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Optical and bandgap study of rare earth doped phosphate phosphor

V. B. Pawade, A. Zanwar, R. P. Birmod, S. J. Dhoble & L. F. Koao

Journal of Materials Science: Materials in Electronics ISSN 0957-4522 J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7536-8

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Author's personal copy J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7536-8

Optical and bandgap study of rare earth doped phosphate phosphor V. B. Pawade1 · A. Zanwar2 · R. P. Birmod2 · S. J. Dhoble3 · L. F. Koao4 

Received: 11 April 2017 / Accepted: 14 July 2017 © Springer Science+Business Media, LLC 2017

Abstract  This article present an reports on optical and band gap study of rare earth doped (RE = Tb3+, ­Pr3+, ­Ce3+, ­Dy3+ and ­ Ce3+/Pr3+ ions) KSr(PO4) nanoparticles. The KSr(PO4) phosphor is synthesized by wet chemical method at 120 °C in a hot oven. Pure phase of KSr(PO4) phosphor material is achieved by sintering it at 950 °C for 2 h in muffle furnace. Photoluminescence investigation shows that KSr(PO4) doped with T ­ b3+, ­Pr3+ ions shows an emission in visible to NIR range after exciting it under UV and visible wavelength. Whereas, KSr(PO4) doped with ­Ce3+/Pr3+ ions shows the emission in NIR range extend from 650 to 800 nm, under 295 nm excitation band. Further ­Dy3+ doped ions, shows the characteristics emission band that corresponds to blue and yellow region, under UV excitation. The observed emission bands of ­Ce3+, ­Tb3+, ­Pr3+ and ­Dy3+ ions are assigned due to d–f and f–f transition respectively. Also the band gap of the prepared phosphor is found to be approximately 4.9 eV. Study on micro and nanocslae nature of phosphor are carried out by SEM, HRTEM analysis, indicating the core shell structure of materials with spherical shaped nano particles.

* V. B. Pawade [email protected] 1

Department of Applied‑Physics, Laxminarayan Institute of Technology, RTMNU, Nagpur 440033, India

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Department of Chemical Engineering, Laxminarayan Institute of Technology, RTMNU, Nagpur 440033, India

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Department of Physics, RTMNU, Nagpur 440033, India

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Department of Physics, University of the Free State (Qwa Qwa Campus), Phuthaditjhaba, Private Bag X13, 9866, South Africa



1 Introduction Today an photovoltaic (PV) solar cells are appealing and promising renewable power sources of twenty-first century, and it is the future substitute for fossil-fuel-based electricity generation systems. In the past decade, rare earth ion doped with NIR DC and DS phosphors have been focused worldwide and deeply investigated from the viewpoint of emission spectroscopy of materials. These kind of rare earth doped inorganic phosphor acts as an active layer for Si-solar cell due to their potential in improving the spectral response of Si solar cells and enhancing the light conversion efficiency of Si solar cells. Among the different lanthanides used in DC/UC, DS process such as ­Eu3+, ­Ce3+, ­Yb3+, ­Er3+, ­Nd3+, ­Tb3+, ­Sm3+, ­Pr3+, ­Ho3+ etc [1–3]. The emission originates from these ions are assigned due to 5d–4f, 4f–4f transitions respectively, also they play a crucial role in the field of modern application such as solid state lighting, displays, solar cells, optoelectronics devices and medical applications. However, the fluorescence arising from forbidden transition (f–f) appears in the narrow absorption band and it has a ability to absorb a wide range of solar spectrum. These characteristics features of rare earth makes them an interesting for the development of solar cell devices. Because it acts as active layer when doped with suitable host lattice to enhance the light conversion efficiency of c-Si solar cells. Among the different lanthanides ions ­Tb3+ is an most important activator for doping because it shows emission in visible to NIR range, also it has a relatively simple energy levels structure that corresponds to 7 FJ, 5D4, and 5D3 states. To intensify the absorption of ­Tb3+ ion in the UV region, one of the prominent way is to introduce sensitizer (such as ­Ce3+, ­Eu2+) to transfer effective energy to ­Tb3+ ion. Therefore C ­ e3+ is an extensive doping ions due to their broad band 5d–4f allowed transitions

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followed by the Laporte parity selection rule [4]. Depending on the crystal structure and lattice symmetry of ­Ce3+ ion, the 5d–4f emissions of the ­Ce3+ ion vary from longwavelength that is UV to red region corresponds to electric dipole transition. Thus, C ­ e3+ ions is an suitable sensitizer in the co-doped phosphors host, such as B ­ aY2Si3O10:Ce3+, 3+ 3+ 3+ ­Tb , ­Ca3Sc2Si3O12:Ce , ­Tb , ­Ba3Gd(PO4)3:Ce3+, ­Tb3+, ­Ca2Al3O6F:Ce3+, ­Tb3+, AlN:Ce3+, ­Tb3+ [5–14]. However, ­Pr3+ is most favourable impurity ions due to its luminescence under VUV excitation and received much attention, also it has two possible emission processes depending on the host lattice, i.e. photon cascade emission (PCE) or f–d emission [15]. From the energy level splitting of ­Pr3+ ions, we know that the lowest 5d state of ­Pr3+ is energetically lower than 1S04f state [16], therefore the emission of P ­ r3+ will occur due to the 4f–5d transition, which is usually at the higher energy range side with respect to that of ­Ce3+ ion. The novelty in P ­ r3+ makes it is an potential activator ions for fast decay scintillator as well as possible sensitizer to improve the luminescence of ­Ce3+. Also it shows NIR emission band when doped with suitable host lattice, which is used as down conversion materials for C–Si–Solar Cell [17, 18]. As activator, D ­ y3+ is well known for its emission centres in blue and yellow peaking at 470–500  nm and 570–600 which are attributed to the 4F9/2–6H13/2 and 4 F9/2–6 ­H15/2 transitions [19]. Further, 5d–4f transition energy of ­Ce3+ is higher than that of ­Dy3+. Thus, the mechanism of energy transfer from C ­ e3+–Dy3+ paired ions can be designed for white light generation from past few years in many host lattices [20]. Among the different families of host, phosphates based phosphor belong to stable host, and it has potential applications as luminescence materials due to the superior physical and chemical properties such as low melting point and high ultra-violet transmission etc [21–23]. Here we have reported the photolumenscence and band gap study of rare earth doped (RE = Tb3+, ­Pr3+, ­Ce3+, ­Dy3+ and ­Ce3+/Pr3+ ions) KSr(PO4) phosphor prepared by wet chemical methods for their possible application in solar cell and other optical devices.

J Mater Sci: Mater Electron

Initially all metal nitrates was dissolved separately in 10  ml double distilled water and them mixed together in beaker. The beaker stir continuously for half an hours to obtain a transparent solution and then kept it inside the oven maintain at 120 °C for 12  h upto the formation of white precipitation. Then precipitation initially crush well for 15 min, a fine powder is obtained and again sintered it at 950 °C for 2 h. Finally the prepared phosphor materials is carried out from the furnace at room temperature and crush well. Then the obtained phosphor materials are used for XRD, SEM, TEM, HRTEM, PL characterization.

3 Results and discussion 3.1 X‑ray diffraction XRD analysis is carried out to confirm the phase purity of the synthesized phosphor material. X-ray diffraction pattern is recorded using X-ray diffractometer with Cu-kα (1.54060  nm) radiation, step size 2θ (deg.) 0.0190, scan step time (s) 31.8152, and measurement temperature 25.00 (°C). Here Fig.  1 shows the X-ray diffraction pattern of KSr(PO4) phosphor prepared by wet chemical method. The prepared phosphor shows good agreement with standard reference File No. 00-033-1045, and the peak position of each diffraction peaks are well matched with stick pattern data of reference file. There is no evidence of other phase observed under XRD investigation. Hence it confirm that wet chemical method is an simple and low temperature synthesis techniques for the development of phosphate based phosphor. 3.2 SEM, TEM and HRTEM In this section we have reported the surface morphology, crystallites size and structure of pure KSr(PO4) phosphor, because crystallite size is an important factors for the device fabrication point of view. Figure 2 shows the SEM

2 Synthesis method A series of rare earth (RE = Tb3+, ­Pr3+, ­Ce3+, ­Dy3+ and ­Ce3+/Pr3+ ions) activated KSr(PO4) phosphor synthesized by wet chemical routes. The starting materials were used as K(NO3)2, Sr(NO3)2, ­NH4H2(PO4) purity 99.9%, ­(NH4)2Ce(NO3)6 (1–10  mol%), ­Tb2O3 (0.1–1  mol%), ­Pr6O11 (0.1–1 mol%) (Purity 99.99%), ­Dy2O3 (0.1–1 mol%), further rare earth oxides were converted into nitrate by adding it into few amount of dil nitric acid. The weight of all ingredients is calculated by using the stoichiometric ratio.

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Fig. 1  XRD pattern of KSr(PO4) phosphor

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Fig. 2  SEM images of KSr(PO4) phosphor

images of the KSr(PO4) phosphor. Phosphor images are observed in sphere shape with an varying crystallites sizes under 10  µm, resolution. Thus, it is seen that the synthesized phosphors acquired irregular shape and the size of the observed particles is found to be in the nanoscale range.

To know the nature of crystallites we have carried out the TEM characterization as shown in Fig.  3a, thus prepared phosphor looks like a core shell structure with varying crystallites size. For the further analysis we have carried out the HRTEM characterization as shown in Fig.  3b–f.

Fig. 3  TEM (a) and HRTEM (b–f) images of KSr(PO4) phosphor

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From this images Fig. 3b shows, the TEM image observed under 20  nm resolution, therefore it clearly indicates the core shell structure with small spherical nanoparticles. Again further resolved it under 1, 2 and 5  nm as shown in Fig.  3c, d, then each particles observed in spherical shape with the separation of lattice planes on the surface as depicted in Fig.  3e. This indicates the crystallographic planes with lattice fringes pattern having the interplanar spacing of 2.2 Å and it is well agreed with indexed plane of (212) in the JCPDS pattern. The diffraction data images of the lattice point is shown in Fig. 3f. 3.3 DRS and photon energy Diffuse reflectance spectroscopy is an excellent sampling tool for powdered or crystalline materials in the mid-IR and NIR spectral ranges. Figure 4 shows the diffuse reflection spectra of KSr(PO4) doped with ­Ce3+ (1–10 mol%), ­Dy3+ 3+ ­ b3+ (0.1–1 mol%) ions indicated (0.1–1 mol%), ­Pr (0.1–1 mol%) and T by different colour lines. Therefore from the measured DRS spectra, it is seen that the drastic drop in reflection observed in the UV range at around 220, 262 and 336 nm for ­Ce3+ ions, which clearly shows the optical band gap of the KSr(PO4):Ce3+ host lattice. The strong absorption bands of KSr(PO4):Ce3+ host is observed at 336  nm, and one weak absorption located at 760  nm. These bands are assigned due to 4f–5d allowed transition of impurity ions. Above 336–750  nm there is no absorption band observed in the spectra. Whereas a ­Dy3+ ion shows reflection at 780, 892, 1080 and 1260  nm in mid IR and IR range respectively. ­Pr3+ ions, shows the reflection at 1560 and 1840 nm and it corresponds to IR region. Also the ­Tb3+ shows the reflections at 590 and 1425  nm respectively. So here we have estimated the band gap of C ­ e3+ doped KSr(PO4)

Fig. 4  DRS spectra of KSr(PO4):RE3+ (RE = Ce3+, ­Dy3+, ­Pr3+, ­Tb3+) phosphor mark by different colours 

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phosphor only due to its 5d–4f allowed broad band emission under UV excitation wavelength. Thus, by using the numerical values of Kubelka–Munk coefficient (K/S) we have calculate the band gap from the measured diffuse reflectance spectra.

(1 − R)2 K = S 2R

(1)

where K represent the absorption coefficient, S is the scattering coefficient and R represents the reflectivity. Figure 5 shows the plot of photon energy versus Kubelka–Munk function. Therefore for KSr(PO4):Ce3+ phosphor the photon energy is found to be 4.93 eV. Thus, it confirmed that KSr(PO4) is an promising host candidates in phosphate family due o it’s large band gap and it may used in photovoltaic application. If we have vary the impurity ions with host lattice, then it does not show much variation in band gap of the materials.

4 Photoluminescence 4.1 Luminescence in ­Ln3+ ­(Ln3+=Tb3+, ­Pr3+, ­Ce3+/ Pr3+, ­Dy3+) doped KSr(PO4) phosphor 4.1.1 KSr(PO4):Tb3+ phosphor Figure  6 shows the photoluminescence excitation spectra of KSr(PO4):Tb3+ phosphor, it observed in the broad range centred at 227  nm, by monitoring the emission constant at 546  nm respectively, this band is originates from the 4f8–4f75d1 transitions allowed by the electric dipolar parity. This band corresponds to T ­ b3+ ← O2− charge–transfer

Fig. 5  Plot of photon energy versus Kubelka–Munk function

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Fig. 6  PLE spectra of KSr(PO4):Tb3+ phosphor (λem = 546 nm)

(CT) transition. The emission spectra are composed of five bands observed at 480, 546, 551, 580, 620 and 680  nm respectively as shown in Fig. 7, which are associated with the 5D4–7FJ = 6, 5, 4, 3, 1 transitions of the ­Tb3+ ions. Among the different emission bands observed in the given PL spectra, the strongest peak appears at 546 and 680 nm respectively, has been observed under 227  nm excitation wavelength. The excitation band extend from 220 to 260 nm is assigned to the f–d transitions of ­Tb3+. It is well known that, the interconfigurational radiative transitions of ­Ln3+ ions have been reported in various host crystals [24–27]. Usually, T ­ b3+ ion promote one electron from the 8 ground state 4f to the 4f75d1 excited states, it induced two kinds of transitions: spin-allowed and spin-forbidden transitions. Thus the band at around 227  nm assigned to the spin-allowed transitions of T ­ b3+ ion, respectively. Similar,

Fig. 7  PL emission spectra of KSr(PO4):Tb3+ phosphor

results is reported in G ­ dNbO4:Tb3+ [28], ­CaYBO4:Tb3+ 3+ [29] and ­CaBPO5:Tb [30]. These overlap might result in good luminescence of GdTaO4:Tb3+ in the UV region. In our reported results the high luminescence intensity of KSr(PO4):Tb3+ phosphor is observed due to the spectral overlaps between host absorption and the CTB of ­Tb3+–O2−, when excited by UV wavelength. Also an extremely small distinction is found in the dominant emission band and we may consider that this type of distinction could be caused by splitting in host lattice [31–33]. Here we mainly focused on the band located at 680  nm (14,705 cm−1) in NIR range under 227 nm (44,052 cm−1) excitation wavelength [34]. Therefore it is clearly seen that one high energy photon is converted into more than one low energy photon, hence the mechanism of NIR down conversion is confimed from the observed excitation and emission spectra. Thus, the prepared KSr(PO4):Tb3+ phosphor may acts as promising host for the downconversion to improve the efficiency of C–Si–Solar Cell. 4.1.2 KSr(PO4):Pr3+ phosphor Figure  8 shows the PLE spectra of KSr(PO4):Pr3+ phosphor measured by keeping emission wavelengths constant at 600  nm. The photoluminescence excitation bands is extend from 300 to 500  nm range, and the corresponding bands were observed at 402 and 448  nm due to the transitions from the 3H4 ground state to the excited states of ­Pr3+. Here the phosphor excited at 448  nm therefore the corresponding emission bands are attributed due to the f–f transitions originating from 3P0 to 1D2 states and these are populated through multiphonon relaxation process to the lower energy level. Due to this the high intense bluishgreen emission band appeared at 480  nm, due to 3P0–3H4

Fig. 8  PLE spectra of KSr(PO4):Pr3+ phosphor

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transitions, and the corresponding yellow–green emission band ranging from 525 to 550  nm, attributed due to the 3 P1–3H5 transitions respectively, and other band observed at around 600, 640, 680 nm which are assigned due to the 3 P0–3H6, 3P0–3F2, 1D2–3H5 transitions of ­Pr3+ ions as shown in Fig.  9. Here, the ­Pr3+ concentration varies from 0.1 to 1  mol% with respect to host and the maximum intensity observed at 0.3  mol%. And further increasing concentration of P ­ r3+ the intensity of the emission band decreased due to concentration quenching. Thus the QC mechanism occurs here by doping single rare-earth ions, because the well-separated energy levels of RE ions can be used to generate more than one visible photon out of one high energy incident photon. In past the QC mechanism in ­Pr3+ was first reported by two research groups in 1970s [35, 36]. In the next section we have carried out the energy transfer study in ­Ce3+/Pr3+ ion doped with KSr(PO4) host lattice. 4.1.3 KSr(PO4):Ce3+ & ­Ce3+/Pr3+ Figure 10a, b shows the PLE and emission spectra of C ­ e3+ doped KSr(PO4) phosphor, therefore emission band extend from 320 to 450  nm in doublet nature centred at 340  nm (29,411 cm−1) and 370 nm (27,827 cm−1) under the 309 nm broad band excitation wavelength. Thus band assigned due to the 5d–4f transition of C ­ e3+ ion, separated by energy dif−1 2 ference of 1584  cm ( F7/2 and 2F5/2 level). On the basis of spectral overlap between excitation band of ­Pr3+ ion and ­Ce3+ emission, here we have consider the probability of effective energy transfer (ET) from ­Ce3+ → Pr3+ and it is strongly expected. Figure  11 shows the PLE spectra of KSr(PO4):Ce3+ (1  mol%), ­Pr3+ (0.1–1  mol%), keeping emission wavelength constant at 690  nm. The excitation observed in the broad range extend from 250–330  nm centred at

Fig. 9  PL emission spectra of KSr(PO4):Pr3+ phosphor

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Fig. 10  a, b PLE and emission spectra of C ­ e3+ doped KSr(PO4) phosphor

295  nm. It is seen that with the increasing concentration ­ e3+ decreases of ­Pr3+ (0.1–1 mol%), the emission intensity of C gradually while increasing the concentration of ­Pr3+ ion the emission intensity further decreases due to quenching of luminescence. This phenomenon also gives strong evidence of ET from ­Ce3+ → Pr3+. From the PL spectra, it is clearly noticed that ET occurs via non- radiative transitions, from the lowest 5d excitation level of C ­ e3+ ion to the 3P2 level of P ­ r3+ ion with conversion of two photons. Figure  11a shows the photoluminescence excitation spectra of KSr(PO4):Ce3+(1 mol%)/Pr3+ (0.–1 mol%) observed in the broad band range from 220 to 350 nm, typically centred at 295  nm, therefore after exciting the phosphors at 295  nm it exhibits red to NIR emissions at around 600–690  nm respectively, as depicted in Fig.  11b. Keeping the NIR emission constant at 690  nm, phosphor shows the same PLE spectra as those observed due to allowed 4f–5d transitions of ­Ce3+ as shown in Fig.  10b and this excitation band assigned due to 4f–4f forbidden transitions of P ­ r3+ ion (Fig.  9). Thus, the appearance of 4f–5d transitions of

Fig. 11  a, b PLE and emission spectra of KSr(PO4):Ce3+ ­Pr3+ (0.1–1 mol%) phosphor

(1  mol%),

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­Ce3+ ion in the PLE spectra of ­Pr3+ provides the information about UV-to-NIR ET from ­Ce3+ → Pr3+ ions. Therefore, this kind of energy transfer process can effectively harvest UV photons in the range of 250–340  nm of the solar spectrum. Such a wide excitation band almost covers the whole UV spectrum of the solar photon flux spectrum and it may potentially reduce the charge thermalization in Si solar cells [2, 37, 38]. Therefore RE doped with KSr(PO4) host shows an excellent luminescence properties for downconversion process. In this process the UV part of the solar spectrum can be harvested by luminescent layer, which then shift towards the longer wavelength, hence the process so called as wavelength downshifting (DS). Recently, Teng-Ming Chen et al. [39], reported the enhancing the performance of photovoltaic cells by using downconverting KCaGd(PO4)2:Eu3+ Phosphors, in this work it revealed that with using DC layer there exist an increase in light conversion efficiency of Si- PV cell, the enhancement in efficiency is found in the range from 16.03 to 16.67% as compare to without coating of DC layer (0.64 + 0.01)%. Thus DC layer is a promising way to enhance the efficiency of C–Si–solar Cell. 4.1.4 KSr(PO4):Dy3+ Figure  12 shows the PLE and emission spectra of KSr(PO4):Dy3+ phosphor extend from 300 to 400 nm, the bands are located at 326, 336, 348 and 386 nm respectively, keeping the emission wavelength constant at 480  nm. Whereas the phosphors shows characteristics emission bands at 480, 490 and 570  nm when excited at 348  nm (6H15/2 → 6F9/2 transition), which corresponds to f → f transition. The transitions involved in blue, yellow bands of ­Dy3+ ion are well known and it has been assigned due to 4 F9/2 → 6H15/2, 6H13/2 transitions, and they are corresponds

Fig. 12  PLE and emission spectra of KSr(PO4):Dy3+ phosphor

to the magnetic and electric dipole transition of D ­ y3+ ions. In our reported work the band corresponds to magnetic dipole is more sensitive than electric dipole band, therefore the 4F9/2 → 6H13/2 is predominant only when ­ Dy3+ ions are located at low-symmetry sites with no inversion centers. Thus the low-symmetry location of ­Dy3+ results in the predominate emission via 4F9/2 → 6H13/2 transition. Hence emission at 575 nm is predominant, it suggests that there is a very little deviation from inversion symmetry in this matrix. The PL intensity of KSr(PO4):Dy3+ phosphor increases with varying the concentration of D ­ y3+ ion due 3+ to this the luminescence spectrum of ­Dy ion and it is slightly influenced by the surrounding ligands of the host lattice, because the electronic transitions of ­Dy3+ involve only the redistribution of electrons within the inner 4f subshell [40]. The optical spectra of the rare earth doped phosphor are often influenced by the structure of the matrix and synthesis technique. And the variation in yellow–blue–red ratio in ­Dy3+ ions is called as the asymmetry ratio. It varies with respect to in different host lattices and increase in color ratio observed due to the change in the local site symmetry around the D ­ y3+ ion. The ionic radius of D ­ y3+ 2+ (91.2 pm) is slightly smaller than that of S ­ r (112 pm) and that of K ­ + (138  pm) ions respectively. Therefore, most of the ­Dy3+ ions were entered in the host lattice and of few of them were located at the surface. Therefore the occupation of ­Dy3+ ion into ­K+ and ­Sr2+ sites in KSr(PO4) host forms the substantial number of vacant sites in the oxygen ion and then it expand the lattice to decrease crystal field density. Also from the luminescence spectra of ­Ce3+ and ­Dy3+ ions it observed that the PLE spectra of D ­ y3+ and PL 3+ emission spectra of ­Ce ions are partially overlap on each others as shown in Fig. 13, which give direct evidence for the probability of energy transfer from C ­ e3+ → Dy3+ which is the future study of this research. Therefore to study the phosphors for any application, first of all it is important to know the basic luminescence mechanism associated with

Fig. 13  Spectral overlap of PLE of KSr(PO4):Dy3+ and emission spectra of KSr(PO4):Ce3+ phosphor

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phosphor host and doping ions and also the effect of crystallites size on the properties of phosphor.

5 Conclusion In summary, the rare earth activated (RE = Pr3+, ­Tb3+, ­Ce3+, ­Dy3+, ­Ce3+/Pr3+) KSr(PO4) nanocrystalline phosphor has been synthesized at low temperature by using simple wet chemical routes. Phase purity, surface morphology, crystallite size and structure of materials has been characterized by XRD, SEM and HRTEM techniques. Therefore it is confirmed that wet chemical routes is an well suited techniques to prepare spherical nanoparticles. Also the optical and band gap study is carried out from the photoluminescence and diffuse reflectance measurements. From this it is seen that materials shows the excellent luminescence properties in visible to NIR range under UV excitation wavelength due to f–f and d–f transition of rare earth ions. Thus present results shows the mechanism of down conversion by emitting more than one low energy photon after exciting it with high energy photon. Here estimated band gap of KSr(PO4):Ce3+ phosphor is found to be 4.93 eV. Hence it confirmed that prepared phosphate based host may be an promising large band gap materials as compared to other families of phosphor host such as vanadates, tungstate, oxides etc. Therefore from the above investigation it reveals that RE doped KSr(PO4) phosphor may be an useful down conversion materials to increase the light conversion efficiency of silicon solar cell.

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