Article pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Stabilizing the CsSnCl3 Perovskite Lattice by B‑Site Substitution for Enhanced Light Emission Ziyan Wu,† Qiqi Zhang,† Binghan Li, Zhifang Shi, Kaimin Xu, Yi Chen, Zhijun Ning, and Qixi Mi* School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
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ABSTRACT: All-inorganic metal halide perovskites are noted for their excellent optoelectronic properties and good chemical stability but also a propensity to transform into nonperovskite phases. A case in point is lead-free CsSnCl3, whose perovskite Phase I is metastable at room temperature and could not be stabilized by partial substitution of Cs+ or Cl−. Judging from the radii of the constituent ions of CsSnCl3, we paid attention to octahedral cations smaller than Sn2+ and selected Mn2+ and In3+ for substitution at the Bsite. A quantitative structural indicator, volume per formula unit (V/Z), guided the syntheses of CsSn0.9Mn0.1Cl3 (CTMC) and CsSn0.9In0.067Cl3 (CTIC), which are shown by single-crystal X-ray diffraction and differential scanning calorimetry to be homogeneous, stable perovskite phases at room temperature. CTMC turns into a disordered perovskite structure at 193 K, lower than the corresponding transition temperature of CsSnCl3 by nearly 100 K. Vacancies in CTIC result in a more flexible crystal structure than CTMC and CsSnCl3. CTMC and CTIC appear bright yellow with a steep absorption edge at ∼450 nm, and their photoluminescence peaks are located at 645 and 484 nm, respectively, with intensities significantly enhanced from CsSnCl3. Energy transfer processes are proposed to account for the Stokes shifts in the photoluminescence of CTMC and CTIC. Thin films of CTMC glow red under ultraviolet excitation and hold promise for electroluminescence devices. This study gives insights into the structural and electronic effects of ionic substitution in CsSnCl3 and can be extended to other metastable perovskite phases.
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CsSnCl3-I include: (1) the ionic radii20 of Cs+, Sn2+, and Cl− are imbalanced, as indicated by a tolerance factor t = 0.87. Either a larger cation than Cs+ (which must be molecular) or a smaller cation than Sn2+ could increase t to 0.90, the lower limit for a cubic perovskite structure.21 (2) The 5s2 lone pair of Sn2+ is prone to stereochemical activity,22 especially when coordinated by light anions, so that the Sn2+ center adopts a trigonal pyramidal geometry rather than an octahedron, the building unit of perovskites. The mild temperature or pressure conditions for transforming CsSnCl3-II to a perovskite lattice suggest the feasibility of stabilizing CsSnCl3-I at room temperature by ionic substitution. In light of the fruitless exploration14 in the entire composition ranges Cs1−xRbxSnCl3 and CsSnCl1−xBrx (0 < x < 1), we considered partial substitution at the perovskite B-site to yield CsSn1−xMxCl3, where M is an octahedral cation smaller than Sn2+. A quick survey of the ionic radii20 of the fourth- and fifth-period metals highlights Mn2+, Fe2+, Co2+, Cd2+, Y3+, and In3+, whose chlorides are known23 to also form eutectic systems with SnCl2. Among these candidates, we chose Mn2+ and In3+ thanks to their ease of handling and presence in perovskite-related materials.24−30 In particular, B-
INTRODUCTION Compared with hybrid organic−inorganic perovskite semiconductors, their all-inorganic counterparts are endowed with higher stability against heat and moisture and inherit the outstanding optoelectronic properties of the perovskite framework.1−6 The structural rigidity of all-inorganic perovskites tends to suppress ionic diffusion driven by internal or applied electric fields7 but also brings in additional crystal polymorphs, or in other words, phase instability. For instance, CH3NH3PbI3 maintains a perovskite structure to variable extents of distortion at 100−352 K,8 but the stable phase of CsPbI3 is nonperovskite below 671 K.9,10 In the perovskite structure, using Sn2+ to replace Pb2+ removes its toxicity but imparts more pronounced chemical and phase instabilities.11 In this work, we focus at CsSnCl3,12−17 a relatively stable yet less studied member of the CsSnX3 family, for a general composition−structure relationship and applications as a luminescent semiconductor material. The perovskite phase of CsSnCl3 (Phase I) is vividly yellow and, like other lead and tin trihalides, features electronic and ionic semiconducting properties.15,18 However, the stable phase of CsSnCl3 at room temperature is white, nonperovskite Phase II; the transition from CsSnCl3-II to CsSnCl3-I is reported15 to take place at 379 K. It is also found19 that pressurizing CsSnCl3-II to 25.3 kbar probably generated a perovskite structure. Reasons for the phase instability of © XXXX American Chemical Society
Received: January 29, 2019 Revised: June 17, 2019 Published: June 18, 2019 A
DOI: 10.1021/acs.chemmater.9b00433 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials site substitution by Mn2+ has been employed to stabilize the perovskite structure of CsPbBr3 and CsPbI3 nanocrystals.25,31
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spectrophotometer against BaSO4 as the standard for 100% reflection. The reflectance R was converted to the Kubelka−Munk function F(R) = (1 − R)2/2R. Photoluminescence. Steady-state photoluminescence (PL) spectra and quantum yields were measured on a Horiba Fluorolog Fluorimeter equipped with a Quanta-φ integrating sphere, using 350 nm excitation. After passing a 400 nm filter, the sample emission was monochromatized and recorded by a CCD detector. Time-resolved PL spectra were measured on an Edinburgh Instruments LP920KS transient spectrometer. A sample was excited by 355 nm, 8 ns laser pulses (Spectra-Physics Quanta Ray Lab 170) and observed by an intensified CCD camera (ANDOR iStar 320T) at various delay times using a gate width of 1 μs. Raman Spectroscopy. Samples contained in a N2-filled cuvette were illuminated by a focused (50× object lens, Leica) 532 nm laser beam, and the scattered light was analyzed by an ANDOR Shamrock 750 spectrograph with a high-dispersion grating (1200 lines per mm at 500 nm). Raman shifts were calibrated to Si chips (520 cm−1). Electron Paramagnetic Resonance. Solid samples were measured on an X-band electron paramagnetic resonance (EPR) spectrometer (Bruker ELEXSYS E580) at room temperature, using 0.2 mT field modulation in the continuous wave mode. The experimental spectra were fitted by the SpinFit software. CTMC Nanoparticles and Thin Films. In a three-necked flask, 171 mg (0.90 mmol) SnCl2, 13 mg (0.10 mmol) MnCl2, 1.0 mL trioctylphosphine (TOP, 97%, Aldrich 718165), 0.5 mL oleic acid (OA, AR, Aladdin O108484), 0.5 mL oleylamine (OLA, 80−90%, Aladdin O106967), and 5.0 mL 1-octadecene (ODE, >90%, Aladdin O109487) were stirred at 110 °C under vacuum for 2 h and then at 170 °C under N2 for 0.5 h. To this clear solution was injected 1.36 mL (0.32 mmol) of a cesium precursor solution,17 similarly prepared from 1.04 g (3.19 mmol) Cs2CO3 (≥99%, Adamas G6331), 3.6 mL OA, 3.6 mL OLA, and 20 mL ODE. The reaction was maintained at 170 °C for 1 min before quenching in an ice bath and transferred to a centrifuge tube under N2. A colloidal suspension of CTMC nanoparticles was obtained by centrifuging the above reaction mixture at 4200 rpm for 5 min and redispersing the precipitation in 2 mL toluene with sonication. Spin-casting on cleaned glass slides at 2000 rpm for 30 s produced transparent CTMC thin films that glowed red under 365 nm excitation.
EXPERIMENTAL SECTION
Purification and Synthesis of Materials. All tin(II) compounds were stored and handled in a N2 glovebox (Vigor Tech) containing ≤1.0 ppm O2 and
