Defect-Interceded Cascading Energy Transfer and Underlying Charge Transfer in Europium-Doped CsPbCl₃ Nanocrystals

Rare-earth ion (RE³⁺) doping in cesium lead chloride (CsPbCl₃) has unlocked novel prospects to explore changes in optical, magnetic, and charge carrier transport properties. This leads to a huge advancement in optoelectronic applications, yet deep understanding of the photophysics governing the energy transfer processes is lacking and demands vital attention. Herein, we probe into the mechanistic transfer processes from the band edge of the host (CsPbCl₃) to the dopant europium ion (Eu³⁺) with the aid of femtosecond fluorescence upconversion and transient absorption (TA) spectroscopy. The upconversion measurement portrays a defect-mediated cascading energy transfer from CsPbCl₃ to Eu³⁺ and further cross-relaxation among Eu³⁺ states. Moreover, TA studies reveal that there is charge transfer from the band edge of CsPbCl₃ to doping-induced shallow defect states. Furthermore, two-photon absorption study establishes no compromise in the transfer mechanism even upon bandgap excitation. This work validates that Eu-CsPbCl₃ is an apt entrant for optoelectronic applications.