Composition dependent optical, structural and photoluminescence characteristics of cesium tin halide perovskites

Abstract

In this article we report the structural and optical properties of varying compositions of lead free inorganic perovskite (CsSnX₃ (X = Br, Cl, I)) and its tunable photoluminescence covering the entire visible to near-IR region. Optical band gap studies for prepared inorganic cesium tin halide compositions show a significant (53.5%) blue shift in the absorption spectra as the halide composition varies from I to Br and Cl. As the halide composition was varied from Cl to I, an intense and tunable PL emission was observed at room temperature covering a wide range from Vis-near IR region (420–950 nm). X-ray diffraction studies revealed that undoped compositions were orthorhombic (CsSnI₃), cubic (CsSnBr₃) and monoclinic (CsSnCl₃) in structure at room temperature. For mixed halides, the lattice constant was found to increase gradually as the size of halide cation increases from Cl⁻ to Br⁻ and I⁻.

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1. Introduction

Organic–inorganic metal halide perovskites show excellent optoelectronic properties and have found applications in photovoltaics and light emitting devices.¹ These metal halides adopt an ABX₃ perovskite structure with a network of corner sharing BX₆ octahedral units, where the B atom is a divalent metal (usually Sn²⁺ or Pb²⁺) and X a monovalent halide (usually F⁻, Cl⁻, Br⁻, or I⁻); the A cation is usually Cs⁺ or a small organic molecular species like alkylamines (R-NH₃⁺). Organic–inorganic metal halide perovskites are identified as an excellent absorber material for solar cells, owing to their tunable band gap, covering the visible to near-IR region, and high absorption co-efficient.^2,3 CH₃NH₃PbI₃ has been used as an absorber in photovoltaic devices with efficiencies up to 18%.^4,5 Most of the studies on organic–inorganic perovskites are focused on the light absorbing properties for solar cell application while less attention has been paid to other optical properties such as those relevant for application in light emitting devices and lasing. Recent research shows application of organic–inorganic metal halide perovskites in electroluminescent devices due to its strong exciton bound photoluminescence (PL).^6,7 Kabra et al. demonstrated application of lead based organometallic halide perovskites as emitter material in near IR to visible electroluminescent diodes.⁶ Towards wider application of these perovskites replacement of hazardous lead is essential. There have been efforts to replace lead with tin in organometallic perovskite solar cells.⁷ Inorganic perovskites, CsSnI₃ and CsSnBr₃ are used as absorber materials in p–i–n type solar cells.⁸ Ogomi et al. reported a mixed Sn–Pb perovskite absorber with a tunable band gap by varying the Sn:Pb ratio, indicating that Sn can be a potential replacement for lead.⁹ Hao et al. have reported CH₃NH₃SnI_3−xBr_x perovskite solar cells with 5.7% efficiency.¹⁰ Recent study by Protesescu et al. demonstrates photoluminescence from CsPbX₃ (X = Cl, Br, I) tunable over the entire visible spectrum (410–700 nm) for application in optoelectronic devices.⁷

Inorganic perovskites, cesium tin halides (CsSnI₃, CsSnBr₃, and CsSnCl₃) show strong photoluminescence at room temperature and are solution-processable at room temperature.^10,11 Very few studies are reported on opto-electronic characteristics and applications of tin based perovskites. CsSnI₃ is reported to have PL emission several orders higher in magnitude than that of commercial InP single-crystalline wafer emitting in near IR region.¹¹ Cesium tin bromide and iodide are electrical conductors at room temperature and have been used as an absorber and as a hole transport materials in solar cells.^8,12 In this article we report the structural and optical properties of varying compositions of CsSnX₃ (X = Br, Cl, I) and its tunable photoluminescence covering the entire visible to near-IR region for application in optoelectronic devices.

2. Experimental

Crystals of CsSnX₃ of different compositions (CsSnI₃, CsSnI₂Br–CsSnIBr₂, CsSnBr₃, CsSnBr₂Cl–CsSnBrCl₂, and CsSnCl₃) were prepared by mixing halide anions and cations in stoichiometric proportions using solution route. Towards this, cesium halide (CsX) (Sigma 99.9%) and tin halide (SnX₂) (Sigma 99.9%) salts were taken in stoichiometric ratios and were dissolved in a polar organic solvent, dimethylformamide (DMF). The solution was kept stirring at 70 °C for 12 hours. For optical studies, glass slides coated with CsSnX₃ of different compositions were used. The diffuse-reflectance spectra were measured at room temperature using UV-Vis-NIR spectrometer-Lambda 950, PerkinElmer operating from 200–2500 nm. Barium sulphate pellet was used as the reflecting reference. Band gaps for different compositions of CsSnX₃ were estimated by converting reflectance to absorbance using Kubelka–Munk equation: α/S = (1 − R)²/2R, where R is the reflectance, α the absorption coefficient and S the scattering coefficient.¹³ To prevent exposure of the perovskite films to ambient conditions during PL and XRD measurements the samples were coated with polymer film. The photoluminescence (PL) spectra were acquired using a HORIBA Jobin Yvon Fluorolog 3 model. X-ray diffraction patterns for the thin film samples were analyzed with a PANalytical X'Pert Pro with an X'celerator detector. Chemical composition of prepared halides was verified from EDX analysis using FE-SEM (JEOL JSM 7600F). The melting points for prepared samples were determined using DTA analysis. It may be noted that all of these perovskite compositions are susceptible to degradation when exposed to ambient conditions as has been quantified in prior studies.^14–16 All experiments, including synthesis and sample preparation of CsSnX₃ perovskites were carried out inside a N₂ atmosphere glove box (mbraun). Characterization like XRD, TGA-DTA was carried out under vacuum and N₂ atmosphere respectively. For photoluminescence measurements a thin transparent polymer (parafilm)/wax coating was applied onto the sample films to prevent exposure to moisture and oxygen.

3. Results and discussion

Optical band gaps as determined from reflectance spectra are shown in Fig. 1a, and are also reported in Table 1. It was observed that as the composition of the CsSnX₃ perovskite is varied from I to Br and to Cl, the absorption edge is blue shifted from 1.3 eV to 2.8 eV. As the halide changes from I to Br to Cl, the color of perovskite film also changes significantly from black for CsSnI₃ to brownish-red for CsSnBr₃, yellow for CsSnCl_xBr_3−x and white for CsSnCl₃ and is shown in Fig. 1b. Atomic ratio for all the prepared sample compositions was confirmed through EDX analysis.

Fig. 1 (a) Optical band gap for different compositions of cesium tin halides (CsSnX₃) as determined from UV-Vis absorption spectra calculated using Kubelka–Munk conversion. (b) Color variation for different compositions of cesium tin halides.

Table 1 Optical and physical properties of cesium tin halide perovskites

Compound name	Band gap (eV)	PL (nm)	Melting point determined from DTA analysis (^oC)	Colour	Crystal structure & lattice parameter (Å)
CsSnI3	1.31	950	452	Black	Orthorhombic (a = 8.65, b = 8.61, c = 12.35)
CsSnI2Br	1.41	880	452	Black	Cubic (a = 6.04)
CsSnIBr2	1.65	771	450	Black	Cubic (a = 5.93)
CsSnBr3	1.75	709	448	Black	Cubic (a = 5.8)
CsSnBr2Cl	1.9	630	425	Dark red	Cubic (a = 5.74)
CsSnBrCl2	2.1	573	415	Orange brown	Monoclinic (a = 16.12, b = 7.48, c = 5.74)
CsSnCl3	2.8	420	368	White	Monoclinic (a = 16.10, b = 7.45, c = 5.72)

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X-ray diffraction patterns for the thin films of different compositions of perovskites (CsSnI₃, CsSnI_1.5Br_1.5, CsSnBr₃, CsSnI_1.5Cl_1.5, and CsSnCl₃) are shown in Fig. 2a. Black films of CsSnI₃ were identified to be orthorhombic (JCPDS file no. 43-1162), CsSnBr₃ cubic (JCPDS file no. 70-1645) and CsSnCl₃ monoclinic (JCPDS file no. 71-2023) at room temperature. Phase analysis of mixed halides was carried out by comparison with earlier reports.^17,18

Fig. 2 (a) XRD patterns for varying halide composition of CsSnX₃ (b) shift in the optical band gap estimated from absorption spectra and emission spectra for varying halide composition of CsSnX₃.

As stated earlier only CsSnBr₃ existed in a cubic phase at room temperature and as reported in literature the other halides (CsSnX₃) existed in cubic form only at high temperatures.^19,20 Lattice parameters were calculated for prepared compositions and are reported in Table 1. In the case of mixed halides, it was observed that when orthorhombic CsSnI₃ was doped with smaller Br⁻ ions, a gradual reduction in lattice parameter occurs from CsSnI₂Br (a = 6.04 Å) to CsSnBr₃ (a = 5.8 Å) and further, the crystal structure converts to cubic. And while doping cubic CsSnBr₃ with Cl, it was found that for compositions with <50 mol% of Br, non-cubic phases (monoclinic) were formed and for >50 mol% of Br, cubic phase formed. This observation of phase change with composition is consistent with earlier studies on Mössbauer spectroscopy of CsSnX₃ by Donaldson et al.¹⁷ As the size of halide ion increases, Cl⁻ (1.81 Å) < Br⁻ (1.96 Å) < I⁻ (2.20 Å) a peak shift towards the lower angle is observed for the major peaks in XRD, and are shown in Fig. 1a, suggesting an increase in the lattice parameters and expansion of unit cells. This size effect arises due to the development of strain in the lattice of mixed halides.²¹ Also, as the halide composition varies from I to Br and Cl the shift in lattice parameters are consistent as shown in the case of other similar inorganic and organic perovskites like CsPbX₃ and CH₃NH₃SnX₃ respectively.^5,22 The melting point of CsSnX₃ perovskites were found to increase as the elemental composition changed from Cl to Br and I and is reported in Table 1. It was also observed that CsSnX₃ compounds undergo phase change accompanied by change in color when exposed to air, which have considerable effect on the optical and electronic properties of CsSnX₃. We observed that CsSnI₃ a black semiconducting material, in orthorhombic form at room temperature which has strong PL emission at 950 nm, converts to a yellow non-luminescent phase (Y) CsSnI₃ when exposed to oxygen or moisture. In the case of CsSnCl₃, at higher temperature (120 °C), CsSnCl₃ forms a yellow cubic phase which converts to white monoclinic phase at room temperature.

PL studies of CsSnX₃ halides show intense emission which is tunable over a broad range from visible to near IR region as shown in Fig. 3a. A blue shift is observed in the PL emission, as the halide composition is varied from I to Br and to Cl, from 950 nm to 420 nm. This shift in band gap is attributable to the variation in lattice constant for different halide compositions along with the changes in electronegativity altering the band structure.²³ Literature on life time studies of PL emission in CsSnX₃ reported emission life time of the order of pico seconds to nanoseconds for undoped CsSnX₃ (ref. 24 and 25). We also observed that the absorption and emission edges for most of the cesium tin halides studied here are very close suggesting a direct band edge recombination leading to strong PL emission at room temperature. XRD and optical studies show that for the series of compositions, continuous contraction of lattice parameter from CsSnI₃ to CsSnBr₃ and CsSnCl₃ results in widening of band gap from 1.3 eV to 2.8 eV. The tunability of the band gap for these halides was also demonstrated in samples prepared by replacing Cs⁺ with organic cation CH₃NH³⁺ and Sn²⁺ with Pb²⁺. The shift in the PL spectra after replacing the cations Cs⁺ and Sn²⁺ with CH₃NH³⁺ and Pb²⁺ respectively is shown in Fig 3b. As Cs⁺ ion (1.67 Å) and Sn²⁺ (1.18 Å) was replaced with a larger cation CH₃NH₃⁺ (2.17 Å) and Pb²⁺ (1.18 Å) the band gap changes from 1.3 eV for CsSnI₃ to 1.23 eV for CH₃NH₃SnI₃. As Sn²⁺ was gradually replaced by Pb²⁺ the band gap changes from CsSnI₃-1.3 eV to CsPbI₃-1.7 eV.

Fig. 3 (a) Photoluminescence spectra for different compositions of CsSnX₃ (b) photoluminescence spectra for CsSnI₃ with different cations (CH₃NH₃⁺ and Pb²⁺).

4. Conclusions

In summary, we have synthesized luminescent cesium tin halides of different compositions with intense and tunable PL emission from visible to near IR region (420–950 nm). The optical band gap shows a 53.5% blue shift in the absorption spectra for composition ranging from CsSnI₃ to CsSnCl₃. All compositions show strong direct band edge photoluminescence emission that is tunable according to the halide compositions. With its excellent optical properties, tin halide perovskite is an extremely promising material which can be used either as lead free perovskites or with reduced lead content for application in optoelectronic devices.

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