Abstract: A novel near-infrared (NIR) quantum cutting KCaGd(PO4)2:Ce3+,Yb3+ phosphor was successfully developed. Due to the cooperative energy transfer from one Ce3+ to two Yb3+, an intense NIR emission around 1021 nm of Yb3+:2F5/2-2F7/2 transition was obtained under 324 nm excitation. Yb3+ concentration dependent quantum efficiency has been calculated and the theoretical maximum efficiency approaches up to 158.2%. Because the emission of Yb3+ around 1021 nm is matched with the band gap of crystalline Si, the phosphors could be a potential candidate for silicon-based solar cells.
© 2014 Optical Society of America
In order to solve the problem of the long-term worldwide energy demand, the photovoltaic cells which can convert sunlight into electricity have become more and more important . Among them, the crystalline Si (c-Si) solar cells occupy a majority of the solar cells market. However, the low energy efficiencies around 15% is the chief obstacles of the crystalline Si (c-Si) solar cells for its mass-application . The mismatch between the solar spectrum and the band gap energy of silicon semiconductor is the main reason for source of energy loss (over 70%) [2, 3]. Due to the lower absorption rate of sunlight, the photoelectric conversion efficiency of solar cells is low. Hence, how to increase the photoelectric energy conversion efficiency of solar cells is very meaningful. To increase the energy efficiency, two general approaches were proposed. One is to develop tandem solar cells consisted of multiple semiconductor layers and each semiconductor layer has a characteristic band gap converting a different part of the solar spectrum. However, the complicated fabrication and high cost limit their practical application [3, 5]. The other one has been achieved by quantum cutting (QC). QC is based on the process that an incident high-energy photon splits into two or more low-energy photons, which can reduce the energy loss related to the thermalization of hot charge carries after the absorption of a high-energy photon. One ultraviolet (UV) or blue photon can be cutted to two near-infrared photons which can both be absorbed by solar cell [4–7]. In this case, the energy loss can be effectively reduced and the near-infrared quantum cutting (NIR-QC) has attracted a lot of research attention. Most notable is the luminescent solar concentrator (LSC) based on the theory of NIR-QC. It has become a unique and efficient photovoltaic optical device which consists of a luminescent material dispersed in a transparent waveguide plate as well as Si solar cells optically matched to the plate edges [2, 6, 7]. It is well known that the QC luminescent materials can modify spectral efficiently by combination of different rare earth ions thanks to their abundant energy levels. NIR-QC was first achieved in YPO4: Tb3+, Yb3+ . In this system, one blue photon of Tb3+ ion from the absorption transition of 7F6-5D4 around 483 nm is cut into two NIR photons of Yb3+ ion by the emission of 2F5/2-2F7/2. The cooperative energy transfer (CET) from the donor (Tb3+) to the acceptor (Yb3+) via downconversion is involved with the NIR-QC process. Therefore, it is expected that NIR-QC luminescent materials would significantly maximize LSC performance with Si solar cells and other application.
So far, similar NIR-QC phenomena with RE/Yb3+ (RE = Tb3+, Pr3+, Tm3+, Er3+, Mn2+, Eu2+) couple were reported in various powder and glass systems [9–14]. Due to the Yb3+ can emit photons around 1000 nm [25, 26], it often introduced as a sensitizer in these materials. Nevertheless, the NIR luminescence of Yb3+ duo to CET process is still weak. It is mainly ascribed to the inefficient excitation of the donors for their parity forbidden 4f-4f transitions, i.e., low absorption cross sections, typically on the order of 10−21 cm2. The absorption of Ce3+ is originated from the allowed electric-dipole transition from the 4f ground state to the 5d excited one, which results in a very high absorption cross section in an order of 10−18 cm2 in the ultraviolet (UV) region [6, 23, 24]. Moreover, the excitation band of Ce3+ is broad and the excitation wavelength of Ce3+ can be tuned by the host materials, which means that high quantum efficiency (QE) may be obtained by choosing appropriate host materials. All these conditions show that strong NIR emission with high QE may be obtained in Ce3+, Yb3+ co-doped materials. The QC of visible light to NIR emission in Ce3+, Yb3+ co-doped Y3Al5O12 , YBO3 , LuBO3  and glasses phosphors  had been reported recently. In these systems, a high QE will be possible at relatively high Yb3+ concentration where the NIR emission is quenching at a low concentration of Yb3+ (<10%). In this paper, we develop a novel NIR-QC KCaGd(PO4)2:Ce3+, Yb3+ phosphor. It can harvest UV photons and emit NIR photons around 1021 nm efficiently. The NIR emission intensity of KCaGd(PO4)2: Ce3+, Yb3+ didn’t quenched at 20% Yb3+ doping and the highest NIR QE is about 158.2%. KCaGd(PO4)2: Ce3+, Yb3+ is a promising solar spectral converter and concentrator for LSC with Si solar cells.
Rare earth ion doped samples of KCaGd(PO4)2 were prepared by the thermal decomposition of the corresponding nitrate. The starting materials were CaCO3 [analytical reagent (A. R.)], K2CO3 (A. R.), Gd2O3 (99.99%), Yb2O3 (99.99%), CeO2 (99.99%) and (NH4)2HPO4 (98.5%). Three steps were necessary for synthesizing the samples. Firstly, stoichiometric amounts of CaCO3, K2CO3, (NH4)2HPO4 and rare earth oxides were thoroughly mixed, dissolved completely in the nitric acid, and a certain amounts of deionized water was added into this solution until it is transparent. Secondly, the transparent solution was heated to evaporate water and excess HNO3, at the same time, the coprecipitation was obtained and then dried at 80°C for 10h. Finally, the resulting mixture is heated at 900°C in a reductive atmosphere (H2 + N2) for 6 h and cooled to the room temperature. The obtained samples were white powder bulk.
The X-ray powder diffraction (XRD) patterns were obtained on Rigaku D/max-2000 powder diffract meter using Cu Kα radiation (1.5405 Å) at 40 kV and 60 mA. The absorption spectra were measured with a visible-NIR spectrophotometer (PE Lambda 950) using BaSO4 as a reference. Emission and excitation measurements were performed using a 450 W Xe lamp as the excitation source. Mean decay lifetime measurements were performed using uF900 as the excitation source. All measurements were carried out at room temperature.
3. Results and discussion
Figure 1 shows the XRD patterns of KCaGd0.99-x(PO4)2: 0.01Ce3+, xYb3+(0≤x≤0.2) samples and the reference data of JCPDS Card No. 34-0125 for pure KCaGd(PO4)2. It was reported that KCaGd(PO4)2 had a hexagonal crystal structure with the space group P6222 [19, 20]. Compared with standard data, all the samples exhibit the peaks of pure hexagonal phase. No second phase is detected in the XRD patterns, revealing the successful doping of Ce3+ and Yb3+ ions in KCaGd(PO4)2. To obtain a detailed crystal structure regarding the host KCaGd(PO4)2, Rietveld refinement was performanced with the crystallographic data taken from the structure of LaPO4 (ISCD NO.33241).
Figure 2 shows the observed (crosses), calculated (solid line), and difference (bottom) XRD profiles for the Rietveld refinment of KCaGd0.98Ce0.01Yb0.01(PO4)2. The Rietveld analysis results indicate that the weighted profile R-factor(Rwp) and the excepted R factor (Re) are 13.37% and 10.35%, respectively. The results of the final refinement data indicate that the powder sample is well crystallized with space group P6222 and lattice parameters a = b = 6.9521 Å, c = 6.3680Å. KCaGd(PO4)2 has the similar crystal structure with hexagonal LaPO4, except that the large 8-coordinated K+ occupies only half of the tunnel sites with the coordination polyhedron [KO8], and La3+ position is statistically occupied by Gd3+ or Ca2+ ions [19, 20]. As shown in the inset of Fig. 2, the unit cell parameters a and c decrease linearly with the increasing Yb3+ concentration, which accords with the Vegard’s rule . The linear decreasing trend of unit cell parameters is due to the substitution of Gd3+ ions by small Yb3+ ions in the host lattice. All these results indicate that the hexagonal phase KCaGd(PO4)2: Ce3+, Yb3+ is fully developed and a small amount of Ce3+ and Yb3+ ions are successfully doped into the host.
The absorption spectra of KCaGd(PO4)2: 0.01Ce3+, KCaGd(PO4)2: 0.0.1Ce3+, 0.2Yb3+ and BaSO4 are shown in Fig. 3. The thickness of sample KCaGd(PO4)2: Ce3+ and KCaGd(PO4)2: Ce3+,Yb3+ was around 1mm. In the range of 850 nm-1100 nm, the absorption intensity of KCaGd(PO4)2: 0.01Ce3+, 0.2Yb3+ is increased compared with Yb3+ undoped sample and BaSO4, so that the characteristic absorption peaks due to Yb3+ can be observed more clearly. And the strong absorption band assigned to the Yb3+: 2F7/2-2F5/2 transition is centered at 980 nm in the Yb3+ doped samples . Other strong absorption band of KCaGd(PO4)2: 0.01Ce3+ and KCaGd(PO4)2: 0.0.1Ce3+, 0.2Yb3+ samples assigned to the 4f-5d transition of Ce3+ is also visible around 315 nm.
In order to give a convincing evidence for the energy transfer of Ce3+ to Yb3+, luminescence measurements of Ce3+ and Yb3+ and decay curves of Ce3+ in KCaGd(PO4)2: Ce3+, Yb3+ phosphors are investigated. Figure 4 presents the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the KCaGd0.99-x(PO4)2: 0.01Ce3+, xYb3+ (0≤x≤0.2) samples . By monitoring the 5d to 4f transition of Ce3+ at 366 nm, two broad bands were observed from 260 to 350 nm with a maximum at about 280 nm and 324 nm, respectively. These two broad bands can be attributed to the f-d transitions of Ce3+ in the host lattice, when Ce3+ enters one specific site, its 5d state will be split into several different components depending on the site symmetry . Under 324 nm excitation, the emission spectra shows the d-f transition band in the region of 340-450 nm. It is noticed that, as the Yb3+ concentration increases from 0% to 20%, the intensity of d-f transition gradually decreased. This indicates that an efficient energy transfer from 5d energy level of Ce3+ to 2F5/2 energy leve of Yb3+ ions could occur.
To verify this energy transfer process, the excitation spectra monitored by 1021 nm emission wavelength are depicted in Fig. 5(a). As shown in the Fig. 5(a), all the excitation spectra with different Yb3+ content are similar to the KCaGd(PO4)2: 0.01Ce3+, 0.1Yb3+ sample except the intensity. The broad excitation band in the region of 240-350 nm owing to the d-f transition of Ce3+ is similar with Fig. 4(a). This result further confirms the existence of energy transfer from Ce3+ to Yb3+. As we known, the energy part bellow 400 nm of the AM1.5 spectrum is only 3% of the solar photons arriving on the surface of our planet. Due to the lower absorptivity of the sunlight, the KCaGd(PO4)2: Ce3+, Yb3+ phosphors also have deficiencies in the practical application. However it also can be considered to be the strength of the material compared to other materials which can absorb the visible light. Such as Tb3+-Yb3+/Pr3+-Yb3+/Tm3+-Yb3+ co-doped phosphors and so on, these materials can absorb the visible light in the range of 440-490 nm. In this materials, due to the inefficient excitation of the donors, the NIR luminescence of Yb3+ duo to energy transfer process is still weak. But the absorption of Ce3+ is originated from the allowed electric-dipole transition from the 4f ground state to the 5d excited one, which results in a very high absorption cross section in the UV region and caused to the efficient energy transfer from Ce3+ to Yb3+. Figure 5(b) shows the NIR emission spectra of KCaGd0.99-x(PO4)2: 0.01Ce3+, xYb3+ under 324 nm excitation. It is worthy to note that a clear emission centered at 1021 nm was also observed, corresponding to the 2F5/2-2F7/2 transition of Yb3+, and the intensity of the Yb3+ emission enhances with the increase of the Yb3+ concentration. The concentration quenching has not been discovered until the Yb3+ content increased to 20%. To add the proof for the energy transfer from Ce3+ to Yb3+, the Yb3+ single doped KCaGd(PO4)2 sample are also excited by 324 nm as reference. It can be seen that the emission of Yb3+ from transition of 2F5/2-2F7/2 cannot be observed in the Yb3+ single doped KCaGd(PO4)2 sample obviously, as might be expected. Thus, we can identify that the efficient CET from Ce3+ to Yb3+ is existent in this system. In this work, we investigated the quantum cutting in KCaGd(PO4)2: Ce3+, Yb3+, whose theory is based on the quantum cutting model of cooperative energy transfer that has been reported in the references [8–11, 15–18]. Therefore, the emission of two NIR photons per absorbed one UV photon is possible with the Ce3+-Yb3+ dual ions via this CET process.
Figure 6 illustrates the schematic energy levels with transitions which may be involved in the CET process from one Ce3+ to two Yb3+. Upon excitation at 324 nm, Ce3+ emissions occur at 366 nm (shown in Fig. 4), which can be assigned to the transition from the lowest 5d level to 2F5/2 and 2F7/2 of Ce3+, respectively. Just as mentioned in the reference 8-11 and 15-18, different donor transfer the energy to two acceptor (Yb3+) from their higt energy level by cooperative energy transfer, because the energy of the donor’s high level is twice as the excited state of acceptor. In the KCaGd(PO4)2: Ce3+, Yb3+ phosphor, the energy of Ce3+ 5d level (around 3000 cm−1) is almost triple as 2F5/2 excited state of Yb3+. Considering the cooperative energy transfer theory mentioned above, we can conclude that two NIR photons (around 1021 nm) are obtained due to the 2F5/2→2F7/2 transition of Yb3+. Under this circumstance, the CET of Ce3+: 5d→Yb3+: 2F5/2 + Yb3+: 2F5/2 is the only possible relaxation route to achieve the Yb3+ NIR emission because the Ce3+: 5d→4f emission is located at approximately twice the energy of the Yb3+: 2F5/2→2F7/2 absorption and Yb3+ has no other levels up to the UV region. Ce3+, Yb3+ co-doped KCaGd(PO4)2 phosphors present two attractive features: firstly, the conversion of one UV photon in the region of 200–350 nm (almost useless in the silicon solar cell) to two NIR photons around 1021 nm (where the silicon solar cell exhibits the greatest spectral response). And secondly, the highly efficient Ce3+ excitation and Yb3+ NIR luminescence benefit from the large absorption cross section of Ce3+ [15–18]. In the cooperative energy transfer mechanism, the transition from lower excited state to higher virtual state is inefficient. But the resonant energy transfer is an efficient mechanism for rare-earth ions due to the energy directly transfer from higher energy level of donor to lower energy level of acceptor by means of resonance. Hence the efficiency of energy transfer is lower, because the cooperative energy transfer is clearly off resonance.
Figure 7(a) shows the Yb3+ concentration dependence of the luminescence decay curves and (b) fitted results of Ce3+ emission for all the powder samples KCaGd0.99-x(PO4)2: 0.01Ce3+, xYb3+ (0≤x≤0.2) under 324 nm excitation at the room temperature. The Ce3+: 5d→4f emission in the Ce3+ single doped sample shows, as expected, a nearly single exponential decay, and the fitting yields a decay time of 25.1 ns. The lifetime in nanosecond order is one of the characteristics of the Ce3+ electric-dipole allowed 5d→4f transition . In the whole Yb3+ doping concentration range 0≤x≤0.2, Ce3+ decays single exponentially, and its lifetime is deceased from 25.1 ns when no Yb3+ is codoped to 10.5 ns when Yb3+ concentration is increased to 20%. Under direct Ce3+ f-d transitions at 324 nm, the decrease of Ce3+ d-f emission intensity along with the decrease of Ce3+ decay time, providing a clear evidence that Ce3+→Yb3+ energy transfer takes place in this system. From the luminescence decay curves in Fig. 7 the energy transfer efficiency and the quantum efficiency can be determined. The energy transfer efficiency ηETE is defined as the ratio of Ce3+ that depopulate by energy transfer to Yb3+ ions over the total number of Ce3+ ions excited. We define the total quantum efficiency, ηQE, as the ratio of the number of photons emitted (visible and infrared) to the number of photons absorbed, assuming that all excited Yb3+ decay radiatively . By dividing the integrated intensity of the decay curves of the Ce3+, Yb3+ codoped samples to the Ce3+ single-doped sample, ηETE is obtained as a function of the Yb3+ concentration as follows: [8,10,]8, 10, 15, 18]:15–18]. As mentioned above, the NIR quantum cutting via cooperative energy transfer model is a double photons emitted process. In this work, we also utilize the theory of cooperative energy transfer to explain the quantum cutting process. Therefore, the QE calculated by the Eq. (2) of the double photon quantum cutting process is limited to 200%. With the increase of Yb3+ content from 1 to 20 mol%, the ηETE increases monotonously from 10% to 58.2%, and the ηQE are calculated to be 110% −158.2%. The results are listed in Table 1.
In Fig. 8, decay lifetime and ηQE are plotted as a function of Yb3+ concentration. It is presented that the decay lifetime decreases from 25.1 ns to 10.5 ns with the increase of Yb3+ content from 0 to 20%, and the quantum efficiency increases from 0 to 158.2%. Supposing there is no loss of nonradiation by defects and impurities, the maximum quantum efficiency of 158.2% is obtained.
We have successfully developed a novel NIR QC KCaGd(PO4)2:Ce3+, Yb3+ phosphor. It has intense wide excitation bands in the UV region and exhibits a greatly enhanced NIR emission of Yb3+ around 1021 nm through CET from one Ce3+ to two Yb3+, perfectly matching the maximum spectral response of Si solar cells. The theoretical highest QE has been found to be 158.2% with the doping concentration 20% of Yb3+. These results indicate that KCaGd(PO4)2:Ce3+, Yb3+ phosphor could improve the efficiency of silicon-based solar cells by means of downconversion.
This work is supported by National Science Foundation for Distinguished Young Scholars (No. 50925206) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120211130003).
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