Down shifting and quantum cutting from Eu³⁺, Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor: a dual mode emitting material

Abstract

We report the quantum cutting (QC) in a Eu³⁺, Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor synthesized through combustion method. The X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements reveal the crystalline nature of the phosphor sample. The photoluminescence (PL) spectra of the different samples have been studied using 266 and 394 nm radiation, which give an intense red emission at 612 nm due to ⁵D₀ → ⁷F₂ transition. The addition of Yb³⁺ in the Eu³⁺ doped sample reduces the emission intensity of Eu³⁺ bands in the visible region continuously whereas in the NIR region, the intensity of the Yb³⁺ band first increases up to 2 mol% and then decreases. This is due to cooperative downconversion energy transfer from Eu³⁺ to Yb³⁺ ions and concentration quenching. The decay curve analyses of different samples reveal an efficient energy transfer from Eu³⁺ to Yb³⁺ ions. The quantum efficiency (QE) has been calculated for different concentrations of Yb³⁺ ions and is estimated to be 197%. The possible mechanisms involved in different transitions and energy transfer processes can be understood using a schematic energy level diagram. The phosphor sample is a potential candidate for enhancing the efficiency of c-Si solar cells.

---

1. Introduction

Rare earth doped phosphor materials have been a subject of focus in recent years due to their broad range of potential applications such as blue light-emitting diodes (LEDs), field emission display devices (FEDs), biosensors, magnetic resonance imaging (MRI), medical diagnostics, solar cells, etc.^1–5 Among these, the solar cell is one of the interesting applications of rare earth ions used for spectral conversion of ultraviolet (UV)/visible (vis) light into near infrared (NIR) light. In a solar cell, electron–hole (e–h) pairs are produced on irradiation with NIR photons having energy higher than the band gap of the solar cell. In the case of UV/vis light the excess energy of the UV/vis photons transferred to the e–h pair is dissipated in the form of heat, which degrades the efficiency of the materials.^6,7 However, the NIR photons obtained after spectral conversion through quantum cutting (QC) contain energy, which is sufficient to generate an e–h pair only. Lot of efforts have been made to improve the solar spectral conversion efficiency but the quantum efficiency is still poor (∼37%). The quantum efficiency determines the optical effectiveness of the material and needs further attention.⁸

In recent years, researches have been focussed on improving efficiency of solar cell by photon conversion process and use the environment friendly energy source to meet demand, which can reduce the utilization of resources of fossil fuels.⁹ It is well known that the theoretical photon conversion efficiency of crystalline Si solar cell is 30% having bandgap of 1.12 eV.^10,11

The research work based on rare earth doped phosphors in recent years is mainly motivated in two broad areas viz. bio-imaging and energy harvesting. In bio-imaging, the upconverted lanthanide ions are used as bio-marker to improve the image quality from different point of view i.e. penetration depth, lower scattering, negligible auto-fluorescence, high signal to noise ratio, etc.^12–15 The second aspect i.e. the energy harvesting in which the photoluminescence (PL), upconversion (UC) and downconversion (DC)/quantum cutting (QC) emission characteristics of lanthanide ions are used. Lanthanides have wide range of absorption of solar light from UV-visible to NIR in better ways. The rare earth doped phosphors emit radiation in NIR region through DC/QC process. The generation of NIR emission is highly useful for solar cell research and are being actively explored worldwide.^16,17 Downshifting (DS) is normal photoluminescence process in which conversion of a high energy absorbed photon (266 or 394 nm) into a low energy photon may be in visible region. Therefore, the conversion efficiency is always less than 100% and the excess energy is lost non-radiatively or by emission of photons with small energy. However, QC is a process in which a high energy absorbed photon is converted into two (or more) low energy photons, which increases the quantum efficiency more than 100%.^7,18 Thus, there is a large demand of QC materials due to their novel application in the field of solar energy.

The QC phenomenon was first observed in singly Pr³⁺ doped fluorides host.^19,20 This approach is further shifted towards two ion systems in which one ion transfers its excitation energy to other through cooperative downconversion process. There are various combinations of rare earth ions such as Tb³⁺–Yb³⁺, Tm³⁺–Yb³⁺, Eu²⁺–Yb³⁺, Nd³⁺–Yb³⁺, etc., co-doped in different fluoride and oxide host materials, which show QC phenomenon to enhance the efficiency of solar cell.^8,10,21–27 The QC mechanism in Eu³⁺–Yb³⁺ combination is reported very little.^28,29 There is no other report seen for the QC mechanisms from Eu³⁺–Yb³⁺ pair in Ca₁₂Al₁₄O₃₃ phosphor, which motivated us to design the experiment and study the QC in this combination. We have used Eu³⁺ and Yb³⁺ rare earth ions combination in which Eu³⁺ acts as donor whereas Yb³⁺ as an acceptor. The lifetime studies confirm that Eu³⁺ ion transfers its energy from ⁵D₂ level to two Yb³⁺ ions in ²F_7/2 level through cooperative energy transfer process to promote them to ²F_5/2 level. Ca₁₂Al₁₄O₃₃ is a host with low phonon frequency (∼800 cm⁻¹) which supports the radiative transitions. This enhances the energy transfer efficiency. Thus, this material can be a potential candidate to improve the efficiency of solar cell.

In this paper, we have synthesized Eu³⁺, Yb³⁺ co-doped single cubic Ca₁₂Al₁₄O₃₃ phosphor. The structural characterizations of the synthesized samples have been carried out using X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements. The photoluminescence spectra of the synthesized sample have been recorded using 266 nm from Nd-YAG laser and 394 nm source from Xe-lamp. The lifetime of ⁵D₀ level of Eu³⁺ in different samples have been measured to verify the energy transfer between Eu³⁺ and Yb³⁺ ions. The possible mechanisms involved in the QC have been discussed using energy level diagram.

2. Experimental

2.1 Preparation of Eu³⁺/Yb³⁺ co-doped calcium aluminate phosphors

The Eu³⁺/Yb³⁺ co-doped calcium aluminate phosphors have been synthesized through solution combustion method. The CaCO₃ (99.0%), Al(NO₃)₃·9H₂O (98%), Eu₂O₃ (99.99%) and Yb₂O₃ (99.9%) were taken as starting materials in stoichiometric ratio. Urea has been used as organic fuel for combustion. A detailed synthesis procedure has been described earlier.^5,25 The concentration of Eu³⁺ was varied from 0.5 mol% to 7 mol% and optimized at 1 mol%. The concentration of Yb³⁺ was varied from 1 mol% to 15 mol% with the optimized concentration of Eu³⁺ ion. These ions have been replaced by Al³⁺ ions in the host.

2.2 Characterization

The identification of phase and crystallinity in the phosphor samples were confirmed using X-ray diffraction pattern (XRD) using Philips Model PW1830, Cu Kα (λ = 1.54056 Å) radiation at 40 kV and 40 mA. XRD pattern of the synthesized sample was recorded in between 15°–80° range at an interval of 3° per minutes. The transmission electron microscope (TEM) and scanning electron microscope (SEM) micrographs of the sample have been monitored to see the morphological behaviour of the sample. The UV-vis-NIR spectra of the samples have been recorded using diffuse reflectance method with Perkin Elmer UV-vis-NIR spectrometer (Lambda 750). The photoluminescence excitation (PLE) and photoluminescence (PL) measurements have been carried out using a Fluorolog-3 spectrofluorometer equipped with 450 W xenon flash lamp and Nd:YAG laser. The lifetime measurements have been carried out using a pulsed xenon lamp (25 W). The downconversion and quantum cutting spectra were recorded using 266/394 nm excitations.

3. Results and discussion

3.1 Structural characterizations

The XRD pattern of 1 mol% Eu³⁺, 2 mol% Yb³⁺ co-doped Ca₁₂Al_l4O₃₃ has been recorded to identify the phase, crystallinity and average crystallite size and is shown in Fig. 1(a). The diffraction patterns match well with pure cubic Ca₁₂Al_l4O₃₃ with lattice constants a = b = c = 11.98 Å and space group I4̄3d (220). The XRD peaks have been assigned using JCPDS file no. 09-0413 (see Fig. 1(b)). This confirms the formation of pure Ca₁₂Al_l4O₃₃ phosphor without any effect of doping concentration. The average crystallite size (D) for the three most intense peaks have been calculated by Debye Scherrer equation:
where, λ is the wavelength of incident X-ray [Cu Kα (1.54056 Å)], β is the FWHM (full width at half maxima) and θ is the diffraction angle. The FWHM of the peaks were taken by their Lorentzian peak fitting. The average crystallite size was found to be 64 ± 1 nm.

Fig. 1 X-ray diffraction (XRD) pattern (a), its JCPDS file no. 09-0413 (b), transmission electron microscopy (TEM) (c) and scanning electron microscopy (d) images of the annealed 1 mol% Eu³⁺, 2 mol% Yb³⁺ co-doped calcium aluminate phosphor sample.

Furthermore, we have recorded the TEM micrograph of the phosphor sample, which shows the nano-particles agglomerated with each other (see Fig. 1(c)). The TEM micrograph further reveals that the particles of Ca₁₂Al_l4O₃₃ co-doped phosphor are spherical in shape with average particle sizes ≈100 ± 5 nm. The spherical shape of the particles is also confirmed by SEM micrograph (see Fig. 1(d)).

3.2 Optical characterizations

3.2.1 UV-visible-NIR diffuse reflection spectroscopy. The UV-vis-NIR diffuse reflectance spectrum of the Eu³⁺/Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor is shown in Fig. 2. The bands of Eu³⁺ are very weak at lower concentration and they could be observed with distinct features only at 7 mol% of Eu³⁺. The absorption band observed in 200–350 nm range is due to the overlap of charge transfer band (CTB) of the host and Eu³⁺–O²⁻ ions. The spectrum contains bands at 363, 382, 397 and 467 nm of Eu³⁺ ions due to ⁷F₀ → ⁵L₈, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂ transitions, respectively and they are shown as inset in figure. The spectrum also contains a peak centred at 978 nm in NIR region, which is attributed due to ²F_7/2 → ²F_5/2 transition of Yb³⁺ ion. This is in good agreement with the excitation spectra of Eu³⁺/Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor.

Fig. 2 The UV-vis-NIR diffuse reflectance spectrum of the 7 mol% Eu³⁺, 2 mol% Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor.

3.2.2 Photoluminescence excitation spectra of Eu³⁺/Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor. The photoluminescence excitation (PLE) spectra of the different samples have been recorded in the range of 220–550 nm at λ_emi = 612 nm and are shown in Fig. 3. The spectra show a broad band in between 230–310 nm due to overlapped charge transfer band (CTB) of Eu³⁺–O²⁻ and Yb³⁺–O²⁻, which is centred at 260 nm.²⁶ The spectra also contain intense peaks at 322, 363, 382, 394, 464 and 535 nm, which are assigned to arise due to the ⁷F₀ → ⁵H₆, ⁷F₀ → ⁵L₈, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₂ and ⁷F₀ → ⁵D₁ transitions of Eu³⁺ ion, respectively. The strongest band is observed at 394 nm due to ⁷F₀ → ⁵L₆ transition.

Fig. 3 Photoluminescence excitation (PLE) spectra of 1 mol% Eu³⁺, xYb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor at different concentration (i.e. x = 0, 1, 2, 4, 6, 10 and 15 mol%) of Yb³⁺ at λ_emi = 612 nm.

3.2.3 Photoluminescence and quantum cutting emission in Eu³⁺/Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor. The photoluminescence spectra of 1 mol% Eu³⁺, 2 mol% Yb³⁺ doped and co-doped Ca₁₂Al₁₄O₃₃ phosphors have been recorded in 550–1030 nm region on excitation with 266 nm laser radiation separately and are shown in Fig. 4. It excites the charge transfer band of Eu³⁺ ion and gives various transitions. The Eu³⁺ doped and Eu³⁺, Yb³⁺ co-doped samples contain several emission peaks at 591, 612, 652 and 700 nm, which are attributed due to ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions of Eu³⁺, respectively, in which ⁵D₀ → ⁷F₂ transition appears with maximum intensity. The Eu³⁺ ions in charge transfer state (CTS) relax through discrete excited states of Eu³⁺ ion non-radiatively to populate ⁵D_n states and ultimately the lowest ⁵D₀ states to give various transitions. On addition of Yb³⁺ in Eu³⁺ the emission intensity of the Eu³⁺ decreases. The Eu³⁺, Yb³⁺ co-doped sample also contains a broad NIR band at 976 nm due to ²F_5/2 → ²F_7/2 transition of Yb³⁺. The CTB of Yb³⁺ is very deep and transfers its excitation energy to ²F_5/2 of Yb³⁺ ion.²⁶ Therefore, the singly Yb³⁺ doped sample on excitation with 266 nm radiation gives also a broad NIR band centred at 982 nm due to ²F_5/2 → ²F_7/2 transition.

Fig. 4 Photoluminescence (PL) spectra of 1 mol% Eu³⁺ doped; 1 mol% Eu³⁺, 2 mol% Yb³⁺ co-doped and 2 mol% Yb³⁺ doped Ca₁₂Al₁₄O₃₃ phosphor on excitation with 266 nm.

As mentioned above when different concentrations of Yb³⁺ are doped with the Eu³⁺ doped sample the emission intensity of Eu³⁺ bands decreases continuously with Yb³⁺ concentrations. This is due to energy transfer from Eu³⁺ to Yb³⁺ ions. As we add Yb³⁺, the emission intensity of the Eu³⁺ peaks initially decreases rapidly upto 2 mol%; and then slowly depending on the energy transfer rate. The PL spectra of phosphor for different concentration of Yb³⁺ ion with a fixed concentration of Eu³⁺ recorded in 550–1030 nm range are shown in Fig. 5. On the other hand, the emission intensity of Yb³⁺ initially increases upto 2 mol% and then decrease for its higher concentration.

Fig. 5 Photoluminescence (PL) spectra of 1 mol% Eu³⁺, xYb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor at different concentration (i.e. x = 0, 1, 2, 4 and 6 mol%) of Yb³⁺ on excitation with 266 nm radiation.

The 266 nm radiation excites the CTB of Eu³⁺ as well as Yb³⁺ ions to give fluorescence. We therefore selected 394 nm radiation, which excites Eu³⁺ ion only. We have recorded the photoluminescence spectra of Eu³⁺, Yb³⁺ doped and co-doped Ca₁₂Al₁₄O₃₃ phosphors at different concentration of Yb³⁺ ion with a fixed concentration of Eu³⁺(at 1 mol%) in 550–1030 nm region on excitation with 394 nm and they are shown in Fig. 6.

Fig. 6 Photoluminescence (PL) spectra of 1 mol% Eu³⁺ doped; 1 mol% Eu³⁺, 2 mol% Yb³⁺ co-doped and 2 mol% Yb³⁺ doped Ca₁₂Al₁₄O₃₃ phosphor on excitation with 394 nm.

It is observed that the emission spectra show similar emission peaks in the visible region with larger emission intensity compared to previous case because Eu³⁺ ion has larger absorption cross section for 394 nm photon. There is no peak seen in NIR region on excitation with 394 nm from the pure Yb³⁺ doped sample because the charge transfer band of Yb³⁺ ion lies far apart from 394 nm excitation. Therefore, the 394 nm wavelength does not excite Yb³⁺ ion. The Yb³⁺ ion is only excited in Yb³⁺:Eu³⁺ sample by cooperative downconversion energy transfer from Eu³⁺ to Yb³⁺ and results a broad band at 976 nm. The emission intensity of Eu³⁺ bands decreases on increasing the concentration of Yb³⁺ ions exactly in the same way as in earlier case and the effect of concentration of Yb³⁺ ions are shown in Fig. 7. The spectrum has no peak in the NIR region in the absence of Yb³⁺ ion. As soon as Yb³⁺ is added in sample, a broad NIR peak appears in 930–1030 nm range centred at 976 nm due to ²F_5/2 → ²F_7/2 transition of Yb³⁺ ion. The emission intensity of Eu³⁺ bands also decreases with Yb³⁺ concentration. The decrease in emission intensity is due to energy transfer from Eu³⁺ to Yb³⁺ ions through QC process. We have also optimized the emission intensity in NIR region with concentration of Yb³⁺ ions and it is maximum at 2 mol%. If the concentration of Yb³⁺ ions is increased further, the emission intensity of Yb³⁺ peak decreases due to concentration quenching.

Fig. 7 Photoluminescence (PL) spectra of 1 mol% Eu³⁺, xYb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor at different concentration (i.e. x = 0, 1, 2, 4 and 6 mol%) of Yb³⁺ on excitation with 394 nm radiation.

As mentioned earlier the intensity of the bands due to Eu³⁺ decreases regularly whereas in the NIR region due to Yb³⁺ initially increases and then decreases on increasing the concentration of Yb³⁺ ions. The variation in the emission intensity strongly depends on the separation between Eu³⁺ and Yb³⁺ ions, which determines the energy transfer efficiency. Thus for a particular concentration of Yb³⁺, the separation between the Eu³⁺ and Yb³⁺ ions are in the range of critical distance. This enhances the energy transfer rate and thereby the emission intensity of Yb³⁺ ions. The emission intensity in NIR region is optimum at 2 mol% concentration of Yb³⁺ ions. When the concentration of Yb³⁺ ions is increased further, the emission intensity decreases due to concentration quenching by Yb³⁺ ion. Actually in presence of Yb³⁺ ions at higher concentration, one Eu³⁺ ion in ⁵D₂ level interacts with two Yb³⁺ ions and transfer its energy to them to promote from ²F_7/2 to ²F_5/2 level. The Yb³⁺ ions when relax radiatively to ²F_7/2, emit photons of 976 nm. The probability of interaction of one Eu³⁺ donor with two Yb³⁺ acceptor ions increases with increasing concentration and leads to QC phenomenon. This enhances the energy transfer and quantum cutting efficiency at higher Yb³⁺ concentration. However, the concentration quenching by Yb³⁺ ions start playing role after 2 mol% concentration of Yb³⁺ and therefore the emission intensity starts decreasing in NIR region.

The mechanism involved in QC can be understood on the basis of schematic energy level diagram shown in Fig. 8. When the sample is excited using 266 nm wavelengths, the ions are promoted to charge transfer states of Eu³⁺ as well as Yb³⁺. The Eu³⁺ ions relax to ⁵L₆ level non-radiatively, which relax further to populate ⁵D₂ excited level. Since the lifetime of this level is relatively large and behaves as a metastable state. The ions in the ⁵D₂ level can relax in two different ways. One, it can relax non-radiatively to populate ⁵D₀ level via ⁵D₁ level. The ions in ⁵D₀ level emit photons of different wavelengths and give transitions to various low lying levels. The other way may be that the ions in ⁵D₂ level transfer their energy to two Yb³⁺ ions simultaneously through cooperative energy transfer process and populates ²F_5/2 level of Yb³⁺ (because the energy of ⁵D₂ level of Eu³⁺ matches well with the energy of two Yb³⁺ ions in ²F_5/2 level). The Yb³⁺ ions when relax to ground state, emit two NIR photons (quantum cutting). A similar result has also been reported by Liu et al.⁷

Fig. 8 Schematic energy level diagram of Eu³⁺/Yb³⁺ co-doped calcium aluminate phosphor, illustrating the probable cooperative energy transfer mechanism of NIR QC from ⁵D₂ level of Eu³⁺ to Yb³⁺ ions in ground state. The solid arrows indicate the excitation and the emission processes and the dotted arrows represent the non-radiative transition and energy-transfer processes.

On the other hand the 266 nm radiation populates the charge transfer state of Yb³⁺ ion also. The charge transfer state of Yb³⁺ is very deep and it crosses the ²F_5/2 level of Yb³⁺ ion. Thus, when the Yb³⁺ ions are excited to its charge transfer state it populates ²F_5/2 level, which gives a band at 982 nm due to ²F_5/2 → ²F_7/2 transition. Thus, the 266 nm excitation gives fluorescence due to Yb³⁺ as well as Eu³⁺.

On the other hand when we excite the sample with 394 nm radiation, it selectively excites the ⁵L₆ level of Eu³⁺ (not CTS of Yb³⁺). Thus, in this case we get bands due to Eu³⁺ ions only. However, if the concentration of Yb³⁺ is increased, the emission due to Yb³⁺ is also seen at 982 nm through cooperative energy transfer process. Since the population in ⁵D₀ level arise due to relaxation from ⁵D₂ level. The population in ⁵D₀ is affected due to the presence of another channel of energy transfer. It will not only affect the intensity of spectral lines arising from ⁵D₀ but also the lifetime of this level.

In order to understand the effect of energy transfer we measured lifetime of the ⁵D₀ state at 612 nm in presence and absence of Yb³⁺ and calculated the corresponding energy transfer and quantum cutting efficiency for different concentrations of Yb³⁺ ion exciting the sample with 266 as well as 394 nm radiations.

3.2.4 Lifetime measurements of Eu³⁺/Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor. We have monitored the fluorescence decay curves of ⁵D₀ → ⁷F₂ transition for Eu³⁺ doped and Eu³⁺, Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphor at different concentration of Yb³⁺ on excitation with 266 and 394 nm and they are shown in Fig. 9. The decay curves were found to fit well with single exponential equation:
where, I₀ and I are the intensities at time 0 and t s, respectively and τ is the lifetime. The lifetimes thus obtained are given in Table 1.

Fig. 9 Decay curves for ⁵D₀ → ⁷F₂ transition of Eu³⁺ ion in Eu³⁺/Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphors for different concentration of Yb³⁺ on excitations with (a) 266 nm and (b) 394 nm.

Table 1 Lifetime, ET efficiency and quantum efficiency of Eu³⁺, Yb³⁺ co-doped Ca₁₂Al₁₄O₃₃ phosphors for different concentration of Yb³⁺ on excitations with 266 and 394 nm

Yb^3+ concentration (mol%)	(λemi = 612 nm)
	(λexc = 266 nm)			(λexc = 394 nm)
	Lifetime (μs)	ηET (in %)	ηQE (in %)	Lifetime (μs)	ηET (in %)	ηQE (in %)
0	1558	0	100	1442	0	100
1	844	45	145	832	42	142
2	610	60	160	527	63	163
6	576	63	163	515	64	164
10	411	73	173	177	87	187
15	85	94	194	42	97	197

---

It is clear from the table that the lifetime of ⁵D₀ level of Eu³⁺ decreases continuously on increasing the concentration of Yb³⁺ ions in the phosphor.^28,29 The QC emission is mainly due to cooperative energy transfer (CET) from ⁵D₂ level of Eu³⁺ to ²F_7/2 level of Yb³⁺. As the concentration of Yb³⁺ ion increases the emission intensity of Eu³⁺ bands decreases and the corresponding lifetime also decreases simultaneously. The decrease in lifetime also depends on population/depopulation rates of ⁵D₂ level. Since the relaxation of ions and energy transfer take place from ⁵D₂ level of Eu³⁺, it is assumed that the rate of relaxation of ions from ⁵D₂ level of Eu³⁺ to ⁵D₀ decrease with the increase of Yb³⁺ ion and thereby the lifetime of ⁵D₀ level also decrease vice versa, a decrease in emission intensity of bands and lifetime of ⁵D₀ level supports the CET from ⁵D₂ level of Eu³⁺ to ²F_7/2 level of Yb³⁺. It is well known that CET process is not efficient at very low or very high concentration due to the intrinsic properties of Yb³⁺ ion. It can be more efficient in between the two. When the concentration of Yb³⁺ is increased from 0 to 2 mol% the corresponding lifetime is decreased from 1558 to 610 μs and from 1442 to 527 μs in the two cases. Therefore, the decay curves show non-exponential behaviour due to involvement of additional decay pathways, which suggests the involvement of CET process (see Table 1). Therefore, the excitation emission and lifetime studies reveal the conversion of one 266/394 photon into two NIR photons.²⁶ The energy transfer efficiency from Eu³⁺ to Yb³⁺ (η_ET) can be calculated by the following equation:

where, τ_x and τ₀ are the fluorescence lifetimes of Eu³⁺/Yb³⁺ co-doped and singly Eu³⁺ doped phosphors, respectively.

The value of η_ET calculated for different concentration of Yb³⁺ ion using above relation is summarized in Table 1.

The table shows that energy transfer (ET) efficiency increases with increasing Yb³⁺ concentration, and its value is found to be 97%, which is maximum for 15 mol% of Yb³⁺. The internal quantum efficiency (η_QE) for the phosphor sample can be calculated by:

η_QE = η_Eu(1 − η_ET) + 2η_ET (1)

The first term in the relation belongs to the visible photons emitted by Eu³⁺ ions and the second term refers to the corresponding NIR photons emitted by Yb³⁺. Here, η_Eu represents the quantum efficiency of emission of Eu³⁺ ions. Assuming that the non-radiative transitions to be zero, η_Eu = 1, the theoretically estimated η_QE for 15 mol% concentration of Yb³⁺ is found to be 197%.

This suggests that the absorption of 100 photons (i.e. 394 nm radiation) produces a total number of 197 photons (NIR and visible both), out of which 194 photons are emitted in 900–1050 nm (i.e. NIR region) and the remaining 3 photons are emitted in visible region for Ca₁₂Al₁₄O₃₃:1% Eu³⁺, 15% Yb³⁺ phosphor. The calculated value for the internal QE increases with increasing concentration of the Yb³⁺ as shown in Table 1.

The UV/visible to NIR QC in phosphor materials are of great interest for c-Si solar cell applications. It is well known that the c-Si solar cells have poor efficiency in UV and visible regions. The maximum spectral conversion efficiency of c-Si solar cell is in 900–1030 nm region.⁶ The NIR QC in the synthesized phosphor can be achieved on excitation with UV/visible photons. The utilization of these NIR photons leads to generate e–h pairs in silicon, which enhances the current in c-Si solar cell, etc. A working model illustrating the quantum cutting process and mechanism to enhance solar cell efficiency is summarized in Fig. 10. If an electron is excited from valence band to conduction band by 266/394 nm, almost half of the excitation energy is lost by thermal vibration/relaxation. However, if the energy of incidence photon is divided to emit two NIR photons by QC process there will not be any energy loss. The two NIR photons (λ = 976 nm, hν = 1.26 eV) emitted by the QC from ⁵D₂ level of Eu³⁺ through cooperative downconversion process populates the ²F_5/2 level of Yb³⁺. As a result two electrons together exceeds the band gap (E_g = 1.12 eV) of single-crystalline Si, and the electric current thus becomes double theoretically. In this way, a highly efficient silicon-based solar cell could be realized through the NIR QC process.

Fig. 10 The working model represents the application of QC phosphor in c-Si solar cell. The NIR photons are emitted through cooperative energy transfer, which matches with the band gap (1.12 eV) of Si. They are absorbed to produce e–h pair, which enhances the flow of solar current.

4. Conclusions

The Eu³⁺/Yb³⁺ co-doped calcium aluminate phosphors have been synthesized through combustion method. The structural studies reveal crystalline nature of the synthesized sample. The phosphor sample gives efficient visible and NIR emissions via downconversion (DC)/quantum cutting (QC) and downshifting (DS) processes on excitation with 266 and 394 nm wavelengths. The presence Yb³⁺ in the Eu³⁺ doped samples decreases the emission intensity of Eu³⁺ in visible region continuously. However in the NIR region, the emission intensity increases initially upto 2 mol% concentration of Yb³⁺ ions and then starts decreasing due to concentration quenching. An efficient cooperative downcoversion energy transfer has been observed from Eu³⁺ to Yb³⁺ ions, which leads to quantum cutting phenomenon. The energy transfer efficiency has been calculated for different concentration of Yb³⁺ ions and is found to be maximum at 15 mol% concentration of Yb³⁺ ions as 97% whereas the corresponding QC efficiency is achieved maximum as 197%. Thus, the Eu³⁺/Yb³⁺ co-doped calcium aluminate phosphor is a promising candidate for energy conversion in c-Si solar cell applications.

Acknowledgements

The authors are thankful to Department of Science and Technology, New Delhi, India (Grant no. SR/S2/LOP-023/2012) for providing financial assistance.

References

1. K. Korthout, P. F. Smet and D. Poelman, Appl. Phys. Lett., 2011, 98, 261919–261921.
2. S. E. Brinkley, N. Pfaff, K. A. Denault, Z. Zhang, H. T. B. Hintzen, R. Seshadri, S. Nakamura and S. P. DenBaars, Appl. Phys. Lett., 2011, 99, 241106–241108.
3. X. Wang, O. S. Wolfbeis and R. J. Meier, Chem. Soc. Rev., 2013, 42, 7834–7869.
4. E. Hemmer, N. Venkatachalam, H. Hyodo, A. Hattori, Y. Ebina, H. Kishimoto and K. Soga, Nanoscale, 2013, 5, 11339–11361.
5. R. V. Yadav, S. K. Singh and S. B. Rai, RSC Adv., 2015, 5, 26321–26327.
6. T. Trupke, M. A. Green and P. Wurfel, J. Appl. Phys., 2002, 92, 1668–1674.
7. X. Huang, S. Han, W. Huang and X. Liu, Chem. Soc. Rev., 2013, 42, 173–201.
8. J. Sun, W. Zhou, Y. Sun and J. Zeng, Opt. Commun., 2013, 296, 84–86.
9. M. Zhang, Y. Lin, T. J. Mullen, W. F. Lin, L. D. Sun, C. H. Yan, T. E. Patten, D. Wang and G. Y. Liu, J. Phys. Chem. Lett., 2012, 3, 3188–3192.
10. J. Sun, Y. Sun, C. Cao, Z. Xia and H. Du, Appl. Phys. B, 2013, 111, 367–371.
11. D. Serrano, A. Braud, J. L. Doualan, W. Bolanos, R. Moncorge and P. Camy, Phys. Rev. B, 2013, 88, 205144–205154.
12. H. Guo and S. Sun, Nanoscale, 2012, 4, 6692–6706.
13. X. Li, R. Wang, F. Zhang, L. Zhou, D. Shen, C. Yao and D. Zhao, Sci. Rep., 2013, 3, 3536–3542.
14. Y. Sun, J. Peng, W. Feng and F. Li, Theranostics, 2013, 3, 346–353.
15. S. K. Singh, RSC Adv., 2014, 4, 58674–58698.
16. Y. Z. Wang, D. C. Yu, H. H. Lin, S. Ye, M. Y. Peng and Q. Y. Zhang, J. Appl. Phys., 2013, 114, 203510–203514.
17. S. Lian, C. Rong, D. Yin and S. Liu, J. Phys. Chem. C, 2009, 113, 6298–6302.
18. L. Li, X. Zhou, X. Wei, C. G. Ma and M. G. Brik, Mater. Chem. Phys., 2014, 147, 860–866.
19. W. W. Piper, J. A. DeLuca and F. S. Ham, J. Lumin., 1974, 8, 344–348.
20. J. L. Sommerdijk, A. Bril and A. W. de Jager, J. Lumin., 1974, 8, 341–343.
21. T. C. Liu, G. Zhang, X. Qiao, J. Wang, H. J. Seo, D. P. Tsai and R. S. Liu, Inorg. Chem., 2013, 52, 7352–7357.
22. X. Liu, S. Ye, Y. Qiao, G. Dong, B. Zhu, D. Chen, G. Lakshminarayana and J. Qiu, Appl. Phys. B, 2009, 96, 51–55.
23. S. Ye, B. Zhu, J. Chen, J. Luo and J. R. Qiu, Appl. Phys. Lett., 2008, 92, 141112–141114.
24. L. Xie, Y. Wang and H. Zhang, Appl. Phys. Lett., 2009, 94, 061905–061907.
25. R. V. Yadav, S. K. Singh, R. K. Verma and S. B. Rai, Chem. Phys. Lett., 2014, 599, 122–126.
26. J. Zhou, Y. Zhuang, S. Ye, Y. Teng, G. Lin, B. Zhu, J. Xie and J. Qiu, Appl. Phys. Lett., 2009, 95, 141101–141103.
27. Z. Liu, N. Dai, L. Yang and J. Li, Appl. Phys. A, 2015, 119, 553–557.
28. J. Liao, D. Zhou, S. Liu, H. R. Wen, X. Qiu and J. Chen, Phys. B, 2014, 436, 59–63.
29. M. K. Lau and J. H. Hao, Energy Procedia, 2012, 15, 129–134.

---