Advanced Nanomaterials for Solar Cells and Light Emitting Diodes

Feng Gao

• Publisher: Elsevier
• Year: 2019
• Print ISBN:
  9780128136478
• eBook ISBN:
  9780128136485
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Advanced Nanomaterials for Solar Cells and Light Emitting Diodes discusses the importance of nanomaterials as the active layers in solar cells and light emitting diodes (LEDs), along with the progress of nanomaterials as the electron and hole transporting layers. Specifically, the book reviews the use of nano-morphology of polymers, small molecules, and the organic-inorganic perovskites as the active layers in solar cells and LEDs. The design, fabrication and properties of metal-oxide-based nano-structures as electron and hole transporting layers are also reviewed. In addition, the development of plasmonic nanomaterials for solar cells and LEDs is discussed. Each topic in this book includes an overview of the materials system from principles to process. The advantages, disadvantages and related methodologies are highlighted. The book includes applications based on materials and emphasize how to improve the performance of solar cells and LEDs by the materials design, with a focus on nanomaterials. - Provides latest research on nanostructured materials including small molecules, polymers, organic-inorganic perovskites, and many other relevant materials systems for solar cells and LEDs - Addresses each promising materials system from principles to process, detailing the advantages and disadvantages of the most relevant methods of processing and fabrication - Looks ahead to most likely techniques to improve performance of solar cells and light emitting diodes

Chapter 1

Fundamentals of Solar Cells and Light-Emitting Diodes

Feng Wang; Xiao-Ke Liu; Feng Gao Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping, Sweden

Abstract

This chapter focuses on introducing basic concepts in solar cell and light-emitting diode (LED) devices. First, the fundamental knowledge about semiconductors and several important materials related to solar cells and LEDs is introduced to help the reader understand the working principle of devices. Second, we describe the working principle and basic terms involving solar cells, the energy loss processes, and several strategies for high-efficiency solar cell devices. Finally, we present the basic terms and the device structure of LEDs, as well as some approaches for high-efficiency white LEDs.

Keywords

Efficiency; Energy loss; Light-emitting diode; Solar cell; Work principle

1.1 Basics of Semiconductors

1.1.1 Theory of Semiconductors

Materials can be classified by the energy gap between their valence band and the conduction band. In the case of conductors, their conduction band and valence band overlap each other; in other words, the energy gap is absent. Thus, electrons require minimum energy, which can be provided by environmental thermal energy, to jump from the valence band to the conduction band and, consequently, become charge carriers. In contrast, the band gap is extremely wide in insulators. It requires a significant amount of the energy to excite an electron from the valence band to the conduction band. Thus, insulators are poor conductors of electricity due to the lack of charge carriers at room temperature. Semiconductors are materials with a level of conductivity between that of a conductor and that of an insulator. They can conduct electricity due to the addition of impurities or because of temperature effects, making them good media for the control of electrical current. In addition, their conductance varies depending on the current, voltage, or the intensity of light irradiation. Semiconductor materials can be classified into two types, intrinsic and extrinsic, which are in pure form and with intentionally added impurity, respectively.

1.1.2 n- and p-Type Semiconductors

Extrinsic semiconductors are classified into n- and p-types based on the electron and hole carrier concentrations in these materials, which are determined by the types of the added impurities. In n-type semiconductors, the donor impurity atoms have more valence electrons than the semiconductor atoms that they replace, and excess valence electrons can be provided to the conduction band, making electrons outnumber holes in these materials. In contrast, holes are the majority charge carriers and electrons are the minority carriers in p-type semiconductors.

Take silicon (Si) for example. Pure Si is an intrinsic semiconductor with four valence electrons. Group V elements with five valence electrons, such as phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi), can be doped into pure Si to create n-type Si. The mechanism is that even after the formation of four covalent bonds due to the sharing of four electrons between the Group V and Group IV elements, one more electron in Group V element is left behind. The extra electron can be easily excited to the conduction band, becoming a charge carrier by harvesting environmental thermal energy. The total number of electrons in the material is the sum of thermally generated electrons and the electrons donated by the donor atoms. However, the number of holes in the same material remains equal to the number of thermally generated electrons. This means that electrons are the majority charge carriers, while holes are the minority carriers. Thus, these materials are named as n-type semiconductors. On the contrary, p-type Si can be obtained by doping Group III elements with fewer valence electrons, such as boron (B), aluminum (Al), nitrogen (N), gallium (Ga), and indium (In), into pure Si.

1.1.3 p-n Junction

When a p-type semiconductor contacts an n-type semiconductor, free electrons on the n-type semiconductor near the heterojunction will diffuse to the p-type semiconductor because the concentration of electrons is much more in the n-type region than in the p-type region. Electrons diffuse across the junction, leaving behind a layer of fixed positive charge in the n-type semiconductor (Fig. 1.1). Similarly, free holes in the p-type semiconductor near the junction will diffuse to the n-type semiconductor and leave a layer of negative charge in the p-type semiconductor. This space charge sets up an electrostatic field to oppose further diffusion across the junction. Equilibrium is established when the diffusion of majority carriers across the junction is balanced by the drift of minority carriers back across the junction by the built-in electrostatic field.

Fig. 1.1 Schematic of carrier transfer close to the p-n junction. Free carriers are diffused close to the junction so that the p-side becomes negatively charged and the n-side becomes positively charged.

When a bias voltage is applied on the p-n junction, of which the p- and n-sides are contacted with the positive and negative terminals, respectively, electrons on the p-side will be attracted to the positive terminal, while holes on the n-side will be shifted to the negative terminal. The junction is forward biased in this case, accompanying the obvious current. Consequently, the width of the depletion region is reduced and finally vanishes altogether. It is worthwhile to note that increased bias voltage may lead to hole injection on the p-side and electron injection on the n-side. In additi...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Preface
• Acknowledgments
• Chapter 1: Fundamentals of Solar Cells and Light-Emitting Diodes
• Chapter 2: Nanostructure of Organic Solar Cells
• Chapter 3: Nanomaterials in Dye-Sensitized Solar Cells
• Chapter 4: Solution-Processed Crystalline Silicon Heterojunction Solar Cells
• Chapter 5: Perovskite Solar Cells Processed by Solution Nanotechnology
• Chapter 6: Solar Cells Based on Hot Carriers and Quantum Dots
• Chapter 7: LEDs Based on Small Molecules
• Chapter 8: Bulk- and Nanocrystalline-Halide Perovskite Light-Emitting Diodes
• Chapter 9: Polymer Light-Emitting Diodes
• Chapter 10: Nanomaterials for Polymer and Perovskite Light-Emitting Diodes
• Chapter 11: Preparation and Characterization of Oxide/Semiconductor Interfaces
• Chapter 12: Highly Conductive Nanocomposites Based on Doped Metal Oxides as Interlayer Materials for Organic/Polymer Solar Cells
• Chapter 13: Nanostructures for Plasmonic Effects in Solar Cells and LEDs
• Index