We report on downconversion of one blue photon to two near-infrared (NIR) photons (~10000 cm−1) in Pr3+/Yb3+ co-doped SrO-La2O3-Al2O3-B2O3-SiO2 glasses and LaBO3 glass ceramics. The Pr3+ ions act as broadband spectral sensitizer in the spectral range of 415-505 nm. Energy transfer occurs subsequently from Pr3+ to Yb3+, followed by re-emission in the NIR spectral range. The transfer efficiency is indicated by the degree of decrease of Pr3+-related photoluminescence (PL) and PL lifetime of the 3P0 and 1D2 levels with increasing Yb3+ concentration. For the present case, we find an optimum dopant concentration of Yb2O3 of ~0.5 mol % for a Pr2O3 concentration of 1.0 mol %. A theoretical maximum of quantum efficiency of 183% is reached for 5 mol % of Yb2O3. PL characteristics (absorption cross section and emission lifetime) are further improved upon precipitation of crystalline LaBO3, where both Pr3+ and Yb3+ ions occupy La3+ sites with an assumedly statistical distribution and a high degree of partitioning.
©2013 Optical Society of America
One of the reasons for the low theoretical quantum efficiency of silicon solar cells is related to the Shockley–Quaisser limit, where due to the spectral mismatch between incident photons and the band-gap of crystalline silicon of ~1.1 eV, only about 32% of the solar spectrum can be used for the generation of photoelectrons. In addition, high energy photons are often lost due to recombination effects and/or active filtering to prevent photobleaching [1–3]. For these reasons, several ways have been explored to convert, by photoluminescence, parts of the solar spectrum to regions where the photovoltaic efficiency might be improved. These comprise three principle routes, i.e., downconversion [4–13] where a near-ultraviolet (NUV) to visible (Vis) photon is cut into two or more near infrared (NIR) photons, up-conversion [14,15], where the energy of two or three NIR photons is absorbed to emit one photon of higher energy, and down-shifting [16,17], where the energy of incident photons is shifted into wavelength regions which can be harvested more effectively. Downconversion provides the additional advantage that the energy loss due to the thermalization of hot charge carriers after absorption of a high-energy photon is minimized. A brief look at the solar irradiance spectrum reveals that, in principal, ~15% of excess energy, the UV-Vis part of the spectrum which would otherwise be lost as heat, is available for downconversion [18,19].
Considering the energy level of all lanthanides, Yb3+ has been recognized as the most suitable candidate for downconversion. That is, the Yb3+ ion has only a single excited state, 2F5/2 at ~10000 cm−1, above the ground state of 2F7/2. This means that Yb3+ centers may act as efficient recipients for energy quanta of ~10000 cm−1 from any other co-dopant to emit photons with a wavelength of about ~1 µm [4,19]. To effectively sensitize Yb3+ to the UV-Vis spectral region, a donor ion with an energy level at ~20000 cm−1 is necessary. For this, Pr3+, Er3+, Nd3+, Ho3+, Tm3+, Tb3+ or Ce3+ may be employed. In the present report, we focus on Pr3+ to sensitize Yb3+ because the absorption bands of Pr3+ cover a broad spectral window in the blue region due to the successive energy levels of Pr3+:3PJ(J = 0, 1 and 2) [Fig. 1(b)]. These absorption bands are located at approximately twice the band level of Yb3+:2F7/2 →2F5/2 [20–22]. As host material, we chose a glass matrix which exhibits virtually universal forming capability and high compositional flexibility for well-controlled and homogeneous doping. To adjust the ligand symmetry and phonon energy, the glass composition may be selected so that in a subsequent annealing procedure, a crystalline species precipitates from the undercooled melt into which dopants are incorporated during crystallization [23–28]. Here we employ a glass of the composition 20 SrO-20 La2O3-10 Al2O3-40 B2O3-10 SiO2 . In this system, LaBO3 crystallites can be precipitated by controlled nucleation in a heat treatment process. Due to the equivalent charge and similar ionic radii of La3+ (1.16 Å, CN = 8), Pr3+ (1.13 Å, CN = 8) and Yb3+ (0.99 Å, CN = 8), we expect that the dopant species can readily be incorporated into the lattice of crystalline LaBO3 .
Precursor glass samples with nominal composition (mol %) 20 SrO-(19-x) La2O3-10 Al2O3-40 B2O3-10 SiO2-1 Pr2O3-x Yb2O3 (SLABS, x = 0, 0.1, 0.2, 0.5, 1, 2, and 5) were produced by conventional melting in alumina crucibles at 1400°C for 2 h (air). Glass slabs were obtained after pouring the melt into a preheated (500°C) graphite mold and subsequent annealing at 550°C for 2 h. From these slabs, disks of 20 × 20 × 3.0 mm3 were cut and polished on both sides. To obtain glass ceramic samples, individual specimen were placed on alumina substrates and isothermally annealed at 800°C for up to 32 h (air).
UV-VIS-NIR absorption spectra were recorded from 300 to 2500 nm with a double-beam photo-spectrometer equipped with a 150 mm integration sphere and a PbS detector (Perkin-Elmer Lambda 950). Static and dynamic photoluminescence (PL) were studied with a high-resolution spectrofluorometer and time correlated single photon counting (TCSPC, Horiba Jobin Yvon Fluorolog FL3-22) using a static Xe lamp (450 W) and a Xe flashlamp (75 W) as excitation sources. NIR PL was observed with a thermoelectrically cooled InP/InGaAs-based photomultiplier tube (Hamamatsu H10330A-75). Photoluminescence excitation (PLE) spectra were corrected over the lamp intensity with a silicon photodiode and PL spectra were corrected by the spectral response of the detector using correction spectra of the employed PMT. The crystallization process of each specimen was first analyzed by differential scanning calorimetry (DSC, Netzsch, Ar atmosphere) with a heating rate of 20 K min−1. To identify the crystalline phases after heat treatment, X-ray diffractometry (XRD, Siemens Kristalloflex D500, Bragg-Brentano, 30 kV/30 mA, Cu Kα) was performed with a step width of 0.02°/s and a counting time of 10 s per step over the 2θ range of 5-70°. All analyses were performed at room temperature.
3. Results and discussion
In Fig. 1 the effect of Yb2O3 concentration on optical absorption is shown. For Pr3+ singly doped SLABS glass, eight absorption peaks centered at 440, 471, 484, 591, 1003, 1416, 1515 and 1915 nm are ascribed to the inhomogeneously broadened 4f–4f transitions from the ground state 3H4 to the excited states 3P2, 3P1, 3P0, 1D2, 1G4, 3F4, 3F3 and 3F2 of Pr3+ (labels in Fig. 1) . As mentioned in the introduction section, three relatively strong and overlapping absorption bands (3H4 → 3P1,2,3) cover a large part of the blue wavelength range of 415-505 nm. For the Pr3+/Yb3+ co-doped samples, the additional absorption band with a maximum at 976 nm and a shoulder peak at ~935 nm is assigned to the transition from the ground state of Yb3+, 2F7/2, to two different Stark levels of the excited state of 2F5/2. As expected, the intensity of this NIR absorption band of Yb3+ increases linearly with increasing Yb3+ doping concentration.
Radiative transitions within the 4fn configurations of trivalent rare earth ions can be predicted by the Judd-Ofelt (J-O) theory. The Judd-Ofelt parameters Ω2, Ω4 and Ω6 as obtained from the measured absorption spectra in Fig. 1 are calculated 1.10 × 10−20, 1.06 × 10−20 and 0.87 × 10−20 cm2 respectively [31–33]. Using these values, the radiative decay rate AJJ', the branching ratio βJJ' and the radiative emission lifetime τrad were estimated. Data are summarized in Table 1.PLE and PL spectra reveal direct evidence of energy transfer from Pr3+ to Yb3+ [Figs. 2(a)–2(d)]. Room temperature PLE spectra are shown for emission at 608 nm (Pr3+) and 976 nm (Yb3+), respectively. Consistent with the absorption spectra (Fig. 1), the PLE bands of Pr3+ comprise three characteristic peaks at 445, 471 and 483 nm, together with a relatively weak band at 591 nm. The former three bands exhibit similar intensity and strong overlap with each other, what lets expect a relatively high excitation efficiency in this spectral range. The PLE spectra of the Yb3+:2F5/2→2F7/2 [Fig. 2(d)] are in full agreement with those of the Pr3+-related PLE [Fig. 2(a)] and with the absorption spectra of Pr3+ (Fig. 1), what is taken as clear evidence for the sensitization of the Yb3+ emission center by Pr3+ donors in the NUV-Vis spectral range. In the following, the maximum PLE peak at 445 nm is selected to monitor the PL spectra as a function of Yb2O3 doping concentration.
The PL spectra of Pr3+-related emission centers [Fig. 2(b)] consist of six characteristic bands which are located at 487, 530, 608, 646, 701 and 730 nm. Their designation is shown in the inset of Fig. 2(a) and labeled in Fig. 2(b) [34–36]. The PL spectra of all samples are dominated by the red band at ~608 nm which is attributed to the transition of Pr3+:3P0→3H6. The intensity of all bands of Pr3+ decreases gradually with increasing Yb3+ concentration over the whole range of observed concentrations [Figs. 2(b) and 2(c)]. Very sharp drop appears to occur in the sample with 5 mol% of Yb2O3.
In Fig. 2(e), PL spectra of the NIR region are presented. For the Pr3+ singly-doped SLABS sample, the broad NIR PL band centered at ~1047 nm with a full width at half maximum (FWHM) of 73 nm and a shoulder centered at ~998 nm is ascribed to the transitions of 1D2→3F4 and 1D2→3F3 in Pr3+, respectively. The relatively weak PL band at ~1475 nm is assigned to 1D2→1G4. The observation of three bands originating from 1D2 implies that relaxation of the 3P1,2,3 levels occurs via a multi-phonon relaxation to 1D2. This indicates that 1D2 is efficiently occupied . After co-doping with a small amount of Yb3+ (x = 0.1), a sharp PL peak appears at 976 nm, accompanied by a broad shoulder at ~997 nm. These two bands are attributed to the transition of the lowest Stark level of 2F5/2 in Yb3+ to two different Stark levels of the ground state of 2F7/2 . The latter two overlap with the Pr3+-related emission band at 1046 nm. To account for these three bands, all NIR PL spectra were deconvoluted into three Gaussian functions [Figs. 3(a)–3(f)]. Regarding the concentration of Yb3+, the intensity of the emission bands at 976 and 997 nm increases for x ≤ 0.5 and decreases sharply for higher amounts Yb3+-doping. We conclude that this is caused by concentration quenching and that, in the present matrix material, x = 0.5 represents the optimal doping concentration. As already seen in the excitation spectra, also the decrease of the intensity of Pr3+-related PL intensity after co-doping with a small amount of Yb3+ and the parallel occurrence of increasingly intense Yb3+-related emission bands indicate energy transfer from Pr3+ to Yb3+. Pr3+ acts as a sensitizer by absorbing NUV-Vis photons and partially transferring their energy to Yb3+ [Figs. 2(e) and 2(f)] [37–39]. Parallel to the intensity decrease of PL bands of Pr3+ in the visible range, also the PL intensity of Pr3+:1D2→3F4 [1047 nm, Figs. 3(e) and 3(f)] and Pr3+:1D2→1G4 [1475 nm, Fig. 2(e)] decrease gradually with increasing Yb3+ concentration. At the same time, the position of the Pr3+:1D2→3F3 band red-shifts from 1047 to 1072 nm with increasing Yb3+ concentration [Fig. 3(e)], what is understood as another consequence of energy transfer from Pr3+ to Yb3+.
Figures 4(a), 4(c), and 4(e) depict normalized PL decay curves for the Pr3+-related transitions of 3P0→3H6 (608 nm) and 1D2→1G4 (1475 nm), and for the Yb3+-related transition of 2F5/2→2F7/2 (976 nm) as a function of Yb3+ concentration after excitation at 445 nm. For Pr3+:3P0→3H6, all decay curves exhibit multi-exponential behavior [Fig. 4(a)]. In the Pr3+ singly-doped sample, this may be due to cross-relaxation between Pr3+ ions, whereas in co-doped samples, this is caused by the increasingly faster depopulation of the Pr3+:3P0 level due to energy transfer from 3PJ of Pr3+ to 2F5/2 of Yb3+ with increasing Yb3+ concentration . The effective PL lifetime of the Pr3+:3P0 state decreases from 7.2 to 1.2 μs when the Yb3+ concentration is increased to 5 mol % [Figs. 4(a) and 4(b)]. Similarly, also the effective PL lifetime of the Pr3+:1D2 state decreases from 7.6 to 2.1 μs with increasing Yb3+ concentration [Figs. 4(c) and 4(d)]. This shows that both states contribute to the energy transfer process. The energy transfer from the 3P0 level results in downconversion of one visible photon into two NIR photons whereas the energy transfer from the 1D2 level results in down-shifting of one NIR photon into another NIR photon. A more detailed discussion will be presented in the following paragraphs. The energy transfer efficiency ηETE is defined as the ratio between the number of depopulation events which result in energy transfer from donor to acceptor and the total population of the excited states in the donor. It can be calculated from the ratio of the lifetimes of the excited state(s) of the donor in the presence of the acceptor as compared to a specimen which contains only the donor species [6,38],
Here, τx%Yb is the lifetime a Pr3+-related state after co-doping with x mol% Yb2O3. For both levels, 3P0 and 1D2, the calculated values of ηET increase with increasing Yb3+ concentration, i.e., from 11.5% and 9.8% for x = 0.1 to 83.0% and 88.1% for x = 5.0, respectively. The total theoretical quantum efficiency is then calculated as a function of Yb3+ concentration [6,40],
In agreement with the PL intensity change of Yb3+:2F5/2→2F7/2 (976 nm) upon Yb3+ concentration increase [Figs. 4(e) and 4(f)], also the effective lifetime of this emission band first increases up to ~56.6 μs for x = 0.5 and subsequently decreases for higher x, i.e., to 10.7 μs for x = 5.0.
To evaluate the potential of further enhancement of the energy transfer efficiency, glass ceramic samples were considered where LaBO3 crystallites were precipitated so that Pr3+ and Yb3+ are incorporated into the crystalline lattice on La3+ sites.
Figure 5(a) exemplarily shows a DSC curve of the SLABS glass sample with a heating rate of 20 K min−1 after baseline correction. The onset of glass transition Tg was observed at 679.5 ± 0.5 °C. Three crystallization peaks were found at 852, 926 and 948 ± 0.5 °C. Targeting the non-isothermal crystallization event at 852 °C, controlled crystallization was then performed isothermally at 800 °C for 32 h. XRD [Fig. 5(b)] of the untreated sample did not reveal any diffraction peaks, showing that the as-made glass was - within the accuracy of measurement - free of any crystalline phases. After thermal annealing, multiple intense diffraction peaks were found. These were indexed to the room-temperature orthorhombic phase of LaBO3 (JCPDS card no. 00-013-0113). In addition, XRD patterns indicate the presence of at least one further, minor crystallite species which we assign as hexagonal SrAl2B2O7 (JCPDS card no. 00-046-0621). For clarity, the tabulated diffraction patterns of both species are shown in Fig. 5(b). The lattice of LaBO3 is illustrated in Fig. 5(c). It comprises an orthorhombic aragonite-type structure and cell parameters of a = 5.104 Å, b = 8.252 Å, c = 5.872 Å, composed of LaO9 polyhedra and BO3 trigonal groups . Due to the aforementioned similarity of ionic radii, Pr3+ and Yb3+ should readily be incorporated on La3+-sites (for comparison, the radius of B3+ is about one third, i.e., 0.41 Å, as compared to the three rare earth species) .
Figures 6(a) and 6(d) show PLE spectra (monitoring 608 nm PL from Pr3+ and 976 nm PL from Yb3+, respectively) of the SLABS glass and glass ceramic for x = 0.5. After crystallization, the characteristic excitation bands of Pr3+ are clearly enhanced. Not surprisingly, annealing at elevated temperature leads to a great change in the PL spectra [Figs. 6(b) and 6(e)] and decay curves [Figs. 6(c) and 6(f)], as well. For Vis PL from Pr3+ [Fig. 6(b)], the peak intensity at 608 nm increased by a factor of ~5 after crystallization. Moreover, all PL bands apparently split and sharpen notably as a result of crystallization. The latter observations indicate that Pr3+ is indeed incorporated into the LaBO3 crystal phase. A comparison of the decay curves of the Pr3+:3P0→3H6 PL band of the SLABS glass and the corresponding glass ceramic is shown in Fig. 6(c). Both curves can be best fit by second-order exponential equations, suggesting that in both cases a slow and a fast decay process contributes to PL. The lifetime values are obtained from the best fit. For both decay processes they increase after crystallization, i.e. from 2.4 to 7.1 μs and from 11.9 to 52.3 μs, respectively. This is taken as further evidence for the incorporation of Pr3+ into the crystallite phase. If Pr3+ is incorporated into the LaBO3 lattice, we expect that Yb3+ would behave similarly. Otherwise, energy transfer between the two species would become highly unlikely due to their spatial and configurational separation. Examining the PL spectra of Yb3+ in the NIR range for the crystallized sample [Fig. 6(e)], a strongly enhanced intensity of the peak at 976 nm is visible as compared to the as-made glass. Additionally, the peak intensity ratio of the two peaks at 976 and 997 nm, I976/I997, decreases from 2.2 to 1.5 after crystallization. We attribute the latter observation to stronger Stark splitting of the ground state of Yb3+, 2F7/2, in the LaBO3 crystalline environment. Analogous to the behavior in the as-made glass, and also similar to the behavior of Pr3+ in the LaBO3 lattice, decay curves follow a second-order exponential equation [Fig. 6(f)]. After crystallization, the two resulting lifetime values increase from 28.7 to 35.3 μs and from 149.0 to 297.3 μs, respectively, for the fast and for the slow decay process. From these observations, we conclude that also Yb3+ is incorporated into the crystallite phase.
Although on the first view, the presence of an intermediate level at about 10000 cm−1 in Pr3+, suggest a relatively simple energy transfer process, at least four different ways may be considered for the pair of Pr3+ and Yb3+, illustrated in Figs. 7(a)–7(d). These are
- a) two-step first-order resonant energy transfer from the 3P0 and 1G4 levels of Pr3+ to the 2F5/2 level of Yb3+, resulting in the generation of two NIR photons at ~1 µm ,
- b) single step first-order resonant energy transfer from 3P0 of Pr3+ to 2F5/2 of Yb3+, and further relaxation of 1G0 to 3H4 or 3H5 of Pr3+, resulting in one photon at ~1 µm and another one at ~1 µm (3H4) or at ~1.33 µm (3H5) ,
- c) single step second-order cooperative energy transfer from the 3P0 level of Pr3+ to two neighboring Yb3+:2F5/2 levels, resulting in the emission of photons from Yb3+ centers with a wavelength of ~1 µm , and
- d) non-radiative relaxation of 3P0 to 1D2 (Pr3+), followed by first-order resonant energy transfer from Pr3+:1P2 to Yb3+:2F5/2, resulting in the emission of a single photon at ~1 µm.
The first two mechanisms (a-b) appear rather unlikely due to the following reasons: firstly, the absence of Pr3+ PL bands from 3P0→1G4 (which should occur ~950 nm), from 1G4→3H6 (~1850 nm) and from 1G4 →3H5 (~1330 nm), what indicates poor population of the 1G4 level of Pr3+. Secondly, the 1G4 level of Pr3+ is ~200 cm−1 lower than the 2F5/2 level of Yb3+. This means that energy transfer should rather occur from Yb3+ to Pr3+ than vice versa. Thirdly, the absorption band which is assigned to the transition from the ground state of Pr3+, 3H4, to the 1G4 level is very weak (Fig. 1). Fourthly, the energy gap between the 1G4 and the lower lying 3F4 level is low (~3000 cm−1) may lead to very high multi-phonon assisted non-radiative transition rates from the 1G4 level. Finally, the branching ratio of the 3P0→1G4 radiative transition and the total radiative rates of the 3P0 level are as low as ~0.06 and 218.07 cm−1, respectively, resulting in a low population of the 1G4 level (Table 1).
As briefly discussed in the previous chapters, the decrease in intensity of all PL bands of Pr3+ in the presence of Yb3+ [Figs. 2(a)–2(f)], and also of PL lifetime of the Pr3+:3P0 and Pr3+:1D2 levels [Figs. 4(a)–4(d)] suggests that energy transfer occurs through both the 3P0 and 1D2 levels of Pr3+. The branching ratio of the 3P0→1D2 radiative transition is close to zero. This indicates the dominating multi-phonon assisted non-radiative transition of 3P0→1D2 (Table 1). The excitation efficiency of Yb3+ via the 3P0 level of Pr3+ is 2.7 times higher than via the 1D2 level [Fig. 2(e)]. This indicates that also energy transfer from the 3P0 state is more efficient than from the 1D2 state. Additionally, the total radiative rate of the 3P0 level (10319.43 cm−1) is much higher than that of the 1D2 level (924.43 cm−1) which confirms our conclusion (Table 1). The absence of 3P0→1G4 Pr3+ PL also indicates that the cooperative energy transfer process cannot be neglected [Fig. 7(c)].
In summary, NIR downconversion of one blue photon into two NIR photons (~10000 cm−1) was observed in Pr3+/Yb3+ co-doped SLABS glasses and corresponding LaBO3 glass ceramics. The decrease of all Pr3+-related PL bands in intensity and lifetime with increasing Yb3+ concentration along with the observation of typical Pr3+ PLE bands by monitoring Yb3+-related PL at 976 nm indicated that energy transfer occurs through both the 3P0 and the 1D2 level of Pr3+ to Yb3+. Thereby, transfer from Pr3+:3P0 to Yb3+ occurs as a downconversion reaction where of one visible photon is cut into two NIR photons, whereas transfer from Pr3+:1D2 to Yb3+ results in a down-shift of the energy of a single NIR. The transfer efficiency from the Pr3+:3P0 level is higher as compared to that from the Pr3+:1D2 level. A theoretical quantum efficiency of 183% was estimated for this process. In the present matrix material, the optimal doping concentration of Yb2O3 for downconversion is ~0.5 mol%. Crystallization of the as-made glass into a LaBO3 glass ceramic leads to improved emission properties. Thereby, Pr3+ ions as well as Yb3+ ions enter the crystallite lattice, assumedly on La3+ sites.
The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence “Engineering of Advanced Materials - EAM”.
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