Res Chem Intermedhttps://doi.org/10.1007/s11164-018-3278-3

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Lanthanide complexes embedded in silicone resin as a spectral converter for solar cells

Jae Hyun Han1,2 · Sung‑Hwan Lee1,3 · Byeong‑Kwon Ju2 · Bok‑Ryul Yoo1 · So‑Hye Cho1,4 · Joon Soo Han1

Received: 6 April 2017 / Accepted: 10 May 2017 © Springer Science+Business Media B.V., part of Springer Nature 2018

Abstract The performance of solar cell devices is dependent on various factors, and the spectral mismatch limits the upper limit of performance. Using a spectral converter to manipulate solar radiation is one way to improve solar cell efficiency. We present herein a spectral converter to move the short-wavelength (<  400  nm) range of solar radiation to the longer-wavelength range. The spectral converter com-prises fluorescent lanthanide complexes uniformly embedded in silicone resin. A successful spectral converter showed transmittance of over 85%, and when applied in a Si solar cell, its relative efficiency was increased up to 4%.

Keywords Silicone resin · Lanthanide complex · Wavelength conversion · Solar cells

Introduction

A solar cell is a device which can convert solar energy to electrical energy. Since solar energy is unlimited and pollution-free, it has received great attention as an alternative energy source. However, the current state of the photoelectric

* Joon Soo Han jshan@kist.re.kr

1 Materials Architecturing Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea

2 Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul 02841, Republic of Korea

3 Department of Chemistry, Hannam University, 70 Hannamro, Daedeok-Gu, Daejeon 34430, Republic of Korea

4 Division of Nano & Information Technology, Korea University of Science and Technology, 217 Gajeong-ro Yuseong-gu, Daejeon 34113, Republic of Korea

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conversion efficiency of single-junction solar cells is quite low (≤~ 26%), hamper-ing their usage as a main energy resource. One of the main reasons for such low efficiency is the spectral mismatch between the incident sunlight and solar cell [1]. The energy of solar radiation reaching the Earth is maximum in the blue region (λ = 470 nm), reduces by 25% at λ = 720 nm, and drops below 50% at λ = 1000 nm (Fig.  1). However, in the case of silicon solar cells, the highest sensitivity occurs at λ  =  950–980  nm while the sensitivity drops to  ~  20% of this maximum at λ = 400–470 nm. Naturally, the maximum theoretical efficiency of silicon solar cells is only around 30% [2]. Recently, research has been carried out to improve solar cell efficiency by lowering loss from such spectral mismatch. In particular, use of spec-tral converters to absorb the UV to blue region of sunlight and convert it a region that the solar cell can efficiently absorb has received much attention [3, 4].

In the spectral conversion technique, two factors have to be considered: (1) prep-aration of spectral conversion materials which can absorb sunlight at wavelengths at which the solar cell does not respond and efficiently convert it to wavelengths at which the solar cell can function well, and (2) fabrication of a transparent layer composed of such spectral conversion materials (a wavelength conversion layer) for minimal loss of incident sunlight [1]. Spectral conversion materials can be chosen from among various luminescent materials such as fluorescent organic compounds [5], inorganic phosphors [6, 7], quantum dots [8, 9], lanthanide complexes [10, 11], etc. Among such materials, fluorescent organic compounds and quantum dots have been demonstrated to significantly improve solar cell efficiency; For example, Rich-ards et al. [5] reported a transparent wavelength conversion layer, viz. a composite film composed of Lumogen® dyes and polyethylene vinyl acetate, and its applica-tion to a CdTe module, reporting relative enhancement of its short-circuit current (Jsc) up to ~ 9.7%. This large enhancement was possible due to the high lumines-cent efficiency of the organic dye (close to 100%) and high transparency of the film. Park et al. [8] used a thin film of semiconductor quantum dots (CdSe/ZnS core/shell quantum dots illuminating green color) formed by spin coating as a spectral con-version layer for a Si solar cell, reporting a relative enhancement of the solar cell

Fig. 1 Solar radiation spectrum overlaid with silicon absorption

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Lanthanide complexes embedded in silicone resin as a spectral…

efficiency of ~ 5.5%. Due to the thin thickness of the film (~ 8 nm), the quantum dot layer located on top of the solar cell did not hamper the incidence of sunlight, and the high luminescent efficiency of the quantum dots enabled the enhancement of the solar cell efficiency. Although organic dyes and quantum dots are excellent materi-als for use in spectral conversion layers, they suffer from intrinsic drawbacks that are hard to overcome, viz. low thermal and light stability and small Stokes shift (the difference between the positions of the band maxima of the absorption and emis-sion spectra of the same electronic transition), which have been an obstacle to their industrial application.1

Inorganic phosphors, therefore, have been most widely studied for use in wave-length conversion layers [3]. Inorganic phosphors, especially those containing rare-earth elements, exhibit large Stokes shift, which enables optimal wavelength con-version (e.g., UV or blue light to red or near-infrared light), and are stable over a wide range of temperature and for long times. Although many different approaches have been applied to form wavelength conversion layers from inorganic phosphors (including direct deposition of nanophosphors and fabrication of a polymer-embed-ded phosphor layer), the enhancement in solar cell efficiency by such inorganic phosphors has not been satisfactory (< 2%). The difficulty lies mainly in two factors: (1) low transparency of inorganic phosphor layers, which causes scattering of inci-dent sunlight for a solar cell, and (2) low luminescent efficiency of inorganic phos-phors when their size is controlled under several hundred nanometers. Although inorganic phosphors are used industrially in many different fields (displays, lighting, etc.), their application to solar cells requires much improvement.

Luminescent lanthanide complexes, on the other hand, have not been explored much for use in solar cell applications. These organometallic compounds are typi-cally composed of light-absorbing organic ligands and trivalent lanthanide metal (e.g., Tb3+ and Eu3+), which are responsible for the luminescence [12]. Since the absorption is by the organic ligands while the luminescence is due to the lanthanide metal, their Stokes shift is large (> 100 nm), and due to their organometallic charac-ter, their stability is much higher than that of organic dyes or quantum dots. Among the lanthanide complexes, europium(III) metal-based tris-complexes of diketonates [such as thenoyltrifuoroacetate (tta) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionate (fod)] have drawn much attention due to their prominent charac-teristics of high luminescent efficiency and chemical and light stability [13, 14]. The main application of these complexes so far has been in nuclear magnetic resonance (NMR) spectroscopy as a shift agent [15] due to their facile chelation properties with many organic molecules, while their application as luminescent materials has been limited to energy transfer studies [16].

To fabricate a transparent layer for optical applications, resins such as poly-acrylate, polycarbonate or polyepoxy are generally used [5, 9]. However, these mate-rials comprise pure organic compounds and therefore often exhibit yellowing over long-term usage and have low thermal and light stability and relatively high gas per-meability. Silicone resins, on the other hand, are organic–inorganic hybrid materials

1 https ://en.wikip edia.org/wiki/Stoke s_shift .

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and hence possess high stability and low gas permeability, making them highly suit-able for use in optical applications [17].

We present herein wavelength conversion layers composed of Eu(III)fod3 com-plexes and silicone resin and their application for a Si solar cell. Use of lanthanide complexes enables fabrication of highly luminescent and transparent layers which substantially enhance the efficiency of the solar cell. In addition, the fact that the layer is formed with silicone resin makes our wavelength conversion technique read-ily applicable industrially.

Experimental

Two kinds of silicone resin were used (see Table  1 for materials properties). A methyl silicone resin (MeSiR) was donated by KCC Inc. [product no. SL3875 A&B, refractive index (R.I.) 1.41] for research purposes, while a phenyl silicone resin (PhSiR) was purchased from Dow Corning Inc. (product number no. OE-6630 A&B, R.I. 1.54). The two components of each product (solution  A and B) were mixed prior to heating for hardening. Europium(III)-tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionate) (Eu(III)fod3, Resolve-Al™, Sievers’ reagent), tri-octylphosphine oxide (TOPO), and (tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethox-ysilane were purchased from Sigma-Aldrich Co., chloroform (99.9%) was purchased from Daejung Chemical Inc., and a refractive index liquid (R.I. 1.642) was pur-chased from Cargille Labs. All the above reagents were used as received. A complex of Eu(III)fod3 and TOPO to afford Eu(III)fod3·TOPO2 (denoted EuFOD hereinafter) was prepared as described previously [14] with some modifications. A crystalline silicon solar cell, equipped with a silver electrode and an 80-nm Si3N4 (R.I. = ~ 2.0 at 630 nm) antireflection layer was homemade.

Stainless-steel molds (1 × 3 cm2) with a 1-mm spacer were used to prepare sili-cone films with uniform thickness. Glass slides surface-treated with a hydrophobic compound, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane, were used on the top and bottom of the mold for easy removal of silicone resins. A premix for the methyl silicone resin was prepared by mixing solution A and solution B of SL3875 at 1:1 weight ratio at room temperature. A premix for the phenyl silicone resin was

Table 1 Materials properties of methyl and phenyl silicone resins (supplied by vendors)

Methyl silicone Phenyl silicone

Typical base polymer

Refractive index 1.40–1.42 1.50–1.54Yellowing < 2% at 400 nm after

3000 h at 200 °C4–5% at 400 nm

after 1000 h at 150 °C

Gas permeability (cm3/m2 day atm)

O2 20,000–35,000 O2 300–3000

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Lanthanide complexes embedded in silicone resin as a spectral…

prepared by mixing solution  A and solution  B of OE-6630 at 1:4 weight ratio at room temperature. To the resin mixture (5 g) was added 15 ml EuFOD dissolved in CHCl3 (0.01 g/ml), followed by stirring for 30 min. The solvent was removed and the mixture was degassed under vacuum for 30 min. Then, the mixture was carefully poured into a mold equipped with a glass slide (hydrophobic surface) on the bottom and covered with another glass slide (hydrophobic surface). A single curing process at 150 °C for 2 h was applied to cure silicone resins in all experiments.

Ultraviolet–visible (UV–Vis) transmittance spectra were measured using a Perki-nElmer Lambda 25 UV–Vis spectrometer in the scan range of 200–900 nm at reso-lution of 1.0  nm. A custom-made film holder with an 8-mm-wide hole was used for measuring the transmittance of films, and a blank holder without any film was used for background transmittance. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at room temperature using a PerkinElmer LS50 spectrometer equipped with a xenon discharge lamp (equivalent to 20 kW for 8 µs duration) as excitation source. All samples were measured with a cutoff filter of < 430 nm. The transmittance of films under solar radiation was obtained using a USB-4000 (Ocean Optics) with illumination by a solar simulator calibrated for 1 sun (Newport, 94023A, class AAA with 450-W xenon lamp and 2 × 2 inch2 illu-minated area). Photovoltaic efficiency was measured using the same solar simulator (calibrated for 1  sun) equipped with a Peltier cooling system and source measur-ing unit (Keithley SourceMeter®). The measurement was repeated five times, and an average value obtained after excluding the highest and lowest values.

Results and discussion

The commercially available europium(III) diketonate complex [Eu(III)fod3] was modified with a strong binding ligand (TOPO) following the research result reported by Nathalia et al. [14]: Eu(III)fod3 was mixed with 2 equiv. of TOPO in chloroform at room temperature, and the solvent was removed to provide EuFOD as pink solid (Scheme 1). The PL spectra (with excitation wavelength of 380 nm) of Eu(III)fod3 before and after addition of TOPO are compared in Fig. 2a. Both

Scheme 1 Synthesis of EuFOD

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complexes showed distinctive emission peaks at 580–360 nm due to 5D0 →  7Fj (j = 0–3) transition of Eu(III) [18]. Typically, this emission is not well observed for Eu inorganic complexes such as Eu(NO3)3 since their absorption coefficients are very low [19]. However, when sensitized by diketonate ligands which have high absorption coefficients, Eu(III) strongly emits red light due to ligand-to-metal charge transfer (LMCT) [16]. Furthermore, when additional coordinating ligands such as phosphine oxide (P=O), sulfoxide (S=O), and pyridine (=N–) were added, the Eu emission becomes much stronger [20]. Accordingly, the emis-sion spectrum of EuFOD was higher than that of Eu(III)fod3 by ~16 times under excitation at � = 380 nm . This enhancement mainly originates from replacement of H2O with TOPO, which eliminates nonradiative relaxation of excited elec-trons caused by coordinated H2O. The PLE spectra of Eu(III)fod3 and EuFOD in Fig. 2b show that both complexes exhibited broad absorption spanning from 340 to 450 nm and a small absorption band from 455 to 475 nm. The former absorp-tion is due to the sensitization effect (or antenna effect) of fod ligands, transfer-ring the absorbed light to Eu(III) by LMCT, while the latter corresponds to the 7F2 →  5D0 transition of Eu(III) [18]. It is notable that the main peaks in the PL and PLE spectra were slightly blue-shifted upon addition of TOPO. This shift may be due to enhanced solvation by increased hydrophobicity or an altered elec-tronic transition due to changes in metal–ligand interactions.

When EuFOD dissolved in chloroform was mixed with silicone resins (MeSiR and PhSiR), it formed a homogeneous solution with no precipitation. The two dif-ferent resins were used to test their compatibility with EuFOD and identify the optimal one for the wavelength conversion layer. The mixture was then dried/degassed in vacuum and cast as a film in a mold with a 1-mm spacer prior to cur-ing. Each silicone resin comprised two components (parts A and B) and undergoes curing by hydrosilylation catalyzed by a Pt catalyst upon mixing (Scheme 2). Fig-ure 3a shows images of MeSiR and PhSiR without EuFOD and with 3 wt% EuFOD (MeSiR–EuFOD and PhSiR–EuFOD), while Fig. 3b shows the same under UV light (wavelength 365  nm, power 5  W). It is clearly seen that both MeSiR and PhSiR

Fig. 2 a PL spectra under excitation at 380 nm and b PLE spectra for emission at 615 nm of Eu(III)fod3 and Eu(III)fod3·TOPO (EuFOD) in chloroform

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Lanthanide complexes embedded in silicone resin as a spectral…

formed highly transparent films with the europium complex, and the films lumi-nesced well (with red color) when EuFOD was present.

To compare the two different resins (MeSiR and PhSiR) in terms of their trans-lucency, the transmittance of four films, MeSiR, MeSiR–EuFOD, PhSiR, and PhSiR–EuFOD, was measured and is presented in Fig. 4. It appeared that all four samples showed high transmittance in the visible range, but MeSiR showed higher transmittance than PhSiR, and there was a transmittance decrease of ~ 3–4% when the europium complex was present in each resin. The order of transmittance was MeSiR  >  PhSiR  >  MeSiR–EuFOD  >  PhSiR–EuFOD. In the UV range, MeSiR showed a transmittance drop below 250 nm, while PhSiR showed a sharp drop at 300 nm due to its phenyl ring. This lower transmittance of PhSiR compared with MeSiR has been reported by the resin vendors and appears to be related to their intrinsic properties. Both MeSiR–EuFOD and PhSiR–EuFOD showed a transmit-tance cutoff below 350 nm, which is related to absorption of EuFOD.

The decreased transmittance of the EuFOD-embedded resins throughout the vis-ible range indicates that the solubility of the Eu complex in the resin is not very

Scheme 2 Typical curing mechanism of silicone resins (R = methyl or phenyl)

Fig. 3 Images of (i) MeSiR, (ii) PhSiR, (iii) MeSiR–EuFOD, and (iv) PhSiR–EuFOD under a natural light and b UV lamp (λ = 365 nm, power 5 W)

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high, so that some degree of aggregation occurred, especially in the phenyl silicone resin when 3 wt% EuFOD was embedded. We chose the concentration of EuFOD in the resin to be 3 wt% so that complete absorption of solar radiation below 350 nm would be possible. To test this notion, the change in the solar spectrum (generated by the solar simulator) after being filtered through the EuFOD-embedded films was measured using a modular spectrometer and is shown in Fig. 5 (with the measure-ment setup illustrated in Fig. 5c). The solar spectrum upon addition of the four films is shown in Fig. 5a (methyl resins) and Fig. 5b (phenyl resins) in comparison with a reference solar spectrum obtained from the solar simulator. Note that both the methyl and phenyl resins showed high transmission of visible light above 400 nm, while significant decreases of transmittance were found below 400 nm for both res-ins with EuFOD, reaching complete blocking of UV light below 350 nm. This result implies that the concentration of EuFOD embedded in the 1-mm-thick film was high enough to absorb all incident light below 350 nm. Note that the red emission at ~ 615 nm was not detected during these measurements, even though we could see the red color emitted from the films upon illumination by the solar simulator. It is surmised that light scattering at both faces of the films, due to their high refractive indexes compared with air (R.I. = 1.0), caused wave-guiding of the emitted light to the edges of the film [11], as also observed in the photographic images under UV light in Fig. 3b (it is known that, to extract light to the forward face, a special tech-nique such as microlensing, surface scattering, or R.I. matching is necessary [21]). Comparing the transmittance of the films carefully in the visible range, both methyl and phenyl resins showed a slight decrease compared with the reference spectrum, especially when the Eu complex was present. However, MeSiR–EuFOD showed less decrease than PhSiR–EuFOD, which is in agreement with the transmittance data shown in Fig. 4. The fact that MeSiR–EuFOD showed higher transmittance of vis-ible light while its absorption of UV light was comparable to PhSiR–EuFOD indi-cates that the methyl resin is probably a more suitable matrix for use in wavelength conversion layers for solar cells.

The PL spectra of the EuFOD-embedded films were obtained at excitation wave-length of 325  nm (Fig.  6a). Both the methyl and phenyl resins provided strong

Fig. 4 Transmittance spectra of MeSiR, PhSiR, MeSiR–EuFOD, and PhSiR–EuFOD

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Lanthanide complexes embedded in silicone resin as a spectral…

Fig. 5 Solar spectra filtered through a methyl silicone films (MeSiR and MeSiR–EuFOD) and b phenyl silicone films (PhSiR and PhSiR–EuFOD) with respect to a solar spectrum (blank); c measurement setup

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red emission at 614  nm, which corresponds to the distinctive emission of Eu(III) 5D0 → 7F2 transition [18]. The emission of MeSiR–EuFOD was found to be higher by ~ 25% than that of PhSiR–EuFOD, which is probably due to better separation of emission centers [Eu(III)], since less aggregation was found in the methyl than phe-nyl resin. The PLE spectra showed that both resins with EuFOD exhibited a broad absorption spanning from 250 to 400 nm, while that of MeSiR–EuFOD showed a stronger and broader absorption than that of PhSiR–EuFOD. The absorption range for both resins was significantly blue-shifted (~ 40 nm) compared with EuFOD in chloroform solution (Fig. 3), which is believed to be influenced by the solvatochro-mic properties (the ability of a chemical substance to change color due to a change in solvent polarity2) of EuFOD. This blue-shift might result from the increased polarity when the solvation medium of EuFOD was changed from chloroform to silicone resin.

Next, the photoelectric conversion efficiency of a Si solar cell (homemade, 1 × 1 cm2) was compared with and without the silicone layer (Fig. 7). To reduce the wave-guiding side-effect, refractive index matching was necessary. One or two drops of refractive index matching liquid (R.I. = 1.642) was placed between the front sur-face of the solar cell and the silicone film to afford an increasing R.I. gradient from top to bottom [R.I. = ~ 1.5 (resin) → ~ 1.64 (R.I. matching liquid) → ~ 2.0 (Si solar cell)] (see measurement setup illustrated in Fig. 7c). Figure 7a and b display the I–V curves of a Si solar cell with methyl and phenyl resin layer, respectively. The short-circuit current density (Jsc) and cell efficiency results are summarized in Table 2. Upon addition of the reference methyl resin (MeSiR), the efficiency increased by ~ 10%. This increase is due to reduced reflection of incident light by the Si sur-face due to the presence of the R.I. gradient. When MeSiR–EuFOD was applied, the efficiency further increased by ~ 4% relative to MeSiR. On the other hand, the reference phenyl resin provided an efficiency increase of ~ 6% and PhSiR–EuFOD

Fig. 6 a PL spectra under excitation at 325 nm and b PLE spectra for emission at 615 nm of MeSiR–EuFOD and PhSiR–EuFOD

2 https ://en.wikip edia.org/wiki/Solva tochr omism .

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Lanthanide complexes embedded in silicone resin as a spectral…

enhanced it further by ~ 4% relative to PhSiR. These enhancements agrees with our expectation based on the transmission and photoluminescence data, as the methyl resin showed higher translucency with stronger red emission. A summary of the short-circuit current density (Jsc) and cell efficiency of the Si solar cells with differ-ent layers is presented in Table 2. It is apparent that the enhanced efficiency origi-nated from enhanced current density rather than open-circuit voltage, indicating that wavelength conversion resulted in the enhancement.

Conclusions

We used highly luminescent europium complexes composed of three diketonate ligands (fod) and two nonionic ligands (TOPO). The complex shows UV absorption below ~ 400 nm and emission at 615 nm, which is ideal for wavelength conversion

Fig. 7 I–V curves of devices with a methyl silicone films (MeSiR and MeSiR–EuFOD) and b phenyl silicone films (PhSiR and PhSiR–EuFOD) with respect to reference (blank); c measurement setup

Table 2 Summary of Jsc and cell efficiency results obtained from I–V curves

Jsc (mA/cm2) Cell efficiency (%)

Blank (no film) 3.58 13.37MeSiR 3.97 14.78MeSiR–EuFOD 4.09 15.30PhSiR 3.76 14.00PhSiR–EuFOD 3.96 14.61

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materials for Si solar cells. To form a wavelength conversion layer from the complex, we chose to use silicone resins due to their chemical and light stability and low gas per-meability, which are important for preserving the embedded luminescent materials dur-ing long-term usage. When the methyl and phenyl resins were compared, it was found that the methyl resin formed a more transparent film with higher luminescence upon addition of the europium complex. The wavelength conversion effect by the europium complex was proved by enhanced photoelectric conversion efficiency of a homemade Si solar cell. Between the two resins, the methyl resin showed higher efficiency by 0.6% (absolute value). This study suggests that the combination of europium diketonates and silicone resin results in an efficient wavelength converter. Further work to increase the solar cell efficiency by optimization of the film thickness and EuFOD doping amount, and reduced surface scattering, is in progress.

Acknowledgements We thank the KIST for institutional funding (project no. 2E28020).

References

1. E. Klampaftis, D. Ross, K.R. McIntosh, B.S. Richards, Sol. Energy Mater. Sol. Cells 93, 1182 (2009) 2. T. Trupke, M.A. Green, P. Wurfel, J. Appl. Phys. 92, 1668 (2002) 3. X.Y. Huang, S.Y. Han, W. Huang, X.G. Liu, Chem. Soc. Rev. 42, 173 (2013) 4. B.C. Rowan, L.R. Wilson, B.S. Richards, IEEE J. Sel. Top. Quantum 14, 1312 (2008) 5. D. Ross, D. Alonso-Alvarez, E. Klampaftis, J. Fritsche, M. Bauer, M.G. Debije, R.M. Fifield, B.S. Rich-

ards, IEEE J. Photovolt. 4, 457 (2014) 6. C.K. Huang, Y.C. Chen, W.B. Hung, T.M. Chen, K.W. Sun, W.L. Chang, Prog. Photovolt. 21, 1507 (2013) 7. A. Le Donne, S.K. Jana, S. Banerjee, S. Basu, S. Binetti, J. Appl. Phys. 113, 014903 (2013) 8. S.W. Baek, J.H. Shim, H.M. Seung, G.S. Lee, J.P. Hong, K.S. Lee, J.G. Park, Nanoscale 6, 12524 (2014) 9. S.D. Hodgson, W.S.M. Brooks, A.J. Clayton, G. Kartopu, V. Barrioz, S.J.C. Irvine, Nano Energy 4, 1

(2014) 10. M.M. Nolasco, P.M. Vaz, V.T. Freitas, P.P. Lima, P.S. Andre, R.A.S. Ferreira, P.D. Vaz, P. Ribeiro-

Claro, L.D. Carlos, J. Mater. Chem. A 1, 7339 (2013) 11. J. Graffion, X. Cattoen, M.W.C. Man, V.R. Fernandes, P.S. Andre, R.A.S. Ferreira, L.D. Carlos, Chem.

Mater. 23, 4773 (2011) 12. G. Vicentini, L.B. Zinner, J. Zukerman-Schpector, K. Zinner, Coord. Chem. Rev. 196, 353 (2000) 13. H. Iwanaga, A. Amano, F. Aiga, K. Harada, M. Oguchi, J. Alloys Compd. 408, 921 (2006) 14. N.B.D. Lima, S.M.C. Goncalves, S.A. Junior, A.M. Simas, Sci. Rep. 3, 2395 (2013) 15. H. Iwanaga, A. Amano, M. Oguchi, Jpn. J. Appl. Phys. 1(44), 3702 (2005) 16. E.R. dos Santos, R.O. Freire, N.B. da Costa, F.A.A. Paz, C.A. de Simone, S.A. Junior, A.A.S. Araujo,

L.A.O. Nunes, M.E. de Mesquita, M.O. Rodrigues, J. Phys. Chem. A 114, 7928 (2010) 17. Y. Yang, H.Q. Shi, W.N. Li, H.M. Xiao, Y.S. Luo, S.Y. Fu, T.X. Liu, Compos. Sci. Technol. 71, 1652

(2011) 18. P.P. Pal, J. Manam, J. Rare Earth 31, 37 (2013) 19. P. Hänninen, H. Härmä, SpringerLink (Online service), Lanthanide Luminescence: Photophysical,

Analytical and Biological Aspects, vol. 7, Springer series on fluorescence, methods and applications (Springer, Berlin, 2011) (online resource (1 volume))

20. K. Binnemans, Handbook on the Physics and Chemistry of Rare Earths, vol. 35 (Elsevier, Amsterdam, 2005), p. 107

21. W.H. Koo, S.M. Jeong, F. Araoka, K. Ishikawa, S. Nishimura, T. Toyooka, H. Takezoe, Nat. Pho-tonics 4, 222 (2010)