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

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

Silicon solar cells: spectral response Spectral mismatch 930 nm X. Huang et al. Chem. Soc. Rev., 2013,42, 173-201

Rare earth ions have suitable energy level structure for energy conversion processes One UV or blue photon Two red / NIR photons (Yb max. emission @ 980 nm) L. Wondraczek et al., Adv. Sci. 2015, 2, 1500218

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 photon @ 488nm Two infrared photons @ 980 nm

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 50’’ @900 °C after each dip 1h stirring 5 min. @900 °C after 20 dips HfOCl2 + EtOH Hf GLASS (G) Tb(NO3)3 Yb(NO3)3 RE 30 min. @ 1000 °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.

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

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

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

Optical characterization: PL excitation

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

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 %

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

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

Optical characterization 𝟓 𝑫 𝟒 𝟕 𝑭 𝑱 lexc = 355 nm lem = 543.5 nm Tb3+ emission lifetime decreases when Yb3+ is added Tb3+ emission lifetime decreases more and more with RE content

Optical characterization

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

Rate equations: direct transfer vs virtual level transfer NaYF4:Tb3+,Yb3+ 50SiO2-20Al2O3-27CaF2:Tb3+,Yb3+ Duan et al., J.Appl.Phys., 2011,110, 113503; Duan et al., Opt. Lett., 2012,37, 521-523

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

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

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

Sample preparation SiO2-ZrO2 glass and glass-ceramics Thermal treatments TEOS + EtOH + H2O + HCl Si 3 min. @700 °C after each dip 1h stirring Zr propoxide + Acac + EtOH 15 min. @700 °C after 10 dips Zr GLASS (G) Na Na-Ac + MeOH 1h @ 1000 °C in order to nucleate ZrO2 nanocrystals RE Tb(NO3)3 Yb(NO3)3 GLASS-CERAMICS (GC) 16h stirring DIP COATING

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

Composition: 70% SiO2 – 30% ZrO2 + additional 10% Na Samples properties SiO2-ZrO2 glass and glass-ceramics Composition: 70% SiO2 – 30% ZrO2 + 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

XRD preliminary check SPIN-COATING deposited samples 4 layers @ 1000 rpm - 1 min.

RBS measurements (Rutherford Backscattering Spectrometry)

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

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

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

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

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

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

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

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

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

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

CNR-Italy / CNRST-Morocco bilateral project 2014-15 MAECI bilateral project PLESC - Centro Fermi Italy / University of Witwatersrand South Africa 2015-18 CNR-Italy / CNRST-Morocco bilateral project 2014-15 VINNMER Marie Curie Incoming Project - Nano2solar 2016-18 37

CNR-Italy / CNRST-Morocco bilateral project 2014-15 Thank you! MAECI bilateral project PLESC - Centro Fermi Italy / University of Witwatersrand South Africa 2015-18 CNR-Italy / CNRST-Morocco bilateral project 2014-15 VINNMER Marie Curie Incoming Project - Nano2solar 2016-18 38

R 5% 7% 9%

Ag doping

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: 985+1180 nm) Er3+/Yb3+ Violet‐Blue‐Green → NIR (2 emitted photons: 1000 + 1500 nm)

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

Optical characterization 𝟓 𝑫 𝟒 𝟕 𝑭 𝑱 lexc = 355 nm lem = 543.5 nm G GC

Optical characterization 𝟓 𝑫 𝟒 𝟕 𝑭 𝑱 lexc = 355 nm lem = 543.5 nm 108 % 110 % 126 % G 125 % 132 % 154 % GC

Optical characterization 𝟓 𝑫 𝟒 𝟕 𝑭 𝑱 lexc = 355 nm lem = 543.5 nm G GC

Optical characterization 𝟓 𝑫 𝟒 𝟕 𝑭 𝑱 lexc = 355 nm lem = 543.5 nm 129 % 138 % 144 % G 145 % 171 % 179 % GC