Cu-deficient CZTS (copper zinc tin sulfide) thin films were grown on soda lime as well as molybdenum coated soda lime glass by reactive cosputtering. Polycrystalline CZTS film with kesterite structure was produced by annealing it at 500°C in Ar atmosphere. These films were characterized for compositional, structural, surface morphological, optical, and transport properties using energy dispersive X-ray analysis, glancing incidence X-ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, UV-Vis spectroscopy, and Hall effect measurement. A CZTS solar cell device having conversion efficiency of ~0.11% has been made by depositing CdS, ZnO, ITO, and Al layers over the CZTS thin film deposited on Mo coated soda lime glass. The series resistance of the device was very high. The interfacial properties of device were characterized by cross-sectional SEM and cross-sectional HRTEM. 1. Introduction The optical and electronic properties of CZTS make it a suitable absorber layer for thin film photovoltaic. CZTS made from earth abundant and nontoxic elements is an ideal candidate to replace Cu(In,Ga)Se2 (CIGS) and CdTe which face material scarcity and toxicity issues for large scale production of solar cells. Therefore, thin film CZTS solar cell has received great attention in recent years [1–5]. CZTS is reported to have a band gap between 1.40 to 1.50？eV [1–5] and a band edge absorption coefficient above 104？cm？1 which makes it highly attractive as a single junction solar cell material. Efficiency up to 8.4% and 12.6% have been achieved for CZTS [6] and Cu2ZnSn(S,Se)4 (CZTS,Se) [7] solar cells, respectively. Efficiency of nearly 6.2% has been achieved using cosputtering of Cu, ZnS, and SnS target in Ar atmosphere [8]. In the case of CIGS and CdTe thin film solar cells, the best efficiency reported is ~21.7% [9] and 20.4% [10], respectively. The CZTS thin film solar cell device property is affected by the presence of phase impurities like Cu2S, Cu2SnS3, ZnS, SnS, and so on and poor quality of interfaces [11–14]. The open circuit voltage of the device which depends on the career generation and recombination process is controlled by the presence of defects [15–18]. The poor interfacial properties also affect the performance of the device [19, 20]. Generally, two-step process is used for the deposition of CZTS thin film (metallic film deposition and then sulfurization). In the two-step process, during sulfurization, the film surface becomes rough which may hinder the formation of proper junction when n-type CdS is coated over it and
References
[1]
H. Katagiri, N. Sasaguchi, S. Hando, S. Hoshino, J. Ohashi, and T. Yokota, “Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E-B evaporated precursors,” Solar Energy Materials and Solar Cells, vol. 49, no. 1–4, pp. 407–414, 1997.
[2]
J. S. Seol, S. Y. Lee, J. C. Lee, H. D. Nam, and K. H. Kim, “Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process,” Solar Energy Materials and Solar Cells, vol. 75, no. 1-2, pp. 155–162, 2003.
[3]
K. Tanaka, N. Moritake, and H. Uchiki, “Preparation of Cu2ZnSnS4 thin films by sulfurizing sol-gel deposited precursors,” Solar Energy Materials and Solar Cells, vol. 91, no. 13, pp. 1199–1201, 2007.
[4]
A. I. Inamdar, S. Lee, K.-Y. Jeon et al., “Optimized fabrication of sputter deposited Cu2ZnSnS4 (CZTS) thin films,” Solar Energy, vol. 91, pp. 196–203, 2013.
[5]
A. Emrani, P. Vasekar, and C. R. Westgate, “Effects of sulfurization temperature on CZTS thin film solar cell performances,” Solar Energy, vol. 98, pp. 335–340, 2013.
[6]
B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey, and S. Guha, “Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber,” Progress in Photovoltaics: Research and Applications, vol. 21, no. 1, pp. 72–76, 2013.
[7]
W. Wang, M. T. Winkler, O. Gunawan et al., “Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency,” Advanced Energy Materials, vol. 4, no. 7, Article ID 1301465, 2014.
[8]
T. P. Dhakal, C. Y. Peng, R. Reid Tobias, R. Dasharathy, and C. R. Westgate, “Characterization of a CZTS thin film solar cell grown by sputtering method,” Solar Energy, vol. 100, pp. 23–30, 2014.
[9]
The best efficiency report is currently from ZSW and corresponds to 21.7%, http://www.pv-tech.org/news/zsw_sets_21.7_cigs_cell_record.
H. Katagiri, K. Jimbo, S. Yamada et al., “Enhanced conversion efficiencies of Cu2ZnSnS4-based thin film solar cells by using preferential etching technique,” Applied Physics Express, vol. 1, no. 4, Article ID 041201, 2008.
[12]
H. Katagiri, K. Saitoh, T. Washio, H. Shinohara, T. Kurumadani, and S. Miyajima, “Development of thin film solar cell based on Cu2ZnSnS4 thin films,” Solar Energy Materials and Solar Cells, vol. 65, no. 1, pp. 141–148, 2001.
[13]
S. Ahmed, K. B. Reuter, O. Gunawan, L. Guo, L. T. Romankiw, and H. Deligianni, “A high efficiency electrodeposited Cu2ZnSnS4 solar cell,” Advanced Energy Materials, vol. 2, no. 2, pp. 253–259, 2012.
[14]
F. Jiang, H. Shen, W. Wang, and L. Zhang, “Preparation and properties of Cu2ZnSnS4 absorber and Cu2ZnSnS4/amorphous silicon thin-film solar cell,” Applied Physics Express, vol. 4, no. 7, Article ID 074101, 2011.
[15]
A. Ennaoui, M. Lux-Steiner, A. Weber et al., “Cu2ZnSnS4 thin film solar cells from electroplated precursors: novel low-cost perspective,” Thin Solid Films, vol. 517, no. 7, pp. 2511–2514, 2009.
[16]
A. Walsh, S. Chen, S.-H. Wei, and X.-G. Gong, “Kesterite thin-film solar cells: Advances in materials modelling of Cu2ZnSnS4,” Advanced Energy Materials, vol. 2, no. 4, pp. 400–409, 2012.
[17]
S. Chen, X. G. Gong, A. Walsh, and S.-H. Wei, “Defect physics of the kesterite thin-film solar cell absorber Cu2ZnSnS4,” Applied Physics Letters, vol. 96, no. 2, Article ID 021902, 2010.
[18]
S. Chen, A. Walsh, X.-G. Gong, and S.-H. Wei, “Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers,” Advanced Materials, vol. 25, no. 11, pp. 1522–1539, 2013.
[19]
O. Gunawan, T. K. Todorov, and D. B. Mitzi, “Loss mechanisms in hydrazine-processed Cu2ZnSn(Se,S)4 solar cells,” Applied Physics Letters, vol. 97, no. 23, Article ID 233506, 2010.
[20]
T. Gokmen, O. Gunawan, T. K. Todorov, and D. B. Mitzi, “Band tailing and efficiency limitation in kesterite solar cells,” Applied Physics Letters, vol. 103, no. 10, Article ID 103506, 2013.
[21]
F. Liu, Y. Li, K. Zhang et al., “In situ growth of Cu2ZnSnS4 thin films by reactive magnetron co-sputtering,” Solar Energy Materials and Solar Cells, vol. 94, no. 12, pp. 2431–2434, 2010.
[22]
J. B. Li, V. Chawla, and B. M. Clemens, “Investigating the role of grain boundaries in CZTS and CZTSSe thin film solar cells with scanning probe microscopy,” Advanced Materials, vol. 24, no. 6, pp. 720–723, 2012.
[23]
J. J. Scragg, T. Ericson, X. Fontané, et al., “Rapid annealing of reactively sputtered precursors for Cu2ZnSnS4 solar cells,” Progress in Photovoltaics: Research and Applications, vol. 22, no. 1, pp. 10–17, 2014.
[24]
O. P. Singh, N. Muhunthan, V. N. Singh, and B. P. Singh, “Effect of annealing time on the composition, microstructure and band gap of copper zinc tin sulfide thin films,” Advanced Materials Letters, 2014.
[25]
O. P. Singh, N. Muhunthan, V. N. Singh, K. Samanta, and N. Dilawar, “Effect of temperature on thermal expansion and anharmonicity in Cu2ZnSnS4 thin films grown by co-sputtering and sulfurization,” Materials Chemistry and Physics, vol. 146, no. 3, pp. 452–455, 2014.
[26]
K. Ito and T. Nakazawa, “Electrical and optical properties of stannite-type quaternary semiconductor thin films,” Japanese Journal of Applied Physics, vol. 27, no. 11, pp. 2094–2097, 1988.
[27]
N. Muhunthan, O. P. Singh, S. Singh, and V. N. Singh, “Growth of CZTS thin films by cosputtering of metal targets and sulfurization in H2S,” International Journal of Photoenergy, vol. 2013, Article ID 752012, 7 pages, 2013.
[28]
F. Jiang, H. Shen, J. Jin, and W. Wang, “Preparation and optoelectronic properties of Cu2ZnSnS4 film,” Journal of the Electrochemical Society, vol. 159, no. 6, pp. H565–H569, 2012.
[29]
T. Tanaka, T. Nagatomo, D. Kawasaki, et al., “Preparation of Cu2ZnSnS4 thin films by hybrid sputtering,” Journal of Physics and Chemistry of Solids, vol. 66, no. 11, pp. 1978–1981, 2005.
[30]
J. C. Gonzalez, G. M. Ribeiro, E. R. Viana et al., “Hopping conduction and persistent photoconductivity in Cu2ZnSnS4 thin films,” Journal of Physics D: Applied Physics, vol. 46, no. 15, Article ID 155107, 2013.
[31]
M. Z. Ansari and N. Khare, “Structural and optical properties of CZTS thin films deposited by ultrasonically assisted chemical vapour deposition,” Journal of Physics D: Applied Physics, vol. 47, no. 18, Article ID 185101, 2014.
[32]
S. J. Wang, H. Wang, K. Du, W. Zhang, M. L. Sui, and S. X. Mao, “Deformation-induced structural transition in body-centred cubic molybdenum,” Nature Communications, vol. 5, article 3433, 2014.
[33]
C.-H. Ruan, C.-C. Huang, Y.-J. Lin, G.-R. He, H.-C. Chang, and Y.-H. Chen, “Electrical properties of CuxZnySnS4 films with different Cu/Zn ratios,” Thin Solid Films, vol. 550, pp. 525–529, 2014.
[34]
S. Sarkar, K. Bhattacharjee, G. C. Das, and K. K. Chattopadhyay, “Self-sacrificial template directed hydrothermal route to kesterite-Cu2ZnSnS4 microspheres and study of their photo response properties,” CrystEngComm, vol. 16, no. 13, pp. 2634–2644, 2014.
[35]
D. Lang, Q. Xiang, G. Qiu, X. Feng, and F. Liu, “Effects of crystalline phase and morphology on the visible light photocatalytic H2-production activity of CdS nanocrystals,” Dalton Transactions, vol. 43, no. 19, pp. 7245–7253, 2014.
