Synthesis and Optical Properties of Colloidal BiI3 Nanocrystals and Cs2AgBiBr6: analogues to Lead Halide Perovskite

Abstract

Current research interest in nano material based optoelectronics is dominated by colloidal lead halide perovskites nanocrystals (NCs) because of their defect tolerant nature. This defect tolerant nature is a consequence of its unique electronic band structure where both valence band maximum and conduction band minimum possess an antibonding character along with the desired crystal structure. Interestingly, due to analogous electronic configuration, Bi3+ halides also adopt a similar electronic structure though a different crystal structure and are therefore expected to yield similar exciting properties. Motivated by this similarity, we investigated optical properties of solution processed BiI3 NCs to test its potential as lead free defect tolerant material. BiI3 NCs with a band-gap of 3.2 eV spanning in the near UV-region, emitted roomtemperature band-edge photoluminescence (PL) in the blue region of visible spectrum with PL quantum yield (PLQY) ~ 14%. Bi-exponential PL decay was obtained for BiI3 NCs with radiative recombination lifetimes of 2.4 ns and 5.6 ns. NMR, 2D-DOSY and FTIR confirmed that BiI3 NCs remain dispersed in solution through oleylamine passivation. BiI3 NCs demonstrate great potential for light emitting diodes and radiation detector owing to their low toxicity, favourable PLQY in blue/violet spectral region. Such a high PL efficiency for high band gap material (>2.5 eV), suggests the defect tolerant nature of our BiI3 NCs. Crystal structure of lead halide perovskite also adds to its defect tolerant nature. Double perovskite Cs2AgBiBr6 have crystal structure similar to lead halide perovskite. Bulk Cs2AgBiBr6 has been synthesized via solution processible method. X-ray diffraction pattern and elemental analysis confirmed the formation of cubic double perovskite structure with many different morphology. Diffuse reflectance measurements reveal band gap of 2.3 eV and that is comparable to analogous lead halide perovskite, CsPbBr3 (2.4 eV). Cs2AgBiBr6 remains stable in environmental condition for at least 20 days with further study in progress. In future we aim to synthesis colloidal double perovskite NCs.These results suggest that Bi based halides can probably act as replacement of lead based perovskite.