This document summarizes research on limiting factors for the storage capacity of persistent phosphors. It finds that optically stimulated detrapping during charging may be a key bottleneck. A simple model is presented to simulate trapping and detrapping simultaneously. The model reveals that optically stimulated detrapping has a large influence on charging dynamics, even at low excitation rates, which had not been considered previously. This could explain discrepancies seen between charging and decharging behaviors. Further careful study is needed to better understand effects of excitation intensity and wavelength on trap filling and emptying dynamics, which is important for applications of these materials.

1. ugent.be lumilab.ugent.be nb-photonics.ugent.be
Optically stimulated detrapping
limiting the storage capacity of persistent phosphors
ICDIM ‘16 – Defects in Insulating Materials
Lyon, France – July 12 2016
Philippe F. Smet
Claude Tydtgat, Dirk Poelman, Katrien W. Meert
philippe.smet@ugent.be
@pfsmet
1

2. The game changing persistent phosphor: SrAl2O4:Eu,Dy
Van den Eeckhout K. et al, Materials 3 (2010) 2536-2566
Persistent Luminescence in Eu2+-Doped Compounds: A Review

3. @pfsmet

4. @pfsmet
thermally, mechanically
or optically driven
release

5. @pfsmet
Back of the envelope calculation
Accessible traps: 10% of recombination centers
2% molar doping by Eu in SrAl2O4
Alkaline battery: 407 J/g
Charging
(filling of traps)
Decharging
(emptying of traps)

6. I (cd/m²)
Time after sunset (h)
Thermally driven (uncontrolled) release
DT = 0°C
DT = -10°C
DT = -20°C
Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution

7. Botterman et al., Acta Materialia 60 (2012) 5494-5500
Mechanoluminescence in BaSi2O2N2:Eu
BaSi2O2N2:Eu
Mechanically driven release

8. Kersemans et al, Applied Physics Letters 107 (2015) 234102
Fast Reconstruction of a Bounded Ultrasonic Beam using Acoustically induced Piezoluminescence
Mechanically driven release: ultrasound detection

9. Whatever the application ( ),
energy storage capacity is crucial.
Where is the bottleneck?
@pfsmet

10. Charging CaAl2O4:Eu,Nd
The trapping probability is very, very high.
Afterglow
… > 72 hours

11. How can we unite
• a (relatively) low storage capacity
• a very high trapping efficiency
?
One possible approach:
Modelling trapping and detrapping simultaneously

12. The experiment... in order to understand (de)trapping
Variation 193K to 353K
Jonas Botterman et al, Physical Review B 90, 085147 (2014)
Trapping and detrapping in SrAl2O4:Eu,Dy persistent phosphors: Influence of excitation…

13. Setting up the model: keep it simple & local
Eu2+
Trap + e
Eu3+
Empty trap
Detrapping
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors

14. pnr
• Boundary conditions for charging and afterglow
• pe (excitation rate) is small
• Two solutions l1 and l2: two exponentials
Differential equations
Eu2+ trap

15. • pe  0 : Eigenvalues for charging and afterglow are 
• Solution for charging:
charging
afterglow
@pfsmet

16. Problem #1
Step in charging curve ≠ Step for afterglow
Charging | Afterglow | TL analysis
Simple system (one Eu site)
Sr2MgSi2O7:Eu,Dy

17. Charging | Afterglow | TL analysis
Problem #2
Eigenvalues are identical for different pe
≠ Charging dynamics strongly depend on pe

18. Problem #3
Absorption  pe(M-me-m) should decrease
<> Absorption increases, depends on pe
Reflected
excitation
light

19. Influence of excitation rate pe not negligible, on the contrary!
Large influence, yet pe is low
Effect is proportional to pe x a
OSL : a pe
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
Eu2+ traps

20. It makes sense for charging and decharging…
Emission intensity
a = 200
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors
Charging | Afterglow

21. … and for the influence of the excitation intensity pe
Claude Tydtgat et al, Optical Materials Express 6 (2016) 844-858
Optically stimulated detrapping during charging of persistent phosphors

22. 22
-60°C
0°C
TL
Heating
Empty all traps
0
10
20
30
-50 -25 0 25 50 75 100
TLintensity(arb.units)
Temperature (°C)
100%

23. 23
-60°C
0°C
TL
Heating
Empty all traps
100% 1% 60%
Thermal barrier
for trapping

24. OSL (at charging l) is compatible with:
• Different initial rise and drop after charging
• Exponentials for charging and afterglow are different
• Absorption increases during charging
• Influence of excitation intensity
• Charging behaviour depends on wavelength (via abs)
• Different trap filling for different wavelengths
This is worrying/hopeful for applications…
… and requires careful study!

25. Resources
Feature issue on Persistent and Photostimulable Phosphors
in Optical Materials Express (published)
http://tiny.cc/OMEXPPP

26. Resources & Acknowledgments
Thank you for your attention !
(and your feedback…)
Presentation can be found at http://www.slideshare.net/pfsmet