AIT textured glass front electrode for thin film solar cells

Applied Surface Science 357 (2015) 651–658

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Optimization of surface morphology and scattering properties of TCO/AIT textured glass front electrode for thin film solar cells M.L. Addonizio ∗ , L. Fusco, A. Antonaia, F. Cominale, I. Usatii ENEA, Portici Research Centre, P.le E. Fermi, 1, 80055 Portici, Napoli, Italy

a r t i c l e

i n f o

Article history: Received 20 July 2015 Received in revised form 3 September 2015 Accepted 6 September 2015 Available online 8 September 2015 Keywords: AIT textured glass Light scattering Zinc oxide Thin film solar cells

a b s t r a c t Aluminium induced texture (AIT) method has been used for obtaining highly textured glass substrate suitable for silicon based thin film solar cell technology. Wet etch step parameters of AIT process have been varied and effect of different etchants and different etching times on morphological and optical properties has been analyzed. The resulting morphology features (shape, size distribution, inclination angle) have been optimized in order to obtain the best scattering properties. ZnO:Ga (GZO) films have been deposited by sputtering technique on AIT-processed glass. Two different ZnO surface morphologies have been obtained, strongly depending on the underlying glass substrate morphology induced by different etching times. Very rough and porous texture ( rms ∼ 150 nm) was obtained on glass etched 2 min showing cauliflower-like structure, whereas a softer texture ( rms ∼ 78 nm) was obtained on glass etched 7 min giving wider and smoother U-shaped craters. The effect of different glass textures on optical confinement has been tested in amorphous silicon based p-i-n devices. Devices fabricated on GZO/high textured glass showed a quantum efficiency enhancement due to both an effective light trapping phenomenon and an effective anti-reflective optical behaviour. Short etching time produce smaller cavities (<1 ␮m) with deep U-shape characterized by high roughness, high inclination angle and low autocorrelation length. This surface morphology promoted a large light scattering phenomenon, as evidenced by haze value and by angular resolved scattering (ARS) behaviour, into a large range of diffraction angles, giving high probability of effective light trapping inside a PV device. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In order to enhance the performances of thin film silicon based solar cells it is strategic to use textured transparent conductive oxide (TCO) as front electrode. Textured substrate, apart from light scattering benefits improving light trapping in thin film solar cell, allows to minimize reflection losses due to an enhanced optical index matching at the front surface of the solar cell. Textured front contact is usually achieved by the use of naturally textured TCO, like ZnO:B obtained by LP-CVD [1,2], SnO2 :F obtained by AP-CVD [3] or sputtered ZnO:Al with post-deposition wet etching [4]. An alternative method to obtain a front electrode with high scattering properties consists in texturing glass substrate before depositing ZnO film by sputtering technique. This fabrication method allows to avoid post-deposition wet etching of ZnO and, in this manner, optical and electrical properties of the ZnO film can be optimized regardless of how to obtain an appropriate textured

∗ Corresponding author. E-mail address: [email protected] (M.L. Addonizio). http://dx.doi.org/10.1016/j.apsusc.2015.09.073 0169-4332/© 2015 Elsevier B.V. All rights reserved.

film surface. Furthermore, ZnO layer thickness can be adjusted only with regard to the desired layer electrical resistance. Textured glass makes more attractive the sputtering technology for TCO production together with related benefits of large area employment and low fabrication cost. Several glass texturing methods have been proposed and developed in recent years for thin film PV applications: chemical etching with hydrofluoric acid [5], liquid surface coating (sol–gel) containing SiO2 spheres [6], sandblasting [7], hot embossing [8], aluminium induced texture (AIT) [9,10], plasma etching or reactive ion etching with or without natural lithography [11,12]. The AIT glass texturization is based on a thermally activated chemical reaction between glass surface and a thin aluminium layer. After the thermally activated chemical reaction, products are removed by wet chemical etching and the result is a randomly textured glass surface. Texture topography of AIT glass can be controlled by varying the process parameters such as the initial Al thickness [10], the annealing conditions [13,14] or the etching conditions [15,16]. The AIT method can create a suitable substrate texture for thin film silicon solar cells [17]. Although several studies have been carried out on optimization of process parameters,

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a comprehensive investigation regarding the correlation between optimized surface morphology and light trapping effectiveness in the device is not again available. In this work we utilized the AIT method on Eagle-XG type glass substrate with the aim of obtaining more efficient scattering surfaces. Special attention has been given to the influence of chemical etching composition and etching time on the light trapping capability of the resulting AIT treated glass. ZnO:Ga thin films were deposited by RF magnetron sputtering on optimized textured glass. Relationship between different surface textures and light trapping obtained in a-Si:H solar cells has been investigated. Optical, structural and morphological analyses were carried out on both textured glass and ZnO/glass textured structures. 2. Experimental AIT glass texturization was carried out through a thermally activated chemical reaction between the glass surface and a thin sacrificial aluminium layer. During thermal annealing, aluminium reacts with SiO2 at the glass/Al interface according to the following redox reaction: 4Al + 3SiO2 → 2Al2 O3 + 3Si

(1)

The reaction products on the glass surface are subsequently removed by wet-chemical etching. Corning Eagle-XG type glass substrates 1.1 mm tick were used. Details of Al deposition and thermal annealing are reported elsewhere [13]. The chemical etching was carried out through a two step process by a basic solution (NaOH 50% by Sigma–Aldrich) in the first step and by two different acid diluted solutions (48% HF and 70% HNO3 by Baker) in the second step. In the first step the sample was dipped in a 10% w sodium hydroxide (NaOH) solution at 70 ◦ C for 10 min, then rinsed in deionised water under magnetic stirrer for few minutes. In order to remove formed alumina, Si and Al, sodium hydroxide solution works according to the following reactions [18]: 2NaOH + Si + H2 O → Na2 SiO3 + 2H2

(2)

2NaOH + 2Al + 2H2 O → 2NaAlO2 + 3H2

(3)

2NaOH + Al2 O3 → 2NaAlO2 + H2 O

(4)

The basic reaction by hot sodium hydroxide solution is characterized by vigorous effervescence due to hydrogen production and high reaction rate. In the second etch step, both the products formed on the glass by sodium hydroxide reaction (sodium aluminate, sodium silicate) and remaining aluminium oxide and silicon (Si) can also be removed with acids, like HF/HNO3 solution mixture or HF solution. In fact HNO3 reacts like vigorous oxidant and HF reacts with oxidized compounds to remove it and with a softly etching effect due to dilution in the oxidizing acid. In order to obtain a greater etching effect we have also used a HF solution alone. Two different acid solutions have been used, a mixture of 5% w HF/HNO3 in the 1:1 volume ratio at room temperature for a duration ranging from 2 to 7 min and/or a 5%w HF solution at room temperature for a duration ranging from 2 to 5 min. On the textured side of the AIT-processed glass gallium doped zinc oxide (GZO) layer 1.0 ␮m thick was deposited by RF magnetron sputtering. Subsequently, GZO layer/textured glass structure (without any post-deposition treatment) was utilized as front electrodes for the realization of a-Si:H solar cells with the following structure: textured glass/ZnO/p-i-n a-Si:H/ZnO/Ag. Ultraviolet–visible–near Infrared (UV–VIS–NIR) reflectance and transmittance measurements were carried out in the range

200–2500 nm utilizing a Perkin Elmer Lambda 900 double beam spectrophotometer. Average spectral haze in transmission HT (defined as diffuse to total transmittance ratio (TT − Tspec )/TT where TT and Tspec are total and specular transmittance respectively) was determined. X-ray diffraction (XRD) analysis has been performed by a Philips X’Pert PROMRD diffractometer working with CuK␣ radiation ( = 0.154056 nm) at a grazing incidence angle of 0.3◦ . A 2 scan range 10–90◦ was used. Morphology and surface roughness properties were studied using a LEO mod. S360 scanning electron microscope (SEM) and a Veeco mod. NSIV atomic force microscope (AFM), respectively. In order to better optimize the AIT process the size distribution and the inclination angle distribution of the texture features were determined using the experimental AFM images and analyzing with MATLAB programming. The software of the Nanoscope IV AFM was also used to perform power spectral density (PSD) analysis. Angular resolved scattering (ARS) measurements were used to determine the preferential scattering angle of the rough surfaces. Light I–V characteristics of the solar cells were measured at AM 1.5 Global under a solar simulator. The photo-generated current Jsc was evaluated in the wavelength range of 350–800 nm by Quantum Efficiency measurements on a-Si:H based devices.

3. Results and discussion 3.1. Etching time effect on morphology and optical properties of AIT glass Several factors affect the AIT texturing glass process such as: Al thickness, annealing temperature and annealing time, etchants and etching time. Thickness of aluminium deposited, annealing temperature and annealing time of the AIT process are very critical parameters and it is necessary an accurate optimization to obtain an uniform and complete reaction at the glass/Al interface. Their optimization is reported elsewhere [13]. On the basis of the previous results, in this work an optimized Al thickness of 200 nm was used with an annealing process carried out at 615 ◦ C/40 min. This thermal step is the same for all the samples. To remove the reaction products we have used an alkaline etching method using NaOH at different concentrations (5, 10 and 15%) and at different etching times (5, 7, 10 and 20 min). The best removal uniformity was obtained with 10% NaOH for 10 min. The main advantages of alkaline removal respect to normally used concentrated orthophosphoric acid [9,13,19] are lower etching temperatures and, when using diluted solutions, also lack of acidic hazards. Low etching temperature of 70 ◦ C was used followed by a dip etch in 5% HF solution and/or 5% HF-HNO3 solution with ratio of 1:1. The dip time was changed from 2 min to 7 min. In Fig. 1a the XRD patterns after annealing, after NaOH etch and after HF etch are compared. After annealing, reflections due to crystalline silicon, Al2 O3 and a little amount of unreacted Al are observable. The first NaOH-based etch remove Si and unreacted Al, whereas peaks related to Al2 O3 , silicate and aluminate compounds (see Eqs. (2)–(4)) are again present. After HF dip these compounds are completely removed as it appears in the third pattern. The absorption spectra of glass after only NaOH etch and after HF dip are also showed in Fig. 1b. After NaOH etch high absorption is again present that decreases after the second etch. These results show complete removal of reaction products and indicate that AIT process does not leave any contamination on the glass surface. In the present paper the role that the acid-etchant, in the second step of AIT process, plays in the formation of the texture has been analyzed. The effects induced by different acid-etching solutions (diluted HF solutions or HF-HNO3 mixed solutions) and by

M.L. Addonizio et al. / Applied Surface Science 357 (2015) 651–658

Intensity (arb. units)

Table 1 Average haze factor, RMS roughness and average inclination angles of the cavities of AIT samples etched with HF-HNO3 solutions for different etching times.

a)

Si

Al

Si Al

Si

after annealing

Etch time (min)

2

4

5

7

Haze factor (%)  RMS (nm) Average inclination angle (◦ )

28.7 140 16

27.1 120 13

26.2 108 11.6

25 71 9.5

50

after NaOH etch after NaOH and HF etch 10

20

30

40

50

60

70

80

Haze Factor (%)

2 theta (°)

b)

Absorption (%)

30

Only NaOH etch NaOH and HF-HNO3 etch

HF-HNO3 etch etching time: 2 min 4 min 5 min 7 min

40

90

50 40

653

30

20

10

20

0 400

10

450

500

550 600 650 700 Wavelenght (nm)

750

800

Fig. 3. Spectral Haze curves of AIT etched glasses with HF-HNO3 solution for different etching times.

0 400

600

800

1000

1200

Wavelength (nm) Fig. 1. (a) XRD patterns of AIT glass after annealing, after NaOH etch only and after HF dip etch and (b) optical absorption of AIT glass prepared by NaOH etch only and by NaOH and HF etch.

etching times on surface morphology modification have been widely studied and correlated to optical scattering properties of glass textured surfaces. In Fig. 2 the surface morphology of HFHNO3 etched glass is shown for treatment time of 2 and 7 min. The glass surface is made of random texture with hemispherical Ushaped craters, several deeper and larger valleys appear at shorter etch times whose inner surface shows an inverted cauliflowerlike shape (see Fig. 2a), with RMS = 140 nm. At the increase of the

etching time a smoothing effect occurs, the initially formed small adjacent craters join together to form larger ones, smoother and large cavities up to 2 ␮m appear with RMS = 70 nm (see Fig. 2b). The average cavity diameter varies from 0.6 ␮m for 2 min etching to 1.1 ␮m for 7 min etched glass. The roughness behaviour evaluated from AFM images confirms a decrease of the RMS roughness at the increase of the etching time as reported in Table 1. In the former case (short etching time) the peak-to-valley distance increases, resulting in higher roughness values, whereas in the latter case (long etching time) it decreases with lower roughness values. Spectral haze factor of textured glasses (measured with light entering from textured glass side) obtained for different etching times is showed in Fig. 3. The optical scattering of the glass surface decreases at the increase of the etching time. The Haze value at the

Fig. 2. AFM images related to surface morphology of AIT etched glass with HF-HNO3 solution for: (a) 2 min and (b) 7 min.

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1 HF-HNO etch 3 Etching time:2 min

0.8

PS (θ) (arb. units)

7 min

0.6

0.4

0.2

0

0

20

40

60

80

Scattering angle (°) Fig. 4. Distribution histograms of inclination angles derived from AFM images of AIT etched glass with HF-HNO3 for different etching durations.

wavelength of 600 nm reaches 22% for texture obtained with 2 min HF-HNO3 etch and 18% for 7 min HF-HNO3 etch. An excellent total transmittance of about 92% is maintained. In order to better understand the light scattering capability of the textured glass, it is important to analyze the surface angles of the texture features and the angle distribution of the surface elements. The AFM images were processed using MATLAB programming. The resulting angle histograms obtained for AIT substrates etched with HF-HNO3 for different times are shown in Fig. 4. The AIT glass sheet dip etched for 2 min shows a wide angle distribution ranging from 0 to 60◦ , with a peak at 16◦ . As expected, higher angles are close to the crater surface. With increasing etching time a slight shift of the angle distribution curve to the left is produced with a narrow curve distribution and the peak of the curve related to 7 min of etching is located at low angle (∼9.5◦ ), indicating that the craters of this sample are shallows. This sample is thus not expected to provide good light scattering. Differently, at short etching time we can observe a peak with largest angle distribution, which means it can scatter the incident light into a large range of diffraction angles and, as a consequence, we can expect a good light scattering behaviour for this textured glass. ARS measurements were carried out for obtaining information about the angular distribution of the scattered light by textured glass. The behaviour of the scattered power PS() derived from transmitted light for the textured surfaces, HF-HNO3 etched for 2 min and 7 min, is shown in Fig. 5. The ARS measurements have been performed with laser illumination at 630 nm and beam incidence through the no-treated glass surface. The resulting intensity profiles were normalized respect to to the maximum intensity of the scattered light and subsequently integrated over the azimuth angle (under assumption of isotropic scattering). All the samples show a broad profile with high scattering intensity at low angles. The different surface texture results in significant changes of the scattering properties, where the PS() curve shifts towards smaller angles with increasing treatment time. At the increase of the etching time, surface morphology and scattering properties of only HF dip etched glass show the same behaviour of HF:HNO3 mixed solutions, in terms of surface texture, depth and size of craters. Both the average inclination angle of craters and the ARS-derived preferential scattering angle are lower respect to HF:HNO3 wet etched glass indicating smoother craters. As a consequence, poorer light scattering properties are expected. We can observe that the etching time is the decisive factor to tune the texture roughness in AIT process. From the surface

Fig. 5. Scattered power of transmitted intensity, derived from sin-weighted ARS measurements, as a function of the scattering angle for AIT processed glasses with different etching times.

morphologies obtained and from their optical properties it can also be concluded that varying wet-chemical etching conditions, the size and shape of the surface texture can be adjusted in order to obtain the best scattering properties. This makes the AIT method potentially well suited for a range of thin film PV technologies in superstrate configuration. 3.2. GZO coated textured glass The surface morphology of TCO-coated AIT glass sheets was studied by SEM and AFM methods. Surface morphology of ZnO:Ga (GZO) films on textured glass is shown in Figs. 6 and 7. GZO grown on glass textured at short etching time (2 min dip in HF-HNO3 solution) (see Fig. 6a) with U-shaped craters and high roughness, exhibits a cauliflower-like structure with smooth valleys and free from sharp peaks or edges, having a final RMS roughness of 150 nm (see Fig. 6b). Differently, for GZO grown on textured glass for long etching time (7 min dip in HF-HNO3 solution) (see Fig. 7a) SEM images reveal smoother craters, low roughness and homogeneous sputtered film growth with good surface coverage of the GZO film on the textured glass sheet (see Fig. 7b). A submicron-scale structure appears over the GZO film surface and such submicron features seem well suited to scatter short-wavelength light. Cross-sectional SEM images (see Fig. 8) of TCO-coated AIT glass sheets at different etching times reveal conformal film growth and good surface coverage of the GZO film on the textured glass sheet. The images also confirm that the randomly textured surface of the AIT sheet is conformally transferred to the sputtered TCO. The ZnO films have been grown on glasses with different textures at the same sputtering conditions, with identical particle energy and with an incidence angle determined only by the local substrate tilt. Sputtered ZnO thin films consist of crystalline columns, generally their orientation in a sputtering process is determined by the angle between the particle flux and the substrate normal [20,21]. If the substrate is quite smooth (see Fig. 8b), less growth disturbances are present and vertical column orientation results more pronounced. On the contrary, when ZnO is grown on textured substrates (see Fig. 8a), the columns are oriented according to the local substrate angles, that resembles oblique sputtering with locally very different substrate angles [21,22]. The average haze values, determined in the wavelength range 380–800 nm, for the structures GZO/text glass (2 min dip) and GZO/text glass (7 min dip) are 33% and 28%, respectively and are

M.L. Addonizio et al. / Applied Surface Science 357 (2015) 651–658

Fig. 6. SEM images of surface morphology, obtained with short etching time, related to: (a) textured glass surface and (b) GZO on textured glass.

Fig. 7. SEM images of surface morphology, obtained with long etching time, related to: (a) textured glass surface and (b) GZO on textured glass.

Fig. 8. Cross-sectional SEM images of GZO-coated AIT glass prepared by HF-HNO3 etch for: (a) 2 min and (b) 7 min.

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Table 2 Average haze factor, RMS roughness and average inclination angles of the cavities of AIT samples etched with HF-HNO3 solutions for different etching times, before and after ZnO deposition.

2 28.7 140 16

7 25 71 9.5

2 33.2 150 17

7 28 78 10.5

10

Frequency of occurrence (%)

10 10 10

AIT glass - 2 min AIT glass - 7 min --- GZO / AIT glass - 2 min --- GZO / AIT glass - 7 min

8

10

GZO/text glass

10 10

6

Asahi-U GZO/AIT glass-2 min GZO/AIT glass-7 min

9

4

Etching time (min) Haze factor (%)  RMS (nm) Average incl. angle (◦ )

10

PSD (nm )

Text glass

10

8

7

6

5

0.1

1

10

Lateral lenght (μm) Fig. 10. Two-dimensional power spectral density function for GZO deposited on AIT textured glass etched by HF-HNO3 solution for two different etching times. For comparison, also the PSD curve of Asahi-U substrate is reported.

4

2

0

0

10

20

30

40

50

Inclination angle (°) Fig. 9. Distribution histograms of inclination angles for different HF etch duration before (solid line) and after (dashed line) GZO deposition.

higher respect to bare textured glass (29% and 25%, respectively). The RMS roughness evaluated from AFM images confirms the trend given by the haze value, both parameters showing a decrease at the increase of the etching time (see Table 2). From the AFM images also the angle histograms were determined for TCO/text glass structures and the resulting curves are shown in Fig. 9. Textured glass at long etching time shows a narrow angle distribution curve with a peak at 10.5◦ and this peak shift at 17◦ , with an angle range from 0◦ to 60◦ , for short etching time glass textured. In the same figure the curves for bare AIT glass sheets are reported. Deposition of 1000 nm thick GZO film onto the textured AIT glass surface produces a slight shift of the angle distribution curve to the right (about 3◦ ). However, RMS roughness gives information exclusively related to depth profile of a rough surface and it is not sufficient to well define texture details of the surface morphology. More detailed information of the surface roughness, as the lateral size of the features characterizing the surface profile, can be obtained by the Fourier transform of the AFM images. The power spectral density (PSD) function is the frequency spectrum of the surface roughness measured in inverse length. It was employed to obtain information about the surface spatial frequencies that produce scattered light. By using AFM images of 20 ␮m × 20 ␮m, the PSD functions have been extracted from AFM software and are plotted in Fig. 10 for GZO coated textured glass etched for 2 and 7 min. In the same graph also the PSD function related to commercial Asahi-U coated glass is reported as a reference. Harvey et al. [23] showed that mid-range spatial frequencies of the PSD function reflect the large lateral feature sizes that greatly contribute to small angle scattering, whereas the PSD at high spatial frequencies reflects small feature sizes which prevalently give the large angle light scattering. GZO on textured glass exhibits higher PSD values at low spatial frequency in comparison with Asahi-U. These spectral powers represent larger lateral feature sizes of AIT randomly textured glass and contribute to lowangle scattering. At high spatial frequencies only the structure on

glass etched for 2 min shows a PSD value near to Asahi-U. In addition, we observe that the spectral power of the sample etched 7 min is always lower than the sample etched 2 min. This result indicates that the textured surface of the sample etched 2 min consists of lateral features with lower size and, as a consequence, this sample should scatter more light into large angles than the sample obtained from AIT glass etched 7 min. This is also confirmed when looking at the corresponding AFM images (see Fig. 2). The larger average lateral feature size (autocorrelation length) for the sample etched 7 min results in lower haze values at low wavelengths. In addition, these results also reflect the presence of the superposition of a micro-texture and a nanotexture, where small features are present inside larger ones, as can be seen by SEM images related to GZO on textured glass. 3.3. Solar cells on textured glass Effectiveness of two different glass textures on the optical absorption in a-Si solar cells was tested. a-Si:H solar cells were deposited on flat and on AIT textured glass etched with different acid solutions and for different times. Reflectance curves of the devices deposited on such different etched glasses are compared in Fig. 11a. The devices on glass etched with HF:HNO3 mixed solution shows lower reflectance respect to only HF dipped glass. At shorter etching times the solar cells show a reflectance lower than that obtained in the case of long etching times and the lowest reflectance mean value, indicating the highest optical absorptance inside the device, is R = 11.6% (see Table 3) related to 2 min HF-HNO3 etched glass. This value is better than the recorded one for the cells fabricated over the commercial Asahi-U substrate (R = 12.7%), as clearly shown in Fig. 11b. By comparing these two reflectance curves, a similar behaviour is observed at short wavelengths, whereas the AIT etched substrate behaves very efficient at wavelengths very close to the absorption edge ( = 700 nm) of a-Si:H. In Fig. 12a the quantum efficiencies of two PV devices fabricated on AIT textured glass for different etching times (HF-HNO3 solutions 2 and 7 min) are compared. High light trapping in the red region of solar spectrum is observable only for the devices realized on glass etched for 2 min where cavities with size lower than 1 ␮m are suitable to give better light scattering properties. In Fig. 12b the quantum efficiencies of PV devices fabricated on flat and on AIT textured glass are reported together with the device grown on the commercial Asahi-U substrate. The cell fabricated on AIT textured

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Table 3 Mean values of the hemispherical reflectance (R) in the range 350–700 nm for the solar cells fabricated on different textured glasses, flat glass and Asahi-U type substrate. Asahi-U

R (%) (350–700)

GZO/flat glass

12.7

24.1

GZO/text glass HF 2 min

5 min

2 min

7 min

14.8

18

11.6

18.7

1

60

GZO / AIT glass --- HF 2 min --- HF -5 min

50

−− HF-HNO3 2 min

a) 0,8

−− HF-HNO3 7 min

Quantum Efficiency

Reflectance (%)

70

40 30 20 10 0

GZO/text glass HF-HNO3

400

500

600

700

0,4

0,2

400

80

700

800

1

b) 0.8

50 40 30 20

0.6

0.4

0.2

10 0

600

b)

Asahi-U GZO / flat glass GZO / text. glass

Quantum Efficiency

Reflectance (%)

60

500

Wavelength (nm)

Wavelength (nm)

70

a)

0,6

0

800

GZO / AIT glass (2 min etch) GZO / AIT glass (7 min etch)

400

500

600

700

800

0

glass shows an appreciably higher quantum efficiency in the entire wavelength region of interest, giving a Jsc value of 13.9 mA/cm2 respect to 10.9 mA/cm2 obtained on flat glass, whereas a Jsc value of 13.7 mA/cm2 has been obtained for Asahi-U substrate. This Jsc enhancement can well be attributed to the appropriate scattering properties of the GZO/textured glass structure that behaves very effective in inducing both light trapping phenomenon and good anti-reflective optical behaviour. The EQE of the device realized on Asahi-U substrate shows, in the red region of the spectrum, a very similar curve respect to the device grown on AIT textured glass etched 2 min, whereas in the range of shorter wavelengths (<600 nm) the EQE obtained with AIT textured glass is slightly higher than that obtained for the Asahi-U, giving a better antireflective optical behaviour. In conclusion, a very rough cauliflower-like structure gave larger light trapping effect and lower reflectance loss respect to a

400

500

600

700

800

Wavelength (nm)

Wavelength (nm) Fig. 11. Comparative reflectance curves of the devices fabricated on: (a) textured glasses by HF and HF-HNO3 solutions at different etch time and (b) textured glass by HF-HNO3 for 2 min, smooth glass and Asahi-U substrate. For reflectance measurements devices were illuminated from substrate side.

Asahi-U GZO / flat glass GZO / text glass (2 min etch)

Fig. 12. Quantum efficiency curves of a-Si:H solar cells realized on: (a) AIT textured glasses etched with HF-HNO3 solutions at different etch time and (b) AIT textured glass by HF-HNO3 for 2 min, smooth glass and Asahi-U substrate.

flattened U-shaped structure. AIT-treated glass obtained at short HF etching time gave evidence for high effectiveness in enhancing light absorption in silicon thin film solar cells to gain higher short circuit current. From the results obtained, it seems that the AIT glass texturing method is well suited for a range of thin-film PV technologies. 4. Conclusions Aluminium induced texture (AIT) method has been used for obtaining highly textured glass substrate suitable for silicon based thin film solar cell technology. The wet etch step parameters of AIT process have been varied and the effect of different etchants and different etching times on morphological and optical properties has been analyzed. The resulting morphology features (shape, size distribution, inclination angle) have been optimized in order to obtain the best scattering properties. Surface morphology of AIT textured

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glass for different etching time shows that at the increase of the etching time the average cavity diameter increases from 0.6 ␮m up to 1.5 ␮m for 2 and 7 min etching time, respectively. Distribution histograms of inclination angles are derived from AFM images processed by MATLAB programming for different etching times. At short etching time a wide angle distribution curve ranging from 0 to 60◦ appears with a peak at 16◦ . With increasing etching time a slight shift of the angle distribution curve to the left is produced with a narrow curve distribution and a peak at 9.5◦ . Short etching times provide larger angle distribution and, as a consequence, incident light is subjected to scattering into a large range of diffraction angles, giving high probability of effective light trapping inside a device. ZnO:Ga (GZO) films have been deposited by sputtering technique on AIT-processed glass. Two different ZnO surface morphologies have been obtained, strongly depending on the underlying glass substrate morphology induced by different etching times. Very rough and porous texture ( rms ∼ 150 nm) is obtained on glass etched 2 min showing cauliflower-like structure, whereas a softer texture ( rms ∼ 78 nm) is obtained on glass etched 7 min giving wider and smoother U-shaped craters. The effect of different glass textures on optical confinement has been tested in amorphous silicon based p-i-n devices. Devices fabricated on GZO/high textured glass show a quantum efficiency enhancement due to both an effective light trapping phenomenon and an effective anti-reflective optical behaviour. Acknowledgement The present work was supported by the Italian Ministry of Economic Development in the framework of the Operating Agreement with ENEA for the Research on the Electric System. References ¨ L. Feitknecht, R. Schlüchter, U. Kroll, E. Vallat-Sauvain, A. Shah, Rough [1] S. Fay, ZnO layers by LP-CVD process and their effect in improving performances of amorphous and microcrystalline silicon solar cells, Sol. Energy Mater. Sol. Cells 90 (2006) 2960–2967. [2] M.L. Addonizio, C. Diletto, Doping influence on intrinsic stress and carrier mobility of LP-MOCVD-deposited ZnO:B thin films, Sol. Energy Mater. Sol. Cells 92 (2008) 1488–1494. [3] W.Y. Kim, A. Shibata, Y. Kazama, M. Konagai, K. Takahashi, Optimum cell design for high-performance A-Si: H solar cells prepared by photo-CVD, Jpn. J. Appl. Phys. 28 (1989) 311.

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