Ubiquitous Near-Band-Edge Defect State in Rare-Earth-Doped Lead-Halide Perovskites

Abstract CsPb­(Cl1–xBrx)3 (0 ≤ x ≤ 1) nanocrystals and thin films doped with a series of trivalent rare-earth ions (RE3+ = Y3+, La3+, Ce3+, Gd3+, Er3+, Lu3+) have been prepared and studied using variable-temperature and time-resolved photoluminescence spectroscopies. We demonstrate that aliovalent (trivalent) doping of this type universally generates a new and often-emissive defect state ca. 50 meV inside the perovskite band gap, independent of the specific RE3+ dopant identity or of the perovskite form (nanocrystals vs thin films). Chloride-to-bromide anion exchange is used to demonstrate that this near-band-edge photoluminescence shifts with changing band-gap energy to remain just below the excitonic luminescence for all compositions of CsPb­(Cl1–xBrx)3 (0 ≤ x ≤ 1). Computations show that this shift stems from the effect of the changing lattice dielectric constants on a shallow defect-bound exciton. Microscopic descriptions of this dopant-induced near-band-edge state and its relation to quantum cutting in Yb3+-doped CsPb­(Cl1–xBrx)3 are discussed.