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This paper reports the continuous fabrication of dual-side nano-structured anti-reflection protective layer for performance enhancement of solar cells using plasma sputtering and infrared assisted roller embossing techniques. Nano-structures were first deposited onto the surface of glass substrates using the plasma sputtering technique. After electroforming, a nickel master mold containing nano-array of 30 nm was obtained. The mold was then attached to the surfaces of the two metallic rollers in an infrared assisted roll-to-roll embossing facility. The embossing facility was used to replicate the nano-structures onto 60 μm thick polyethylene terephthalate (PET) films in the experiments. The embossed films were characterized using UV–vis spectrophotometer, atomic force microscope (AFM), and scanning electron microscope (SEM); its total conversion efficiency for solar cells was also measured by a solar simulator. The experimental results showed that the fabricated films could effectively reduce the reflectance and increase the conversion efficiency of solar cells. The proposed method shows great potential for fast fabrication of the anti-reflection protective layer of solar cells due to its simplicity and versatility.
©2012 Optical Society of America
1. Introduction
Photovoltaic device has gradually become an important device for a new source of power generation. To increase the conversion efficiency of solar energy, the reduction of reflection at the surface of a concentrated photovoltaic device is required [1]. An anti-reflection protective layer, i.e., a coating applied to the surface of a material to reduce light reflection and to increase light transmission, is highly desirable. Typical applications of anti-reflection layers include solar cell, planar displays, glasses, prisms, videos, and camera monitors. Surface-relief gratings with a size smaller than the wavelength of light, i.e., named sub-wavelength structure, can behave as anti-reflection surfaces. By employing a continuous wavelike grating (e.g., pyramidal, triangular, conical shapes), the sub-wavelength structured grating acts as a surface possessing a gradually and continuously changing refractive index profile from the air to the substrate. Furthermore, deeper sub-wavelength structured grating can greatly enhance the anti-reflection effect, since the refractive index value changes smoothly and continuously.
As of now, several surface anti-reflection treatment methods have been developed, including techniques with surface texturing [2], single-layer interference coating [3], multi-layer coating [4], and moth-eye structure forming [5–9]. However, most methods employ complex processes and require expensive equipment. Among them, hot embossing [10,11] is a relatively low-cost replication method for fabricating nano-structured parts. During the embossing step, the original pattern is directly transferred onto a thermoplastic, which acts as resistance. When heated above its glass transition temperature, the polymer becomes viscous and conforms exactly to the embossing shim by filling the cavities of the surface relief. After it has cooled down, the replica is demolded from the master. The heating and cooling processes in hot embossing are, however, time-consuming, and the cores of the plates are unnecessarily softened. The long cycle time caused by such heating and cooling systems makes the hot embossing an inefficient method for mass production.
This study reports a simple and effective infrared assisted hot roller embossing process for continuous fabrication of dual-side nano-structured anti-reflection films for the protective layer of solar cells. Master mold for hot roller embossing was first fabricated by plasma sputtering of aluminum nano-particles onto the surface of glass substrates. After electroforming, nickel (Ni) master molds containing nano-array of 30 nm were obtained. The molds were then attached to the surfaces of the two rollers in an infrared assisted roll-to-roll embossing facility to replicate the nano-structures onto 60 μm thick polyethylene terephthalate (PET) films in the experiments. The patterned protective layer was characterized using UV–vis spectrophotometer, scanning electron microscope (SEM), and atomic force microscope (AFM). Furthermore, the total conversion efficiency of solar cells with the layers was also measured by solar simulator.
2. Experimental setup
2.1 Materials
The material used in this study was polyethylene terephthalate (PET) films (PET C-X, Shinkong Textile Co., Taiwan) with a thickness of 60 μm.
2.2 Manufacture of master molds
The plasma sputtering experiment was completed on a commercially available sputtering machine (Sputter Demos, Kao Duen Tech., Taiwan) equipped with a gas mass flow control (MFC KD-1000, Kao Duen Tech., Taiwan), a vacuum gauge controller (Terranova Model 934, Duniway Stockroom Corp, Canada), and a DC current plasma generator (MDX 500, Advanced Energy, U.S.A). The sputtering gas used was Argon (Ar), while an aluminum plate (MAK Sputter Source-L400A01, MeiVac, Inc., U.S.A.) was used as the target. During sputtering, the gas ions pass the energy to the target atoms, knocking them out and generating film growth on the glass substrate (Microscope Cover Glasses, Deckglaer, Germany), as shown schematically in Fig. 1 . Various processing conditions, as listed in Table 1 , were employed to identify the optimal processing condition. The shape, height, and width of the microstructures on the glass substrates were inspected using scanning electronic microscopy (FEG-SEM 6500F, Jeol Ltd., U.S.A.). Figure 2 shows the images of fabricated nano-structures on the substrates. The results suggested that glass substrate, which was plasma sputtered by using condition G (i.e., a power of 230 watts, a substrate rotating speed of 6 rpm, an Ar flow rate of 25 sccm, and a sputtering time of 60 mins), could have the most uniformly distributed nano-structures of 30 nm in diameter. It was then selected for the subsequent procedure. After electroforming, Ni master molds bearing nano-arrays of 30 nm in size were obtained as shown microscopically in Fig. 3a . The mold was characterized using an atomic force microscope (AFM 5100, Agilent Tech., U.S.A.), and the measured result in Fig. 3b suggests that the surface roughness (Ra) was 21.5 nm.
Table 1. Processing parameters used for the plasma sputtering experiments
Fig. 2 SEM photographs of glass substrates subjected to different sputtering conditions (as listed in Table 1).
2.3 Roller embossing experiments
An infrared (IR) assisted roll-to-roll embossing facility, which includes two metallic rollers and two IR heaters with temperature controls, was designed and built in our lab. Figure 4a shows photographically the setup of the IR assisted roll-to-roll embossing facility and the embossing rollers. The power of the IR-heater (Elstein Model FRS/2, Germany) was 500 watts and the highest working temperature of the heater was 750°C. To emboss microstructures on two sides of the plastic films, the Ni mold bearing nanostructures were attached onto the surfaces of the rollers by glues. During roller embossing, the metallic rollers are driven by a servo-motor and heated by the IR heaters before coming into contact with the plastic plate. The distance between the heater and the roller surface (Fig. 4b) can be adjusted by the Z-stage, located above and beneath the top and bottom heaters respectively. A few test trials were first carried out to find out the correlation between the emitting temperatures of the IR heater and the temperature at the roller surface. The relevant emitting temperature of the heater was then set to obtain the desired surface temperature of the Ni molds. After the plastic films came into contact with the rollers, the top roller was pushed against the plastic film and the bottom roller by the pneumatic actuator located at top of the embossing facility. The infrared energy was converted into heat at the rollers’ surface and was sufficiently high enough to melt thermoplastics at the roller/plastic film interfaces and cause the melt to flow and conform to the microstructures. Anti-reflection films with two-sided nanostructures are thus obtained.
3. Results and discussion
3.1 Effects of processing parameters on the replication of nanostructures
After roller embossing, the nanostructures embossed on the plastic films were characterized. Various processing parameters were studied in terms of their influence on the replicability of IR assisted embossed films: roller temperature, embossing pressure, and rolling speed. Table 2 lists these processing variables as well as the values used in the experiments. To roller emboss the nano-features, one central processing condition was chosen as a reference (the bolded ones in Table 2). By varying one of the parameters in each test while keeping the others fixed, we were able to understand the effect of each factor on the replicability of embossed films.
Table 2. Optimal processing parameters for roller embossing of the anti-reflection films
The results in Fig. 5a suggest that the replicated depths of nanostructures increased with the roller temperature to some optimum value (75°C), after which the replicability decreased. When a force is applied at the rollers, it is easier for the plastic materials with a higher temperature to conform to the surface of the embossing rollers. The replicability increases accordingly. However, if the roller temperature is too high, the plastic film may be overmelt and lead to excessive flow of the polymeric materials when it comes into contact with the roller. Replicated quality thus decreased.
Fig. 5 Influence of (a) roller temperature, (b) embossing pressure, and (c) rolling speed on the replication quality of embossed nanostructures (the other processing parameters were fixed during each test trial).
The measured result in Fig. 5b suggests that the proper embossing pressure for the PET films is 8 bars. Embossing pressure that is too low may not be able to replicate the surface nanostructure well. In the embossing process, the conformability of the part to the roller’s microstructures is the major concern for the embossed films. By applying a higher embossing pressure, the plastic films can be made to conform to the roller to replicate the nanostructure. Furthermore, applying the embossing pressure to the samples also causes the molten polymer to flow and to fill the embossing interface. Increasing the embossing pressure should therefore increase the replicability of the embossed films.
As far as the rolling speed is concerned, the proper rolling speed is 0.6 mm/s (Fig. 5c) for the PET films that were hot embossed in this study. When the plastic films come into contact with the hot rollers, energy is transferred to the film/roller interface to melt the materials. After the melting of the polymeric materials, the plastic film is held against the rollers for some time for the purpose of microstructure conformity. If the rolling speed is too high, the materials at the film/roller interface may not be completely melted. The replicability of the embossed films decreases accordingly. On the other hand, during embossing, the plastic film is held against the roller for some time for the purpose of microstructure conformity. Reducing the rolling speed increases the hold time and thus increases the replicability of the embossed plates.
With a combination of proper processing conditions, i.e., the values in bold in Table 2, plastic films with nanostructures of good dimensional uniformity could be obtained, as shown in Fig. 6a . This confirms the replicating capability by the IR assisted embossing technique proposed in this study. An atomic force microscope (AFM 5100, Agilent Tech., U.S.A.) is also used to measure the surface morphology of the diffusers. Figure 6b shows an AFM image and surface roughness of the fabricated diffusers. The average surface roughness (Ra) is 20.9 nm. When compared to the surface roughness of the Ni mold (Ra=21.5 nm), the experimental result suggests the IR assisted roller embossing method can successfully replicate the nanostructures onto the PET films.
3.2 Characterization of fabricated anti-reflection films
To further verify and inspect the optical properties of the fabricated anti-reflection films, an UV/VIS spectrometer with Integration Sphere (V-650, Jasco Co., U.S.A.) was employed, while unpolarized UV-visible light was used as the light source. Figure 7 shows the transmittance and reflectance of the flat and nano-structured PET films. While the solar cell without the nanostructures showed 7.4% of average reflectance in the wavelength range of 300 to 700 nm, the average reflectance of the nano-structured solar cell decreased to 4.4% in the same range. Furthermore, at the wavelength of 630 nm, the transmittance increased from 89.7% to 93.5% with the nano-structured pattern. The measured result suggests that the overall performance of the films with nanostructures is very much improved when compared to that of the flat films. Nanostructure-patterned films can serve as effective anti-reflective layers over the 300-700 nm spectrum range.
Fig. 7 Transmittance and reflectance of the solar cell comparing the flat layer and the nano-structured layer.
To further inspect the enhancement capacity of the anti-reflection films, photocurrent-voltage measurements were performed using a solar simulator (YSS-50A, Yamashita Denso, Japan) equipped with a xenon lamp. Figure 8 shows the measured I–V characteristic of the solar cells with and without the nano-structured patterns. While the flat PET film shows 1.222V of open circuit voltage (Voc), 65.3% of fill factor (FF), 2.46 mA of short circuit current (Isc), and 6.5% of efficiency, the solar cell with nanostructures shows 1.224V, 71.8%, 2.39 mA, and 7.2% of Voc, FF, Isc and efficiency, respectively. The plastic film with nanostructures displays better energy conversion efficiency than the flat film. The results demonstrate that the fabricated anti-reflection films can effectively enhance the performance of solar cells.
4. Conclusions
This paper discussed an innovative and effective IR assisted roll-to-roll embossing method for the rapid fabrication of dual-side nano-structured anti-reflection films for performance enhancement of solar cells. An infrared (IR) assisted roll-to-roll embossing facility, which includes two metallic rollers and two IR heaters with temperature controls, was designed and built. The embossing facility was used to replicate the nano-structures onto 60 μm thick polyethylene terephthalate (PET) films in the experiments. The embossed films were characterized using UV–vis spectrophotometer, atomic force microscope (AFM), and scanning electron microscope (SEM); its total conversion efficiency for solar cells was also measured by a solar simulator. The experimental results showed that the fabricated films could effectively reduce the reflectance and increase the conversion efficiency of solar cells. The proposed method shows great potential for fast fabrication of the anti-reflection protective layer of solar cells due to its simplicity and versatility.
Acknowledgments
The National Science Council of Taiwan, R.O.C. under the grant NSC100-2221-E-182-020-MY3, has supported this work financially.
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