1. Introduction

Inspired by the hexagonal structures of a moth’s eye [1,2], many artificial nanostructures have been fabricated on various substrates to implement broadband and omnidirectional antireflective (AR) structures. Over the last few years, many different structures such as nanorods [3,4], nanocones [5,6], pyramids [7], nanotips [8], and porous structures [9,10] have been created by using top-down or bottom-up processes, reporting great reductions in surface reflection. Nanostructures on a sample surface can sometimes cause a gradual change in the effective refractive index of an interface between two different materials, leading to minimized reflection. For solar cells, broadband AR property is a desirable feature in order to efficiently collect light and omnidirectional absorption is another favorable attribute leading to efficient harvesting of solar energy regardless of the incident angle of the sunlight [4]. However, it has not generally been demonstrated whether reductions in surface reflection are accompanied by improved absorption, since scattering can also play a role.

To demonstrate enhanced transparency by AR layer, it is necessary to generate a graded refractive index layer on optically low-loss and transparent substrate. In practice, a gradual change of the refractive index can be achieved by changing the filling fraction of materials in the nanostructures [11]. A close-packed design is commonly used to achieve a nominally continuous variation of filling fraction throughout a nanostructure such as colloidal self-assemblies [12,13]. However, most of these methods rely on pillar or sphere type nanostructures which have discontinuous effective refractive indices at the bottoms or tops of the nanostructure, inducing an abrupt change in the refractive index, thus an increase in reflection [13].

Herein, we demonstrate a gradual variation of the refractive index by creating V-shaped nanoholes in a glass surface using a simple bottom-up fabrication process. The tapered profile of the V-shape nanohole arrays provides more continuous variation of the refractive index on the glass. These V-shaped structures in glass are similar conceptually to the conical-shaped structures that have been used for AR coatings on light emitting diode surfaces [14]. Besides, direct patterning of glasses, unlike other methods where residual organic materials play a role as AR structures [15,16], exhibits higher mechanical stability and better durability than coatings since no foreign materials are involved, and also can be used over a broad thermal range [17,18]. The broadband and omnidirectional AR effect of the proposed nanostructures is confirmed by measuring the transmittance of the patterned glasses and also tested with a solar cell. To the authors' knowledge, this is the first study which correlates direct patterning of solar cell packaging glass with solar cell efficiency enhancement as well as demonstrates the self cleaning effect from directly patterned glass after two weeks of outdoor testing.

2. Methods

The schematic diagram of the fabrication process for the AR structures is shown in Fig. 1 . Bare borosilicate glass substrates are cleaned with acetone in an ultrasonic bath followed by isopropyl alcohol (IPA) and deionized (DI) water rinse. After cleaning, 250 nm thick aluminum film is deposited by electron beam evaporation on cleaned glass substrates (Fig. 1b), followed by anodization under a constant voltage of 40 V in 0.3 M oxalic acid solution to created porous aluminum oxide with randomly distributed pore patterns [10] (Fig. 1c). The size of the nanopores is controlled by soaking the sample in a solution of 5 wt% phosphoric acid for different durations (Fig. 1d). The anodized aluminum oxide (AAO) structures are used as an etching mask to create nanoholes in the glass substrates. Thin AAO layer is also etched slowly during the etching process, however, it is thick enough to be used as an etching mask. Reactive ion etching using SF₆ gas is performed in an inductively coupled plasma-reactive ion etching (ICP-RIE) system, and the pattern depths are controlled by adjusting the etching time from 1 to 5 min (Fig. 1e). Finally, AAO template is removed by phosphoric acid (Fig. 1f).

 

Fig. 1 Schematic illustration of the process for preparing AR structures.

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Surface morphology of AAO layers and etched glasses is investigated by a scanning electron microscopy (SEM). To overcome the charging effect on the glass surface, 10 nm gold film is deposited on the patterned surface prior to SEM imaging. The images obtained from the SEM are analyzed by using Image J. The spectral transmittance over the wavelength range 400 - 1200 nm is carried out using a spectrophotometer for light of normal incidence. The angle dependent transmission is evaluated in a dark room using a home-made equipment consisting of a silicon photodetector operating at the wavelength ranging from 350 to 1100 nm and a tungsten halogen light source with a color temperature of 2900 K. The characteristic of solar cells is measured by Oriel Xenon arc lamp solar simulators with Air Mass (AM) 1.5 filter. The filter is used to reduce spectrum mismatch between lamp and solar light of AM 1.5.

3. Results and discussion

3.1 Fabrication

Figure 2 shows SEM images of nanohole arrays formed by the AAO process after the pore widening process (Fig. 1d). Although it is reported that a highly ordered pattern can be generated by a two step anodizing method [19], we used an one step anodizing method to simplify the fabrication and to generate randomly distributed nanostructures. The randomized structures are reported to have a better broadband transmission which is undesirable for filter applications, but very useful for solar cell applications [16]. The prepared samples in Fig. 2 have an array of nanoholes with diameters ranging from 51.5 to 85.6 nm. The holes are distributed randomly on the whole surface with an average spacing of 100 nm between the holes regardless of the pore widening time. Table 1 shows the dependence of the hole diameter on the pore widening time. The air filling fraction of the AAO templates changes from 23.33 to 54.66% as the soaking time, thus the hole diameter, increases. Though larger holes are preferred in the AAO patterns in order to obtain an effective refractive index close tothat of the air after transferring the patterns to glass, longer duration of pore widening causes the thinning of AAO layers, which is undesirable for etch masks.

 

Fig. 2 SEM images of the AAO nanohole structures formed by different pore widening conditions, leading to various diameters of 51.5 (a), 66.8 (b), 74 (c), and 85.6 nm (d).

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Table 1. Mean diameters, standard deviations of diameters, and filling fraction of holes at the top surface of AAO layers for different pore widening times in Fig. 2

View Table

Figure 3a and 3b show the top and cross-sectional view of the etched patterns, respectively, after 4 minutes of etching. The inset of Fig. 3a shows the size distribution of nanoholes with a mean diameter of 48 nm, which is smaller than the diameter (51.5 nm) of the AAO template. The profile in Fig. 3b demonstrates the 400 nm deep nanoholes are generated. The nanohole arrays are normal to the surface with a V-shape profile, which creates the gradual variation of refractive index at the both ends of nanostructures. Fluorine based gas has been known to generate etch inhibitor during the etching process and the inhibitor is deposited on the side wall to protect the wall from reactive ions [20]. Excessive use of SF₆ prohibits side wall etching which relatively increases the etching reaction at the center of nanoholes, leading to the V-shaped patterns [21].

 

Fig. 3 (a) SEM images of glass surface after etching for 4 min. AAO pattern is formed with 50 min of pore widening time and removed after etching (inset: size distribution of nanoholes on glass). (b) 45° oblique view of etched pattern in glass substrate.

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3.2 Broadband and omnidirectional antireflection

In Fig. 4 , the spectral transmittance of glass substrates patterned with different widening times and etching times is compared with that of the bare glass. The optical transmission through the glass increases from 92 to 94.5% with nanopatterning with 4 min of etching (Fig. 4a). By increasing pore widening time, the samples exhibit a higher transmission of 93%, 94%, and 94.5% corresponding to the hole diameters of 51.5 nm, 66.8 nm, and 85.6 nm in the AAO layer, respectively, in the wavelength range above 500 nm. Close-packed nanoholes with a larger diameter show an enhanced AR behavior due to more smooth change of the refractive index between the glass and the air. Figure 4b shows the depth dependence of AR property of nanohole structures with a pore widening time of 60 min. The samples with more than 3 min etching time show similar transmittance of 94.5% in the wavelength range above 500 nm. It indicates that an etch depth of 300 nm (3 min etching) is sufficient due to the gradual variation of the refractive index at the bottom of V-shaped nanoholes. It is reported that the height of pyramidal nanostructure has less dependence on AR characteristic [22]. For a better transparency double sided patterning in glasses can be employed to reduce the transmission loss at both interfaces [17]. The reduction of transmission at a range less than 500 nm is explained by light scattering and absorption loss, induced by the fabrication process and defects on the surface [17].

 

Fig. 4 (a) Optical transmittance of patterned glass at normal incidence for different diameters adjusted by pore widening time from 50 to 80 min followed by 4 min of etching. The bare glass data is shown as a reference. (b) Transmittance comparison for different hole depths etched for 1 to 5 min. Pore widening time is 60 min. (c) Transmission comparison between the experimental data (symbols) and the simulated data (lines) of the optimal AR structure. (d) Incident angle dependence of transmittance (inset: the schematic illustration of a simulated structure).

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Figure 4c shows the comparison of spectral transmission for the bare glass and the optimized patterned glass, along with the simulation results of the same structures in the case of normal incident angle. The dimensions of the nanohole array in the optimized sample are ~60 nm diameter and 300 nm depth. In the finite difference time domain (FDTD) simulations, the periodicity is fixed at 100 nm and the height of the structure is 300 nm with a refractive index of 1.5, which represents that the 60% of surface is glass and 40% is filled with nanoholes in two dimensional simulation structures. The computational geometries are shown in the inset of Fig. 4d. Another advantage of the AR structures with gradually varying refractive index is an omnidirectional AR property. We have performed angle dependent transmission measurements, where a photodetector is placed under the glass and a white light source is located above the sample simulating the movement of the sunlight. In Fig. 4d, the enhancement of transmission of the patterned glass is clearly shown at all angles. There is at least 3% better transmission in the AR structures at all incident angles. The measured results are not perfectly matched with the simulation result at the angles larger than 55° due to the guided loss of light through the glass substrate.

3.3 Outdoor testing of AR layers with solar cells

The change of transmission is measured after exposing both the bare and the optimized patterned glasses for two weeks in outdoor environments. The nanostructured glass shows a much less decrease in transmission after two weeks compared to the bare one as shown in Fig. 5a . This property can be attributed to the self cleaning effect from the nanostructures [23]. Finally the nanostructured glass is tested with a solar cell under the AM 1.5 illumination condition and is compared to the cell with a bare glass. The measured current-voltage characteristics are shown in Fig. 5b. The short-circuit current (I _SC) and the open-circuit voltage (V _OC) are enhanced by 3.5% and 1.44%, respectively, due to the nanostructured glass. The light conversion efficiency (η) of the solar cells is improved from 10.66% to 11.32% with the use of the nanoholes. After two weeks of exposure to outdoor environments, I _SC and V _OC are improved by 4.04% and 3.8%, respectively, and η is enhanced from 10.47 to 11.20% with nanostructures.

 

Fig. 5 (a) Transmittance comparison after 2 weeks of outdoor testing. (b) Current-voltage characteristics of solar cells without and with the nanostructured glass.

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4. Conclusion

In summary, AR coatings based on an AAO method are studied for fabricating hole-type nanostructures on the glass substrates combined with a dry etching technique. The glasses with tapered nanoholes of sub-100 nm diameter show enhanced transmission due to a continuous variation of filling fraction at both ends of nanostructures. The array of randomly distributed nanoholes also shows prominent improvement in optical transmittance in wide spectral and wide angular ranges which can be directly applied in solar cell packaging for efficient light harvesting.