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The structural, morphological and optical–electrical characteristic of Cu2XSnS4 (X:Cu,Mg) thin films fabricated by novel ultrasonic co-spray pyrolysis
Materials Letters 172 (2016) 68–71
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
The structural, morphological and optical–electrical characteristic of Cu2XSnS4 (X:Cu,Mg) thin films fabricated by novel ultrasonic co-spray pyrolysis Yixin Guo a, Wenjuan Cheng b,n, Jinchun Jiang a,c,nn, Shaohua Zuo a,c, Fuwen Shi a,c, Junhao Chu a,c,d a
Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China Department of Physics, East China Normal University, Shanghai 200241, China c Shanghai Center for Photovoltaics, Shanghai 201201, China d Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 December 2015 Received in revised form 19 February 2016 Accepted 20 February 2016 Available online 23 February 2016
In this work,a simple and low cost route is proposed to fabricate pure Cu2SnS3 and Cu2XSnS4 (X:Cu,Mg) thin films.The Cu2XSnS4 (X:Cu,Mg) film was synthesized by a novel dual source ultrasonic co-spray method for the first time.The structural, morphological, optical and electrical properties of the films were well investigated.All the films show different structure,satisfactory morphology and p-type conductivity with a minimum resistivity of 4.82 10 3 Ω cm for Cu2CuSnS4. Optical properties suggest the samples have band gap values of 1.4 eV, 1.65 eV and 1.76 eV for Cu2SnS3,Cu2CuSnS4 and Cu2MgSnS4 respectively, which is the optimal value for high efficient single or tandom thin film solar cell. & 2016 Elsevier B.V. All rights reserved.
Keywords: CTS Co-spray Thin films Optical-electrical properties Solar energy Materials
1. Introduction With the increase of world's energy needs, great efforts have been made in searching for cheaper and non-toxic photovoltaic materials. Photovoltaic cell using Cu2ZnSnSxSe4 x (CZTS(Se)) as the absorber material has achieved a highest record efficiency up to 12.6% [1]. However, this efficiency is still lower than the requirement of commercial solar cell device.The existence of antisite defects(CuZn or ZnCu) in the CZTS(Se) absorber layer could be an important factor that deteriorates the solar cell performances [2,3]. In order to eliminate the antisite defects,substituting other elements for Zn is an alternative approach to suppress the defects formation and improve the device performance. Substitution of Fe, Mn and Cd for Zn to form Cu2FeSnS4 [3] Cu2MnSnS4 [4] and Cu2CdSnS4 [5] thin film had been experimentally achieved. Whereas,these elements are toxic and may bring undesirable magnetism into PV device.Cu2CuSnS4 (CCTS) was proved to be a promising candidate as absorber layer in thin film heterojunction n
Corresponding author. Corresponding author at: Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China. nn
http://dx.doi.org/10.1016/j.matlet.2016.02.088 0167-577X/& 2016 Elsevier B.V. All rights reserved.
solar cells [6]. However, there were few works reported on Cu2CuSnS4 thin films. Cu2MgSnS4(CMTS) was previously believed as an unstable material, however, recent theoretic and experimental results [7,8] showed Cu2MgSnS4 could be an thermodynamically stable material for photovoltaic application. CZTS thin films were synthesized by many techniques such as magnetron sputtering [9], electrochemical deposition [10], sol-gel spinning method [11], spray pyrolysis [12] and pulsed lased laser deposition (PLD) [13]. Among them, ultrasonic spray pyrolysis has attracted much interest due to its low cost, no need of vacuum process, simplicity and easy control on the element ratio. Previous reports on spray deposition of CZTS thin films used aqueous solution with three metal salt precursors, therein it was very difficult in controlling the compositional ratio and in reducing spray rate of CZTS thin film due to the varied spray powers for different metal salt precursors. In this work, we developed a facile and novel ultrasonic cospray pyrolysis for the Cu2XSnS4 (X:Cu,Mg) thin film fabrication for the first time. To the best of our knowledge, there are no similar reports in previous works. The structural, morphological and optical-electrical properties of prepared films by this novel method were well investigated in detail.
Y. Guo et al. / Materials Letters 172 (2016) 68–71
69
2. Material and methods
3. Results and discussion
2.1. Preparation of spray solution and deposition detail
3.1. Crystalline structure and morphology properties
2.1.1. Preparation of Cu2SnS3 precursor solution Copper chloride(CuCl2 H2O),tin chloride (SnCl4 H2O) and thiourea (CS(NH2)2) were dissolved in distilled water as precursor. The tin chloride concentration was 0.1 M while the copper chloride concentration was 0.18 M. The thiourea concentration was fixed at 1.4 M, excess thiourea more than standard stoichiometric was used in order to avoid the loss of sulfur during preparation process.
Fig. 1 reveals the XRD patterns of the pure Cu2SnS3 and Cu2XSnS4 films (X: Cu,Mg). As for pure Cu2SnS3(CTS), three characteristic diffraction peaks located around 2θ ¼28.4°,32.9°,47.2° and 56.1° can be found, which match with monoclinic Cu2SnS3 phase (PDF#04-010-5719) ( 1¯ 31),( 2¯ 02),( 3¯ 31) and (4̄ 4¯ 02) plane. As shown in Fig. 1, when the Cu2 þ or Mg2 þ was incorporated into CTS film, the XRD diffraction peaks have a slightly shift to right. The XRD patterns of Cu2CuSnS4(CCTS) match with orthorhombic Cu3SnS4 (PDF#00-036-0128) (222),(400),(044) and (262) plane and the weak peak at 31° may be related to Cu2 xS phase.The XRD patterns of Cu2MgSnS4 show close resemblance to tetragonal Cu2SnS3 phase (PDF#89-4714 ) (112),(220) and (312) plane and no peaks associated with magnesium or magnesium compound were found, which may indicate the magnesium ions were successfully incorporated into the CTS film. Fig. 2 shows the Raman spectra of Cu2MSnS4 (X: Cu,Mg) thin films. As seen in Fig. 2, the CCTS film exhibits characteristic Raman peaks at 265,290,316,330 and 348 cm 1 belonging to orthorhombic Cu3SnS4 phase [14].On the other hand, as for CMTS, the domain peaks at 338 and 351 cm 1 match with tetragonal Cu2SnS3 phase [15], which is in accordance with the XRD result. The weak peaks at 290 cm 1 and 318 cm 1 may correspond to monoclinic Cu2SnS3 [15] and SnS2 phase [15]. Fig. 3(a-b) depicts surface SEM morphology of the CMTS and CCTS film. It can be seen the films exhibit smooth surface with small grains. The CMTS film consists of irregular small particles whereas the CCTS thin film exhibits a microstructure consisting of cauliflower-buds like particles. Table 1 shows the metal composition of two films. Both films exhibit relatively proper metal elemental component ratio (Cu/(Mg þ Sn)) with 1 and 2.89 respectively, which is close to the standard stoichiometric of CMTS (1:1) and CCTS (3:1).
2.1.2. Preparation of Cu2 þ or Mg2 þ precursor solution Magnesium chloride (MgCl2 6H2O) or Copper chloride (CuCl2 H2O) was dissolved in distilled water as precursor. The magnesium chloride or the copper chloride concentration was 0.1 M. 2.1.3. Preparation of Cu2XSnS4 films Cu2XSnS4 films were prepared by a facile dual source ultrasonic co-spray pyrolysis equipment with Cu2SnS3 precursor solution and Cu2 þ /Mg2 þ precursor solution as source materials. During the deposition, two nozzles had the same spray rate and the distance from substrate to nozzle with 5 mL/min and 5 cm, respectively. The angle between nozzle and substrate was 45°. All films were deposited on soda-lime glass substrates at temperature of 350 °C. During the deposition, the substrate holder was rotated using a rotary drive mechanism for uniform deposition of the films. Prior to deposition process, all the substrates were cleaned by alcohol, acetone and distilled water in ultrasonic bath and then dried by nitrogen gas. 2.2. Characterization The structure of the films were investigated using X-ray diffraction (XRD, Rigku D/max 2550 V) with Cu Ka radiation. The Raman spectra was taken using Raman spectrometer (Renishaw inVia plus) with 514 nm laser.The surface morphology and composition of the films was obtained by Scanning electron microscopy (SEM,JEOL) equipped with energy dispersive X-ray spectroscopy(EDX).The optical parameters of the films were measured by UV–vis-NIR spectrophotometer (Cary5000). The electrical studies of the films were taken by Hall Effect measurement (Ecopia, HMS3000).
3.2. Optical properties The band gap of the films can be calculated by the relationship between absorption coefficient a and excitation photon frequency v:
(ahv)n = (hv − Eg ) where h is the Planck constant, Eg is the band gap. Cu2SnS3 is the direct band material, so n¼ 2.
Fig. 1. X-ray diffraction patterns of pure CTS and Cu2XSnS4 (X: Cu,Mg) films.
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Y. Guo et al. / Materials Letters 172 (2016) 68–71
Fig. 2. Raman spectrum of CCTS and CMTS films.
Fig. 3. The scanning electron microscopic images for (a) CCTS film; (b) CMTS film.
Table 1 The metal component of CCTS and CMTS films.
Table 2 Electrical properties of pure CTS and Cu2XSnS4(X:Cu,Mg) films.
Sample
Cu (At%)
Mg (At%)
Sn (At%)
Cu/(Mg þ Sn)
CMTS CCTS
26.66 36.89
15.07 –
11.61 12.77
1 2.89
Sample Mobility (cm2 V 1 S 1)
Carries concentration(cm 3) Resistivity (Ω cm)
CTS CMTS CCTS
5.556 1019 5.295 1018 1.302 1021
1.198 8.346 0.997
9.365 10 2 1.412 10 1 4.810 10 3
The relationship between (ahv )2 and hv is shown in Fig. 4. For pure CTS film, the band gap is around 1.4 eV,while the band gap of the CCTS film increases to 1.65 eV which is in good agreement with the reported value [14]. Compared to CZTS with band gap of 1.5 eV [11], the band gap of CMTS shows a small increase (around 1.76 eV). The conduction band of CZTS film is formed by the antibonding component of hybridization between the Sn s and S s states. Compared to Zn2 þ ,Mg2 þ may enhance interaction between Sn and S and improve the bottom of the conduction band,which leads to the increase of band gap [8]. The higher band gap (Z1.65 eV) is the optimum value for tandem solar cell with higher conversion efficiency (455%) than that of single solar cell (o33%). 3.3. Electrical properties
Fig. 4. The optic band gap of pure CTS and Cu2XSnS4 (X:Cu,Mg) films.
The electrical properties of pure Cu2SnS3 and Cu2XSnS4 (X:Cu, Mg) films were measured by Hall Effect measurement system using Van der Pauw method and four-point probes at room temperature. As shown in Table 2, all films are p-type semiconductors.
Y. Guo et al. / Materials Letters 172 (2016) 68–71
Similar to CZTS film, the conductivity of CTS film may come from defects, such as Vcu or SnCu, in the film. CuS was a p-type and narrow band gap semiconductor [16] with low electrical resistivity caused by copper vacancies in the film. The incorporation of CuS into CTS film may produce more copper vacancies and increase the carries concentration and decrease the resistivity of the film, as shown in Table 2. In contrast, CMTS shows the lowest carrier concentration and the highest electrical resistivity among all samples because of the high resistivity of MgS.
4. Conclusion In summary, pure CTS and Cu2XSnS4(X:Cu,Mg) thin films were synthesized by a novel ultrasonic co-spray method for the first time. The structural, morphological, optical and electrical properties of the obtained samples were studied. XRD and Raman analysis confirm all the films exhibit different structure due to the incorporation of Mg2 þ or Cu2 þ into CTS film. The calculated value of the optical band gap for the CTS, CCTS and CMTS film is 1.4, 1.65 and 1.76 eV which is close to the optimum value for an efficient single or tandem solar cell material. The Hall Effect measurement shows all the films have p-type conductivity and the lowest resistivity is 4.82 10 3 Ω cm for CCTS and the highest resistivity is 1.412 10 1 Ω cm for CMTS.The results indicate the pure CTS and Cu2XSnS4 (X:Cu,Mg) films prepared by ultrasonic co- spray method can be suitable candidates for using in PV devices.
Acknowledgments This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Y2K4401DG0).
71
References [1] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, et al., Adv. Energy Mater. 4 (2014) 1301465. [2] T. Gokmen, O. Gunawan, T.K. Todorov, D.B. Mitzi, Appl. Phys. Lett. 103 (2013) 103506. [3] F. Ozel, M. Kus, A. Yar, E. Arkan, M. Can, A. Aljabour, et al., J. Mater. Sci. 50 (2015) 777–783. [4] L. Chen, H. Deng, J. Tao, H. Cao, L. Huang, L. Sun, et al., RSC Adv. 5 (2015) 84295–84302. [5] L. Meng, Y.F. Li, B. Yao, Z. Ding, G. Yang, R. Liu, et al., J. Phys. D Appl. Phys. 48 (2015) 445105. [6] H. Guan, H. Shen, C. Gao, X. He, J. Mater. Sci. Mater. Electron. 24 (2013) 1490–1494. [7] M. Wei, Q. Du, R. Wang, G. Jiang, W. Liu, C. Zhu, Chem. Lett. 43 (2014) 1149–1151. [8] G. Zhong, K. Tse, Y. Zhang, X. Li, C. Yang, J. Zhu, et al., arXiv Prepr. arXiv 1509 (2015) 06230. [9] J. He, L. Sun, Y. Chen, J. Jiang, P. Yang, J. Chu, J. Power Sources 273 (2015) 600–607. [10] J. Tao, J. Liu, J. He, K. Zhang, J. Jiang, L. Sun, et al., RSC Adv. 4 (2014) 23977–23984. [11] K. Zhang, J. Tao, J. He, W. Wang, L. Sun, P. Yang, et al., J. Mater. Sci. Mater. Electron. 25 (2014) 2703–2709. [12] T. Martini, C. Chubilleau, O. Poncelet, A. Ricaud, A. Blayo, C. Martin, et al., Sol. Energy Mater. Sol. Cells 144 (2016) 657–663. [13] N.S.C. Sulaiman, C.H. Nee, S.L. Yap, Y.S. Lee, T.Y. Tou, S.S. Yap, Appl. Surf. Sci. 354 (2015) 42–47. [14] U. Chalapathi, Y.K. Kumar, S. Uthanna, V.S. Raja, Thin Solid Films 556 (2014) 61–67. [15] U. Chalapathi, Y. Jayasree, S. Uthanna, V. Sundara Raja, Phys. Status Solidi A 210 (2013) 2384–2390. [16] H. Lei, G. Fang, F. Cheng, W. Ke, P. Qin, Z. Song, et al., Sol. Energy Mater. Sol. Cells 128 (2014) 77–84.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
The structural, morphological and optical–electrical characteristic of Cu2XSnS4 (X:Cu,Mg) thin films fabricated by novel ultrasonic co-spray pyrolysis Yixin Guo a, Wenjuan Cheng b,n, Jinchun Jiang a,c,nn, Shaohua Zuo a,c, Fuwen Shi a,c, Junhao Chu a,c,d a
Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China Department of Physics, East China Normal University, Shanghai 200241, China c Shanghai Center for Photovoltaics, Shanghai 201201, China d Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 December 2015 Received in revised form 19 February 2016 Accepted 20 February 2016 Available online 23 February 2016
In this work,a simple and low cost route is proposed to fabricate pure Cu2SnS3 and Cu2XSnS4 (X:Cu,Mg) thin films.The Cu2XSnS4 (X:Cu,Mg) film was synthesized by a novel dual source ultrasonic co-spray method for the first time.The structural, morphological, optical and electrical properties of the films were well investigated.All the films show different structure,satisfactory morphology and p-type conductivity with a minimum resistivity of 4.82 10 3 Ω cm for Cu2CuSnS4. Optical properties suggest the samples have band gap values of 1.4 eV, 1.65 eV and 1.76 eV for Cu2SnS3,Cu2CuSnS4 and Cu2MgSnS4 respectively, which is the optimal value for high efficient single or tandom thin film solar cell. & 2016 Elsevier B.V. All rights reserved.
Keywords: CTS Co-spray Thin films Optical-electrical properties Solar energy Materials
1. Introduction With the increase of world's energy needs, great efforts have been made in searching for cheaper and non-toxic photovoltaic materials. Photovoltaic cell using Cu2ZnSnSxSe4 x (CZTS(Se)) as the absorber material has achieved a highest record efficiency up to 12.6% [1]. However, this efficiency is still lower than the requirement of commercial solar cell device.The existence of antisite defects(CuZn or ZnCu) in the CZTS(Se) absorber layer could be an important factor that deteriorates the solar cell performances [2,3]. In order to eliminate the antisite defects,substituting other elements for Zn is an alternative approach to suppress the defects formation and improve the device performance. Substitution of Fe, Mn and Cd for Zn to form Cu2FeSnS4 [3] Cu2MnSnS4 [4] and Cu2CdSnS4 [5] thin film had been experimentally achieved. Whereas,these elements are toxic and may bring undesirable magnetism into PV device.Cu2CuSnS4 (CCTS) was proved to be a promising candidate as absorber layer in thin film heterojunction n
Corresponding author. Corresponding author at: Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, China. nn
http://dx.doi.org/10.1016/j.matlet.2016.02.088 0167-577X/& 2016 Elsevier B.V. All rights reserved.
solar cells [6]. However, there were few works reported on Cu2CuSnS4 thin films. Cu2MgSnS4(CMTS) was previously believed as an unstable material, however, recent theoretic and experimental results [7,8] showed Cu2MgSnS4 could be an thermodynamically stable material for photovoltaic application. CZTS thin films were synthesized by many techniques such as magnetron sputtering [9], electrochemical deposition [10], sol-gel spinning method [11], spray pyrolysis [12] and pulsed lased laser deposition (PLD) [13]. Among them, ultrasonic spray pyrolysis has attracted much interest due to its low cost, no need of vacuum process, simplicity and easy control on the element ratio. Previous reports on spray deposition of CZTS thin films used aqueous solution with three metal salt precursors, therein it was very difficult in controlling the compositional ratio and in reducing spray rate of CZTS thin film due to the varied spray powers for different metal salt precursors. In this work, we developed a facile and novel ultrasonic cospray pyrolysis for the Cu2XSnS4 (X:Cu,Mg) thin film fabrication for the first time. To the best of our knowledge, there are no similar reports in previous works. The structural, morphological and optical-electrical properties of prepared films by this novel method were well investigated in detail.
Y. Guo et al. / Materials Letters 172 (2016) 68–71
69
2. Material and methods
3. Results and discussion
2.1. Preparation of spray solution and deposition detail
3.1. Crystalline structure and morphology properties
2.1.1. Preparation of Cu2SnS3 precursor solution Copper chloride(CuCl2 H2O),tin chloride (SnCl4 H2O) and thiourea (CS(NH2)2) were dissolved in distilled water as precursor. The tin chloride concentration was 0.1 M while the copper chloride concentration was 0.18 M. The thiourea concentration was fixed at 1.4 M, excess thiourea more than standard stoichiometric was used in order to avoid the loss of sulfur during preparation process.
Fig. 1 reveals the XRD patterns of the pure Cu2SnS3 and Cu2XSnS4 films (X: Cu,Mg). As for pure Cu2SnS3(CTS), three characteristic diffraction peaks located around 2θ ¼28.4°,32.9°,47.2° and 56.1° can be found, which match with monoclinic Cu2SnS3 phase (PDF#04-010-5719) ( 1¯ 31),( 2¯ 02),( 3¯ 31) and (4̄ 4¯ 02) plane. As shown in Fig. 1, when the Cu2 þ or Mg2 þ was incorporated into CTS film, the XRD diffraction peaks have a slightly shift to right. The XRD patterns of Cu2CuSnS4(CCTS) match with orthorhombic Cu3SnS4 (PDF#00-036-0128) (222),(400),(044) and (262) plane and the weak peak at 31° may be related to Cu2 xS phase.The XRD patterns of Cu2MgSnS4 show close resemblance to tetragonal Cu2SnS3 phase (PDF#89-4714 ) (112),(220) and (312) plane and no peaks associated with magnesium or magnesium compound were found, which may indicate the magnesium ions were successfully incorporated into the CTS film. Fig. 2 shows the Raman spectra of Cu2MSnS4 (X: Cu,Mg) thin films. As seen in Fig. 2, the CCTS film exhibits characteristic Raman peaks at 265,290,316,330 and 348 cm 1 belonging to orthorhombic Cu3SnS4 phase [14].On the other hand, as for CMTS, the domain peaks at 338 and 351 cm 1 match with tetragonal Cu2SnS3 phase [15], which is in accordance with the XRD result. The weak peaks at 290 cm 1 and 318 cm 1 may correspond to monoclinic Cu2SnS3 [15] and SnS2 phase [15]. Fig. 3(a-b) depicts surface SEM morphology of the CMTS and CCTS film. It can be seen the films exhibit smooth surface with small grains. The CMTS film consists of irregular small particles whereas the CCTS thin film exhibits a microstructure consisting of cauliflower-buds like particles. Table 1 shows the metal composition of two films. Both films exhibit relatively proper metal elemental component ratio (Cu/(Mg þ Sn)) with 1 and 2.89 respectively, which is close to the standard stoichiometric of CMTS (1:1) and CCTS (3:1).
2.1.2. Preparation of Cu2 þ or Mg2 þ precursor solution Magnesium chloride (MgCl2 6H2O) or Copper chloride (CuCl2 H2O) was dissolved in distilled water as precursor. The magnesium chloride or the copper chloride concentration was 0.1 M. 2.1.3. Preparation of Cu2XSnS4 films Cu2XSnS4 films were prepared by a facile dual source ultrasonic co-spray pyrolysis equipment with Cu2SnS3 precursor solution and Cu2 þ /Mg2 þ precursor solution as source materials. During the deposition, two nozzles had the same spray rate and the distance from substrate to nozzle with 5 mL/min and 5 cm, respectively. The angle between nozzle and substrate was 45°. All films were deposited on soda-lime glass substrates at temperature of 350 °C. During the deposition, the substrate holder was rotated using a rotary drive mechanism for uniform deposition of the films. Prior to deposition process, all the substrates were cleaned by alcohol, acetone and distilled water in ultrasonic bath and then dried by nitrogen gas. 2.2. Characterization The structure of the films were investigated using X-ray diffraction (XRD, Rigku D/max 2550 V) with Cu Ka radiation. The Raman spectra was taken using Raman spectrometer (Renishaw inVia plus) with 514 nm laser.The surface morphology and composition of the films was obtained by Scanning electron microscopy (SEM,JEOL) equipped with energy dispersive X-ray spectroscopy(EDX).The optical parameters of the films were measured by UV–vis-NIR spectrophotometer (Cary5000). The electrical studies of the films were taken by Hall Effect measurement (Ecopia, HMS3000).
3.2. Optical properties The band gap of the films can be calculated by the relationship between absorption coefficient a and excitation photon frequency v:
(ahv)n = (hv − Eg ) where h is the Planck constant, Eg is the band gap. Cu2SnS3 is the direct band material, so n¼ 2.
Fig. 1. X-ray diffraction patterns of pure CTS and Cu2XSnS4 (X: Cu,Mg) films.
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Y. Guo et al. / Materials Letters 172 (2016) 68–71
Fig. 2. Raman spectrum of CCTS and CMTS films.
Fig. 3. The scanning electron microscopic images for (a) CCTS film; (b) CMTS film.
Table 1 The metal component of CCTS and CMTS films.
Table 2 Electrical properties of pure CTS and Cu2XSnS4(X:Cu,Mg) films.
Sample
Cu (At%)
Mg (At%)
Sn (At%)
Cu/(Mg þ Sn)
CMTS CCTS
26.66 36.89
15.07 –
11.61 12.77
1 2.89
Sample Mobility (cm2 V 1 S 1)
Carries concentration(cm 3) Resistivity (Ω cm)
CTS CMTS CCTS
5.556 1019 5.295 1018 1.302 1021
1.198 8.346 0.997
9.365 10 2 1.412 10 1 4.810 10 3
The relationship between (ahv )2 and hv is shown in Fig. 4. For pure CTS film, the band gap is around 1.4 eV,while the band gap of the CCTS film increases to 1.65 eV which is in good agreement with the reported value [14]. Compared to CZTS with band gap of 1.5 eV [11], the band gap of CMTS shows a small increase (around 1.76 eV). The conduction band of CZTS film is formed by the antibonding component of hybridization between the Sn s and S s states. Compared to Zn2 þ ,Mg2 þ may enhance interaction between Sn and S and improve the bottom of the conduction band,which leads to the increase of band gap [8]. The higher band gap (Z1.65 eV) is the optimum value for tandem solar cell with higher conversion efficiency (455%) than that of single solar cell (o33%). 3.3. Electrical properties
Fig. 4. The optic band gap of pure CTS and Cu2XSnS4 (X:Cu,Mg) films.
The electrical properties of pure Cu2SnS3 and Cu2XSnS4 (X:Cu, Mg) films were measured by Hall Effect measurement system using Van der Pauw method and four-point probes at room temperature. As shown in Table 2, all films are p-type semiconductors.
Y. Guo et al. / Materials Letters 172 (2016) 68–71
Similar to CZTS film, the conductivity of CTS film may come from defects, such as Vcu or SnCu, in the film. CuS was a p-type and narrow band gap semiconductor [16] with low electrical resistivity caused by copper vacancies in the film. The incorporation of CuS into CTS film may produce more copper vacancies and increase the carries concentration and decrease the resistivity of the film, as shown in Table 2. In contrast, CMTS shows the lowest carrier concentration and the highest electrical resistivity among all samples because of the high resistivity of MgS.
4. Conclusion In summary, pure CTS and Cu2XSnS4(X:Cu,Mg) thin films were synthesized by a novel ultrasonic co-spray method for the first time. The structural, morphological, optical and electrical properties of the obtained samples were studied. XRD and Raman analysis confirm all the films exhibit different structure due to the incorporation of Mg2 þ or Cu2 þ into CTS film. The calculated value of the optical band gap for the CTS, CCTS and CMTS film is 1.4, 1.65 and 1.76 eV which is close to the optimum value for an efficient single or tandem solar cell material. The Hall Effect measurement shows all the films have p-type conductivity and the lowest resistivity is 4.82 10 3 Ω cm for CCTS and the highest resistivity is 1.412 10 1 Ω cm for CMTS.The results indicate the pure CTS and Cu2XSnS4 (X:Cu,Mg) films prepared by ultrasonic co- spray method can be suitable candidates for using in PV devices.
Acknowledgments This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Y2K4401DG0).
71
References [1] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, et al., Adv. Energy Mater. 4 (2014) 1301465. [2] T. Gokmen, O. Gunawan, T.K. Todorov, D.B. Mitzi, Appl. Phys. Lett. 103 (2013) 103506. [3] F. Ozel, M. Kus, A. Yar, E. Arkan, M. Can, A. Aljabour, et al., J. Mater. Sci. 50 (2015) 777–783. [4] L. Chen, H. Deng, J. Tao, H. Cao, L. Huang, L. Sun, et al., RSC Adv. 5 (2015) 84295–84302. [5] L. Meng, Y.F. Li, B. Yao, Z. Ding, G. Yang, R. Liu, et al., J. Phys. D Appl. Phys. 48 (2015) 445105. [6] H. Guan, H. Shen, C. Gao, X. He, J. Mater. Sci. Mater. Electron. 24 (2013) 1490–1494. [7] M. Wei, Q. Du, R. Wang, G. Jiang, W. Liu, C. Zhu, Chem. Lett. 43 (2014) 1149–1151. [8] G. Zhong, K. Tse, Y. Zhang, X. Li, C. Yang, J. Zhu, et al., arXiv Prepr. arXiv 1509 (2015) 06230. [9] J. He, L. Sun, Y. Chen, J. Jiang, P. Yang, J. Chu, J. Power Sources 273 (2015) 600–607. [10] J. Tao, J. Liu, J. He, K. Zhang, J. Jiang, L. Sun, et al., RSC Adv. 4 (2014) 23977–23984. [11] K. Zhang, J. Tao, J. He, W. Wang, L. Sun, P. Yang, et al., J. Mater. Sci. Mater. Electron. 25 (2014) 2703–2709. [12] T. Martini, C. Chubilleau, O. Poncelet, A. Ricaud, A. Blayo, C. Martin, et al., Sol. Energy Mater. Sol. Cells 144 (2016) 657–663. [13] N.S.C. Sulaiman, C.H. Nee, S.L. Yap, Y.S. Lee, T.Y. Tou, S.S. Yap, Appl. Surf. Sci. 354 (2015) 42–47. [14] U. Chalapathi, Y.K. Kumar, S. Uthanna, V.S. Raja, Thin Solid Films 556 (2014) 61–67. [15] U. Chalapathi, Y. Jayasree, S. Uthanna, V. Sundara Raja, Phys. Status Solidi A 210 (2013) 2384–2390. [16] H. Lei, G. Fang, F. Cheng, W. Ke, P. Qin, Z. Song, et al., Sol. Energy Mater. Sol. Cells 128 (2014) 77–84.