luminous enhancement was obtained for device with nanomesh anode and microlens as compared to ITO and planar silver thin film anode. In the experiments, The overall current efficiency of device with nanomesh anode was 87.7% higher than traditional ITO based device. Currently, we applied the nanosphere lithography technique to prove the concept of combinational light enhancement by internal metallic nanomesh anode and external BEF sheet. The metallic nanomesh structures can be made by using nanoimprint technique for mass production and reduce the fabrication cost.
Acknowledgments
This work was supported by National Science Council, Taipei, Taiwan, under Contract No. NSC-101-2627-E-002-005, NSC-100-2120-M-007-006 and NSC-100-2221-E-001-010-MY3.
Reference and links
1. C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes,” Appl. Phys. Lett. 51(12), 913–915 (1987). [CrossRef]
2. J. Huang, G. Li, E. Wu, Q. Xu, and Y. Yang, “Achieving high-efficiency polymer white-light-emitting devices,” Adv. Mater. 18(1), 114–117 (2006). [CrossRef]
3. M. Izaki and T. Omi, “Transparent zinc oxide films prepared by electrochemical reaction,” Appl. Phys. Lett. 68(17), 2439–2440 (1996). [CrossRef]
4. R. Hoffman, B. Norris, and J. Wager, “ZnO-based transparent thin-film transistors,” Appl. Phys. Lett. 82(5), 733–735 (2003). [CrossRef]
5. X. Jiang, F. Wong, M. Fung, and S. Lee, “Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices,” Appl. Phys. Lett. 83(9), 1875–1877 (2003). [CrossRef]
6. G. Jain and R. Kumar, “Electrical and optical properties of tin oxide and antimony doped tin oxide films,” Opt. Mater. 26(1), 27–31 (2004). [CrossRef]
7. F. Yang and S. Forrest, “Organic solar cells using transparent SnO2–F anodes,” Adv. Mater. 18(15), 2018–2022 (2006). [CrossRef]
8. C. D. Williams, R. O. Robles, M. Zhang, S. Li, R. H. Baughman, and A. A. Zakhidov, “Multiwalled carbon nanotube sheets as transparent electrodes in high brightness organic light-emitting diodes,” Appl. Phys. Lett. 93(18), 183506 (2008). [CrossRef]
9. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi, and B. H. Hong, “Large-scale pattern growth of graphene films for stretchable transparent electrodes,” Nature 457(7230), 706–710 (2009). [CrossRef] [PubMed]
10. Y. H. Ho, K. Y. Chen, S. W. Liu, Y. T. Chang, D. W. Huang, and P. K. Wei, “Transparent and conductive metallic electrodes fabricated by using nanosphere lithography,” Org. Electron. 12(6), 961–965 (2011). [CrossRef]
11. J. Zhu, X. Zhu, R. Hoekstra, L. Li, F. Xiu, M. Xue, B. Zeng, and K. L. Wang, “Metallic nanomesh electrodes with controllable optical properties for organic solar cells,” Appl. Phys. Lett. 100(14), 143109 (2012). [CrossRef]
12. N. K. Patel, S. Cinà, and J. H. Burroughes, “High-efficiency organic light-emitting diodes,” IEEE J. Sel. Top. Quantum Electron. 8(2), 346–361 (2002). [CrossRef]
13. C. F. Madigan, M.-H. Lu, and J. C. Sturm, “Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification,” Appl. Phys. Lett. 76(13), 1650–1652 (2000). [CrossRef]
14. Y. R. Do, Y. C. Kim, Y. W. Song, and Y. H. Lee, “Enhanced light extraction efficiency from organic light emitting diodes by insertion of a two-dimensional photonic crystal structure,” J. Appl. Phys. 96(12), 7629–7636 (2004). [CrossRef]
15. Y. R. Do, Y. C. Kim, Y. W. Song, C.-O. Cho, H. Jeon, Y. J. Lee, S. H. Kim, and Y. H. Lee, “Enhanced light extraction from organic light-emitting diodes with 2D SiO2/SiNx photonic crystals,” Adv. Mater. 15(14), 1214–1218 (2003). [CrossRef]
16. S. Y. Hsu, M. C. Lee, K. L. Lee, and P. K. Wei, “Extraction enhancement in organic light emitting devices by using metallic nanowire arrays,” Appl. Phys. Lett. 92(1), 013303 (2008). [CrossRef]
17. D. K. Gifford and D. G. Hall, “Emission through one of two metal electrodes of an organic light-emitting diode via surface-plasmon cross coupling,” Appl. Phys. Lett. 81(23), 4315–4317 (2002). [CrossRef]
18. K. Ishihara, M. Fujita, I. Matsubara, T. Asano, S. Noda, H. Ohata, A. Hirasawa, H. Nakada, and N. Shimoji, “Organic light-emitting diodes with photonic crystals on glass substrate fabricated by nanoimprint lithography,” Appl. Phys. Lett. 90(11), 111114 (2007). [CrossRef]
19. Y. Sun and S. Forrest, “Enhanced light out-coupling of organic light-emitting devices using embedded low-index grids,” Nat. Photonics 2(8), 483–487 (2008). [CrossRef]
20. C. Cong, T. Yu, Z. Ni, L. Liu, Z. Shen, and W. Huang, “Fabrication of graphene nanodisk arrays using nanosphere lithography,” J. Phys. Chem. C 113(16), 6529–6532 (2009). [CrossRef]
21. K. Y. Wu, S. Y. Yu, and Y. T. Tao, “Continuous modulation of electrode work function with mixed self-assembled monolayers and its effect in charge injection,” Langmuir 25(11), 6232–6238 (2009). [CrossRef] [PubMed]
22. Y. H. Ho, Y. T. Chang, S. W. Liu, H. H. Lai, C. W. Chu, C. I. Wu, W. C. Tian, and P.-K. Wei, “Optimization of polymer light emitting devices using TiOx electron transport layers and prism sheets,” Org. Electron. 13(11), 2667–2670 (2012). [CrossRef]
