SYNTHESIS AND SPECTRAL INVESTIGATIONS OF RARE EARTH DOPED METASILICATE PHOSPHOR FOR OPTOELECTRONIC APPLICATIONS

Abstract

In the recent era, the usage of conventional light sources such as incandescent and fluorescent bulbs have diminished significantly due to the enormous benefits of solid state lighting (SSL) technology. These conventional sources rely on either heat or discharge of gases and associated with large energy losses due to high temperature and large stokes shifts. Phosphor converted light emitting diodes (pc-LEDs) are strongly acknowledged due to their unprecedented features which include longer lifetime, high efficiency, swift response, affordable, stable and environmentally benign. In general, the pc-LEDs are designed by coating the phosphor on the LED chip and produce the emission output in the desirable spectral region upon specific excitation. On the basis of previously mentioned advantages, SSL based pc-LEDs have emerged as a long-term solution for unceasing advancement of human civilization and will bring about a revolutionary shift in the lighting sector. Especially, white light emitting pc-LEDs (or w-LEDs) have been the most endorsed SSL devices for various indoor and outdoor lighting applications. Moreover, the phosphor based w-LEDs can be fabricated via layering a mixture of red (R), green (G) and blue (B) phosphors over the UV LED chip and coating a yellow emitting phosphor on blue LED chip. However, the former approach lead to inadequacy of reabsorption of blue color by red and green phosphor while later one reflects white light with deteriorated color rendering index (CRI), high correlated color temperature (CCT) and low color saturation due to lack of red color. Thus, the above-mentioned flaws can be eliminated by developing single phase phosphor activated with the combination of bi- or trivalent rare earth ions to produce efficient white light via energy transfer mechanism with improved luminous efficiency, significant CCT and CRI values. Moreover, phosphor materials possess noteworthy qualities to be employed in versatile applications. One of the most crucial and promising applications is solar cell, which rely on abundant and renewable solar energy to get an electricity. In solar cells, the contribution of phosphor materials has shown prodigious potential to substitute other non-renewable sources of energy. In general, the solar cell efficiency has been limited due to the spectral mismatch between vi the incident solar spectrum and spectral response of the solar cell, which mainly arises due to two reasons; (i) transmittance loss and (ii) thermalization loss. Although, these losses can be exceeded by the use of phosphor material as a conversion layer to improve the conversion efficiency of the solar cell. The RE doped phosphor materials are frequently sought for the purpose of transformation of light by means of photon down-conversion (DC) approach. Thus, illuminating high brightness, multimodal spectral region, effective color purity and thermal stability are the implausible features of phosphor materials which encourages them to utilize in solar cell applications to improve the luminous and conversion efficiency, respectively. In general, phosphor materials are essentially associated with the host lattice (inorganic crystal) and an activator ion (luminescent centre) possess excellent chemical, physical, thermal and luminescent properties. Usually, most inorganic hosts are chosen as an effective host lattice since the contribution of effective host is to provide proficient luminescent properties. Among inorganic hosts, silicates have been the preferential choice as they possess good physical, thermal, chemical stability with wide band gap and low thermal expansion coefficient. Also, silicates can be synthesized via physical, physico-chemical and chemical routes at significantly low temperature using economical precursors. Recently, RE doped silicates have been speculated as the most promising contender for w-LED and solar cell applications as they provide proficient luminescent characteristics due to probability of f-f transitions in RE ions. By considering above factual advantages, silicates especially metasilicate (Na4Ca4Si6O18) phosphors doped with specific RE ions have been studied for w-LEDs and solar cell applications. Indeed, the incorporation of suitable RE ions into host (Na4Ca4Si6O18) has yielded the favourable results for both proposed applications. Based on the detailed analysis of thermal, structural, morphological and luminescent properties, the outcomes of the research undertaken to achieve research objectives has been organized in various chapters. The brief summary of each chapter is outlined below: Chapter 1 preferably summarize the introduction of basic terminology, brief history and recent development, challenges and influential opportunities in w-LEDs and solar cell vii applications. This chapter focused on the fundamental concepts of luminescence and their classifications. The luminescence mechanisms appear in the RE ions and luminescent properties of RE ions have been described in detail. The chapter outlines the brief explanation about the targeted applications, challenges, prerequisites and alternative strategies to improve the materials quality. This chapter also describes various parameters such as chromaticity coordinates, CCT, color purity and activation energy, which are necessarily required to analyse the luminescence performance of the synthesized phosphor. Moreover, importance of the selection of host and activator ions in the current research work have been discussed. Finally, the objectives of the thesis have been framed on the basis of literature review. Chapter 2 makes a comprehensive discussion on the synthesis technique chosen to synthesize single phase sodium calcium silicate (Na4Ca4Si6O18: NCMS) phosphors activated with distinct as well as combination of rare earth ions (Tb3+, Pr3+, Dy3+, Eu3+ and Sm3+). In this chapter, experimental procedure and merits of adopted synthesis approach have been explained in detail. This chapter elucidates the characterization techniques used to investigate thermal, structural, morphological, optical and photoluminescent features of NCMS phosphor along with their principle, instrumentation, formulation and graphical data analysis. Moreover, thermal, structural and morphological studies have been accomplished via thermogravimetric analysis (TGA), x-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) techniques. Moreover, optical properties have been analysed through diffuse reflectance spectral (DRS) measurement. Excitation, emission, chromaticity coordinates, color purity and decay studies associated with photoluminescent (PL) properties of NCMS phosphors have been executed through spectrofluorophotometer. Chapter 3 emphasizes on the green emitting Tb3+ activated Na4Ca4Si6O18 phosphor synthesized via solid state reaction method. XRD patterns identified the phase and confirmed the phase purity by comparing the diffraction patterns with standard JCPDS card no.: 75-1687 for NCMS compound. The morphology and size of the particles have been illustrated with FE-SEM viii micrographs. PLE spectrum of Tb3+ doped Na4Ca4Si6O18 phosphor depicts the strong excitation peak obtained in UV spectral region. The trivalent terbium activated NCMS phosphors excited under UV region (λex=232 nm) exhibit intense emission in the blue (350-470 nm) and green (470- 650 nm) spectral regions. With increasing the concentration of Tb3+ ions in the host matrix, the emission color shifts from blue to green region due to cross relaxation (CR) mechanism and shows the tunable behaviour of Tb3+ activated as-synthesized NCMS phosphor. The aforementioned results manifest that Tb3+ activated sodium calcium metasilicate phosphor has an immense potential to contribute as a green and blue-green emitting component in lighting and display device applications. This research work has been published in Luminescence, 37 (2022) 1465 (Impact Factor: 2.90) Chapter 4 describes about the synthesis, characterizations and the outcome analysis of Pr3+ activated NCMS phosphor. Microcrystalline pure phase praseodymium doped Na4Ca4Si6O18 (NCMS: Pr3+) phosphors have been synthesized and systematically characterized for implementation in w-LED applications. In this chapter, high temperature solid state reaction methodology has been opted to synthesize a series of NCMS: xPr3+ (x= 0.0, 1.0, 2.0, 3.0 and 4.0 mol%) phosphor. X-ray diffraction patterns confirmed the phase purity and crystallinity of the as synthesized phosphor via comparing all the diffraction peaks with the standard pattern. Luminescent studies have been carried out in n-UV and visible regions to illustrate the excitation and emission spectra of NCMS: Pr3+ phosphor. PL spectrum exhibits the radiative emission at 611 nm under the most intense excitation peak at 480 nm for 1.0 mol% of Pr3+ doped NCMS phosphors. The optimized concentration of the dopant ion has been achieved by following the energy transfer based concentration quenching mechanism and found to be 1.0 mol% for NCMS crystal. The chromaticity diagram represents the integrated emission color of the optimized NCMS:1.0 mol% Pr3+ phosphor falls in the red region with a color purity of 97.0% when stimulated with the blue light. Temperature dependent luminescence studies are evident that the thermal stability of the prepared phosphor is quite high when utilized under the operating temperature of LEDs. Thus, the ix investigated outcomes encourage that the promising red-emitting NCMS phosphor can be utilized in w-LED applications. This research work has accepted in the Bulletin of Material Science (Accepted: In Press, 2023) (Impact Factor: 1.80) Chapter 5 demonstrates the structural and spectroscopic analysis of thermally stable yellow emitting component and its usage in optoelectronic applications. In modern days, researchers are focused into developing the strategies for production of white light having high quantum efficiency, better performance, and high CRI. Recently, high quality white light has been achieved via adopting fundamental phosphor-based approaches in lighting industries. The white light can be generated by stimulating the yellow emitting phosphor with blue LED excitation. Thus, Dy3+ activated phosphors have great potential to contribute as thermally stable yellow emitting phosphor in the w-LED applications. In this chapter, a series of Dy3+ doped sodium calcium metasilicate (Na4Ca4Si6O18) phosphors have been synthesized by adopting the conventional solid-state reaction route. Phase identification has been carried out through XRD technique. Under near-UV excitation, PL spectra exhibit two characteristic bands with blue and yellow colour emitting light. The concentration quenching was achieved beyond the 5.0 mol% of Dy3+ ion in NCMS host lattice. The temperature-dependent PL studies display that the as-synthesized phosphor has high thermal stability. All investigations listed above demonstrate the tremendous potentiality of yellow emitting NCMS phosphor for optoelectronic device applications. This research work has been published in Journal of Materials Science: Materials in Electronics 33 (2022) 19218 (Impact Factor: 2.80) Chapter 6 proposes the description of energy transfer induced color tunability behaviour of bi-activated NCMS phosphor for luminescent device applications. As it is commonly known that commercial w-LEDs are fabricated by coating a yellow emitting YAG: Ce3+ (Ce3+ ion doped yttrium aluminium garnet) phosphor with reasonably broad spectrum on blue emitting InGaN LED chip. One such fabricated w-LED shows deprived value of color rendering index and correlated x colour temperature caused by the dearth of red colour emitting component. In order to achieve sufficient CRI and lower CCT with improved luminous stability, researchers and scientists have to delve into an alternative approach with the suitable amalgamation of RGB (red: R, green: G and blue: B) mono phase phosphor. Solid state reaction methodology has been adopted to synthesize Dy3+/Eu3+ co-activated thermally persistent Na4Ca4Si6O18 (NCMS) phosphors and investigated their structural, morphological and luminescent characteristics. In PL studies, as-synthesized NCMS phosphors co-activated with Dy3+/Eu3+ ions have been excited with near-ultraviolet light (λex=348 nm) and shows the utmost energy transfer up to 97.80% from sensitizer to activator. Dy3+ activated NCMS phosphor shows the illumination shift from yellow to red region with varying the Eu3+ ions concentration and also observed colour tunability by altering the excitation energy. Thus, Dy3+/Eu3+ co-doped NCMS phosphors embrace tremendous thermally stable behaviour with flexible color tunability to emerge as a promising contender for w-LED applications. This research work has been published in RSC Advances 13 (2023) 21105 (Impact Factor: 3.90) Chapter 7 discusses on the effect of sensitizer on the luminescence of Eu3+ activated metasilicate phosphor for solar cell applications. Solar energy has been ascended as a leading renewable energy source to accomplish the massive demand through amalgamating this energy source into technology. The luminescent materials especially downconversion/downshifting (DC/DS) materials have the immense potential to be utilized as an efficient energy conversion source, which absorbs energy in UV/n-UV region and releases energy in the visible region (without heat dissipation). Thus, down-converting features of thermally stable NCMS: Sm3+/Eu3+ phosphors are highly preferred for solar cells. Conventional solid state reaction methodology has been opted to synthesize microcrystalline pure phase Sm3+/Eu3+ co-doped Na4Ca4Si6O18 (NCMS) phosphors. Pure phase has been obtained and confirmed via x-ray diffraction technique. Photoluminescence studies of Sm3+ activated and Sm3+/Eu3+ co-activated NCMS phosphors have been thoroughly investigated and discussed. The energy transfer mechanism in NCMS: Sm3+/Eu3+ phosphors have been studied using PL and lifetime data to facilitate the emission in red region xi under n-UV excitation. The chromaticity coordinates of NCMS: Sm3+/Eu3+ phosphors exhibit the color tunability from orange to red region excited with n-UV/blue light as the concentration of activator ion increases. Moreover, enhanced luminescence and superior thermal stability of Sm3+/Eu3+ co-activated NCMS phosphor demonstrates the viability of the phosphor to be utilized in solar cell applications. This research work has been published in Journal of Materials Science: Materials in Electronics 34 (2023) 1999. (Impact Factor: 2.80) Chapter 8 explicates the summary of the results obtained in the chapters from 3 to 7 and also highlights the scope of the thesis for future applications.