High Photoluminescence Quantum Yield Downconversion in Ytterbium-Doped Metal Halide Perovskites

Redshifting the solar spectrum received by a solar cell with an overlayer that creates two near-infrared (NIR) photons from each ultraviolet (UV) and blue photon via quantum cutting can increase their power conversion efficiencies while reducing their degradation by UV light and heating. Yb-doped CsPb(Cl_1-xBr_x)₃ (x&lt;0.65) has emerged as a potential quantum cutting material for this application because the Yb³⁺ emission via ²F_5/2 → ²F_7/2 electronic transition at 1.24 eV is close to the bandgap of silicon (~ 1.1 eV) and photoluminescence quantum yields (PLQY) approaching 200% (2 NIR photons per UV photon) has been demonstrated in colloidal dispersions, and thin films synthesized using solution-based approaches.¹ These films, however, exhibit significant sub-bandgap absorption. We are using physical vapor deposition (PVD), specifically evaporation, as an alternative for depositing films with high PLQY and low sub-bandgap absorption. We explore the PVD of thin Yb-doped CsPb(Cl_1-xBr_x)₃ (x&lt;0.65) and Yb-doped lead-free double perovskite films (e.g., Cs₂AgBiX₆ with B = Bi, In, Sb and X=Cl, Br) via co-deposition from metal halide precursors. Yb is doped into the host halide perovskite, which absorbs in the UV and visible regions of the electromagnetic spectrum and then transfers energy to two Yb³⁺, exciting them from the ²F_7/2 ground state to the ²F_5/2 state. The excited Yb³⁺ emits NIR photons at ~1.24 eV upon relaxation. In this scenario, the two Yb³⁺ that receive the energy must be close to each other and likely also to a defect that captures the exciton. A likely candidate is a charge-compensated defect complex involving three corner-connected PbX₆ octahedra where one octahedron has a Pb vacancy (V_Pb), and two neighboring octahedra have Yb³⁺ ions that substitute in Pb²⁺ lattice sites (Yb_Pb).² PLQY from PVD CsPb(Cl_1-xBr_x)₃depends strongly on the annealing environment implicating surfaces and their importance in achieving quantum cutting and PLQY. To replace Pb and contrast CsPb(Cl_1-xBr_x)₃with a material where Pb is heterosubstituted to form a double perovskite, we deposited Yb-doped Cs₂AgBiBr₆ films using PVD. Yb³⁺ ions substitute into the double perovskite structure of Cs₂AgBiBr₆, specifically the AgBr₆^5- and/or BiBr₆^3- octahedra. Robust, reproducible, and stable PLQY as high as 95% are achieved with Cs₂AgBiBr₆ films doped with 8% Yb. This high PLQY indicates facile and efficient energy transfer from the perovskite host, Cs₂AgBiBr₆, to Yb, making Cs₂AgBiBr₆the most promising lead-free down-conversion material.³ However, downconversion is achieved even when the excitation energy is less than ~2.5 eV, indicating that the mechanism is not via quantum cutting. It is interesting to note that Yb³⁺substitution into neither Ag⁺ nor Bi³⁺ lattice positions has the potential to form a defect like Yb_Pb- V_Pb - Yb_Pbin CsPb(Cl_1-xBr_x)₃. PLQY of Yb-doped Cs₂AgBiBr₆thin films synthesized via physical vapor deposition depends strongly on how the substrate temperature changes during the deposition because it determines the amount of Bi incorporated into the film. Yb-doped Cs₂AgBiBr₆ films with PLQY as high as 95% are obtained only with excess BiBr₃ and by ramping substrate temperature during the deposition. Ramping the substrate temperature reduces BiBr₃ loss from the film by promoting reactions that form Cs₂AgBiBr₆.<br/><br/>1. D. M. Kroupa, J. Y. Roh, T. J. Milstein, S. E. Creutz and D. R. Gamelin, <i>ACS Energy Lett</i>., 2018, <b>3</b>, 2390-2395.<br/>2. T. J. Milstein, D. M. Kroupa and D. R. Gamelin, <i>Nano Lett.,</i> 2018, <b>18</b>, 3792-3799.<br/>3. M. N. Tran, I. J. Cleveland, J. R. Geniesse and E. S. Aydil, <i>Materials Horizons</i> (2022). https://dx.doi.org/10.1039/D2MH00483F