Open Access
Add to BibSonomyAbstract
We have characterized antireflection (AR) moth-eye films placed on top of crystalline silicon photovoltaic (PV) modules by indoor and outdoor experiments and examined improvements in conversion efficiency. The effects of the ratio of diffuse solar irradiation to total solar irradiation (diffusion index) and incident angle on efficiency have been quantitatively analyzed. Using computer simulations, yearly efficiency improvements under different installation conditions have been projected. We have shown that the use of AR moth-eye films offers the best advantages. Further, vertical tilt angle installation leads to the highest efficiency improvement, whereas spectral matching with the PV modules influences the efficiency improvement.
©2011 Optical Society of America
1. Introduction
Photovoltaic (PV) systems that use solar cells to convert solar power to electricity are well known to have considerable potential to mitigate the current energy and environmental crisis; however, it is important to improve their efficiency for maximum benefits. The efficiency of PV systems can be directly increased by reducing the reflections from the surface of solar cells. As a result, antireflection (AR) coatings have now become essential components of PV module systems. Crystalline silicon (c-Si) PV cells are presently the most important PV cells because they have long life and they can be recycled. Many types of AR coatings on c-Si PV cells have been developed (for example [1–9], ).
A biomimetic moth-eye structure is a superior AR coating that exhibits ultralow reflections for broadband wavelengths and omnidirectional light incidences [10–15]. The first artificial moth-eye structure was created by recording the interference patterns of two coherent laser beams on a photoresist [11]. Recently, various methods have been developed to create moth-eye-like micro/nanostructures. Master structures with a surface area of 0.5 m2 have been produced by employing a complex holographic optical process or plasma treatment [13,14]. A dual-scale hierarchical structure, which not only exhibits excellent AR but also superhydrophobic properties, has been fabricated by a template-mediated UV replica molding [15]. In a recent study, a high throughput nanoimprint technique using anodic porous alumina molds [16] was developed in order to deposit a moth-eye structure made of acrylic resin on a substrate, as shown in Fig. 1 . Large-area low-cost moth-eye films can be fabricated with a roll-to-roll process by using this method. A similar technique has also been reported by [17]. Some researchers have designed the shape of the moth-eye structure for applications to c-Si PV cells in such a manner that the reflection and absorption can be minimized and transmission can be maximized over a spectrum range that matches that of c-Si PV cells [18].
Fig. 1 Moth-eye structure made of acrylic resin. (a) Fabricated moth-eye structure observed using scanning electron microscopy (SEM). (b) Fabricated rolls of moth-eye film; the rolls appear green due to the color of the protection film.
Despite such excellent studies, the characterization of a practical scale PV module with moth-eye AR coating has not been adequately reported yet. In this paper, we report the characterization of a c-Si PV module having a moth-eye film on its top surface by indoor and outdoor experiments; further, we report the resulting conversion efficiency improvements. In particular, the effects of the ratio of diffuse solar radiation to total solar radiation (hereafter, diffusion index) and incident angle on the conversion efficiency have been quantitatively analyzed, which, to our knowledge, have never been previously reported till date. Furthermore, computer simulations based on the experimental results are presented to project yearly efficiency improvements by the application of the moth-eye film under several installation conditions.
2. Indoor experiment
Figure 2 shows comparisons of the reflectances of the moth-eye film and the commercially available conventional multilayered AR film. Incident angle dependency of spectral reflectance was measured by using spectrophotometer JASCO V-670 equipped with absolute reflectance measurement accessory JASCO ARSN-733 which can adjust incident angle to the film. In order to avoid reflections, the back surfaces of the measured films were roughened by sandpaper and painted matte-black. It is obvious that the moth-eye film has broadband and omnidirectional AR performance, that is, very low reflectance. The reflectance has the maximum values of 1.2% and 5.7% at small (θ in = 5°, 1100nm) and large (θ in = 60°, 1100nm) incident angles, respectively, for the primary spectral response range (approximately 400–1200 nm; peak: around 1000 nm) of the c-Si PV cells [19]. This property implies that the moth-eye film has the potential to improve the c-Si PV module efficiency during sunrise-to-sunset operations.
Fig. 2 Spectral reflectance of moth-eye antireflection (AR) film and the conventional multilayered AR film. Solid line: moth-eye AR; Dashed line: conventional AR; red: θ in = 5°; blue: θ in = 30°; green: θ in = 60°.
This moth-eye film was practically applied to the top surface of a typical c-Si PV module; the effective area was 25 cm × 18 cm. The film was manually glued to the top of the PV module by using a roller device. Figure 3 shows photos and cross-sectional schematics of the modules with and without the moth-eye film. The average geometry of the moth-eye shape is also shown. By comparing the photos in Fig. 3(c), it can be observed that the module surface without the moth-eye film is reflecting the hands of the camera operator, whereas the one with the moth-eye film is not.
Fig. 3 Photos and cross-sectional schematics of the tested c-Si PV modules (a) with moth-eye film; (b) without moth-eye; (c) upper: with moth-eye; lower: without moth-eye.
First, an indoor experiment was conducted using a solar simulator. Figure 4(a) shows a photo of the solar simulator used in the experiment. The modules with and without the moth-eye film were set on a horizontal test table. The light incident on the modules was diffusive, or, in other words, the ratio of the diffuse radiation to direct (beam) radiation was approximately 0.9. To verify the individual differences among the tested modules with respect to the electric conversion efficiency, the current–voltage (I–V) curve characteristics of each module were tested before the moth-eye film was applied to each module. A fairly low individual difference (less than 1.5% of the absolute value of the efficiency) was confirmed. Individual differences among the measurement systems were also confirmed to be negligible.
Fig. 4 Indoor experiment using 1.2 m × 1.2 m solar simulator that meets Class-C ASTM / IEC / JIS standards; (a) solar simulator; the temperature inside the room was controlled to be 25 °C; (b) comparison of conversion efficiency of c-Si PV modules with and without moth-eye film. The average efficiencies of the same modules in outdoor experiments are also shown for comparison.
Figure 4(b) shows the conversion efficiency at the maximum power with and without the moth-eye film. The efficiency was averaged by changing the module position on the test table in order to eliminate the possible effects of the non-uniformity of irradiance. As a result, it was found that the module efficiency with the moth-eye film was 1.05 times higher than that of the module without the moth-eye film. During the experiments, the average room and module temperatures were 25 °C and 41 °C, respectively. For comparison, the resultant efficiencies in the outdoor experiment (next section) are also shown in Fig. 4(b). The outdoor efficiency was higher than the indoor efficiency because the indoor module temperature was approximately 11–16 °C higher than the outdoor module temperature. This trend is consistent with the reported correlation between the operating temperature and conversion efficiency of the module [20].
3. Outdoor experiment
An outdoor experiment was conducted using the same test modules as the indoor experiment. Figure 5 shows the schematic diagram and photo of the outdoor experiment system. The modules were placed on a 40° tilted surface facing the south at Nagaoka University of Technology (lat.: 37.44°N, long.: 138.85°E). A sun tracker EKO STR-21 equipped with a pyrheliometer (full opening view angle: 5° ± 0.2°) and a pyranometer were used to measure the direct normal irradiance (DNI) and global normal irradiance (GNI), respectively. The global irradiation on the tilted surface was also measured by using a pyranometer placed on the tilted surface with the modules. The incident angle to the modules, θ in, was calculated using the orientation of the tracked surface. The diffusion index was calculated by (GNI−DNI)/GNI.
Fig. 5 Apparatus of outdoor experiment system. (a) Schematic diagram of the system. (b) Photo of the system. Tilt angle of PV modules is 40°, facing southward.
Figure 6(a) shows the daily variations in the conversion efficiency of the modules with/without moth-eye film on May 21, 2010, a clear day. The moth-eye film obviously improved the efficiency during the entire measurement period. Measurements were also carried out on the other 7 days in May and June. Figure 6(b) shows a histogram of the measurement hour and efficiency improvement Χ for all measurement data; here, Χ is the relative improvement ratio of the conversion efficiency, i.e., X = 5 shows that the conversion efficiency of the module with the moth-eye film is 1.05 times higher than that without the moth-eye film. The average value of Χ is 5.5. 81% of the measurement hour exists in the range 3 ≤ Χ ≤ 7.
Fig. 6 Results of outdoor experiment. (a) Daily variations in conversion efficiency of the modules with and without moth-eye film on May 21, 2010. (b) Histogram of efficiency improvement Χ for 8-day experiment. Vertical axis represents measurement hour with respect to X value.
The incident-angle dependency of the reflectance of the moth-eye film, which is shown in Fig. 2, indicates that the efficiency improvement Χ can be attributed to the relationship between the incident angle of the beam solar radiation and the diffusion index. To characterize this attribution, the experimental data was statistically analyzed. Figure 7(a) shows the relationship among X, diffusion index, and incident angle of beam solar radiation θ in. Not only experimental results but also projected results for both the moth-eye and conventional AR films have been plotted for comparison. In computer simulations, the measured incident-angle dependencies of the reflectance for both films were used to project efficiency improvements taking into consideration the spectral responses of c-Si cells [19]. In the case of the solar spectrum, the same fraction as the AM1.5 standard solar spectrum [21] was assumed in the simulations. Obviously, the projected results are consistent with the experimental results. The moth-eye film exhibits the best results up to X = 7 with a large incident angle >60° for low diffusion index <0.2, whereas it tends to converge to X = 5 with small incident-angle dependency for high diffusion index >0.5. In addition, an interesting characteristic can be observed in Fig. 7(a). The lowest X of the moth-eye AR film appears in the middle incident angle range 30° < θ in ≤ 60° for any diffusion index, whereas in contrast, the lowest X of the multilayered AR film appears in the lowest incident angle range 0° ≤ θ in ≤ 30°. To explain the reasons for this characteristic, we plotted the spectral breakdown of the relationship between X and θ in for the moth-eye film; see Fig. 7(b). X for a wavelength of 1000 nm shows the lowest improvement in the range 30° < θ in < 50°. This is caused by the higher reflectance trend of the moth-eye film for a longer wavelength range, as shown in Fig. 2. In addition, the spectral response of c-Si is rather high at around 1000 nm due to its band gap, and this leads to improved AR performance. Thus, further efficiency improvements can be achieved by the optimization of spectral matching.
Fig. 7 Relationship among efficiency improvement X, incident angle, and diffusion index range. (a) Overall relationship; points: experiment; lines: guide line for the calculated points; Each experiment and calculation point is mean value over a range of incident angle and diffusion index; Color legend indicates that each point and line corresponds to a range of incident angle. (b) Spectral breakdown of the relationship for the moth-eye film.
4. Estimation of yearly efficiency improvement
The computer simulations were in good agreement with the abovementioned experimental results; further, by using the simulations, the yearly efficiency improvement X was estimated for two installation places, Tokyo and Phoenix (Table 1 ). The latitudes of these places are similar, whereas in contrast, their yearly diffusion indexes are different. The tilt angle of the module was assumed to be 0°, 30°, 60°, and 90° facing the equator. The available weather data [22,23] were used. In the simulation, the effects of module temperature were neglected for the sake of simplicity. Table 1 summarizes the estimated yearly efficiency improvement X. The most relevant finding is that for both places, the tilt angle of 90° (vertical to horizontal) exhibits the highest improvement. In the case of other tilt angles, the improvement is almost similar to that in Tokyo and Phoenix. On the other hand, the highest gain of the solar irradiation trapped by the AR moth-eye film was at the tilt angle of 30°, whereas the lowest gain was at the tilt angle of 90° for both places. Figure 8 shows the monthly breakdown of the estimated X and diffusion index. A close correlation can be observed between X and diffusion index in the monthly variations. The diffusion index in Tokyo is higher than that in Phoenix, especially in summer. In the case of the larger tilt angle, this trend caused the lower efficiency improvement in summer in Tokyo.
Table 1. Estimated Yearly Efficiency Improvement X of the PV Module Using the Moth-eye Film.
Fig. 8 Estimated monthly efficiency X of PV module with moth-eye film. (a) Monthly average of diffusion index; Tilt angle of the module: (b) 0°, (c) 30°, (d) 60°, (e) 90°.
5. Conclusions
The performance of a practical-scale moth-eye AR when applied to the top surface of a c-Si PV module was quantitatively characterized by experiments and computer simulations. The efficiency improvements obtained when the moth-eye film was used were remarkable when compared with the performance of non-AR and conventional AR modules. The efficiency improvement is attributed to an incident-angle characteristic, i.e., the ratio of diffuse solar irradiation to beam solar irradiation. When the moth-eye film was used, the vertical tilt angle installation of the photovoltaic module gave the highest efficiency improvement with the diffusion index ranging from 0.21 to 0.47, whereas the tilt angle close to the latitude led to the highest gain of the trapped solar irradiation. For the case of the highest gain, efficiency was improved to approximately 1.05 times higher than that without moth-eye film. Moreover, the spectral matching with the photovoltaic cell also affected the efficiency improvement. It may be possible to improve the efficiency further by tuning and optimizing the spectral incident-angle dependency of the reflectance of the moth-eye film for spectral responses of the coupled photovoltaic cells. Further research is required to demonstrate the long-term reliability and durability of moth-eye films and popularize them.
References and links
1. J. Kim, D. Inns, K. Fogel, and D. K. Sadana, “Surface texturing of single-crystalline silicon solar cells using low density SiO2 films as an anisotropic etch mask,” Sol. Energy Mater. Sol. Cells 94(12), 2091–2093 (2010). [CrossRef]
2. D. Kumar, S. K. Srivastava, P. K. Singh, M. Husain, and V. Kumar, “Fabrication of silicon nanowire arrays based solar cell with improved performance,” Sol. Energy Mater. Sol. Cells (2010), doi:.
3. V. V. Iyengar, B. K. Nayak, and M. C. Gupta, “Optical properties of silicon light trapping structures for photovoltaics,” Sol. Energy Mater. Sol. Cells 94(12), 2251–2257 (2010). [CrossRef]
4. S. A. Boden and D. M. Bagnall, “Sunrise to sunset optimization of thin film antireflective coatings for encapsulated, planar silicon solar cells,” Prog. Photo. 17(4), 241–252 (2009). [CrossRef]
5. M. F. Schubert, F. W. Mont, S. Chhajed, D. J. Poxson, J. K. Kim, and E. F. Schubert, “Design of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials by genetic algorithm,” Opt. Express 16(8), 5290–5298 (2008). [CrossRef] [PubMed]
6. H. Sai, Y. Kanamori, K. Arafune, Y. Ohshita, and M. Yamaguchi, “Light trapping effect of submicron surface textures in crystalline Si solar cells,” Prog. Photo. 15(5), 415–423 (2007). [CrossRef]
7. S. A. Boden and D. M. Bagnall, “Optimization of moth-eye antireflection schemes for silicon solar cells,” Prog. Photo. 18(3), 195–203 (2010). [CrossRef]
8. S. A. Boden and D. M. Bagnall, “Nanostructured biomimetic moth-eye arrays in silicon by nanoimprint lithography,” Proc. SPIE 7401, 7410J (2009).
9. C.-H. Sun, P. Jiang, and B. Jiang, “Broadband moth-eye antireflection coatings on silicon,” Appl. Phys. Lett. 92(6), 061112 (2008). [CrossRef]
10. C. G. Bernhard, “Structural and functional adaptation in a visual system,” Endeavor 26, 79–84 (1967).
11. P. B. Clapham and M. C. Hutley, “Reduction of lens reflection by the moth-eye principle,” Nature 244(5414), 281–282 (1973). [CrossRef]
12. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies,” Proc. R. Soc. B-Biol,” Science 273, 661–667 (2006).
13. A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999). [CrossRef]
14. A. Kaless, U. Schulz, P. Munzert, and N. Kaiser, “NANO-moth-eye antireflection pattern by plasma treatment of polymers,” Surf. Coat. Tech. 200(1-4), 58–61 (2005). [CrossRef]
15. S. J. Choi and S. Y. Huh, “Direct structuring of a biomimetic anti-reflective, self-cleaning surface for light harvesting in organic solar cells,” Macromol. Rapid Commun. 31(6), 539–544 (2010). [CrossRef] [PubMed]
16. T. Yanagishita, K. Yasui, T. Kondo, Y. Kawamoto, K. Nishio, and H. Masuda, “Antireflection polymer surface using anodic porous alumina molds with tapered holes,” Chem. Lett. 36(4), 530–531 (2007). [CrossRef]
17. Q. Chen, G. Hubbard, P. A. Shields, C. Liu, D. W. E. Allsopp, W. N. Wang, and S. Abbott, “Broadband moth-eye antireflection coatings fabricated by low-cost nanoimprinting,” Appl. Phys. Lett. 94(26), 263118 (2009). [CrossRef]
18. N. Yamada, O. N. Kim, T. Tokimitsu, Y. Nakai, and H. Masuda, “Optimization of anti-reflection moth-eye structures for use in crystalline silicon solar cells,” Prog. Photo. 18, 195–203 (2010).
19. H. Field, “Solar cell spectral response measurement errors related to spectral band width and chopped light waveform,” Twenty-Sixth IEEE Photovol. Spec. Conf., 471–474 (1997).
20. E. Skoplaki, A. G. Boudouvis, and J. A. Palyvos, “A simple correlation for the operating temperature of photovoltaic modules of arbitrary mounting,” Sol. Energy Mater. Sol. Cells 92(11), 1393–1402 (2008). [CrossRef]
21. C. Honsberg, and S. Bowden, “PVCDROM, Appendices: Standard Solar Spectra,” http://www.pveducation.org/pvcdrom/appendicies/standard-solar-spectra.
22. H. Akasaka, N. Hideyo, K. Soga, S. Matsumoto, K. Emura, N. Miki, E. Emura, and K. Takemasa, “Development of Expanded AMeDAS weather data for building calculation in Japan,” ASHRAE Transactions,” Symposia 106, 455–465 (2000).
23. W. Marion, and K. Urban, “National Solar Radiation Data Base,” http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/tmy2/.
