Bulk single crystal growth and optoelectronic properties of the quasi-two-dimensional perovskites (CH3NH3)3Bi2X9 (X− = Br− and I−)

Abstract

Two-dimensional perovskites have demonstrated superior application prospects in optoelectronic fields, such as X-ray detection, light emitting diodes, and photovoltaics, because of their tunability of band structures, improved stability, fast responses, and suppressed noise signal. In this work, bulk single crystals of MA₃Bi₂Br₉ (MA⁺ = CH₃NH₃⁺) and the two mixed crystals MA₃Bi₂(Br_0.74I_0.26)₉ and MA₃Bi₂(Br_0.62I_0.38)₉ were successfully grown using the bottom-seeded solution growth method. MA₃Bi₂I₉ crystals were also prepared and characterized for comparisons. X-ray diffraction analysis revealed that all the crystals had quasi-two-dimensional (2D) structures and the partial substitution of I⁻ for Br⁻ did not alter the space group symmetry. However, with the increase of I⁻ concentrations, the incongruent melting temperature gradually decreased from 342 °C for MA₃Bi₂Br₉ to 328 °C for MA₃Bi₂(Br_0.62I_0.38)₉, respectively. For the optoelectronic properties, the MA₃Bi₂Br₉ was measured to have a direct bandgap of 2.60 eV, while the values decreased to 2.15 eV for MA₃Bi₂(Br_0.74I_0.26)₉, 2.04 eV for MA₃Bi₂(Br_0.62I_0.38)₉, and 1.97 eV for MA₃Bi₂I₉, respectively. Similarly, the photoluminescence peaks blue-shifted from 550 nm for MA₃Bi₂Br₉, to 453 nm for MA₃Bi2(Br_0.74I_0.26)₉, 445 nm for MA₃Bi₂(Br_0.62I_0.38)₉, and 444 nm for MA₃Bi₂I₉, respectively. Due to the strong dielectric confinement effects and anisotropic exciton transportation characteristics of the 2D structures, the targeted crystals studied all exhibited ultrafast fluorescence decay characteristics, where a decay time as short as 0.65 ns, sub-nanosecond, for MA₃Bi₂Br₉, 1.06 ns for MA₃Bi₂(Br_0.74I_0.26)₉, 3.00 ns for MA₃Bi₂(Br_0.62I_0.38)₉, and 3.12 ns for MA₃Bi₂I₉, respectively, was observed. Besides, the low-dimensional structures also brought the crystals superior environmental stability because of the suppressed ion migrations. This work provides a simple, straightforward method for the large-sized crystal growth of low-dimensional MA₃Bi₂X₉ (X⁻ = Br⁻, I⁻), and reveals their promising application prospects in novel optoelectronic fields.