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Fabrication of homogeneous CIGS thin film by plasma-enhanced Se vapor selenization coupled with etching process
Materials Letters 190 (2017) 276–279
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Fabrication of homogeneous CIGS thin film by plasma-enhanced Se vapor selenization coupled with etching process Xiaoqing Zhang, Yunxiang Huang, Wei Yuan ⇑, Yong Tang, Lin Li Guangdong Engineering Research Center of Green Manufacturing for Energy-Saving and New-Energy Technology, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 30 August 2016 Received in revised form 17 December 2016 Accepted 7 January 2017 Available online 9 January 2017 Keywords: Thin films Cu(In, Ga)Se2 Plasma-enhanced Etching Selenization Solar energy materials
a b s t r a c t In this study, a novel plasma-enhanced Se vapor selenization coupled with etching (PESVSE) technique is demonstrated to achieve a uniform depth distribution of Ga in the Cu(In, Ga)Se2 (CIGS) film. The significant increase of Ga concentration on the surface of the CIGS film was ascribed to the removal of the excessive metallic-indium by the Ar-plasma during the PESVSE process and a single phase CIGS film was obtained without phase separation. No small grains or cracks were observed in the CIGS film based on the PESVSE process. The CIGS device obtained from the PESVSE process showed an enhanced conversion efficiency of 8.79%, compared to that of the device obtained from the TASVS process (6.18%). Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Cu(In, Ga)Se2 (CIGS) solar cells are considered to be the most promising thin film solar cells with a conversion efficiency of up to 22.6% [1]. However, the CIGS solar cells with high efficiency are usually obtained by co-evaporation [1,2] and magnetron sputtering [3,4] with high cost, which is considered to be a main obstacle for mass production. Post-selenization of metallic precursors is an alternative economic method to fabricate the CIGS films so that the nontoxic Se vapors can be used to replace the toxic H2Se gas [5]. Nevertheless, due to the preferential reaction between In and Se [6], Ga usually accumulates at the bottom of the CIGS film during the selenization process, resulting in a phase separation between CuInSe2 (CIS) and CuGaSe2 (CGS) in a severe case [7]. In order to increase the Ga content on the surface of the CIGS films, some new kinds of fabrication processes were recently developed. Lin et al. [8] etched the excessive metallic-indium in the cosputtered Cu-In-Ga precursors by the Ar-ion plasma to increase the Ga content on the surface of the CIGS films. Wu et al. [9] increased the amount of Ga on the surface of the CIGS films by pre-annealing and hydrogen-assisted solid Se vapor selenization of the sputtered precursors with multi-stacking CuGa layers at the top. Although a significant increase of Ga content on the sur-
⇑ Corresponding author. E-mail address: [email protected] (W. Yuan). http://dx.doi.org/10.1016/j.matlet.2017.01.036 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
face of the films is observed in their studies, a further improvement is still necessary to achieve a uniform depth distribution of Ga. In this study, a novel plasma-enhanced Se vapor selenization coupled with etching (PESVEE) technique is proposed to achieve a homogeneous depth distribution of Ga in the CIGS film.
2. Experimental The schematic design of the PESVSE process is shown in Fig. 1. The Ar gas (discharge gas) was adjusted by a mass flow controller to flow into the chamber. A radio-frequent (RF) power supply with a frequency of 13.56 MHz was adopted to generate the capacitive coupled plasma (CCP) at the RF power of 50 W by two electrodes, i.e. the stainless steel shower head and the sample. The distance between two electrodes is 60 mm. The tank filled with Se pellets was heated up to 240 °C to provide Se vapors that flowed through the shower head into the plasma region to generate the Se radicals with the enhanced reaction activity [6]. Then the chamber was filled with the Se radicals which reacted with the precursors. Meanwhile, the Ar-plasma etched the precursors to remove the excessive amount of the metallic-indium in the film during the PESVSE process [8]. The Cu, In and Ga layers were successively electrodeposited on the Mo-coated soda-lime glass (SLG) substrates using a pulsed direct-current (DC) deposition, a pulsed DC deposition and a DC deposition, respectively, at room temperature to obtain Cu/In/Ga
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conventional thermal-assisted Se vapor selenization process (TASVS, sample A) and the PESVSE process (sample B) at 550 °C for 30 min to form the CIGS films, respectively. The corresponding CIGS solar cells were completed with a 50 nm CdS buffer layer, a window layer composed of 50 nm i-ZnO and 300 nm ZnO:Al and a Ni/Al grid contact. The CIGS films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectrometer (XPS), X-ray fluorescent (XRF), Raman spectrometer and secondary ion mass spectrometer (SIMS), respectively. The current density-voltage (J-V) measurements of the solar cells were performed under the standard AM1.5 conditions (1000 W/m2). 3. Results and discussion Fig. 1. Schematic design of the plasma-enhanced Se vapor selenization coupled with etching process.
precursors. The detailed electrodeposition parameters can be found in Table S1 (Supporting information). Next, the Cu/In/Ga precursors were annealed at 250 °C for 30 min, followed by the
The XPS surface scan data for the CIGS films are shown in Fig. 2a. In addition to the C 1 s and O 1 s signals related to the surface contamination and oxidation, the Cu 2p, In 3d and Se 3d signals were clearly observed for both CIGS films in Fig. 2a. Due to the accumulation of most Ga at the back of the CIGS film [9], a very weak Ga 2p signal was detected for sample A. However, the intensity of Ga 2p signal increased significantly for sample B, indicating
Fig. 2. XPS surface scans (a), XRD patterns (b) and Raman spectra (c) of the CIGS films.
Table 1 Surface composition and bulk composition of the CIGS films. Sample
A B
Surface composition (at.%)
Stoichiometry
Cu
In
Ga
Se
24.11 23.11
25.31 21.21
0.45 4.12
50.45 51.56
Cu0.96In1.01Ga0.02Se2.02 Cu0.92In0.85Ga0.16Se2.06
Bulk composition (at.%)
Stoichiometry
Cu
In
Ga
Se
23.39 23.04
23.04 21.18
4.21 4.15
49.36 51.63
Cu0.94In0.92Ga0.17Se1.97 Cu0.92In0.85Ga0.16Se2.07
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Fig. 3. The surface and cross-sectional FESEM images of the CIGS films.
Fig. 4. SIMS depth profiles of elements for the CIGS films (a)–(b) and J-V curves (c) of the solar cells.
X. Zhang et al. / Materials Letters 190 (2017) 276–279
an increase of Ga concentration on the surface of the CIGS film. Furthermore, the intensity of In 3d signal for sample B decrease significantly, compared to that for sample A. The surface composition and bulk composition of the CIGS films are shown in Table 1. The content of indium on the surface of sample B decreased but that of Ga increased, compared to that of sample A. The content of indium in the bulk composition also decreased when the Arplasma was used. This was because the Ar-plasma etched the excessive metallic-indium in the film, leading to an increase of Ga concentration on the surface of the CIGS film. The significant increase of Se content in the bulk composition was attributed to the enhancement of the reaction between Ga and Se during the PESVSE process. It was noteworthy that the surface composition matched very well with the bulk composition for sample B. These results indicated that the PESVSE process could effectively facilitate the diffusion of Ga towards the surface of the CIGS film, realizing a uniform depth distribution of the composition in the whole film. Finally, a CIGS film with a stoichiometry of Cu0.92In0.85Ga0.16Se2.07 was obtained by using the PESVSE process. Fig. 2b shows the XRD patterns of the CIGS films. A phase separation was observed for sample A in Fig. 2b due to Ga aggregation [9]. However, three single diffraction peaks corresponding to (112), (220) and (312) of CIGS were observed without phase separation for sample B [4]. Meanwhile, the intensity of each CIGS peak increased significantly due to the enhanced reaction activity of the Se radicals [6]. The CIGS films were further characterized using Raman spectroscopy, as shown in Fig. 2c. Because of a low Ga concentration on the surface of the CIGS film, a Raman shift at 172 cm 1 was observed for sample A, which is assigned to the A1 mode of CIGS [10]. However, the A1 mode for sample B was observed at higher Raman shift of 175 cm 1, confirming the increase of Ga concentration on the surface of the CIGS film. The surface and cross-sectional FESEM images of the CIGS films are shown in Fig. 3. A few of small grains were observed on the surface of sample A. But this is not the case of sample B which exhibited the faceted grains, as shown in Fig. 3b. The cross-sectional FESEM images demonstrate that the large grains covered the upper area of the film while many small grains distributed at the bottom due to Ga aggregation for sample A [9]. Furthermore, some voids and cracks were also observed due to the loss of materials during the TASVS process. However, due to the homogeneous depth distribution of Ga in the whole film, no small grains or cracks were observed for sample B based on the PESVSE process, as shown in Fig. 3d. Fig. 4a and b show the SIMS depth profiles of elements for the CIGS films. As shown in Fig. 4(a), a steep grading trend of Ga was observed for sample A. However, due to the removal of the excessive metallic-indium in the film by the Ar-plasma etching during the PESVSE process, the intensity profile of In was observed to decrease and a homogeneous depth distribution of Ga was achieved in the whole film, as shown in Fig. 4b. The J-V curves of the solar cells are presented in Fig. 4c. The device based on the PESVSE process achieved Voc = 513 mV, Jsc = 27.9 mA/cm2,
279
FF = 61.4% and an enhanced conversion efficiency of 8.79%, respectively, while the one obtained from the TASVS process yielded Voc = 428 mV, Jsc = 26.8 mA/cm2, FF = 53.9%, respectively, resulting in a conversion efficiency of 6.18%. 4. Conclusion CIGS thin film with a homogeneous depth distribution of Ga was prepared from the PESVSE process in this work. XPS and Raman spectra analysis indicated that the Ar-plasma could facilitate the diffusion of Ga towards the surface of the CIGS film by removing the excessive metallic-indium in the film. Owning to the uniform depth distribution of Ga, a single phase CIGS film was obtained from the PESVSE process without phase separation. Furthermore, no small grains or cracks were observed in the CIGS film when the Ar-plasma was introduced during the selenization process. An enhanced conversion efficiency of 8.79% was achieved for the CIGS device based on the PESVSE process, in comparison to that (6.18%) of the CIGS device obtained from the TASVS process. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51475172), Team Program (No. 2014A030312017) funded by the Natural Science Foundation of Guangdong Province, Guangdong Science Fund for Distinguished Young Scholars (No. 2015A030306013) and Science and Technology Plan Project (No. 2014B090921004) funded by Guangdong Province. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2017.01. 036. References [1] P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, M. Powalla, Phys. Status Solidi RRL 10 (2016) 583–586. [2] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photo. : Res. Appl. 19 (2011) 894–897. [3] H. Kong, J. He, L. Huang, L. Zhu, L. Sun, P. Yang, J. Chu, Mater. Lett. 116 (2014) 75–78. [4] H. Kong, J. He, X. Meng, L. Zhu, J. Tao, L. Sun, P. Yang, J. Chu, Mater. Lett. 118 (2014) 21–23. [5] W. Li, Y. Sun, W. Liu, L. Zhou, Sol. Energy 80 (2006) 191–195. [6] T.T. Wu, C.h. Chang, C.H. Hsu, W.C. Tsai, H.S. Tsai, Y.T. Yen, C.H. Shen, J.M. Shieh, Y.L. Chueh, Nano Energy 24 (2016) 45–55. [7] Z. Baji, Z. Lábadi, G. Molnár, B. Pécz, A.L. Tóth, J. Tóth, A. Csik, I. Bársony, Vacuum 92 (2013) 44–51. [8] W.T. Lin, S.H. Chen, S.H. Chan, S.C. Hu, W.X. Peng, Y.T. Lu, Vacuum 99 (2014) 1–6. [9] T.T. Wu, J.H. Huang, F. Hu, C.h. Chang, W.L. Liu, T.H. Wang, C.H. Shen, J.M. Shieh, Y.L. Chueh, Nano Energy 10 (2014) 28–36. [10] S. Mandati, B.V. Sarada, S.R. Dey, S.V. Joshi, J. Power Sources 273 (2015) 149– 157.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Fabrication of homogeneous CIGS thin film by plasma-enhanced Se vapor selenization coupled with etching process Xiaoqing Zhang, Yunxiang Huang, Wei Yuan ⇑, Yong Tang, Lin Li Guangdong Engineering Research Center of Green Manufacturing for Energy-Saving and New-Energy Technology, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 30 August 2016 Received in revised form 17 December 2016 Accepted 7 January 2017 Available online 9 January 2017 Keywords: Thin films Cu(In, Ga)Se2 Plasma-enhanced Etching Selenization Solar energy materials
a b s t r a c t In this study, a novel plasma-enhanced Se vapor selenization coupled with etching (PESVSE) technique is demonstrated to achieve a uniform depth distribution of Ga in the Cu(In, Ga)Se2 (CIGS) film. The significant increase of Ga concentration on the surface of the CIGS film was ascribed to the removal of the excessive metallic-indium by the Ar-plasma during the PESVSE process and a single phase CIGS film was obtained without phase separation. No small grains or cracks were observed in the CIGS film based on the PESVSE process. The CIGS device obtained from the PESVSE process showed an enhanced conversion efficiency of 8.79%, compared to that of the device obtained from the TASVS process (6.18%). Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Cu(In, Ga)Se2 (CIGS) solar cells are considered to be the most promising thin film solar cells with a conversion efficiency of up to 22.6% [1]. However, the CIGS solar cells with high efficiency are usually obtained by co-evaporation [1,2] and magnetron sputtering [3,4] with high cost, which is considered to be a main obstacle for mass production. Post-selenization of metallic precursors is an alternative economic method to fabricate the CIGS films so that the nontoxic Se vapors can be used to replace the toxic H2Se gas [5]. Nevertheless, due to the preferential reaction between In and Se [6], Ga usually accumulates at the bottom of the CIGS film during the selenization process, resulting in a phase separation between CuInSe2 (CIS) and CuGaSe2 (CGS) in a severe case [7]. In order to increase the Ga content on the surface of the CIGS films, some new kinds of fabrication processes were recently developed. Lin et al. [8] etched the excessive metallic-indium in the cosputtered Cu-In-Ga precursors by the Ar-ion plasma to increase the Ga content on the surface of the CIGS films. Wu et al. [9] increased the amount of Ga on the surface of the CIGS films by pre-annealing and hydrogen-assisted solid Se vapor selenization of the sputtered precursors with multi-stacking CuGa layers at the top. Although a significant increase of Ga content on the sur-
⇑ Corresponding author. E-mail address: [email protected] (W. Yuan). http://dx.doi.org/10.1016/j.matlet.2017.01.036 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
face of the films is observed in their studies, a further improvement is still necessary to achieve a uniform depth distribution of Ga. In this study, a novel plasma-enhanced Se vapor selenization coupled with etching (PESVEE) technique is proposed to achieve a homogeneous depth distribution of Ga in the CIGS film.
2. Experimental The schematic design of the PESVSE process is shown in Fig. 1. The Ar gas (discharge gas) was adjusted by a mass flow controller to flow into the chamber. A radio-frequent (RF) power supply with a frequency of 13.56 MHz was adopted to generate the capacitive coupled plasma (CCP) at the RF power of 50 W by two electrodes, i.e. the stainless steel shower head and the sample. The distance between two electrodes is 60 mm. The tank filled with Se pellets was heated up to 240 °C to provide Se vapors that flowed through the shower head into the plasma region to generate the Se radicals with the enhanced reaction activity [6]. Then the chamber was filled with the Se radicals which reacted with the precursors. Meanwhile, the Ar-plasma etched the precursors to remove the excessive amount of the metallic-indium in the film during the PESVSE process [8]. The Cu, In and Ga layers were successively electrodeposited on the Mo-coated soda-lime glass (SLG) substrates using a pulsed direct-current (DC) deposition, a pulsed DC deposition and a DC deposition, respectively, at room temperature to obtain Cu/In/Ga
277
X. Zhang et al. / Materials Letters 190 (2017) 276–279
conventional thermal-assisted Se vapor selenization process (TASVS, sample A) and the PESVSE process (sample B) at 550 °C for 30 min to form the CIGS films, respectively. The corresponding CIGS solar cells were completed with a 50 nm CdS buffer layer, a window layer composed of 50 nm i-ZnO and 300 nm ZnO:Al and a Ni/Al grid contact. The CIGS films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectrometer (XPS), X-ray fluorescent (XRF), Raman spectrometer and secondary ion mass spectrometer (SIMS), respectively. The current density-voltage (J-V) measurements of the solar cells were performed under the standard AM1.5 conditions (1000 W/m2). 3. Results and discussion Fig. 1. Schematic design of the plasma-enhanced Se vapor selenization coupled with etching process.
precursors. The detailed electrodeposition parameters can be found in Table S1 (Supporting information). Next, the Cu/In/Ga precursors were annealed at 250 °C for 30 min, followed by the
The XPS surface scan data for the CIGS films are shown in Fig. 2a. In addition to the C 1 s and O 1 s signals related to the surface contamination and oxidation, the Cu 2p, In 3d and Se 3d signals were clearly observed for both CIGS films in Fig. 2a. Due to the accumulation of most Ga at the back of the CIGS film [9], a very weak Ga 2p signal was detected for sample A. However, the intensity of Ga 2p signal increased significantly for sample B, indicating
Fig. 2. XPS surface scans (a), XRD patterns (b) and Raman spectra (c) of the CIGS films.
Table 1 Surface composition and bulk composition of the CIGS films. Sample
A B
Surface composition (at.%)
Stoichiometry
Cu
In
Ga
Se
24.11 23.11
25.31 21.21
0.45 4.12
50.45 51.56
Cu0.96In1.01Ga0.02Se2.02 Cu0.92In0.85Ga0.16Se2.06
Bulk composition (at.%)
Stoichiometry
Cu
In
Ga
Se
23.39 23.04
23.04 21.18
4.21 4.15
49.36 51.63
Cu0.94In0.92Ga0.17Se1.97 Cu0.92In0.85Ga0.16Se2.07
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X. Zhang et al. / Materials Letters 190 (2017) 276–279
Fig. 3. The surface and cross-sectional FESEM images of the CIGS films.
Fig. 4. SIMS depth profiles of elements for the CIGS films (a)–(b) and J-V curves (c) of the solar cells.
X. Zhang et al. / Materials Letters 190 (2017) 276–279
an increase of Ga concentration on the surface of the CIGS film. Furthermore, the intensity of In 3d signal for sample B decrease significantly, compared to that for sample A. The surface composition and bulk composition of the CIGS films are shown in Table 1. The content of indium on the surface of sample B decreased but that of Ga increased, compared to that of sample A. The content of indium in the bulk composition also decreased when the Arplasma was used. This was because the Ar-plasma etched the excessive metallic-indium in the film, leading to an increase of Ga concentration on the surface of the CIGS film. The significant increase of Se content in the bulk composition was attributed to the enhancement of the reaction between Ga and Se during the PESVSE process. It was noteworthy that the surface composition matched very well with the bulk composition for sample B. These results indicated that the PESVSE process could effectively facilitate the diffusion of Ga towards the surface of the CIGS film, realizing a uniform depth distribution of the composition in the whole film. Finally, a CIGS film with a stoichiometry of Cu0.92In0.85Ga0.16Se2.07 was obtained by using the PESVSE process. Fig. 2b shows the XRD patterns of the CIGS films. A phase separation was observed for sample A in Fig. 2b due to Ga aggregation [9]. However, three single diffraction peaks corresponding to (112), (220) and (312) of CIGS were observed without phase separation for sample B [4]. Meanwhile, the intensity of each CIGS peak increased significantly due to the enhanced reaction activity of the Se radicals [6]. The CIGS films were further characterized using Raman spectroscopy, as shown in Fig. 2c. Because of a low Ga concentration on the surface of the CIGS film, a Raman shift at 172 cm 1 was observed for sample A, which is assigned to the A1 mode of CIGS [10]. However, the A1 mode for sample B was observed at higher Raman shift of 175 cm 1, confirming the increase of Ga concentration on the surface of the CIGS film. The surface and cross-sectional FESEM images of the CIGS films are shown in Fig. 3. A few of small grains were observed on the surface of sample A. But this is not the case of sample B which exhibited the faceted grains, as shown in Fig. 3b. The cross-sectional FESEM images demonstrate that the large grains covered the upper area of the film while many small grains distributed at the bottom due to Ga aggregation for sample A [9]. Furthermore, some voids and cracks were also observed due to the loss of materials during the TASVS process. However, due to the homogeneous depth distribution of Ga in the whole film, no small grains or cracks were observed for sample B based on the PESVSE process, as shown in Fig. 3d. Fig. 4a and b show the SIMS depth profiles of elements for the CIGS films. As shown in Fig. 4(a), a steep grading trend of Ga was observed for sample A. However, due to the removal of the excessive metallic-indium in the film by the Ar-plasma etching during the PESVSE process, the intensity profile of In was observed to decrease and a homogeneous depth distribution of Ga was achieved in the whole film, as shown in Fig. 4b. The J-V curves of the solar cells are presented in Fig. 4c. The device based on the PESVSE process achieved Voc = 513 mV, Jsc = 27.9 mA/cm2,
279
FF = 61.4% and an enhanced conversion efficiency of 8.79%, respectively, while the one obtained from the TASVS process yielded Voc = 428 mV, Jsc = 26.8 mA/cm2, FF = 53.9%, respectively, resulting in a conversion efficiency of 6.18%. 4. Conclusion CIGS thin film with a homogeneous depth distribution of Ga was prepared from the PESVSE process in this work. XPS and Raman spectra analysis indicated that the Ar-plasma could facilitate the diffusion of Ga towards the surface of the CIGS film by removing the excessive metallic-indium in the film. Owning to the uniform depth distribution of Ga, a single phase CIGS film was obtained from the PESVSE process without phase separation. Furthermore, no small grains or cracks were observed in the CIGS film when the Ar-plasma was introduced during the selenization process. An enhanced conversion efficiency of 8.79% was achieved for the CIGS device based on the PESVSE process, in comparison to that (6.18%) of the CIGS device obtained from the TASVS process. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51475172), Team Program (No. 2014A030312017) funded by the Natural Science Foundation of Guangdong Province, Guangdong Science Fund for Distinguished Young Scholars (No. 2015A030306013) and Science and Technology Plan Project (No. 2014B090921004) funded by Guangdong Province. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2017.01. 036. References [1] P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, M. Powalla, Phys. Status Solidi RRL 10 (2016) 583–586. [2] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photo. : Res. Appl. 19 (2011) 894–897. [3] H. Kong, J. He, L. Huang, L. Zhu, L. Sun, P. Yang, J. Chu, Mater. Lett. 116 (2014) 75–78. [4] H. Kong, J. He, X. Meng, L. Zhu, J. Tao, L. Sun, P. Yang, J. Chu, Mater. Lett. 118 (2014) 21–23. [5] W. Li, Y. Sun, W. Liu, L. Zhou, Sol. Energy 80 (2006) 191–195. [6] T.T. Wu, C.h. Chang, C.H. Hsu, W.C. Tsai, H.S. Tsai, Y.T. Yen, C.H. Shen, J.M. Shieh, Y.L. Chueh, Nano Energy 24 (2016) 45–55. [7] Z. Baji, Z. Lábadi, G. Molnár, B. Pécz, A.L. Tóth, J. Tóth, A. Csik, I. Bársony, Vacuum 92 (2013) 44–51. [8] W.T. Lin, S.H. Chen, S.H. Chan, S.C. Hu, W.X. Peng, Y.T. Lu, Vacuum 99 (2014) 1–6. [9] T.T. Wu, J.H. Huang, F. Hu, C.h. Chang, W.L. Liu, T.H. Wang, C.H. Shen, J.M. Shieh, Y.L. Chueh, Nano Energy 10 (2014) 28–36. [10] S. Mandati, B.V. Sarada, S.R. Dey, S.V. Joshi, J. Power Sources 273 (2015) 149– 157.