Extraordinary high broadband specular transmittance of sodalime glass substrate by vapor phase etching

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ScienceDirect Solar Energy 129 (2016) 147–155 www.elsevier.com/locate/solener

Extraordinary high broadband specular transmittance of sodalime glass substrate by vapor phase etching Arvind Kumar a, Soumik Siddhanta b, Harish C. Barshilia a,⇑ a b

Nanomaterials Research Laboratory, Surface Engineering Division, CSIR – National Aerospace Laboratories, Bangalore 560017, India Jawaharlal Nehru Centre for Advanced Scientific Research, Chemistry and Physics of Materials Unit, Jakkur, Bangalore 560064, India Received 10 September 2015; received in revised form 9 January 2016; accepted 14 January 2016

Communicated by: Associate Editor Alessio Bosio

Abstract In this paper, we present a simple method to fabricate the antireflective porous surface on sodalime glass using a single step HF-vapor phase etching method. Under optimal conditions, both-sides etched glass substrate exhibited a broadband enhancement in the transmittance with maximum transmittance as high as 99.2% at
500 nm with extremely low diffusive scattering. The measured transmittance exceeds by
7.6% as compared to plain glass (91.6%). X-ray photoelectron spectroscopy results confirmed the formation of a fluoride layer comprising of NaF and CaF2 on sodalime glass substrate after etching. Field emission scanning electron microscopy results showed the formation of porous structure with randomly distributed pores of size <150 nm. The refractive index of the porous fluoride layer was found to be 1.28 and lowest reflectance of 0.6% has been achieved. Moreover, reflection (measured at 500 nm) remains below 1.5% over a range of incident angles (8–48°), which is ascribed to the fact that refractive index follows a gradual change in the nanoporous surface. The theoretical transmittance of the optimized etched glass determined by finite difference time domain simulation shows a good agreement with the experimental results. The silicon solar cell covered with both-sides optimized etched glass showed a relative increase of
4% in power conversion efficiency as compared to a solar cell covered with a plain glass. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: Vapor phase etching; Antireflection; Specular transmittance; Nanoporous surface

1. Introduction Glass substrate has been extensively used in numerous optical and electronic devices such as optical lenses, eye glasses, display devices, mobile phone screen, light emitting diodes (LED), and photovoltaic (PV) modules, because of its advantageous properties such as low cost, high transmittance, excellent stability against water and UV radiation (Xiong et al., 2010; Yeo et al., 2014; Leem et al., 2015). However,
8% of incident light is lost due to Fresnel reflection at air/glass interface (Mahadik et al., 2015; Yoldas, ⇑ Corresponding author. Tel.: +91 80 2508 6494; fax: +91 80 2521 0113.

E-mail address: [email protected] (H.C. Barshilia). http://dx.doi.org/10.1016/j.solener.2016.01.044 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.

1980), which causes a performance deterioration of many optical and electronic devices. Therefore, this reflection loss cannot be ignored and should be minimized. Antireflection (AR) coatings are a solution to eliminate the reflection loss, and have been successfully used in numerous applications such as: (i) PV and solar thermal applications to enhance the device efficiency (Groep et al., 2015; Kalogirou, 2004), (ii) display applications for elimination of the undesirable ghost image or veil glares to achieve high brightness and contrast (Hiller et al., 2002; Groep et al., 2015) and (iii) laser and sensor applications (Yoldas and Partlow, 1985). For single layer coating, two conditions should be satisfied to achieve the minimum reflection from glass surface (Xiong et al., 2010; Raut et al., 2011) (1) First, the optical thickness

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A. Kumar et al. / Solar Energy 129 (2016) 147–155

of the coating should be one fourth of the incident wavelength (t
k/4). (2) Second, the refractive index (RI) of pffiffiffiffiffiffiffiffiffiffi the coating should be equal to nglass , where nglass is RI of glass and its value is 1.52. Therefore, the ideal RI value of coating material should be
1.23. No dense coating material is available in nature which has the RI close to the ideal value (1.23). However, some fluoride materials such as LiF (n = 1.39), MgF2 (n = 1.38) and CaF2 (n = 1.42) have low value of RI as compared to nglass, but these values are still large to achieve the minimum reflection from glass surface (Xiong et al., 2010; Groep et al., 2015). The ideal value of RI can be achieved by creating a nanoporous microstructure of dense materials or by depositing a layer of nanoparticles having controlled fraction of empty space (porosity/voids). But, size of pores/particles may enhance the diffuse scattering that decreases the specular transmittance, thus, lowering the quality of AR coatings (Park et al., 2005; Yancey et al., 2006; Kim et al., 2007). Over the last decades a variety of methods have been demonstrated to fabricate the AR surface on glass substrate such as electron beam evaporation, chemical vapor deposition (CVD), reactive ion etching, sputtering, spin coatings, layer by layer deposition, sol–gel process, nanophase separation of polymer and nano-imprint lithography, (Walheim et al., 1999; Raut et al., 2011; da Silva et al., 2011; Askar et al., 2013; Shin et al., 2013). Apart from all above mentioned techniques, chemical etching has also been widely used as it is the most convenient and scalable approach to produce the excellent broadband AR surface on glass (Chinyama et al., 1993; Du and He, 2012; Yao and He, 2013). Moreover, etched glass surface showed good durability and outstanding antifogging properties as well. Although, chemical etching was started around seven decades before, its usage to improve the transmittance is still being reported because of its simplicity and scalable approach (Du and He, 2012; Liu et al., 2012; Wang and Zhao, 2014). Chemical etching can be classified into two categories: dip or liquid phase etching and dry or vapor phase etching. Vapor phase etching is more controlled process than dip etching but it requires either very long etching time (Cathro et al., 1984; Yao and He, 2013) or lithographic methods to create the AR nanostructured surface on glass (Wang and Zhao, 2014). Based on dry etching, numerous lithography-free methods have also been used for realizing the nanostructured AR surface on glass substrate. Lee et al., fabricated the tapered subwavelength antireflection structure on Si substrate using capacitively coupled plasma reactive ion etching (CCP-RIE) with Cl2 and N2 gases (Lee et al., 2009). Prior to etching thin film of thermally dewetted Pt/Pd nanodots was used an etching mask. Hien et al. fabricated the nanostructured surface on glass substrates by two steps lithography free Ar/CF4-based plasma etching process (Hien et al., 2011). In first step, a metal film of 10 nm was deposited on glass substrate as a sacrificial layer. An AR grassy surface was fabricated using a single step lithography free CF4/O2

based reactive ion etching process (Song et al., 2013). After etching, an enhancement of the 4.15% in the transmittance (with Tmax
97%) has been achieved. Yu et al. demonstrated a two steps lithography-free method to create the nanostructured surface on sodalime glass substrates by CF4 plasma etching a sacrificial layer of SiO2 (Yu et al., 2015). But all these methods require vacuum based techniques, generation of plasma and use of sacrificial layer, therefore, these methods are costly, time consuming and inappropriate for large area applications. To the best of our knowledge, a non-vacuum base single step, fast and lithography/mask free dry etching method to produce the AR surface with reflectance as low as <1% (Tmax >99%) has not been reported so far. In this paper, we report a simple, non-lithographic and single step method to fabricate broadband AR nanoporous surface on sodalime glass substrate using hydrofluoric (HF)-vapor phase etching. Compared to other AR fabrication methods, it requires a very less fabrication time and does not use any costly apparatus. Both-sides treated glass substrate exhibits a broadband enhancement in the transmittance with a maximum transmittance as high as 99.2%
500 nm with remarkable stability in the transmittance spectra against 30 days of outdoor exposure. We also demonstrate the effect of broadband high transmittance on solar cell power conversion efficiency. 2. Experimental details Sodalime glass slides (Blue Star) were used for the present study. Before etching process, they were rinsed with DI water followed by sonicating them in acetone and propanol solutions for 15 min. The etching experiment was performed in a Teflon beaker and the schematic diagram of the etching setup is shown in Fig. 1(a). Cleaned sodalime glass substrates were etched using HF (40%) vapor at different substrate temperatures (room temperature to 100 °C). Transmittance (specular and total) and reflectance spectra were recorded with a UV–Vis–NIR spectrophotometer (PerkinElmer, Lambda 950) over a spectral range of 300– 800 nm using 150 mm integrating sphere detector. To study the angle dependent AR property, reflectance was measured at different incident angles (8–68°) using universal reflectance accessary. The surface morphology and the roughness of the etched sodalime glass were investigated by field emission scanning electron microscopy (FESEM, Carl Zeiss, SUPRA 40VP) and atomic force microscopy (AFM, Bruker, Nano). X-ray photoelectron spectroscopy (XPS, SPECS) was used to examine the composition of the glass surface before and after etching. For the theoretical analysis, finite difference time domain (FDTD, Lumerical Solutions, Inc.) trial license was used to verify the AR properties of etched surface. For photovoltaic characterization, current–voltage (I–V) measurements were performed on mono-crystalline silicon solar cell of area is 4 cm2 under incident power (Pi) of 100 mW/cm2.

A. Kumar et al. / Solar Energy 129 (2016) 147–155

Thermo couple

149

100

(a)

(b) 96

Heater T (%)

92 Plain glass RT 60 οC 75 οC 90 οC 100 οC 110 οC

88

Glass Substrate

84

Acid solution

80

400

500

600

700

800

Wavelength (nm) 100

100

(c)

96

92 Plain glass 2 min 5 min 7 min 10 min 12 min

88 84 80

T (%)

T (%)

96

(d)

400

500

600

92 Plain glass One side etched glass Both sides etched glass

88 84

700

800

Wavelength (nm)

80

400

500

600

700

800

Wavelength (nm)

Fig. 1. (a) Schematic representation of etching setup. Transmittance spectra of: (b) glass substrates etched at different Tsub, (c) glass substrates etched with different tE, and (d) plain glass, one side and both-sides etched glass.

3. Results and discussion Fig. 1(b) shows the total transmittance spectra of glass substrate etched at different substrate temperatures (Tsub) with constant etching time (tE = 7 min). It can be seen that the maximum transmittance increases with increasing Tsub and reaches to its maximum value of 99.2% for 100 °C. With further increase in Tsub, the maximum transmittance decreases. The trend in the transmittance spectra is ascribed to the fact that the vapor phase etching rate decreases with increasing substrate temperature (Wang and Zhao, 2014). Moreover, higher Tsub avoids the formation of etchant layer and etching reaction takes place in gas phase regime via solid–gas interaction (Han, 1999). The transmittance spectra of glass substrate etched at 100 °C with different tE are depicted in Fig. 1(c). The maximum transmittance increases with increasing the tE up to 7 min. If the etching time is further prolonged, the transmittance decreases. Therefore, the glass substrate etched under optimized conditions (Tsub = 100 °C, tE = 7 min) was further studied along with plain glass. The total transmittance spectra of one and both-sides optimized etched glass along with plain glass are shown in Fig. 1(d). It is clear that transmittance increased over a wide range of wavelength after etching. One side etched glass substrate shows a maximum transmittance of
95%. In case of both-sides etched glass, the transmittance was further increased and it remains >98% over a range 390–585 nm with a maximum transmittance of 99.2%. In

order to demonstrate the broadband AR property of etched surface over 350–800 nm, the solar weighted average transmittance (Tsw) was calculated using the following equation (Leem et al., 2011; Raut et al., 2013): R 800 S k T k dk ð1Þ T sw ¼ 350 R 800 S dk 350 k where Sk, Tk are the spectral irradiance (air mass 1.5 G) and total transmittance at wavelength k, respectively. The calculated values of Tsw were found to be 90.4%, 93.1% and 97.1%, for plain, one and both-sides etched glass, respectively. These results indicate that etched glass shows enhancement of 6.7% in Tsw. It is well known that the total transmittance is the sum of diffusive and specular transmittance. If the random scattering occurs in a greater extent the contribution of diffusive transmittance will dominate, which leads to the high haze value. For many applications such as display, and lenses, the haze is unacceptable property of AR coating/structure as it degrades the visibility, therefore, it should be minimized. To determine the diffusive scattering or haze, we have measured the specular transmittance of the optimized etched glass, which is shown in Fig. 2(a) along with total transmittance. The specular transmittance remains almost equal to total transmittance, which indicates extremely low diffusive scattering of light. A photograph of plain and optimized etched glass placed over a white paper with dark color letters is shown in Fig. 2(b). It is clearly seen that the etched surface

150

A. Kumar et al. / Solar Energy 129 (2016) 147–155 100

(a)

(b) Etched glass

T (%)

96

Plain glass

92

Total Specular

88

84

400

500

600

700

800

Wavelength (nm) Fig. 2. (a) Total and specular transmittance spectra of the etched glass and (b) photographs of the optimized etched glass exposed to white light along with plain glass.

improves the clarity and readability of the letters under the exposure of light due to the AR effect of the etched surface. Human eye is sensitive to the wavelength greater than 400 nm and shorter than 700 nm, therefore, for display and eye lenses applications only this spectral range is important. The calculated value of Tsw over 400–700 nm (using specular transmittance spectra) was found to be 90.5% and 98% for plain and optimized etched glass, respectively. AFM image of plain glass is shown in Fig. 3(a). The morphology of plain glass is smooth with some spikes having very small height
6 nm and it shows root mean square roughness (rrms) of 3.1 nm. Fig. 3(b) and (c) shows the AFM and FESEM images of optimized etched glass, respectively. These results clearly show that a nanoporous layer with random distribution of pores size and height was formed on etched glass substrate. The formation of a nanoporous surface on glass can reduce the reflection of light over a broad range of wavelength (Yao and He, 2013). In addition, if the porous surface has big pores and/or high roughness value, the diffusive scattering will take place. FESEM image shows that most of the pores are smaller than 150 nm, which avoid the random scattering of light as the size of pores is smaller than the incident light wavelength. Due to the formation of porous layer, the value of rrms increased to 4.5 nm as compared to plane glass, but this value of rrms is too short to produce the

(a)

diffusive scattering (Krc et al., 2003). Therefore, the specular contribution was dominated in total transmittance, which indicates the formation of high quality AR surface on glass. Moreover, to understand trends in transmittance spectra (Fig. 1(b) and (c)), the AFM images of glass substrates etched under different conditions were taken and shown in Fig. S1(a)–(d) (see supplementary information). Fig. S1(a) and (b) shows the surface morphology of etched glass for tE = 5 and 12 min with Tsub = 100 °C. It can be seen that for etching time tE = 5, the porosity is less as compared to optimized sample (tE = 7 min, Fig. 3(b)). On increasing tE from 7 to 10 min, the pore size becomes larger, which enhances the light scattering thus reducing the transparency. To achieve the minimum reflection (maximum transmittance), the porosity should be high by restricting the pore size to the sub-wavelength dimension (Park et al., 2005). This is the reason that transmittance increases with increasing tE up to 7 min due increase in the porosity. With further increasing tE, transmittance decreases due to the light scattering from the larger pores. AFM images of glass substrates etched at Tsub = 60 and 110 °C for tE = 7 min are presented in Fig. S1(c) and (d). For Tsub = 60 °C, the pores are wider and longer, which reduces the transmittance. If Tsub is increased to 110 °C the porosity decreases compared to the optimized sample. The reflectance spectra of plain and optimized etched glass are shown in Fig. 4(a). For optimized sample, the

(c)

(b)

500 nm

σrms= 3.1 nm

σrms= 4.5 nm

Fig. 3. AFM images of (a) plain glass and (b) etched glass. (c) FESEM image of the etched glass prepared under optimized conditions.

A. Kumar et al. / Solar Energy 129 (2016) 147–155

reflectance significantly reduces over the wide range of wavelength as compared to plain glass with minimum reflectance of 0.6% at
498 nm. However, it has been encountered that for etched glass, minimum reflectance and maximum transmittance were obtained at different wavelengths (Yao and He, 2013). But in our case, both occur at the same wavelength, which indicates etched surface acts as effective medium rather than a scattering medium. The effective RI (neff) of the AR coating can be extracted from reflectance and transmittance data using Essential Macleod software for optical simulation (Selvakumar et al., 2015). neff plotted for plain and the optimized etched glass are shown in the inset of Fig. 4(a). It is evident that neff for the optimized etched glass remains lower in the whole visible range as compared to the plain glass. According to the effective medium theory, neff of the porous layer has the lower value as compared to solid layer due to existence of air pockets, which can be obtained from the following equation (Yao and He, 2013): n2eff ¼ n2layer ð1  f p Þ þ n2air f p

ð2Þ

where nlayer and nair are the RIs of the solid layer material and air, respectively. fp is pore filling fraction. To calculate fp of the etched surface, the FESEM image was analyzed by image J software (version 1.48v). The approximated value of pore fraction was found to be
0.21. By taking, fp
0.21%, nair
1, and nlayer
1.35 (that will be discussed later), the estimated value of neff was found to be 1.28, which is very close to the ideal value of RI for minimum reflectivity. It is well known that Fresnel reflection from the interface of two different media strongly depends on incident angle (h), and it increases with increasing h. For a single layer coating, the AR property is effective only for normal incident as RI follows a sharp change at air/coating interface (Chattopadhyay et al., 2010). To determine the angle dependent AR effect, the reflectance of optimized etched glass was measured over a wide range of angle of incidence varying from 8° to 68° along with the plain glass. Reflectance values of optimized and plain glass at 500 nm are shown in Fig. 4(b). It is clear that the reflectance of the

optimized etched glass changes slowly as compared to plain glass and it remains <1.5% up to 48°. At higher incident angle (h
68°), the reflection reduces to
8.5% in contrast to 26.1% for plain glass. This effect is attributed to the fact that the neff of the nanoporous surface follows a gradual change from air (n = 1) to glass substrate (n = 1.53) rather than a steep discontinuity in the RI. These results are consistent with the reported literature for single layer gradient index AR film fabricated by etching process (Chattopadhyay et al., 2010; Minot, 1977). Moreover, it is reported that for h
70°, a nonporous MgF2 quarterwave film (n = 1.38) on glass (n = 1.474) reflects 13.3% of the incident light at 500 nm (Minot, 1977). In contrast, the optimized etched glass shows less sensitive to the incident angles and reflects
8.5% incident light at 68°. The AR property of the etched surface was further corroborated by FDTD simulation. To design the model, the FESEM image of the etched surface was imported on the both sides of the glass. A RI value of 1.28 was used and a constant thickness of the etched surface (50 nm) was taken. The thickness of the glass was kept 10 lm due to the computational limitations. The periodic boundary conditions were taken along the x-axis and y-axis and a mesh size of 0.5 nm was used. The plane wave source of wavelength 350–800 nm was introduced perpendicular to etched surface. For glass, a constant RI of 1.53 was used and simulation was performed in a similar way. Simulated and experimental transmission spectra are plotted in Fig. 5(a) for plain and the optimized etched glass, respectively. The simulated and experimental spectra show the enhancement in the transmittance enhancement over a wide range of wavelength (350–800 nm). Additionally, the theoretical transmittance was found to be
99% at 500 nm, which is in good agreement with the experimental result. However, the simulated results show some deviation from the experimental results for both plain and optimized etched glass, which is ascribed to the fact that sodalime glass absorbs the light due the presence of iron oxide or other impurities (Ceglia et al., 2015). Moreover, the RI value taken for the etched surface may be different from the actual value. Nevertheless, it is clear from the experimental as well as the simulated spectra that the 30

Plain glass

(a)

(b)

8

25 Plain glass

R (%)

4

λ = 500 nm

20

6

neff

R (%)

1.6

151

1.4 1.2

Both sides etched glass

400

2

500

600

700

15 Plain Glass

10

800

Wavelength (nm)

5

Etched glass

Both sides etched glass 0

0

400

500

600

Wavelength (nm)

700

800

10

20

30

40

50

60

70

Incident angle (ο)

Fig. 4. (a) Reflectance spectra of plain and the optimized etched glass. Inset of figure represents neff of plain and the optimized etched glass. (b) Reflectance values at different incident angles for plain and the optimized etched at 500 nm.

152

A. Kumar et al. / Solar Energy 129 (2016) 147–155 100

(a)

96

|E|2 at transmitted surface

|E|2 at transmitted surface

88

Plain glass simulation Optimized sample simulation Plain glass experimetal Optimized sample experimental

84 80 300

(c)

(b) y (10-2 µm)

T (%)

0.98897

92

0.98893

4 0.95498

0.98889

3 0.98885 0.98881

400

500

600

700

800

-9

-6

-3

0

3

6

Wavelength (nm)

9

-9

x (10-2 µm)

-6

0

-3

3

6

9

SiO2 þ 4HF ! SiF4 þ 2H2 O ðat higher temperature
60  CÞ ðIÞ Na2 O þ 2HF ! 2NaF þ H2 O CaO þ 2HF ! CaF2 þ H2 O

ðIIÞ ðIIIÞ

SiF4 is a gas product, therefore, it is possible that a composite layer comprising of NaF and CaF2, is formed on glass substrate after etching. Fig. 6 shows the XPS survey scans of sodalime glass substrate before and after etching. It can be seen that a strong peak at 685.2 eV of F-1s was present after etching, which indicates a large accumulation of fluorine on etched surface. The peak at 1072.4 eV corresponds to Na-1s peak, which is ascribed to the presence of Na2O in the sodalime glass. It is clear that Na-1s peak shifts toward high binding energy and becomes more intense after etching. This shift is attributed to the fact that HF-vapor reacts with Na2O to form NaF (Reaction II). The high intensity in the Na-1s peak is due to the accumulation NaF on glass surface after etching. A similar behavior was found for Ca-2p1/2 and Ca-2p3/2 peaks present in sodalime glass, which is in accordance with the Reaction III. Therefore, a thin fluoride layer comprising of NaF and CaF2 is formed on sodalime glass after etching. The RI values of NaF and CaF2 are 1.33 and 1.39, respectively; therefore, their average value was taken to calculate neff

Na-1s

Plain glass

F-1s

F-1s

Ca-2p1/2 & 3/2

Si-2p

Intensity

Na-1s

O-1s

Ca-2p1/2 & 3/2

Etched glass

Si-2p

etched surface of glass enhances the transmittance over a broad range of wavelength with maximum transmittance as high as 99% at
500 nm. The electric field distribution data for a light with wavelength of 685 nm are depicted in Fig. 5(b) and (c) for plain glass and the optimized sample, respectively. Plain glass substrate shows uniform distribution of electric field. On contrary, electric field distribution is non-uniform in case of the optimized etched glass, which is attributed to the random distribution of pores of different sizes. The mechanism of the formation of AR surface on sodalime glass after etching can be explained on the basis of HF reaction with sodalime glass components. HF vapor phase etching of sodalime glass leads to the formation tetrafluorosilicate (SiF4), sodium fluoride (NaF) and calcium fluoride (CaF2) according to the following equations (Iliescu et al., 2008; Kolli et al., 2009):

O-1s

Fig. 5. (a) The simulated and the experimental transmittance spectra for plain and the optimized etched glass. Distribution of |E|2 for (b) plain glass and (c) the optimized etched glass.

0

200

400

600

800

1000

1200

Binding energy (eV) Fig. 6. XPS survey scan spectra of sodalime glass before and after etching.

using Eq. (2). In addition, the formation of SiF4 gas may generate the porosity in the composite fluoride layer. At higher Tsub (100 °C), SiF4 gets evaporated, which helps to create the nanopores in the fluoride layer. In order to investigate the effect of high transmittance on photovoltaic efficiency, the silicon solar cell was covered with plain and the optimized etched glass, as depicted in Fig. 7(a). The photogenerated current (Iph) determines the photovoltaic characteristics (I–V) of the solar cell, which can be related to transmittance by the following equation (Raut et al., 2013): Z   I ph ¼ e F k ð1  Rk ÞgIQE dk ð3Þ k

where Fk and Rk are the photon flux and reflectance at the each wavelength, respectively. gIQE is the internal quantum efficiency. Eq. (3) states that the enhancement in the transmittance (or reduction in the reflectance) of the etched glass can produce a proportional current gain in the solar cell. The I–V characteristics of of silicon solar cell for both the configrations are shown in Fig. 7(b). It can be seen that an improvement was obseved in saturated current (Isc) for the optimized etched glass. The values of Isc were found to be 119 and 127 mA for solar cell covered with plain glass and optimized etched glass, respectively. This increase in

A. Kumar et al. / Solar Energy 129 (2016) 147–155

(a)

PInci

(b)

Pref

PInci

153

Pref

Etched glass

Plain glass Silicon solar cell

(c)

120

Current (mA)

80

With plain glass With etched glass 40

0

-40 -1. 0

-0. 5

0. 0

0 .5

1.0

Voltage (V) Fig. 7. (a) Schematic representation of cell solar cell covered with plain glass and the optimized etched glass. (b) Comparison of I–V characteristics of silicon solar cell covered with plain and the optimized etched glass.

Table 1 Photovoltaic parameters of silicon solar cell covered with plain and etched glass. Solar cell

Jsc (mA/cm2)

VOC

FF

g (%)

Covered with plain glass Covered with etched glass

29.9 31.72

0.596 0.598

0.39 0.38

6.95 7.21

the Isc corresponds to a relative gain of
6.5% in Isc, which shows a good agreement with the enhancement in the solar weighted average transmittance (Tsw). The power conversion efficiency (g) of the solar cell was calculated using following equation (Tammy et al., 2007): gð%Þ ¼

J sc  V OC  FF  100 P in

ð4Þ

where Jsc, VOC and FF are the saturation current density, open circuit voltage and fill factor, respectively. The measured photovoltaic parameters are listed in Table 1. The estimated values of g solar cell covered with plain glass and the optimized etched glass were found to be 6.95% and 7.21%, respectively. This improvement of the g corresponds to a relative gain of
4%. A simple test was performed to examine the effect of outdoor exposure on the transmittance. For this purpose the optimized etched glass was exposed to open surrounding for 30 days and the total transmittance spectra before and after the outdoor exposure test are presented in Fig. 8(a) at different exposure times. It can be seen that the maximum transmittance was decreased by 0.8%, 1.2% and 1.8% after 7, 14 and 30 days outdoor exposure test, respectively. Intrestingly, the maximum transmittance

100

100

(b)

96

96

92

92

Plain glass Before test After 7 days After 14 days After 30 days

88 84 80

T (%)

T (%)

(a)

400

500

600

Wavelength (nm)

Before heating o 100 C o 200 C o 300 C

88 84

700

800

80

400

500

600

700

80 0

Wavelength (nm)

Fig. 8. The total transmittance spectra of the etched glass before and after (a) the outdoor exposure test (30 days), and (b) thermal stability test.

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A. Kumar et al. / Solar Energy 129 (2016) 147–155

shifts toward higher wavelenghth and the transmittance decreases more rapidly at the shorter wavelength. This change is attribured to the accumulation of dust particles that may produce the scattering, which ocuurs in greater extent at shorter wavelength. The pores can also absorb moisture (nwater
1.33), resulting an increase in neff. Addtionally, the etched glass was annealed up to 300 °C for 1 h and the corresponding transmittance data are shown in Fig. 8(b). It can be seen that no detectable drop was observed in the transmittance spectra, which shows excellent thermal stability of the etched glass surface. 4. Conclusions In conclusion, we have demonstrated a low cost, time effective and scalable method to produce the wide range AR nanoporous surface on sodalime glass substrate. Maximum total transmittance of 99.2% at
500 nm has been achieved for both-sides etched glass under the optimized conditions with extremely low diffusive scattering. It was found that after etching a porous fluoride layer comprising of NaF and CaF2 is formed on glass surface, which acts as an antireflective layer with RI of 1.28. The fabricated nanoporous surface on sodalime glass substrate exhibited low reflectance (<1.5%) over a wide range of incidence angles (8–48°). AR surface shows less sensitivity with angle of incident as compared to plain due to gradual change in neff. FDTD simulation of the etched surfaces confirmed a good agreement of AR property with the experimental results. The transmittance of the etched sample decreased only by 1.8% after 30 days of outdoor test. A relative gain of 4% in power conversion efficiency of a silicon solar cell was observed in case of the etched glass, which is attributed to the broadband AR effect of the etched surface of sodalime glass. Acknowledgements The authors are thankful to the Director, CSIR-NAL for his support and encouragement. Thanks are also due to Mr. Praveen Kumar, Mr. Siju, Mr. G. Srinivas for AFM, FESEM and optical characterization, respectively. We also thank Mr. Jakeer Khan for the help rendered during this work. Financial support from BRNS (Project # U1-125) and TAPSUN (NWP0054) is highly acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.solener.2016.01.044. References Askar, K., Phillips, B.M., Fang, Y., Choi, B., Gozubenli, N., Jiang, P., Jiang, B., 2013. Self-assembled self-cleaning broadband anti-reflection coatings. Colloids Surf., A 439, 84–100.

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