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.
