Femtosecond Upconversion Study of Interfacial Electron Transfer from Photoexcited CsPbBr₃ Perovskite Nanocrystal to Rhodamine 6G

Photoinduced electron transfer (PET) from an excited-state CsPbBr₃ nanocrystal (NC) to rhodamine 6G (r6G) is studied in toluene using different fluorescence-based techniques. Because of weak solubility of r6G in toluene, excess r6G molecules adsorb at NC surface which result in a much slower rotational diffusion time scale of r6G in the presence of NCs. Study of intrinsic PET benefits from the soft molecular interactions leading to donor (NC)-acceptor (r6G) complex formation, where solvent diffusion parameters would not play any role in the PET kinetics. Femtosecond transients of NCs are nicely fit to a Poisson expression originally proposed by Tachiya. Conclusive fittings to the temperature dependence quenching data reveal two interesting observations: (1) Even though the average number of surface trap state in a NC does not change with temperature (5–60 °C), the trap-state-induced quenching time scale is accelerated with increase in temperature, pointing toward a more efficient trapping at higher temperature. (ii) In the presence of r6G, a fast (∼150 ps per r6G molecule) interfacial PET time scale is observed, which remains unaffected by temperature (5–60 °C). Our findings demonstrate that even a simple “perovskite NC–electron acceptor” composite like that in the present study can ensure a rapid interfacial charge separation. Such information will help us to realize the actual potential of perovskites NCs in their real applications.