AU2021102696A4 - A solar cell efficiency enhancement by downshifting layer of kalf4:dy3+, eu3+ co-activated downconversion phosphor as spectral converters - Google Patents

Abstract

A SOLAR CELL EFFICIENCY ENHANCEMENT BY DOWNSHIFTING LAYER OF KALF4:DY 3+, EU3+ CO-ACTIVATED DOWNCONVERSION PHOSPHOR AS SPECTRAL CONVERTERS The present invention relates to asolar cell efficiency enhancement by downshifting layer of KALF 4:DY 3 , EU 3 co-activated downconversion phosphor as spectral converters. In present invention rare earth activated fluoride phosphors have attention in recent years in the field of solid-state lighting and solar cell efficiency enhancement. In the present study, RE (RE = Dy Eu3+) activated/ co-activated KAlF4 phosphor has been synthesized by low temperature wet chemical method. Photoluminescence (PL) properties of this sample were investigated by RF 5301 PC Spectrofluorophotometer. The structural feature of the samples was analyzed by X-ray Diffraction (XRD), which showed that the samples were crystallized in a well-known structure.Photoluminescence (PL) measurement of KalF4:Dy 3 phosphors we find that they can be efficiently excited at 351 nm and possess dominant emission bands centered at 482 nm(blue) and 576 nm(yellow). From Photoluminescence (PL) measurement of KaF4:Eu phosphors a strong orange emission at the peak wavelength of 594 nm and 616nm were observed which could be attributed to the D-> 7 Fi and Do-> 7 F2 transition in Eu ions. Solar cell efficiency has been checked by I-V characteristics of the coated and blank cell. 1/4 KAF,:Pure [ ICSD#98-00-0285 11 22 33 44 55 66 77 88 20(Degrees) Figure 1 p 7 se-+-YO Bragg Poston 10 20 30 40 50 60 70 80 90 2theta (C) Figure 2

Description

1/4

KAF,:Pure

[ ICSD#98-00-0285

11 22 33 44 55 66 77 88 20(Degrees)

Figure 1

p 7 se-+-YO

Bragg Poston

10 20 30 40 50 60 70 80 90 2theta (C)

Figure 2

A SOLAR CELL EFFICIENCY ENHANCEMENT BY DOWNSHIFTING LAYER OF KALF 4 :DY 3+, EU3+ CO-ACTIVATED DOWNCONVERSION PHOSPHOR AS SPECTRAL CONVERTERS

Technical field of invention

Present invention, in general, relates to the field of solid state of lighting and morespecifically to a solar cell efficiency enhancement by downshifting layer of KALF 4 :DY , EU co activated downconversion phosphor as spectral converterswhich precisely shows more than 37% efficiency under sunlight.

Background of the invention

The background information herein below relates to the present disclosure but is not necessarily prior art.

In recent years, the solar cell efficiency enhancement is the trending research theme in the current time.To accomplish this goal two options can be investigated. The first is to utilize new materials and interaction to improve the electrical reaction of photovoltaic cell, and the subsequent one is to deal with the solar spectrum to coordinate with the incident radiation on the photovoltaic cell with its sensibility. In the first investigation, the refreshed consequences of examination about solar cells efficiencies are consistently stated to build up the condition of specialty of the photovoltaic solar cell innovation.

In the second investigation, the downconversion and upconversion procedures are engaged to transmute photons in the UV region, which are absorbed and thermalized growing the solar cell temperature, and NIR photons, that are not absorbed, to photons with energy close to the band gap of the semiconductor photovoltaics correspondingly. The down/up conversion procedures in compound phosphorsdoped withlanthanides are broadly concentrated in the articles to depict and to improve its luminescence for applications in photonics like white light production, solar cells and optical waveguides. The suitable alternative of the lanthanide ion depends on the application. Such that, Er3 + ions are examined for activated fiber amplifiers, terbium/tulium/holmium ions for white light production, and europium for composites covering solar cells.

Rare earth activated fluoride materials are gaininga lot of consideration owing to diverse potential applications. Therefore, these applications are the use of lanthanide ions to enhance the efficiency of Si solar cells.Tb -Yb 3+ couple activated inorganic phosphors have been broadly examined for solar cell application. The transition unequivocally relies upon the crystal field of the host.

Efficient energy transfer between a sensitizer and a donor can only occur when the emission band of the sensitizer overlap with the excitation band of the donor. For example, in a YAG host the emission band of Ce 3+ appears at a longer wavelength[13], which could not overlap the Tb3 +excitation levels. However, very less times Dy to Eu energy transfer has been reported to enhance the luminescence efficiency as well as applications in solar cell efficiency enhancement. In the present study, Dy3 , Eu activated/ co-activated potassium tetrafluoroaluminate has been reported for the solar cell efficiency enhancement. Proposed phosphor synthesized by sol gel method for the first time, crystallinity and morphological study of the proposed phosphor has been done. All these results evidences that proposed phosphor is suitable candidate for the white light generation and solar cell efficiency enhancement.

Objective of the invention

An objective of the present invention is to attempt to overcome the problems of prior art and provide a solar cell efficiency enhancement by downshifting layer of KALF 4:DY , EU co activated downconversion phosphor as spectral converters.

In the present invention Dy33- Eu3 activated/ co-activated KAlF4phosphors is synthesized by sol-gel method.

It is therefore an object of the invention showscomplete white light emission whose color co ordinates are too close to the ideal white light emission.

These and other objects and characteristics of the present invention will become apparent from the further disclosure to be made in the detailed description given below. Summary of the invention

Accordingly, the following invention provides a solar cell efficiency enhancement by downshifting layer of KALF 4 :DY3 , EUS3co-activated downconversion phosphor as spectral converterswhich precisely shows efficiency more than 37% under sunlight. In present invention a series of Dy3 , Eu3 doped/co-doped potassium tetrafluoro aluminate phosphor has been synthesized by sol gel method. All the starting materials were weighed in the stochiometric ratio. Here Polyethylene glycol and citric acid were used as a fuel. Transparent solutions of the stoichiometric amounts of precursors were prepared separately and then mixed one by one at regular intervals. The solution was kept on a hot plate and stirred using a magnetic needle at a medium pace to avoid the solution from spilling out of the beaker. The temperature of the hot plate was set to 50 °C. After 30 min of stirring, a citric acid solution was added to the main solution and this was followed by the addition of a PEG solution. The proportion of the metal ions, citric acid and PEG was maintained as 1:1.5:1, respectively. Thereafter, the temperature of the hot plate was raised to 100 °C. The solution was continued to stir until the solution transformed into a gel. This gel was then kept in the oven at 120 °C for drying overnight. The dried gel was annealed in an open atmosphere at 650 °C for 4 h set at a heating rate of 1 C/ min.

Brief description of drawing

This invention is described by way of example with reference to the following drawing where,

Figure Ishows a graph of XRD patterns of pure KAF 4phosphor. Figure 2 shows a graph of (XRD profiles derived from the Rietveld refinement of the KAlF 4 phosphor. Figure 3 showscrystal structure of KAlF 4phosphor obtained from the Rietveld refinement. Figure 4 shows SEM analysis of KAlF 4 phosphor. Figure 5 shows a graph of PL excitation spectra of KAlF 4 : X mol % Eu3 phosphor at emission of 591 nm (X=0.05, 0.1, 0.2, 0.5, 1).

Figure 6 shows a graph of PL emission spectra of KAlF 4 : X mol % Eu 3 phosphor at excitation of 395 nm (X=0.05, 0.1, 0.2, 0.5, 1). Figure 7 shows a) Silicon solar cell without coating, b) Silicon solar cell coated with Dy 3 Eu 3 co-activated KAF 4 down-conversion phosphor. Figure 8 shows CIE chromaticity diagram of KAF 4 : 1 % Dy , xEu phosphor.

Detailed description of the invention

Exemplary embodiments theXRD pattern of theKAlF 4hosts phosphor are shown in Fig. 1. It can be perceived from curve that utmost peaks can be indexed to the KAlF 4 phase, which is in good agreement and well matched with standard ICSD card numbered 98-000-0285. When matching the diffractograms, the X-ray diffraction pattern of the sample confirms a single phase isotopic with KAlF 4. It can be established that the obtained phases are iso-structure. Due to approximation in ionic radii. To validate the statement, crystal structure of KAlF 4 phosphor were refined using Rietveld refinement method. FullProf suite was used to refine the structure using the Pseudo-Voigt peak profile function and linear interruption of the background. Fig 2 demonstrates the observed, calculated Powder XRD profiles resulted from the Rietveld refinement KAlF 4 phosphor and their difference profile along with the Bragg positions. The consistency factors united with the factors Rwp = 14.34, Rp = 7.58, Rexp = 9.81 and X2 = 1.39, thereby, fulfilling the manifestation conditions. The refinement analysis verifies that KAlF 4 phosphor crystallizes into Monoclinicstructure with space group P 21/m and lattice parameters, a = 7.340A b = 7.237 A, c = 6.407 A and a = 90.00°'p= 106.8010, y = 90.00. The volume of the unit cell is 325.8241A 3 .

Table 1 demonstrates the structural parameters of the KAlF 4 phosphor obtained from the Rietveld refinement. Fig. 3 shows crystals structure of KAlF 4 phosphor obtained from Rietveld refinement. The details of position of atoms alongwith their respective occupancies are summarized in Table 1 Table 1: Position of atoms alongwith their respective occupancies.

x Y z Occ. B Site Sym. 1 Al All 0 0 0 1 1 2a -1

2 Al Al2 0.5 0 0 1 1 2b -1 3 K KI 0.131 0.25 0.541 1 1 2e m 4 K K2 0.633 0.25 0.544 1 1 2e m

F Fl 0.2581 0.0205 0.0077 1 1 4f 1 6 F F2 0.029 0.25 0.051 1 1 2e m 7 F F3 0.4874 0.25 0.044 1 1 2e m 8 F F4 0.0712 -0.037 0.2836 1 1 4f 1 9 F F5 0.5738 0.0361 0.284 1 1 4f 1

In present invention morphology of KAlF 4 phosphor was studied using scanning electron microscope (SEM) displayed in fig. 4. It can be seen that proposed phosphor is calcinated at high temperature and shows porous nature. The cause of pores in the phosphor may be the improper allocation of temperature and accumulation flow during synthesis and calcination process. Particle sizes were in the range 1 m to a 3 pm as demonstrated in fig. 4; the irregularity of the particle size was due to non-uniform allocation of temperature during the sintering process. Porous particles were probably created due to emission of gas molecules during synthesis of the sample in the combustion process. Sol gel is a standard method for synthesis of fine powder samples in a comparatively short time.

The excitation spectrum for 614 nm red emission of 1 mol % Eu 2 03 in KAlF 4phosphoris shown in Fig. 5 that exhibitsa several absorptionbands ranging from 250 to 550 nm. The band initially peaking at 258 nmascribedtoO(2p)--Eu 3 chargetransferwhichis the key bandfortheUVexcitationwhenusedasaphosphor. Fine absorptionbandslocatedbeyond350nm are characteristicoftypicalEu 3 4f-4f parity forbiddentransitions.There are four recognizable excitation peaks parking at 258, 395, 466 and 536 nm, respectively, which shields250-550 nm spectral range well, signifying that UV, violet, blue and green laser diodes (LDs)and light-emitting diodes (LEDs) are competent pumping sources in obtaining Eu3 emissions.Moreover in present investigation the detailed analysis is carried out by selecting distinct excitation peaks to study the effect of excitation wavelength on enhancement of luminescence intensity of phosphor. And it has been observed that there is an essential enhancement in emission intensity of phosphor host as described beneath.

The luminescence properties of trivalent europium ion activated phosphors strongly depend on its valence states. Typically, Eu3 ions can give luminescence from orange to red originated from 4f-4f transitions.The PL emission spectrum for above mentioned excitation peaks consists of two identical peaks which is a splendor of this work. The first emission peak is located at 591 nm (orange) corresponding to 5Do-- 7Fi transition of Eu3 3ion and another intense peak is located at 614 nm (red)) which can be associated with the

Do-- 7F 2transition of Eu3+ion.

Fig. 6 presents the emission spectra of KAF 4 :Eu 3+ phosphor excited by 395 nm wavelength. There are groups of sharp lines assigned to the transitions of5 Do- 7Fj (J = 0, 1,2, 3, 4) levels of Eu 3+.The emission at about 580 nm is shallow arise due to the transition5 Do-- 7Fo. The emission around 591 nm initiates from the magnetic dipole transition5 Do-- 7F1 . The emission corresponding to the transition of 5 D0 - 7F 3 , 4 are feeble. The dominated red emission band at 614 nm is ascribed to the electric dipole transition5 Do-- 7F 2, demonstrating that Eu"3is situated at the non-inversion site symmetry.

In common, when the Eu3 ion is situated at the crystallographic site without inversion symmetry, its hypersensitive transition Do- 7F2 red emission will dominate in the emission spectrum. In KAF 4 :Eu 3+ phosphor, the Eu 3 will replace the principal host sitesbecause of their valence state and similar sizes. Consequently, the Eu 3 3is positioned at a site without inversion center. Accordingly, 5 D 7 0 -F 2 red emission band at 614 nm presents the most prominent intensity. With the increasing concentration of Eu3 the luminescence intensity has been also increased. It is fascinating that the luminescence intensity at 0.5 mol % of Eu 3is maximum and beyond that it tends to decreased for KAlF 4phosphors.

Fig.7 shows the PL excitation spectra of KAF 4 :Dy phosphor, monitoring emission wavelength at 484 nm, the excitation bands were observed at 326 nm, 351 nm, 366 nm and 6 6 6 391 nm respectively due to the 6His/2_>6D 1 /2, 6H5/2_> P 7/ 2, 6H5 /2> P 5 /2 and H5 /2> 4 F 7/2

transitions of Dy 3 ions, the stronger bands at 351 nm ( 6HI5/2> 6P7/ 2 ) yields near-white emission that's why we monitored PL emission spectra by this excitation. Fig. 8 shows the characteristic emission spectra of KAlF 4:Dy 3 phosphors, the emission bands peaking at 484 nm and 575 nm respectively, are due to 4F 9/2>6H15 /2 and 4F9/2>6H 13/2 transitions of Dy3 ions. The electronic configuration of Dy 3ions, consists of 4f electron in its outermost shell, when one electron is promoted from ground states of (4f ) to excited states of (45d'), it gives rise to two groups of transitions, one is spin-allowed (SA) which is generally stronger and has higher energies and other is spin-forbidden (SF) transitions.

The emission bands of Dy 3 in blue (484 nm) and yellow (575 nm) are assigned to 4F 9 /2 -6H 1 /52due to magnetic dipole moment corresponding to the hypersensitive transition (AL=2, AJ= 2) and ( 4 F9 / 2 -+ 6H 13 / 2 ) due to electric dipole transition. Thus, preparedKAlF 4 :Dy3 phosphor may have potential application in the production of white light. Fig. 9 is the PLE of KAlF 4 : 0.5 % Eu3 3and the PL of KAlF 4: 1 %Dy 3. As can be seen from the picture that the PLE of KAlF 4 : 0.5 % Eu3 consists of two parts, the strongest and broad band, which results from the 02-Eu3+ charge transfer, the others are the characteristic exciting peaks of Eu 3 +, 395 nm and 466 nm belong to the7 F o- 5 L6 and 7 Fo- 5 D 2 respectively. The right of the picture is PL of KAlF 4 : 1 % Dy*, the obvious bands are from Dy3+. Moreover, it is worth noting that the presence of the overlap between the PLE of KAlF 4: 0.5 % Eu3 and the PL of KAlF 4 : 1

% Dy . According to the rule of Dexter, there may be energy transfer from Dy to Eu .The PL spectra of KAlF 4 : 1 % Dy , xEu 3 with 350 nm excitation are shown in Fig. 10. The fixed content of Dy 33is 1 % and the amount of Eu"3is varied. The emission 484 nm and 575 nm

respectively, are due to 4F 9 /2>6H1 5/2and 4F9 /2>6H 13 /2 transitions of Dy 3 3ions.The KAlF4: 1 % Dy , xEu 3 downshifting phosphor can absorb the photons in the high energy range and convert this energy to lower range which is not absorbed effectively by solar cell results in thermal losses.

A high energy photon having large part of the energy is lost by thermalization of highly excited charge carriers. The KAlF 4 : 1 % Dy , xEu3 spectral converter can be easily reduced these thermalization losses by converting high energy photons into low energy photons which can easily absorb by the solar cell for generation of electron hole pair. The thin layer coating of spectral converter as luminescent downshifting layer can be applied on top of the solar cell which could enhance the efficiency of Si-solar cell. Fig. 11 a, b illustrate a proposed coating of KAlF 4: 1 % Dy3 , xEu33downconversion phosphor on Si-solar cell to enhance the ultimate efficiency of solar cell. Additionally, the proposed spectral converter can also reduce the reflection losses due to its well-known used as antireflection coating. Fig. 12 shows the spectral converter for KAlF 4 : 1 % Dy , xEu 3 phosphor for efficiency enhancement of solar cellThus, the obtained spectroscopy results revels that the KAlF 4: 1 % Dy , xEu phosphor could be a good candidate as spectral converter for next generation solar cells.

The International commission on Illumination (CIE) chromaticity coordinates is calculated on the basis of PL emission spectra of KAF 4 : 1 % Dy , xEu phosphorrepresented in fig.. The value of CIE coordinates are (0.3158, 0.3470) which falls in white region in the CIE 1931 chromaticity diagram. As seen the PL spectra of KAlF 4 : 1 % Dy , xEu 3 phosphors has red and white emission under different excitations. In order to determine the solar cell efficiency in terms of I-V characteristics, experiments were performed on solar simulator as well as in open conditions at 40 0 C of temperature in the department of Physics, RTM Nagpur University, Nagpur at 12:30 pm when sunlight is most intense. The ambient outdoor temperature is measured by simple thermometer. Firstly I-V characteristics of blank silicon solar cell were measured on solar simulator by maintaining its temperature at 40 °C and again same experiments were performed outdoor under the direct sunlight at same temperature. This cell is then coated by Dy33- Eu3 3co-activated KAlF 4downconversion phosphor by doctor blade method using terpineol, ethyl cellulose, ethanol and acetic acid as a combine binder by taking them in a stochiometric ratio. The blank solar cell and solar cell after coating by Dy33- Eu3 3co-activated KAlF4 down-conversion phosphor are displayed in fig. 11.-V characteristics of this coated solar cell are again performed at both the conditions i.e. on solar simulator and under direct sunlight at same temperature. It is observed that solar cell efficiency is enhanced after coating of sample by 35.42 % under solar simulator and 37.73 under direct sunlight. The solar cell efficiency enhancement of both the cells is determined %

for both the conditions by following formula;

Slope of coated cell EfficiencyS% = * 100 Slope of without coatedcell %

Claims (3)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS

1.A series of Dy , Eu doped/co-doped potassium tetrafluoroaluminate phosphor has been synthesized by sol gel method, wherein the polyethylene glycol (stochiometric ratio) and citric acid (stochiometric ratio) were used as a fuel, transparent solutions of the stoichiometric amounts of precursors were prepared separately and then mixed one by one at regular intervals.

2. The KALF 4 :DY 3 +, EU3+ co-activated downconversion phosphor as claimed in claim 1 wherein the solution was kept on a hot plate (50 C) and stirred using a magnetic needle at a medium pace to avoid the solution from spilling out of the beaker; After 30 min of stirring, a citric acid solution was added to the main solution and this was followed by the addition of a PEG solution.

3. The proportion of the metal ions, citric acid and PEG was maintained as 1:1.5:1, respectively; thereafter, the temperature of the hot plate was raised to 100 °C. The solution was continued to stir until the solution transformed into a gel; this gel was then kept in the oven at 120 °C for drying overnight; the dried gel was annealed in an open atmosphere at 650 °C for 4 h set at a heating rate of 1 C/ min.