How many electron traps are formed in persistent phosphors?

Abstract

Persistent luminescence is caused by a charge carrier detrapping process from the carrier traps filled by excitation light. Thus, the carrier storage capacity is an important factor in determining the persistent luminescence intensity. Here, the electron storage capacity was investigated in the YAGG:Ce³⁺-Yb³⁺ transparent ceramic persistent phosphor, in which the Ce³⁺ ion is the luminescence center and the Yb³⁺ ion acts as the electron trap. The number density of the Yb²⁺ electron trapping center was estimated to be approximately 1.6 × 10¹⁸ ions cm⁻³ from the absorption coefficient spectrum of Yb²⁺:4f–5d photochromic absorption center and the Yb-LIII edge XANES spectrum, which means that approximately 12% of the Yb³⁺ ions in the sample were changed to the divalent state after charging. Although the maximum energy density of 0.61 J cm⁻³ was calculated as a storage property of persistent phosphors from the Yb²⁺ number density and the photon energy of Ce³⁺:5d₁−4f luminescence at 520 nm, the actual energy density which was detected as persistent luminescence was 0.011 J cm⁻³. It is suggested that the recombination efficiency of the detrapped electrons from the Yb²⁺ ions with the hole-trapped Ce³⁺ ions is approximately a few percent.