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Preparation of perfect chalcopyrite ordering CuInS2 thin films by high-temperature sulfurization of metal oxide nanoparticles
Materials Letters 156 (2015) 153–155
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of perfect chalcopyrite ordering CuInS2 thin films by high-temperature sulfurization of metal oxide nanoparticles Jiwan Liu, Jianmin Li, Guoshun Jiang, Weifeng Liu n, Changfei Zhu n CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2015 Accepted 28 April 2015 Available online 7 May 2015
In this letter, a low-cost non-vacuum process is introduced to fabricate the pure crystalline chalcopyrite (CH) CuInS2 thin films. Since the Chalcopyrite (CH) and CuAu (CA) ordering phases have very close formation energy and structures and tend to coexist in CIS films, it is very difficult to get single-phase chalcopyrite CIS in normal way. Simply, in this study, the high-temperature sulfurization of copper indium oxide nanoparticles precursor layers has been taken out and the presence of the metastable CuAu (CA) ordering phase is successfully avoided in the chalcopyrite CIS thin films. In addition, films sulfurized at high temperature show pure chalcopyrite phase, good crystallization and reliable optical properties, which are suitable for preparation of high efficiency solar cells. & 2015 Elsevier B.V. All rights reserved.
Keywords: Thin films Semiconductors Raman Solar energy materials
1. Introduction Thin film deposition of chalcopyrites has become an important technology to realize low production costs of solar cells with high conversion efficiency. I–III–VI2 semiconducting materials of the chalcopyrite structure have attracted intensive attention on account of their excellent photovoltaic properties. Among ternary chalcopyrite semiconductors, CuInS2 (CIS) is one of the most promising chalcopyrites on consideration of its stability and less toxicity. CIS has good structural defects tolerance, large absorption coefficient of the order of 105 cm 1 and direct band gap nearly 1.53 eV coming near the ideal band gap (1.5 eV) for highest conversion efficiency [1,2]. With theoretically high power conversion efficiency (PCE) of 28.5% [3], current CuInS2 based cells solar energy conversion efficiencies just 12.5% has been obtained by a sputtering process [4]. Further improvements have not been reported. The main reason for this rather low efficiency is that CIS is a multicomponent compound with a complex phase diagram. Such kinds of compounds are of particular interest for the formation of the optoelectronic materials due to specific anionic structural fragments [5]. As the main polymorphs, CH and CA ordering phases have very close formation energy and structures. The difference of formation energy between these two phases is only 2 meV/atom [6], as a consequence, CAordering and CH-ordering coexist in CIS films frequently. It is very difficult to get pure chalcopyrite CIS. There are some intrinsic cationic defects always presented in such kinds of compounds, such as V 0cu , V ″in' , Cu″in , In″cu et al. Specifically, CA ordering is associated with the
n
Corresponding authors. E-mail addresses: [email protected] (W. Liu), [email protected] (C. Zhu).
http://dx.doi.org/10.1016/j.matlet.2015.04.146 0167-577X/& 2015 Elsevier B.V. All rights reserved.
concentration of sulfur vacancies ðV ″S Þ. A high concentration of V ″S is connected to a high concentration of anti-site defects, i.e. Cu″in and In″cu [7]. The related anti-site defects damage the quality of the CIS layers and the overall efficiency of the cells [8]. Up to now, many techniques have been proposed and investigated for the deposition of CIS absorber layer such as co-evaporation [8], sputtering [4], electrodeposition [2], spraying [9] and doctor blading [10]. Among them, the doctor blading method would be promising in terms of a fast process, efficient use of the material and application of continuous roll-to-roll deposition. Especially, it is also a low-cost and simple attractive non-vacuum technique for sulfurizing metal oxides precursors films. However, there are two problems with using metal oxides as precursor materials for CIS solar cells. One problem is the presence of CA ordered second phase, which is widespread in a variety of deposition techniques. Another difficulty is the conversion of oxide precursors to chalcogenide films. The compound In2O3 is stable and difficult to reduction [11], which would result in poor quality of CIS films and detrimental the solar cells efficiency. In this study, we aim to synthesize high quality CIS thin films with single CH structure. Annealing process was carried out at seven temperatures from 450 to 680 1C. The relation between annealing temperatures and the properties of sulfurized CIS films has also been studied in details. Through increasing sulfurization temperature, not only In2O3 was removed easily but also pure CHordering phase was obtained successfully.
2. Experimental Oxides precursors were prepared free mechanochemical method. The
by a novel solventprocess started with
154
J. Liu et al. / Materials Letters 156 (2015) 153–155
stoichiometric copper nitrate hydrate Cu(NO3)2 3H2O and indium nitratehyd rate In(NO3)3 4.5H2O and superfluous ammonium bicarbonate (NH4HCO3). The quantified amount of mixed nitrates and excess ammonium bicarbonate were mixed completely and ground in a mortar and pestle for 20 min and then baked at 380 1C for half an hour. The mixing CuO, In2O3 powder product was put in dispersant alcohol. After stirring evenly, the slurry for precursor film was prepared. Knife coating was used to deposit the films. After coating, the oxide precursor thin films were sulfurized using a commercial two zone furnace system in the S2(s) atmosphere. The substrate temperatures varied from 450 to 680 1C for 30 min. In this process, heating rate was about 50 1C/min and the two zone furnace temperature status was kept identically. The crystalline quality and the phases of the films were measured by x-ray diffraction (XRD) (D/Max-rA) patterns. Raman scattering (LABRAM-HR micro-Raman system, JY, Paris, France, 514 nm) analysis was used for further identifying the phases in the films. The morphologies of the films were observed by scanning electron microscopy (SEM) (SEM: JEOL-JSM-6700F). The optical absorbance of the CIS thin films were measured with a UV–visible spectrophotometer (SOLID 3700) from 400 nm to 1600 nm.
main modes in Raman spectra [12,13]. Annealing at temperatures below about 620 1C, both phases are visible (Fig. 2) with characteristic peak at 290 cm 1 for CH phase (space group I42d), and at 303 cm 1 (þ/ 2 cm 1) characteristic for CuAu phase that is consistent well with literatures. In the work of Alvarez-Garcia [8], a simple linear law is used to determine the ratio XCA/XCH of each film depending on the intensity of Raman peaks. The ratio of the two peak intensity gets bigger gradually along with increasing of the temperature. When sulfurized temperature surpass 620 1C, the peak at 290 cm 1 of CH-ordering phase becomes very strong with small FWHM and the peak at 303 cm 1 belonging to CA-ordering is almost disappeared, which means better crystallization quality of CHordering phase. This result suggests that higher sulfurized temperature can facilitate a transformation from CA-ordering to CH-ordering phase. Particularly, the 290 cm 1 peak is approximately symmetric while the annealing temperature around 680 1C. Single CH-ordering phase crystal can be obtained by higher annealing temperature, which is in good agreement with the literature [12]. Besides, Raman peak at 494 cm 1 is detected expectedly for the films annealed under temperature 500 1C. This information
3. Results and discussion X-ray diffraction analysis allows the estimation of the relative amount of the phase domains. All samples show (112) preferential orientation corresponding to PDF#47-1372. No second phase is observed. As temperature increasing, the (112) peak gets sharper obviously. Fig. 1(a) shows the X-ray diffraction patterns of the CuInS2 absorber thin films. The FWHM values of the peaks are shown in Fig. 1(b). The full-width at half maximum (FWHM) of (112) peak was analyzed with Miller index. With the increasing of temperature, the FWHM decreases significantly indicating that annealing temperature has a crucial influence on the crystal growth. As we know, smaller FWHM means larger grain size and better crystallinity. For 680 1C, the film has the best crystallization quality. Since the crystallographic CuAu and CH structures are very similar [6], it is difficult to use XRD for determination of the phase proportion in non-epitaxial films or crystals. Raman spectroscopy is more appropriate to confirm CuInS2 thin film. Raman spectra of samples annealed at seven different temperatures were revealed in Fig. 2. A1 modes of CIS crystals are reported to be observed as the
Fig. 2. Raman spectra of CuInS2 films with different sulfurization temperatures. Peaks of In2O3 and both chalcopyrite and CuAu-ordered phases of A1 modes are marked.
Fig. 1. (a) XRD patterns for the CuInS2 films with different sulfurization temperature; (b) FWHM curve corresponding to XRD plot.
J. Liu et al. / Materials Letters 156 (2015) 153–155
155
Fig. 3. SEM micrographs of CIS films annealed at 450–680 1C (a–g).
As is shown, Eg distributes at a range of 1.36–1.41 eV, which is in good agreement with the values of 1.21–1.45 eV reported by other researchers [10].
4. Conclusion
Fig. 4. (ahν)2 as a function of hν curves of CuInS2 thin films at various annealing temperatures. The absorb spectra are shown in inset.
illustrates the existence of In2O3 secondary phase, just as Kaelin et al. have revealed, In2O3 is very stable and hard to be reduced [11]. Fortunately, In2O3 phase disappears for films annealed under temperature upon 550 1C. The reactions can take place because of the high sulfur vapor pressure at high sulfurization temperature. There are three weak peaks at 240, 323, and 345 cm 1 for all samples. They can be assigned to E3TO and B22TO , E1TO and B12TO , B12LO and E1LO modes of the chalcopyrite phase of CuInS2 [9]. Therefore, the pure CH phase of CIS thin film without In2O3 second phase can be obtained by using high temperature sulfurization. The films can be a good absorber layer for high efficiency solar cell preparation. Fig. 3 shows SEM micrographs of CIS films annealed at different temperatures. As sulfurization temperature increasing from 450 to 680 1C, crystal grains of the CIS films grow up obviously. The result coincides with the results of XRD and Raman. Films show a good crystallization upon 620 1C. The optical band-gap (Eg) is determined from the absorption spectra of the CIS layers. For the direct band gap semiconductors α related as (αhν)2 ¼A(αhν Eg), where A is a constant which depends on the transition probability, Eg is the optical band gap energy, h is Plank's constant and ν is the frequency. The intercept of the linear portion on energy axis at (αhν)2 equal to zero gives the band gap energy. Fig. 4 is the plot of (αhν)2 versus photon energy (hν) of the CIS absorber thin films with different sulfurization temperatures. The inset shows the optical absorption spectra.
In this work, CIS films have been fabricated by sulfursulfurization of oxide based precursor films prepared through doctor blading the Cu, In oxides nanoparticles synthesized via a solvent-free solid-state reaction. The effects of sulfurization temperatures on the properties of CIS films have been studied. According to XRD and SEM, higher-temperature sulfurization can easily improve the crystallinity of films, and no obviously binary phases such as CuS and InS are observed. Raman spectroscopy is introduced to detect the CH and the CA phases, and the results demonstrate that In2O3 impurity phase and CA-ordering CIS second phases are removed effectively by high-temperature sulfurization. The band gaps at the range of 1.36–1.41 eV are determined by absorption spectroscopy measurement. In general, the pure chalcopyrite structure CIS thin films with good crystallinity have been obtained by this novel way, which can be expected to be used for preparation of high-efficiency solar cells.
Acknowledgment This work was supported by National Basic Research Program of China (973 Program) 2012CB922001. References [1] Klenka R, Klaera J, Scheera R, Lux-Steinera MCh, Luckb I, et al. Thin Solid Films 2005;480–481:509–14. [2] Martinez AM, Fernández AM, Arriaga LG, Canob U. Mater Chem Phys 2002;403–404:1–8. [3] Siebentritt Susanne. Thin Solid Films 2002;403–404:1–8. [4] Scheer R, Klenk R, Klaer J, Luck I. Sol Energy 2004;77:777–84. [5] Romanyuk YE, Marushko LP, Piskach LV, Kityk IV, Fedorchuk AO, Pekhnyo VI, et al. CrystEngComm 2013;15:4838–43. [6] Su DS, Wei Su-Huai. Appl Phys Lett 1999;74:17. [7] Nanu M, Schoonman J, Goossens A. Thin Solid Films 2004;451–452:193–7. [8] Alvarez-Garcıá J, Rudigier E, Rega N. Thin Solid Films 2003;431–432:122–5. [9] Ojaa I, Nanub M, Katerskia A, Krunksa M, Merea A, Raudojaa J, et al. Thin Solid Films 2005;480–481:82–6. [10] Xu JG, Wang YL. Mater Lett 2013;99:90–3. [11] Kaelin M, Rudmann D, Tiwari AN. Sol Energy 2004;77:749–56. [12] Rudigier E, Barcones B, Luck I, Jawhari-Colin T, Pérez-Rodríguez A, Scheer R. J Appl Phys 2004;95:5153. [13] Wu KJ, Wang DL. Phys Status Solidi A 2011;208(12):2730–6.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of perfect chalcopyrite ordering CuInS2 thin films by high-temperature sulfurization of metal oxide nanoparticles Jiwan Liu, Jianmin Li, Guoshun Jiang, Weifeng Liu n, Changfei Zhu n CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2015 Accepted 28 April 2015 Available online 7 May 2015
In this letter, a low-cost non-vacuum process is introduced to fabricate the pure crystalline chalcopyrite (CH) CuInS2 thin films. Since the Chalcopyrite (CH) and CuAu (CA) ordering phases have very close formation energy and structures and tend to coexist in CIS films, it is very difficult to get single-phase chalcopyrite CIS in normal way. Simply, in this study, the high-temperature sulfurization of copper indium oxide nanoparticles precursor layers has been taken out and the presence of the metastable CuAu (CA) ordering phase is successfully avoided in the chalcopyrite CIS thin films. In addition, films sulfurized at high temperature show pure chalcopyrite phase, good crystallization and reliable optical properties, which are suitable for preparation of high efficiency solar cells. & 2015 Elsevier B.V. All rights reserved.
Keywords: Thin films Semiconductors Raman Solar energy materials
1. Introduction Thin film deposition of chalcopyrites has become an important technology to realize low production costs of solar cells with high conversion efficiency. I–III–VI2 semiconducting materials of the chalcopyrite structure have attracted intensive attention on account of their excellent photovoltaic properties. Among ternary chalcopyrite semiconductors, CuInS2 (CIS) is one of the most promising chalcopyrites on consideration of its stability and less toxicity. CIS has good structural defects tolerance, large absorption coefficient of the order of 105 cm 1 and direct band gap nearly 1.53 eV coming near the ideal band gap (1.5 eV) for highest conversion efficiency [1,2]. With theoretically high power conversion efficiency (PCE) of 28.5% [3], current CuInS2 based cells solar energy conversion efficiencies just 12.5% has been obtained by a sputtering process [4]. Further improvements have not been reported. The main reason for this rather low efficiency is that CIS is a multicomponent compound with a complex phase diagram. Such kinds of compounds are of particular interest for the formation of the optoelectronic materials due to specific anionic structural fragments [5]. As the main polymorphs, CH and CA ordering phases have very close formation energy and structures. The difference of formation energy between these two phases is only 2 meV/atom [6], as a consequence, CAordering and CH-ordering coexist in CIS films frequently. It is very difficult to get pure chalcopyrite CIS. There are some intrinsic cationic defects always presented in such kinds of compounds, such as V 0cu , V ″in' , Cu″in , In″cu et al. Specifically, CA ordering is associated with the
n
Corresponding authors. E-mail addresses: [email protected] (W. Liu), [email protected] (C. Zhu).
http://dx.doi.org/10.1016/j.matlet.2015.04.146 0167-577X/& 2015 Elsevier B.V. All rights reserved.
concentration of sulfur vacancies ðV ″S Þ. A high concentration of V ″S is connected to a high concentration of anti-site defects, i.e. Cu″in and In″cu [7]. The related anti-site defects damage the quality of the CIS layers and the overall efficiency of the cells [8]. Up to now, many techniques have been proposed and investigated for the deposition of CIS absorber layer such as co-evaporation [8], sputtering [4], electrodeposition [2], spraying [9] and doctor blading [10]. Among them, the doctor blading method would be promising in terms of a fast process, efficient use of the material and application of continuous roll-to-roll deposition. Especially, it is also a low-cost and simple attractive non-vacuum technique for sulfurizing metal oxides precursors films. However, there are two problems with using metal oxides as precursor materials for CIS solar cells. One problem is the presence of CA ordered second phase, which is widespread in a variety of deposition techniques. Another difficulty is the conversion of oxide precursors to chalcogenide films. The compound In2O3 is stable and difficult to reduction [11], which would result in poor quality of CIS films and detrimental the solar cells efficiency. In this study, we aim to synthesize high quality CIS thin films with single CH structure. Annealing process was carried out at seven temperatures from 450 to 680 1C. The relation between annealing temperatures and the properties of sulfurized CIS films has also been studied in details. Through increasing sulfurization temperature, not only In2O3 was removed easily but also pure CHordering phase was obtained successfully.
2. Experimental Oxides precursors were prepared free mechanochemical method. The
by a novel solventprocess started with
154
J. Liu et al. / Materials Letters 156 (2015) 153–155
stoichiometric copper nitrate hydrate Cu(NO3)2 3H2O and indium nitratehyd rate In(NO3)3 4.5H2O and superfluous ammonium bicarbonate (NH4HCO3). The quantified amount of mixed nitrates and excess ammonium bicarbonate were mixed completely and ground in a mortar and pestle for 20 min and then baked at 380 1C for half an hour. The mixing CuO, In2O3 powder product was put in dispersant alcohol. After stirring evenly, the slurry for precursor film was prepared. Knife coating was used to deposit the films. After coating, the oxide precursor thin films were sulfurized using a commercial two zone furnace system in the S2(s) atmosphere. The substrate temperatures varied from 450 to 680 1C for 30 min. In this process, heating rate was about 50 1C/min and the two zone furnace temperature status was kept identically. The crystalline quality and the phases of the films were measured by x-ray diffraction (XRD) (D/Max-rA) patterns. Raman scattering (LABRAM-HR micro-Raman system, JY, Paris, France, 514 nm) analysis was used for further identifying the phases in the films. The morphologies of the films were observed by scanning electron microscopy (SEM) (SEM: JEOL-JSM-6700F). The optical absorbance of the CIS thin films were measured with a UV–visible spectrophotometer (SOLID 3700) from 400 nm to 1600 nm.
main modes in Raman spectra [12,13]. Annealing at temperatures below about 620 1C, both phases are visible (Fig. 2) with characteristic peak at 290 cm 1 for CH phase (space group I42d), and at 303 cm 1 (þ/ 2 cm 1) characteristic for CuAu phase that is consistent well with literatures. In the work of Alvarez-Garcia [8], a simple linear law is used to determine the ratio XCA/XCH of each film depending on the intensity of Raman peaks. The ratio of the two peak intensity gets bigger gradually along with increasing of the temperature. When sulfurized temperature surpass 620 1C, the peak at 290 cm 1 of CH-ordering phase becomes very strong with small FWHM and the peak at 303 cm 1 belonging to CA-ordering is almost disappeared, which means better crystallization quality of CHordering phase. This result suggests that higher sulfurized temperature can facilitate a transformation from CA-ordering to CH-ordering phase. Particularly, the 290 cm 1 peak is approximately symmetric while the annealing temperature around 680 1C. Single CH-ordering phase crystal can be obtained by higher annealing temperature, which is in good agreement with the literature [12]. Besides, Raman peak at 494 cm 1 is detected expectedly for the films annealed under temperature 500 1C. This information
3. Results and discussion X-ray diffraction analysis allows the estimation of the relative amount of the phase domains. All samples show (112) preferential orientation corresponding to PDF#47-1372. No second phase is observed. As temperature increasing, the (112) peak gets sharper obviously. Fig. 1(a) shows the X-ray diffraction patterns of the CuInS2 absorber thin films. The FWHM values of the peaks are shown in Fig. 1(b). The full-width at half maximum (FWHM) of (112) peak was analyzed with Miller index. With the increasing of temperature, the FWHM decreases significantly indicating that annealing temperature has a crucial influence on the crystal growth. As we know, smaller FWHM means larger grain size and better crystallinity. For 680 1C, the film has the best crystallization quality. Since the crystallographic CuAu and CH structures are very similar [6], it is difficult to use XRD for determination of the phase proportion in non-epitaxial films or crystals. Raman spectroscopy is more appropriate to confirm CuInS2 thin film. Raman spectra of samples annealed at seven different temperatures were revealed in Fig. 2. A1 modes of CIS crystals are reported to be observed as the
Fig. 2. Raman spectra of CuInS2 films with different sulfurization temperatures. Peaks of In2O3 and both chalcopyrite and CuAu-ordered phases of A1 modes are marked.
Fig. 1. (a) XRD patterns for the CuInS2 films with different sulfurization temperature; (b) FWHM curve corresponding to XRD plot.
J. Liu et al. / Materials Letters 156 (2015) 153–155
155
Fig. 3. SEM micrographs of CIS films annealed at 450–680 1C (a–g).
As is shown, Eg distributes at a range of 1.36–1.41 eV, which is in good agreement with the values of 1.21–1.45 eV reported by other researchers [10].
4. Conclusion
Fig. 4. (ahν)2 as a function of hν curves of CuInS2 thin films at various annealing temperatures. The absorb spectra are shown in inset.
illustrates the existence of In2O3 secondary phase, just as Kaelin et al. have revealed, In2O3 is very stable and hard to be reduced [11]. Fortunately, In2O3 phase disappears for films annealed under temperature upon 550 1C. The reactions can take place because of the high sulfur vapor pressure at high sulfurization temperature. There are three weak peaks at 240, 323, and 345 cm 1 for all samples. They can be assigned to E3TO and B22TO , E1TO and B12TO , B12LO and E1LO modes of the chalcopyrite phase of CuInS2 [9]. Therefore, the pure CH phase of CIS thin film without In2O3 second phase can be obtained by using high temperature sulfurization. The films can be a good absorber layer for high efficiency solar cell preparation. Fig. 3 shows SEM micrographs of CIS films annealed at different temperatures. As sulfurization temperature increasing from 450 to 680 1C, crystal grains of the CIS films grow up obviously. The result coincides with the results of XRD and Raman. Films show a good crystallization upon 620 1C. The optical band-gap (Eg) is determined from the absorption spectra of the CIS layers. For the direct band gap semiconductors α related as (αhν)2 ¼A(αhν Eg), where A is a constant which depends on the transition probability, Eg is the optical band gap energy, h is Plank's constant and ν is the frequency. The intercept of the linear portion on energy axis at (αhν)2 equal to zero gives the band gap energy. Fig. 4 is the plot of (αhν)2 versus photon energy (hν) of the CIS absorber thin films with different sulfurization temperatures. The inset shows the optical absorption spectra.
In this work, CIS films have been fabricated by sulfursulfurization of oxide based precursor films prepared through doctor blading the Cu, In oxides nanoparticles synthesized via a solvent-free solid-state reaction. The effects of sulfurization temperatures on the properties of CIS films have been studied. According to XRD and SEM, higher-temperature sulfurization can easily improve the crystallinity of films, and no obviously binary phases such as CuS and InS are observed. Raman spectroscopy is introduced to detect the CH and the CA phases, and the results demonstrate that In2O3 impurity phase and CA-ordering CIS second phases are removed effectively by high-temperature sulfurization. The band gaps at the range of 1.36–1.41 eV are determined by absorption spectroscopy measurement. In general, the pure chalcopyrite structure CIS thin films with good crystallinity have been obtained by this novel way, which can be expected to be used for preparation of high-efficiency solar cells.
Acknowledgment This work was supported by National Basic Research Program of China (973 Program) 2012CB922001. References [1] Klenka R, Klaera J, Scheera R, Lux-Steinera MCh, Luckb I, et al. Thin Solid Films 2005;480–481:509–14. [2] Martinez AM, Fernández AM, Arriaga LG, Canob U. Mater Chem Phys 2002;403–404:1–8. [3] Siebentritt Susanne. Thin Solid Films 2002;403–404:1–8. [4] Scheer R, Klenk R, Klaer J, Luck I. Sol Energy 2004;77:777–84. [5] Romanyuk YE, Marushko LP, Piskach LV, Kityk IV, Fedorchuk AO, Pekhnyo VI, et al. CrystEngComm 2013;15:4838–43. [6] Su DS, Wei Su-Huai. Appl Phys Lett 1999;74:17. [7] Nanu M, Schoonman J, Goossens A. Thin Solid Films 2004;451–452:193–7. [8] Alvarez-Garcıá J, Rudigier E, Rega N. Thin Solid Films 2003;431–432:122–5. [9] Ojaa I, Nanub M, Katerskia A, Krunksa M, Merea A, Raudojaa J, et al. Thin Solid Films 2005;480–481:82–6. [10] Xu JG, Wang YL. Mater Lett 2013;99:90–3. [11] Kaelin M, Rudmann D, Tiwari AN. Sol Energy 2004;77:749–56. [12] Rudigier E, Barcones B, Luck I, Jawhari-Colin T, Pérez-Rodríguez A, Scheer R. J Appl Phys 2004;95:5153. [13] Wu KJ, Wang DL. Phys Status Solidi A 2011;208(12):2730–6.