Yb3+ co-activated Silica-Hafnia glass ceramic waveguides

Optical Materials 33 (2010) 227–230

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Tb3+/Yb3+ co-activated Silica-Hafnia glass ceramic waveguides G. Alombert-Goget a,⇑, C. Armellini a,b, S. Berneschi c,d, A. Chiappini a, A. Chiasera a, M. Ferrari a, S. Guddala a,e,f, E. Moser e, S. Pelli c, D.N. Rao f, G.C. Righini c a

CNR-IFN, CSMFO Lab., via Alla Cascata 56/c Povo, 38123 Trento, Italy Fondazione Bruno Kessler FBK, via Sommarive 18 Povo, 38123 Trento, Italy c CNR-IFAC, MDF Lab, via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy d Centro Studi e Ricerche ‘‘Enrico Fermi”, Piazza del Viminale 1, 001845 Roma, Italy e Dipartimento di Fisica, Università di Trento, via Sommarive 14 Povo, 38123 Trento, Italy f School of Physics, University of Hyderabad, Hyderabad 500046, India b

a r t i c l e

i n f o

Article history: Received 1 June 2010 Received in revised form 31 August 2010 Accepted 30 September 2010 Available online 25 October 2010 Keywords: Quantum cutting Down-conversion Rare earth Glass ceramic Energy transfer

a b s t r a c t 70SiO2-30HfO2 glass ceramic planar waveguides co-activated by Tb3+/Yb3+ ions were fabricated by sol gel route using a top-down approach. The energy transfer from Tb3+ to Yb3+ ions was investigated as a function of the Tb3+/Yb3+ molar ratio as well as of the total amount of rare earth ions. In particular, the conversion of absorbed photons at 476 nm into photons at 980 nm is considered, looking for potential application in photovoltaic systems. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction A main limitation of a photovoltaic device is related to the mismatch between the incident solar spectrum and the spectral absorption properties of the material of the cell [1]. Several routes were proposed in the past decades to modify the incident spectrum and improve the conversion efficiency of a semiconductor solar cell [1,2]. Among these, the down-conversion, also called quantum cutting, permits to generate more than one low energy photon exploiting the energy of one incident high energy photon [1]. The most used solar cells are based on crystalline Si (c–Si). In this context, the down-conversion would be beneficial if a photon with a wavelength shorter than approximately 500 nm (around twice the energy of the silicon band gap) could be converted in two photons with wavelengths around 1000 nm (with a energy just above the band gap of c–Si) [2]. Rare-earth ions, where absorption and emission take place via a number of energy levels, allow down-conversion processes [3,4]. Several publications report the possibility to obtain a down-conversion process using terbium and ytterbium ions [5,6]. The down-conversion process using cooperative energy transfer between a Tb3+ ion and two Yb3+ ions permits to cut high energy pho⇑ Corresponding author. E-mail address: [email protected] (G. Alombert-Goget). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.09.030

ton at wavelength shorter than 488 nm into two low energy photons around 980 nm. The cooperative energy transfer between a Tb3+ ion and two Yb3+ ions can be the main relaxation route to achieve the NIR luminescence of the Yb3+. The choice of the matrix is a crucial point to obtain an efficient down-conversion process. The materials need to be chosen to minimize non-radiative transition process from the rare-earth ions to the host matrix [2]. Recently, some studies have demonstrated that the transparent glass ceramics may be a valid system to support an effective quantum cutting process [7,8]. Sol gel-derived silica-hafnia is a reliable and flexible system that has proved to be suitable for rare earth doping and fabrication of glass ceramic planar waveguides with excellent optical and spectroscopic properties for photonic applications [9]. In particular, previous works on silica-hafnia glass ceramic using X-ray absorption fine structure (EXAFS) and X-ray photoelectron spectroscopy (XPS) confirmed that Er3+ ions remain incorporated in HfO2 nanocrystals due to substitution of Hf4+ by Er3+ in the crystalline lattice [10,11]. An important consequence of this structural arrangement is that the rare-earth ions undergo a low cut off vibrational energy (700 cm1) so that the non-radiative contribution to the relaxation mechanism is reduced compared to that of the parent glass where the high phonon energy of 1100 cm1 due to the silica are effective [9,12]. For these reasons the silica-hafnia glass ceramic seems a suitable matrix to produce rare-earth activated

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films suitable for down-conversion process. Different parameters, however, remain to be determined before synthesizing a layer potentially useable in a photovoltaic system. The aim of this work is to assess the energy transfer efficiency between the Tb3+ and Yb3+ ions in sol–gel derived SiO2–HfO2 glass ceramic planar waveguides. We therefore produced two samples series of various rare earth contents. In a first step, the synthesis process permits to obtain an amorphous glass planar waveguide, while subsequent thermal treatments allow to fabricate a glass ceramic waveguide. We will shortly mention the results obtained for the glass to compare the energy transfer efficiencies in the two systems, but we will focus the discussion on the results obtained on the glass ceramics samples.

2. Synthesis protocol Two series of 70SiO2 – 30 HfO2 samples activated by different molar concentrations of terbium and ytterbium ions were prepared by sol–gel route using the dip-coating technique with the experimental procedure reported in [13]. Terbium and ytterbium were added as Tb(NO3)35H2O and Yb(NO3)35H2O. Final films, 1 lm in thickness, were obtained after 30 coatings. The films were stabilized by a treatment for 5 min at 900 °C in air to achieve a full densification of the material. As a result of the procedure, transparent and crack-free amorphous glass films were obtained. An additional heat treatment was performed in air at a temperature of 1,000 °C for 30 min in order to nucleate nanocrystals inside the film. 70SiO2-30HfO2 glass ceramic planar waveguides doped with rare-earth ions were thus produced. Table 1 gives the compositional and optical parameters of the obtained silica-hafnia glass ceramic planar waveguides. The refractive index of the different samples at 543.5 and 632.8 nm were measured by an m-line apparatus [14]. Photoluminescence spectroscopy was performed using the 476 nm line of an Ar+ ion laser as excitation source. The luminescence spectrum in the region of the transition 2F5/2 ? 2F7/2 of Yb3+ ion was analyzed by a single grating monochromator with a resolution of 2 nm and detected using a Si/InGaAs two-color photodiode and standard lock-in technique. Luminescence decay measurements of the 5D4 state of Tb3+ ion were performed after excitation with the third harmonic of a pulsed Nd-YAG laser. The visible emission was collected by a double monochromator with a resolution of 5 cm1 and the signal was analyzed by a photon-counting system. Decay curves were obtained recording the signal by a multichannel analyzer Stanford SR430. X-ray diffraction measurements (XRD) were performed using a X-Pert pro equipment with the Cu Ka radiation at 1.5426 Å. The diffracted intensity was measured in the 2h range between 10°

and 100°, with a step of 0.05°. The phases of the nanocrystals were determined with the help of software which identifies the crystal phase by fitting to the diffraction peaks.

4. Results and discussion Fig. 1 shows the XRD pattern obtained for the B1 glass ceramic waveguide. The broad hump around 2h = 22° is due to the diffraction from the amorphous silica substrate. The fitting procedure of the spectrum indicates that the dominant structure is the tetragonal phase HfO2. The main peaks are indicated in Fig. 1. The size of the nanocrystals in the amorphous matrix was estimated about 5 ± 2 nm by the Scherrer formula. This value is compatible with the nanocrystals of about 4–6 nm in size, homogeneously dispersed in the amorphous matrix, already observed by HRTEM in the samples fabricated with the same protocol [9,11]. The first series of samples, labeled as ‘‘A”, allowed us to perform a systematic study of the effect of the ytterbium amount in the material on the performance of the system. In particular, it enabled us to determine the best molar ratio between the two rare-earths used. Four glass ceramic samples of composition 70SiO2–30HfO2 doped with 0.5 mol% of terbium and with different concentration of ytterbium have been prepared: AR1 with 0 mol% (reference sample without ytterbium), A1 with 1 mol%, A2 with 2 mol%, A3 with 3 mol%. The photoluminescence spectra of the Tb3+–Yb3+co-doped A series under the 476 nm excitation are shown in Fig. 2. The intense emission band centered at 977 nm, with a shoulder at 1027 nm, is attributed to the 2F5/2 ? 2F7/2 transition of Yb3+ ions. The emission of the Yb3+ ion after excitation in the blue region is an indication of the presence of an efficient energy transfer from Tb3+ to Yb3+ and so of an effective down-conversion. However, it is not possible to evaluate the conversion efficiency only on the base of the photoluminescence spectra. Assessment of the conversion efficiency is obtained from the estimation of the energy transfer efficiency between terbium and ytterbium. The evaluation of the energy transfer efficiency between Tb3+ and Yb3+ can be obtained by comparing the luminescence decay of terbium with and without ytterbium co-doping ions. In the inset of Fig. 2, the decay curves of the 5D4 ? 7F5 emission of Tb3+ at 543.5 nm are plotted for the different samples of the A series. Nearly single exponential luminescence decay is observed for the sample without Yb3+ (AR1). The fast luminescence decay observed for the co-doped samples is attributed to the energy transfer from the Tb3+: 5D4 to the Yb3+: 2F5/2 [6]. The not exponential behavior of the decay can be explained by different distributions of Yb3+ ions around the Tb3+ ions, which lead to different energy transfer rates for the different Tb3+ ions [15]. The energy transfer efficiency gTb–Yb can be obtained experimentally by dividing the integrated intensity of the decay curves

Table 1 Rare earth concentration, refractive index and layer thickness of the prepared silica-hafnia glass ceramic planar waveguides. Sample label

Terbium Concentration in mol%

Ytterbium Concentration in mol%

[email protected] nm TE polarization [±0.001]

[email protected] nm TE polarization [±0.001]

Layer thickness [±0.2lm]

AR1 Al A2 A3 BR1 Bl BR3 B3 BR5 B5

0.5 0.5 0.5 0.5 0.2 0.2 0.6 0.6 1 1

0 1 2 3 0 0.8 0 2.4 0 4

1.621 1.626 1.631 1.633 1.623 1.604 1.624 1.630 1.626 1.638

1.616 1.621 1.626 1.628 1.617 1.608 1.620 1.625 1.621 1.633

1.0 1.0 1.0 1.1 1.1 0.9 1.1 1.2 1.2 1.1

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Fig. 1. X-ray diffraction pattern for the B1 glass ceramic planar waveguide. The peaks assigned to tetragonal HfO2 crystals are evidenced.

Fig. 2. Room temperature photoluminescence spectra of the 2F5/2 ? 2F7/2 transition of Yb3+ ions after excitation at 476 nm for the three samples of the first series: (a) A1; (b) A2; (c) A3. Each spectrum was normalized to the maximum of the luminescence intensity. The inset shows the decay curves of the luminescence from the 5D4 metastable state of Tb3+ ion.

of the Tb3+–Yb3+ co-doped glass ceramics by the integrated intensity of the Tb3+ single doped curve [6]:

R

dt I gTbYb ¼ 1  RTbYb ITb dt

ð1Þ

In some papers, an effective quantum efficiency is employed. The relation between the transfer efficiency and the effective quantum efficiency is linear [6] and is defined as:

gEQE ¼ gTbr ð1  gTbYb Þ þ 2gTbYb

ð2Þ

where the quantum efficiency for Tb3+ ions, gTbr, is set equal to 1. The evaluated values of energy transfer efficiency and effective quantum efficiency for the A series samples are reported in Table 2. The results on these samples indicate an increase of the transfer efficiency with the increase of the ytterbium/terbium molar ratio. For a given concentration of donors (Tb3+), increasing the number of acceptors (Yb3+) located near to the Tb3+ ion, also the Tb–Yb transfer probability increases. However, when increasing the density of active ions, detrimental effects, due to cross-relaxation mechanism, become important too. This seems to be the case for

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Table 2 Estimated transfer efficiency and estimated effective quantum efficiency as a function of Yb3+ molar concentration for A glass–ceramic waveguide, where Tb3+ content is fixed at 0.5 mol%. The results obtain for the parent glass matrix are also reported. Composition (Yb concentration in mol%) Estimated transfer efficiency (in glass ceramic) Estimated effective quantum efficiency (in glass ceramic) Estimated transfer efficiency (in glass) Estimated effective quantum efficiency (in glass)

1% 14% 114%

2% 24% 124%

3% 25% 125%

 the Tb–Yb energy transfer efficiency increases with the increase of the molar ratio Yb/Tb;  the energy transfer efficiency does not exceed 24–25%.

2% 102%

4% 104%

6% 106%

For compositions with constant molar ratio Yb/Tb = 4 and different total amount of rare-earth ions, we observed that:

Table 3 Estimated transfer efficiency and estimated effective quantum efficiency as function of (Yb3+ + Tb3+) molar concentration for B glass ceramic waveguide, with constant molar ratio Yb/Tb = 4. Composition (Tb+Yb concentration in mol %) Estimated transfer efficiency Estimated effective quantum efficiency

1% 1% 101%

For compositions with Tb3+ content kept constant at 0.5 mol% and increasing Yb3+ molar concentration, we observed that:

3% 18% 118%

5% 38% 138%

 the Tb–Yb energy transfer efficiency increases with the increase of the total Yb + Tb concentration. The highest transfer efficiency, equal to 38%, has been achieved so far in 70 SiO2 – 30 HfO2 glass ceramic films activated by 1% of terbium and 4% of ytterbium. We plan to test samples with higher rare-earth concentration in order to optimize the energy transfer efficiency. Acknowledgements

the A samples, where the energy transfer efficiency does not exceed 24–25% (Table 2). The results obtained for the precursor glasses are also reported in Table 2. The values obtained for the glassy system are very low in comparison to those of the glass ceramics. As expected, [2] the estimated effective quantum efficiency strongly depends on the matrix. The reduction of non-radiative relaxation channels due to the low cut off frequency of HfO2 nanocrystals plays a capital role in the energy transfer efficiency between Tb3+ and Yb3+ions. On the basis of these observations we synthesized a second series of samples, labeled ‘‘B”, having constant molar ratio Yb/Tb = 4. The B samples are 70 SiO2 – 30 HfO2 thin films doped with a total amount (Yb3+ + Tb3+) equal to 1 mol% (sample B1), 3 mol% (sample B3) or 5 mol% (sample B5). For each composition a reference sample without Yb3+ ions was prepared; these samples are labeled BR1, BR3, and BR5, respectively. The photoluminescence spectra of the B samples, like the A samples, present the emission of the 2F5/2 ? 2F7/2 transition of the Yb3+ ion upon excitation at 476 nm [14], indicating that Tb– Yb energy transfer is effective. In order to evaluate the energy transfer efficiency between terbium and ytterbium ions, the Tb3+ 5 D4 decay curves of the co-doped samples were compared with those of the reference samples. The evaluated values of energy transfer efficiency and effective quantum efficiency for the B series samples are reported in Table 3. The Tb–Yb energy transfer efficiency increases with the increase of the total amount of rare-earth ions. The sample B5, with the highest concentration of rare earths (5%), presents an energy transfer efficiency of 38%. The fabrication of samples with higher rare earth content is in progress. However, the role of the rare-earth ions in the formation of the glass matrix cannot be ignored for higher contents. The fabrication protocols will be revised in order to preserve the good optical quality of the samples. 5. Conclusion In summary, efficient Tb3+: Yb3+ co-doped 70 SiO2 – 30 HfO2 glass ceramic down-conversion layers have been developed. In order to optimize the rare-earth ions content, two series of samples were synthesized. The transfer efficiency was evaluated for each sample by decay curves analysis. It is demonstrated that glass ceramic increases the effective quantum efficiency compared to the parent glass.

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