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Microstructures and electronic structures of Cu2ZnSnS4 (CZTS) grain-boundaries (GB) are studied by the first-principles electronic structure method. Some special twist grain-boundaries have low grain-boundary energies and exhibit similar electronic structure as that in a perfect crystal. The twist grain-boundaries such as 3[221] and 6[221] have grain-boundary planes parallel to (112) plane, the easiest cleavage plane, so that they have small damages to the crystal structure and small influence on the properties of the materials. Grain-boundary plays two roles in CZTS thin-films: (1) capturing and trapping holes from p-n junctions, and (2) providing fast channels for transportation of majority carriers. As the majority of carriers, the positively charged holes need override a barrier before being trapped by a potential-well in the grain-boundary region. For the minority of carriers, the grain boundary is a high barrier to prevent electrons from transporting across it. The intrinsic nature of the potential barrier is not very clear. By calculating the distributions of static potentials across different grain boundaries of CZTS and also by comparing them with those across different surfaces, we find that the potential barriers at grain boundaries are the remnants of the potential barriers of surfaces, which trap the electrons in the bulk and prevent the electrons from escaping from the bulk to vacuum. When two surfaces get contact to form a grain boundary the corresponding surface barriers will be merged together as one potential barrier of the grain boundary. It is obvious that if a grain boundary intersects with the surface, the escaping work function near the grain boundary is lower than that near the prefect crystal surface. Experiment shows the coexistence of Sn4+ and Sn2+ions. The Sn4+ ions are located in the bulk by bonding 4 S atoms as neighbors. Our results show that Sn2+ ions can appear in the grain-boundary regions, on the surfaces or in the bulk with lattice defects so that Sn2+ ions have the lower coordination number by bonding 3 S atoms. The Sn atom is favored to be at the center of S octahedron with six neighboring S (or O) atoms in most sulfides (oxides) of tin. In CZTS, Sn atom is at the center of tetrahedron with 4 neighboring S atoms so that Sn atom is very active to move by structural relaxations. Most importantly the conduction-bands in CZTS are formed by the hybridizations between the s electrons of Sn and p electrons of S so that the conduction-bands of CZTS are sensitively dependent on the distributions and properties of Sn atoms. The appearing of Sn2+ ions and the strong structural relaxations of Sn atoms in grain-boundary regions and on surfaces induce extra in-gap states as a new source for the recombination of electron-hole pairs that are un-favored to the photo-voltage effects. Generally, the grain boundary plays a negative role in brittle photo-voltage materials such as Si and GaAs, and the positive role in ductile photo-voltage materials such as CdTe and CIGS (Cu(InGa)Se2). It means that the growth of the hard and brittle films is very difficult, the micro-cracks and micro-pores are easily created. Our calculations show that CdTe, CIGS and CZTS are all ductile with Poisson-ratio greater than 0.33. This means that CZTS can be used as the absorber of flexible solar cell. By comparing the optical absorption-coefficients of crystals, grain-boundaries, surfaces and nano-particles, we find that the internal surfaces in thin-films with high pore-ratio can create new energy-levels in band-gap, which enhances the recombination between electrons and holes and decreases the optical absorption-coefficients (1.3 eV). As a result, the high dense CZTS thin-film is required for high-efficient CZTS solar-cell. The positive role of grain boundary is more important if the CZTS film has the large, unique oriented grains and the uniform distribution of grain sizes. The simple and regular grain-boundary network is more beneficial to the coherent transport of majority carriers.
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Keywords:
- kesterite Cu2ZnSnS4 /
- solar cell /
- density functional theory /
- grain boundary
[1] Ito K, Nakazawa T 1988 Japanese Journal of Applied Physics 27 2094
[2] Shin B, Gunawan O, Zhu Y, Bojarczuk N A, Chey S J, Guha S 2013 Prog. Photovolt: Res. Appl. 21 72
[3] Xu Jia-Xiong, Yao Ruo-He 2012 Acta Phys. Sin. 61 187304 (in Chinese) [许佳雄, 姚若河 2012 61 187304]
[4] Guo Q, Ford G M, Yang W C, Walker B C, Stach E A, Hillhouse H W, Agrawal R 2010 J. Am. Chem. Soc. 132 17384
[5] Wang W, Winkler M T, Gunawan O, Gokmen T, Todorov T K, Zhu Y, Mitzi D B 2014 Adv. Energy Mater. 4 1301465
[6] Chen Q M, Li Z Q, Ni Y, Cheng S Y, Dou X M 2012 Chin. Phys. B 21 038401
[7] Strohm A, Eisenmann L, Gebhardt R K, Harding A, Schlötzer T, Abou-Ras D, Schock H W 2005 Thin Solid Film 480-481 162
[8] Jiang C -S, Noufi R, AbuShama J A, Ramanathan K, Moutinho H R, Pankow J, AI-Jassim M M 2004 Applied Physics Letters 84 3477
[9] Azulay D, Millo O, Balberg I, Schock H W, Visoly-Fisher I, Cahen D 2007 Solar Energy Materials & Sollar Cells 91 85
[10] Azulay D, Balberg I, Millo O 2012 Phys. Rev. Lett. 108 076603
[11] Li J B, Chawla V, Clemens B M 2012 Adv. Mater. 24 720
[12] Jeong A R, Jo W, Jung S, Gwak J, Yun J H 2011 Applied Physics Letters 99 082103
[13] Haight R, Shao X Y, Wang W, Mitzi D B 2014 Applied Physics Letters 104 033902
[14] Kosyak V, Karmarkar M A, Scarpulla M A 2012 Applied Physics Letters 100 263903
[15] Mendis B G, Goodman M C J, Major J D, Tayler A A, Durose K, Halliday D P 2012 Journal of Applied Physics 112 124508
[16] Persson C, Zunger A 2003 Phys. Rev. Lett. 91 266401
[17] Persson C, Zunger A 2005 Applied Physics Letters 87 211904
[18] Schmidt S S, Abou-Ras D, Sadewasser S, Yin W J, Feng C B Yan Y F 2012 Phys. Rev. Lett. 109 095506
[19] Yan Y F, Jiang C S, Noufi R, Wei S H, Moutinho H R, Al-Jassim M M 2007 Phys. Rev. Lett. 99 235504
[20] Li J W, Mitzi D B, Shenoy V B 2011 ACSNANO 5 8613
[21] Medvedeva N I, Shalaeva E V, Kuznetsov M V, Yakushev M V 2006 Phys. Rev. B 73 035207
[22] Dong Z Y, Li Y F, Yao B, Ding Z H, Yang G, Deng R, Fang X, Wei Z P, Liu L 2014 J. Phys. D: Appl. Phys. 47 075304
[23] Bao W, Ichimura M 2012 International Journal of Photoenergy,ArticleID 61982
[24] Xu P, Chen S Y, Huang B, Xiang H J, Gong X G, Wei S H 2013 Phys. Rev. B 88 045427
[25] Kresse G, Joubert D 1999 Phys. Rev. B 59 1758
[26] Blöchl P E 1994 Phys. Rev. B 50 17953
[27] Perdew J P, Burke K, Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
[28] Gajdoš M, Hummer K, Kresse G, Furthmller J, Bechstedt F 2006 Phys. Rev. B 73 045112
[29] Sutton A P, Balluffi R W 1995 Interface in Crystalline Materials, Clarendon Press, Oxford
[30] Balluffi R W 1982 Metall. Trans. B 13 527
[31] Fan W, Liu D Y, Zeng Z 2014 Physica C 497 110
[32] Grossberg M, Raadik T, Raudoja J, Krustok J 2014 Current Applied Phsyics 14 447
[33] Henkelman G, Arnaldsson A, Jónsson H 2006 Comput. Mater. Sci. 36 354
[34] Yang C Y, Qin M S, Wang Y M, Wan D Y, Huang F Q, Lin J H 2013 Sci. Rep. 3, 1286
[35] Fan Wei, Zeng Zhi 2014 Acta Phys. Sin. 63 047503 (in Chinese) [范巍, 曾雉 2014 63 047503]
[36] Liu Hao, Xue Yu-Ming, Qiao Zai-Xiang, Li Wei, Zhang Chao, Yin Fu-Hong, Feng Shao-Jun 2015 Acta Phys. Sin.64 068801 (in Chinese) [刘浩, 薛玉明, 乔在祥, 李微, 张超, 尹富红, 冯少君 2015 64 068801]
[37] Sun K W, Su Z H, Han Z L, Liu F Y, Lai Y Q, Li J, Liu Y X 2014 Acta Phys. Sin. 63 018801 (in Chinese) [孙凯文, 苏正华, 韩自力, 刘芳洋, 赖延清, 李劼, 刘业翔 2014 63 018801]
[38] Momma K, Izumi F 2008 J. Appl. Crystallogr. 41 653
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[1] Ito K, Nakazawa T 1988 Japanese Journal of Applied Physics 27 2094
[2] Shin B, Gunawan O, Zhu Y, Bojarczuk N A, Chey S J, Guha S 2013 Prog. Photovolt: Res. Appl. 21 72
[3] Xu Jia-Xiong, Yao Ruo-He 2012 Acta Phys. Sin. 61 187304 (in Chinese) [许佳雄, 姚若河 2012 61 187304]
[4] Guo Q, Ford G M, Yang W C, Walker B C, Stach E A, Hillhouse H W, Agrawal R 2010 J. Am. Chem. Soc. 132 17384
[5] Wang W, Winkler M T, Gunawan O, Gokmen T, Todorov T K, Zhu Y, Mitzi D B 2014 Adv. Energy Mater. 4 1301465
[6] Chen Q M, Li Z Q, Ni Y, Cheng S Y, Dou X M 2012 Chin. Phys. B 21 038401
[7] Strohm A, Eisenmann L, Gebhardt R K, Harding A, Schlötzer T, Abou-Ras D, Schock H W 2005 Thin Solid Film 480-481 162
[8] Jiang C -S, Noufi R, AbuShama J A, Ramanathan K, Moutinho H R, Pankow J, AI-Jassim M M 2004 Applied Physics Letters 84 3477
[9] Azulay D, Millo O, Balberg I, Schock H W, Visoly-Fisher I, Cahen D 2007 Solar Energy Materials & Sollar Cells 91 85
[10] Azulay D, Balberg I, Millo O 2012 Phys. Rev. Lett. 108 076603
[11] Li J B, Chawla V, Clemens B M 2012 Adv. Mater. 24 720
[12] Jeong A R, Jo W, Jung S, Gwak J, Yun J H 2011 Applied Physics Letters 99 082103
[13] Haight R, Shao X Y, Wang W, Mitzi D B 2014 Applied Physics Letters 104 033902
[14] Kosyak V, Karmarkar M A, Scarpulla M A 2012 Applied Physics Letters 100 263903
[15] Mendis B G, Goodman M C J, Major J D, Tayler A A, Durose K, Halliday D P 2012 Journal of Applied Physics 112 124508
[16] Persson C, Zunger A 2003 Phys. Rev. Lett. 91 266401
[17] Persson C, Zunger A 2005 Applied Physics Letters 87 211904
[18] Schmidt S S, Abou-Ras D, Sadewasser S, Yin W J, Feng C B Yan Y F 2012 Phys. Rev. Lett. 109 095506
[19] Yan Y F, Jiang C S, Noufi R, Wei S H, Moutinho H R, Al-Jassim M M 2007 Phys. Rev. Lett. 99 235504
[20] Li J W, Mitzi D B, Shenoy V B 2011 ACSNANO 5 8613
[21] Medvedeva N I, Shalaeva E V, Kuznetsov M V, Yakushev M V 2006 Phys. Rev. B 73 035207
[22] Dong Z Y, Li Y F, Yao B, Ding Z H, Yang G, Deng R, Fang X, Wei Z P, Liu L 2014 J. Phys. D: Appl. Phys. 47 075304
[23] Bao W, Ichimura M 2012 International Journal of Photoenergy,ArticleID 61982
[24] Xu P, Chen S Y, Huang B, Xiang H J, Gong X G, Wei S H 2013 Phys. Rev. B 88 045427
[25] Kresse G, Joubert D 1999 Phys. Rev. B 59 1758
[26] Blöchl P E 1994 Phys. Rev. B 50 17953
[27] Perdew J P, Burke K, Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
[28] Gajdoš M, Hummer K, Kresse G, Furthmller J, Bechstedt F 2006 Phys. Rev. B 73 045112
[29] Sutton A P, Balluffi R W 1995 Interface in Crystalline Materials, Clarendon Press, Oxford
[30] Balluffi R W 1982 Metall. Trans. B 13 527
[31] Fan W, Liu D Y, Zeng Z 2014 Physica C 497 110
[32] Grossberg M, Raadik T, Raudoja J, Krustok J 2014 Current Applied Phsyics 14 447
[33] Henkelman G, Arnaldsson A, Jónsson H 2006 Comput. Mater. Sci. 36 354
[34] Yang C Y, Qin M S, Wang Y M, Wan D Y, Huang F Q, Lin J H 2013 Sci. Rep. 3, 1286
[35] Fan Wei, Zeng Zhi 2014 Acta Phys. Sin. 63 047503 (in Chinese) [范巍, 曾雉 2014 63 047503]
[36] Liu Hao, Xue Yu-Ming, Qiao Zai-Xiang, Li Wei, Zhang Chao, Yin Fu-Hong, Feng Shao-Jun 2015 Acta Phys. Sin.64 068801 (in Chinese) [刘浩, 薛玉明, 乔在祥, 李微, 张超, 尹富红, 冯少君 2015 64 068801]
[37] Sun K W, Su Z H, Han Z L, Liu F Y, Lai Y Q, Li J, Liu Y X 2014 Acta Phys. Sin. 63 018801 (in Chinese) [孙凯文, 苏正华, 韩自力, 刘芳洋, 赖延清, 李劼, 刘业翔 2014 63 018801]
[38] Momma K, Izumi F 2008 J. Appl. Crystallogr. 41 653
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