Broadband White Emission in Cs₂AgIn_1–<i>x</i>Bi_<i>x</i>Cl₆ Phosphors

Abstract

The photoluminescent properties of the lead-free double perovskite solid solution Cs₂AgIn_1-<i>x</i>Bi_<i>x</i>Cl₆ have been investigated. The In³⁺ end member, Cs₂AgInCl₆, is a direct gap semiconductor that absorbs UV light (λ < 350 nm) and shows little to no photoluminescence. Incorporation of Bi³⁺ leads to a strong sub-band gap absorption that peaks in the near UV (∼360 nm) and extends into the visible. This absorption, which is thought to originate from localized 6s² → 6s¹p¹ transitions on Bi³⁺ ions, is split by a Jahn-Teller distortion of the excited state. In-rich samples show strong photoluminescence that is attributed to radiative decay of self-trapped excitons, with a broad emission peak of significant intensity from 450 to 750 nm. The color of the emitted light is best described as yellow-white (λ_max ≈ 625 nm), due to the extreme breadth of the emission peak (fwhm ≈ 217(2) nm). The excitation spectrum extends out to 450 nm for samples near <i>x</i> = 0.25, while the photoluminescent quantum yield (PLQY) reaches a maximum of 39 ± 3% in the <i>x</i> = 0.167 sample. The emission characteristics, which include a correlated color temperature (CCT) of 3119 K and a color rendering index (CRI) of 85, coupled with an excitation spectrum that can be driven by visible photons emitted from a Ga_1-<i>x</i>In_<i>x</i>N LED, make Cs₂AgIn_1-<i>x</i>Bi_<i>x</i>Cl₆ phosphors promising for use in solid state white lighting applications.