Defect structure in quantum-cutting Yb 3 + -doped CsPb Cl 3 perovskites probed by x-ray absorption and atomic pair distribution function analysis

Kluherz, Kyle T.; Mergelsberg, Sebastian T.; Sommer, David E.; Roh, Joo Yeon D.; Saslow, Sarah A.; Biner, Daniel; Krämer, Karl W.; Dunham, Scott T.; De Yoreo, James J.; Gamelin, Daniel R. (2022). Defect structure in quantum-cutting Yb 3 + -doped CsPb Cl 3 perovskites probed by x-ray absorption and atomic pair distribution function analysis. Physical review materials, 6(7) American Physical Society 10.1103/PhysRevMaterials.6.074601

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Ytterbium doping in all-inorganic lead-halide perovskites [CsPb(Cl1−xBrx )3] generates interesting properties
including quantum cutting and narrow line emission, making these materials attractive spectral down converters
for solar photovoltaics. The relationship between this optical efficiency and the defect structure(s) associated with
Yb3+ dopants within perovskites is not well understood. Various charge-neutral doping motifs have previously
been proposed and studied computationally, including clusters involving two substitutional Yb3+ ions charge
compensated by a single local Pb2+ vacancy. Near-band-edge defect states associated with such motifs are
believed to play an important mechanistic role in quantum cutting itself. Here, we report the results of x-ray
absorption and x-ray total-scattering measurements on ytterbium-doped CsPbCl3. XANES shows that the dopant
oxidation state is exclusively Yb3+, and a combination of Yb L3 and Pb L3 extended x-ray absorption fine
structure (EXAFS) shows that this Yb3+ substitutes exclusively at Pb2+ sites, where it adopts a pseudo-octahedral
[YbCl6]
3− coordination environment. Shell-by-shell fits to the data show a short Yb-Cl bond distance of 2.58
Å compared to the Pb-Cl bond distance of 2.83 Å. We confirm this finding by x-ray pair distribution function
analysis, which also shows evidence of additional Pb2+ vacancy formation induced by Yb3+ doping. We evaluate
whether this is the primary mechanism of charge compensation using simulated EXAFS and pair distribution
function data for several computed defect structures. Together, these results resolve the local dopant structures
and charge-compensation mechanisms in lanthanide-doped all-inorganic lead-halide perovskites, and, as such,
significantly advance the understanding of structure-function relationships in this important class of materials.

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