Energy Fuels 2010, 24, 3743–3747 Published on Web 01/13/2010
: DOI:10.1021/ef901327c
Using ZnS Nanostructured Thin Films to Enhance Light Extraction from Organic Light-Emitting Diodes† Lifang Lu, Zheng Xu,* Fujun Zhang,* Suling Zhao, Liwei Wang, Zuliang Zhuo, Dandan Song, Haina Zhu, and Yongsheng Wang Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University, Ministry of Education, Beijing 100044, China Received November 10, 2009. Revised Manuscript Received January 2, 2010
Wide band gap semi-conductor zinc sulfide (ZnS) nanocolumn arrays were prepared on the glass side of indium tin oxide (ITO) substrates by glancing angle deposition (GLAD) technology. The scanning electron microscopy (SEM) images show the formation of ZnS nanocolumn arrays when the glancing angle was set to 85°; however, continuous ZnS films without any evident nanostructures were fabricated under normal deposition. The transmitting ability of ITO substrates coated with ZnS nanocolumn arrays is improved in comparison to bare ITO substrates and continuous ZnS films coated ITO substrates in the visible range. Organic lightemitting diodes (OLEDs) were simultaneously fabricated on these three kinds of substrates. The electroluminescence (EL) intensity of OLEDs based on the ITO substrate coated with ZnS nanocolumn arrays was about 1.2 times bigger than the devices based on the other substrates (bare ITO substrates and continuous ZnS films coated ITO substrates) under the same driving voltage. The improvement of EL intensity should be ascribed to the enhancement of the light extraction by the nanocolumn arrays effect.
We have successfully fabricated Alq3 thin films with different morphology by field modified thermal deposition.7 The devices with Alq3 thin films prepared with electromagnetic field modification as the active layer show lower luminescence intensity and greater current density under the same driving voltage. Robbie et al.8,9 reported an effective physical vapor deposition method called glancing angle deposition (GLAD) technology, by which a wide variety of thin film morphologies can be created easily, including cylindrical columns, zigzags, helices, and graded-width structures.10-13 Fu et al.14 designed catalytic nanomotors with a variety of geometries capable of performing multiple desired motions in a fuel solution using the GLAD technology to coat a thin catalyst layer asymmetrically on the side of a nanorod backbone. The organic and inorganic nanocolumns were prepared by thermal deposition and electron beam deposition through GLAD, respectively.15,16
Introduction Nanostructured materials and thin films are attractive in many application fields ranging from medicine to various industrial products from catalysts and electronics to paints, etc., for their novel properties of small size and particular shape, large surface area, and surface activity.1,2 In recent years, nanostructured materials have attracted more attention because of their potential application to improve the performance of solar cells, organic light-emitting diodes (OLEDs), and biosensors.3-6 The realization of oriented nanometer thin films prepared by physical vapor deposition is considered as an effective way to improve the performance of devices. † This paper has been designated for the Asia Pacific Conference on Sustainable Energy and Environmental Technologies (APCSEET) special section. *To whom correspondence should be addressed. Telephone: 0086-1051688605. E-mail:
[email protected] (Z.X.);
[email protected] (F.Z.). (1) Landsiedel, R.; Kapp, M. D.; Schulz, M.; Wiench, K.; Oesch, F. Genotoxicity investigations on nanomaterials: Methods, preparation and characterization of test material, potential artifacts and limitations;Many questions, some answers. Mutat. Res. 2009, 681, 241–258. (2) Colvin, V. L. The potential environmental impact of engineered nanomaterials. Nat. Biotechnol. 2003, 21, 1166–1170. (3) Tian, B. Z.; Zheng, X. L.; Kempa, T. J.; Fang, Y.; Yu, N. F.; Yu, G. H.; Huang, J. L.; Lieber, C. N. Coaxial silicon nanowires as solar cells and nanoelectronic power sources. Nature 2007, 449, 885–810. (4) Chiu, C. H.; Lee, C. E.; Lo, M. H.; Huang, H. W.; Lu, T. C.; Kuo, H. C.; Wang, S. C. Metal organic chemical vapor deposition growth of GaN-based light emitting diodes with naturally formed nano pyramids. Jpn. J. Appl. Phys. 2008, 47, 2954–2956. (5) Fu, J. X.; Park, B.; Siragusa, G.; Jones, L.; Tripp, R.; Zhao, Y. P.; Cho, Y. J. An Au/Si hetero-nanorod-based biosensor for Salmonella detection. Nanotechnology 2008, 19, 155502. (6) Sun, X. W.; Huang, J. Z.; Wang, J. X.; Xu, Z. A ZnO nanorod inorganic/organic heterostructure light-emitting diode emitting at 342 nm. Nano Lett. 2008, 8, 1219–1223. (7) Zhang, F. J.; Xu, Z.; Zhao, D. W.; Zhao, S. L.; Jiang, W. W.; Yuan, G. C.; Song, D. D.; Wang, Y. S.; Xu, X. R. Influence of evaporation conditions of Alq3 on the performance of organic light emitting diodes. J. Phys. D: Appl. Phys. 2007, 40, 4485–4488.
r 2010 American Chemical Society
(8) Robbie, K.; Brett, M. J.; Lakhtakia, A. First thin film realization of a helicoidal bianisotropic medium. J. Vac. Sci. Technol., A 1995, 13 (6), 2991–2993. (9) Robbie, K.; Brett, M. J.; Lakhtakia, A. Chiral sculptured thin films. Nature 1996, 384, 616. (10) Robbie, K.; Brett, M. J. Sculptured thin films and glancing angle deposition: Growth mechanics and applications. J. Vac. Sci. Technol., A 1997, 15 (3), 1460–1465. (11) Kennedy, S. R.; Brett, M. J. Porous broadband antireflection coating by glancing angle deposition. Appl. Opt. 2003, 42, 4573–4579. (12) Toader, O.; John, S. Proposed square spiral microfabrication architecture for large three-dimensional photonic band gap crystals. Science 2001, 292, 1133–1135. (13) Zhao, Y. P.; Ye, D. X.; Wang, G. C.; Lu, T. M. Novel nanocolumn and nano-flower arrays by glancing angle deposition. Nano Lett. 2002, 2 (4), 351–354. (14) Fu, J. X.; He, Y. P.; Zhao, Y. P. Fabrication of heteronanorod structures by dynamic shadowing growth. IEEE Sens. J. 2008, 8, 989– 997. (15) Zhang, J.; Salzmann, I.; Rogaschewski, S.; Rabe, J. P.; Koch, N.; Zhang, F. J.; Xu, Z. Arrays of crystalline C60 and pentacene nanocolumns. Appl. Phys. Lett. 2007, 90, No. 193117. (16) Karabacak, T.; Wang, G. C.; Lu, T. M. Quasi-periodic nanostructures grown by oblique angle deposition. J. Appl. Phys. 2003, 94, 7723–7728.
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Energy Fuels 2010, 24, 3743–3747
: DOI:10.1021/ef901327c
Lu et al.
Figure 1. Schematic diagram of the GLAD setup.
In this paper, zinc sulfide (ZnS) thin films with different nanostructures were grown on the glass side of indium tin oxide (ITO) substrates at different oblique angles by GLAD technology. The ZnS nanocolumn arrays were formed when the oblique angle was 85° . The light extraction of OLEDs has been improved by coating these nanocolumn arrays on the glass side of ITO substrates. The experimental demonstrations were carried out from the transmittance spectra of different nanostructured ZnS films and the current density-voltage and electroluminescence (EL) intensity versus current density characteristics of OLEDs based on them.
Figure 2. SEM photographs of the ZnS films deposited on ITO/ glass substrates at different oblique angles: (a) top view and (c) sectional view of ZnS films at R = 0° and (b) top view and (d) sectional view of ZnS films at R = 85°.
Results and Discussion The morphology of the ZnS thin films prepared under different oblique angles with a constant rotation rate was measured by SEM (Figure 2). From the top view of the SEM micrographs of the ZnS films, it is obvious that the surface of the film is continuous, without pores or voids when R is set to 0°, as shown in Figure 2a. In contrast, the surface of an obliquely deposited (R = 85°) film on the ITO/glass substrates shows convexities and pores, as shown in Figure 2b. To investigate the detailed structure of these two kinds of thin films, the sectional views of SEM images were given in panels c and d of Figure 2. The cross-section view of films deposited at 85° (Figure 2d) shows densely populated nanocolumns lying almost vertical to the substrate normal direction. It is very apparent that the nanocolumns grow together near the substrate surface and become disjunctive as they grow. This phenomenon may be attributed to the inconspicuous shadowing effect on the atomic-scale roughness of the glass substrate during the deposition process. However, the cross-section of the film deposited at normal incidence (R = 0°) is still without any evident nanostructures and abruptions (Figure 2c). To obtain further information about the surfaces of the thin films, the morphology of these two kinds of thin films was also characterized by AFM (Figure 3). The surface roughness of these films was characterized by the root-mean-square of the surface height variation (Rrms). The Rrms values are 8.835 and 1.456 nm for the films prepared at the oblique angles of 85° and 0°, respectively. The bigger Rrms can further demonstrate the porosity of the films deposited with R = 85°. The AFM images show results similar to the SEM photographs. The crystalline structures of these films were characterized by XRD, as shown in Figure 4. The (111) diffraction peak is obvious for the film deposited at R = 85°, but the peak in the direction of (220) is weaker. In comparison to the situation of 85°, there is a strong diffraction peak along the direction of (220) in the condition of R = 0° and the (111) peak becomes weak, as the black line shows. The XRD spectra indicate that there are two kinds of preferred orientations of ZnS films under different oblique angles, which is in good agreement with the results obtained from the SEM and AFM images. The periodic structure may influence the optical characteristics of the thin films; therefore, the optical transmittance spectra of these ZnS thin films under different deposition conditions on the ITO/glass substrate and the bare ITO/glass
Experimental Section ITO-coated glass substrates were cleaned consecutively in an ultrasonic bath with acetone, ethanol, and deionized water. The ZnS films were deposited by GLAD technology. The schematic diagram of the GLAD setup is shown in Figure 1. Substrates were mounted on a substrate holder attached to a stepper motor, which can rotate the substrate holder. The oblique angle R of the substrate normal relative to the incoming particle flux could be adjusted and was set to 0° and 85° off the substrate normal. The center of the substrate surface was 26 cm away from the deposition sources. Inorganic semi-conductor material ZnS was evaporated onto the glass side of the ITO substrates by the electron beam deposition method in an EVA-450 vacuum deposition system under a pressure of 2 10-6 Torr at a rate of 0.2 nm/s. The rotation speed of the stepper motor was 0.05 revolutions/s. The film thickness and deposition rate were monitored by a quartz crystal oscillator. The morphology of the thin films was measured using a scanning electron microscope (SEM, XL30SFEG) and an atomic force microscope (AFM, Nanoscope III). The crystallinity of the thin films was measured by X-ray diffraction (XRD). The transmittance spectrum was measured using a UV-3101PC absorption spectrometer. To investigate the effect of the ZnS layer deposited under different conditions to the OLEDs, three series of devices with different substrates were fabricated. The organic materials, 40 nm thick N,N0 -bisnaphthalen-1-yl-N,N0 -bis(phenyl)-benzidine (NPB) as the hole transporting layer and 60 nm thick aluminum(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq) as the electron transporting and emitting layer, were sequentially deposited on the bare and different ZnS layer coated ITO/glass substrates. The deposition rate for organic layers was about 0.4 A˚/s in a high vacuum of 2 10-7 Torr. The thickness and deposition rate of films were simultaneously monitored by the quartz crystal oscillator. The Al cathode was deposited by thermal evaporation with a shadow mask of active area of 7 mm2 under a vacuum of 2 10-6 Torr. The EL intensity and current density-voltage characteristics were measured with an I-V-L measurement system and a Keithley source meter 2410 in atmosphere at room temperature. The electroluminescence spectra were measured by chargecoupled device (CCD) spectrometers. 3744
Energy Fuels 2010, 24, 3743–3747
: DOI:10.1021/ef901327c
Lu et al.
Figure 3. AFM images of the ZnS films deposited on ITO/glass substrates at different oblique angles: (a and c) images of ZnS films at R = 0° and (b and d) images of ZnS films at R = 85°.
Figure 5. Transmittance spectra of ZnS thin films deposited on ITO/glass substrates with different oblique angles. Figure 4. XRD spectra of the ZnS films deposited on ITO/glass substrates at different oblique angles.
of the crystal structure may lead to a different refractive index. The refractive index strongly influences the transmitting ability of the films.17,18 From Figure 2d, we find that the nanocolumns grow together near the substrate surface and become disjunctive as they grow and the film becomes more porious; therefore, the refractive index becomes lower. It could be considered that a graded-refractive-index layer may be formed because of the changes of its nanostructure. This graded-refractive-index layer could reduce the reflection of the incident light, and then the transmitting ability could be enhanced. This phenomenon can be used to enhance the light extraction of EL devices, as shown in the following EL experimental results. The efficiency of OLEDs ηex can be expressed as ηex = ηinηext,19 where ηin is the internal quantum efficiency and
substrate were measured. Figure 5 shows that the transmitting ability of the continuous ZnS film is lower than the bare ITO/ glass substrate in the range from 350 to 500 nm, while the transmitting ability of the nanocolum ZnS thin film is greater than that of the bare substrate in the 450-750 nm range. The enhancement may be ascribed to the diffraction effect of the periodic structure of nanocolumns formed by GLAD technology. Another factor that should be considered is that the change (17) Hawkeye, M. M.; Brett, M. J. Narrow bandpass optical filters fabricated with one-dimensionally periodic inhomogeneous thin films. J. Appl. Phys. 2006, 100, No. 044322. (18) Wang, S. M.; Xia, G. D.; Fu, X. Y.; He, H. B.; Shao, J. D.; Fan, Z. X. Preparation and characterization of nanostructured ZrO2 thin films by glancing angle deposition. Thin Solid Films 2007, 515, 3352– 3355.
(19) Tang, C. W.; Van Slyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913–915.
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Energy Fuels 2010, 24, 3743–3747
: DOI:10.1021/ef901327c
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Figure 6. Energy level diagram of the used materials.
Figure 8. EL spectra of devices under a driving voltage of 26 V (the intensity of all devices was normalized by the value of device C).
The current density-voltage and EL intensity versus the current density characteristics of all devices were measured to demonstrate the effect of ZnS nanostructured thin films on the light extraction, as shown in panels a and b of Figure 7. All devices have an identical current densityvoltage performance obviously (Figure 7a). It is very interesting that the EL intensity of the devices with nanocolumn ZnS thin films shows the biggest value and is about 1.2 times compared to the devices without ZnS films on the substrate, and the value of device B with a continuous ZnS thin film is a little lower than that of device C (Figure 7b) under the same current density. Figure 8 shows the relative EL intensity and EL spectra of devices A, B, and C under the driving voltage of 26 V. All of the EL emissions are located at about 500 nm, which is attributed to the emission of BAlq exciton. Device A shows a 1.18 times bigger EL intensity than the other two, which are nearly the same. This trend is consistent with EL intensity dependence on the current density, as shown in Figure 7b. It is known that the EL intensity of OLEDs strongly depends upon the current density of devices. Why do the devices show different EL intensity at the same current density? According to the above experimental results, it should be attributed to the enhancement effect of the nanocolumn ZnS film on the light extraction from the emitting layer to air. As a consequence, there is potential to apply this ZnS nanocolumn array for the improvement of light extraction for OLEDs and light in coupling into the active layer for solar cells.
Figure 7. (a) Current density-voltage characteristics of devices. (b) EL intensity-current density curves of devices (the EL intensity of all devices was normalized by the value of device C).
ηext is the extraction efficiency. It is known that the efficiency of OLEDs is typically only 20% of the internal quantum efficiency.20 Thereby, how to increase the light extraction is a crucial problem for improving the efficiency of OLEDs. From the transmittance spectra of these nanocolumn thin films, it should take a very active effect to improve the light extraction for organic electroluminescence devices. Therefore, we fabricated three series of devices with different configurations: nanocolumn ZnS thin film (deposited at 85°)/glass/ITO/NPB/BAlq/Al for device A, continuous ZnS thin film (deposited at 0°)/glass/ ITO/NPB/BAlq/Al for device B, and glass/ITO/NPB/ BAlq/Al for device C. The energy level diagram of these used materials is shown in Figure 6.
Conclusion ZnS nanocolumn arrays were prepared on the glass side of ITO substrates by GLAD technology when the glancing angle was set to 85°. The transmitting ability of ITO substrates coated with ZnS nanocolumn arrays is improved in comparison to bare ITO substrates and continuous ZnS films coated ITO substrates in the visible range. The EL intensity of OLEDs based on ITO substrates coated with ZnS nanocolumn arrays was about 1.2 times bigger than the devices based on the other substrates (bare ITO substrates and continuous ZnS films coated ITO substrates) at the same current density. The improvement of the EL intensity should be ascribed to the enhancement of the light extraction by the nanocolumn arrays effect.
(20) Tong, B.; Mei, Q.; Wang, S.; Fang, Y.; Meng, Y.; Wang, B. Nearly 100% internal phosphorescence efficiency in a polymer lightemitting diode using a new iridium complex phosphor. J. Mater. Chem. 2008, 18, 1636–1639.
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Energy Fuels 2010, 24, 3743–3747
: DOI:10.1021/ef901327c
Lu et al.
The improvement of device performance is promising for developing OLED-based displays with lower power consumption.
the 111 project (B08002), the Research Fund for the Doctoral Program of Higher Education in China (RFDP) (20070004024 and 20070004031), the Beijing Natural Science Foundation of China (BNSFC) (1102028), the Major State Basic Research Development Program of China (973 Program) (2010CB327705), and Excellent Doctoral Science and Technology Innovation Foundation of Beijing Jiaotong University (No. 141036522 and 141028522).
Acknowledgment. The authors express their thanks for the support from the National Natural Science Foundation of China (NSFC) (Grants 10774013, 10804006, 10974013, and 60825407),
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