受到石化能源耗竭與其對環境及人體的危害影響，綠色能源是現今重要的發展方向，而有機無機混成型鈣鈦礦材料的出現大大改變了傳統的太陽能電池，它不僅在結構與能隙方面具可調性，低成本、高轉換效率的優點更吸引了廣泛的研究，至2017年其光電轉換效率已達22.7%。然而，儘管MAPbI3已經被證實擁有相當高的轉換效率，但鉛存在的毒性及不穩定性仍成為大規模應用上的限制，錫因此被提出為無鉛鈣鈦礦材料的第一候選元素，期待能得到雙贏的效果。
我們利用第一原理計算研究有機無機混成鈣鈦礦ASnX3（A = CH3NH3+簡寫MA， NH2CHNH2+簡寫FA，X = Cl, Br, I）之幾何與能帶結構。觀察鈣鈦礦框架中Sn及不同鹵素原子X對於能帶、軌域、態密度以及實空間電荷分佈的影響，也探討內部有機分子如何影響鈣鈦礦的主要架構，藉由這些結果有系統地分析鈣鈦礦材料的結構穩定性及電性；同時在本研究中，也計算了六種不同鈣鈦礦材料之輸運特性，進一步探討電子傳輸的情況。
我們發現，有機分子與鹵素原子混成導致鈣鈦礦系統的幾何優化結構產生扭曲、對稱性下降，其中因為電負度差異的影響又以ASnCl3最為嚴重；能帶結構以SnX3的離子性質影響為主，有機分子A的主要作為是提供電子，但其分子的不安定特性會導致整體結構對稱下降而間接造成能隙的改變，當鹵素X原子由Cl, Br換至I時能隙會降低，同時使吸光範圍增大且產生紅移。由態密度圖的分析可發現，此類鈣鈦礦材料的價帶主要由鹵素X的p軌域貢獻；而導帶則由Sn的p軌域主導。觀察電荷密度投影圖會發現，FA比起MA的電子密度分布是較均勻的，所以能夠均勻的支撐起鈣鈦礦框架，得到穩定性較高的立方體結構。
在輸運性質的部分，我們發現此類鈣鈦礦材料主要為電子傳輸，其中以鹵素原子I系統之傳遞效果較好，這與Sn-I間電負度差異最小有關。混成鈣鈦礦材料中的有機分子是成為雙極性半導體材料的重要關鍵，它可以使電子與電洞對有效分離並減少再結合反應的發生，且透過與鹵素間的氫鍵作用力間接影響Sn-X間軌域的混成改變價電帶型態，同時，計算結果也顯示材料內之有機分子的方向，並不直接影響整體傳輸特性，然而，散射中心的空穴缺陷仍會有影響。FASnI3是作為理想的太陽能電池吸收層材料的最佳選擇。

Solar energy is the most abundant and clean form of energies offering a solution to the increasing concern of energy exhaustion and greenhouse gas by fossil fuels. The solar cells based on hybrid organic-inorganic halide perovskites as light absorbers are new players in the field of third generation photovoltaics, owing to their wide variability and multiple opportunities for fine-tuning performance. Otherwise, they are simple solution-processable and low-cost materials. For practical applications, the toxicology issue of Pb needs to be avoided for large scale commercialization, and the most likely substitution is Sn which is nontoxic and belongs to the same group in the periodic table. In this work, we systematically investigate the structural and electronic properties of simple cubic Sn-based hybrid perovskite, ASnX3 (A=FA+, MA+; X= Cl, Br, I), based on density functional theory within local density approximation. Consequently, we find that the electronic properties mainly depend on the SnX3 cage size and ionicity, while the organic molecules play the role as an electron donor and indirectly affect the band gap values by modifying the symmetry of cells. As X changes from Cl to Br to I, the band gap decreases, and the absorption edge have a red shift. It turns out from projected density of state analysis that the main contributions of VBM is from p states of X atoms, while in CBM, the essentially component is p states of Sn atoms. Additionally, the charge density distribution of MA and FA cations also be studied. For FA cations, they are more uniform separated than MA, that is, the FA cations interact with the SnX3 frame intensely possessing more stable structures. According to the charge transport calculations, we found that this kind of perovskite materials have a strong electron transfer capacity. Because of the smallest electronegativity difference between Sn-I, ASnI3 systems own the largest transmission spectra. The organic cations discussed in our research are the primary cause for ambipolarity of perovskite giving rise to desirable semiconductor properties. Their orientations do not really affect the transport properties, but a vacuum defect inside the scattering region can do so. Considering as an optimal absorber material of solar cells, FASnI3 would be the best choice.

[1] 台電系統歷年發電量統計, https://www.taipower.com.tw/tc/.
[2] Renewable 2016 Global Status Report, https://natgrp.wordpress.com/2017/02/10/renewables-2016-global-status-report/.
[3] NREL Efficiency Chart, https://www.nrel.gov/pv/assets/images/efficiency-chart.png.
[4] I. Mesquita, L. Andrade, and A. Mendes, "Perovskite solar cells : Materials, configurations and stability," Renewable and Sustainable Energy Reviews, vol. 82, pp. 2471-2489, 2018.
[5] G. R. Berdiyorov, M. E. Madjet, and F. El-Mellouhi, "Improved Electronic Transport Properties of Tin-Halide Perovskites," Solar Energy Materials and Solar Cells, vol. 170, pp. 8-12, 2017.
[6] A. Kojima, K. Teshima, Y. Shirai, and T. Miyasaka, "Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells," Journal of the American Chemical Society, vol. 131, pp. 6050-6051, 2009.
[7] Y. Zhou, M. Yang, W. Wu, A. L. Vasiliev, K. Zhu, and N. P. Padture, "Room-temperature crystallization of hybrid-perovskite thin films via solvent–solvent extraction for high-performance solar cells," Journal of Materials Chemistry A, vol. 3, pp. 8178-8184, 2015.
[8] V. A. Hintermayr et al., "Tuning the Optical Properties of Perovskite Nanoplatelets through Composition and Thickness by Ligand-Assisted Exfoliation," Advanced Materials, vol. 28, pp. 9478-9485, 2016.
[9] A. Toshniwal and V. Kheraj, "Development of organic-inorganic tin halide perovskites : A review," Solar Energy, vol. 149, pp. 54-59, 2017.
[10] C. Zhang, L. Gao, S. Hayase, and T. Ma, "Current Advancements in Material Research and Techniques Focusing on Lead-free Perovskite Solar Cells," Chemistry Letters, vol. 46, pp. 1276-1284, 2017.
[11] Z. Yang, Y. Wang, and Y. Liu, "Stability and charge separation of different CH 3 NH 3 SnI 3 /TiO 2 interface: A first-principles study," Applied Surface Science, vol. 441, pp. 394-400, 2018.
[12] M. C. Payne, M. P. Teter, D. C. Allan, T. A. Arias, and J. D. Joannopoulos, "Iterative minimization techniques forab initiototal-energy calculations: molecular dynamics and conjugate gradients," Reviews of Modern Physics, vol. 64, pp. 1045-1097, 1992.
[13] H. J. Monkhorst and J. D. Pack, "Special points for Brillouin-zone integrations," Physical Review B, vol. 13, pp. 5188-5192, 1976.
[14] J. Even et al., "Solid-State Physics Perspective on Hybrid Perovskite Semiconductors," The Journal of Physical Chemistry C, vol. 119, pp. 10161-10177, 2015.
[15] K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J. Fujisawa, and M. Hanaya, "Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes," Chem Commun vol. 51, pp. 15894-15897, 2015.
[16] P. Pinpithak, A. Kulkarni, H. W. Chen, M. Ikegami, and T. Miyasaka, "Solid-State Thin-Film Dye-Sensitized Solar Cell Co-Sensitized with Methylammonium Lead Bromide Perovskite," Bulletin of the Chemical Society of Japan, vol. 91, pp. 754-760, 2018.
[17] H. S. Kim et al., "Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%," Scientific Reports, vol. 2, pp. 591-597, 2012.
[18] F. Brivio, A. B. Walker, and A. Walsh, "Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics from first-principles," APL Materials, vol. 1, pp. 042111-042115, 2013.
[19] Q. Chen et al., "Under the spotlight: The organic–inorganic hybrid halide perovskite for optoelectronic applications," Nano Today, vol. 10, pp. 355-396, 2015.
[20] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, and H. J. Snaith, "Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites," Science vol. 338, pp. 643-647, 2012.
[21] J. Burschka et al., "Sequential deposition as a route to high-performance perovskite-sensitized solar cells," Nature, vol. 499, pp. 316-319, 2013.
[22] N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, and S. I. Seok, "Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells," Nature Materials, vol. 13, pp. 897-903, 2014.
[23] J. Nakazaki and H. Segawa, "Evolution of organometal halide solar cells," Journal of Photochemistry and Photobiology C: Photochemistry Reviews, vol. 35, pp. 74-107, 2018.
[24] H. S. Kim, S. H. Im, and N. G. Park, "Organolead Halide Perovskite : New Horizons in Solar Cell Research," The Journal of Physical Chemistry C, vol. 118, pp. 5615-5625, 2014.
[25] I. Borriello, G. Cantele, and D. Ninno, "Ab initio investigation of hybrid organic-inorganic perovskites based on tin halides," Physical Review B, vol. 77, pp. 235214-235222, 2008.
[26] C. C. Stoumpos, C. D. Malliakas, and M. G. Kanatzidis, "Semiconducting tin and lead iodide peraovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties," Inorganic Chemistry, vol. 52, pp. 9019-9038, 2013.
[27] I. E. Castelli, J. M. G. Lastra, K. S. Thygesen, and K. W. Jacobsen, "Bandgap calculations and trends of organometal halide perovskites," APL Materials, vol. 2, pp. 081514-081520, 2014.
[28] M. A. Green, A. H. Baillie, and H. J. Snaith, "The emergence of perovskite solar cells," Nature Photonics, vol. 8, pp. 506-514, 2014.
[29] J. H. Im, J. Chung, S. J. Kim, and N. G. Park, "Synthesis, structure, and photovoltaic property of a nanocrystalline 2H perovskite-type novel sensitizer (CH3CH2NH3)PbI3," Nanoscale Research Letters, vol. 7, pp. 353-359, 2012.
[30] M. Ye et al., "Recent advances in dye-sensitized solar cells: from photoanodes, sensitizers and electrolytes to counter electrodes," Materials Today, vol. 18, pp. 155-162, 2015.
[31] M. A. Loi and J. C. Hummelen, "Hybrid solar cells: Perovskites under the Sun," Nature Materials, vol. 12, pp. 1087-1089, 2013.
[32] A. Binek, F. C. Hanusch, P. Docampo, and T. Bein, "Stabilization of the Trigonal High-Temperature Phase of Formamidinium Lead Iodide," Journal of Physical Chemistry Letters, vol. 6, pp. 1249-1253, 2015.
[33] J. Seo, J. H. Noh, and S. I. Seok, "Rational Strategies for Efficient Perovskite Solar Cells," Accounts of Chemical Research, vol. 49, pp. 562-572, 2016.
[34] I. Levchuk et al., "Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X = Cl, Br, I) Colloidal Nanocrystals," Nano Letters, vol. 17, pp. 2765-2770, 2017.
[35] L. Jiang et al., "First-Principles Screening of Lead-Free Methylammonium Metal Iodine Perovskites for Photovoltaic Application," The Journal of Physical Chemistry C, vol. 121, pp. 24359-24364, 2017.
[36] N. K. Noel et al., "Lead-free organic–inorganic tin halide perovskites for photovoltaic applications," Energy Environmental Science, vol. 7, pp. 3061-3068, 2014.
[37] Elizabeth S. Parrott, Rebecca L. Milot, T. Stergiopoulos, H. J. Snaith, M. B. Johnston, and L. M. Herz, "Effect of Structural Phase Transition on Charge-Carrier Lifetimes and Defects in CH3NH3SnI3 Perovskite," Journal of Physical Chemistry Letters, vol. 7, pp. 1321-1326, 2016.
[38] Z. Zhao et al., "Mixed-Organic-Cation Tin Iodide for Lead-Free Perovskite Solar Cells with an Efficiency of 8.12%," Advanced Science, vol. 4, pp. 1700204-1700210, 2017.
[39] T. B. Song, T. Yokoyama, S. Aramaki, and M. G. Kanatzidis, "Performance Enhancement of Lead-Free Tin-Based Perovskite Solar Cells with Reducing Atmosphere-Assisted Dispersible Additive," ACS Energy Letters, vol. 2, pp. 897-903, 2017.
[40] T. Yokoyama, T. B. Song, D. H. Cao, C. C. Stoumpos, S. Aramaki, and M. G. Kanatzidis, "The Origin of Lower Hole Carrier Concentration in Methylammonium Tin Halide Films Grown by a Vapor-Assisted Solution Process," ACS Energy Letters, vol. 2, pp. 22-28, 2016.
[41] M. C. Jung, S. R. Raga, and Y. Qi, "Properties and solar cell applications of Pb-free perovskite films formed by vapor deposition," RSC Advances, vol. 6, pp. 2819-2825, 2016.
[42] T. Yokoyama et al., "Overcoming Short-Circuit in Lead-Free CH3NH3SnI3 Perovskite Solar Cells via Kinetically Controlled Gas-Solid Reaction Film Fabrication Process," Journal of Physical Chemistry Letters, vol. 7, pp. 776-782, 2016.
[43] C. Liu, J. Fan, H. Li, C. Zhang, and Y. Mai, "Highly Efficient Perovskite Solar Cells with Substantial Reduction of Lead Content," Scientific Reports, vol. 6, pp. 35705-35712, 2016.
[44] Z. Yang et al., "Stable Low-Bandgap Pb-Sn Binary Perovskites for Tandem Solar Cells," Advanced Materials, vol. 28, pp. 8990-8997, 2016.
[45] G. E. Eperon et al., "Perovskite-perovskite tandem photovoltaics with optimized band gaps," Science, vol. 354, pp. 861-865, 2016.
[46] J. e. M. Soler et al., "The SIESTA method for ab initio order-N materials simulation," Journal of Physics: Condensed Matter, vol. 14, pp. 2745-2779, 2002.
[47] J. Taylor, H. Guo, and J. Wang, "Ab initiomodeling of quantum transport properties of molecular electronic devices," Physical Review B, vol. 63, pp. 245407-245419, 2001.
[48] Y. Yuan, R. Xu, H.-T. Xu, F. Hong, F. Xu, and L.-J. Wang, "Nature of the band gap of halide perovskites ABX3 (A = CH3NH3, Cs; B = Sn, Pb; X = Cl, Br, I): First-principles calculations," Chinese Physics B, vol. 24, pp. 116302-116306, 2015.
[49] O. Granas, D. Vinichenko, and E. Kaxiras, "Establishing the limits of efficiency of perovskite solar cells from first principles modeling," Scientific Reports, vol. 6, pp. 36108-36113, 2016.
[50] Y. Takahashi et al., "Charge-transport in tin-iodide perovskite CH3NH3SnI3: origin of high conductivity," Dalton Transactions, vol. 40, pp. 5563-5568, 2011.
[51] Z. Q. Ma, H. Pan, and P. K. Wong, "A First-Principles Study on the Structural and Electronic Properties of Sn-Based Organic–Inorganic Halide Perovskites," Journal of Electronic Materials, vol. 45, pp. 5956-5966, 2016.
[52] Y. Y. Pan, Y.-H. Su, C.-H. Hsu, L. W. Huang, and C. C. Kaun, "The electronic structure of organic–inorganic hybrid perovskite solar cell: A first-principles analysis," Computational Materials Science, vol. 117, pp. 573-578, 2016.
[53] L. Lang, J. H. Yang, H. R. Liu, H. J. Xiang, and X. G. Gong, "First-principles study on the electronic and optical properties of cubic ABX3 halide perovskites," Physics Letters A, vol. 378, pp. 290-293, 2014.
[54] T. Shi et al., "Effects of organic cations on the defect physics of tin halide perovskites," Journal of Materials Chemistry A, vol. 5, pp. 15124-15129, 2017.
[55] W. J. Yin, J. H. Yang, J. Kang, Y. Yan, and S. H. Wei, "Halide perovskite materials for solar cells: a theoretical review," Journal of Materials Chemistry A, vol. 3, pp. 8926-8942, 2015.
[56] Z. Xiao, Y. Zhou, H. Hosono, T. Kamiya, and N. P. Padture, "Bandgap Optimization of Perovskite Semiconductors for Photovoltaic Applications," Chemistry a European Journal, vol. 24, pp. 2305-2316, 2018.
[57] L. Z. Wang, Y. Q. Zhao, B. Liu, L. J. Wu, and M. Q. Cai, "First-principles study of photovoltaics and carrier mobility for non-toxic halide perovskite CH3NH3SnCl3: theoretical prediction," Physical Chemistry Chemical Physics, vol. 18, pp. 22188-22195, 2016.
[58] Q. Y. Chen, Y. Huang, P. R. Huang, T. Ma, C. Cao, and Y. He, "Electronegativity explanation on the efficiency-enhancing mechanism of the hybrid inorganic–organic perovskiteABX3from first-principles study," Chinese Physics B, vol. 25, pp. 027104-027109, 2016.
[59] F. Chiarella et al., "Combined Experimental and Theoretical Investigation of Optical, Structural, and Electronic Properties of CH3NH3SnX3 Thin Films (X = Cl , Br)," Physical Review B, vol. 77, pp. 045129-045134, 2008.
[60] J. Feng and B. Xiao, "Effective Masses and Electronic and Optical Properties of Nontoxic MASnX3 (X = Cl, Br, and I) Perovskite Structures as Solar Cell Absorber: A Theoretical Study Using HSE06," The Journal of Physical Chemistry C, vol. 118, pp. 19655-19660, 2014.
[61] S. Yun, X. Zhou, J. Even, and A. Hagfeldt, "Theoretical Treatment of CH3 NH3 PbI3 Perovskite Solar Cells," Angewandte Chemie International Edition, vol. 56, pp. 15806-15817, 2017.
[62] G. Giorgi, J. I. Fujisawa, H. Segawa, and K. Yamashita, "Cation Role in Structural and Electronic Properties of 3D Organic–Inorganic Halide Perovskites: A DFT Analysis," The Journal of Physical Chemistry C, vol. 118, pp. 12176-12183, 2014.
[63] J. Xiang, K. Wang, B. Xiang, and X. Cui, "Sn(2+)-Stabilization in MASnI3 perovskites by superhalide incorporation," The Journal of Chemical Physics, vol. 148, pp. 124111-124116, 2018.