Lead free halide perovskite Cs₃Bi₂I₉ bulk crystals grown by a low temperature solution method

As a member of the bismuth iodide family compounds, Cs₃Bi₂I₉ perovskite shows potential applications in photovoltaic devices and radiation detectors, due to the absence of toxic lead and its high air and moisture stability. In this work, we report Cs₃Bi₂I₉ bulk crystals grown by a low temperature solvent volatilized crystallization method. The morphology of the single crystals exhibits a well symmetric hexagonal shape, with a layered structure along the [00l] direction. Three defect related transitions (iodine substituting bismuth (I_Bi) to the valence band maximum (VBM), the conduction band minimum (CBM) to caesium substituting bismuth (Cs_Bi ^1-/2-), and the CBM to caesium substituting iodine (Cs_I ^1+/2+)) are identified in 10 K photoluminescence spectra, with dissociation energies of 47.87 meV, 45.28 meV, and 52.01 meV, respectively. The anisotropic resistivities along the [00l] and [l00] directions are compared, with values of 11.08 GΩ cm and 0.77 GΩ cm, respectively, which is attributed to the difference of carrier mobility. This is further certified by the first principles calculations, which demonstrate that the effective masses along the [00l] and [l00] directions are 3.22m_e (electrons)/2.14m_e (holes) and 0.327m_e (electrons)/0.946m_e (holes), respectively. The Cs₃Bi₂I₉ device exhibits fast photoresponse to optical light, with an on-off ratio of ∼24 along [l00], which is fifteen times higher than that along [00l]. This photoelectric anisotropy dictates the potential applications in photovoltaic devices and radiation detectors.