Light conversion material: LiBaPO₄:Eu²⁺, Pr³⁺, suitable for solar cell

Abstract

A light conversion material LiBaPO₄:Eu²⁺, Pr³⁺ is successfully developed as a solar spectral converter for Si solar cells. The photoluminescence excitation and emission spectra from 3 K to room temperature, the lifetime and energy transfer mechanism are all systematically investigated. The results show that Eu²⁺ ions absorb and transfer the energy of UV-Vis photons to Pr³⁺ ions, which exhibit an intense near-infrared (NIR) emission between 950–1060 nm, matching well with the maximum spectral response of Si solar cells. We believe that this NIR emitting phosphor may open a new route to design advanced UV-Vis-to-NIR phosphors, with promising applications for solar spectral convertors.

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1. Introduction

Phosphors doped with rare earth ions (Reⁿ⁺) are used for many devices such as white light emitting diodes (WLED), displays and so on.^1–3 Recently, they have also attracted much interest for photovoltaic applications to improve solar cell efficiency by modifying the solar spectrum. Si solar cells effectively convert near infrared (NIR) photons of lower energy close to the semiconductor band gap (E_g ≈ 1.12 eV, λ ≈ 1000 nm). However, the strongest emission of the solar spectrum is located in the UV-Vis region. The mismatch between the strongest emission in the solar spectrum and the spectral response of Si solar cells, will reduce the photoelectric conversion efficiency of the cell.^4,5

Recently, light conversion material is used to modify the solar spectrum and enhance the external quantum efficiency of Si solar cell.^6–8 Many researches have been developed phosphors doped by Yb³⁺ ion, Eu³⁺–Yb³⁺, Tb³⁺–Yb³⁺, Dy³⁺–Yb³⁺, Eu²⁺–Yb³⁺ and so on.^9–15 However, Yb³⁺ usually has no direct absorptions in UV-Vis (300–800 nm) range since it only has a single excited state ²F_5/2 approximately 10 000 cm⁻¹ above the ground state ²F_7/2. Therefore, Re³⁺ (Eu³⁺, Tb³⁺, Dy³⁺ and so on) is selected as a sensitizer in Re³⁺–Yb³⁺ co-doped phosphor. But this kind of materials usually have low absorption efficiency in UV-Vis light range, due to the forbidden 4f–4f transitions by the parity selection rule.

As we know, Pr³⁺ ion has the ability to absorb visible photons and emits NIR photons, because of its abundant energy levels in UV-Vis-NIR range. For instance, Pr³⁺ ion doped phosphors can be excited at the range of 440–700 nm due to the transitions from ³H₄ to ¹D₂ (∼600 nm), ³P_J (J = 0, 1, 2) and ¹I₆ (440–490 nm), and emit NIR phonons at ∼1000 nm ascribed to the ³P₀ → ¹G₄ and/or ¹G₄ → ³H₄, matching well with the optimal spectral response of Si solar cells.^16–19 To enhance the absorption of Pr³⁺ in ultraviolet region, here we explored a promising light conversion phosphor LiBaPO₄:Eu²⁺, Pr³⁺ for Solar Spectral Convertor. It can exhibit an intense NIR emission of Pr³⁺ ion at about 1000 nm upon excitation with allowed 4f–5d absorption of Eu²⁺ ions.

2. Experiment

2.1 Syntheses

All phosphors were prepared by a conventional solid-state method. The stoichiometric amounts of LiH₂PO₄ (97%), BaCO₃ (A.R.), NH₄H₂PO₄ (A.R.), Eu₂O₃ (99.99%) and Pr₆O₁₁ (99.99%) were used as raw materials. Then the ingredients were thoroughly grinded in an agate mortar and transferred into crucibles. Finally, the mixtures were sintered at 1200 °C for 3 h under N₂/H₂ (10 : 1) atmosphere and grounded homogeneously after cooling.

2.2 Measurements

The phase purity of the as-prepared phosphors was investigated by a Rigaku D/max-IIIA X-ray Diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA. The XRD patterns were collected in range of 15° ≤ 2θ ≤ 58°.

The photoluminescence (PL) and photoluminescence excitation (PLE) spectra from 3 K to room temperature as well as the decay curves were measured by FSP920 Time Resolved and Steady State Fluorescence Spectrometers (Edinburgh Instruments) equipped with a 450 WW Xe lamp, a 100 W μF920H lamp, TM300 excitation monochromator and double TM300 emission monochromators, red sensitive PMT and R5509-72 NIR-PMT in a liquid nitrogen cooled housing (Hamamatsu Photonics K.K). The spectral resolution for the steady measurements is about 0.05 nm in UV-vis and about 0.075–0.01 nm in NIR, and the experimental conditions for the transient measurements are a pulse width of 1–2 μs, a repetition rate of 50 Hz and the lifetime range of 100 μs–200 s. For PL and PLE measurements at 3 K, the sample was mounted in a Optistat AC-V12 actively cooled optical cryostat, based on 0.25 W @ 4 K PTR (pulse tube refrigerator), with a ITC503 temperature controller and a water cooled compressor.

3. Results and discussion

3.1 Phase and structure characterization

The XRD patterns of polycrystalline samples LiBaPO₄:Eu_x²⁺, Pr_y³⁺ (x = 0, y = 0; x = 0.005, y = 0, 0.005; x = 0, y = 0.01) are shown in Fig. 1a. The results indicate that all the peaks of rare earth ions doped LiBaPO₄ can be indexed to the Joint Committee On Powder Diffraction Standard card of a pure LiBaPO₄ (JCPDS 14-0270), and also match well with the reference data.^20,21 No detectable impurity phases were observed, which indicated that the introduction of dopants has no obvious influence on the crystalline structure of the host, which crystallize in a stuffed tridymite structure with a hexagonal unit cell and space group P6₃. Each PO₄ tetrahedron is jointed with four LiO₄ tetrahedron, while each LiO₄ is jointed whit four PO₄ tetrahedrons, and two series of tetrahedron are linked by corner sharing,²¹ as shown in Fig. 1b and c. Furthermore, the ionic radius of Eu²⁺, Pr³⁺, Ba²⁺ and Li⁺ are 1.25 Å, 1.13 Å, 1.42 Å and 0.92 Å, respectively. According to the ionic size and valence state of cationic, Eu²⁺ and Pr³⁺ ions are expected to replace the site of Ba²⁺ ion, since the radius of Eu²⁺ and Tb³⁺ are closed to that of Ba²⁺.

Fig. 1 XRD patterns of LiBaPO₄:Eu_x²⁺, Pr_y³⁺ (x = 0, y = 0; x = 0.005, y = 0, 0.005; x = 0, y = 0.01) (a), and schematic diagram of the structure of LiBaPO₄ along the a- (b) and c-direction (c).

3.2 Luminescence properties of LiBaPO₄:Pr³⁺

Fig. 2 shows the PL (a and b) and PLE (c) spectra, and decay curves of LiBaPO₄:Pr_0.01³⁺ at 3 K. The selection of low test temperature is to reduce the effects of the host vibration or phonons on the optical properties of Pr³⁺ ion. As shown in Fig. 2c, the excitation spectrum mainly contains four sharp peaks at 444 nm, 468 nm, 483 nm and 588 nm, which are due to the absorption transitions of ³H₄ → ³P₂, ³H₄ → ³P₁ and ¹I₆, ³H₄ → ³P₀, and ³H₄ → ¹D₂ of Pr³⁺ ion, respectively, monitoring the emission at 617 nm. Fig. 2a and b are the emission spectra of Pr³⁺ ion under excitation from the ground state to the ³P₂ and ¹D₂ levels, respectively. It is obviously that the emission spectrum excited to ¹D₂ levels (Fig. 2a) consists of only one emission peak at 599 nm, due to the transitions from ¹D₂ to ³H₄. Whereas exited to ³P₂ level (Fig. 2b), the emission spectrum consists of not only the emission from ³P₀ to ³H₄, ³H₆ and ³F₂ peaked at 482 nm, 606 and 640 nm, but also the emission from ¹D₂ to ³H₄ at 599 nm.

Fig. 2 PL (a and b) and PLE (c) spectrum, and decay curves (d) of LiBaPO₄:Pr_0.01³⁺ at 3 K.

In order to clarify the assignment of the emissions peaks at 599 nm and 606 nm as shown in Fig. 2a, the luminescence decay curves at 3 K (λ_ex = 483 nm and 599 nm, λ_em = 606 nm) of LiBaPO₄:Pr_0.01³⁺ were measured and presented in Fig. 2d. Generally, the decay time of the emission from ¹D₂ is 50–250 μs, longer than that from ³P₀, which is about 0.135–50 μs.²² It is clear that the decay curves have different characteristics and the average lifetimes are calculated to be about 84.8 μs and 3.8 μs when excited to ³P₀ and ¹D₂ level, respectively. Therefore, it is reasonable to assign the emission peaks at 599 nm and 606 nm to ¹D₂ → ³H₄ and ³P₀ → ³H₆ transitions.

However, why can the emission from ¹D₂ to ³H₄ be detected when excited into ³P₂ level? The energy transfer through multi-phonon relaxation and cross-relaxation process may give a clear interpretation. Fig. 3 presents the schematic diagrams of Pr³⁺ in LiBaPO₄. It is widely accepted that the electrons excited to ³P₂ level (2a) are able to relax to ³P₀ level assisted by phonons (process 2b), because of the small energy difference between ³P₂ and ³P₀ level. There are two possible non-radiative path to generate the ¹D₂ → ³H₄ transition (process 2c and 2d). One is the non-radiative relaxation from ³P₀ to ¹D₂ level assisted by phonons, which is labeled as process 2c. The energy difference between ³P₀ and ¹D₂ level is about 3697 cm⁻¹. The maximum vibration frequency of phosphate is about 1037 cm⁻¹.²³ It means that three phonons are required to bridge this energy gap. Generally, phonons assisted non-radiative relaxation process from the upper level to the lower level is possible, if the energy gap is less than 5 times the energy of the highest energy phonon. Therefore, it is expected that multi-phonon relaxation process from ³P₀ to ¹D₂ level may occur in LiBaPO₄:Pr³⁺. The other is cross-relaxation process (2d), [³P₀, ³H₄] → [³H₆, ¹D₂]. The energy gaps between ³P₀ and ³H₆ level, and ¹D₂ and ³H₄ level are 16 502 cm⁻¹ and 17 007 cm⁻¹, respectively. The difference between them is only about 505 cm⁻¹. Therefore, the cross-relaxation [³P₀, ³H₄] → [³H₆, ¹D₂] (2d) is also efficient. In a word, the whole blue-to-red ET process could be expressed as (2a → 2b → 2c/2d → 599 nm) for the red emissions from the ¹D₂ level and (2a → 2b → 606 nm) from the ³P₀ level.

Fig. 3 Schematic diagrams of the cross-relaxation processes and ET from Eu²⁺ to Pr³⁺ in LiBaPO₄ and from Pr³⁺ to Si solar cells. (2d: [³P₀, ³H₄] → [³H₆, ¹D₂]; 2e: [¹D₂, ³H₄] → [¹G₄, ³F₄]; 3b: the lattice thermalization loss).

To further study the effect of phonon to luminescence property of Pr³⁺ in LiBaPO₄, Fig. 4a presents the normalized emission spectra of LiBaPO₄:Pr_0.01³⁺ at different temperature. It's interesting to observe the temperature dependence of ³P₁ → ³H₅ transition peaked at 523 nm when excited into ³P₀ level. At 3–100 K, Pr³⁺ ion gives no emission at 523 nm whereas above 100 K the ³P₁ → ³H₅ transition is clearly observed. Moreover, its emission intensity regularly increases with the increase of temperature. This phenomenon is due to the increase of electron–phonon interaction, which leads to more and more electrons populated from ³P₀ up to ³P₁ level with the rise of the temperature. In addition, the intensity ratio of ¹D₂ → ³H₄ to ³P₀ → ³H₆ increases gradually with temperature increases, as shown in Fig. 3b, which strongly proves cross-relaxation or multi-phonon relaxation process (Fig. 3, 2d and c) from ³P₀ to ¹D₂ level is efficient especially at room temperature. However, it is very hard to make a decision which one is dominant between multi-phonon relaxation process (2c) and cross-relaxation process (2d).

Fig. 4 Normalized emission spectra of LiBaPO₄:Pr_0.01³⁺ at 3–300 K (a). Inset is the intensity ratio of ¹D₂ → ³H₄ to ³P₀ → ³H₆ as a function of temperature (b) and the energy diagram of LiBaPO₄:Pr_0.01³⁺ (c).

The energy difference between transition ¹G₄ → ¹D₂ (7203 cm⁻¹) and ³H₄ → ³F₄ (6854 cm⁻¹), is calculated to be only about 349 cm⁻¹, as shown in Fig. 3. Thus, the cross-relaxation (2e), [¹D₂, ³H₄] → [¹G₄, ³F₄] will happen due to the small energy mismatch. In other words, electrons populate to the ¹G₄ level by two steps of cross-relaxation (2d–2e) when excited into ³P₀ level. Consequently, it is theoretically expected that the NIR1 emissions from the ¹G₄ level could be observed. Fig. 5a presents the emission spectra of LiBaPO₄:Pr_0.01³⁺ in NIR region at room temperature. Up to ³P₀ (483 nm) and ¹D₂ (590 nm) level, the strong NIR emission at 1020 nm is observed and attributed to the transition of ¹G₄ → ³H₄. The above results further prove that the cross-relaxation (2e), [¹D₂, ³H₄] → [¹G₄, ³F₄], is occurred in LiBaPO₄:Pr_0.01³⁺. Beside the prominent NIR1 emission at 1020 nm, Pr³⁺ ion also gives the NIR2 emissions at 976 nm and 993 nm, attributed to the transitions from different sublevels of ¹D₂ to ³F₄ level. To sum up, the process of NIR emissions from ¹G₄ and ¹D₂ levels could be expressed as (2b → 2c/2d → 2e → NIR1 and 2b → 2c/2d → NIR2), as shown in Fig. 3.

Fig. 5 Normalized PLE (a) and PL spectrum (b), decay curves (c) and absorption spectrum (d) of LiBaPO₄:Pr_0.01³⁺ at room temperature.

For the sake of verifying the assignment of emission at 976 nm, 993 nm and 1020 nm, Fig. 5b shows normalized decay curves and lifetime of LiBaPO₄:Pr_0.01³⁺ at room temperature. If the emission transitions from the different stark levels of a excited level to a ground level, it is expect to have the approximated lifetime when up to the same excited levels. The lifetimes of emission at 1020 nm are 0.125 ms and 0.136 ms excited to ³P₀ level (483 nm) and ¹D₂ level (588 nm), respectively. Similarly, the lifetimes of emissions at 993 nm and 976 nm are 0.085 ms and 0.068 ms excited to ¹D₂ level (588 nm). That is to say the emissions at 993 nm and 976 nm belong to the same transition, different to the 1020 nm. It is clearly that the lifetime of emission at 1020 nm is longer than that of the 993 nm and 976 nm because of the two steps of cross-relaxation processes. Therefore, the emissions at 976 nm (or 993 nm) and 1020 nm are attributed to the transitions of ¹D₂ → ³F₄ and ¹G₄ → ³H₄, respectively.

According to the report of A. Meijerink,²⁴ the absorption of a photon to the ³P_J or ¹I₆ levels would be followed by the emission of two NIR photons, whereas absorption to ¹D₂ would result in emission of only one NIR photon. If the area ratio (R_E) of the ³P_J and ¹I₆ band to ¹D₂ band in the excitation spectrum is twice as large as the ratio (R_A) similarly obtained in the absorption spectra, the quantum cutting will be efficient. Fig. 5c and d show the excitation spectrum and the absorption spectrum of LiBaPO₄:Pr_0.01³⁺. It can be found in Fig. 5c and d that the R_E and R_A are 4.0 and 4.4, respectively. It means that the blue-to-NIR quantum cutting process from the ³P₀ level does not occur in LiBaPO₄.

As mentioned above, the ideal light conversion material for Si solar cells should utilize the UV-vis region of the solar spectrum and possess an intense NIR emission, matching well with the maximum spectral response (λ ≈ 1000 nm) of Si semiconductor. As shown in Fig. 5a and c, LiBaPO₄:Pr³⁺ meets the requirement of excitation and emission characteristics of light conversion material. However, it can only absorb the photons in blue and red region of the solar spectrum. Unfortunately, the intensity of the excitation peaks is very weak because the 4f–4f absorption transitions are forbidden by the parity selection rule. In order to widen the utilization of the solar spectrum towards UV region, we further investigate the luminescent properties of LiBaPO₄:Eu²⁺,Pr³⁺ and explore the possibility of Eu²⁺ ion as donor absorbing UV photons, which greatly enhances the NIR emission of Pr³⁺ ion through efficient energy feeding by allowed 4f–5d absorption of Eu²⁺ ion with high oscillator strength.

3.3 Luminescence properties of LiBaPO₄:Eu_0.005²⁺, Pr_0.005³⁺

As discussed above, LiBaPO₄:Eu_0.005²⁺, Pr_0.005³⁺ phosphor shows the optimal NIR performance. In order to explore its possibility of potential application in Si solar cells, as shown in Fig. 6c. It is obviously seen that Si solar cells most effectively convert photons of energy close to the semiconductor band gap (E_g ≈ 1.12 eV, 1000 cm⁻¹), but the incident solar spectrum dominates in UV-Vis region. This mismatch between the incident solar spectrum and the spectral response of Si solar cells is one of the main reasons to limit the efficiency of cell. For instance, a UV photon with high energy (250–425 nm) directly creates an electron–hole (e–h) pair in Si solar cells (Fig. 3a). However, the photo-excited pair quickly loses energy in excess of band gap of Si and the extra energy is lost as heat within the device (Fig. 3b). Consequently, the heat energy leads to the thermalization of the charge carriers. In summary, the efficiency of Si solar cells is limited because of the energy loss resulting from thermalization of the charge carriers generated by the absorption of high-energy photons. But if UV-vis photons can be converted into NIR photons prior to absorption into Si solar cells (as shown in Fig. 3c), the charges thermalization of Si solar cells will be greatly decreased and the efficiency of Si solar cells will be improved.

Fig. 6 PLE and PL spectra of LiBaPO₄:Eu_0.005²⁺ (a), LiBaPO₄:Eu_0.005²⁺, Pr_0.005³⁺, Ca₂BO₃Cl:Ce_0.002³⁺, Tb_0.01³⁺, Yb_0.01³⁺ (b) at room temperature and the solar spectral and spectral response of c-Si (c).

Fig. 6a exhibits the PLE and PL spectra of LiBaPO₄:Eu_0.005²⁺, which contains 4f–5d allowed transition of Eu²⁺ ions at 250–425 nm and a blue emission band at 480 nm.²¹ It is worth noting that the Eu²⁺ single doped LiBaPO₄ has no emission band at NIR range. Fig. 6b gives the PLE and PL spectra of LiBaPO₄:Eu_0.005²⁺, Pr_0.005³⁺. Monitoring the emission at 1020 nm, the PLE spectrum contains a broad band and some sharp peaks. Comparing the PLE spectra of Eu²⁺ or Pr³⁺ singly doped in LiBaPO₄ phosphors (Fig. 6a and 2c), the intense broad band centered at 338 nm should be attributed to 4f–5d allowed transition of Eu²⁺ ions. The other peaks at 444 nm, 468 nm, 483 nm and 590 nm are due to f–f transitions of Pr³⁺ ions. Similar as Fig. 5a, the emission band peaked at 1020 nm is attributed to transition of ¹G₄ → ³H₄ of Pr³⁺. The appearance of 4f → 5d transitions of Eu²⁺ ions in the PLE spectrum and the existence of the ¹G₂ → ³H₄ transitions of Pr³⁺ ion in the PL spectrum of LiBaPO₄:Eu_0.005²⁺,Pr_0.005³⁺ indicate that the energy transfer from Eu²⁺ to Pr³⁺ occurs, as shown in Fig. 3. It demonstrates that LiBaPO₄:Eu²⁺, Pr³⁺ can harvest UV-Vis photons of the incident solar spectral and significantly reduce the charges thermalization of Si solar cells by UV/vis-to-NIR spectral modification. At the same time, we compared our material with Ca₂BO₃Cl:Ce_0.002³⁺, Tb_0.01³⁺, Yb_0.01³⁺, which was promising as useful light convert material for solar cell.²⁵ It is obviously that the emission intensity of our phosphor is about 5 times as intense as Ca₂BO₃Cl:Ce_0.002³⁺, Tb_0.01³⁺, Yb_0.01³⁺ in NIR area. Therefore, it can be a promising light conversion material to increase the efficiency of Si solar cells.

4. Conclusions

Light conversion material LiBaPO₄:Eu²⁺, Pr³⁺ was systematically investigated. We studied the spectral assignment of Pr³⁺ ions, and proved that the emissions at 599 nm and 617 nm are attributed to the ¹D₂ → ³H₄ and ³P₀ → ³H₆ transfer according to the study of luminescence decay and spectra at low temperature. It is demonstrated that the energy transfer from Eu²⁺ to Pr³⁺ occurs in this matrix. Therefore, this phosphor has a wide excitation band in UV-vis region and emits intense NIR emitting around 1000 nm, which is matching well with the maximum spectral of Si solar cells. From the viewpoint of spectral modification, it may be a new light conversion material to match the solar spectrum and the spectral response of Si solar cells.

Acknowledgements

This work was financially supported by grants from National Nature Science Foundation of China (51602227), Scientific Foundation for Yong Teachers of Wuyi University (2016zk06), Doctor' Start-up Foundation of Wuyi University (2016BS10), Innovative Research Team in university of Guangdong (2015KCXTD027), Science Foundation for Yong Teachers of Wuyi University (2014td01) and the Science and Technology Projects of Guangdong Province (2015A090905010).

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