Abstract
Tin halide perovskites are promising candidates for lead-free photovoltaic and optoelectronic materials, but not all of them have been well characterized. It is essential to determine how the bulk photophysical properties are correlated with their structures at both short and long ranges. Although CsSnCl3 is normally stable in the cubic perovskite structure only above 379 K, it was prepared as a metastable phase at room temperature. The transition from the cubic to the monoclinic phase, which is the stable form at room temperature, was tracked by solid-state 133Cs NMR spectroscopy and shown to take place through a first-order kinetics process. The complete solid solution CsSn(Cl1−xBrx)3 (0 ≤ x ≤ 1) was successfully prepared, exhibiting cubic perovskite structures extending between the metastable CsSnCl3 and stable CsSnBr3 end-members. The NMR spectra of CsSnBr3 samples obtained by three routes (high-temperature, mechanochemical, and solvent-assisted reactions) show distinct chemical shift ranges, spin-lattice relaxation parameters and peak widths, indicative of differences in local structure, defects and degree of crystallinity within these samples. Variable-temperature 119Sn spin-lattice relaxation measurements reveal spontaneous mobility of Br atoms in CsSnBr3. The degradation of CsSnBr3, exposed to an ambient atmosphere for nearly a year, was monitored by NMR spectroscopy and powder X-ray diffraction, as well as by optical absorption spectroscopy.
Highlights
The proportion of the cubic phase decreases monotonically in an exponential manner, with 65% remaining a er 10 days and 14% a er 72 days (Fig. 3b and S2c†). This process is much slower than a similar one that we have recently reported for the transformation of metastable gCsPbI3 perovskite to d-CsPbI3 non-perovskite at room temperature, which is complete within 2 hours, as monitored by 133Cs nuclear magnetic resonance (NMR) spectroscopy.[41]
The long-range structure was elucidated by powder X-ray diffraction (XRD) and the local structure and dynamics by 133Cs and 119Sn solid-state NMR spectroscopy
CsSnBr3 can be prepared by various synthetic routes that apparently have similar long-range structures, as ascertained by powder XRD and optical properties, the local structure and degree of crystallinity differ, as revealed by NMR spectroscopy; in particular, samples prepared by mechanochemical synthesis tend to show greater local polyhedral disorder and vacancies, resulting in a reduction of the spin-lattice relaxation of 119Sn
Summary
The mixed-halide perovskites CsSn(Cl1–xBrx)[3] have been prepared in the form of nanocrystals or thin lms, in which the band gap can be adjusted with composition, making them suitable for optoelectronic applications such as light emitting diodes and lasers.[19,20] some of the structural details of these mixed-halide perovskites are unclear because the end-members are known to exhibit multiple phase transitions: CsSnCl3 adopts a monoclinic structure at room temperature (own type; space group originally reported as P21/n, but standardized as P21/c) and transforms to the cubic perovskite structure (space group Pm3m) above 379 K,21 and CsSnBr3 undergoes complicated phase transitions at low temperature, but attains the cubic structure above 292 K.22 Because the physical properties depend sensitively on the structure and stability of these mixed-halide perovskites, it is essential to determine the local and long-range atomic arrangement, to unravel the dynamics of halogen mobility, and to evaluate changes entailed by different synthetic methods and exposure to ambient conditions. Because the physical properties depend sensitively on the structure and stability of these mixed-halide perovskites, it is essential to determine the local and long-range atomic arrangement, to unravel the dynamics of halogen mobility, and to evaluate changes entailed by different synthetic methods and exposure to ambient conditions. Powder XRD and NMR spectroscopy were carried out to determine the long-range structure and the local coordination around the Cs and Sn sites by Cl and Br atoms, which could be ordered or disordered. Given the ambiguity about the nature of CsSnBr3, we evaluate whether samples prepared by various methods show important structural differences which could in uence their optical properties. The stability of CsSnBr3 under ambient conditions was assessed by examining the products and pathways of its degradation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
