Electrical stimulation for brighter persistent luminescence

In a new paper published in Light Science & Application, a team of scientists, led by Professor Yuhua Wang from the School of Materials and Energy, Lanzhou University, China, and co-workers have successfully developed a phosphor/electrode layer structure for high brightness long afterglow emission using in-situ electric field stimulation.

 

Since the introduction of the new yellow-green luminescent material SrAl₂O₄:Eu²⁺,Dy³⁺ in 1996, several new types of persistent luminescent materials have been developed through the relentless research of scientists, such as CaAl₂O₄: Eu²⁺,Nd³⁺ (blue), Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ (blue), Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ (cyan), Ca₂BO₃Cl:Eu²⁺,Dy³⁺ (yellow), Sr₃SiO₅:Eu²⁺,Nb⁵⁺ (yellow), Y₂O₂S:Eu³⁺,Mg²⁺,Ti⁴⁺ (red), Sr₂Si₅N₈:Eu²⁺,Tm³⁺ (red), and so on. Some of these have been effectively commercialized.

 

However, SrAl₂O₄:Eu²⁺,Dy³⁺ is still the most widely used and no more powerful long afterglow material has been developed, so the growing industrial demand for “brighter afterglow” has not been met. In the event of a sudden power failure in a building such as a factory, commercial premises or office, people need a brighter, clearer indication to guide them quickly and safely to an exit or shelter in the dark. This requires bright afterglow for a short period, such as 30 minutes, rather than for a relatively long time, such as 1-10 hours.

 

To achieve higher brightness in long afterglow materials, it is necessary to start with the afterglow mechanism. In the research process of long afterglow materials, the afterglow mechanism of Eu²⁺ activated/Ln³⁺ co-doped long afterglow phosphor represented by SrAl₂O₄:Eu²⁺,Dy³⁺ is also gradually progressing. Many afterglow mechanism models have been proposed, such as the Matsuzawa model, the Aitasalo model, the Dorenbos-Nakazawa model and the Clabau model. Among these, the Dorenbos-Nakazawa model is recognized as a better explanation of the afterglow behaviours. However, the model still lacks sufficient experimental evidence to infer carrier/trap assignments, energy transport pathways, etc. More experimental data and new methods are needed to fully understand the persistent luminescence process and solve the problem of low afterglow emission intensity.

 

The research team first synthesised single-phase SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors and designed and assembled devices consisting of molybdenum oxide/silver electrodes, phosphors and transparent conductive FTO substrates, which can be connected to external circuits for spectral measurement when voltage is applied. The spectra results show that the application of voltages of 3 V and 6 V both lead to an increase in afterglow luminance, and with the voltage increased to 12 V, the initial luminance of the SrAl₂O₄:Eu²⁺,Dy³⁺ can also be increased from 0.409 cd m^-2 to 0.538 cd m^-2. Then the afterglow process was simulated using the Arrhenius and rate equations, and the depth of the voltage-stimulated ultra-shallow traps is derived from the fitted relationship between the initial afterglow intensity and the applied voltage, which is about 0.022 eV. In addition, the external electric field stimulation of shallow trap release is further confirmed using thermoluminescence techniques. Finally, a typical application demonstration based on electric field excitation of long afterglow luminescence is provided.

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