Synthesis, optoelectronic properties, and charge carrier dynamics of colloidal quasi-two-dimensional Cs3Bi2I9 perovskite nanosheets

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 band structure calculated using density functional theory. The nanosheets show no detectable photoluminescence at room temperature at near bandgap 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 emission is observed, indicating the phonon- and trap-assisted recombination of charge carriers. We study the ultrafast charge carrier dynamics in Cs3Bi2I9 nanosheets using femtosecond transient absorption spectroscopy. The samples are excited with photon energies higher than their bandgap, and the results are interpreted in terms of hot carrier generation (<1 ps), thermalization with local phonons (similar to 1 ps), and cooling (>30 ps). Further, a relatively slow relaxation of excitons (greater than or similar to 3 ns) at the band edge suggests the formation of stable polarons which decay nonradiatively by releasing phonons.

Automated summary

Non-toxic and stable, sub-micron size colloidal quasi-2D Cs3Bi2I9 perovskite nanosheets were synthesized and their charge carrier generation and relaxation were studied. Indirect bandgap of 2.07 eV was observed, and no detectable photoluminescence at room temperature was found, but cathodoluminescence in a broad range from 500 nm to 750 nm was observed, indicating phonon- and trap-assisted recombination of charge carriers. Ultrafast charge carrier dynamics were studied using femtosecond transient absorption spectroscopy, revealing hot carrier generation, thermalization with local phonons, and cooling processes, as well as the formation of stable polarons and their nonradiative decay by releasing phonons.