An Introduction to the Physics and Electrochemistry of Semiconductors

Fundamentals and Applications

Maheshwar Sharon

• Publisher: Wiley
• Year: 2016
• ISBN:
  9781119274353
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This book has been designed as a result of the author's teaching experiences; students in the courses came from various disciplines and it was very difficult to prescribe a suitable textbook, not because there are no books on these topics, but because they are either too exhaustive or very elementary. This book, therefore, includes only relevant topics in the fundamentals of the physics of semiconductors and of electrochemistry needed for understanding the intricacy of the subject of photovoltaic solar cells and photoelectrochemical (PEC) solar cells. The book provides the basic concepts of semiconductors, p: n junctions, PEC solar cells, electrochemistry of semiconductors, and photochromism.

Researchers, engineers and students engaged in researching/teaching PEC cells or knowledge of our sun, its energy, and its distribution to the earth will find essential topics such as the physics of semiconductors, the electrochemistry of semiconductors, p: n junctions, Schottky junctions, the concept of Fermi energy, and photochromism and its industrial applications.

"The topics in this book are explained with clear illustration and indispensable terminology. It covers both fundamental and advanced topics in photoelectrochemistry and I believe that the content presented in this monograph will be a resource in the development of both academic and industrial research".
—Professor Akira Fujishima, President, Tokyo University of Science, and Director, Photocatalysis International Research Center, Tokyo University of Science, Japan

Chapter 1
Our Universe and the Sun

All forms of energy stored inside our planet or available throughout the earths atmosphere are a consequence of processes involved in forming the universe. The sun is the main source of energy for the earth; its energy sustains the evolution and growth of living beings. The ongoing survival of human beings on earth depends on the life of the sun. Therefore, before learning about methods for utilizing solar energy, it is appropriate to describe in brief how the universe, and consequently our solar system, was formed and the factors that control the release of solar energy. The processes by which this energy arrives at our planet also need to be understood.

1.1 Formation of the Universe

Many theories have been put forward to explain the formation of the universe [1]. However, none of these theories can be experimentally confirmed. One theory advocates that the universe was formed as it exists today. George Gamov, conversely, theorized the big bang, which proposes that the universe was originally concentrated into a very small volume and that its temperature was 10¹⁶ K. The radiation pressure in this volume increased so greatly that the universe exploded with a big bang. Masses started to move away from their point of reference. Finally, these masses took the shape of various planets, stars, and other bodies. The big bang is theorized to have occurred approximately 10¹⁰ years ago.

Many stars (on the order of 10¹¹) are believed to exist in our galaxy, the Milky Way, of which our sun is one. The name “Milky Way” is derived from the galaxy’s appearance as a dim, glowing band arching across the night sky in which the naked eye cannot distinguish individual stars. The term is a translation of the Latin via lactea. From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. The diameter of the Milky Way is approximated to be 10⁵ light-years. To appreciate the magnitude of the Milky Way’s diameter, it is worth mentioning that light takes 100,000 years to cross the galaxy from one end to the other. Within a radius of 2 × 10⁶ light-years from the Milky Way are approximately 15 galaxies, and the universe is approximated to contain 1 billion galaxies.

1.2 Formation of Stars

How can the big bang theory explain the formation of these stars? In the process of cosmic dust moving away from the nucleus of the big bang, a star (i.e., a sun) was formed by the mutual gravitational attraction of its constituents. The individual particles condensed from the cloud of cosmic dust and fell together to one central point. The particles’ kinetic energy increased as they moved toward the central point. Owing to this process, the size of the dust cloud started to decrease. The gravitational energy was converted into energy of motion, heating the interior of the star to a very high temperature (of the order of 2 × 10⁷ K). This high temperature initiated a thermonuclear reaction involving the burning of hydrogen. The fusion reaction of hydrogen thus became the source of the energy we experience as solar energy.

An explanation is needed as to why only stars accumulated hydrogen gas so that the fusion of hydrogen could release energy. According to the Vedas of Indian scripture, in the beginning of the formation of the universe were speed and sound, which collided to generate positive and negative charges. On collision of these charges, a great sound was made, which we know as the sound of om. This sound is like what we call the big bang, through which the universe was formed. It is also believed that the entire universe is moving toward one focal point. This means that when all materials meet at their destination point in space, there will be another big bang. Furthermore, according to the Vedas, there have been several collisions of the universe. This means that the universe has been formed several times and destroyed several times. Hindu philosophy also puts forth so-called satyug (when people believed in honesty), treta, dwyeta and kalyug. Kalyug will be an era when people will be very dishonest and the world will be destroyed, and when yug will start afresh. This comparison also suggests that there is great similarity between Western and Hindu philosophy.

1.2.1 Formation of Energy in the Sun

Now we shall discuss the formation and release of energy from the sun. The energy released within the core of the sun is a consequence of nuclear fusion. From the atomic weights of a proton and a neutron, it is possible to calculate theoretically the weight of any atom. For example, ₁H² contains one proton and one neutron, so the weight of ₁H² (Deuterium) should be the sum of the weight of one proton and one neutron, that is,

But the actual weight of ₁H² is 2.01355 amu, which is lower than the added weight of one proton and one neutron by 0.0024 amu—this is known as excess mass. This weight (i.e., 0.0024 amu) is converted into energy (931 × 0.0024 = 2.23 MeV) to keep the proton and neutron together, which is known as the binding energy. The factor 931 is used to convert amu into MeV units. The binding energy per nucleon becomes 2.23/2 = 1.1172 MeV. In other words, this is the energy used up to keep hydrogen in its stable form. If this exercise is carried out with all elements of the periodic table and a graph is plotted between the binding energy per nucleon and the corresponding atomic number, one obtains a graph such as that shown in Figure 1.1.

Figure 1.1 Variation of binding energy ...

Table of contents

• Cover
• Title page
• Copyright page
• Dedication
• Foreword
• Preface
• Chapter 1: Our Universe and the Sun
• Chapter 2: Solar Energy and Its Applications
• Chapter 3: Theory of Junction Formation
• Chapter 4: Effect of Illumination of a PEC Cell
• Chapter 5: Electrochemistry of the Metal–Electrolyte Interface
• Chapter 6: Electrochemistry of the Semiconductor–Electrolyte Interface
• Chapter 7: Impedance Studies
• Chapter 8: Photoelectrochemical Solar Cell
• Chapter 9: Photoelectrochromism
• Chapter 10: Dye-Sensitized Solar Cells
• Index
• End User License Agreement