Quantum Cutting via Broadband Sensitization in Ytterbium-Doped Lead Halide Perovskites

Abstract

The trivalent lanthanide (Ln3+) family has long been utilized for optical applications such as in lasing materials, lighting, and fiber optics. Their electronic structure is dominated by coulombic interactions and spin-orbit-split valence 4f states that have a minimal dependence on the ligand field environment. As a result, these 4f states are nearly the same energy between coordination in crystalline materials and as free ions. Yb3+ has one f-f transition, 2F5/2 --> 2F7/2, energetically well-matched to the bandgap of crystalline silicon making it a desirable phosphor for solar spectral shifting, a method of redistributing solar irradiance to a more desirable energetic regime. Furthermore, Yb3+ demonstrates the ability to participate in quantum cutting, a unique photophysical process in which high energy photons are converted into multiple lower-energy photons. While this process has been demonstrated numerous times in bulk morphologies using other lanthanides, transition metals, or dyes as sensitizers, it had not yet been demonstrated to occur from broadband sensitization originating from a semiconducting material until 2017 when Yb3+:CsPbCl¬3 was shown to have a host-sensitized photoluminescent quantum yield (PLQY) over 100%. The sensitization of this quantum cutting process is proposed to be facilitated by a defect formed upon doping in which two Yb3+ substitute with lattice Pb2+ adjacent to a charge-compensating Pb2+ vacancy. This defect motif results in a shallow, observable, state that rapidly localizes photoexcitation energy and cooperatively excites the two adjacent Yb3+ luminophores. Upon anion exchange to from the large bandgap chloride to the smaller bandgap bromide, the high quantum yields are maintained until the bandgap is smaller than twice the energy of the 2F7/2 --> 2F5/2 transition, the energy threshold at which quantum cutting is energetically possible. Further reduction of the bandgap precipitously decreases the Yb3+ PLQY to nearly 0% verifying the proposed cooperative excitation scheme. This process occurs in the same manner irrespective of the surface area-to-volume ratio of the host CsPbX3 (X = Cl, Cl/Br) material implicating its potential use in bulk morphologies as optically-coupled layers for solar spectral shifting on solar cells and in nano morphologies as phosphors in solar concentrators.