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We fabricated the parabola-shaped subwavelength grating (SWG) nanostructures on indium tin oxide (ITO) films/Si and glass substrates using laser interference lithography, dry etching, and subsequent re-sputtering processes. The efficiency enhancement of an a-Si:H/μc-Si:H tandem thin film solar cell was demonstrated theoretically by applying the experimentally measured data of the fabricated samples to the simulation parameters. Their wetting behaviors and effective electrical properties as well as optical reflectance properties of ITO SWGs, together with theoretical prediction using a rigorous coupled-wave analysis method, were investigated. For the parabola-shaped ITO SWG/ITO film, the solar weighted reflectance (SWR) value was ~10.2% which was much lower than that (i.e., SWR~20%) of the conventional ITO film, maintaining the SWR values less than 19% up to a high incident angle of 70° over a wide wavelength range of 300-1100 nm. Also, the ITO SWG with a superhydrophilic surface property (i.e., water contact angle of 6.2°) exhibited an effective resistivity of 2.07 × 10−3 Ω-cm. For the a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer, the conversion efficiency (η) of 13.7% was theoretically obtained under AM1.5g illumination, indicating an increased efficiency by 1.4% compared to the device with the conventional ITO film (i.e., η = 12.3%).
©2012 Optical Society of America
1. Introduction
Recently, there has been a major study to improve the cell efficiency of solar cells around the world, especially for silicon (Si)-based thin film solar cells due to the little material usage and low cost [1,2]. However, the Si-based thin film solar cells with thicknesses of < 10 μm make it difficult to harvest the solar spectrum over a broadband wavelength region because of its thin absorption layer and weak optical absorption. Thus, it is necessary to employ an antireflection coating (ARC) layer in solar cell structures for efficient light absorption and trapping, which can enhance the cell efficiency. The biomimetic subwavelength grating (SWG) nanostructures with a period smaller than the incident light wavelength have attracted a great interest as an alternative of the conventional thin film ARCs for achieving high-efficiency solar cells [3–6]. As observed from the moth eye effect [7], the SWG with a parabola shape leads to a nearly linearly graded effective refractive index distribution between air and the semiconductor, which can considerably suppress the Fresnel surface reflection losses in the wide ranges of incident wavelengths and angles, showing a long-term thermal stability as well as a good durability [8–11]. Among periodic nanoscale patterning techniques, the laser interference lithography is a relatively simple, fast, cost-effective, and large-scale method compared to other ones such as e-beam lithography and nanoimprint lithography [12].
Meanwhile, indium tin oxide (ITO), which is one of common transparent conducting oxide materials, can be widely used as an antireflective and transparent electrode layer in solar cells due to its relatively low refractive index as well as good optical and electrical properties [13–16]. Besides, the ITO surface has a hydrophilic property which can lead to the removal of dust particles and surface contaminants, i.e., self cleaning [17,18]. The hydrophilicity makes water droplets to spread out evenly like a thin film on the surface. When a water droplet flows on the hydrophilic surface, it can wedge into the space between the dusts and the surface, and takes the dusts away. The hydrophilicity can be enhanced if the roughness is sufficiently large [19]. The ITO nanostructures (i.e., nanowires, nanopillars, and nanowhisker etc.) have been demonstrated for high-efficiency devices [20–22]. But, there is very little work reported on the use of the periodic SWGs with a parabola shape on ITO films for the ARC layer and electrode in solar cell applications. Also, it is necessary and crucial to optimize and analyze theoretically the device performance using the numerical modeling and simulation of solar cell structures, which have been often performed using a Silvaco ATLAS device simulator [23,24]. In this work, we fabricated the parabola-shaped SWGs on ITO films using the laser interference lithography/dry etching patterning and the subsequent re-sputtering process and investigated their effect as an antireflective electrode layer on the cell efficiency of a-Si:H/μc-Si:H tandem thin film solar cells by applying the experimentally measured data to the material parameters in Silvaco ATLAS simulations. For optical reflectance characteristics, the theoretical analysis was also performed using the rigorous coupled-wave analysis (RCWA) method.
2. Experimental and simulation modeling details
2.1. Fabrication of the parabola-shaped ITO SWG nanostructures
Figure 1 shows the schematic illustration of process steps for the fabrication of the parabola-shaped ITO SWG nanostructures on ITO films/Si and glass substrates. The side- and top-view SEM images of ITO SWGs as a function of re-sputtering time for the order of taper (OT) and the ratio of bottom diameter of SWGs to period between SWGs (RBdP) are also shown in Fig. 1. The structural parameters of SWGs were defined in our previous work [10]. To fabricate the parabola-shaped SWG structures on the surface of ITO films, the ITO films were prepared on the squared (100) Si and glass (Corning Eagle 2000) substrates with a size of 20 × 20 mm2 by using an RF magnetron sputtering system (KVS-2000L, Korea Vac. Tech. Ltd.). Prior to the sputtering, the substrates were ultrasonically cleaned in acetone, methanol, and de-ionized water for 10 min, respectively, and then dried with nitrogen (N2) gas. The dilute nitric acid rinse was also performed to remove metal contaminants from the surface of substrates. The 2 inch 99.99% purity ITO (In2O3:SnO2 = 90:10 wt.%) sputtering target was used. The distance between the sample and target was kept at 60 mm. A turbomolecular pump maintained a base pressure of less than 10−6 Torr before the sputtering. The ITO sputtering was done with an RF power of 100 W at the process pressure of 7 mTorr in an Ar gas flow of 30 sccm. For a good uniformity, the samples were rotated with 10 rpm during the sputtering process. The thickness of deposited ITO films was approximately 460 nm. For the fabrication of SWG structures, the AZ 5206-E photoresist (PR) diluted with an AZ 1500 thinner was spin-coated on the ITO films. The dilution ratio and rotation speed were determined to obtain a PR thickness of ~190 nm. The prebaking on a hot plate was carried out at 90 °C for 90 s. For the subwavelength scale etch masks with two-dimensional (2D) periodic hexagonal patterns, the PR was exposed twice with 60° sample rotation between exposures by the interference of two laser beams using a 363.8 nm Ar ion laser. After the development, the cone-shaped SWG structure on the surface of ITO films with a non-closely packed hexagonal pattern can be obtained through the pattern transfer by an inductively coupled plasma (ICP: Multiplex ICP, STS) etcher system using the overall etching until the PR on ITO films is completely removed. The average OT, RBdP, height, and period of conical ITO SWGs were approximately 0.9, 0.38, 170 nm, and 460 nm, respectively, exhibiting the ITO film thickness of ~250 nm. The dry etching was performed with 30 W RF power with an additional 50 W ICP power at 10 mTorr in Cl2/CF4 gas mixture flow rate of 45 sccm/5 sccm for 3 min. The parabola-shaped ITO SWG nanostructures can be obtained from the cone-shaped SWGs by the ITO re-sputtering method. As illustrated in Fig. 1, the SWGs became more parabolic and closely packed with increasing the re-sputtering time from 10 to 20 min, indicating the increase of average OT and RBdP from 1.3 and 0.5 to 1.8 and 0.72, respectively, which leads to a linearly gradual change in the effective refractive index profile from air to the ITO [8]. The average height of SWGs and thickness of the film were also increased from 190 nm and 370 nm at 10 min re-sputtering to 210 nm and 480 nm at 20 min, respectively. Subsequently, the fabricated samples were annealed at 500 °C for 5 min in an air atmosphere furnace.
Fig. 1 Schematic illustration of process steps for the fabrication of the parabola-shaped ITO SWGs on ITO films/Si and glass substrates. The side- and top-view SEM images of ITO SWGs as a function of re-sputtering time for OT and RBdP are also shown.
The etched and deposited profiles of the fabricated parabola-shaped SWG nanostructures on ITO films were observed by using a scanning electron microscope (SEM, LEO SUPRA 55, Carl Zeiss). The optical reflectance was measured by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) at near-normal incidence (θi = 8°). For angle-dependent reflectance measurements, the spectroscopic ellipsometry (V-VASE, J. A. Woollam Co. Inc.) was used at incident angles of 20-70° using a linearly polarized incident light. The water contact angles were measured and averaged at three different areas on the surface of samples by using a contact angle measurement system (Phoenix-300, SEO Co., Ltd.). The effective resistivity, carrier concentration, and Hall mobility were measured by using a Hall effect measurement system (HL5500PC, Accent) at room temperature. For Hall effect measurements, the indium contacts were deposited onto the corners of square-shaped samples with a size of 10 × 10 mm2 in the Van der Pauw geometry [25]. Then, the samples were heated on a hot plate at 180 °C for 3 min.
2.2. Simulation modeling of the parabola-shaped ITO SWG nanostructures and a-Si:H/μc-Si:H tandem thin film solar cell structures
The theoretical analysis of the reflectance property in parabola-shaped ITO SWG nanostructures was performed by the RCWA method [26]. To design the theoretical models, the periodic geometry of the SWGs on ITO films/Si substrate was roughly represented in the Cartesian coordinate system by a scalar-valued function of three variables, f(x, y, z). We assumed that the linearly polarized incident light entered from air into the structure and the thickness of Si substrate was set to 525 μm.
For the numerical modeling of a Si-based thin film solar cell, the experimentally reported a-Si:H/μc-Si:H tandem thin film solar cell structure was employed [27]. We carried out the simulation of the a-Si:H/μc-Si:H tandem thin film solar cell structure with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer by applying the experimentally measured results under 1-sun standard air mass 1.5 global (AM1.5g [28], 100 mW/cm2) illumination using the 2D Silvaco ATLAS simulation [29]. For constituent materials used in simulations, the physical information was obtained in refs [29–33] including their refractive indices and extinction coefficients. From the theoretical calculations, we obtained the solar cell parameters, i.e., J-V characteristics (short circuit current density, Jsc; open circuit voltage, Voc), fill factor (FF), and conversion efficiency (η). Further details of theoretical optical modeling and solar cell simulations can be found in our previous works [10,16].
3. Results and discussion
Figure 2 shows the contour plots of the calculated reflectance variation as a function of wavelength at different (a) OT and (b) RBdP for ITO SWG nanostructures. The 2D and 3D simulation models used in these calculations are also shown in Figs. 2(a) and (b), respectively. In RCWA simulations, the period and height of SWGs were set to 460 nm and 210 nm and the thickness of the ITO film was kept at 480 nm. For the ITO SWGs with the RBdP of 0.72, the reflectance is generally reduced over a wide wavelength region of 300-1100 nm when the OT becomes larger from 0.6 to 2, as shown in Fig. 2(a). This is because the parabola-shaped structure provides a more linearly graded effective refractive index profile from air (nair = 1) and the ITO (nITO~1.96 at a wavelength of 550 nm) than the cone-shaped one [8,10]. At OT = 1.8, the average reflectance (Rave) value is ~9.6% in the wavelength range of 300-1100 nm. This value is lower than Rave~11.9% at OT = 0.9 and Rave~10.5% at OT = 1.3. The reflectance is also strongly affected by the RBdP, as can be seen in Fig. 2(b). The reflectance of the parabola-shaped ITO SWG nanostructure with an OT of 1.8 tends to decrease with increasing the RBdP due to the increase in the density of SWGs [34]. For RBdP values less than 0.3, there are strong and high oscillations in the reflectance spectra at wavelengths of 300-1100 nm. This is due to the constructive or destructive interference caused by multiple reflections of the light at interfaces of the air/ITO film and ITO film/Si substrate. It is noticeable that there is little effect on the reflectance property even if the SWG nanostructure is formed on the ITO film/Si substrate. While, at RBdP values higher than 0.7, the reflectance values of < 20% are maintained in the wavelength region of 300-1100 nm. At RBdP values of > 0.5, the low reflection region of < 5% is shifted toward the longer wavelength region at wavelengths of > 550 nm. As expected, the parabola-shaped ITO SWG/ITO film at RBdP = 0.72 has the lower Rave value than those with RBdP = 0.38 and 0.5 (i.e., Rave~14.5% and ~12.4%, respectively). Thus, the parabola-shaped ITO SWG nanostructures with a high density can efficiently reduce the optical loss caused by the Fresnel surface reflection at the surface of the ITO films/Si substrate.
Fig. 2 Contour plots of the calculated reflectance variation as a function of wavelength at different (a) OT and (b) RBdP for ITO SWG nanostructures. The 2D and 3D simulation models used in these calculations are also shown in (a) and (b).
Figure 3(a) shows the measured reflectance spectra at different incident angles of θi = 8-70° for the parabola-shaped ITO SWG/ITO film on the Si substrate. For comparison, the reflectance spectra of Si substrate (θi = 8°) and conventional 460 nm-thick ITO film on Si substrate (θi = 8-70°) are also shown in the left inset of Fig. 3(a). At near-normal incident angle of θi = 8°, although the 460 nm-thick ITO film exhibited the lower Rave value of ~19.4% than that of Si substrate (i.e., Rave~39.5%) at wavelengths of 300-1100 nm, there are strong oscillations in the reflectance spectrum due to the constructive or destructive light interference at interfaces of air/ITO and ITO/Si, which forms an abruptly changed effective refractive index profile from air (nair = 1) to the Si substrate (nSi~4) via the ITO film (nITO~1.96). In contrast, for the parabola-shaped ITO SWG/ITO film, the reflectance spectrum as well as its oscillations were reduced compared to the conventional ITO film, exhibiting the Rave~10.8%. This results from a linearly gradual change in the effective refractive index distribution between air and the ITO via ITO SWGs though there is an abrupt change in refractive index at the interface of ITO film/Si substrate. Moreover, for the periodic grating structure with a period of Λ, the angle of the reflected diffraction waves, θr,m, in the m-th diffraction order is given by the grating equation at normal incidence [35]:
where λ is the wavelength of incident light and n is the refractive index of the incident medium. If the grating period becomes much shorter than the wavelength of incident light, there exists only a zero diffraction order (i.e., m = 0) in the reflection and all the higher orders of diffracted light waves are evanescent [22]. For the parabola-shaped ITO SWG/ITO film with a period of 460 nm, there may exist the higher orders (i.e., m> 0) of diffractive light waves in the shorter wavelength region than the period. As shown in Fig. 3(a), however, the ITO SWG exhibited a lower reflectance compared to that of ITO film at wavelengths of < 460 nm because the structure provides a linearly graded effective refractive index profile between air and the ITO film, as mentioned above. From these reasons, the periodic parabola-shaped structure can suppress significantly the optical loss caused by the Fresnel surface reflection compared to the flat film in the broadband wavelength range. For solar cell applications, the solar weighted reflectance (SWR) [20], i.e., the ratio of the useable photons reflected to the total useable photons, can be evaluated by normalizing the measured reflectance and the terrestrial AM1.5g spectra integrated over a wavelength range of 300-1100 nm. The estimated SWR values of the ITO film and ITO SWG/ITO film on Si substrates as a function of incident angle are shown in the right inset of Fig. 3(a). For the ITO film, the SWR value was 20% but it was largely decreased to 10.2% for the parabola-shaped ITO SWG/ITO film. As the incident angle was increased, the SWR values of both structures exhibited an increasing trend. At the high incident angle of θi = 70°, the SWR value of ITO film was 27% while it was 19% for the parabola-shaped ITO SWG/ITO film, which is lower than that of the ITO film at θi = 8°. For both structures, the angle-dependent reflectance calculations were performed by the RCWA simulation. Figure 3(b) shows the contour plots of the variation of calculated reflectance spectra of the (i) ITO film and (ii) parabola-shaped ITO SWG/ITO film on Si substrates as a function of the incident angle of light. The 3D simulation models of the corresponding structures are shown in Fig. 3(b). In simulations, the OT, RBdP, and height of SWGs were set to 1.8, 0.72, and 210 nm, respectively. The film thicknesses of the conventional ITO film and parabola-shaped ITO SWG/ITO film were 460 and 480 nm, respectively. As can be seen in Fig. 3(b), as the incident angle is increased, the reflectance is also increased. Clearly, it can be observed that the reflectance of the parabola-shaped ITO SWG/ITO film is less affected by the incident angle than the ITO film. There exists a difference between the experimentally measured and theoretically calculated results at some wavelengths and incident angles. This may be ascribed to the geometrical difference in the simulation model and the fabricated structure as well as the refractive index mismatch of the ITO or Si used in these experiment and calculation. Nevertheless, the calculated reflectance results roughly provide a similar tendency to the measured results in wide ranges of wavelengths and incident angles.Fig. 3 (a) Measured reflectance spectra at different incident angles of θi = 8-70° for the parabola-shaped ITO SWG/ITO film on the Si substrate and (b) contour plots of the variation of calculated reflectance spectra of the (i) ITO film and (ii) parabola-shaped ITO SWG/ITO film on Si substrates as a function of the incident angle of light. For comparison, the reflectance spectra of Si substrate (θi = 8°) and conventional 460 nm-thick ITO film on Si substrate (θi = 8-70°) are also shown in the left inset of (a). The estimated SWR values of the ITO film and ITO SWG/ITO film on Si substrates as a function of incident angle are shown in the right inset of (a). The 3D simulation models of the corresponding structures are shown in (b).
Figure 4 shows the photographs of (a) conventional ITO film and parabola-shaped ITO SWG/ITO film on Si substrates and (b) water droplets on the samples and (c) 30°-tilted oblique-view SEM images of the parabola-shaped ITO SWG/ITO film. The side-view SEM image of the parabola-shaped ITO SWG/ITO film is shown in the inset of Fig. 4(c). The ITO film surface had a dark orange color. For the fabricated parabola-shaped ITO SWG/ITO film, it exhibited the black surface. This can be explained by the fact that the sample absorbs the light at wide ranges of wavelengths and incident angles, and thus it does not reflect the light at visible wavelengths, which indicates a low surface reflectivity as confirmed in Fig. 3(a). The SWGs with a parabola shape can be obtained from the cone-shaped ones by the re-sputtering method [11]. Also, it can be observed that the parabola-shaped ITO SWG structure is very uniform and closely-packed over a large area on the ITO film/Si substrate, as shown in the SEM images of Fig. 4(c). The parabola-shaped ITO SWG nanostructure produces the more hydrophilic (i.e., superhydrophilic) surface with a contact angle (θc) of 6.2° compared to the surface of the ITO film (i.e., θc~27°). This means that the hydrophilic property can be larger by the roughness on the hydrophilic surface as proposed by the Wenzel’s equation though it is also related to the surface energy of materials [19]. On the superhydrophilic surface, the dust and other contaminants are easily wiped away with the flowing water which can squeeze into the space between the dusts and the surface. Therefore, this ITO SWG nanostructure may self-clean the dust particles and quickly dry the rainwater on the surface of Si-based thin film solar cells in real environments.
Fig. 4 Photographs of (a) conventional ITO film and parabola-shaped ITO SWG/ITO film on Si substrates and (b) water droplets on the samples and (c) 30°-tilted oblique-view SEM images of the parabola-shaped ITO SWG/ITO film. The side-view SEM image of the parabola-shaped ITO SWG/ITO film is shown in the inset of (c).
The effective electrical properties, i.e., resistivity, carrier concentration, and Hall mobility, of the conventional 460 nm-thick ITO film and parabola-shaped ITO SWG/ITO film with the film thickness of 480 nm on glass substrates are summarized in Table 1 . The effective resistivity of the ITO film was also dependent on its surface morphology. For the ITO film, the resistivity was 1.4 × 10−3 Ω-cm, indicating the carrier concentration and Hall mobility of 2.24 × 1020 cm−3 and 20 cm2V−1s−1, respectively. On the other hand, the resistivity of the parabola-shaped ITO SWG/ITO film was 2.07 × 10−3 Ω-cm. This value is higher than that of the ITO film due to the slight decrease of carrier concentration and Hall mobility (i.e., 1.91 × 1020 cm−3 and 15.8 cm2V−1s−1, respectively). This is probably ascribed to the increased surface area due to the SWGs on the ITO film, which leads to the surface recombination loss. Nevertheless, the electrical properties of the parabola-shaped ITO SWG/ITO film are not distinctly degraded compared to the ITO film, exhibiting the resistivity difference of 0.67 × 10−3 Ω-cm.
Table 1. Effective Electrical Properties of Conventional ITO Film and Parabola-shaped ITO SWG/ITO Film
To investigate the effect of the parabola-shaped ITO SWG nanostructure as an antireflective electrode layer in Si-based thin film solar cells, the theoretical modeling and simulation of the experimentally reported a-Si:H/μc-Si:H tandem thin film solar cell structure were performed using the 2D Silvaco ATLAS software in the wavelength region of 400-1100 nm under AM1.5g illumination. Figure 5(a) shows the (i) schematic diagram of a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer, (ii) calculated effective refractive index profile of the ITO SWG, and (iii) the refractive index (n) and extinction coefficient (k) of the ITO used in this simulation. In simulations, the experimentally measured results of the parabola-shaped ITO SWG/ITO film were applied to the physical parameters of the antireflective electrode layer in the solar cell structure. We assumed that the SWGs were considered as an effective medium, composed of 21 multi-layers, with the linearly graded refractive index profile, as shown in the (i) and (ii) of Fig. 5(a). The effective refractive index and extinction coefficient of ITO SWGs were estimated at each wavelength by an average volume fraction of the refractive index and extinction coefficient of the individual constituents [36]. In this case, for the parabola-shaped ITO SWG/ITO film, the reflectance calculated in the Silvaco ATLAS simulation could differ from that calculated by the RCWA method because the period of the SWGs is not considered in the effective medium theory. However, it is clear that the reflectance of the parabola-shaped ITO SWG/ITO film would be much lower than that of the ITO film due to the more linearly graded refractive index profile between air and the ITO film via the ITO SWG as an effective medium in the effective medium calculation. The optical energy bandgap (Eg) of the ITO was obtained from the absorption coefficient (α = 4πk/λ) and (αhν)2 = A(hν-Eg) [37], where k is the extinction coefficient of the ITO, λ is the wavelength, A is a constant related to the material, and hν is the photon energy, exhibiting the Eg~3.81 eV.
Fig. 5 (a) (i) Schematic diagram of a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer, (ii) calculated effective refractive index profile of the ITO SWG, and (iii) the refractive index (n) and extinction coefficient (k) of the ITO used in this simulation and (b) J-V characteristics of a-Si:H/μc-Si:H tandem thin film solar cells with the conventional ITO film and parabola-shaped ITO SWG/ITO film as an antireflective electrode layer. The parameters of a-Si:H/μc-Si:H tandem thin film solar cells with the ITO film and parabola-shaped ITO SWG/ITO film are summarized in the inset of (b), respectively.
Figure 5(b) shows the J-V characteristics of a-Si:H/μc-Si:H tandem thin film solar cells with the conventional ITO film and parabola-shaped ITO SWG/ITO film as an antireflective electrode layer. The parameters of a-Si:H/μc-Si:H tandem thin film solar cells with the ITO film and parabola-shaped ITO SWG/ITO film are summarized in the inset of Fig. 5(b), respectively. For the solar cell structure with the ITO SWG/ITO film, the Jsc of 13.43 mA/cm2 was obtained, which is higher than that with the ITO film (i.e., Jsc = 12 mA/cm2), thus leading to the increase of the conversion efficiency (η) from 12.3% for ITO film to 13.7% for ITO SWG/ITO film. This is attributed to the fact that the SWR (i.e., 10.2%) of the parabola-shaped ITO SWG/ITO film is much lower than that (i.e., SWR~20%) of the ITO film due to the linearly graded refractive index profile between air and the ITO film via ITO SWGs while the effective electrical resistivity is a little bit degraded, as shown in Fig. 3(a) and Table 1. Meanwhile, the Voc of both structures were maintained to approximately 1.37 V while their FF was reduced from 74.8% for ITO film to 74.45% for ITO SWG/ITO film because of a slightly degraded effective electrical property.
4. Conclusion
We fabricated the broadband and wide-angle antireflective parabola-shaped ITO SWG nanostructures using the laser interference lithography, ICP etching, and subsequent RF magnetron re-sputtering techniques and investigated their optical reflectance characteristics, together with the theoretical analysis using the RCWA method. The effective electrical properties and wetting behaviors were also studied. The use of parabola-shaped ITO SWG/ITO film as an antireflective electrode layer in the a-Si:H/μc-Si:H tandem thin film solar cell structure was explored using the Silvaco ATLAS simulation. The parabola-shaped ITO SWG/ITO film with a superhydrophilic surface (i.e., water contact angle of 6.2°) considerably reduced the reflectance compared to the conventional ITO film over a wide wavelength region of 300-1100 nm, exhibiting the SWR value of ~10.2% (i.e., SWR~20% for ITO film). Furthermore, its effective resistivity was not distinctly degraded rather than that of the ITO film. For the a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the parabola-shaped ITO SWG/ITO film as an antireflective electrode layer, the η of 13.7% was obtained under AM1.5g illumination, indicating an efficiency improvement by 1.4% compared to the solar cell with the ITO film (i.e., η = 12.3%). These results can provide a better insight into the ITO SWG nanostructures with the broadband and wide-angle antireflective surface as well as the self-cleaning function for high-efficiency Si-based thin film solar cells.
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2011-0026393) and by the National Research Foundation of Korea Grant funded by the MEST (No. 2011-0031508).
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