COLOR TUNABLE AND ENERGY TRANSFER STUDIES IN SINGLE PHASE RARE EARTH DOPED CALCIUM BISMUTH ORTHOPHOSPHATE PHOSPHOR FOR ENERGY EFFICIENT OPTOELECTRONIC APPLICATIONS

Abstract

In the current scenario, successively increasing global electricity demand, consumption and energy saving strategies are prime subjects captivating ample attention. This electricity demand and consumption can resolve by opting two practical approaches, i.e., by energy-saving strategy and enhancing the power generation efficiency of green renewable energy sources. One of the important practical approaches is advanced artificial lighting sources based on phosphor converted white light emitting diodes (pc-WLEDs), which reduce the world’s energy consumption. The pc-WLEDs have been considered as a next-generation indoor and outdoor light sources. All over the place around the globe, artificial lighting has inclusive influences on environmental surroundings, biotic societies, and human development as they have superior advantages such as high efficiency, environmental friendly, huge amount of energy saving, compact size as compared to fluorescent and outdoor illumination devices. The second practical approach is solar cells, one of the potential green renewable technologies to accomplish the worldwide energy crises, which could possibly reduce the dependency on traditional fossil fuel based power generation. As the solar spectrum is an abundant and valuable green renewable energy resource that is freely accessible in every part of the earth. These two practical approaches have some shortcomings such as commercially available pc-WLEDs illuminate the bluish-white light with high correlated color temperature due to the deficiency of the red color component with less thermal stability and the silicon based solar cell operated with limited conversion efficiency as most of the spectrum was unexploited, i.e., spectral mismatch. Several scientists and researchers have been viii uninterruptedly working on this area and try to formulate methods to overcome the shortcomings in pc-WLEDs and enhance solar cell energy conversion efficiency by utilizing the unused part of the solar spectrum. The commercially available sulfide and nitride-based near-ultraviolet/blue pumped red phosphors have some disadvantages, such as low chemical stability and efficiency compared with other primary colors. Furthermore, phosphors used in lighting devices require high thermal stability as the UV/blue LED chips generate temperature around 120 to 150 °C while operating time, which affects the luminescent properties of phosphor and degrades emission intensity as well as the performance of the pc-WLEDs. Phosphors based devices have emerged as an alternative to conventional energy sources. They offer to save huge amounts of electrical energy and reduce carbon emissions globally. Furthermore, phosphors have been found in an extensive range of applications in our daily life, including solid-state lighting, advanced optical displays, optical waveguides, solar cells and many more. As a result, tremendous effort has been made to date to investigate novel phosphor materials. The desire for new phosphor materials has driven scientific and technological attempts to enhance luminous materials' current properties. The current research work is mainly focused on the color tunable and energy transfer studies in single-phase rare earth-doped calcium bismuth orthophosphate phosphor for energy-efficient optoelectronic applications. Rare earth-doped orthophosphate phosphors have attracted much attention for light emitting diode and display applications. The eulytite type orthophosphate with the general formula A3B(PO4)3 (A = divalent alkaline earth & B = trivalent Bi or RE ion) has gained attention due to their excellent optical, magnetic, and dielectric property, along with thermal and chemical stability. Among all the eulytite structures, calcium-based eulytite type orthophosphate [Ca3Bi(PO4)3] phosphor can be synthesized at a moderate temperature. Eulytite-type materials doped with rare earth ion exhibit promising luminescence properties due to the disordered ix structure. Based on the literature survey, it was recognized that eulytite-type materials may perform as significant host matrices for efficient optoelectronic applications. The current research work has been deeply focused on the synthesis of strategic luminescent materials to explore their morphology, structural optical characterizations and enhancement of luminescent properties for optoelectronic applications. Therefore, the main objectives of the present proposal are to produce the most efficient phosphor with improved luminescent properties by scaling up the process for comfortable industrial acclimatization. The results of the research effort have been divided into seven chapters in order to achieve the research goals of the current work. Here is a quick overview of each chapter: Research usually initiates with self-motivation and recognition of a contemporary problem and then followed by strategies to perform and prove the methodology for resolving it. Here, chapter 1 states the motivation, outline of the problem, literature survey and origin of research objectives for the development of efficient visible light emitting phosphors. It includes the basics of photoluminescence and comprehensive studies about luminescent crystalline materials (phosphors). Furthermore, the role of phosphor in the advancement of smart optoelectronic devices has been discussed. Finally, every step to accomplish the present research work, such as the selection of the host material (Ca3Bi(PO4)3), activator, sensitizer and optimization of emission and thermal stability for direct utility in a variety of optoelectronic devices such as lighting, solar cell and optical sensors have been included in this chapter. Chapter 2 focuses on the experimental work and characterization techniques required for the preparation of an efficient phosphor with excellent PL characteristics. The preparation of efficient multifunctional material needs appropriate acquaintance of numerous synthesis procedures along with the proper characterization techniques for the potential utility of the selected RE ions activated Ca3Bi(PO4)3 phosphor in optoelectronic devices. The characteristics x of RE ions doped phosphors were scrutinized by employing the numerous characterization techniques used to explore their structural, vibrational, morphological and spectroscopic characteristics for their utility in advanced optoelectronic devices. To understand the instruments better, working principles, device descriptions, and operational controls of characterization tools used in the present research work are described in the current chapter. Chapter 3 covers the luminescence properties of eulytite-type crystalline structure of Eu3+ activated Ca3Bi(PO4)3 phosphors synthesized via a solid-state reaction method in an ambient atmosphere. The diffraction pattern of the synthesized phosphor confirmed the formation of pure and single-phase crystalline with a cubic structure of Ca3Bi(PO4)3 microparticles. The SEM image of Ca3Bi(PO4)3 illustrates the growth of heterogeneous microstructures with some agglomeration. The Ca3Bi(PO4)3 host shows the broad emission peak at 434 nm (blue region) under the excitation wavelength of 326 nm ascribing to 3P1→1S0 electronic transition of Bi3+ ions. The Eu3+ activated Ca3Bi(PO4)3 phosphors exhibit an intense red emission band centered at λem = 612 nm (5D0 →7F2) at excitation wavelengths of 393 & 465 nm and perceived that the optimized Eu3+ ion concentration is 8.0 mol%. The host blue emission intensity diminished with increasing Eu3+ engagement, whereas the intensity enhanced for the characteristic peaks of Eu3+ ions are located in the 550-725 nm range under the host excitation wavelength (λex = 326 nm). This suggests that part of the host emission energy was transferred to the activator when the host was doped with Eu3+ activator ions. The CIE color coordinates for the Ca3Bi(PO4)3 host lie in the blue region, which has been modulated towards the red part with increasing Eu3+ ions concentration. However, the CIE coordinate values for Eu3+ doped Ca3Bi(PO4)3 phosphor fall in the red region at λex = 393 & 465 nm with high color purity. The average decay time of the optimized phosphor was in the range of milliseconds at λex = 393 nm λem = 612 nm. The PL intensity persists up to 75.45% at 200 °C of ambient temperature, xi assuring the excellent thermal stability of phosphor. The combination of the above-revealed results recommends that Ca3Bi(PO4)3: Eu3+ phosphor can be a probable contestant in near UV/blue excited luminescent devices. [Part of this work has been published in the Journal of Luminescence 227 (2020) 117570] (IF: 3.6) Chapter 4 depicts the structural, vibrational, morphological luminescent properties of white light emitting dysprosium doped Ca3Bi(PO4)3 phosphor for solid-state lighting applications. The X-ray diffraction (XRD) and structural refinement studies reveal that the synthesized phosphors consist of a single phase with a cubic structure. The field emission scanning electron microscopy images reveal that the as-synthesized phosphor has a micron size with an irregular shape. Under near ultraviolet (n-UV) and blue excitation, the phosphor exhibits white light emission via a combination of blue (~451 nm) and yellow (~575 nm) emission bands. The optimized concentration of Dy3+ ions is 6.0 mol%, after which the concertation quenching occurs. The energy transfer process between Dy3+ ions is due to dipole dipole interaction, which was confirmed by applying Dexter and Reisfeld’s Energy Transfer (ET) formula. The CIE chromaticity coordinates for the optimized phosphor were (0.329, 0.377), which lie in the white light region. The emission intensity remains at 83.41% at 373 K to that at room temperature, which indicates good thermal stability. The results mentioned above demonstrate that Ca3Bi(PO4)3 is a potential phosphor for solid-state lighting applications. [Part of this work has been published in the Journal of the American Ceramic Society 102 (2019) 6087-6099] (IF: 4.186) Chapter 5 describes about the numerous characteristics of eulytite type Ca3Bi(PO4)3 (CBP) phosphors activated with Sm3+ and co-activated with Sm3+ & Eu3+ ions for solar cell, pc-w-LEDs and photonic devices. Phase and crystal structure have been recognized using X ray diffraction analysis and Rietveld refinement. The Sm3+ doped CBP phosphor exhibit three xii emission peaks at 562, 598 and 644 nm under 400 nm excitation and the highest emission intensity has been observed for 5.0 mol% Sm3+ doped CBP phosphor. The co-doping of Sm3+ and Eu3+ ions in CBP phosphor has expanded the excitation as well as emission spectra. The ET between sensitizer ions (Sm3+) to activator ions (Eu3+) in CBP phosphor has been studied based on the photoluminescence (PL) and decay curves using the Dexter ET formula and Reisfeld's theory. The PL quantum yield was obtained as high as 86.64%, indicating a potential red constituent to use in photonic devices. The CBP:Sm3+/Eu3+ emission intensity persisted at 81.27% at 150 °C compared to the emission intensity at room temperature, confirming the excellent thermal stability. Moreover, the color coordinates and decay curves have enhanced the suitability of CBP:Sm3+, Eu3+ for accomplishing spectral conversion in solar cells, pc-w LEDs and other photonic devices. [Part of this work has been published in the Journal of Materials Science: Materials in Electronics, 33 (2022) 5201-5213] (IF: 2.8) Chapter 6 focuses on the structural, optical, morphological, luminescent characteristics of the CBP:Er3+ phosphors for optical thermometry and optoelectronic applications. The diffraction patterns of the samples are matched well with the standard data of single-phase cubic CBP structure and did not change after doping. The reflectance spectrum shows the various absorption peaks owing to Er3+ ions. The field emission scanning electron microscopy images disclose that the sample particles have asymmetrical spherical particles with some aggregation. Elemental analysis and mapping confirm the existence of various compositional elements. The PL spectra specify that the phosphor can be effectively excited by near-ultraviolet (n-UV) LED, which was well-observed from the excitation spectrum. As synthesized phosphor emits the visible emission at 523 (2H11/2→ 4 I15/2), 552 (4S3/2→ 4 I15/2) and 662 nm (4F9/2→ 4 I15/2) under n UV excitation (376 nm). The calculated color coordinates reveal the green emission from the phosphor under 376 nm excitation with high purity. PL variation and fluorescence intensity xiii ratio from the thermally coupled 4S3/2 and 2H11/2 levels of Er3+ ions can be utilized to explore applications in optical thermometry and found good thermal sensitivity in the range of 300-473 K. Hence, the mentioned structural and optical characteristics of the prepared CBP: xEr3+ phosphors will find fascinating applicability in optical thermometry and other optoelectronic devices. [Part of this work has been published in Solid State Sciences, 131 (2022) 106956] (IF: 3.5) Chapter 7 summarizes the outcomes of all the working research chapters from 3 to 6. The revolutionary scope of the research work has also been explained based on the probable optoelectronic device applications of these phosphors in numerous fields of science and technology.