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

Two-dimensional perovskites have demonstrated superior application prospects in optoelectronic fields, such as X-ray detection, light emitting diodes, and photovoltaics, as their tunability of band structures, improved stability, fast responses, and suppressed noise signal. In this work, bulk single crystals of MA3Bi2Br9 (MA+=CH3NH3+) and its two mixed crystal MA3Bi2(Br0.74I0.26)9, MA3Bi2(Br0.62I0.38)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 incongruently melting temperature gradually decreased from 342 oC for MA3Bi2Br9 to 328 oC for MA3Bi2(Br0.62I0.38)9, respectively. For 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(Br0.74I0.26)9, 2.04 eV for MA3Bi2(Br0.62I0.38)9, and 1.97 eV for MA3Bi2I9, respectively. Similarly, the photoluminescence peaks blue-shifted from 550 nm for MA3Bi2Br9, to 453 nm for MA3Bi2(Br0.74I0.26)9, 445 nm for MA3Bi2(Br0.62I0.38)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 decay time as short as 0.65 ns the sub-nanosecond, for MA3Bi2Br9, 1.06 ns for MA3Bi2(Br0.74I0.26)9, 3.00 ns for MA3Bi2(Br0.62I0.38)9, and 3.12 ns for MA3Bi2I9, respectively, were observed. Besides, the low dimensional structures also brought the crystals superior environmental stability as the suppressed ion migrations. This work provides 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.