Materials Concepts for Solar Cells

Thomas Dittrich

• Publisher: WSPC (EUROPE)
• Year: 2018
• eBook ISBN: 9781786344502
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

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A modern challenge is for solar cell materials to enable the highest solar energy conversion efficiencies, at costs as low as possible, and at an energy balance as sustainable as necessary in the future. This textbook explains the principles, concepts and materials used in solar cells. It combines basic knowledge about solar cells and the demanded criteria for the materials with a comprehensive introduction into each of the four classes of materials for solar cells, i.e. solar cells based on crystalline silicon, epitaxial layer systems of III-V semiconductors, thin-film absorbers on foreign substrates, and nano-composite absorbers. In this sense, it bridges a gap between basic literature on the physics of solar cells and books specialized on certain types of solar cells.

The last five years had several breakthroughs in photovoltaics and in the research on solar cells and solar cell materials. We consider them in this second edition. For example, the high potential of crystalline silicon with charge-selective hetero-junctions and alkaline treatments of thin-film absorbers, based on chalcopyrite, enabled new records. Research activities were boosted by the class of hybrid organic-inorganic metal halide perovskites, a promising newcomer in the field.

This is essential reading for students interested in solar cells and materials for solar cells. It encourages students to solve tasks at the end of each chapter. It has been well applied for postgraduate students with background in materials science, engineering, chemistry or physics.

--> Contents:

• Preface
• Symbols and Abbrevations
• Basics of Solar Cells and Materials Demands:
  • Basic Characteristics and Characterization of Solar Cells
  • Photocurrent Generation and the Origin of Photovoltage
  • Influence of Recombination on the Minimum Lifetime
  • Charge Separation Across pn -Junctions
  • Ohmic Contacts for Solar Cells
  • Maximum Efficiency of Solar Cells
• Materials Specific Concepts:
  • Solar Cells Based on Crystalline Si
  • Solar Cells Based on III–V Semiconductors
  • Thin-Film Solar Cells
  • Nano-Composite Solar Cells
• Solutions to Tasks
• Bibliography
• Index

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--> Readership: Advanced undergraduates and graduate students in photovoltaics, as well as students with background in materials science, engineering, chemistry or physics. -->
Keywords:Solar Cells;Solar Cell Materials;Photovoltaics;Absorber;Ohmic Contact;Silicon;GaAs;III–V Semiconductors;Thin-Film Solar Cells;Nano-Composite Solar Cells;Organic Solar CellsReview:

Reviews of the First Edition:

"The book offers a well-balanced treatment of physical principles and materials-related concepts of solar cells, and considers both classical and new trends in this rapidly developing field... The book is perfectly structured, with a concise summary of the most important points provided for every chapter, and the description of the concepts well complemented by the tasks. I strongly recommend this book for students and scientists attracted to the renewable energy and the materials science fields."

Andrey Rogach
Chair Professor of Photonic Materials
City University of Hong Kong

“The book is of good pedagogical value. Students as well as teachers can make use of this either as a main textbook or as a support for their lessons. In general, the book is well-written and provides a solid basis for studying solar cells.”

MRS Bulletin
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Part I

Basics of Solar Cells and Materials Demands

1

Basic Characteristics and Characterization of Solar Cells

Solar cells convert power of sunlight into electric power. As an introduction, therefore, Chapter 1 is devoted to a brief characterization of sunlight and basic electric parameters of solar cells. The power of sun is given in terms of the solar constant, the power spectrum and power losses in earth atmosphere expressed by the so-called air mass. The basic characteristics of a solar cell are the short-circuit current (I_SC), the open-circuit voltage (V_OC), the fill factor (FF) and the solar energy conversion efficiency (η). The influence of both the diode saturation current density and of I_SC on V_OC, FF and η is analyzed for ideal solar cells. The importance of concentrated sunlight for increasing η is shown. Tolerable series and parallel resistances are introduced as an evaluation criterion for resistive losses in real solar cells. The influence of the series resistance (R_s) and parallel resistance (R_p) on I_SC, V_OC, FF and η is investigated. The specific role of R_s and R_p is discussed in detail for the dependence of η on I_SC. Concepts are described for measuring the basic characteristics of solar cells and their dependencies on light intensity, temperature and light spectra. Attention is paid to principle work with various kinds of load resistances, to the function of a pyranometer, of a sun simulator and to the measurement of the quantum efficiency of solar cells.

1.1Solar Radiation and Two Fundamental Functions of a Solar Cell

The sun is a hot sphere radiating energy in form of light or photons into space. The absolute temperature (T) of the outer photo sphere of the sun (T of the sun, T_S) is about 5800 K. The radius of the sun (R_sun) is 6.96 · 10⁵km. The power of thermal radiation and therefore the total power emitted by the sun

can be calculated by using the Stefan–Boltzmann equation and the surface area of the sun.

The Stefan–Boltzmann constant (σ_S) is 5.67 · 10^–8 Wm^–2K^–4.

The sun’s power received on earth (P_e) is proportional to the cross-section of the earth and to the reciprocal area of a sphere with the radius equal to one astronomical unit (AU), the distance between the sun and the earth. This is shown schematically in Figure 1.1. The shortest and longest distances between the sun and the earth, the so-called perihelion and aphelion, are equal to 1.47 and 1.52 · 10⁸ km, respectively. The radius of the earth (R_e) is about 6400 km.

The solar constant (J_s) is defined as the power of the sun (P_sun) received on earth over 1 m².

Figure 1.1.Sun emitting photons and earth receiving a proportion of photons emitted by the sun. The radius of the sun, the radius of earth and the distance between sun and earth are denoted by R_sun, R_e and AU, respectively.

In reality, the solar constant is not a constant since the distance between the earth and the sun and the temperature distribution at the surface of the sun are not constant. A solar constant of 1356 W/m² will be taken into account in the following.

A photon is the smallest portion or quantum of light, the energy of which is proportional to the frequency of the light (υ). The factor between the energy of a photon (photon energy, E_ph) and the frequency is called the Planck constant (h = 6.626 · 10^–34 Js).

The power spectrum of the sun, i.e. the dependence of power emitted within an interval of photon energies (E_ph, E_ph + dE_ph), can be approximated by the radiation of a blackbody with the temperature T_S which is given by the Planck equation.

The velocity of light (c) is equal to 2.99 · 10⁸ m/s and the Boltzmann constant (k_B) is given as 1.38 · 10^–23 J/K. The spectrum of blackbody radiation for 5800 K is shown in Figure 1.2.

Spectra are usually measured in units of wavelength (λ). The λ of light is proportional to the reciprocal frequency while the proportionality factor is the velocity of light.

The E_ph is given in units of eV, which means that the energy is divided by the elementary charge (q = 1.6 · 10^–19 As). Therefore, the product of the photon energy and the wavelength is given by

The λ can be easily transformed into the E_ph by using Equation (1.7). The Eph and the λ in the maximum of the corresponding spectra of blackbody radiation are obtained in tasks T1.3 and T1.4 (see end of this chapter), respectively, for a blackbody with T of 5800 K.

Figure 1.2.Spectrum of sunlight (sun spectrum) outside the earth’s atmosphere (air mass (AM) 0, thick solid line), on earth for a zenith angle of 48.2° (AM1.5, thin solid line) and of a blackbody with a temperature of 5800 K (dashed line).

The power received over a certain area on earth depends on geographical location, on the rotation of the earth (day–night cycle), on the inclination angle of the earth’s axis (sun in summer and winter) and on the given distance between sun and earth. Further, light is scattered and absorbed by molecules and particles in the earth’s atmosphere. The average power of sunlight received on earth, for example, is reduced by a factor of π as a result of the day–night cycle.

The optical path of sunlight through the earth’s atmosphere normalized to the thickness of the earth’s atmosphere defines the so-called air mass (AM). The power of the sun corresponds at zenith to AM1 and at a zenith angle of 48.2° to AM1.5. It is agreed worldwide that P_sun is equal to 1kW/m² at AM1.5G (IEC, 2008) (G denotes global radiation including direct light and scattered light, see also the AM1.5 spectrum (the sun spectrum can be downloaded, for example from (RReDc, 2013)) in Figure 1.2; where direct radiation is less than the global radiation by a factor of about 1.1). The average power of the sun on earth (

P_su...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• Contents
• Symbols and Abbreviations
• Part I. Basics of Solar Cells and Materials Demands
• Part II. Materials Specific Concepts
• Appendix. Solutions to Tasks
• Bibliography
• Index