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Influence of Acid and Alkali Etching on Sputtered Aluminium Doped Zinc Oxide Films
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 9726–9732
www.materialstoday.com/proceedings
IC-FNM 2016
Influence of Acid and Alkali Etching on Sputtered Aluminium Doped Zinc Oxide Films Jayasree Roy Sharmaa*, Sukanta Bosea, Sourav Mandala, Gourab Dasa, Sumita Mukhopadhyaya, A.K. Baruaa a
CEGESS, IIEST, Shibpur, Howrah, West Bengal 711103, India.
Abstract In order to increase the efficiency of silicon based Thin Film Solar (TFS) cells, it is necessary to use better light management techniques. Texturization of sputtered aluminium doped zinc oxide (Al:ZnO or AZO) films has opened up a variety of ways for optimization of light trapping schemes. Here, AZO has been etched with acid as well as alkali. A difference in optical and electrical characteristics of acid and alkali etched AZO has been observed. By etching AZO with potassium hydroxide (KOH), 82% transmittance (in the visible range), high rms roughness and ~13Ω/□ sheet resistance have been achieved. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016. Keywords: AZO; etching; ion; light trapping;
1. Introduction In thin-film photovoltaic technology, transparent conductive oxides (TCOs) are abundantly exploited. Due to the very thin active absorber layer, thin-film solar cells require a more effective light management scheme [1, 2]. Better light trapping assists in improving the short-circuit current density (Jsc) and thus increasing the overall efficiency of thin-film Si solar cells. Common TCO materials are indium-doped tin oxide (ITO), fluorine-doped tin oxide (FTO) and impurity-doped (e.g. Al, Ga, B) zinc oxide [3]. Nowdays, AZO films are becoming increasingly appreciated as the front contact for thin-film Si solar cells as it consists of the advantages such as wide band gap, non-toxic, and has good thermal
* Corresponding author. Tel.:033-64551644; fax: 033-26682964. E-mail address: [email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016.
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stability against hydrogen plasma environment, easy post-deposition texturing, easy fabrication, enormous amounts of raw materials, and low expense [4]. The electronic properties as well as the surface texture of the AZO films after etching mostly depend on the preparation method of the AZO. A wide variety of thin film deposition methods have been studied for the growth of doped and as-deposited ZnO and they are: magnetron sputtering, chemical vapor deposition (CVD), spray pyrolysis, pulsed-laser deposition, evaporation, chemical solution deposition, electrochemical deposition, molecular beam epitaxy and atomic layer deposition, sol-gel processes. Among these techniques, magnetron sputtering and CVD have shown the most promise for large scale thin-film silicon solar cell production, as these techniques can yield large-area thin-films that can be texturized effectively to introduce light trapping features. In this work, AZO films have been prepared by magnetron sputtering. In pin configuration, the TCO front contacts have to meet a number of requirements which include (i) High transparency in the active region of the absorber layer (between 400 - 900 nm in case of µc-Si:H), to minimize optical losses by absorption and/or reflection,(ii) High conductivity to minimize the series resistance of the solar cells and avoid parasitic losses, (iii) Excellent light scattering properties through a rough surface, (iv) favourable physico-chemical properties for the growth of silicon. Besides the above mentioned criterions, a TCO with a suitable texture has to be developed which scatters the light very efficiently in order to elongate the effective path length within the active silicon layers. Thus, a special design of the TCO is required to fulfil all these requirements. Chemical etching is widely applied to texture the surface of initially smooth sputtered AZO for light scattering. Kluth et al. [5] related the influence of pressure and substrate temperature during radio frequency sputter deposition of AZO. They showed that, depending on sputter parameters, crater-like surface topography with typical lateral length scales of 1 to 2 μm and depths of about 200 to 400 nm developed in a self-organized fashion. Yan et al. [6] developed AZO texturing method by either two step chemical etching or mixed etching with diluted hydrochloric acid (HCl) and hydrofluoric acid (HF). The haze value also increased from 36% to 45% at 600 nm wavelength. Here, we address different types of structures of sputtered AZO surfaces by wet chemical etching. The asdeposited surface has been treated with acid as well as alkali. The effect of two different pH etchant, on AZO surface has been compared through structural and optical characterizations. The slower etchant alkali provides more controllable etching of AZO than acid. The physical reason of this kind of behaviour has been explained through this work. 2. Experimental Procedure AZO films were deposited in a RF magnetron sputtering system (Millman, Beckhoff, Pune) (base pressure 9.0 x 10-6 bar) with 4'' ceramic ZnO: Al2O3 (2 wt. %) target. The substrate holder is kept rotating at the time of deposition. After sputtering the substrate has been etched with acid and alkali such as hydrochloric acid (HCl) and potassium hydroxide (KOH). The thicknesses of the films were measured by Surface Profilometer (Bruker, Dektak XT). Sheet resistance of the samples has been measured by Four Point Probe method (Jandel). Optical transmittance measurements were carried out in the wavelength range 330–800 nm using UV-Vis analyser (Jasco V-750). The surface topography was analyzed by Atomic Force Microscopy (AFM) (NT XDT, Solver Next) and Field Emission Scanning Electron Microscope (FESEM) (Carl Zeiss Microscopy Ltd, Sigma 02-87). 3. Results and Discussions The AZO films have been etched by 0.5% HCl solution for 30 sec at room temperature (RT). Another AZO film has been etched by 1% KOH solution for 20 min at 55-60 °C. The thickness of the AZO has been reduced by 246 nm by only 30 sec HCl etching whereas 250 nm by 20 min KOH etching. From these data it has been found that the etch rate of alkali is much lower than acid. The etch rate of HCl (0.5%) is ~250 nm/min and of KOH (1%) is 12.5 nm/min. So, alkali etching provides more controllable etching and thereby ability to create desired structure. The sheet resistance of KOH etched AZO is quite low i.e. 13.38 (Ω/□), whereas of HCl etched one is quite high 20.5 (Ω/□) [Table 1]. This will shed an effect in series resistance at the time of development of p-i-n cell on the etched substrate.
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Sharma et. al. / Materials Today: Proceedings 5 (2018) 9726–9732 Table 1. Film Thickness and Sheet Resistance for AZO (etched and as-deposited).
Conditions
Thickness (nm)
Sheet Resistance (Ω/□)
As-deposited
1000
7-8
After Etching by HCl
754
20.5
After Etching by KOH
750
13.38
3.1. Morphological Analysis
Fig.1. Diagrams of: a) interaction of the etchants with the ZnO film depending on the etchant size, at grain boundaries of different etch potentials, b) resulting crater shape for large and small etchants with vertically (left) and laterally (right) limited etch rates [7]
In case of acid etching crater like structures have been found on the surface of etched AZO (Fig .2b), whereas in case of alkali etching nanorod like structures have been found on the surface (Fig. 2c). This difference of structures can be explained by Fig. 1. Aquous HCl solution in a reaction with zinc oxide (ZnO) provides hydronium ion (H3O+). On the other side, aquous KOH solution in reaction with ZnO provides hydroxide ion (OH-). These can be properly understood by equations [1-6].
Sharma et. al./ Materials Today: Proceedings 5 (2018) 9726–9732
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(b)
(a)
(c)
Fig. 2. FESEM images of AZO film. (a) As-deposited, (b) HCl etched, (c) KOH etched
HCl +
-
ZnO + H + Cl
H+ + Cl-
(1)
ZnCl2 + H2O
(2)
+
H3O
KOH
+
H + H2O
+
K + OH +
ZnO + K + OH OH- + H2O
-
(3) -
(4)
K2[Zn(OH)4]
(5)
OH- + heat
(6)
HCl with high dissociation constant (ka=1X104) have a tendency to create large water-hydronium clusters as it dissociates well in water. The size of water-hydronium cluster is so high that it cannot penetrate through all the grain boundaries, thereby etchant etches laterally more than vertically and vertical limited etching occurs. On the other side KOH with less dissociation constant (ka=0.12X10-6) have the tendency to produce smaller water-hydroxide clusters as it does not properly dissociate in water. These smaller clusters easily penetrate through the grain boundaries and thereby the etchant etches vertically, limiting horizontal etching. It also has been observed from AFM measurements (Fig. 3a, 3b, 3c) that, the rms roughness of the surface etched by alkali is much higher (53.041 nm) than acid etching (29.998 nm) whereas for as-deposited (unetched) AZO, it is 3.167 nm only.
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Fig. 3. 2D (left side) and 3D (right side) AFM images of AZO film. (a,b) as-deposited, (c,d) HCl etched, (e,f) KOH etched
3.2. Optical Analysis
Fig.4. Transmittance spectra of AZO film (etched and as-deposited)
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The transmittance of the HCl etched AZO film is better than as-deposited as well as the KOH etched one (Fig. 4). From the transmittance curve data, it has been calculated that, acid (HCl) and alkali (KOH) etched film has an average value of 89% and 82% respectively, whereas for as-deposited film it is 78% within the visible range.
Fig.5. Absorbance spectra of AZO film (etched and as-deposited)
From the absorbance spectra (Fig. 5) it has been observed that, the absorbance of as-deposited AZO film is lesser than both of the etched films within the visible range. This results show that, in this case etching has no such influence on AZO surface, but if we compare between the two etchants, the KOH etched AZO film shows lesser absorbance than the HCl etched one.
Fig.6. Reflectance spectra of AZO film (etched and as-deposited)
The reflectance of etched AZO is also less than the as-deposited one (Fig. 6). In between acid and alkali, the alkali (KOH) etched surface shows very less reflectance, which is good for front TCO application.
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4. Conclusion Through this study we have observed that there is an influence of acid and alkali etching on the AZO substrate, which have been found from morphological and optical analysis. A nanorod like structure has been achieved through etching without using any kind of nanorod growth processes, we conclude that, in between acid and alkali etching, AZO etching by alkali (KOH) is better than acid (HCl) as it is better controllable due to its lower etching rate. The sheet resistance of KOH etched film is about 13 (Ω/□) which will help to reduce the series resistance of the developed cell. Moreover, the KOH etched substrate surface has high rms roughness due to its nanorod like sharp structure. This will provide high haze TCO for better light trapping by effective carrier collection. Although the HCl etched AZO film shows slightly higher (7%) transmittance than KOH etched, the reflectance and absorbance are better for KOH etched AZO films than HCl etched. So, the overall optical characteristics are better for KOH etched AZO film than HCl etched one. Finally, we can say that, if KOH etched AZO film is further applied in a pin cell as TCO; an efficiency enhancement could be achieved to some extent. Acknowledgements The authors would like to thank Ministry of Non-Conventional Renewable Energy (MNRE) and Department of Science and Technology- Science and Technology Research Board (DST-SERB) for financial support. References 1. 2.
3. 4. 5.
Müller J, Rech B, Springer J, Vanecek M. “TCO and light trapping in silicon thin film solar cells.” Solar Energy2004; 77: 917-30. Hüpkes J, Pust SE, Böttler W, Gordijn A, Wyrsch N, Güttler D, Tiwari AN, Gordon L, Qiu Y. “Light scattering and trapping in different thin film photovoltaic device.” Proc. 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, 2009, pp. 2766-2769. Castañeda L. “Present status of the development and application of transparent conductors oxide thin solid films.” Materials Sciences and Applications2011; 2:1233-42. Kluth O, Rech B, Houben L et al (1999) “Thin Solid Films” 351:247 O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, “Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behaviour“, Thin Solid Films, 442,80 - 85, 2003.
6.
Xia Yan,*, Selvaraj Venkataraj, Armin G. Aberle, “Modified Surface Texturing of Aluminium-Doped Zinc Oxide (AZO) Transparent Conductive Oxides for Thin-Film Silicon Solar Cells”, doi: 10.1016/j.egypro.2013.05.053 PV Asia Pacific Conference 2012,Solar Energy Research Institute of Singapore, National University of Singapore.
7.
J. Hupkes, J. I. Owen, S. E. Pust, and E. Bunte, “Chemical etching of zinc oxide for thin films silicon solar cells”. Chemphyschem : a European journal of chemical physics and physical chemistry, 13(1):66-73, Jan. 2012.
ScienceDirect Materials Today: Proceedings 5 (2018) 9726–9732
www.materialstoday.com/proceedings
IC-FNM 2016
Influence of Acid and Alkali Etching on Sputtered Aluminium Doped Zinc Oxide Films Jayasree Roy Sharmaa*, Sukanta Bosea, Sourav Mandala, Gourab Dasa, Sumita Mukhopadhyaya, A.K. Baruaa a
CEGESS, IIEST, Shibpur, Howrah, West Bengal 711103, India.
Abstract In order to increase the efficiency of silicon based Thin Film Solar (TFS) cells, it is necessary to use better light management techniques. Texturization of sputtered aluminium doped zinc oxide (Al:ZnO or AZO) films has opened up a variety of ways for optimization of light trapping schemes. Here, AZO has been etched with acid as well as alkali. A difference in optical and electrical characteristics of acid and alkali etched AZO has been observed. By etching AZO with potassium hydroxide (KOH), 82% transmittance (in the visible range), high rms roughness and ~13Ω/□ sheet resistance have been achieved. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016. Keywords: AZO; etching; ion; light trapping;
1. Introduction In thin-film photovoltaic technology, transparent conductive oxides (TCOs) are abundantly exploited. Due to the very thin active absorber layer, thin-film solar cells require a more effective light management scheme [1, 2]. Better light trapping assists in improving the short-circuit current density (Jsc) and thus increasing the overall efficiency of thin-film Si solar cells. Common TCO materials are indium-doped tin oxide (ITO), fluorine-doped tin oxide (FTO) and impurity-doped (e.g. Al, Ga, B) zinc oxide [3]. Nowdays, AZO films are becoming increasingly appreciated as the front contact for thin-film Si solar cells as it consists of the advantages such as wide band gap, non-toxic, and has good thermal
* Corresponding author. Tel.:033-64551644; fax: 033-26682964. E-mail address: [email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016.
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stability against hydrogen plasma environment, easy post-deposition texturing, easy fabrication, enormous amounts of raw materials, and low expense [4]. The electronic properties as well as the surface texture of the AZO films after etching mostly depend on the preparation method of the AZO. A wide variety of thin film deposition methods have been studied for the growth of doped and as-deposited ZnO and they are: magnetron sputtering, chemical vapor deposition (CVD), spray pyrolysis, pulsed-laser deposition, evaporation, chemical solution deposition, electrochemical deposition, molecular beam epitaxy and atomic layer deposition, sol-gel processes. Among these techniques, magnetron sputtering and CVD have shown the most promise for large scale thin-film silicon solar cell production, as these techniques can yield large-area thin-films that can be texturized effectively to introduce light trapping features. In this work, AZO films have been prepared by magnetron sputtering. In pin configuration, the TCO front contacts have to meet a number of requirements which include (i) High transparency in the active region of the absorber layer (between 400 - 900 nm in case of µc-Si:H), to minimize optical losses by absorption and/or reflection,(ii) High conductivity to minimize the series resistance of the solar cells and avoid parasitic losses, (iii) Excellent light scattering properties through a rough surface, (iv) favourable physico-chemical properties for the growth of silicon. Besides the above mentioned criterions, a TCO with a suitable texture has to be developed which scatters the light very efficiently in order to elongate the effective path length within the active silicon layers. Thus, a special design of the TCO is required to fulfil all these requirements. Chemical etching is widely applied to texture the surface of initially smooth sputtered AZO for light scattering. Kluth et al. [5] related the influence of pressure and substrate temperature during radio frequency sputter deposition of AZO. They showed that, depending on sputter parameters, crater-like surface topography with typical lateral length scales of 1 to 2 μm and depths of about 200 to 400 nm developed in a self-organized fashion. Yan et al. [6] developed AZO texturing method by either two step chemical etching or mixed etching with diluted hydrochloric acid (HCl) and hydrofluoric acid (HF). The haze value also increased from 36% to 45% at 600 nm wavelength. Here, we address different types of structures of sputtered AZO surfaces by wet chemical etching. The asdeposited surface has been treated with acid as well as alkali. The effect of two different pH etchant, on AZO surface has been compared through structural and optical characterizations. The slower etchant alkali provides more controllable etching of AZO than acid. The physical reason of this kind of behaviour has been explained through this work. 2. Experimental Procedure AZO films were deposited in a RF magnetron sputtering system (Millman, Beckhoff, Pune) (base pressure 9.0 x 10-6 bar) with 4'' ceramic ZnO: Al2O3 (2 wt. %) target. The substrate holder is kept rotating at the time of deposition. After sputtering the substrate has been etched with acid and alkali such as hydrochloric acid (HCl) and potassium hydroxide (KOH). The thicknesses of the films were measured by Surface Profilometer (Bruker, Dektak XT). Sheet resistance of the samples has been measured by Four Point Probe method (Jandel). Optical transmittance measurements were carried out in the wavelength range 330–800 nm using UV-Vis analyser (Jasco V-750). The surface topography was analyzed by Atomic Force Microscopy (AFM) (NT XDT, Solver Next) and Field Emission Scanning Electron Microscope (FESEM) (Carl Zeiss Microscopy Ltd, Sigma 02-87). 3. Results and Discussions The AZO films have been etched by 0.5% HCl solution for 30 sec at room temperature (RT). Another AZO film has been etched by 1% KOH solution for 20 min at 55-60 °C. The thickness of the AZO has been reduced by 246 nm by only 30 sec HCl etching whereas 250 nm by 20 min KOH etching. From these data it has been found that the etch rate of alkali is much lower than acid. The etch rate of HCl (0.5%) is ~250 nm/min and of KOH (1%) is 12.5 nm/min. So, alkali etching provides more controllable etching and thereby ability to create desired structure. The sheet resistance of KOH etched AZO is quite low i.e. 13.38 (Ω/□), whereas of HCl etched one is quite high 20.5 (Ω/□) [Table 1]. This will shed an effect in series resistance at the time of development of p-i-n cell on the etched substrate.
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Sharma et. al. / Materials Today: Proceedings 5 (2018) 9726–9732 Table 1. Film Thickness and Sheet Resistance for AZO (etched and as-deposited).
Conditions
Thickness (nm)
Sheet Resistance (Ω/□)
As-deposited
1000
7-8
After Etching by HCl
754
20.5
After Etching by KOH
750
13.38
3.1. Morphological Analysis
Fig.1. Diagrams of: a) interaction of the etchants with the ZnO film depending on the etchant size, at grain boundaries of different etch potentials, b) resulting crater shape for large and small etchants with vertically (left) and laterally (right) limited etch rates [7]
In case of acid etching crater like structures have been found on the surface of etched AZO (Fig .2b), whereas in case of alkali etching nanorod like structures have been found on the surface (Fig. 2c). This difference of structures can be explained by Fig. 1. Aquous HCl solution in a reaction with zinc oxide (ZnO) provides hydronium ion (H3O+). On the other side, aquous KOH solution in reaction with ZnO provides hydroxide ion (OH-). These can be properly understood by equations [1-6].
Sharma et. al./ Materials Today: Proceedings 5 (2018) 9726–9732
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(b)
(a)
(c)
Fig. 2. FESEM images of AZO film. (a) As-deposited, (b) HCl etched, (c) KOH etched
HCl +
-
ZnO + H + Cl
H+ + Cl-
(1)
ZnCl2 + H2O
(2)
+
H3O
KOH
+
H + H2O
+
K + OH +
ZnO + K + OH OH- + H2O
-
(3) -
(4)
K2[Zn(OH)4]
(5)
OH- + heat
(6)
HCl with high dissociation constant (ka=1X104) have a tendency to create large water-hydronium clusters as it dissociates well in water. The size of water-hydronium cluster is so high that it cannot penetrate through all the grain boundaries, thereby etchant etches laterally more than vertically and vertical limited etching occurs. On the other side KOH with less dissociation constant (ka=0.12X10-6) have the tendency to produce smaller water-hydroxide clusters as it does not properly dissociate in water. These smaller clusters easily penetrate through the grain boundaries and thereby the etchant etches vertically, limiting horizontal etching. It also has been observed from AFM measurements (Fig. 3a, 3b, 3c) that, the rms roughness of the surface etched by alkali is much higher (53.041 nm) than acid etching (29.998 nm) whereas for as-deposited (unetched) AZO, it is 3.167 nm only.
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Fig. 3. 2D (left side) and 3D (right side) AFM images of AZO film. (a,b) as-deposited, (c,d) HCl etched, (e,f) KOH etched
3.2. Optical Analysis
Fig.4. Transmittance spectra of AZO film (etched and as-deposited)
Sharma et. al./ Materials Today: Proceedings 5 (2018) 9726–9732
9731
The transmittance of the HCl etched AZO film is better than as-deposited as well as the KOH etched one (Fig. 4). From the transmittance curve data, it has been calculated that, acid (HCl) and alkali (KOH) etched film has an average value of 89% and 82% respectively, whereas for as-deposited film it is 78% within the visible range.
Fig.5. Absorbance spectra of AZO film (etched and as-deposited)
From the absorbance spectra (Fig. 5) it has been observed that, the absorbance of as-deposited AZO film is lesser than both of the etched films within the visible range. This results show that, in this case etching has no such influence on AZO surface, but if we compare between the two etchants, the KOH etched AZO film shows lesser absorbance than the HCl etched one.
Fig.6. Reflectance spectra of AZO film (etched and as-deposited)
The reflectance of etched AZO is also less than the as-deposited one (Fig. 6). In between acid and alkali, the alkali (KOH) etched surface shows very less reflectance, which is good for front TCO application.
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4. Conclusion Through this study we have observed that there is an influence of acid and alkali etching on the AZO substrate, which have been found from morphological and optical analysis. A nanorod like structure has been achieved through etching without using any kind of nanorod growth processes, we conclude that, in between acid and alkali etching, AZO etching by alkali (KOH) is better than acid (HCl) as it is better controllable due to its lower etching rate. The sheet resistance of KOH etched film is about 13 (Ω/□) which will help to reduce the series resistance of the developed cell. Moreover, the KOH etched substrate surface has high rms roughness due to its nanorod like sharp structure. This will provide high haze TCO for better light trapping by effective carrier collection. Although the HCl etched AZO film shows slightly higher (7%) transmittance than KOH etched, the reflectance and absorbance are better for KOH etched AZO films than HCl etched. So, the overall optical characteristics are better for KOH etched AZO film than HCl etched one. Finally, we can say that, if KOH etched AZO film is further applied in a pin cell as TCO; an efficiency enhancement could be achieved to some extent. Acknowledgements The authors would like to thank Ministry of Non-Conventional Renewable Energy (MNRE) and Department of Science and Technology- Science and Technology Research Board (DST-SERB) for financial support. References 1. 2.
3. 4. 5.
Müller J, Rech B, Springer J, Vanecek M. “TCO and light trapping in silicon thin film solar cells.” Solar Energy2004; 77: 917-30. Hüpkes J, Pust SE, Böttler W, Gordijn A, Wyrsch N, Güttler D, Tiwari AN, Gordon L, Qiu Y. “Light scattering and trapping in different thin film photovoltaic device.” Proc. 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, 2009, pp. 2766-2769. Castañeda L. “Present status of the development and application of transparent conductors oxide thin solid films.” Materials Sciences and Applications2011; 2:1233-42. Kluth O, Rech B, Houben L et al (1999) “Thin Solid Films” 351:247 O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, “Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behaviour“, Thin Solid Films, 442,80 - 85, 2003.
6.
Xia Yan,*, Selvaraj Venkataraj, Armin G. Aberle, “Modified Surface Texturing of Aluminium-Doped Zinc Oxide (AZO) Transparent Conductive Oxides for Thin-Film Silicon Solar Cells”, doi: 10.1016/j.egypro.2013.05.053 PV Asia Pacific Conference 2012,Solar Energy Research Institute of Singapore, National University of Singapore.
7.
J. Hupkes, J. I. Owen, S. E. Pust, and E. Bunte, “Chemical etching of zinc oxide for thin films silicon solar cells”. Chemphyschem : a European journal of chemical physics and physical chemistry, 13(1):66-73, Jan. 2012.