  Non-toxicity and stability make two-dimensional (2D) bismuth halide
perovskites better alternatives to lead-based ones for optoelectronic
applications and catalysis. In this work, we synthesize sub-micron size
colloidal quasi-2D Cs3Bi2I9 perovskite nanosheets and study their generation
and relaxation of charge carriers. Steady-state absorption spectroscopy reveals
an indirect bandgap of 2.07 eV, which is supported by the density functional
theory calculated band structure. The nanosheets do not show detectable
photoluminescence at room temperature at band-edge excitation which is
attributed to the indirect bandgap. However, cathodoluminescence spanning a
broad range from 500 nm to 750 nm with an asymmetric and Stokes-shifted
spectrum is observed, indicating the phonon- and trap-assisted recombination.
We study the ultrafast charge carrier dynamics in Cs3Bi2I9 nanosheets using a
femtosecond transient absorption spectroscopy. The samples are excited with
pump energies higher than their bandgap, and the results are interpreted in
terms of hot carrier generation (<1 ps), thermalization with local phonons (~1
ps), and cooling (>30 ps). Further, a relatively slow relaxation of excitons
(>3 ns) at the band edge suggests the formation of stable polarons which decay
nonradiatively by emitting phonons.
