1. Sol-gel rare-earth-doped glasses for solar cells
Francesco Enrichi
Research Fellow, Centro Studi e Ricerche Enrico Fermi, Roma (Italy)
Vinnmer Marie Curie Incoming Fellow, Luleå University of Technology, Luleå (Sweden) 1

2. Outline
Introduction
Downconversion in Tb3+/Yb3+ co-doped silica-hafnia glasses and glass ceramics
Preliminary results on Ag-exchanged Tb3+/Yb3+ co-doped zirconia glasses and glass ceramics
Conclusions and perspectives

3. Silicon solar cells: spectral response
Spectral mismatch
930 nm
X. Huang et al. Chem. Soc. Rev., 2013,42,

4. Rare earth ions have suitable energy level structure for energy conversion processes
One UV or blue photon
Two red / NIR photons
(Yb max. 980 nm)
L. Wondraczek et al., Adv. Sci. 2015, 2,

5. Two infrared photons @ 980 nm
Choice of the rare earths
Choice of the host material
SiO2-HfO2 glass-ceramics
SiO2-ZrO2 glass-ceramics
Tb3+-Yb3+
Combine the advantages of glasses and the better spectroscopic properties of crystals
Silica-based coatings, waveguides, fibers
Low phonon energies:
HfO2 ~ 700 cm-1 ; ZrO2 ~ 470 cm-1
High refractive index (500 nm):
HfO2 ~ 2.13 ; ZrO2 ~ 2.18
One blue 488nm
Two infrared 980 nm

6. 30 min. @ 1000 °C in order to nucleate HfO2 nanocrystals
Sample preparation
SiO2-HfO2 glass and glass-ceramics
Thermal treatments
TEOS + EtOH + H2O + HCl Si
°C after each dip
1h stirring
5 °C after 20 dips
HfOCl2 + EtOH Hf
GLASS (G)
Tb(NO3)3
Yb(NO3)3 RE
°C in order to nucleate HfO2 nanocrystals
16h stirring
GLASS-CERAMICS (GC)
DIP COATING
G. Alombert-Goget et al., Proc. SPIE 7598 (2010) 75980P–1/9.
G. Alombert-Goget et al., Opt. Mater. 33 (2010) 227–230.

7. Composition: 70% SiO2 – 30% HfO2
SiO2-HfO2 glass and glass-ceramics
Samples properties
Composition: 70% SiO2 – 30% HfO2
[Tb] : [Yb] = 1 : 4 (best rate from previous studies)
[Tb]+[Yb] = 5 ÷ 21 % (previous studies 1 ÷ 5 %)
G (900 °C ann.) and GC (1000 °C ann.) samples
Layer thickness ~ 0,7 – 0,8 µm
G. Alombert-Goget et al., Proc. SPIE 7598 (2010) 75980P–1/9.
G. Alombert-Goget et al., Opt. Mater. 33 (2010) 227–230.
A. Bouajaj et al., Opt. Mat., 2016,52, 62-68

8. Structural characterization: XRDG GC * *
Phase separation and formation of HfO2 nanocrystals of about 3-4 nm in GC samples
* tetragonal HfO2 (ICSD card No 85322)

9. Structural characterization: TEM
9% G
9% GC
Phase separation and formation of HfO2 nanocrystals of about 3-4 nm in GC samples
A. Bouajaj et al., Opt. Mat., 2016,52, 62-68

10. Optical characterization: PL excitation

11. Optical characterization: PL emission
9% GC
Tb3+ emission decrease when Yb3+ is added
Indication of energy-transfer

12. 𝑷𝑳 𝒅𝒆𝒄𝒓𝒆𝒂𝒔𝒆 = 𝟏 − 𝑷𝑳 𝑻𝒃−𝒀𝒃 𝑷𝑳 𝑻𝒃
Optical characterization: intensity decrease
𝑷𝑳 𝒅𝒆𝒄𝒓𝒆𝒂𝒔𝒆 = 𝟏 − 𝑷𝑳 𝑻𝒃−𝒀𝒃 𝑷𝑳 𝑻𝒃
Sample
Glass
Glass-ceramics
RE 5
34,5 %
40,2 %
RE 7
37,9 %
61,8 %
RE 9
67,7 %

13. Optical characterization: intensity decrease
9% G and GC
Different Yb3+ emission in G and GC samples
Indication of different RE environment

14. Optical characterization: energy-transfer
5 𝐷 𝟒 : 𝟏 𝝉 𝒎𝒆𝒂𝒔 = 𝟏 𝝉 𝒓 𝟏 𝝉 𝒏𝒓
Energy-transfer efficiency
𝜼 𝑬𝑻 = 𝑰 𝑻𝒃+𝒀𝒃 𝒅𝒕 𝑰 𝑻𝒃 𝒅𝒕
TOTAL efficiency
𝜼 𝒕𝒐𝒕 = 𝜼 𝑽𝑰𝑺 𝜼 𝑵𝑰𝑹 = 𝜼 𝒓, 𝑻𝒃 (1 - 𝜼 𝑬𝑻 ) + 𝟐 𝜼 𝑬𝑻 ≈ 𝜼 𝑬𝑻

15. Optical characterization
𝟓 𝑫 𝟒
𝟕 𝑭 𝑱
lexc = 355 nm lem = nm
Tb3+ emission lifetime decreases when Yb3+ is added
Tb3+ emission lifetime decreases more and more with RE content

16. Optical characterization

17. Optical characterization
Maximum efficiency
for [Tb+Yb] = 19%
ET = 94%
EQE = 194%

18. Rate equations: direct transfer vs virtual level transfer
NaYF4:Tb3+,Yb3+
50SiO2-20Al2O3-27CaF2:Tb3+,Yb3+
Duan et al., J.Appl.Phys., 2011,110, ; Duan et al., Opt. Lett., 2012,37,

19. Optical characterization
Main contribution in our samples seems direct transfer from the 5D4 Tb3+ excited state
Possible role of non-accounted processes: back-transfer, excited state absorption, cross-relaxation plus multiphonon ?
Direct quantum efficiency measurements could help in better understanding this point

20. Outline
Introduction
Downconversion in Tb3+/Yb3+ co-doped silica-hafnia glass and glass ceramics
Preliminary results on Ag-exchanged Tb3+/Yb3+ co-doped zirconia glass and glass ceramics
Conclusions and perspectives

21. Why Ag doping? Ag introduction by ion exchange
Broadband excitation of Er3+ ions
1.54 µm emission

22. Sample preparation SiO2-ZrO2 glass and glass-ceramics
Thermal treatments
TEOS + EtOH + H2O + HCl Si
3 °C after each dip
1h stirring
Zr propoxide + Acac + EtOH
15 °C after 10 dips Zr
GLASS (G) Na
Na-Ac + MeOH
°C in order to nucleate ZrO2 nanocrystals RE
Tb(NO3)3
Yb(NO3)3
GLASS-CERAMICS (GC)
16h stirring
DIP COATING

23. Ag+-Na+ ion-exchange Na-doped coating
Molten salt bath 1 mol% of AgNO3 in NaNO3
Ion-exchange process
350 °C \
Ag Na+
Ag+
Followed by thermal annealing in air
Ag aggregation
380 °C
440 °C

24. Composition: 70% SiO2 – 30% ZrO2 + additional 10% Na
Samples properties
SiO2-ZrO2 glass and glass-ceramics
Composition: 70% SiO2 – 30% ZrO additional 10% Na
G (700 °C ann.) and GC (1000 °C ann.) samples
Layer thickness ~ 0,5 µm
G0 / GC0 : [Tb] = 0% [Yb] = 0%
G1 / GC1 : [Tb] = 1% [Yb] = 0%
G4 / GC4 : [Tb] = 0% [Yb] = 4%
G5 / GC5 : [Tb] = 5% [Yb] = 4%
+ Ag exchange
(8 samples)
+ two annealing T
(16 samples)
8 starting samples

25. XRD preliminary check SPIN-COATING deposited samples
rpm - 1 min.

26. RBS measurements (Rutherford Backscattering Spectrometry)

27. RBS spectra comparison
Tb, Yb are detected and follow the expected trend
Ag is clearly detected in the exchanged samples

28. Quantitative analysis
Matrix nominal: Si 70, Zr 30, O 200, Na 10
G0: Si 68, Zr 29, O 200, Na 10, Hf 0.25

29. Quantitative analysis
Matrix nominal: Si 70, Zr 30, O 200, Na 10
G0: Si 68, Zr 29, O 200, Na 10, Hf 0.25
G1: Si 65, Zr 29, O 200, Na 10, Hf 0.25, TbYb 0.9

30. Quantitative analysis
Matrix nominal: Si 70, Zr 30, O 200, Na 10
G0: Si 68, Zr 29, O 200, Na 10, Hf 0.25
G1: Si 65, Zr 29, O 200, Na 10, Hf 0.25, TbYb 0.9
G4: Si 65, Zr 29, O 200, Na 10, Hf 0.25, TbYb 4.2

31. Quantitative analysis
Matrix nominal: Si 70, Zr 30, O 200, Na 10
G0: Si 68, Zr 29, O 200, Na 10, Hf 0.25
G1: Si 65, Zr 29, O 200, Na 10, Hf 0.25, TbYb 0.9
G4: Si 65, Zr 29, O 200, Na 10, Hf 0.25, TbYb 4.2
G5: Si 67, Zr 30, O 200, Na 10, Hf 0.25, TbYb 5.3

32. Quantitative analysis
Matrix nominal: Si 70, Zr 30, O 200, Na 10
G0: Si 68, Zr 29, O 200, Na 10, Hf 0.25
G1: Si 65, Zr 29, O 200, Na 10, Hf 0.25, TbYb 0.9
G4: Si 65, Zr 29, O 200, Na 10, Hf 0.25, TbYb 4.2
G5: Si 67, Zr 30, O 200, Na 10, Hf 0.25, TbYb 5.3
GC0: Si 67, Zr 32, O 200, Na 10, Hf 0.25

33. Quantitative analysis
Matrix nominal: Si 70, Zr 30, O 200, Na 10
G0A: Si 68, Zr 29, O 200, Na 10, Hf 0.25, Ag x
Thickness obtained by using 1.9 g/cm3 for sol-gel SiO2
280 nm
Ag 3
40 nm
Ag 5.5

34. Quantitative analysis
Matrix nominal: Si 70, Zr 30, O 200, Na 10
GC0A: Si 67, Zr 32, O 200, Na 10, Hf 0.25, Ag x
Thickness obtained by using 1.9 g/cm3 for sol-gel SiO2
140 nm
Ag 1
180 nm
Ag 2

35. Quantitative analysis
Matrix nominal: Si 70, Zr 30, O 200, Na 10
G5A: Si 70, Zr 29, O 220, Na 0, Hf 0.25, TbYb 5.5, Ag x
280 nm
Ag 2.5
80 nm
Ag 5
Thickness obtained by using 1.9 g/cm3 for sol-gel SiO2

36. Conclusions and perspectives
Downconversion in Tb3+ + Yb3+ codoped SiO2-HfO2 glasses and glass-ceramics
Maximum efficiency 194 % for 19 at.% [Tb+Yb]
deeper investigation of the ET process via rate-equation modelling / simulations
direct efficiency measurement by an integrating sphere
Preparation of Na-doped SiO2-ZrO2 glasses and glass-ceramics
Introduction of Ag in the films by Ag+-Na+ ion-exchange process up to 3-5%
optimization of ion-exchange conditions (homogeneity, higher Ag concentration)
study and optimization of the post-exchange annealing
optical characterization

37. CNR-Italy / CNRST-Morocco bilateral project 2014-15
MAECI bilateral project PLESC - Centro Fermi Italy / University of Witwatersrand South Africa
CNR-Italy / CNRST-Morocco bilateral project
VINNMER Marie Curie Incoming Project - Nano2solar 37

38. CNR-Italy / CNRST-Morocco bilateral project 2014-15
Thank you!
MAECI bilateral project PLESC - Centro Fermi Italy / University of Witwatersrand South Africa
CNR-Italy / CNRST-Morocco bilateral project
VINNMER Marie Curie Incoming Project - Nano2solar 38

40. R 5% 7% 9%

41. Ag doping

42. Choice of rare-earth couples
Donor: absorbs incident photons from the sun and transfer its energy to the acceptor rare‐earth : Pr3+, Tm3+, Tb3+, relatively strong absorption in the blue
Er3+ : many mean or weak absorption bands in the UV‐blue‐green
Acceptor: emits the photons that will be absorbed by the PV cell
Yb3+, single excited state ‐ Emission at ~1000 nm absorbed efficiently by Si solar cell
Donor/acceptor
Frequency conversion
Pr3+/Yb3+
Blue (440nm) → NIR (2 emitted photons: 1000 nm)
Tm3+/Yb3+
Blue (478nm) → NIR (2 emitted photons: 1000 nm)
Tb3+/Yb3+
Blue (485nm) → NIR (2 emitted photons: 1000 nm)
Ce3+/Yb3+
UV‐blue (4f‐5d 330 borate to 450 nm YAG) → NIR
Ho3+/Yb3+
Blue (~450 nm)→NIR (2 emitted photons: nm)
Er3+/Yb3+
Violet‐Blue‐Green → NIR (2 emitted photons: nm)

44. Energy Spatial extension Filling orbital (optical properties)
Outer electrons
(chemical properties)
Spatial extension

45. Optical characterization
𝟓 𝑫 𝟒
𝟕 𝑭 𝑱
lexc = 355 nm lem = nm G GC

46. Optical characterization
𝟓 𝑫 𝟒
𝟕 𝑭 𝑱
lexc = 355 nm lem = nm
108 %
110 %
126 % G
125 %
132 %
154 % GC

47. Optical characterization
𝟓 𝑫 𝟒
𝟕 𝑭 𝑱
lexc = 355 nm lem = nm G GC

48. Optical characterization
𝟓 𝑫 𝟒
𝟕 𝑭 𝑱
lexc = 355 nm lem = nm
129 %
138 %
144 % G
145 %
171 %
179 % GC