Photoredox-Mediated Sensitization of Lanthanide Dopants by Perovskite Nanocrystals

Trivalent lanthanide ions (Ln3+) have electronically isolated f-orbitals that support excited states, which, due to their ultra-narrow emission and long spin coherence lifetimes, are potentially suitable as hosts for spin qubits. These excited states must however be generated through photosensitization due to the low-absorption cross sections of Ln3+. This paper describes the mechanism of photosensitization of Yb3+ and Sm3+ ions by their CsPbCl3 perovskite nanocrystal (NC) host matrices, through examination of the photophysical processes in NCs doped with seven different Ln3+ ions. An observed dependence of the NC's band-edge photoluminescence (PL) on the potential of the Ln3+/Ln2+ redox couple implies the presence of a charge transfer intermediate in the sensitization mechanism. Ultrafast transient absorption (TA) experiments indicate that (i) a previously identified Ln3+-Cl-VPb2-Cl-Ln3+ defect is formed within 1 ps of photoexcitation of the NC regardless of the yield of photosensitization, (ii) the first step of photosensitization, an electron transfer, occurs in 10s of picoseconds, and (iii) the resulting charge-separated state forms the emissive species, Ln3+*, through a slower subsequent hole transfer from the NC. A mechanistic understanding of Ln3+ dopant sensitization provides a framework for choosing the right combination of host matrix and Ln3+ species for efficient photosensitization.