Crystal Growth, Structural Phase Transitions, and Optical Gap Evolution of CH 3 NH 3 Pb(Br 1-x Cl x ) 3 Perovskites

Abstract

Chemically tuned inorganic-organic hybrid halide perovskites based on bromide and chloride anions CH 3 NH 3 Pb(Br 1-x Cl x ) 3 have been crystallized and investigated by synchrotron X-ray diffraction (SXRD), scanning electron microscopy, and UV-vis spectroscopy. CH 3 NH 3 PbBr 3 and CH 3 NH 3 PbCl 3 experience successive phase transitions upon cooling, which are suppressed for intermediate compositions probably due to compositional disorder. For CH 3 NH 3 PbCl 3 , a transient phase, formerly described as tetragonal, was identified at 167.5 K; the analysis of SXRD data demonstrated that it is indeed orthorhombic, with space group Pnma, and a ≈ 2a p ; b ≈ 2a p ; c ≈ 2a p (a p is the ideal cubic perovskite unit-cell parameter). The band gap engineering brought about by the chemical management of CH 3 NH 3 Pb(Br 1-x Cl x ) 3 perovskites can be controllably tuned: the gap progressively increases with the concentration of Cl ions from 2.2 to 2.9 eV, and shows a concomitant variation with the unit-cell parameters of the cubic phases at 295 K. This study provides an improved understanding of the structural and optical properties of the mixed CH 3 NH 3 Pb(Br 1-x Cl x ) 3 perovskites.