Physical vapor deposition of Yb-doped CsPbCl3 thin films for quantum cutting

Ytterbium-doped CsPbCl3 is the leading candidate for a quantum-cutting coating on silicon solar cells to increase their efficiencies and durability by converting each incident ultraviolet and blue photon with energies >2.5 eV to two 1.25-eV near-infrared (NIR) photons. This approach can potentially increase silicon solar cell efficiencies above the Quessier limit. While photoluminescence quantum yields (PLQYs) approaching 200% have been demonstrated with thin films made using colloidal particle synthesis and nanocrystal dispersions, there is increased interest in physical vapor deposition as a large-area scalable method. We investigated the effect of Yb concentration and the annealing environment on the film structure, morphology, and optical properties, including the NIR PLQY. We found that subtle differences in the postdeposition annealing protocol significantly affect PLQY. Specifically, the highest PLQY (∼70%) is achieved by annealing the films in an N2-filled glovebox first, followed by a second annealing step in air, both at 350∘C. X-ray photoelectron spectroscopy reveals that this approach anneals bulk defects, brings Yb to the surface, and forms a passivating oxide layer on the surface whose major component appears to be Yb2O3. Both surface oxidation and Yb segregation to the surface suppress grain growth during annealing. In addition, annealing only in air stops Yb segregation toward the surface. Annealing in N2 brings Yb to the surface but does not form a robust oxide. In contrast, annealing in N2, followed by annealing in air, brings Yb toward the surface and forms a passivating oxide, resulting in the highest PLQY.