Visible to NIR downconversion process in Tb3+-Yb3+ codoped silica-hafnia glass and glass-ceramic sol-gel waveguides for solar cells

Visible to NIR downconversion process in Tb3+-Yb3+ codoped silica-hafnia glass and glass-ceramic sol-gel waveguides for solar cells

Journal of Luminescence 193 (2018) 44–50 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate...

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Journal of Luminescence 193 (2018) 44–50

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Visible to NIR downconversion process in Tb3+-Yb3+ codoped silica-hafnia glass and glass-ceramic sol-gel waveguides for solar cells

MARK



F. Enrichia,b, , C. Armellinic, S. Belmokhtard, A. Bouajajd, A. Chiappinic, M. Ferraria,c, A. Quandta,e, G.C. Righinia,f, A. Vomierob, L. Zura,c a

Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi, Piazza del Viminale 1, 00184 Roma, Italy Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, 971 87 Luleå, Sweden c CSMFO Lab., Istituto di Fotonica e Nanotecnologie CNR, Via alla Cascata 56/C, 38123 Povo-Trento, Italy d Laboratoire des Technologies Innovantes, LTI, Université Abdelmalek Essâadi, Tanger, Morocco e School of Physics, CoE-SM and MERG, University of the Witwatersrand, Johannesburg, South Africa f MipLAB, Nello Carrara Institute of Applied Physics, CNR-IFAC, Sesto Fiorentino 50019, Italy b

A R T I C L E I N F O

A B S T R A C T

Keywords: Rare earths Glass Glass-ceramics Down-conversion Energy transfer Sol-gel films

The efficiency of photovoltaic solar cells is strongly related to the spectral absorption and photo-conversion properties of the cell's active material, which does not exploit the whole broadband solar spectrum. This mismatch between the spectrum of the solar light and the wavelength dependent cell's response can be partially overcome by using luminescent conversion layers in front or in the back of the solar cell. In this paper, the investigation of Tb3+-Yb3+ co-doped SiO2-HfO2 glass and glass-ceramic waveguides is presented. Due to a down-conversion process based on cooperative energy transfer between one Tb3+ ion and two Yb3+ ions, a blue photon at 488 nm can be divided in two NIR photons at 980 nm. Films with different molar concentrations of rare earths, up to a total amount of [Tb + Yb] = 15%, were prepared by a sol-gel route, using dip-coating deposition on SiO2 substrates. For all the films, the molar ratio [Yb]/[Tb] was taken equal to 4. The comparison of the energy-transfer efficiency between Tb3+ and Yb3+ ions in the glass and in the glass-ceramic materials demonstrated the higher performance of the glass-ceramic, with a maximum quantum transfer efficiency of 179% for the highest rare earth doping concentration. Moreover, experimental results and comparison with proper rate equations modelling showed a linear dependence of the photoluminescence emission intensity for the Yb3+ ions 2F5/2 → 2F7/2 transition at 980 nm on the excitation power, indicating a direct transfer process from Tb3+ to Yb3+ ions. The reported waveguides could find applications not only as downconverting filters in transmission but also as efficient solar concentrators in the near-IR spectral region.

1. Introduction Nowadays, photovoltaics (PV) has become a solid technology, which is still in further progression and gaining larger portions of the electricity market. While the total renewable power capacity (in GW) increased by 18% from 2014 to 2015, the solar PV capacity increased by more than 28% [1]. The evolution of PV technology is driven by the constant reduction of the cost per watt of the solar modules, and this can be achieved by increasing the conversion efficiency per unit area at affordable costs, with the theoretical possibility to approach the thermodynamic limit up to 93% [2]. One of the major limitations to the efficiency of photovoltaic solar cells is the spectral mismatch between the limited absorption and photo-conversion properties of the cell's active material and the



broadband spectrum of the incoming solar radiation. Indeed, the absorption of radiation and its electrical conversion in PV solar cells is spectrally controlled by the bandgap of the semiconductor material. This corresponds, for instance, to 1127 nm for c-Si (Eg = 1.1 eV) and to 690 nm for amorphous silicon (a-Si, Eg = 1.8 eV). Radiation with energy lower than the bandgap is not able to generate electron-hole pairs and it is lost. Radiation with energy much higher than the gap is also not efficient because most of the energy is lost in thermalization processes [3]. These two phenomena are the main sources of losses in PV solar cells, accounting respectively for a loss in power of 24% by the non-absorption of low energy photons and 32% by thermalization [4], which also increases the temperature of the cell, further reducing its efficiency [5]. Therefore, improving the match between the solar spectrum and the band gap of the semiconductor would result in a

Corresponding author at: Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi, Piazza del Viminale 1, 00184 Roma, Italy. E-mail address: [email protected] (F. Enrichi).

http://dx.doi.org/10.1016/j.jlumin.2017.08.027 Received 29 April 2017; Received in revised form 18 July 2017; Accepted 14 August 2017 Available online 17 August 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

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and GC materials) at increasing values of rare earths' molar concentrations up to [Tb+Yb]/[Si+Hf] = 15%, still keeping the optimal rate [Yb]/[Tb] = 4. We also aim at obtaining a deeper understanding of the mechanisms involved in the process, in particular for the GC samples, which exhibit a higher performance.

significant increase of the cell's efficiency [2]. This could be achieved, for example, by using a series of different bandgap semiconductors (as in multi-junction solar cells), each able to absorb a different part of the solar spectrum, but increasing significantly the technology complexity and finally the overall cost of the PV module. A different approach consists in using luminescent materials for downconverting and upconverting solar radiation [6–9], to transfer as much of the solar spectrum close to the energy gap of the semiconductor, where absorption and efficiency of the cell are maximum. Indeed, theoretical calculations indicate that it could be possible to increase the cell's efficiency to about 80% by concentrating the 300–1500 nm solar spectrum into a single narrow emission at the semiconductor bandgap [10]. Thus, the present work is focused on the development of rare earth doped waveguides for spectral downconversion. A detailed review of materials and methods for obtaining optical downconversion in solar cells applications was recently reported by de la Mora and co-workers [11]. In particular, rare earth ions are good candidates for spectral modification, thanks to their wide variety of electronic levels [12,13]. Examples of downconverting lanthanides materials are Pr3+-Yb3+ codoped KY3F crystals [14], Nd:SrTiO thin films [15], LiGdF4:Eu3+ and LiGdF4:Er3+, Tb3+ [16], Tb3+-Eu3+ codoped CaF2 nanocrystals [17]. Among rare earths, the combination of Tb3+ and Yb3+ ions can allow the occurrence of a quantum cutting process. Tb3+-Yb3+ couple has been used in (Y,Yb) PO4:Tb3+ [18], GdAl3(BO3)4 [19], GdBO3 [20], Y2O3 [21], CaF2 nanocrystals [22], lanthanum borogermanate glasses [23], nanostructured glass-ceramic materials [24–31]. In Tb3+-Yb3+ codoped systems, the relaxation 2F5/2 → 2F7/2 between the excited state and the fundamental level of Yb3+ ions produces a near infrared (NIR) photon at 980 nm, which is close to the edge of the silicon band gap. Tb3+ is used as sensitizer, with absorption in the blue at 488 nm through the 7F6 → 5D4 energy levels and cooperative transfer to Yb3+ ions. The result of this process is the conversion of one high-energy blue photon at 488 nm in two lower-energy NIR photons at 980 nm. The choice of the matrix is also of paramount importance. Glass-ceramic materials can combine the advantages of the glass with the better spectroscopic properties of the crystals [24–26]. Based on previous experiments [27–31], the binary oxide composition 70% SiO2 – 30% HfO2 was chosen. Sol-gel derived silica-hafnia is a reliable and flexible system suitable for rare earth doping and fabrication of planar waveguides. It was demonstrated by XRD and EXAFS studies that in silica-hafnia glass ceramics the rare-earth ions are embedded in the ceramic HfO2 nanocrystals [27], which have a high refractive index, excellent transparency and low phonon cutoff frequency of about 700 cm−1. The presence of hafnia nanocrystals, therefore, provides a strong reduction of the non-radiative transition processes, making the silica-hafnia glass-ceramic a suitable matrix to produce rare-earth activated films for downconversion. As an additional feature, glass-ceramic waveguides allow to obtain high radiation confinement, opening new possibilities for integrating downconversion processes and light concentration. Previous studies on 70% SiO2 – 30% HfO2 downconverting waveguides [28–30] codoped with Tb3+ and Yb3+ ions showed that, for a given concentration of donors (Tb3+), the increase of the number of acceptors (Yb3+) located close to the Tb3+ ions can have detrimental effects, due to concentration quenching. The optimal concentration rate resulted to be [Yb]/[Tb] = 4. On the other hand, the transfer efficiency increases with the total rare earth concentration and it is much higher in glass-ceramics than in glasses [30]. A maximum transfer efficiency value of about 55% was reported for glass-ceramic films activated by 1.8% of terbium and 7.2% of ytterbium ([Yb+Tb] = 9%), compared with only 26% for the glass counterpart with the same composition. It is worth observing that the transition from glass (G) to glass-ceramic (GC) is obtained by only changing the final annealing treatment: at 900 °C for the glass and 1000 °C for the glass-ceramics. In the current paper, we focus on the energy-transfer process between the two rare earth ions in different structural environments (G

2. Energy-transfer considerations Although numerous studies on Tb3+-Yb3+ co-doped materials for downconversion have been reported in the literature, the full understanding of the mechanisms underlying the energy transfer process is still debated, and different solutions have been used to explain different systems [28–38]. Most of the papers deal with the comparison of the lifetime of the 5D4 excited state of Tb3+ ions with and without Yb3+ acceptor ions, to evaluate the transfer efficiency, assuming the absence of non-radiative processes. The energy transfer efficiency η can be obtained experimentally by dividing the integrated intensity of the decay curves of the Tb3+-Yb3+ co-doped systems by the integrated intensity of the Tb3+ single doped curve [28–30]:

ηTb − Yb = 1−

∫ ITb− Yb dt ∫ ITb dt

(1)

The effective quantum efficiency is defined by the ratio between the number of emitted photons and the number of photons absorbed by the material. In our case, a perfect downconversion system would have an effective quantum efficiency value of 200%, corresponding to the emission of two photons for one absorbed. The relation between the transfer efficiency and the effective quantum efficiency is linear and is defined as:

ηEQE = ηTb − r (1−ηTb − Yb) + 2ηTb − Yb ≈ 1 + ηTb − Yb

(2)

3+

ions, ηTb − r , is set equal to 1. where the quantum efficiency for Tb It must be noted that the previous expressions lead to an overestimation of the efficiency value, and some authors are critical on this approach, finding that for some oxyfluoride glasses the absolute measurement of the total efficiency could be much lower than what is estimated through the lifetime analysis [32]. A different approach to investigate the transfer process can be obtained by focusing on the power law dependence of the downconversion emission versus the excitation intensity. According to some researchers, this dependence should be linear [3,7,33,34]; for others, on the contrary, a nonlinear downconversion process would be responsible for the cooperative quantum cutting, and the slope n of the power dependence curve of Yb3+ emission was reported to be 0.5 [35] or between 0.5 and 1 [36,37]. In addition, it is interesting to note that Strek et al. [38] have observed a temperature dependence for the slope, with n = 0.5 at room temperature and n = 1 at lower temperatures (77 K), indicating the non-resonant nature of the quantum cutting energy transfer process. In Fig. 1a comprehensive energy level scheme for the two ions is reported. In a general description, after the absorption of one 488 nm blue photon, the excited Tb3+ ion in the 5D4 energy level can transfer his energy to two Yb3+ ions in the ground state, resulting in the emission of two 980 nm photons. The energy transfer mechanism between these ions can be described either by a direct process or by the involvement of an intermediate virtual state (v). As reported in [37], if one considers only the Tb3+ system, the rate equations can be written as follows:

dN1 = σϕN0 − ATb N1 − W1 N1 N0 + W2 Nv2 dt

(3)

dNv = 2W1 N1 N0 − 2W2 Nv2 dt

(4)

In the previous equations, N0, N1, Nv are the populations of the 45

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the direct population transfer and it is proportional to the pump power, the second comes from the virtual state mediated transfer and it is proportional to the square root of the pump power. 3. Experimental The synthesis protocol for the preparation of 70SiO2–30HfO2 glass and glass-ceramic films was reported in previous papers [28–30]. In general, the samples are obtained by sol-gel technique and by dipcoating deposition. Tb3+ and Yb3+ co-doped samples were obtained by keeping constant the molar ratio [Yb]/[Tb] = 4 and changing the total rare earth molar concentration [Tb + Yb] / [Si + Hf]. Previous studies demonstrated an increase of the transfer efficiency from Tb3+ ions to Yb3+ ions by increasing their total molar concentration from 1% to 9%; in this paper, therefore, we have focused on 9%, 12% and 15% total concentrations, to clarify if there is an upper limit. Reference Tb3+doped samples were also prepared to compare the lifetime of the Tb3+ ions 5D4 → 7F5 transition at 543.5 nm with and without Yb3+ ions. A final annealing at 900 °C or 1000 °C was done to complete the preparation of glass and glass-ceramic samples, respectively. The refractive indices at 543.5 nm and 632.8 nm and the thickness of the waveguides were measured for both transverse electric (TE) and transverse magnetic (TM) polarization by an m-line apparatus (Metricon, model 2010) based on the prism-coupling technique and using two He-Ne lasers operating at these wavelengths. X-ray Diffraction (XRD) spectra were collected in continuous scan mode in the 2θ range 10–100°, with a scanning step of 0.1° and counting time of 60 s. The photoluminescence characterization in the visible was performed by a Horiba JobinYvon Fluorolog-3 spectrofluorometer. The light of a 450 W Xenon lamp excites the sample after passing a doublegrating Czerny–Turner monochromator to select the desired wavelength. The optical emission of the sample is analysed by a single grating monochromator coupled to an R928 Hamamatsu PMT detector. The PL emission in the NIR was obtained by exciting the sample with the 488 nm of an Ar laser and analysing the emission by a single grating monochromator coupled to a Si/InGaAs photodiode and using standard lock-in technique. Luminescence decay measurements of the 5D4 state of Tb3+ ions were performed after excitation with the third harmonic of a pulsed Nd:YAG laser. The visible emission was collected by a double monochromator and the signal was analysed by a photon-counting system. Decay curves were obtained recording the signal by a multichannel analyser Stanford SR430.

Fig. 1. Energy level scheme of Tb3+ and Yb3+ ions. After absorption of a 488 nm blue photon by one Tb3+ ion, the process of energy transfer to two Yb3+ ions can be modelled by a direct transfer mechanism or by the involvement of a virtual intermediate state (v). These two approaches can be described by different rate equations.

Tb3+ ground state, excited state and virtual state, respectively, σ is the absorption cross section of the Tb3+ ground state, ϕ is the pumping photon flux, ATb is spontaneous radiative emission rate of Tb3+, and W1, W2 are the coupling coefficients for the population and depopulation of the virtual state. Looking at the steady state solution of these equations, it is possible to obtain the population of the 5D4 Tb3+ excited level (N1) and of the virtual state (Nv). If we start considering N1, remembering that the total number of Tb3+ ions is fixed (NTb = N0 + N1), the result is:

N1 =

σϕ NTb ∝ ϕ σϕ + ATb

(5)

where the last passage holds because, for the small absorption cross section of rare earth ions and under standard pumping conditions, σ ϕ < < ATb. This also means that N0 ≈ NTb. More details on this approximation and experimental investigations on analogous Er3+ doped glasses are reported in [40]. The calculation for virtual state population gives: 1/2

WN Nv = ⎛ 1 0 N1⎞ ⎝ W2 ⎠ ⎜



∝ ϕ1/2

(6)

where the last passage was obtained remembering that N0 ≈ NTb and N1 ∝ ϕ. Considering the complete interacting system and neglecting other processes like energy back-transfer from Yb3+ to Tb3+ or excited state absorption within Tb3+ ions, the rate equations for the excited states become:

dN1 2 = σϕN0 − ATb N1 − W1 N1 N0 − kD NYb 0 dt

(7)

dNYb1 2 = 2k v Nv NYb0 + 2kD NYb 0 − AYb NYb1 dt

(8)

4. Results and discussion All the waveguides synthesized have refractive index values around 1.6 and thickness between 0.7 and 0.9 µm. Given these values and the wavelengths of the two He-Ne lasers, two guiding modes were obtained for all the waveguides in both the TE and TM polarization. Table 1 reports the optical parameters of the prepared samples. Fig. 2 shows the XRD patterns of the Tb3+/Yb3+-doped 70SiO2–30HfO2 waveguides, treated at 900 °C (glass) and 1000 °C (glass ceramic). All XRD spectra contain contributions from amorphous structures (the reflection centred at 2θ ≈ 21°), originating from the silica substrate and from the SiO2 component of the waveguides (70 mol%). According to XRD, all the waveguides treated at 900 °C are fully amorphous. Crystallization can be detected for all the samples treated at 1000 °C. From the comparison between XRD data and the ICD database, we attribute the waveguides' crystalline phase to the metastable tetragonal hafnium-oxide (t-HfO2). The effect of the crystallization increases with the increase of the rare-earth concentration, a trend which is in agreement with the fact that in silica–hafnia GC, the RE ions are embedded in hafnia nanocrystals, according to previous studies [27].

where kD is the parameter for the cooperative ET from Tb to Yb and kv the energy transfer rate constant for the ET from the virtual state of Tb to Yb. The steady state solution allows to calculate the population of the Yb3+ 2F5/2 excited state, which is responsible for the emission at 980 nm:

NYb1 =

2 2kD NYb 2k v NYb0 0 N1 + Nv ∝ Aϕ + Bϕ1/2 . AYb AYb

(9)

The last passage in Eq. (9) derives from (5) and (6). Assuming the correctness of the model, the 980 nm PL coming from the population of the Yb3+ excited state has two contributions: the first one comes from 46

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Table 1 Optical parameters of the prepared samples. In the labels, G and GC indicate glass and glass ceramics, respectively, and the number is the total rare-earth concentration. R labels correspond to samples containing Tb only, in a concentration which is 1/5 of the total concentration of the series. Sample label

G9R G9 GC9R GC9 G12R G12 GC12R GC12 G15R G15 GC15R GC15

n @543.5 nm

n @632.8 nm

TE

TM

TE

TM

1.602 1.598 1.642 1.610 1.597 1.582 1.627 1.623 1.604 1.605 1.631 1.655

1.590 1.592 1.637 1.583 1.590 1.574 1.620 1.617 1.598 1.596 1.626 1.651

1.590 1.593 1.636 1.598 1.592 1.577 1.622 1.618 1.559 1.599 1.626 1.650

1.576 1.507 1.632 1.583 1.585 1.569 1.615 1.611 1.593 1.590 1.681 1.645

Layer thickness (μm)

0.8 0.8 0.8 0.8 0.8 0.9 0.8 0.8 0.7 0.9 0.7 0.8

Fig. 4. PL emission spectra in the near IR region for the 9% co-doped GC sample under 488 nm excitation.

overlapping bands centred at around 270 nm and 305 nm. The origin of these bands is associated with the charge transfer (CT) from the orbitals 2p of O2- to the 4f of Tb3+, or with the 4f–5d intra-band transitions of Tb3+. Beyond 310 nm, the bands are due to 4f intra-band transitions of Tb3+ ions, corresponding to transitions from the 7F6 ground state to the 5 L7 (317 nm), 5L9 (339 nm), 5L10 (351 nm), 5G6 (369 nm), 5D3 (378 nm), 5D4 (486 nm). The PL emission spectra are reported for the GC9R and GC9 samples, with and without Yb3+ ions respectively. The PL emissions show the characteristic 5D4 → 7FJ transitions (J = 3, 4, 5, 6) of Tb3+, with a maximum peak in the green at about 543.5 nm corresponding to the 5 D4 → 7F5 transition. It is worth observing that the intensity of the spectrum has a significant decrease after Yb3+ co-doping, which is a strong indication in favour of the occurrence of an energy transfer process. This was observed also for the other samples and it is more significant in GC than in G, in agreement with previous studies [30]. Co-doped samples exhibit a significant emission at 980 nm, as reported in Fig. 4, corresponding to the Yb3+ 2F5/2 → 2F7/2 transition. The spectral difference between G and GC samples is more evident in this spectral region and indicates the different environment around the rareearth ions, with a maximum peak at 978 nm (GC) or 981 nm (G). The investigation of the energy transfer dynamics to obtain the effective quantum efficiency can be done by comparing the PL decay curves of the 5D4 state for the Tb3+-Yb3+ co-doped sample with those of the Tb3+ doped reference sample, according to Eqs. (1) and (2). The

Fig. 2. XRD reflections of G and GC samples for different total rare earth concentrations.

Fig. 3 reports the photoluminescence excitation (PLE) and emission spectra (PL) of Tb3+ ions in the GC9R and GC9 samples, containing respectively only Tb (1.8%) or both Tb (1.8%) and Yb (7.2%) rare earths. PLE spectrum, reported for GC9R, was obtained by monitoring the 5D4 → 7F5 transition at λem = 543.5 nm. Let us consider two spectral regions in the PLE spectrum, below and above 310 nm [39]. Below 310 nm, in agreement with other works, we expected two

Fig. 3. Left: PLE for the Tb3+ doped 1.8% GC sample at 543.5 nm (Tb3+ 5D4 → 7F5 transition). This sample is the reference for the 9% series. Right: PL emission spectra in the visible under 377 nm for the 9% GC series, with or without Yb3+ co-doping.

47

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Fig. 5. Decay curves of the luminescence from the 5D4 metastable state of Tb3+ ions upon excitation at 355 nm for glass (left) and glass-ceramic (right) samples. The decay curves for the samples without Yb3+ resulted, within the same group of samples (G or GC), very similar for all the concentrations; in the plots, therefore, only one curve per group (G9R and GC9R, respectively) is shown.

predominant direct transfer mechanism, but it also suggests some limitation in the model prediction, and other non-accounted processes could be occurring. A deeper investigation in the low-pump-power regime, between 4 and 100 mW (not reported), was performed on the GC9 sample, confirming the previous result (slope about 1.14) and should exclude the role of excited state absorption processes, that take place typically under high photon fluxes; further studies, however, are ongoing to clarify this point. As a final consideration, the high efficiency measured for the energy transfer process could make these materials suitable as downconverting filters for the solar applications. To this respect, one of the main limitations of RE3+ ions, their small absorption cross section, could be partially overcome by the high rare earths concentration that was possible to reach. Moreover, the possibility to realize a high-quality glass-ceramic waveguide points out the even more promising alternative of making an efficient solar concentrator in the near-IR spectral region.

Table 2 1/e lifetime values for G and GC samples with constant molar ratio [Yb] / [Tb] = 4. The lifetime values for the samples without Yb3+ resulted, within the same group of samples (G or GC), very similar for all the concentrations, and it is indicated as “reference” in the following table. Sample

Reference 9 12 15

1/e lifetime (ms) Glass

Glass-ceramics

1,80 1,30 0,93 0,73

2,10 1,00 0,40 0,26

curves for both G and GC samples are reported in Fig. 5 and the 1/e lifetime values are reported in Table 2. It was observed that the lifetime value is almost the same in the samples containing only Tb3+ without Yb3+ (R series, Tb concentration is 1.8%, 2.4% and 3%), therefore only one reference lifetime is reported in the table and one decay curve in each graph. Moreover, the lifetime is longer for the GC samples than for G samples, in agreement with the rare-earth confinement in the HfO2 crystal which has less non-radiative transition processes. After incorporation of Yb3+ the lifetime is strongly reduced, and the effect is more evident in the GC samples. The shortening of the luminescence decay observed for the co-doped samples is due to the energy transfer from Tb3+ to Yb3+. The values of effective quantum efficiency are reported in Table 3, calculated according to Eqs. (1) and (2). The energy transfer process is very efficient in our system, reaching 179% for the 15% rare earth concentration GC sample. As discussed before, further investigation of the energy transfer mechanism can be obtained by investigating the power dependence behaviour of the Yb3+ 980 nm NIR emission. This was done for the GC samples, as reported in Fig. 6, as a function of the 488 nm excitation power intensity. The slope of the linear fit is very similar for the three samples, between 1.12 and 1.15. This value, slightly higher than unity, seems to indicate a

5. Conclusions 70% SiO2 – 30% HfO2 glasses and glass-ceramics co-doped with Tb3+-Yb3+ at different molar concentrations of rare earths, namely [Tb + Yb] / [Si + Hf] = 9%, 12%, 15%, were prepared by sol-gel method and dip coating processing. Visible emission from 5D4 → 7FJ transitions (J = 3, 4, 5, 6) of Tb3+ ions at different wavelengths and near-infrared emission from 2F5/2 → 2F7/2 transition of Yb3+ ions at 980 nm were observed. The decrease of Tb3+ excited state lifetime indicates a strong energy-transfer from Tb3+ to Yb3+ ions, which increases to a maximum of 144% in glass samples and 179% in glass-ceramics, both obtained for the highest rare earth doping concentration of 15%. The power dependence of the 980 nm Yb3+ emission in the glassceramic samples follows a power law that is slightly super linear (exponent 1.12–1.14), indicating that the main contribution to the population of Yb3+ excited state in these conditions is the direct energy transfer from Tb3+. Further investigations, however, are ongoing to clarify this point. The reported waveguides could find applications not only as downconverting filters in transmission but also as efficient solar concentrators in the near-IR spectral region.

Table 3 Effective quantum efficiency as function of [Tb + Yb] molar concentration for G and GC samples with constant molar ratio [Yb] / [Tb] = 4. Total concentration (%)

9 12 15

Acknowledgements

Effective quantum efficiency (%) Glass

Glass-ceramics

129 138 144

145 171 179

This work was funded by the Centro Fermi's PLANS project and by the PLESC project (Plasmonics for a better efficiency of solar cells) between South Africa and Italy (contributo del Ministero degli Affari Esteri e della Cooperazione Internazionale, Direzione Generale per la Promozione del Sistema Paese). F.E. acknowledges VINNOVA for 48

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Fig. 6. Power dependence of the Yb3+ NIR emission under 488 nm excitation in the GC samples with 9%, 12% and 15% total rare-earth concentration, respectively. The linear fit line is shown.

support, under the Vinnmer Marie Curie Incoming – Mobility for Growth Programme (project “Nano2solar” Ref. N. 2016-02011). A.Q. thanks the Materials for Energy Research Group (MERG) and the DSTNRF Centre of Excellence in Strong Materials (CoE-SM) at the University of the Witwatersrand for support. A.V. acknowledges Knut & Alice Wallenberg Foundation and Kempe Foundation for financial support.

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