Sol gel derived durable antireflective coating for solar glass

Transcription

1 J Sol-Gel Sci Technol (2010) 53: DOI /s y ORIGINAL PAPER Sol gel derived durable antireflective coating for solar glass Xiaodong Wang Jun Shen Received: 5 August 2009 / Accepted: 7 October 2009 / Published online: 27 October 2009 Ó Springer Science+Business Media, LLC 2009 Abstract In this paper, we present the design and preparation of a type of high-strength SiO 2 /TiO 2 AR coatings used in solar glass by dip coating method. The average transmittance of glass coated in this way is increased by more than 6% in the wavelength from 400 to 800 nm, which fits well with the theoretical expectation. The mechanical performance and atmospheric exposure tests prove that the coatings have scratch resistance, erosion resistance and long-time stability. The AFM morphology shows that the surface of the coating is very smooth with the experimental result of RMS roughness nm. These phenomena indicate that the SiO 2 /TiO 2 AR coatings have high potential commercialization for low-cost solar glass. Keywords Sol gel Antireflective coatings Mechanical properties Solar glass 1 Introduction The amount of solar energy that strikes earth in a few days is greater than the amount of fuel burnt over the whole human history, which encourages scientists to think about ways to harness solar energy to satisfy ever-increasing demand for energy. Among these ways, photovoltaic (PV) is a simple and elegant method. PV devices (solar cells) directly convert the incident solar radiation into electricity, X. Wang J. Shen (&) Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Pohl Institute of Solid State Physics, Tongji University, Shanghai, China shenjj@online.sh.cn; shenjun67@tongji.edu.cn X. Wang uestcwxd@yahoo.com.cn with no noise, pollution or moving parts, making them robust, reliable and long lasting [1]. Definitely, it will play an important role in the world s future energy trends. However, the major hurdle we face is the photoelectric conversion efficiency of the solar cells is not high enough. In the study and application of silicon solar cells, it is very difficult to increase even 1% of the photoelectric conversion efficiency due to the limitation of the silicon s band gap. Luckily, solar glass, a kind of glass used for solar application to protect the cells as top cover, still has 8 9% reflection losses during the photoelectric conversion process, which can be used by some suitable technologies such as antireflection. Optical reflection is a fundamental phenomenon occurring when light propagates across a boundary between two media which have different refractive indices [2]. Due to different refractive indices of glass and air, part of solar energy radiation reflects from the solar glass surface, which leads to the loss of solar energy. Therefore, selective antireflective coatings should be applied to the glass surface which could prevent the loss of solar energy. Compared with the conventional PVD (physical vapor deposition) coating methods, the sol gel method is inexpensive and easily adaptable to industry scale. It is possible to work in normal atmospheric conditions and get high homogeneity of the final coatings. The parameters of the coatings can be controlled by temperature or chemical contents of the raw materials such as their molar ratios [3 5]. So far, no commercial AR coated glass has been developed for photovoltaic applications yet. The main problem lies in stability and reproducibility of sol gel process as well as adhesion and mechanical stability of the coatings. Colloidal method with basic hydrolysis has been widely used to produce porous silica AR coatings [6 11]. The excessive high porosity of the coatings has the advantage of

2 J Sol-Gel Sci Technol (2010) 53: low refractive index, but also has the disadvantage of bad performance of adhesion and mechanical properties, and such disadvantage is crucial for the solar glass. In this study, a double-layer visible (VIS) AR coating was prepared by using sol gel dip coating method on both sides of the glass substrate. The acid catalyzed SiO 2 and TiO 2 were respectively selected as low-index and high-index layer. The sol used here could be easily fabricated and showed the stability for long-term storage. Besides, the coatings obtained by this process were dense and scratch-resistant. Generally, most of solar energy concentrates in visible and infrared region, and only around 7% is in ultraviolet region. In infrared region, the efficient harnessing of solar radiation can be ignored, because in this area the light transmittance in silicon is very high. Moreover, the heating effect of the infrared light will decrease conversion efficiency of the cells. And the absorption edge of the glass is in the ultraviolet region, which prevents the transmittance of the ultraviolet light. Due to the above-mentioned disadvantages of infrared and ultraviolet radiation, this paper focuses on the antireflection in the wavelength region of nm. 2 Experimental procedure 2.1 Sol preparation and dip coating process The flow chart of the AR coating synthesis was shown in Fig. 1. The precursor for the SiO 2 sol was teraethylorthosilicate (Si(OC 2 H 5 ) 4, TEOS). TEOS was mixed with ethanol (C 2 H 5 OH, EtOH) as solvent, distilled water (H 2 O) for hydrolysis and hydrochloric acid (HCl) [ph = 1] as catalyst. The molar ratio was TEOS:EtOH:H 2 O:HCl = 1:40:2:0.5. The solutions were stirred at room temperature for 2 h and further aged for 120 h in stable environment (with humidity TPOT+EtOH+AcAc+HAc+H 2 O Stirring Aged TiO 2 Sol First Dip Coating Heating the Coating TEOS+EtOH+HCl+H 2 O Stirring Aged SiO 2 Sol Second Dip Coating Annealing the Coating Fig. 1 Flow chart of the AR coating synthesis process lower than 30% and temperature of C) to form transparent gel before use. TiO 2 sol was prepared from tetrabutyl titanate (Ti(OC 4 H 9 ) 4, TPOT). TPOT was mixed with ethanol as solvent, distilled water for hydrolysis, acetylacetone (CH 3 COCH 2 COCH 3, AcAc) as chelating agent and acetic acid (CH 3 COOH, HAc) as catalyst for 2 h. The molar ratio was TPOT:EtOH:H 2 O:HAc:AcAc = 1:50:3:2:1. The mixture was aged in stable environment (as mentioned above) for 72 h. The SiO 2 and TiO 2 coatings were then dip coated on the glass substrate from the sols. The desired thickness could be adjusted by the withdrawal rate (0 12 inch/min). Each film was annealed at 300 C for 1 h in electric muffle furnace. 2.2 Design of the double-layer AR coating There are two double-layer AR filters, V-shaped (k/4 k/4) and W-shaped (k/4 k/2) in thin film optics [12 14]. The V-shaped AR structure can only cover a very narrow bandwidth which is not applied in this paper, while W-shaped AR structure has two minimum values of the reflectivity at both sides of the reference wavelength, which forms a relatively broader band of antireflection. In order to obtain information about the component layers of the AR structure, which is crucial for the film design and theoretical simulation, each of the individual layers mentioned above for the AR filters was prepared on a separate substrate. The preparation technique was exactly the same as the one applied for the corresponding layer in the double-layer coating. Therefore, the optical constants (n, k) of SiO 2 and TiO 2 films could be determined by spectroscopic ellipsometer (Ellip-A). Cauchy model was used to extract n, k from the measured data (Psi, Delta). The final results are shown in Fig. 2. It was found that the extinction coefficient of each film after annealing was much less than 0.1 between 400 and 800 nm. This indicated that the weak absorption of the sol gel deposited SiO 2 and TiO 2 films was suitable for the AR coatings of solar glass. We constructed a simple theoretical model in order to simulate and optimize the optical parameters of the doublelayer coating. The schematic diagram of the double-layer coating is shown in Fig. 3. Giving the thickness, refractive index n(k), and extinction coefficient k(k) of each layer in our system, the total reflectance as a function of wavelength can be calculated, and it can be compared with the spectrophotometer result. Since the total thickness of the films on either side is about nm, which is around the same order as the wavelength of the light, the multiple reflected and transmitted beams as the light pass through the films are essentially coherent. The reflection R, for a beam passing through our double-layer stack from outside in, can be

3 324 J Sol-Gel Sci Technol (2010) 53: Fig. 2 Refractive indices and extinction coefficients of the component layers in the AR filter Refractive index 2.0 SiO TiO Extinction coefficient 0.1 SiO TiO 2 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E E E-10 1E-11 SiO 2 (n 1 ) TiO 2 (n 2 ) Glass Substrate(n s ) Fig. 3 Design diagram of AR coating d 1 d 2 Incident light + R a + T a R a - T a R T b b a b easily determined through the standard transfer matrix method as follows. " # B ¼ cos d 1 i sin d 1 =N 1 C in 1 sin d 1 cos d 1 " # cos d 2 i sin d 2 =N 2 1 in 2 sin d 2 cos d 2 N s R ¼ N 0B C N0 B C N 0 B þ C N 0 B þ C where N 0, N 1, N 2, N s are the complex refractive indices of air, SiO 2, TiO 2, and the glass substrate, respectively. The phase thickness d i ¼ 2pN i d i =k (i = 1, 2), where d i is the physical thickness of the layer, k is the wavelength of the light, and the light is normal incidence. B and C are the two elements of the characteristic matrix of the assembly. In the experiment, both sides of the glass substrate were coated. Usually the thickness of the glass substrate will be much greater than the wavelength of the light, so the beams, whose path difference involves one or more traversals through the substrate, will be treated as incoherent. The symbols used are illustrated in Fig. 4. Waves are reflected successively at the front and rear surfaces. The sums of the irradiances are given by h R ¼ R þ a þ Tþ a Rþ b T a 1 þ R a Rþ b þ i 2þ R a Rþ b ¼ Rþ a þ Rþ b Tþ a T a Rþ a R a : 1 R a Rþ b where R is the reflectance, T is the transmittance. To the transmitted light, we denote waves in the direction of Fig. 4 Symbols used in calculation of incoherent reflection at two surfaces incidence by? (positive-going), and waves in the opposite direction by - (negative-going). To the reflected light, it is just the opposite. The symbols a, b represent the front surface and rear surface of the substrate, respectively. If there is no absorption in the layers, R þ a ¼ R a ¼ R a, Ta þ ¼ T a ¼ T a; and 1 ¼ R a þ T a ; Therefore, R ¼ R a þr b 2R a R b 1 R a R b : For the particularity of the dipping coating method, coatings of both sides of the substrate are the same. Then, R b ¼ R a, therefore, R ¼ 2R a 1þR a. For the substrate with antireflective coatings, 0 R a 1, then, R 2R a. The simulation and optimization were carried out with the optical coating design program Essential Macleod. To achieve a minimum reflection of the double-layer AR filter, refinement and synthesis [15] are used to get best design parameters of this AR structure. In order to refine, we set the reflectance from 400 to 800 nm as zero, the reference wavelength k 0 = 510 nm, the starting optical thickness of SiO 2 and TiO k 0, 0.5k 0, respectively. And then the AR filter was optimized through simulated annealing method [16]. After that, the optimum physical thickness d 1, d 2 was calculated as 92, 136 nm. The reflectance spectrum of the double-layer AR structure was also calculated by this method as shown in Fig. 5. The valley values of the reflectance are inclined to 0 at around 433 and 665 nm. At the same time, the comparatively successful broadband AR effect has been reached, that is, the average reflectance between 400 and 800 nm decreased to 2.415%.

4 J Sol-Gel Sci Technol (2010) 53: Expiremental Result Theoretical Result time 100 times 200 times Uncoated Glass Reflectance(%) Transmittance(%) Fig. 5 Reflectance in theory and practice Fig. 6 Transmittances before and after coating and transmittances of different scratch times 2.3 Characterization UV VIS-NIR Spectrophotometer (Jasco-570) was used to measure transmittance and reflectance of the coated glass. Deuterium lamp and tungsten halogen light were used as light source in the spectrophotometer. The refractive index n, extinction coefficient k and film thickness were determined by the spectroscopic ellipsometer (Ellip-A). The mechanical properties of the coatings were evaluated by pencil hardness tester, tape and rubber tests. AFM (Atomic force microscope, Nanoscope III) was used for the characterization of surface morphology of the coatings. 3 Results and discussion 3.1 Optical properties Solar glass made of low iron glass has a transmittance of approximately 90%, in which 8 9% losses reflection and the rest, 1 2%, is absorbed. Figure 6 shows the measured transmittance spectra of the double-layer AR coating on glass substrate. In the experiment, two maximum values of transmittance can be obtained, which are 98.67% at 435 nm and 98.09% at 668 nm. The average transmittance is 96.4% in the wavelength between 400 and 800 nm, indicating an increase of transmitted light of approximately 6.4%. Based on the results mentioned above, the solar radiation directly incident to the solar cell panels can increase. It is therefore very suitable for application in solar cells to reduce energy loss and improve efficiency. Figure 5 presents the measured and calculated reflectance spectra of the optimized double-layer AR coating glass. The minimum value of the reflectance hereby is lower than 0.1%. The reflectance of the bare coated glass is only slightly decreasing linearly in the range of nm from 8.33% to 7.81%. Compared with the transmittance shown in Fig. 6, there is absorption of glass substrate which results in about 1.5% decrease of the transmittance. The curves in Fig. 5 show good consistency between the theoretical and experiment results about the reflectance. Only a little difference is found in the short wavelength region due to the slight deviations of the thickness and optical constants from the target values, and the parameters of the glass used in experiment are different from what we used for simulation. 3.2 Mechanical performance The mechanical properties of the films were evaluated qualitatively with the pencil hardness tester, tape and rubber tests [17, 18]. The surface of the coating was rubbed by the cotton ball immersed with dust and ethanol. After several times of rubber tests, it was found that there wasn t any damage of the coatings. The reason is that the dense acid catalyzed silica film leads to a better scratch resistant property than that of the colloidal silica film. The peak values of the transmittance of the coating scratched before and after were made a comparison. In Fig. 6, there is little difference among the original coating, the 100 times scratched coating, and the 200 times scratched coating. The peak values of the coating transmission after scratching are reduced by only 0.04% and 0.13%, respectively. The tape tests were applied to check the layer adhesion. The tape (Brand Panda) was laid across the coated surface, and then was slowly pulled away. This process may be repeated again and again with a fresh tape each time. No change was observed on the films after the tests. It indicated the good adherence of the SiO 2 layer to the TiO 2 layer and the intensive binding force of the TiO 2 coating to

5 326 J Sol-Gel Sci Technol (2010) 53: the substrate. The result of pencil hardness test shows that the hardness of the two-layer coating is 5H, which is much higher than the coating prepared with the colloidal sol. Compared with other AR coatings via sol gel process [19], the coating prepared by this method shows better mechanical properties and easily commercial application. 3.3 Atmospheric exposure test Solar glasses in use are always exposed in atmosphere. Therefore, AR layers must have sufficient atmospheric exposure properties to avoid damages result from acid erosion and long-time using. In order to evaluate the atmospheric exposure properties of the coatings, acid erosion tests and long-time exposure tests have been performed. The coating was immersed in the mixture of sulfuric acid and nitric acid solution for 60 h to test its erosion resistance. The ph value of the acid solution is 3, lower than the ph value of common rainwater. After comparing the transmittances of the coating before and after erosion, no considerable change was found as shown Transmittance(%) as-deposited after the acidic erosion test after 1 year of air exposure Fig. 7 Transmittances after the erosion test and after 1 year of air exposure in Fig. 7. It proved that the erosion resistance of this kind of coating was very well. Figure 7 also shows the transmittance contrast of the coatings after 1 year exposure in the air. The result presented that there was only 0.05% reducing of the average transmittance. The film still maintained good optical antireflection and mechanical performance. 3.4 Surface morphology A 3D AFM image of the coated glass surface and a scan line of this surface are shown in Fig. 8a and b, respectively. From the Fig. 8a it can be concluded that the surface of the coating is very smooth and covered by small particles. Under the acid catalysis circumstance, with fast hydrolysis and slow condensation, the growth of clusters can be described as a RLCA (Reaction Limited Cluster cluster Aggregation) model [20], which presents linearly or randomly branched-chains so that the particles and clusters may be fully cross-linked. Therefore, the formed film tends to be dense and the pore volume is extremely low. It also can be observed that the surface is fairly smooth and without obvious fluctuation from the scan line of this surface (Fig. 8b) because the RMS (Root Mean Square) roughness of the surface was only nm, much less than the wavelength of the light wave. 4 Conclusions SiO 2 /TiO 2 double-layer AR coatings have been successfully prepared via sol gel process. The valley value of the reflectance of the two-layer coated glass was determined to be 0.1%. A 6.4% enhancement in the average transmittance between 400 and 800 nm was obtained in the solar glass. The mechanical properties and atmospheric exposure tests showed that the scratch-resistance, erosion-resistance and atmospheric exposure properties can meet the demands of the practical application. Moreover, the sol used in this experiment can be stored stably and fabricated easily. All Fig. 8 AFM image of the coating a surface, b a scan line from the AFM of the entire surface

6 J Sol-Gel Sci Technol (2010) 53: these results prove that this kind of coating is very suitable for the antireflection of the solar glass. With a combination of dip coating and rapid thermal annealing processes, the double-layer AR coatings present potential for continuous mass production of solar glass with low cost. Acknowledgments This work was supported by National High Technology Research and Development Program of China (Grant No. 2008AA ), National Natural Science Foundation of China (Grant Nos & ) and Shanghai Key Laboratory Program (Grant No. 07DZ22302). References 1. Richards BS (2002) Novel uses of titanium dioxide for silicon solar cells, University of New South Wales, Sydney, pp Chen DG (2001) Sol Energy Mater Sol Cells 68: Hinczewski DS, Hinczewski M, Tepehan FZ, Tepehan GG (2005) Sol Energy Mater Sol Cells 87: Chen CC, Lin DJ, Don TM, Huang FH, Cheng LP (2008) J Non- Cryst Solids 354: Bautista MC, Morales A (2003) Sol Energy Mater Sol Cells 80: Floch HG, Belleville PF (1994) J Sol Gel Sci Technol 1: Nostell P, Roos A, Karlsson B (1998) Sol Energy Mater Sol Cells 54: Thomas IM (1992) Appl Opt 28: Zhang QY, Wang J, Wu GM, Shen J, Buddhudu S (2001) Mater Chem Phys 72: Zhang Y, Wu D, Sun YH, Peng SY (2005) J Sol Gel Sci Technol 33: Cathro KJ, Constable DC, Solaga T (1984) Sol Energy 32(5): Tang JF, Zheng Q (1984) Applied Film Optics. Shanghai Science and Technology Press, Shanghai, p Pedrotti FL, Pedrotti LS (1993) Introduction to Optics. Prentice- Hall, London, p Macleod HA (2001) Thin-film optical filters, 3rd edn. Institute of Physics Publishing, Bristol and Philadelphia, p Thin Film Center ( ) Essential macleod optical coating design program user s manual. Thin Film Center Inc., p Kirkpatrick S, Gelatt CD, Vecchi MP (1983) Science 220: Shen J, Yang TH, Zhang QY, Wang J (2003) J Sol Gel Sci Technol 26: Shen J, Xie ZY, Wu XX, Ouyang L, Xiao YQ (2008) Rare Metal Mater Eng 37(Suppl 2): Shen J, Xie ZY, Ouyang L, Wu D, Luo AY, Wang SZ, Wang XD (2007) J Wuhan Univ Technol 29(Suppl 1): Vold MJ (1963) J Colloid Interface Sci 18: