Controlled synthesis of highly stable lead-free bismuth halide perovskite nanocrystals: Structures and photophysics

Recently, cesium bismuth halide perovskites have emerged as potential substitutes to their counterparts, cesium lead halide perovskites, owing to their low toxicity. However, the photophysics of cesium-bismuth halides nanocrystals (NCs) have not yet been fully rationalized because their structures remain highly debated. The ultraviolet-visible (UV-vis) absorption along with other photophysical properties such as the nature and lifetime of the excited states vary considerably across the previous reports. Here, we successfully synthesize pure Cs$_3$BiBr$_6$ and Cs$_3$Bi$_2$Br$_9$ NCs via a modified hot-injection method, where the structure can be easily controlled by tuning the reaction temperature. The UV-vis absorption spectrum of the pure Cs$_3$Bi$_2$Br$_9$ NCs features two characteristic peaks originating from the absorption of the first exciton and second exciton, respectively, which ultimately clarifies the debate in the previous reports. Using femtosecond transient absorption spectroscopy, we systematically investigate the excited state dynamics of the Cs$_3$Bi$_2$Br$_9$ NCs and reveal that the photoexcited carriers undergo a self-trapping process within 3 ps after excitation. More intriguingly, the Cs$_3$Bi$_2$Br$_9$ NCs prepared by this method show much better photostability than those prepared by the ligand-assisted reprecipitation process. Photodetectors based on these Cs$_3$Bi$_2$Br$_9$ NCs show a sensitive light response, demonstrating the definite potential for breakthrough optoelectronic applications.