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Study on cobalt doped tin based perovskite material with enhanced air stability
Materials Science in Semiconductor Processing 57 (2017) 95–98
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short communication
Study on cobalt doped tin based perovskite material with enhanced air stability
crossmark
⁎
Jiongliang Yuana, , Bo Lib, Cunjiang Haoc a
Department of Environmental Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Department of Experimental Teaching, Tianjin University of Traditional Chinese Medicine, and Tianjin Key Laboratory of Chemistry and Analysis of Chinese Materia Medica, Tianjin 300193, PR China b c
A R T I C L E I N F O
A BS T RAC T
Keywords: Semiconductors Tin based perovskite Cobalt doping Air stability Bandgap
Although the power conversion efficiency of the lead-halide perovskite solar cells boost to > 20%, the toxicity of lead hinders their large-scale implementation. Tin based perovskite have less toxicity, however its air stability is a big challenge. In this study, the air stability of tin based perovskite is improved by doping cobalt. The cobalt doped tin based perovskite thin films (GASn1−xCoxI2−2xCl1+2x) with cubic structure are prepared by one-step spin coating method. All GASn1−xCoxI2−2xCl1+2x (0.025≤x≤0.15) perovskite materials exhibit strong absorption in the visible light spectrum range. According to voltammetric measurement, the highest occupied molecule orbit (HOMO) and the lowest unoccupied molecule orbit (LUMO) of GASn1−xCoxI2−2xCl1+2x (x=0.15) are −5.45 and −3.99 eV, respectively, and the bandgap is calculated to be 1.46 eV. The binding energy increase of Sn3d and I3d for the cobalt doped tin based perovskite thin film might be the reason for enhanced air stability.
1. Introduction Due to their higher power conversion efficiency (PCE), silicon based solar cells dominate the photovoltaic market at present. However, their high cost and heavily polluted manufacturing procedure decrease the feasibility of widespread use [1]. Therefore there have been intensive efforts to develop the alternatives to silicon based solar cells [2,3]. An innovative photovoltaic material, lead based organic-inorganic hybrid perovskite (CH3NH3PbX3 (X=I, Br, Cl)), emerged in 2009, and the PCE of perovskite solar cells boost to > 20% in 2015 via solvent engineering, interface engineering and composition engineering [3–5]. Owing to their lower cost and their comparable efficiency to silicon based solar cells, perovskite solar cells are expected to replace silicon based solar cells in future [3,4]. Nevertheless, the toxicity of lead in the perovskite will hinder their practical application. Replacing the lead element in the perovskite with less toxic metal elements is a big challenge. Tin based perovskite solar cells have been developed with the initial PCE of over 6% under 1 sun illumination, but their stability is a serious problem: their PCE decreases by 64.0% in 24 h when the solar cell devices are stored in nitrogen glove box after careful sealing [6,7]. It is because Sn2+ ion will be rapidly oxidized to its more stable Sn4+ analogue in the atmosphere environment. Although other nontoxic perovskite materials, eg. Cs2AgBiBr6, have been explored recently,
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the materials with similar photoelectronic properties have not yet developed [8–10]. In order to inhibit the oxidation of Sn2+ ion, Co2+ or Fe2+ is usually introduced to the electroplating solution in electroplating industry [11]. Inspired by this, it is expected that the oxidation of Sn2+ in tin based perovskite materials in air is inhibited by doping Co2+. Methylammonium (MA) is usually used as the organic cation in lead or tin based perovskite; however, it is reported that the replacement of MA by guanidinium (GA) leads to suppressed hysteresis, higher device open voltage and PCE, owing to a nearly zero dipole moment of GA cation and suppressed nonradiative carrier loss in perovskites [12,13]. According to the empirical Goldschmidt tolerance factor, it is predicted that GA cation is too large to be incorporated into the lead or tin iodide cavity [14,15]. However, since all of three NH2 groups in GA can form hydrogen bonds with I atoms, the lead or tin iodide cavity will be compressed, leading to higher stability of perovskites with GA [13]. Therefore, GA is used as the cation in tin based perovskite materials in this study. 2. Experimental The perovskite thin films were prepared by one-step spin coating method. SnI2, CoCl2 and guanidine hydrochloride were dissolved in N,
Corresponding author. E-mail address: [email protected] (J. Yuan).
http://dx.doi.org/10.1016/j.mssp.2016.09.029 Received 30 May 2016; Received in revised form 7 September 2016; Accepted 25 September 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 57 (2017) 95–98
J. Yuan et al.
Fig. 1. (a) XRD patterns of the perovskite thin films with various cobalt content x. (b) Lattice parameters of the perovskite thin films as a function of cobalt content x.
Fig. 2. (a) UV–visible absorbance spectra of the perovskite thin films with various cobalt content x. Inset: Tauc plot showing the characteristics of bandgap. (b) Cyclic voltammetric curve of the GASn1−xCoxI2−2xCl1+2x (x=0.15) perovskite thin film.
N′- dimethylformamide (DMF) and stirred at 60 °C for 12 h. The total concentration of SnI2 and CoI2 was 0.5 M, and the concentration of GA was 0.5 M. The precursor solution was spin-coated on soda-lime glass at 2000 rpm for 30 s, and the resulting thin films were annealed at 100 °C for 30 min on the hot plate, thus the dark brown, cobalt doped tin based perovskite (GASn1−xCoxI2−2xCl1+2x) materials were obtained. All of the fabrication process was conducted in the atmosphere environment, not in the glove box filling nitrogen. The crystal structure of thin films was determined by X-ray diffractometry (XRD, D8 Advance, Bruker, Germany). The thin films of GASn1−xCoxI2−2xCl1+2x perovskite were stored in the drier for 60 days, and the XRD patterns of the as-prepared and stored samples were compared. The optical absorption spectra were measured by the ultravioletvisible spectrophotometer (UV-3600, Shimazu, Japan), and the scanning wavelength was ranged from 220 to 900 nm. The x-ray photoelectron spectrum (XPS) was measured by ESCALAB 250 (ThermoFisher Scientific, USA) equipped with x-ray source of twin anode Al Kα 300 W, and all binding energies were calibrated to C1s at 284.6 eV. The cyclic voltammetric measurement was conducted in a threeelectrode cell, and the potential scan rate was 0.01 V s−1. In the threeelectrode cell, the GASn1−xCoxI2−2xCl1+2x perovskite thin film on indium tin oxide (ITO) conductive glass, platinum foil and saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. The photocurrent was measured in a quartz glass beaker containing acetonitrile with 100 mM
Fig. 3. XRD patterns of the as-prepared and stored perovskite thin films (x=0.15). The diffraction peaks of SnI4 become stronger for the stored sample.
NaClO4. The visible light irradiation was emitted from xenon lamp (AULTT, Beijing, P. R. China) with the irradiation intensity of 100 mW cm−2 on the thin film. 3. Results and discussion CH3NH3SnX3 (X=Br, I) perovskite thin films fade within seconds when exposed in atmosphere environment [6,7]; in contrast, there is 96
Materials Science in Semiconductor Processing 57 (2017) 95–98
J. Yuan et al.
Fig. 4. XPS spectra for the cobalt doped tin perovskite thin films (x=0.15). (a) XPS spectrum of Sn 3d, (b) XPS spectrum of I 3d.
60 days, and the XRD patterns for the as-prepared and stored samples were compared (Fig. 3). The stored thin film exhibits obvious diffraction peaks of SnI4, indicating the perovskite is partially oxidized. However, the perovskite structure of the thin film still retains, indicating that the doping of cobalt enhances the air stability. In addition, the diffraction peaks of the stored sample at 20.5° and 24.3° become stronger, while those at 14.6° and 28.7° become weaker. It might be due to the crystal transition of the perovskite structure.. The XPS spectrum for the GASn1−xCoxI2−2xCl1+2x (x=0.15) perovskite thin film is studied. The binding energies of Sn3d3/2 and Sn3d5/2 are 495.75 and 487.40 eV, respectively (Fig. 4a), which is about 2.8 eV higher than those for CH3NH3SnI3 (492.91 and 484.52 eV) [22]. In addition, the binding energies of I3d3/2 (631.10 eV) and I3d5/2 (619.35 eV) for the perovskite thin film are higher than that for CH3NH3SnI3 (628.43 and 616.94 eV) (Fig. 4b) [22]. The binding energy increase of Sn3d and I3d for the perovskite thin film might be the reason for enhanced air stability..
undetectable color change for the resulting GASn1−xCoxI2−2xCl1+2x thin films for 3 h, suggesting higher stability. The XRD patterns of the thin films with various cobalt content x are shown in Fig. 1a. It can be seen that all thin films have good crystallinity and similar crystal structure. The structure is identical to that of the more widely used CH3NH3SnX3 perovskite [16,17]; and the diffraction peaks at 14.6°, 20.5°, 24.3°, 28.7°, 32.3° and 43.9° can be indexed to (110), (112), (202), (220), (312) and (314) facets of perovskite, respectively, indicating all of the thin films with various cobalt content x have cubic structure. Although the diffraction peaks of SnI4 occur, the peaks are very weak; and they become much weaker at higher cobalt content x, revealing that the oxidation process of Sn2+ ion is inhibited.. Fig. 1b exhibits the lattice parameter a of the cubic GASn1−xCoxI2−2xCl1+2x perovskite as a function of cobalt content x. Since the ionic radii of Co2+(72 pm) and Cl− (136 pm) are smaller than those of Sn2+(93 pm) and I− (216 pm), the lattice parameter a decreases with increasing cobalt content x. According to Vegard's law, the relationship of the lattice parameter in the alloy is linear with the composition in the absence of strong electronic effects [18,19]. The lattice parameter a shows a linear relationship to cobalt content x in the range from 0.025 to 0.15, therefore the GASn1−xCoxI2−2xCl1+2x perovskite is formed by a simple solution-mixing process. The UV–visible absorbance spectra of GASn1−xCoxI2−2xCl1+2x perovskite thin films are shown in Fig. 2a. All samples exhibit strong absorption in the visible light spectrum range. With increasing cobalt content x, the absorption edge shifts slightly to longer wavelength. The bandgap of the GASn1−xCoxI2−2xCl1+2x is calculated to be ca.1.55 eV when cobalt content x ranges from 0.025 to 0.10 by Tauc plot (Fig. 2a inset), and the bandgap is estimated to be 1.51 eV at x=0.15. The bandgap will become higher by replacing of I− by Cl− [20,21], while it will become smaller by replacing Sn2+ by Co2+, therefore it varies slightly with cobalt content x.. The cyclic voltammetric curve of the GASn1−xCoxI2−2xCl1+2x (x=0.15) perovskite thin film is shown in Fig. 2b. According to the onset potentials of oxidation and reduction peaks, the highest occupied molecule orbit (HOMO) and the lowest unoccupied molecule orbit (LUMO) can be estimated to be −5.45 and −3.99 eV, thus the bandgap is calculated to be 1.46 eV, which is close to the value estimated from UV–visible absorbance spectrum measurement (1.51 eV). Due to rapid oxidation of Sn2+, tin based perovskite materials usually exhibit poor stability in air. The black/brown color of MASnI3 or GASnI3 thin film fades in several minutes in air, and the black color of GASnI2Cl thin film fades in 30 min in air. Even stored in nitrogen glove box after careful sealing, 64.0% PCE loss is observed within 24 h for MASnI3 solar cells [7]. In order to examine the anti-oxidation property of the cobalt doped tin perovskite materials, the thin film of GASn1−xCoxI2−2xCl1+2x (x=0.15) perovskite was stored in the drier for
4. Conclusion The cobalt doped tin based perovskite GASn1−xCoxI2−2xCl1+2x thin films were prepared by one-step spin coating method. The thin films at 0.025≤x≤0.15 have cubic perovskite structure. The lattice parameter a shows a linear relationship to cobalt content x in the range from 0.025 to 0.15. All GASn1−xCoxI2−2xCl1+2x (0.025≤x≤0.15) perovskite thin films exhibit strong absorption in the visible light spectrum range. With increasing cobalt content x, the absorption edge shifts slightly to longer wavelength. According to voltammetric measurement, the HOMO and LUMO of GASn1−xCoxI2−2xCl1+2x (x=0.15) are −5.46 and −3.97 eV, respectively, and the bandgap is calculated to be 1.46 eV. The doping of cobalt in the tin perovskite enhances its air stability. The binding energy increase of Sn3d and I3d for the perovskite thin film might be the reason for enhanced anti-oxidation property. Now the photoelectronic property of the GASn1−xCoxI2−2xCl1+2x thin films is under investigation. Acknowledgements This work was supported by Beijing Natural Science Foundation (grant number 2102034); and the Special Fund of Basic Research in Central Universities (grant number JD1107). References [1] [2] [3] [4]
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Materials Science in Semiconductor Processing 57 (2017) 95–98
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[13] G. Giorgi, J.-I. Fujisawa, H. Segawa, K. Yomashita, J. Phys. Chem. C 119 (2015) 4694–4701. [14] V.M. Goldschmidt, Naturwissenschaften 14 (1926) 477–485. [15] G. Kieslich, S. Sun, A.K. Cheetham, Chem. Sci. 5 (2014) 4712–4715. [16] J.M. Ball, M.M. Lee, A. Hey, H. Snaith, Energy Environ. Sci. 6 (2013) 1739–1743. [17] B.-W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo, E.M.J. Johansson, Adv. Mater. 27 (2015) 6806–6813. [18] L. Vegard, Z. Phys. 5 (1921) 17–26. [19] J. Xu, Y.-B. Tang, X. Chen, C.-Y. Luan, W.-F. Zhang, J.A. Zapien, W.-J. Zhang, H.L. Kwong, X.-M. Meng, S.-T. Lee, C.-S. Lee, Adv. Funct. Mater. 20 (2010) 4190–4195. [20] B. Suarez, V. Gonzalez-Pedro, T.S. Ripolles, R.S. Sanchez, L. Otero, I. Mora-Sero, J. Phys. Chem. Lett. 5 (2014) 1628–1635. [21] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J.P. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Science 342 (2013) 341–344. [22] Y. Ogomi, A. Morita, S. Tsukamoto, T. Saitho, N. Fujikawa, Q. Shen, T. Toyoda, K. Yoshino, S.S. Pandey, T. Ma, S. Hayase, J. Phys. Chem. Lett. 5 (2014) 1004–1011.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short communication
Study on cobalt doped tin based perovskite material with enhanced air stability
crossmark
⁎
Jiongliang Yuana, , Bo Lib, Cunjiang Haoc a
Department of Environmental Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Department of Experimental Teaching, Tianjin University of Traditional Chinese Medicine, and Tianjin Key Laboratory of Chemistry and Analysis of Chinese Materia Medica, Tianjin 300193, PR China b c
A R T I C L E I N F O
A BS T RAC T
Keywords: Semiconductors Tin based perovskite Cobalt doping Air stability Bandgap
Although the power conversion efficiency of the lead-halide perovskite solar cells boost to > 20%, the toxicity of lead hinders their large-scale implementation. Tin based perovskite have less toxicity, however its air stability is a big challenge. In this study, the air stability of tin based perovskite is improved by doping cobalt. The cobalt doped tin based perovskite thin films (GASn1−xCoxI2−2xCl1+2x) with cubic structure are prepared by one-step spin coating method. All GASn1−xCoxI2−2xCl1+2x (0.025≤x≤0.15) perovskite materials exhibit strong absorption in the visible light spectrum range. According to voltammetric measurement, the highest occupied molecule orbit (HOMO) and the lowest unoccupied molecule orbit (LUMO) of GASn1−xCoxI2−2xCl1+2x (x=0.15) are −5.45 and −3.99 eV, respectively, and the bandgap is calculated to be 1.46 eV. The binding energy increase of Sn3d and I3d for the cobalt doped tin based perovskite thin film might be the reason for enhanced air stability.
1. Introduction Due to their higher power conversion efficiency (PCE), silicon based solar cells dominate the photovoltaic market at present. However, their high cost and heavily polluted manufacturing procedure decrease the feasibility of widespread use [1]. Therefore there have been intensive efforts to develop the alternatives to silicon based solar cells [2,3]. An innovative photovoltaic material, lead based organic-inorganic hybrid perovskite (CH3NH3PbX3 (X=I, Br, Cl)), emerged in 2009, and the PCE of perovskite solar cells boost to > 20% in 2015 via solvent engineering, interface engineering and composition engineering [3–5]. Owing to their lower cost and their comparable efficiency to silicon based solar cells, perovskite solar cells are expected to replace silicon based solar cells in future [3,4]. Nevertheless, the toxicity of lead in the perovskite will hinder their practical application. Replacing the lead element in the perovskite with less toxic metal elements is a big challenge. Tin based perovskite solar cells have been developed with the initial PCE of over 6% under 1 sun illumination, but their stability is a serious problem: their PCE decreases by 64.0% in 24 h when the solar cell devices are stored in nitrogen glove box after careful sealing [6,7]. It is because Sn2+ ion will be rapidly oxidized to its more stable Sn4+ analogue in the atmosphere environment. Although other nontoxic perovskite materials, eg. Cs2AgBiBr6, have been explored recently,
⁎
the materials with similar photoelectronic properties have not yet developed [8–10]. In order to inhibit the oxidation of Sn2+ ion, Co2+ or Fe2+ is usually introduced to the electroplating solution in electroplating industry [11]. Inspired by this, it is expected that the oxidation of Sn2+ in tin based perovskite materials in air is inhibited by doping Co2+. Methylammonium (MA) is usually used as the organic cation in lead or tin based perovskite; however, it is reported that the replacement of MA by guanidinium (GA) leads to suppressed hysteresis, higher device open voltage and PCE, owing to a nearly zero dipole moment of GA cation and suppressed nonradiative carrier loss in perovskites [12,13]. According to the empirical Goldschmidt tolerance factor, it is predicted that GA cation is too large to be incorporated into the lead or tin iodide cavity [14,15]. However, since all of three NH2 groups in GA can form hydrogen bonds with I atoms, the lead or tin iodide cavity will be compressed, leading to higher stability of perovskites with GA [13]. Therefore, GA is used as the cation in tin based perovskite materials in this study. 2. Experimental The perovskite thin films were prepared by one-step spin coating method. SnI2, CoCl2 and guanidine hydrochloride were dissolved in N,
Corresponding author. E-mail address: [email protected] (J. Yuan).
http://dx.doi.org/10.1016/j.mssp.2016.09.029 Received 30 May 2016; Received in revised form 7 September 2016; Accepted 25 September 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 57 (2017) 95–98
J. Yuan et al.
Fig. 1. (a) XRD patterns of the perovskite thin films with various cobalt content x. (b) Lattice parameters of the perovskite thin films as a function of cobalt content x.
Fig. 2. (a) UV–visible absorbance spectra of the perovskite thin films with various cobalt content x. Inset: Tauc plot showing the characteristics of bandgap. (b) Cyclic voltammetric curve of the GASn1−xCoxI2−2xCl1+2x (x=0.15) perovskite thin film.
N′- dimethylformamide (DMF) and stirred at 60 °C for 12 h. The total concentration of SnI2 and CoI2 was 0.5 M, and the concentration of GA was 0.5 M. The precursor solution was spin-coated on soda-lime glass at 2000 rpm for 30 s, and the resulting thin films were annealed at 100 °C for 30 min on the hot plate, thus the dark brown, cobalt doped tin based perovskite (GASn1−xCoxI2−2xCl1+2x) materials were obtained. All of the fabrication process was conducted in the atmosphere environment, not in the glove box filling nitrogen. The crystal structure of thin films was determined by X-ray diffractometry (XRD, D8 Advance, Bruker, Germany). The thin films of GASn1−xCoxI2−2xCl1+2x perovskite were stored in the drier for 60 days, and the XRD patterns of the as-prepared and stored samples were compared. The optical absorption spectra were measured by the ultravioletvisible spectrophotometer (UV-3600, Shimazu, Japan), and the scanning wavelength was ranged from 220 to 900 nm. The x-ray photoelectron spectrum (XPS) was measured by ESCALAB 250 (ThermoFisher Scientific, USA) equipped with x-ray source of twin anode Al Kα 300 W, and all binding energies were calibrated to C1s at 284.6 eV. The cyclic voltammetric measurement was conducted in a threeelectrode cell, and the potential scan rate was 0.01 V s−1. In the threeelectrode cell, the GASn1−xCoxI2−2xCl1+2x perovskite thin film on indium tin oxide (ITO) conductive glass, platinum foil and saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. The photocurrent was measured in a quartz glass beaker containing acetonitrile with 100 mM
Fig. 3. XRD patterns of the as-prepared and stored perovskite thin films (x=0.15). The diffraction peaks of SnI4 become stronger for the stored sample.
NaClO4. The visible light irradiation was emitted from xenon lamp (AULTT, Beijing, P. R. China) with the irradiation intensity of 100 mW cm−2 on the thin film. 3. Results and discussion CH3NH3SnX3 (X=Br, I) perovskite thin films fade within seconds when exposed in atmosphere environment [6,7]; in contrast, there is 96
Materials Science in Semiconductor Processing 57 (2017) 95–98
J. Yuan et al.
Fig. 4. XPS spectra for the cobalt doped tin perovskite thin films (x=0.15). (a) XPS spectrum of Sn 3d, (b) XPS spectrum of I 3d.
60 days, and the XRD patterns for the as-prepared and stored samples were compared (Fig. 3). The stored thin film exhibits obvious diffraction peaks of SnI4, indicating the perovskite is partially oxidized. However, the perovskite structure of the thin film still retains, indicating that the doping of cobalt enhances the air stability. In addition, the diffraction peaks of the stored sample at 20.5° and 24.3° become stronger, while those at 14.6° and 28.7° become weaker. It might be due to the crystal transition of the perovskite structure.. The XPS spectrum for the GASn1−xCoxI2−2xCl1+2x (x=0.15) perovskite thin film is studied. The binding energies of Sn3d3/2 and Sn3d5/2 are 495.75 and 487.40 eV, respectively (Fig. 4a), which is about 2.8 eV higher than those for CH3NH3SnI3 (492.91 and 484.52 eV) [22]. In addition, the binding energies of I3d3/2 (631.10 eV) and I3d5/2 (619.35 eV) for the perovskite thin film are higher than that for CH3NH3SnI3 (628.43 and 616.94 eV) (Fig. 4b) [22]. The binding energy increase of Sn3d and I3d for the perovskite thin film might be the reason for enhanced air stability..
undetectable color change for the resulting GASn1−xCoxI2−2xCl1+2x thin films for 3 h, suggesting higher stability. The XRD patterns of the thin films with various cobalt content x are shown in Fig. 1a. It can be seen that all thin films have good crystallinity and similar crystal structure. The structure is identical to that of the more widely used CH3NH3SnX3 perovskite [16,17]; and the diffraction peaks at 14.6°, 20.5°, 24.3°, 28.7°, 32.3° and 43.9° can be indexed to (110), (112), (202), (220), (312) and (314) facets of perovskite, respectively, indicating all of the thin films with various cobalt content x have cubic structure. Although the diffraction peaks of SnI4 occur, the peaks are very weak; and they become much weaker at higher cobalt content x, revealing that the oxidation process of Sn2+ ion is inhibited.. Fig. 1b exhibits the lattice parameter a of the cubic GASn1−xCoxI2−2xCl1+2x perovskite as a function of cobalt content x. Since the ionic radii of Co2+(72 pm) and Cl− (136 pm) are smaller than those of Sn2+(93 pm) and I− (216 pm), the lattice parameter a decreases with increasing cobalt content x. According to Vegard's law, the relationship of the lattice parameter in the alloy is linear with the composition in the absence of strong electronic effects [18,19]. The lattice parameter a shows a linear relationship to cobalt content x in the range from 0.025 to 0.15, therefore the GASn1−xCoxI2−2xCl1+2x perovskite is formed by a simple solution-mixing process. The UV–visible absorbance spectra of GASn1−xCoxI2−2xCl1+2x perovskite thin films are shown in Fig. 2a. All samples exhibit strong absorption in the visible light spectrum range. With increasing cobalt content x, the absorption edge shifts slightly to longer wavelength. The bandgap of the GASn1−xCoxI2−2xCl1+2x is calculated to be ca.1.55 eV when cobalt content x ranges from 0.025 to 0.10 by Tauc plot (Fig. 2a inset), and the bandgap is estimated to be 1.51 eV at x=0.15. The bandgap will become higher by replacing of I− by Cl− [20,21], while it will become smaller by replacing Sn2+ by Co2+, therefore it varies slightly with cobalt content x.. The cyclic voltammetric curve of the GASn1−xCoxI2−2xCl1+2x (x=0.15) perovskite thin film is shown in Fig. 2b. According to the onset potentials of oxidation and reduction peaks, the highest occupied molecule orbit (HOMO) and the lowest unoccupied molecule orbit (LUMO) can be estimated to be −5.45 and −3.99 eV, thus the bandgap is calculated to be 1.46 eV, which is close to the value estimated from UV–visible absorbance spectrum measurement (1.51 eV). Due to rapid oxidation of Sn2+, tin based perovskite materials usually exhibit poor stability in air. The black/brown color of MASnI3 or GASnI3 thin film fades in several minutes in air, and the black color of GASnI2Cl thin film fades in 30 min in air. Even stored in nitrogen glove box after careful sealing, 64.0% PCE loss is observed within 24 h for MASnI3 solar cells [7]. In order to examine the anti-oxidation property of the cobalt doped tin perovskite materials, the thin film of GASn1−xCoxI2−2xCl1+2x (x=0.15) perovskite was stored in the drier for
4. Conclusion The cobalt doped tin based perovskite GASn1−xCoxI2−2xCl1+2x thin films were prepared by one-step spin coating method. The thin films at 0.025≤x≤0.15 have cubic perovskite structure. The lattice parameter a shows a linear relationship to cobalt content x in the range from 0.025 to 0.15. All GASn1−xCoxI2−2xCl1+2x (0.025≤x≤0.15) perovskite thin films exhibit strong absorption in the visible light spectrum range. With increasing cobalt content x, the absorption edge shifts slightly to longer wavelength. According to voltammetric measurement, the HOMO and LUMO of GASn1−xCoxI2−2xCl1+2x (x=0.15) are −5.46 and −3.97 eV, respectively, and the bandgap is calculated to be 1.46 eV. The doping of cobalt in the tin perovskite enhances its air stability. The binding energy increase of Sn3d and I3d for the perovskite thin film might be the reason for enhanced anti-oxidation property. Now the photoelectronic property of the GASn1−xCoxI2−2xCl1+2x thin films is under investigation. Acknowledgements This work was supported by Beijing Natural Science Foundation (grant number 2102034); and the Special Fund of Basic Research in Central Universities (grant number JD1107). References [1] [2] [3] [4]
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