Phosphors for Energy Saving and Conversion Technology

Vijay B. Pawade, Sanjay J. Dhoble

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781138598171
• eBook ISBN: 9780429943225
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

This text deals with the advantages of rare earth activated phosphors for the development of solid state lighting technology and in enhancing the light conversion efficiency of Si solar cells. The book initiates with a short overview of the atomic and semiconductor theory followed by introduction to phosphor, its working mechanism, role of rare earth ions in the lighting and PV devices and host materials being used. Further, it introduces the applications of inorganic phosphor for the development of green energy and technology including advantages of UP/DC conversion phosphor layers in the enhancing the cell response of PV devices.

Key Features:

• Focuses on discussion of phosphors for both solid state lighting and photovoltaics applications
• Provides introduction for practical applications including synthesis and characterization of phosphor materials
• Includes broad, in-depth introduction of semiconductors and related theory
• Enhances the basic understanding of optical properties for rare earth phosphors
• Covers up-conversion and down-conversion phosphor for energy harvesting applications

Part I

Fundamentals of Atoms and Semiconductors

1

Short Review of Atomic and Semiconductor Theory

1.1 Atomic Theory

In the sciences, especially chemistry, physics, materials science, and electronics, atomic theory is a scientific theory that provides basic knowledge about matter, which is composed of tiny units called atoms.

This section gives a short description of the atom. We know that the atom may be considered as the smallest unit of matter that can have different chemical and physical properties [1]. Matter is composed of atoms and molecules. It has four different states: solid, liquid, gas, and plasma, which is the fourth state of matter and consists of neutral and ionized atoms. The typical size of an atom is very small, approximately 100 pm [2, 3]. The atom has no definite size or boundaries, but there are different ways to define the size of an atom. The behavior of a small atom is well understood using the concepts of classical physics. Quantum theory has played an important role in the development of atomic theory and models, and it is also applicable to exploring and predicting the nature of an atom. In basic science, the word atom is a philosophical concept from ancient Greece, and it was discovered in the nineteenth century in the field of modern chemistry that matter is indeed composed of atoms. Chemists used the word atom in connection with the study of new chemical elements. Later, in the twentieth century, various experiments were carried out based on electromagnetism and radioactivity, for example. Physicists explained that an atom is nothing but a combination of various subatomic particles, which can exist separately from each other. The atomic structure consists of a nucleus at the center, formed by the combination of protons and neutrons, also called nucleons, bonded with one or more electrons around the nucleus of the atom. According to the literature, the nucleus makes up 99.94% of the mass of an atom. Protons, which lie inside the nucleus, carry a positive charge, and electrons have a negative charge, such that the atom is electrically neutral. Atoms can be attached to each other by chemical bonds to form a compound. The ability for an atom to associate or dissociate is responsible for the physical changes observed in nature, so it is directly linked with the branch of chemistry.

1.1.1 Short Overview of Atomic Model and Theory

1.1.1.1 Dalton’s Atomic Theory

The existence of atoms was first suggested by Democritus, but it took almost two millennia to explain it. However, it gained a foothold with the assumption of an atom as a fundamental object, explained by John Dalton (1766–1844).

Two decades later, Dalton’s theory came into existence during the development of modern chemistry. Dalton carried out a most important investigation into the theory of atoms based on chemistry. Hence, the theory was named after Dalton, but its origin was not fully understood [4]. This theory was proposed when Dalton was researching ethylene, methane and analyzing nitrous oxide and NO₂ under the direction of Thomas Thomson [5, 6]. After that he explained the law of multiple proportions based on the idea that the interactions of atoms consist of a chemical combination of definite and characteristic weight [7]. So, while during the study of the properties of atmospheric gases an idea of the atom was in Dalton’s mind, it was nothing more than a simple physical concept. He published his idea in 1805. He asked: “Why does not water admit its bulk of every kind of gas alike? This question I have duly considered and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases.” Some of the important points discussed by Dalton are as follows:

1. An element is made up of extremely small particles called atoms.

2. They are identical in mass, size, and properties. Also, an atom of a different element has different mass, size, and properties.

3. An atom can be neither created nor destroyed.

4. Chemical compounds are formed by the combination of atoms of different elements.

5. Atoms can be combined, separated or rearranged using a chemical reaction.

1.1.1.2 Rutherford Model

This model was discovered by Ernest Rutherford in 1909. He concluded that the plum pudding model of the atom discovered by J. J. Thomson was not correct. Thus, in 1911, Rutherford worked on the Thomson model [8], and with the help of a gold foil experiment, stated that the atom has a small and heavy nucleus. Later, he designed an experiment in which he used α-particles emitted from a radioactive substance as a probe to investigate the unseen atomic structure. According to this experiment, he predicted that if the beam of emitted α-particles was bombarded and passed in a straight line through the gold foil, then the Thomson model would be correct. But it was found that although most of the radioactive α-rays passed through the gold foil, some of the rays were deflected. Therefore, to give an interpretation of the unexpected experimental result, Rutherford applied his own physical model to the study of the subatomic structure of an atom. In this model, the atom is made up of a central charged nucleus surrounded by an orbiting electron [9]. Rutherford’s model deals with charge and the atomic mass of an atom within a very small core but says nothing about the structure of the remaining electron and atomic mass.

The key points of Rutherford’s model are as follows:

• An electron moving around the nucleus does not influence the scattering of α particles.

• In many atoms, the positive charge is situated at the center of the atom in a relatively small volume called a nucleus. The magnitude of charge and mass are proportional to each other. Therefore, the concentrated central mass and charge of an atom cause the deflection of α and β particles.

• Elements of high atomic mass do not deflect high-speed α particles, which carry high momentum compared with electrons.

• The nucleus is about 10⁵ times smaller than the diameter of the atom. This is like putting a grain of sand in the center of a football [10, 11].

1.1.1.3 Bohr Model

In atomic theory, the Rutherford–Bohr model, also called the Bohr atomic model, was discovered in 1913. Bohr predicted that the atom has a small, positively charged nucleus and the electrons travel in a circular orbit around it, similarly to a solar system. This is due to the presence of electrostatic and gravitational forces of attraction. The Rutherford model was further improved using the quantum physical interpretation of the atom. The modern mechanical model of the atom has been developed following the Bohr atomic model. According to the laws of mechanics, an electron revolving around the nucleus should release electromagnetic radiation, and due to loss of energy from the electron, finally, it will spiral inward toward the nucleus. Hence, as the orbit becomes smaller and faster, the frequency of radiation should increase with the emission of radiation. Therefore, it would release continuous electromagnetic radiation. However, in the nineteenth century, an experiment was performed with electric discharge showing that atoms emit electromagnetic radiation at certain frequencies.

Therefore in 1913, Bohr proposed a different model, known ...

Table of contents

• Cover
• Half-Title
• Title
• Copyright
• Contents
• Preface
• About the Authors
• Part I Fundamentals of Atoms and Semiconductors
• Part II Phosphors—An Overview
• Part III Roles of Phosphors
• Part IV Efficient and Eco-Friendly Technology
• Index