Growth mechanism, phase transition and optical properties of large-size and high-quality perovskite CH₃NH₃PbCl₃ single crystal

Abstract

According to the phenomenon that the solubility of CH₃NH₃PbCl₃ decreases with the increase of temperature in different solvents, CH₃NH₃PbCl₃ perovskite single crystal with a maximum dimension of 11 mm × 11 mm × 2 mm is grown by introducing a high-quality seed crystal via the seed-induced inverse temperature crystallization method in this work. X-ray diffraction and Rietveld refinements show that the full widths at half maximum (FWHM) of CH₃NH₃PbCl₃ single crystal diffraction peaks are 0.1527°, 0.1353°, 0.2295° and 0.3452°, corresponding to the crystal plane indices of (100), (200), (300) and (400), respectively. And there are no miscellaneous peaks, indicating a good crystal quality. As a result, CH₃NH₃PbCl₃ single crystal is of cubic phase at room temperature, its space group belongs to <i>Pm</i><inline-formula><tex-math id="M3">\begin{document}$ \bar{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230097_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230097_M3.png"/></alternatives></inline-formula><i>m</i>, and the lattice constant is <i>a</i> = 0.56877 nm. The surface morphology and growth mechanism of CH₃NH₃PbCl₃ crystal are investigated by using a polarizing microscope. It is found that its growth mechanism follows the step horizontal growing mechanism of smooth interface, and its growth direction (that is, step movement direction) is along the outward normal direction of the step. The structural symmetry of CH₃NH₃PbCl₃ crystal is studied by variable temperature Raman spectroscopy, which reveals an orthogonal-tetragonal phase transition at 160 K. But the tetragonal phase structure is not stable, and its temperature range is very narrow. As temperature rises gradually, the tetragonal phase again transforms into a cubic phase (<i>Pm</i><inline-formula><tex-math id="M4">\begin{document}$\bar{3}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230097_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230097_M4.png"/></alternatives></inline-formula><i>m</i>). Results of UV-Vis-NIR absorption and photoluminescence spectra show that the absorption cutoff of CH₃NH₃PbCl₃ crystal is about 442 nm, and the photoluminescence peak is 450 nm. Thereupon, its band gap is obtained to be about 2.93 eV by a linear fit of Tauc formula, which is slightly higher than the theoretical value of 2.55 eV calculated by first principles simulation. We believe that it is related to the seed crystal, which is introduced into the crystal growth process as the core of heterogeneous nucleation and thus making the lattice more distorted. The lower the lattice symmetry of CH₃NH₃PbCl₃, the larger the band gap is, that is, the lattice symmetry determines the degree of distortion for inorganic PbCl₆ octahedral frameworks, resulting in an increase of band gap for CH₃NH₃PbCl₃.