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Rapid synthesis of sphere-like Cu2ZnSnS4 microparticles by microwave irradiation
Materials Letters 86 (2012) 174–177
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Rapid synthesis of sphere-like Cu2ZnSnS4 microparticles by microwave irradiation R. Saravana Kumar, Beo Deul Ryu, S. Chandramohan, Jin Kyung Seol, Sang-Kwon Lee, Chang-Hee Hong n Semiconductor Physics Research Center, School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2012 Accepted 18 July 2012 Available online 25 July 2012
In the present work, a pioneering attempt has been made for the rapid synthesis of Cu2ZnSnS4 (CZTS) microspheres by the microwave-assisted solution method. X-ray diffraction (XRD) and Raman spectroscopy confirmed the formation of single phase CZTS with tetragonal crystal structure. Scanning electron microscope (SEM) and transmission electron microscope (TEM) showed the formation of sphere-like CZTS microparticles. Optical absorption studies revealed that CZTS microparticles have an optical band gap of 1.76 eV, which is optimal for photovoltaic applications. The simplicity, rapidity, and commercial feasibility of the current synthesis strategy would be beneficial for the low-cost and large-scale industrial production of CZTS. & 2012 Elsevier B.V. All rights reserved.
Keywords: Solar energy materials Cu2ZnSnS4 Microwave synthesis Microparticles X-ray diffraction Electron microscopy
1. Introduction Low cost, high efficiency and less environmental pollution have been regarded as the indispensable properties for the next generation solar cells materials. Chalcopyrite semiconductors such as CdTe, Cu(In,Ga)S2 and Cu(In,Ga)Se2 were the most successful commercialized thin-film photovoltaic materials with demonstrated power conversion efficiency (PCE) of nearly 20% [1]. However, concerns over their toxicity, scarcity and increasing prices of rare metals (In, Te and Ga) have motivated the research community to seek an alternate earth abundant, environmentally benign material with adequate photovoltaic properties. In search of a chalcopyrite photovoltaic absorber free of scarce and toxic elements, CZTS has sparkled tremendous research interest due to its excellent thermal, chemical, electronic, optical and mechanical properties in addition to less environmental damaging and cost effectiveness candidature [2]. As a quaternary Cu2–II–IV–VI4 compound, CZTS has emerged as a promising absorber material with intriguing properties like low-cost, direct band gap (1.5 eV), p-type conductivity, high absorption coefficient (4104 cm 1) in the visible wavelength region. In addition, all the constituent elements of CZTS are non-toxic and aplenty on the earth-crust, making it a potential candidate for the thin-film photovoltaics [3,4]. Hence, the incorporation of CZTS thin films as the absorber layer in solar cells will be a promising solution for the realization of solar cells free from both the resource saving problem and environmental pollution. To date, the reported PCE of solar cells based on CZTS is as high as 8.4% under AM1.5G illumination, while its
n
Corresponding author. E-mail address: [email protected] (C.-H. Hong).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.059
theoretical limit is 32.2% [5]. Therefore, a great amount of work has to be done in order to bring the PCE closer to the theoretical limits. Hitherto, variety of methods based on vacuum and non-vacuum techniques such as pulsed laser deposition [2], chemical vapor deposition [3], electro-deposition [4], thermal evaporation [5], sputtering [6], successive ionic layer and adsorption reaction [7], and sol– gel [8] has been utilized to fabricate CZTS thin films. Recently, a new method based on ink printing technology for scaling up solar cell production using nanoparticle inks that can be printed, sprayed, or dip-coated onto large area substrates has been developed for the chalcogenide-based absorber layer deposition [9,10]. Consequently, synthesis of CZTS nanoparticles by hot-injection [9], high-temperature arrested precipitation [10], solvothermal [11], and hydrothermal [12] methods have been reported previously. Nevertheless, the preparation of CZTS particles by microwave-assisted solution method has not yet been reported. Microwave irradiation is an emerging versatile technique for the controlled synthesis of various nanomaterials due to its distinct advantages like homogeneous nucleation, high reproducibility, improved yields, smaller particle size, and relatively narrow size distribution in a shorter crystallization time [13]. Herein, we report a facile, one-pot synthesis of sphere-like CZTS microparticles by microwave-assisted solution method. The structural, morphological and optical properties of the synthesized CZTS microparticles are investigated in detail.
2. Experimental technique All the chemicals used in the present work are of analytical grade and used without further purification. In a typical synthesis, 4 mmol CuCl2 2H2O, 2 mmol ZnCl2, 2 mmol SnCl4 5H2O and
R. Saravana Kumar et al. / Materials Letters 86 (2012) 174–177
9 mmol NH2CSNH2 were added in sequence to 50 ml of ethylene glycol at room temperature under constant stirring. After stirring for 15 min, the mixture was placed in a commercial microwave oven (2.45 GHz, maximum power of 800 W), and cyclic irradiation was carried out for 50 cycles (here one cycle refers to ON/OFF states of 10/15 sec). The resulting black product was retrieved from the solution by centrifugation (4000 rpm for 10 min) and washed with ethanol, deionized water and acetone several times to remove the by-products. Finally the product was dried in a vacuum oven at 70 1C for 3 h. The crystallinity and phase purity of the synthesized CZTS were investigated using X-ray diffraction (XRD, PANalytical ‘X’ PERT-PRO X-ray diffractometer equipped with Cu Ka radiation, ˚ l ¼1.5405 A), and micro-Raman spectroscopy (Nanofinder 30, Tokyo instruments). The morphology of the synthesized product was analyzed using scanning electron microscopy (SEM, Hitachi, S-4300) and transmission electron microscopy (TEM, Onus, JEM2010). The elemental composition was determined by energy dispersive spectroscopy (EDS) attached to a SEM (JEOL, JSM6400). The optical absorption spectra were recorded using UV– vis double beam spectrophotometer (JASCO, V-670). The CZTS microparticles were dispersed in ethanol and the absorption spectra was recorded in the wavelength ranging from 300 to 1000 nm.
3. Results and discussion Fig. 1 shows the XRD and Raman spectra of CZTS microspheres. The diffraction peaks observed in Fig. 1(a) at 28.631, 32.871, 47.531, 56.311, and 76.821 correspond respectively to (112), (200), (220), (312), and (332) planes of CZTS with kesterite crystal structure (JCPDS card no.: 26-0575). Kesterite crystal structure has tetragonal unit cell inside which each sulfur anion is bonded to four cations and each cation is bonded to four sulfur anions, and the cation layers alternate with sulfur anion layers along the crystallographic c-direction as CuZn/SS/CuSn/SS [9]. The mean crystallite size of CZTS nanoparticles was estimated for (112) and (220) planes using the well-known Debye-Scherrer’s formula [11], and found to be 7 nm. Although the XRD pattern does not show diffraction peaks of binary impurities such as Cu2 xS, CuS, SnS2 and SnS, there is always a probability of formation of other binary and ternary phases. It should be noted that the diffraction peaks of cubic ZnS (JCPDS card no.: 5-0566) and orthorhombic-Cu3SnS4 (JCPDS card no.: 36-
175
0218) have diffraction features similar to that of CZTS. Therefore, XRD analysis alone cannot be adequate for the validation of the existence of pure tetragonal CZTS. To further confirm the phase purity of the as-synthesized CZTS microparticles, Raman measurements was carried out and the results are shown in Fig. 1(b). The spectrum exhibits a single intense peak at 332 cm 1, which is attributed to the A1 symmetry of the quaternary CZTS and is generally related to the vibration of the S atoms in CZTS lattice [14]. Also, the Raman peak is red shifted compared to bulk CZTS (336 cm 1) due to the smaller crystallites size. The absence of vibrational modes corresponding to cubic ZnS (352 and 275 cm 1) and orthorhombic-Cu3SnS4 (318 cm 1) substantiates the formation of single phase CZTS [6]. Fig. 2 shows the SEM and TEM images of the as-synthesized CZTS powder. SEM images (Fig. 2(a) and (b)) indicate that the synthesized product consists of large amount of sphere-like particles with relatively uniform sizes ranging from 1 to 2 mm. Higher magnification in Fig. 2(b) clearly shows that the CZTS particles are composed of large number of nanoparticles. Furthermore, the sphere-like CZTS microparticles are linked with the adjacent ones to form chain-like network, as evidenced by the TEM images in Fig. 2(c). From the EDS of CZTS microparticles (Fig. 2(d)) the relative elemental ratio of Cu:Zn:Sn:S is estimated to be 2:0.98:1.1:4, which is very close to the stoichiometric chemical composition. The slightly Sn rich and Zn poor composition may be ascribed to the reactivities of different metal precursor. The optical absorption spectrum of CZTS microparticles is shown in Fig. 3. CZTS microspheres exhibit high absorbance in the visible region, suggesting their potential for solar cells applications. Based on the direct allowed inter band transition theory, the optical band gap of CZTS is determined by extrapolating the linear part of the curve to zero absorption coefficient (a ¼0) as shown in the inset of Fig. 3. The band gap energy estimated from the plot was 1.76 eV, which is slightly higher than the bulk values (1.4–1.6 eV) reported for the CZTS [10,11]. The slightly higher band gap could be attributed to size effect since the structural and morphological analyses have shown strong evidences that the microspheres are composed of nanoparticles. Furthermore, the estimated band gap value is close to the values reported for CZTS nanocrystals of diameter 5–7 nm [12,15]. It is worthy to recall here that the estimated particle diameter in the present work is 7 nm. Optical absorption further confirms the formation of single phase CZTS, since the presence of secondary phases like Cu2SnS3 (0.96 eV) and ZnS (3.6 eV) will
Fig. 1. (a) XRD, and (b) Raman spectra of CZTS microparticles.
176
R. Saravana Kumar et al. / Materials Letters 86 (2012) 174–177
Fig. 2. (a) Low, and (b) high magnification SEM images of CZTS microparticles, (c) TEM images of CZTS microparticles, and (d) EDS of CZTS microparticles.
absorption were investigated by XRD, SEM, TEM, micro-Raman, and UV–vis spectroscopy. XRD and Raman analyses confirmed the formation of single phase CZTS having kesterite crystal structure. SEM and TEM studies revealed that the CZTS particles are spherical in shape, and the particles are connected with the adjacent ones to form chain-like network. The synthesized CZTS microspheres had high absorbance in the visible region with a direct band gap of 1.76 eV, signifying its potential to harvest maximum photon energy when used as an absorbing layer in solar cells. The present investigation provides a wealth of information that microwave-assisted solution method would be a promising route for the rapid synthesis of single phase CZTS.
Acknowledgments
Fig. 3. UV–vis absorption spectrum of CZTS microparticles. The inset is the typical Tauc plot showing the estimation of direct band gap energy of CZTS microparticles.
decrease and increase the band gap of the synthesized CZTS respectively.
4. Conclusion Microwave-assisted rapid synthesis of sphere-like CZTS microspheres has been demonstrated. The physical properties of CZTS microspheres such as structural, morphological, and optical
This work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011– 0031400) and BK21 Center for Future Energy Materials and Devices, Chonbuk National Unverisity. References [1] Jackson P, Hariskos D, Lotter E, Paetel S, Wuerz R, Menner R, et al. Prog Photovoltaics Res Appl 2011;19:894–7. [2] Moholkar AV, Shinde SS, Babar AR, Sim KU, Kwon YB, Rajpure KY, et al. Sol Energy 2011;85:1354–63. [3] Ramasamy K, Malik MA, O’Brien P. Chem Sci 2011;2:1170–2. [4] Jeon M, Shimizu T, Shingubara S. Mater Lett 2011;65:2364–7. [5] Shin B, Gunawan O, Zhu Y, Bojarczuk NA, Chey SJ, Guha S. Prog Photovoltaics Res Appl 2011http://dx.doi.org/10.1002/pip.1174. [6] Fernandes PA, Salome PMP, Cunha AF. J Alloys Compd 2011;509:7600–6. [7] Su Z, Yan C, Sun K, Han Z, Liu F, Liu J, et al. Appl Surf Sci 2012;258:7678–82. [8] Tanaka K, Oonuki M, Moritake N, Uchiki H. Sol Energy Mater Sol Cells 2009;93:583–7.
R. Saravana Kumar et al. / Materials Letters 86 (2012) 174–177
[9] Guo Q, Hilhouse HW, Agrawal R. J Am Chem Soc 2009;131:11672–3. [10] Steinhagen C, Panthani MG, Akhavan V, Goodfellow B, Koo B, Korgel BA. J Am Chem Soc 2009;131:12554–5. [11] Cao M, Shen Y. J Cryst Growth 2011;318:1117–20. [12] Wang C, Cheng C, Cao Y, Fang W, Zhao L, Xu X. Jpn J Appl Phys 2011;50: 065003-1–3.
177
[13] Ge S, Shui Z, Zheng Z, Zhang L. Opt Mater 2011;33:1174–8. [14] Himmrich M, Haeuseler H. Spectrochim Acta Part A 1991;47:933–42. [15] Khare A, Wills AW, Ammerman LM, Norrisz DJ, Aydil ES. Chem Commun 2011;47:11721–3.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Rapid synthesis of sphere-like Cu2ZnSnS4 microparticles by microwave irradiation R. Saravana Kumar, Beo Deul Ryu, S. Chandramohan, Jin Kyung Seol, Sang-Kwon Lee, Chang-Hee Hong n Semiconductor Physics Research Center, School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2012 Accepted 18 July 2012 Available online 25 July 2012
In the present work, a pioneering attempt has been made for the rapid synthesis of Cu2ZnSnS4 (CZTS) microspheres by the microwave-assisted solution method. X-ray diffraction (XRD) and Raman spectroscopy confirmed the formation of single phase CZTS with tetragonal crystal structure. Scanning electron microscope (SEM) and transmission electron microscope (TEM) showed the formation of sphere-like CZTS microparticles. Optical absorption studies revealed that CZTS microparticles have an optical band gap of 1.76 eV, which is optimal for photovoltaic applications. The simplicity, rapidity, and commercial feasibility of the current synthesis strategy would be beneficial for the low-cost and large-scale industrial production of CZTS. & 2012 Elsevier B.V. All rights reserved.
Keywords: Solar energy materials Cu2ZnSnS4 Microwave synthesis Microparticles X-ray diffraction Electron microscopy
1. Introduction Low cost, high efficiency and less environmental pollution have been regarded as the indispensable properties for the next generation solar cells materials. Chalcopyrite semiconductors such as CdTe, Cu(In,Ga)S2 and Cu(In,Ga)Se2 were the most successful commercialized thin-film photovoltaic materials with demonstrated power conversion efficiency (PCE) of nearly 20% [1]. However, concerns over their toxicity, scarcity and increasing prices of rare metals (In, Te and Ga) have motivated the research community to seek an alternate earth abundant, environmentally benign material with adequate photovoltaic properties. In search of a chalcopyrite photovoltaic absorber free of scarce and toxic elements, CZTS has sparkled tremendous research interest due to its excellent thermal, chemical, electronic, optical and mechanical properties in addition to less environmental damaging and cost effectiveness candidature [2]. As a quaternary Cu2–II–IV–VI4 compound, CZTS has emerged as a promising absorber material with intriguing properties like low-cost, direct band gap (1.5 eV), p-type conductivity, high absorption coefficient (4104 cm 1) in the visible wavelength region. In addition, all the constituent elements of CZTS are non-toxic and aplenty on the earth-crust, making it a potential candidate for the thin-film photovoltaics [3,4]. Hence, the incorporation of CZTS thin films as the absorber layer in solar cells will be a promising solution for the realization of solar cells free from both the resource saving problem and environmental pollution. To date, the reported PCE of solar cells based on CZTS is as high as 8.4% under AM1.5G illumination, while its
n
Corresponding author. E-mail address: [email protected] (C.-H. Hong).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.059
theoretical limit is 32.2% [5]. Therefore, a great amount of work has to be done in order to bring the PCE closer to the theoretical limits. Hitherto, variety of methods based on vacuum and non-vacuum techniques such as pulsed laser deposition [2], chemical vapor deposition [3], electro-deposition [4], thermal evaporation [5], sputtering [6], successive ionic layer and adsorption reaction [7], and sol– gel [8] has been utilized to fabricate CZTS thin films. Recently, a new method based on ink printing technology for scaling up solar cell production using nanoparticle inks that can be printed, sprayed, or dip-coated onto large area substrates has been developed for the chalcogenide-based absorber layer deposition [9,10]. Consequently, synthesis of CZTS nanoparticles by hot-injection [9], high-temperature arrested precipitation [10], solvothermal [11], and hydrothermal [12] methods have been reported previously. Nevertheless, the preparation of CZTS particles by microwave-assisted solution method has not yet been reported. Microwave irradiation is an emerging versatile technique for the controlled synthesis of various nanomaterials due to its distinct advantages like homogeneous nucleation, high reproducibility, improved yields, smaller particle size, and relatively narrow size distribution in a shorter crystallization time [13]. Herein, we report a facile, one-pot synthesis of sphere-like CZTS microparticles by microwave-assisted solution method. The structural, morphological and optical properties of the synthesized CZTS microparticles are investigated in detail.
2. Experimental technique All the chemicals used in the present work are of analytical grade and used without further purification. In a typical synthesis, 4 mmol CuCl2 2H2O, 2 mmol ZnCl2, 2 mmol SnCl4 5H2O and
R. Saravana Kumar et al. / Materials Letters 86 (2012) 174–177
9 mmol NH2CSNH2 were added in sequence to 50 ml of ethylene glycol at room temperature under constant stirring. After stirring for 15 min, the mixture was placed in a commercial microwave oven (2.45 GHz, maximum power of 800 W), and cyclic irradiation was carried out for 50 cycles (here one cycle refers to ON/OFF states of 10/15 sec). The resulting black product was retrieved from the solution by centrifugation (4000 rpm for 10 min) and washed with ethanol, deionized water and acetone several times to remove the by-products. Finally the product was dried in a vacuum oven at 70 1C for 3 h. The crystallinity and phase purity of the synthesized CZTS were investigated using X-ray diffraction (XRD, PANalytical ‘X’ PERT-PRO X-ray diffractometer equipped with Cu Ka radiation, ˚ l ¼1.5405 A), and micro-Raman spectroscopy (Nanofinder 30, Tokyo instruments). The morphology of the synthesized product was analyzed using scanning electron microscopy (SEM, Hitachi, S-4300) and transmission electron microscopy (TEM, Onus, JEM2010). The elemental composition was determined by energy dispersive spectroscopy (EDS) attached to a SEM (JEOL, JSM6400). The optical absorption spectra were recorded using UV– vis double beam spectrophotometer (JASCO, V-670). The CZTS microparticles were dispersed in ethanol and the absorption spectra was recorded in the wavelength ranging from 300 to 1000 nm.
3. Results and discussion Fig. 1 shows the XRD and Raman spectra of CZTS microspheres. The diffraction peaks observed in Fig. 1(a) at 28.631, 32.871, 47.531, 56.311, and 76.821 correspond respectively to (112), (200), (220), (312), and (332) planes of CZTS with kesterite crystal structure (JCPDS card no.: 26-0575). Kesterite crystal structure has tetragonal unit cell inside which each sulfur anion is bonded to four cations and each cation is bonded to four sulfur anions, and the cation layers alternate with sulfur anion layers along the crystallographic c-direction as CuZn/SS/CuSn/SS [9]. The mean crystallite size of CZTS nanoparticles was estimated for (112) and (220) planes using the well-known Debye-Scherrer’s formula [11], and found to be 7 nm. Although the XRD pattern does not show diffraction peaks of binary impurities such as Cu2 xS, CuS, SnS2 and SnS, there is always a probability of formation of other binary and ternary phases. It should be noted that the diffraction peaks of cubic ZnS (JCPDS card no.: 5-0566) and orthorhombic-Cu3SnS4 (JCPDS card no.: 36-
175
0218) have diffraction features similar to that of CZTS. Therefore, XRD analysis alone cannot be adequate for the validation of the existence of pure tetragonal CZTS. To further confirm the phase purity of the as-synthesized CZTS microparticles, Raman measurements was carried out and the results are shown in Fig. 1(b). The spectrum exhibits a single intense peak at 332 cm 1, which is attributed to the A1 symmetry of the quaternary CZTS and is generally related to the vibration of the S atoms in CZTS lattice [14]. Also, the Raman peak is red shifted compared to bulk CZTS (336 cm 1) due to the smaller crystallites size. The absence of vibrational modes corresponding to cubic ZnS (352 and 275 cm 1) and orthorhombic-Cu3SnS4 (318 cm 1) substantiates the formation of single phase CZTS [6]. Fig. 2 shows the SEM and TEM images of the as-synthesized CZTS powder. SEM images (Fig. 2(a) and (b)) indicate that the synthesized product consists of large amount of sphere-like particles with relatively uniform sizes ranging from 1 to 2 mm. Higher magnification in Fig. 2(b) clearly shows that the CZTS particles are composed of large number of nanoparticles. Furthermore, the sphere-like CZTS microparticles are linked with the adjacent ones to form chain-like network, as evidenced by the TEM images in Fig. 2(c). From the EDS of CZTS microparticles (Fig. 2(d)) the relative elemental ratio of Cu:Zn:Sn:S is estimated to be 2:0.98:1.1:4, which is very close to the stoichiometric chemical composition. The slightly Sn rich and Zn poor composition may be ascribed to the reactivities of different metal precursor. The optical absorption spectrum of CZTS microparticles is shown in Fig. 3. CZTS microspheres exhibit high absorbance in the visible region, suggesting their potential for solar cells applications. Based on the direct allowed inter band transition theory, the optical band gap of CZTS is determined by extrapolating the linear part of the curve to zero absorption coefficient (a ¼0) as shown in the inset of Fig. 3. The band gap energy estimated from the plot was 1.76 eV, which is slightly higher than the bulk values (1.4–1.6 eV) reported for the CZTS [10,11]. The slightly higher band gap could be attributed to size effect since the structural and morphological analyses have shown strong evidences that the microspheres are composed of nanoparticles. Furthermore, the estimated band gap value is close to the values reported for CZTS nanocrystals of diameter 5–7 nm [12,15]. It is worthy to recall here that the estimated particle diameter in the present work is 7 nm. Optical absorption further confirms the formation of single phase CZTS, since the presence of secondary phases like Cu2SnS3 (0.96 eV) and ZnS (3.6 eV) will
Fig. 1. (a) XRD, and (b) Raman spectra of CZTS microparticles.
176
R. Saravana Kumar et al. / Materials Letters 86 (2012) 174–177
Fig. 2. (a) Low, and (b) high magnification SEM images of CZTS microparticles, (c) TEM images of CZTS microparticles, and (d) EDS of CZTS microparticles.
absorption were investigated by XRD, SEM, TEM, micro-Raman, and UV–vis spectroscopy. XRD and Raman analyses confirmed the formation of single phase CZTS having kesterite crystal structure. SEM and TEM studies revealed that the CZTS particles are spherical in shape, and the particles are connected with the adjacent ones to form chain-like network. The synthesized CZTS microspheres had high absorbance in the visible region with a direct band gap of 1.76 eV, signifying its potential to harvest maximum photon energy when used as an absorbing layer in solar cells. The present investigation provides a wealth of information that microwave-assisted solution method would be a promising route for the rapid synthesis of single phase CZTS.
Acknowledgments
Fig. 3. UV–vis absorption spectrum of CZTS microparticles. The inset is the typical Tauc plot showing the estimation of direct band gap energy of CZTS microparticles.
decrease and increase the band gap of the synthesized CZTS respectively.
4. Conclusion Microwave-assisted rapid synthesis of sphere-like CZTS microspheres has been demonstrated. The physical properties of CZTS microspheres such as structural, morphological, and optical
This work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011– 0031400) and BK21 Center for Future Energy Materials and Devices, Chonbuk National Unverisity. References [1] Jackson P, Hariskos D, Lotter E, Paetel S, Wuerz R, Menner R, et al. Prog Photovoltaics Res Appl 2011;19:894–7. [2] Moholkar AV, Shinde SS, Babar AR, Sim KU, Kwon YB, Rajpure KY, et al. Sol Energy 2011;85:1354–63. [3] Ramasamy K, Malik MA, O’Brien P. Chem Sci 2011;2:1170–2. [4] Jeon M, Shimizu T, Shingubara S. Mater Lett 2011;65:2364–7. [5] Shin B, Gunawan O, Zhu Y, Bojarczuk NA, Chey SJ, Guha S. Prog Photovoltaics Res Appl 2011http://dx.doi.org/10.1002/pip.1174. [6] Fernandes PA, Salome PMP, Cunha AF. J Alloys Compd 2011;509:7600–6. [7] Su Z, Yan C, Sun K, Han Z, Liu F, Liu J, et al. Appl Surf Sci 2012;258:7678–82. [8] Tanaka K, Oonuki M, Moritake N, Uchiki H. Sol Energy Mater Sol Cells 2009;93:583–7.
R. Saravana Kumar et al. / Materials Letters 86 (2012) 174–177
[9] Guo Q, Hilhouse HW, Agrawal R. J Am Chem Soc 2009;131:11672–3. [10] Steinhagen C, Panthani MG, Akhavan V, Goodfellow B, Koo B, Korgel BA. J Am Chem Soc 2009;131:12554–5. [11] Cao M, Shen Y. J Cryst Growth 2011;318:1117–20. [12] Wang C, Cheng C, Cao Y, Fang W, Zhao L, Xu X. Jpn J Appl Phys 2011;50: 065003-1–3.
177
[13] Ge S, Shui Z, Zheng Z, Zhang L. Opt Mater 2011;33:1174–8. [14] Himmrich M, Haeuseler H. Spectrochim Acta Part A 1991;47:933–42. [15] Khare A, Wills AW, Ammerman LM, Norrisz DJ, Aydil ES. Chem Commun 2011;47:11721–3.