(428b) Halide Perovskites for Downconversion and Quantum Cutting to Surpass Solar Cell Efficiency Limits

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 the solar cell power conversion efficiencies above the Quessier limit (~33%) while reducing their degradation by UV and heating and thus improving their service lifetimes. Both efficiency and lifetime improvements are needed in viable technological paths to 2 ¢/kWh, the United States Department of Energy’s Levelized Cost of Electricity (LCOE) goal for silicon modules for 2030. Yb-doped CsPb(Cl_1-xBr_x)₃ (x<0.65) has emerged as a potential quantum cutting material because the Yb³⁺ emission at 1.25 eV is close to the bandgap of silicon 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. However, these solution-synthesized films exhibit problematic large sub-bandgap absorption. Reactive physical vapor deposition (RPVD) is emerging as a promising alternative production method for quantum cutting films, though RPVD CsPb(Cl_1-xBr_x)₃ films have not yet consistently matched the high PLQY of nanocrystals, with reproducible records reaching 95%, in CsPbCl₃, still below the 100% mark to prove beyond doubt that the downconversion is via quantum cutting. In RPVD, the precursor metal halides (e.g., CsCl, PbCl_2, and YbCl₃) are evaporated onto the substrate in a high vacuum and subsequently react either during deposition or during post-annealing to form the desired halide perovskite. Lead toxicity remains a significant concern for CsPb(Cl_1-xBr_x)₃, motivating the development of lead-free quantum-cutting materials. To address the latter issue, in addition to CsPb(Cl_1-xBr_x)₃, we are exploring the RPVD of Yb-doped lead-free double perovskite films with chemical formula Cs2BB’X6 with B = Ag, Na, B’ = Bi, In, Sb, and X=Cl, Br from the corresponding metal halide precursors. The double perovskites are derived by hetero substituting alternating Pb atoms in CsPb(Cl_1-xBr_x)₃ octahedra with alternating monovalent and trivalent ions. This talk will summarize our group’s past and ongoing efforts in pursuing lead-free quantum-cutting materials.^1-7 While reproducible and stable, PLQY as high as 95% are achieved with Cs₂AgBiBr₆.^1,2 This high PLQY indicates facile and efficient energy transfer from the perovskite host, Cs₂AgBiBr₆, to Yb, making Cs₂AgBiBr₆ a promising lead-free downconversion material. However, the downconversion is unlikely to be quantum cutting because the band gap for this material is lower than 2.5 eV, less than the energy required to excite two Yb³⁺ ions simultaneously. Substituting Cl for Br to blue shift the band gap to above 2.5 eV is one strategy to overcome this limitation of Cs₂AgBiBr₆. To that end, we have also been depositing Cs₂AgBiCl₆, doping it with Yb, and studying its suitability as a quantum-cutting material.⁷ The record NIR PLQY for this material to date is 70%, and a talk in this Meeting by Pulkita Jain will describe the progress to date. Some other candidate materials are thermodynamically unstable (e.g., Cs₂AgInBr₆),^3,5, while others can be formed but degrade in the air, possibly exacerbated by water vapor.⁶ We demonstrate significant progress toward developing efficient, lead-free NIR downconversion materials with PLQY approaching 100%. However, whether we can surpass 100% to demonstrate quantum cutting in these materials beyond any doubt remains an open question. We will address this question and the models guiding our materials design and synthesis choices.

1. N. Tran, I. J. Cleveland, J. R. Geniesse, and E. S. Aydil, “High Photoluminescence Quantum Yield Near-Infrared Emission from a Lead-Free Ytterbium-Doped Double Perovskite,” Materials Horizons 2191-2197 (2022). https://dx.doi.org/10.1039/D2MH00483F
2. N. Tran, I. J. Cleveland, and E. S. Aydil, “Reactive Physical Vapor Deposition of Yb-Doped Lead-Free Double Perovskite Cs₂AgBiBr₆ with 95% Photoluminescence Quantum Yield,” ACS Appl. Electron. Mater. 4, 4588–4594 (2022). https://doi.org/10.1021/acsaelm.2c00788
3. Liu, I. J. Cleveland, M. N. Tran, and E. S. Aydil, “Stability of the Halide Double Perovskite Cs₂AgInBr₆,” J. Phys. Chem. Lett. 14, 3000-3006 (2023). https://doi.org/10.1021/acs.jpclett.3c00303
4. Liu, P. Jain, I. J. Cleveland, M. Tran, S. Sarp, K. Sandrakumar, R. S. Rodriguez, and E. S. Aydil, “Optical properties of ytterbium-doped and undoped Cs₂AgInCl₆ thin films deposited by co-evaporation of chloride salts,” J. Mater. Chem. A 11, 21099-21108 (2023). https://doi.org/10.1039/D3TA04335E
5. N. Tran, R. S. Rodriguez, J. R. Geniesse, K. Sandrakumar, I. J. Cleveland, and E. S. Aydil, “Stability of Cs₂NaBiBr₆ and Cs₂NaBiCl₆,” Inorganic Chemistry 63, 12818−12825 (2024). https://doi.org/10.1021/acs.inorgchem.4c01299
6. Liu, P. Jain, I. J. Cleveland, S. Sarp, and E. S. Aydil, “Vapor deposition and stability of the lead-free halide double perovskite Cs₂AgInBr_6-xCl_x,” J. Vac. Sci. Technol. A 43, 023410 (2025). https://doi.org/10.1116/6.0004128
7. Jain, M. N. Tran, I. J. Cleveland, Y. Liu, S. Sarp, and E. S. Aydil, “Vapor deposition and optical properties of Cs₂AgBiCl₆ thin films,” J. Phys. Chem. C 129, 5301-5311(2025). https://doi.org/10.1021/acs.jpcc.4c06622