Presentation on opportunities and limitations of energy storage phosphors, which can be used for glow-in-the-dark roads or safety illumination. Loss mechanisms in phosphors. Presented at the Phosphor Global Summit and Quantum Dot Forum 2019 in San Diego, La Jolla, California. March 19-21.

1. Philippe Smet
philippe.smet@ugent.be
@pfsmet
http://LumiLab.UGent.be
@UGentLumiLab
Energy Storage Phosphors:
From Glowing Roads to Pressure Memory
Presented at the Phosphor Global Summit 2019, San Diego, March 21 2019.

2. Lisa – SEM-CL spectroscopy Jonas – ab initio calculations
Jiaren – NIR persistent phosphors
Robin – ALD coating of QDs
Ying – thermoluminescence
Ang - ML
David – Metrology, TL
Simon – Ultrasound imaging
Reinert – Phosphor stability
Jieqi - OSL
Andreas – SmS thin films
Ewoud – photocatalysis
José – persistent phosphors

3. Highlight 1 – Red fluoride phosphors
Excitation
Moon et al. , Opt. Mater. Express, 2016, 6, 782.
Optical properties are excellent… But what about stability?
Emission

4. Highlight 1 – Red fluoride phosphors
Cation exchange
Mn valence state issues
Slow (limited reactive area)
Mn7+  Mn4+
Sijbom et al., Optical Materials Express 2017, 7, 3332-3365.
K2SiF6:Mn4+ as a red phosphor for displays and warm-white LEDs: a review of properties and perspectives

5. Highlight 1 – Red fluoride phosphors
• Many synthesis parameters: risk of impurity formation
• RT-XRD is not able to detect all impurities (KHF2, Mn3+)
• Impurities severely affect the chemical stability
• XRD and DRS provide a quick tool check purity
• Encapsulation of fluoride phosphors is needed to enhance the long term stability
Verstraete et al., Journal of Materials Chemistry C 2017, 5, 10761-10769
Verstraete et al., ACS applied materials & interfaces 2018, 10, 18845-18856

6. KHF2 and Mn3+ correlated
vs.
vs.
6Verstraete et al., ACS applied materials & interfaces 2018, 10, 18845-18856
Case I – Pure phosphor Case II – Impure phosphor
In-situ XRD
Diffuse reflectance
In-situ XRD
Diffuse reflectance

7. Highlight 1 – Red fluoride phosphors
Inorganic coatings using
Atomic Layer Deposition
for long term stability
In collaboration with CoCooN research group at Ghent University – Christophe Detavernier

8. Highlight 2 - Cathodoluminescence for phosphor research
Temperature
stage
EDX detector
Electron detectors
Spectrograph
ICCD
SEM chamber
Optical fiber
Beam blanker
Pulse
generator

9. 51200 spectra
Highlight 2 - Cathodoluminescence for phosphor research
Poelman and Smet, Physica B 439 (2014) 35–40
Time resolved microscopic cathodoluminescence spectroscopy for phosphor research

10. 5µm
Total CL intensity
Peak emission wavelength (nm) FWHM nm)
SrGa2S4:Eu2+
Highlight 2 - Cathodoluminescence for phosphor research
(0.22eV)
(0.17eV)
(0.195eV)

11. 5µm
Recording consecutive CL mappings, at different temperature
Highlight 2 - Cathodoluminescence for phosphor research
Impact on phosphor research: doping homogeneity in SrGa2S4:Eu2+
Martin et al., ECS Journal of Solid State Science and Technology, 7 (1) R3052-R3056 (2018)
Microscopic Study of Dopant Distribution in Europium Doped SrGa2S4: Impact on Thermal Quenching and Phosphor Performance

12. Energy storage phosphors

13. The early days – ZnS:Cu,Co
The disruptive compound
SrAl2O4:Eu,Dy ...
From LEGO’s Black Knight’s Castle (1992)
(Patent Nemoto 1994)
An example of energy storage phosphors: persistent phosphors

14. Eu2+
exc
em trap/defect
thermal barrier DE Research questions.
Trapping routes
Nature of defects
Trapping capacity
Loss mechanisms
Advanced sensing
Inside energy storage phosphors

15. Glow-in-the-dark road marks
(Oss, NL, 2013, by Studio Roosegaarde)
Large scale applications of energy storage phosphors
Glowing bicycle path
(2017, Poland)

16. Glowing road marks: status and prospects
Q1: Why did the test in the Netherlands fail?
Q2: Can it actually work?
Q3: What are the main limiting factors?
Q4: What is the impact?
Q5: Out-of-the-box: Where is the impact?

17. Q1: Why did the test in the Netherlands fail?
Insufficient chemical stability + unmatched expectations

18. Q2: Can it actually work?

19. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
Brightness curve
0.3 mcd/m²
(100x limit eye sensitivity)

20. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
T(°C)
TLintensity
7h wait, TL
immediate TL
7h wait, TL
immediate TL
Tcharge
0°C
30°C
Saved by the trap distribution

21. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
Overnight temperature drop?
I (cd/m²)
Time after sunset (h)
DT = 0°C
DT = -10°C
DT = -20°C
Traps become
thermally
inaccessible

22. Botterman et al, Optics Express 23 (2015) A868
Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution
Total brightness?
I (cd/m²)
Time after sunset (h)
DT = 0°C
DT = -10°C
DT = -20°C
1h
x10 is sufficient

23. Q3: What are the main limiting factors?
SrAl2O4:Eu,Dy as an example.

24. Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors

25. A fully charged persistent phosphor can store per gram
about 200,000,000,000,000,000 electrons in traps
(i.e. a few % of the number of Eu2+ ions)
Number of traps?
Materials 10 (2017) 867
Counting the photons: determining the absolute trapping capacity of persistent phosphors

26. Quantum efficiencies
Internal quantum efficiency:
𝜂𝑖𝑛 = 𝑁𝑒𝑚/𝑁𝑎𝑏𝑠
ACS Photonics 5 (2018) 4529–4537
Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors

27. But then we expect an intensity
dependent absorption!
Trap induced optical absorptions (OSL)

28. Trap induced optical absorptions (OSL)
𝝀 𝒆𝒙 (nm) 𝒂 𝑬𝒖 𝟐+ (cm²) 𝒂 𝒕𝒓 (cm²)
375 3.6 x 10-18 2.2 x 10-17
445 4.4 x 10-19 1.2 x 10-17
ACS Photonics 5 (2018) 4529–4537
Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors

29. Q4: What is the impact?

30. A road map for glowing roads
One key message:
Tailor phosphor design to application (e.g. charging wavelengths and efficiency)
Sustainable solution
Large phosphor quantities (!)
Limits light
pollution
Brightness increase needed
Limit losses
Optical design
Make full use of solar spectrum

31. Loss mechanism in (high brightness) LED phosphors.
ACS Photonics 5 (2018) 4529–4537
Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors
Internal quantum efficiency:
𝜂𝑖𝑛 = 𝑁𝑒𝑚/𝑁𝑎𝑏𝑠

32. Loss mechanism in (high brightness) LED phosphors.
Flicker reduction in AC LEDs.
ACS Photonics 5 (2018) 4529–4537
Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors

33. Q5: Out-of-the-box: Where is the impact?

34. C.-N. Xu et al. N. Terasaki and C.-N. Xu, IEEE Sens. J., 2013, 13, 3999.
Mechanoluminescence
BaSi2O2N2:Eu2+
Adv. Mater. 2015, 27, 2324–2331

35. C.-N. Xu et al.
N. Terasaki and C.-N. Xu, IEEE Sens. J., 2013, 13, 3999.
Materials 2018, 11, 484

36. Mechanoluminescence: a memory effect
WRITE READINITIALIZE (fill traps) < - - up to 72 h - - >

37. IRML
Thermal detrapping: keeps it local
Mechanical detrapping: reshuffling
OSL: reshuffling
Use the sensitivities of the traps
Mechanoluminescence: a memory effect

38. Conclusions
Don’t expect the impossible
(e.g. for powering solar panels at night)
Many safety applications within reach
(with a little push and optimization)
Make energy storage phosphors smart
(and use them as advanced sensors)

39. I am looking forward to
your feedback!
Presentations can be found at http://www.slideshare.net/pfsmet
@pfsmet @UGentLumiLabphilippe.smet@ugent.be