- Email: [email protected]
Influence of Se supply for selenization of Cu(In,Ga)Se2 precursors deposited by sputtering from a single quaternary target
Materials Letters 118 (2014) 21–23
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Influence of Se supply for selenization of Cu(In,Ga)Se2 precursors deposited by sputtering from a single quaternary target Hui Kong a,b, Jun He a, Xiankuan Meng a, Liping zhu a, Jiahua Tao a, Lin Sun a, Pingxiong Yang a,n, Junhao Chu a a Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China b Shanghai Solar Energy Technology Co., Ltd., Shanghai 200241, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 November 2013 Accepted 11 December 2013 Available online 18 December 2013
Cu(In,Ga)Se2 (CIGS) precursors were deposited on Mo-coated soda lime glass substrates by a radio frequency magnetron sputtering process from a single target. The selenization of CIGS precursor layer was performed by using a rapid thermal process. Energy dispersive X-ray analysis results show CIGS thin films are in a Cu-poor state. The films selenized with 0 mg and 5 mg are deficient in the Se element. However, films selenized using 20 mg show a slightly rich Se content. The results of X-ray diffraction and Raman spectra analysis indicate that the samples are the chalcopyrite-type structures and in pure phase. Both XRD and Raman scattering results indicate the samples selenized using 5 mg and 10 mg of the Se powder have better crystallinity. Furthermore, the samples selenized using Se powders have better surface morphology. In summary, 10 mg of Se powder for usage is the desired candidate. & 2013 Elsevier B.V. All rights reserved.
Keywords: CIGS Sputtering Solar energy materials Thin films
1. Introduction Cu(In,Ga)Se2 (CIGS) has attracted much attention as one of the most promising absorber materials for thin film solar cells due to its tunable band gap (1.04–1.67 eV), high absorption coefficient ( 105 cm 1), high conversion efficiency and excellent antiirradiation performance [1]. CIGS thin films have been prepared by a variety of approaches, such as co-evaporation [1,2], sputtering [3,4], and chemical deposition [5,6]. Until now, co-evaporation has been the most successful technique for the preparation of CIGS, which has achieved the highest efficiency CIGS-based solar cells [1]. However, it seems difficult to scale up nowadays because of its complicated processing steps and difficulties in scaling up for large area devices with uniform compositions. On the other hand, among these methods, sputtering technique is potentially suitable for obtaining good quality, large area CIGS precursor films since the sputtering method typically provides a uniform composition of thin film, smooth surface, and is a simple process [3,7]. The result of our previous work [11] has shown that CIGS thin films deposited by the RF magnetron sputtering process from a single target without the selenization process are deficient in the Se element. It can be assigned to the lower sputtering yield and the volatilization of the Se element during the deposition process. CIGS thin film in a Se-poor state could impact the microstructure and optical
n
Corresponding author. Tel.: þ 86 21 54345157; fax: þ86 21 54345119. E-mail address: [email protected] (P. Yang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.050
properties. Hence, in this work the selenization process was used to improve the Se content in the thin films. Se supply is one of the important parameters for one-target radio frequency (RF) magnetron sputtering following the selenization using the rapid thermal process (RTP). RTP at higher temperatures with a low thermal budget has been widely used in the thin film manufacturing industry to activate doping impurities while at the same time minimizing the doping impurity diffusion that occurs during thermal processing. As a powerful annealing technique, the RTP process offers several advantages such as short cycle time, reduced thermal exposure and lot size flexibility compared to conventional furnaces. The RTP technique has been successfully applied to the fabrication of CIGS thin film solar cells [8]. The characteristics of one-target sputtered CIGS thin film following the selenization using different Se supplies were discussed. The influence of Se supply on compositional, structural and morphological properties of Cu(In,Ga)Se2 thin film has been studied. 2. Experimental CIGS precursors were deposited by RF magnetron sputtering process from a quaternary CuIn0.7Ga0.3Se2 target (2 in. and 4 mm thick). The soda lime glass substrates (25 mm 25 mm) were ultrasonically cleaned in acetone, distilled water, and ethanol, and dried in a nitrogen gas stream before being put into a vacuum chamber. The deposition chamber was evacuated to a background pressure of 2.0 10 4 Pa, using a turbo molecular pump (TMP).
22
H. Kong et al. / Materials Letters 118 (2014) 21–23
Soda lime glass samples with a 1 μm thick layer of sputtered Mo were used as substrates. The substrates were placed on a rotating pallet and the distance between the target and the substrate was fixed as 6 cm. The CIGS precursors were deposited on Mo-coated soda lime glass using high-purity argon (20 sccm) discharged with an RF power of 80 W. The working pressure was at 0.4 Pa. Before the deposition process, the pre-sputter process was done. The deposition time for the CIGS thin films was one hour. The selenization of CIGS precursors was performed by using a rapid thermal process in a customized halogen lamp furnace with a quartz tube as reaction chamber. The samples were rapidly heated in a graphite box by radiation from the front to about 550 1C and kept at that stage for 20 min. The internal volume of the graphite box was 100 ml3. The pressure in the furnace was adjusted by nitrogen flow rate and rotary pump valve. At the start of selenization, the base pressure in the furnace was maintained at about 10 mTorr. In this work, to optimize the initial usage amount of the Se powder (0 mg, 5 mg, 10 mg, and 20 mg), the external selenization pressure in the furnace was fixed at 2 Torr. The crystalline structures of the CIGS thin films were analyzed by X-ray diffraction (XRD) using Cu Kα radiation (D/MAX-2550 V, Rigaku Co.) from 101 to 701. Raman scattering experiments were performed with a micro-Raman spectrometer (Jobin–Yvon LabRAM HR 800UV). The surface micrographs and the composition of these thin films were determined by a field emission scanning electron microscopy (FESEM: Philips XL30FEG) with an energy dispersive X-ray (EDX) analyzer.
Fig. 1. XRD patterns of CIGS thin films selenized using different Se powders. Inset: The values of FWHM of (1 1 2) for all CIGS films.
3. Results and discussion Table 1 shows the chemical composition of CIGS thin films selenized using different Se powders. These average element compositions were measured by EDX at 2 different areas. It can be seen in Table 1 that the film selenized using 0 mg and 5 mg is deficient in the Se element. It can be assigned to the lower sputtering yield of the Se element and the volatilization of the Se element during the deposition process and the selenized process. The films selenized using 20 mg are slightly Se-rich. Thus, 10 mg of Se powder for usage is the desired candidate. Furthermore, the one-target sputtering process usually results in Cu-poor in CIGS thin films. It is due to the fact that the sputtering yield of the Cu element is lower than that of In and Ga according to the sputtering theory, which was first proposed by Sigmund [9]. Fig. 1 presents XRD patterns of CIGS thin films selenized using different Se powders. As shown in the figure, three diffraction peaks are located at 26.81, 44.51, and 53.01for all the thin films. These peaks can be indexed as (112), (204) and (312) of chalcopyrite-type CIGS structure (JCPDS35-1102). All the thin films exhibit a dominant diffraction peak corresponding to CIGS (112) plane, indicating preferred orientation along the (112) plane of the film. Note that no impurity phase can be found in Fig. 1 implying that all thin films are single phase. Fig. 1 inset plots the values of Table 1 Element composition values of CIGS films determined from EDX analysis. Sample
In/Ga ratio
Metals/Se ratio
25.29 18.89 8.46 47.36 0.92 22.93 18.95 8.23 49.89 0.84 22.52 18.67 8.54 50.27 0.83
2.23 2.30 2.18
1.11 1.00 0.99
20.97 18.63 8.19 52.21 0.78
2.27
0.92
Cu 0 mg Se 5 mg Se 10 mg Se 20 mg Se
Cu/(In þ Ga) ratio
At% In
Ga
Se
Fig. 2. Raman spectra of CIGS thin films selenized using different Se powders. Inset: The values of FWHM of A1 mode peak for all CIGS films.
full width at half maximum (FWHM) of (112) for these CIGS films selenized using different amounts of the Se powder. As shown, the samples selenized using 5 mg and 10 mg of the Se powder have better crystallinity due to the low values of FWHM. However, the films grown using 0 mg and 20 mg of the Se powder show degraded crystallinity based on their bigger FWHM. Raman scattering results for CIGS thin films are shown in Fig. 2. Evidence of the single phase character can also be found in the Raman spectra. A spectra exhibits two Raman shifts, one is located around 176 cm 1. Beside this, a weak and broad peak within 215–230 cm 1 was observed. This peak corresponds to B2 mode of chalcopyrite (CH) CIGS thin film [10]. The peak located around 176 cm 1 is assigned to A1 mode of CH CIGS thin film, which represents the vibration of the Se anions in the x–y plane with the cations at rest [10]. No impurity phase can be found in Fig. 2, implying that all thin films are single phase. The effect of Se supply on crystalline quality can also be better understood by comparing the FWHM values of A1 mode peaks. Fig. 2 inset plots the values of FWHM of A1 mode peak for these CIGS films selenized using different amounts of the Se powder. As shown in Fig. 2 inset, the same results with the XRD analysis, the samples selenized using 5 mg and 10 mg of Se powder have better crystallinity due to the low values of FWHM. 5 mg and 10 mg of the Se powder would induce better crystallinity. This may be related to the Se content of the CIGS thin films. For the sample selenized using 0 mg, the sample is significantly deficient in the Se element. For the sample
H. Kong et al. / Materials Letters 118 (2014) 21–23
23
Fig. 3. Surface morphology of CIGS films selenized using different Se powders.
selenized using 20 mg, the sample is in Se-rich state. However, the Se content of the sample selenized using 5 mg and 10 mg is near stoichiometric. Hence, Se-poor and Se-rich state could impact the microstructure of the thin films. In order to investigate the influence of the Se supply on surface morphological features of CIGS films, SEM was carried out on films as depicted in Fig. 3. As shown in Fig. 3, the morphology of the film selenized using 0 mg of Se powder is harsh. It may be due to significant deficiency of the Se element in the film. However, the film morphology selenized using 5 mg, 10 mg and 20 mg of the Se powder was evidently improved, exhibiting visible edges and facets as well as satisfactory compactness and surface. No voids can be seen in the SEM images on the surface. The grain size of the selenized films reached positively 700 nm range, which is positive for the photocurrent collection in the cell.
the samples selenized using 5 mg and 10 mg of the Se powder have better crystallinity. Furthermore, the samples selenized using Se powders have better surface morphology. In summary, 10 mg of the Se powder for usage is the desired candidate.
Acknowledgments This work was supported by the National Natural Science Foundation of China (60990312 and 61076060) and Science,Shanghai RisingStar Program (11QB1403300),Technology Commission of Shanghai Municipality (10JC1404600).We thank Shanghai Topsola Green Energy Co. Ltd. for the SEM measurements.
References 4. Conclusions CIGS thin films were deposited on Mo-coated soda lime glass (SLG) substrates by RF magnetron sputtering process from a single target. All the CIGS thin films were deposited at room temperature with post-selenization using the RTP process with different Se supply. Energy dispersive X-ray analysis results show CIGS thin films are in a Cu-poor state. The film selenized using 0 mg and 5 mg is deficient in the Se element. However, the films selenized using 20 mg are slightly Se-rich. The results of X-ray diffraction and Raman spectra analysis indicated that the samples have the chalcopyrite-type structure and are in pure phase. All the thin films exhibit a dominant diffraction peak corresponding to CIGS (112) plane, indicating preferred orientation along the (112) plane of the film. Both XRD and Raman scattering results indicated that
[1] Green MA, Emery K, Hishikawa Y, Warta W. Prog Photovolt Res Appl 2010;18:346–52. [2] Gabor AM, Tuttle JR, Albin DS, Contreras MA, Noufi R. Appl Phys Lett 1994;65:198–200. [3] Shi JH, Li ZQ, Zhang DW, Liu QQ, Sun Z, Huang SM. Prog Photovolt Res Appl 2011;19:160–4. [4] Li X, Liu W, Jiang G, Wang D, Zhu C. Mater Lett 2012;70:116–8. [5] Shirakawa S, Kannnaka Y, Hasegawa H, Kariya T, Isomura S. Jpn J Appl Phys 1999;38:4997–5002. [6] Lin PY, Fu YH. Mater Lett 2012;70:65–7. [7] He J, Sun L, Zhang K, Wang W, Jiang J, Chen Y, et al. Appl Surf Sci 2013;264:133–8. [8] Wanga X, Lia SS, Kimb WK, Yoonb S, Craciunc V, Howardc JM, et al. Sol Energy Mater Sol Cells 2006;90:2855–66. [9] Sigmund P. Phys Rev 1969;184:383–416. [10] Zhou AJ, Mei D, Kong XG, Xu XH, Feng LD, Dai XY, et al. Thin Solid Films 2012;520:6068–74. [11] Kong H, He J, Huang L, Zhu LP, Sun L, Yang PX, et al. Mater Lett 2014;116:75–8.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Influence of Se supply for selenization of Cu(In,Ga)Se2 precursors deposited by sputtering from a single quaternary target Hui Kong a,b, Jun He a, Xiankuan Meng a, Liping zhu a, Jiahua Tao a, Lin Sun a, Pingxiong Yang a,n, Junhao Chu a a Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China b Shanghai Solar Energy Technology Co., Ltd., Shanghai 200241, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 November 2013 Accepted 11 December 2013 Available online 18 December 2013
Cu(In,Ga)Se2 (CIGS) precursors were deposited on Mo-coated soda lime glass substrates by a radio frequency magnetron sputtering process from a single target. The selenization of CIGS precursor layer was performed by using a rapid thermal process. Energy dispersive X-ray analysis results show CIGS thin films are in a Cu-poor state. The films selenized with 0 mg and 5 mg are deficient in the Se element. However, films selenized using 20 mg show a slightly rich Se content. The results of X-ray diffraction and Raman spectra analysis indicate that the samples are the chalcopyrite-type structures and in pure phase. Both XRD and Raman scattering results indicate the samples selenized using 5 mg and 10 mg of the Se powder have better crystallinity. Furthermore, the samples selenized using Se powders have better surface morphology. In summary, 10 mg of Se powder for usage is the desired candidate. & 2013 Elsevier B.V. All rights reserved.
Keywords: CIGS Sputtering Solar energy materials Thin films
1. Introduction Cu(In,Ga)Se2 (CIGS) has attracted much attention as one of the most promising absorber materials for thin film solar cells due to its tunable band gap (1.04–1.67 eV), high absorption coefficient ( 105 cm 1), high conversion efficiency and excellent antiirradiation performance [1]. CIGS thin films have been prepared by a variety of approaches, such as co-evaporation [1,2], sputtering [3,4], and chemical deposition [5,6]. Until now, co-evaporation has been the most successful technique for the preparation of CIGS, which has achieved the highest efficiency CIGS-based solar cells [1]. However, it seems difficult to scale up nowadays because of its complicated processing steps and difficulties in scaling up for large area devices with uniform compositions. On the other hand, among these methods, sputtering technique is potentially suitable for obtaining good quality, large area CIGS precursor films since the sputtering method typically provides a uniform composition of thin film, smooth surface, and is a simple process [3,7]. The result of our previous work [11] has shown that CIGS thin films deposited by the RF magnetron sputtering process from a single target without the selenization process are deficient in the Se element. It can be assigned to the lower sputtering yield and the volatilization of the Se element during the deposition process. CIGS thin film in a Se-poor state could impact the microstructure and optical
n
Corresponding author. Tel.: þ 86 21 54345157; fax: þ86 21 54345119. E-mail address: [email protected] (P. Yang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.050
properties. Hence, in this work the selenization process was used to improve the Se content in the thin films. Se supply is one of the important parameters for one-target radio frequency (RF) magnetron sputtering following the selenization using the rapid thermal process (RTP). RTP at higher temperatures with a low thermal budget has been widely used in the thin film manufacturing industry to activate doping impurities while at the same time minimizing the doping impurity diffusion that occurs during thermal processing. As a powerful annealing technique, the RTP process offers several advantages such as short cycle time, reduced thermal exposure and lot size flexibility compared to conventional furnaces. The RTP technique has been successfully applied to the fabrication of CIGS thin film solar cells [8]. The characteristics of one-target sputtered CIGS thin film following the selenization using different Se supplies were discussed. The influence of Se supply on compositional, structural and morphological properties of Cu(In,Ga)Se2 thin film has been studied. 2. Experimental CIGS precursors were deposited by RF magnetron sputtering process from a quaternary CuIn0.7Ga0.3Se2 target (2 in. and 4 mm thick). The soda lime glass substrates (25 mm 25 mm) were ultrasonically cleaned in acetone, distilled water, and ethanol, and dried in a nitrogen gas stream before being put into a vacuum chamber. The deposition chamber was evacuated to a background pressure of 2.0 10 4 Pa, using a turbo molecular pump (TMP).
22
H. Kong et al. / Materials Letters 118 (2014) 21–23
Soda lime glass samples with a 1 μm thick layer of sputtered Mo were used as substrates. The substrates were placed on a rotating pallet and the distance between the target and the substrate was fixed as 6 cm. The CIGS precursors were deposited on Mo-coated soda lime glass using high-purity argon (20 sccm) discharged with an RF power of 80 W. The working pressure was at 0.4 Pa. Before the deposition process, the pre-sputter process was done. The deposition time for the CIGS thin films was one hour. The selenization of CIGS precursors was performed by using a rapid thermal process in a customized halogen lamp furnace with a quartz tube as reaction chamber. The samples were rapidly heated in a graphite box by radiation from the front to about 550 1C and kept at that stage for 20 min. The internal volume of the graphite box was 100 ml3. The pressure in the furnace was adjusted by nitrogen flow rate and rotary pump valve. At the start of selenization, the base pressure in the furnace was maintained at about 10 mTorr. In this work, to optimize the initial usage amount of the Se powder (0 mg, 5 mg, 10 mg, and 20 mg), the external selenization pressure in the furnace was fixed at 2 Torr. The crystalline structures of the CIGS thin films were analyzed by X-ray diffraction (XRD) using Cu Kα radiation (D/MAX-2550 V, Rigaku Co.) from 101 to 701. Raman scattering experiments were performed with a micro-Raman spectrometer (Jobin–Yvon LabRAM HR 800UV). The surface micrographs and the composition of these thin films were determined by a field emission scanning electron microscopy (FESEM: Philips XL30FEG) with an energy dispersive X-ray (EDX) analyzer.
Fig. 1. XRD patterns of CIGS thin films selenized using different Se powders. Inset: The values of FWHM of (1 1 2) for all CIGS films.
3. Results and discussion Table 1 shows the chemical composition of CIGS thin films selenized using different Se powders. These average element compositions were measured by EDX at 2 different areas. It can be seen in Table 1 that the film selenized using 0 mg and 5 mg is deficient in the Se element. It can be assigned to the lower sputtering yield of the Se element and the volatilization of the Se element during the deposition process and the selenized process. The films selenized using 20 mg are slightly Se-rich. Thus, 10 mg of Se powder for usage is the desired candidate. Furthermore, the one-target sputtering process usually results in Cu-poor in CIGS thin films. It is due to the fact that the sputtering yield of the Cu element is lower than that of In and Ga according to the sputtering theory, which was first proposed by Sigmund [9]. Fig. 1 presents XRD patterns of CIGS thin films selenized using different Se powders. As shown in the figure, three diffraction peaks are located at 26.81, 44.51, and 53.01for all the thin films. These peaks can be indexed as (112), (204) and (312) of chalcopyrite-type CIGS structure (JCPDS35-1102). All the thin films exhibit a dominant diffraction peak corresponding to CIGS (112) plane, indicating preferred orientation along the (112) plane of the film. Note that no impurity phase can be found in Fig. 1 implying that all thin films are single phase. Fig. 1 inset plots the values of Table 1 Element composition values of CIGS films determined from EDX analysis. Sample
In/Ga ratio
Metals/Se ratio
25.29 18.89 8.46 47.36 0.92 22.93 18.95 8.23 49.89 0.84 22.52 18.67 8.54 50.27 0.83
2.23 2.30 2.18
1.11 1.00 0.99
20.97 18.63 8.19 52.21 0.78
2.27
0.92
Cu 0 mg Se 5 mg Se 10 mg Se 20 mg Se
Cu/(In þ Ga) ratio
At% In
Ga
Se
Fig. 2. Raman spectra of CIGS thin films selenized using different Se powders. Inset: The values of FWHM of A1 mode peak for all CIGS films.
full width at half maximum (FWHM) of (112) for these CIGS films selenized using different amounts of the Se powder. As shown, the samples selenized using 5 mg and 10 mg of the Se powder have better crystallinity due to the low values of FWHM. However, the films grown using 0 mg and 20 mg of the Se powder show degraded crystallinity based on their bigger FWHM. Raman scattering results for CIGS thin films are shown in Fig. 2. Evidence of the single phase character can also be found in the Raman spectra. A spectra exhibits two Raman shifts, one is located around 176 cm 1. Beside this, a weak and broad peak within 215–230 cm 1 was observed. This peak corresponds to B2 mode of chalcopyrite (CH) CIGS thin film [10]. The peak located around 176 cm 1 is assigned to A1 mode of CH CIGS thin film, which represents the vibration of the Se anions in the x–y plane with the cations at rest [10]. No impurity phase can be found in Fig. 2, implying that all thin films are single phase. The effect of Se supply on crystalline quality can also be better understood by comparing the FWHM values of A1 mode peaks. Fig. 2 inset plots the values of FWHM of A1 mode peak for these CIGS films selenized using different amounts of the Se powder. As shown in Fig. 2 inset, the same results with the XRD analysis, the samples selenized using 5 mg and 10 mg of Se powder have better crystallinity due to the low values of FWHM. 5 mg and 10 mg of the Se powder would induce better crystallinity. This may be related to the Se content of the CIGS thin films. For the sample selenized using 0 mg, the sample is significantly deficient in the Se element. For the sample
H. Kong et al. / Materials Letters 118 (2014) 21–23
23
Fig. 3. Surface morphology of CIGS films selenized using different Se powders.
selenized using 20 mg, the sample is in Se-rich state. However, the Se content of the sample selenized using 5 mg and 10 mg is near stoichiometric. Hence, Se-poor and Se-rich state could impact the microstructure of the thin films. In order to investigate the influence of the Se supply on surface morphological features of CIGS films, SEM was carried out on films as depicted in Fig. 3. As shown in Fig. 3, the morphology of the film selenized using 0 mg of Se powder is harsh. It may be due to significant deficiency of the Se element in the film. However, the film morphology selenized using 5 mg, 10 mg and 20 mg of the Se powder was evidently improved, exhibiting visible edges and facets as well as satisfactory compactness and surface. No voids can be seen in the SEM images on the surface. The grain size of the selenized films reached positively 700 nm range, which is positive for the photocurrent collection in the cell.
the samples selenized using 5 mg and 10 mg of the Se powder have better crystallinity. Furthermore, the samples selenized using Se powders have better surface morphology. In summary, 10 mg of the Se powder for usage is the desired candidate.
Acknowledgments This work was supported by the National Natural Science Foundation of China (60990312 and 61076060) and Science,Shanghai RisingStar Program (11QB1403300),Technology Commission of Shanghai Municipality (10JC1404600).We thank Shanghai Topsola Green Energy Co. Ltd. for the SEM measurements.
References 4. Conclusions CIGS thin films were deposited on Mo-coated soda lime glass (SLG) substrates by RF magnetron sputtering process from a single target. All the CIGS thin films were deposited at room temperature with post-selenization using the RTP process with different Se supply. Energy dispersive X-ray analysis results show CIGS thin films are in a Cu-poor state. The film selenized using 0 mg and 5 mg is deficient in the Se element. However, the films selenized using 20 mg are slightly Se-rich. The results of X-ray diffraction and Raman spectra analysis indicated that the samples have the chalcopyrite-type structure and are in pure phase. All the thin films exhibit a dominant diffraction peak corresponding to CIGS (112) plane, indicating preferred orientation along the (112) plane of the film. Both XRD and Raman scattering results indicated that
[1] Green MA, Emery K, Hishikawa Y, Warta W. Prog Photovolt Res Appl 2010;18:346–52. [2] Gabor AM, Tuttle JR, Albin DS, Contreras MA, Noufi R. Appl Phys Lett 1994;65:198–200. [3] Shi JH, Li ZQ, Zhang DW, Liu QQ, Sun Z, Huang SM. Prog Photovolt Res Appl 2011;19:160–4. [4] Li X, Liu W, Jiang G, Wang D, Zhu C. Mater Lett 2012;70:116–8. [5] Shirakawa S, Kannnaka Y, Hasegawa H, Kariya T, Isomura S. Jpn J Appl Phys 1999;38:4997–5002. [6] Lin PY, Fu YH. Mater Lett 2012;70:65–7. [7] He J, Sun L, Zhang K, Wang W, Jiang J, Chen Y, et al. Appl Surf Sci 2013;264:133–8. [8] Wanga X, Lia SS, Kimb WK, Yoonb S, Craciunc V, Howardc JM, et al. Sol Energy Mater Sol Cells 2006;90:2855–66. [9] Sigmund P. Phys Rev 1969;184:383–416. [10] Zhou AJ, Mei D, Kong XG, Xu XH, Feng LD, Dai XY, et al. Thin Solid Films 2012;520:6068–74. [11] Kong H, He J, Huang L, Zhu LP, Sun L, Yang PX, et al. Mater Lett 2014;116:75–8.