Yb3+ doped LaSi3N5 and YSi3N5 with low energy charge transfer for near-infrared light-emitting diode and solar cell application

Abstract

Yb³⁺ doped LaSi₃N₅ phosphors have been prepared by solid-state synthesis in a high temperature furnace directly coupled to a glove box. The charge transfer (CT) band of Yb³⁺ in LaSi₃N₅ was observed at an unusually low energy of 3.1 eV. Excitation of this CT band results in f–f emission of Yb³⁺ at 1.2 eV, which could make Yb³⁺ doped LaSi₃N₅ suitable for application as a spectral conversion material for infrared light-emitting diodes or solar cells. Substitution of La³⁺ by the smaller Y³⁺ ion shifts the CT to higher energy (3.9 eV). A clear relation between the CT energy and the Yb³⁺ f–f emission intensity was found. With the help of a detailed energy level diagram that contains the position of the 4f and 5d levels of all divalent and trivalent ions with respect to the valence and conduction band, all hitherto observed luminescence properties of divalent and trivalent lanthanide dopants in LaSi₃N₅ are reviewed and explained.