Efficient Semitransparent Bulk-Heterojunction Organic ... - IEEE Xplore

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 6, NOVEMBER/DECEMBER 2010

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Efficient Semitransparent Bulk-Heterojunction Organic Photovoltaic Cells With High-Performance Low Processing Temperature Indium–Tin Oxide Top Electrode Xizu Wang, Ging-Meng Ng, Jian-Wei Ho, Hoi-Lam Tam, and Furong Zhu (Invited Paper)

Abstract—An efficient semitransparent bulk-heterojunction zinc phthalocyanine (ZnPc): fullerene (C6 0 )-based photovoltaic cell with a transparent cathode of Ag/LiF/indium–tin oxide (ITO) is demonstrated. The top ITO layer serves not only as an index matching layer to enhance the light in-coupling in semitransparent small molecule photovoltaic cells, but also improves current spreading due to its superior optical transparency and high electric conductivity. In order to avoid causing damages to the underlying functional photoactive organic layers, the ITO top electrode was formed at room temperature without intentional heating. Optimization of light distribution in the semitransparent ZnPc:C6 0 photovoltaic cells was performed using an optical admittance analysis. The performance of the semitransparent organic photovoltaic cells is optimized over the two competing parameter of power conversion efficiency (PCE) and optical transparency. Semitransparent bulk-heterojunction ZnPc:C6 0 photovoltaic cells with an average transmission of more than 40% in the visible light region and a PCE of ∼3.0% measured under simulated AM1.5G illumination of 100 mW/cm2 were obtained. Index Terms—Bulk heterojunction, low processing temperature, organic photovoltaic (OPV), semitransparent.

I. INTRODUCTION HE DEVELOPMENT of emerging organic photovoltaic (OPV) cells has attracted a lot of interest due to its great potential for low-cost photovoltaic technology [1], [2]. The organic semiconductor materials used also exhibit remarkable flexibility in material properties, such as molecular weight, light absorption, band gap energy, etc. These can be tuned for new device concepts. Hence, OPV technology creates an avenue for the design and synthesis of new molecules that can be eventually integrated with organic–organic and nanostructure–organic composites for novel solar cells. For OPV cells, current efforts

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Manuscript received March 8, 2010; revised April 12, 2010; accepted April 18, 2010. Date of publication June 21, 2010; date of current version December 3, 2010. The work of F. Zhu was supported by the Hong Kong Baptist University Strategic Development Fund under Grant SDF10-0118-P01. X. Wang, G.-M. Ng, J.-W. Ho, and H.-L. Tam are with the Institute of Materials Research and Engineering, Singapore 117602 (e-mail: [email protected]. edu.sg; [email protected]; [email protected]; [email protected]). F. Zhu is with the Physics Department and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2010.2049004

are centered on improving the materials’ properties and the devices’ performance. In addition, OPV cells can be made flexible, large area, and light weight [3]–[9], thereby offering additional opportunities for applications in new markets, such as mobile electronics, disposable electronics, smart cards, power generating windows, outdoor lifestyle, etc. Through innovations in device architecture, development and processing of materials, OPV cells can even be made semitransparent. The potential for semitransparency in OPV cells does not exist for conventional opaque silicon photovoltaic cells. This potential is still being developed. One of the key components in semitransparent OPV cells is the transparent top electrode. An ultrathin layer or a bilayer of semitransparent metal cathode often used in semitransparent OPV cells exhibits very low resistance to moisture and oxygen resulting in short lifetimes. This aside, the ultrathin metal cathode also causes a large amount of reflection at the metal/air interface due to its high refractive index. A compound semitransparent cathode consisting of ultrathin metal/optical capping layer is required. A variety of transparent electrodes has been applied for applications in top-emitting or stacked organic LEDs (OLEDs) [10], [11], inverted, tandem, and semitransparent organic solar cells [12]–[19]. An inverted OPV cell with a top cathode, which was made using a combination of an ultrathin metal contact and a tris-(8-hydroxyquinoline) aluminum (Alq3 ) capping layer was reported [20]. It was demonstrated that the use of an Alq3 capping layer enhances the light harvesting in the inverted organic cells. However, the organic capping layer is not stable if the device is operated in air. In this study, a compound semitransparent cathode with a high-performance low processing temperature indium–tin oxide (ITO) capping layer was used as the top electrode for our semitransparent OPV cells. An optical admittance analysis is used to study the optical enhancement in zinc phthalocyanine (ZnPc): fullerene (C60 )-based photovoltaic cells. The results of this study have yielded an optical transparency of >40% and a power conversion efficiency of ∼3% for a semitransparent small molecule photovoltaic cell of glass/ITO/ZnPc:C60 (35 nm)/ C60 (25 nm)/4,7-diphenyl-1,10-phenanthroline (BPhen)(7 nm)/ Ag(20 nm)/LiF(5 nm)/ITO(40 nm). The possible designs to improve the performance of semitransparent OPV cells over

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the two competing indexes of power conversion efficiency and transmittance are discussed. II. EXPERIMENTAL DETAILS An oxidized target (6 in in diameter) with In2 O3 and SnO2 in a weight ratio of 9:1 was employed for the deposition of the top ITO electrode. The substrate was not heated during and after the film deposition. The top ITO electrode was prepared by dc magnetron sputtering with a power of 10 W, the deposition rate was estimated to be about 2 nm/min. The base pressure in the sputtering system was approximately 2.0 × 10−4 Pa. During the film deposition, an argon–hydrogen gas mixture was employed. The argon partial pressure was set at ∼2.9 × 10−1 Pa, the hydrogen partial pressure was varied from 1.1 × 10−3 to 4.0 × 10−3 Pa to modulate and optimize the properties of ITO films. The thickness of the top ITO electrode was controlled by varying the deposition time. Bulk-heterojunction ZnPc:C60 -based photovoltaic cells were fabricated on commercial ITO-coated (thickness ∼150 nm and sheet resistance ∼15 Ω/sq) glass substrates. The ITO substrates were cleaned using acetone, isopropanol, and deionized water in an ultrasonicator. This was followed by oxygen plasma treatment. A 35-nm-thick photoactive blend layer of ZnPc (donor):C60 (acceptor) in a volume ratio of 1:1 was deposited via coevaporation on the ITO substrate. Next, a 25-nm-thick C60 acceptor layer was deposited on the ZnPc:C60 blend followed with a 7-nm-thick (BPhen) exciton-blocking layer. The fabrication of the semitransparent OPV cells was then completed by overlaying a multilayer semitransparent top cathode consisting of Ag(20 nm)/LiF(5 nm)/ITO(40 nm). A control OPV cell with an identical organic device structure, but finished with a 100-nm-thick Ag cathode was also fabricated for comparison studies. The ZnPc, C60 , BPhen, LiF, and metal layers were thermally evaporated in a vacuum chamber with a base pressure of ∼ 10−5 Pa. Both semitransparent and control OPV cells had an active area of 3 mm × 3 mm. The current density–voltage (J–V) characteristics of the OPV cells were measured under AM1.5G illumination at 100 mW/cm2 (300 W SAN-EI XEC-301 S solar simulator; beam size: 4 in diameter). The light intensity was determined by a monosilicon detector (with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory to minimize spectral mismatch. III. RESULTS AND DISCUSSIONS Traditional ITO thin film often requires high processing temperature of over 200 ◦ C and is thus not suitable for application in semitransparent OPV cells. In this study, the substrate was not heated during the ITO deposition. The substrate temperature induced by the plasma was lower than 60 ◦ C [10], [21]. The sheet resistance and resistivity of ITO films as a function of hydrogen partial pressure are plotted in Fig. 1. Both the sheet resistance and resistivity decreased dramatically after introducing hydrogen into the sputtering gas mixture. The use of a hydrogen–argon gas mixture allowed a broader process window for preparation of ITO films with high electric conductivity. For instance, ITO films with a thickness of 130 nm and sheet

Fig. 1. Sheet resistance and resistivity of low processing temperature ITO films as a function of hydrogen partial pressure.

resistance of ∼30 Ω/sq can be fabricated over hydrogen partial pressures from about 1.0 − 3.0 × 10−3 Pa at room temperature. The optical transparency of low processing temperature ITO top electrode has an average transmittance of above 85%. The ITO-based top electrode for application in semitransparent OPV cells must meet the requirements of high conductivity, good stability in film conductivity, good contact with the underlying organic/inorganic layer and made at a low process temperature. The optimal structure of the OPV cells was also investigated using the optical admittance analysis. The simulation uses the dispersive refractive index and extinction coefficient of the materials to calculate the optical absorbance and transmittance of the OPV cells. The wavelength dependent refractive index (n) and extinction coefficient (k) of thin films of ITO, ZnPc, C60 , BPhen, and ZnPc:C60 mixture were measured using variable angle spectroscopic ellipsometry, n and k as a function of the wavelength measured for the photoactive layer of ZnPc:C60 are plotted in Fig. 2(a). In this study, the dispersive complex refractive index of Ag and LiF used for the simulation of the optical properties of devices were taken from the [22]. The bulk-heterojunction OPV cells have a typical layer structure of glass/ITO/ZnPc :C60 /C60 /BPhen/cathode. In a previous study, an ITO-based translucent cathode made with RF magnetron sputtering was demonstrated for application in semitransparent polymeric solar cells [16]. Although RF magnetron sputtering can be used for making ITO-based translucent cathode for small size transparent OPV cells, a scalable ITO cathode that can be fabricated with dc magnetron sputtering has a great impact for potential mass production at a low cost, a thin film deposition process that is widely adopted by ITO coating industry. In this study, it was found that a transparent cathode of Ag(20 nm)/LiF(5 nm)/dc-sputtered-ITO was more effective as compared with Ag(20 mm)/dc-sputtered-ITO as top transparent cathode for semitransparent OPV cells. It is possible that a combined Ag(20 nm)/LiF(5 nm) layer may act as an effective buffer layer to reduce the possible damage at the organic/cathode interface, induced due to ITO deposition by dc magnetron sputtering, resulting in a better contact property at organic/cathode interface. Fig. 2(b) illustrates the optical simulation results of the light distribution at a wavelength of 630 nm in a semitransparent ZnPc:C60 -based photovoltaic cell. The layer thicknesses have been optimized for application in semitransparent OPV

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Fig. 3. Calculated results showing the effect of the top ITO electrode thickness on the integrated absorptance of the ZnPc:C6 0 layer and the overall transmittance of semitransparent OPV cells of the type: glass/ITO/ZnPc:C6 0 (35 nm)/C6 0 (25 nm)/BPhen(7 nm)/Ag(20 nm)/LiF(5 nm)/ITO(0–160 nm).

Fig. 2 (a) Plots of dispersive refractive index and extension coefficient of thin film of ZnPc:C6 0 mixture as a function of wavelength, measured by the variable angle spectroscopic ellipsometry. (b) Calculated square of electromagnetic field distribution (at λ = 630 nm) from glass substrate/ITO interface in a semitransparent bulk-heterojunction OPV device of the type: glass/ITO/ZnPc:C6 0 /C6 0 /BPhen/cathode.

cells. For the given material system, maximum light absorption occurs at the 35-nm-thick ZnPc:C60 active region. The top ITO electrode is optimized to enhance the light harvesting in the active layer as well as the transmission of the semitransparent OPV cells. The calculated total transmittance and integrated absorptance in a 35-nm-thick ZnPc:C60 as a function of top ITO electrode thickness for semitransparent OPV cells of glass/ITO/ZnPc:C60 (35 nm)/C60 (25 nm)/BPhen (7 nm)/Ag(20 nm)/LiF(5 nm)/ITO(0–160 nm) are plotted in Fig. 3. The shaded area in Fig. 3 indicates a region, where high-transparency OPV cells with reasonable light absorption can be expected if the top ITO layer is chosen within this film thickness range. The results indicate that high-transparency semitransparent OPV cells can be achieved at an ITO layer thickness of 40 nm, whereas relative maximum light absorption in active layer occurs at an ITO thickness of 140 nm. In order to achieve high transparency, a combination of Ag(20 nm)/LiF (5 nm)/ITO(40 nm) cathode was selected for application in the semitransparent OPV cells, resulting in a total transmittance of 42% and an integrated absorptance of 26%. The light transmission of a semitransparent OPV cell of glass/ITO/ZnPc:C60 (35 nm)/C60 (25 nm)/BPhen(7 nm)/Ag(20 nm)/LiF(5 nm)/ITO(40 nm) was also measured, as shown in Fig. 4(a). The measured transmission agrees well with the one calculated using the optical admittance method. A good quality top ITO capping layer serves as an index matching layer that

Fig. 4. (a) Comparison of calculated and measured transmission spectra of semitransparent bulk-heterojunction OPV of the type: glass/ITO/ZnPc:C6 0 (35 nm)/C6 0 (25 nm)/BPhen(7 nm)/Ag(20 nm)/LiF(5 nm)/ITO(40 nm). (b) Semitransparent bulk-heterojunction ZnPc:C6 0 -based OPV cell showing its transparency and possibility for dual-sided illumination.

enhances the light in-coupling in semitransparent OPV cells and also improves the current spreading due to its superior optical transparency over the visible light wavelength and high electric conductivity. Fig. 4(b) shows an actual semitransparent OPV cell showing its green tint, transparency, and possibility for dual-sided illumination. Semitransparent small molecule photovoltaic cells, as illustrated in Fig. 4(a) and (b), also block most UV and IR irradiation from sunlight. This is because the transmission of UV (800 nm) in this type of semitransparent OPV cells is less than 20% and 30%, respectively. The color, semitransparency, shielding from UV and IR

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TABLE I SUMMARY OF PHOTOVOLTAIC CHARACTERISTICS MEASURED FOR A SEMITRANSPARENT PHOTOVOLTAIC CELL WITH A CONFIGURATION OF GLASS/ITO/ZnPc:C6 0 (35 nm)/C6 0 (25 nm)/BPhen(7 nm)/Ag(20 nm)/ LiF(5 nm)/ITO(40 nm) AND A CONTROL DEVICE

Fig. 5. J–V characteristics measured in the dark and under simulated AM1.5G illumination of 100 mW/cm2 for a semitransparent OPV cell of glass/ITO/ZnPc:C6 0 (35 nm)/C6 0 (25 nm)/BPhen(7 nm)/Ag(20 nm)/LiF(5 nm)/ITO(40 nm) and a control device of glass/ITO/ZnPc:C6 0 (35 nm)/C6 0 (25 nm)/BPhen(7 nm)/Ag(100 nm).

irradiation add a decorative, aesthetic, and practical dimension to these solar cells. The semitransparent OPV cells thus developed can be used for power-generating coatings or windows and even made on curved and irregular surfaces. The J–V characteristics measured in the dark and under simulated AM1.5G illumination of 100 mW/cm2 for a semitransparent OPV cell of glass/ITO/ZnPc:C60 (35 nm)/C60 (25 nm)/BPhen(7 nm)/Ag(20 nm)/LiF(5 nm)/ITO(40 nm) and a control device of glass/ITO/ZnPc:C60 (35 nm)/C60 (25 nm)/BPhen(7 nm)/Ag(100 nm) are plotted in Fig. 5. The control devices had a power conversion efficiency (PCE) of 4.0 ± 0.2%, a shortcircuit current (JSC ) of 11.8 ± 0.3 mA/cm2 , a fill factor (FF) of 60 ± 1% and an open-circuit voltage (VOC ) of 0.56 V. The errors correspond to the standard deviation calculated from five sets of OPV devices fabricated under identical conditions. In comparison, the semitransparent OPV cells had a PCE of 3.0 ± 0.1%, a JSC of 9.6 ± 0.2 mA/cm2 , a FF of 57 ± 1%, and a VOC of 0.55 V. As both semitransparent and control OPV devices had an identical organic layer structure and were made in the same system using the same sets of the process conditions, the same VOC value obtained for both semitransparent and control OPV devices suggests that the contact property at the organic/cathode interface for charge collection in semitransparent OPV and control cells is very similar. It is envisaged that the quality of semitransparent cathode of Ag(20 nm)/LiF(5 nm)/ITO(40 nm) developed in this study is very suitable for application in semitransparent OPV cells, as there was no observable deterioration in VOC for semitransparent OPV cells. There is ∼20% decrement in JSC measured for semitransparent OPV cells as compared to the control device. It is anticipated that such a reduction in photocurrent is attributable to the transparency of the top electrode used, as there is more than 35% of the incident light being transmitted through the semitransparent OPV cells [see Fig. 4(a)]. A summary of photovoltaic characteristics measured for structurally identical OPV cells with cathodes of Ag(20 nm)/LiF(5 nm)/ITO(40 nm) and Ag(100 nm) is listed in Table I.

OPV cells can be used for applications in a variety of portable consumer electronics, digital electronics, and home appliances. The semitransparent OPV cells can be also made in different colors for application in power-generating coatings. They allow light to pass through, and yet absorb enough light to generate electricity. Semitransparent OPV cells can one day serve as both windows and energy generators for our homes, offices, and even greenhouses. IV. CONCLUSION An efficient semitransparent small molecule photovoltaic cell with an average optical transparency of 40% in the visible light region and a power conversion efficiency of ∼3.0% measured under AM1.5G illumination of 100 mW/cm2 was demonstrated. The high-performance semitransparent cathode of Ag/LiF/low processing temperature ITO is very suitable for semitransparent OPV cells. The insertion of a LiF interlayer between the Ag contact and ITO top electrode also offers greater freedom for device fabrication and avoids possible damage to the underlying organic materials that could be induced during the preparation of the top transparent electrode [18]. The high-performance top ITO electrode developed in this study is also suitable for application in other novel organic devices including inverted or tandem OPV cells, translucent white light OLED lighting, organic sensors, etc. REFERENCES [1] C. W. Tang, “Two-layer organic photovoltaic cell,” Appl. Phys. Lett., vol. 48, pp. 183–185, 1986. [2] J. Xue, S. Uchida, B. P. Rand, and S. R. Forrest, “Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions,” Appl. Phys. Lett., vol. 85, pp. 5757–5759, 2004. [3] M. Granstrom, K. Petritsch, A. C. Arias, A. Lux, M. R. Andersson, and R. H. Friend, “Laminated fabrication of polymeric photovoltaic diodes,” Nature, vol. 395, pp. 257–260, 1998. [4] N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, “Photoinduced electron transfer from a conducting polymer to buckminsterfullerene,” Science, vol. 258, pp. 1474–1476, 1992. [5] C. J. Brabec, J. A. Hauch, P. Schilinsky, and C. Waldauf, “Production aspects of organic photovoltaics and their impact on the commercialization of devices,” MRS Bull., vol. 30, pp. 50–52, 2005. [6] R. A. J. Janssen, J. C. Hummelen, and N. S. Sariciftci, “Polymer-fullerene bulk heterojunction solar cells,” MRS Bull., vol. 30, pp. 33–36, 2005. [7] P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” J. Appl. Phys., vol. 93, pp. 3693–3695, 2003. [8] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, “High-efficiency solution processable polymer photovoltaic cells by selforganization of polymer blends,” Nat. Mater., vol. 4, pp. 864–868, 2005.

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[9] J. Huang, G. Li, and Y. Yang, “Influence of composition and heat-treatment on the charge transport properties of poly(3-hexylthiophene) and [6,6]phenyl C61-butyric acid methyl ester blends,” Appl. Phys. Lett., vol. 87, pp. 112105–112107, 2005. [10] Y. Q. Li, L. W. Tan, X. T. Hao, K. S. Ong, F. R. Zhu, and L. S. Hung, “Flexible top-emitting electroluminescent devices on polyethylene terephthalate substrates,” Appl. Phys. Lett., vol. 86, pp. 153508–153508, 2005. [11] C. W. Law, K. M. Lau, M. K. Fung, M. Y. Chan, F. L. Wong, C. S. Lee, and S. T. Lee, “Effective organic-based connection unit for stacked organic light-emitting devices,” Appl. Phys. Lett., vol. 89, pp. 133511–133513, 2006. [12] V. Shrotriya, E. H.-E. Wu, G. Li, Y. Yao, and Y. Yang, “Efficient light harvesting in multiple-device stacked structure for polymer solar cells,” Appl. Phys. Lett., vol. 88, pp. 064104–064106, 2006. [13] T. Oyamada, Y. Sugawara, Y. Terao, H. Sasabe, and C. Adachi, “Top lightharvesting organic solar cell using ultrathin Ag/MgAg layer as anode,” Jpn. J. Appl. Phys. Part 1, vol. 46, pp. 1734–1736, 2007. [14] G. Li, C.-W. Chu, V. Shrotriya, J. Huang, and Y. Yang, “Efficient inverted polymer solar cells,” Appl. Phys. Lett., vol. 88, pp. 253503–253505, 2006. [15] R. F. Bailey-Salzman, B. P. Rand, and S. R. Forrest, “Semitransparent organic photovoltaic cells,” Appl. Phys. Lett., vol. 88, pp. 233502–233504, 2006. [16] G.-M. Ng, E. L. Kietzke, T. Kietzke, L.-W. Tan, P.-K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cells,” Appl. Phys. Lett., vol. 90, pp. 103505–103507, 2007. [17] F.-C. Chen, J.-L. Wu, K.-H. Hsieh, W.-C. Chen, and S.-W. Lee, “Polymer photovoltaic devices with highly transparent cathodes,” Org. Electron., vol. 9, pp. 1132–1135, 2008. [18] H. Schmidt, H. Fl¨ugge, T. Winkler, T. B¨ulow, T. Riedl, and W. Kowalsky, “Efficient semitransparent inverted organic solar cells with indium tin oxide top electrode,” Appl. Phys. Lett., vol. 94, pp. 243302–243304, 2009. [19] J. Ouyang and Y. Yang, “Conducting polymer as transparent electric glue,” Adv. Mater., vol. 18, pp. 2141–2144, 2006. [20] J. Meiss, N. Allinger, M. K. Riede, and K. Leo, “Improved light harvesting in tin-doped indium oxide (ITO)-free inverted bulk-heterojunction organic solar cells using capping layers,” Appl. Phys. Lett., vol. 93, pp. 103311– 103313, 2008. [21] J. Q. Hu, J. S. Pan, F. R. Zhu, and H. Gong, “Evidence of nitric-oxideinduced surface band bending of indium tin oxide,” J. Appl. Phys., vol. 95, pp. 6273–6276, 2004. [22] E. D. Palik, Handbook of Optical Constants of Solids. vol. I, pp. 355– 356, 688–690, San Diego, CA: Academic, 1998.

Xizu Wang received the B.Sc. degree in physics with specialization in optical electronics from the Xi’an Jiaotong University, Xi’an, China, in 2000, and the Ph.D. degree in condensed matter physics (experimental) from Fudan University, Shanghai, China, in 2007. He is currently a Research Engineer at the Institute of Materials Research and Engineering, Singapore. From 2007 to 2008, he was a Postdoctoral Fellow at the Nanoscience Center, Copenhagen University, Copenhagen, Denmark, where he was involved for the surface and interface of organic/inorganic ultrathin films. His research interests include new organic semiconductor devices, organic surfaces, and interface research.

Ging-Meng Ng received the B.Eng (Hons.) and Ph.D. degrees in materials science and engineering from Nanyang Technological University, Singapore, in 2005 and 2009, respectively. He is currently a Research Engineer at the Institute of Materials Research and Engineering, Singapore, where he is involved in device optimization of organic LED (OLED) and organic photovoltaic (OPV) cells. He was also engaged in optical admittance analysis to study the light distribution in an organic thin film system, to design an optimum device structure for OLED and OPV cells. His research interests include the charge transport properties and degradation process in OPV cells.

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Jian-Wei Ho received the B.Eng. (first class Hons.) degree in mechanical engineering from the National University of Singapore, Singapore, in 2008, where he specialized in microsystems technology. He is currently a Research Officer at the Institute of Materials Research and Engineering, Singapore, where he is involved in the integration of organic optoelectronic devices with waveguides for sensor applications. His research interests include photovoltaics for solar cell applications and optoelectronic devices.

Hoi-Lam Tam received the B.Sc. (first class Hons.) and Ph.D. degrees in applied physics from the Hong Kong Baptist University, Kowloon Tong, Hong Kong, in 2000 and 2004, respectively. He is currently a Research Engineer at the Institute of Materials Research and Engineering, Singapore. From 2004 to 2009, he was a Research Assistant in the Physics Department, Hong Kong Baptist University. He has authored or coauthored more than 26 papers in journals. He holds two U.S. patents. His research interests include stability studies in organic LEDs, light extraction from metal/dielectric interfaces through plasmonic coupling, superlens effect on subwavelength photolithography, and photonic crystals.

Furong Zhu received the B.Sc. and M.Sc. degrees in physics from Fudan University, Shanghai, China, in 1983 and 1987, respectively, and the Ph.D. degree in applied physics from Charles Darwin University, Darwin, Australia, in 1993. He is currently a Professor in the Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong. From 1993 to 1995, he was a Postdoctoral Researcher in the Department of Electrical and Electronic Engineering, Kyoto University, Kyoto, Japan. From 1995 to 1997, he was a Research Fellow in the Department of Physics, Murdoch University, Perth, Australia. From 1997 to 2009, he was a Senior Scientist at the Institute of Materials Research and Engineering (IMRE), Singapore, where he was an IMRE Program Manager from 2005 to 2009 and leading the organic LED and organic photovoltaic R&D activities at IMRE.