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 OSA
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 . 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 . In a recent study, a high throughput nanoimprint technique using anodic porous alumina molds  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 . 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 .
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 . This property implies that the moth-eye film has the potential to improve the c-Si PV module efficiency during sunrise-to-sunset operations.
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.
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.
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 .
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.
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.
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 . In the case of the solar spectrum, the same fraction as the AM1.5 standard solar spectrum  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.
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.
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.
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