The worldwide demand for energy may rise to 30 terawatts by 2050. To meet this demand, solar energy can contribute substantially: 120,000 terawatts come from the sun at any given time. However, materials must be developed that can efficiently convert solar radiation to electricity, are available in large quantities, inexpensive, and safe to handle. Current technology is based on silicon, which has a solar conversion efficiency of 25%, but processing it is expensive because (1) it requires high temperatures and (2) the material has to be thick to absorb light due to the small absorption coefficients. Alternative materials—gallium arsenide, cadmium telluride, and copper indium gallium selenide—contain elements that are much less abundant in the earth’s crust than silicon, which is the second most abundant element and constitutes 28% of the earth’s crust. Hence research on safer materials prepared from cheaper and more easily available materials such as perovskites and dye-sensitized solar cells is gaining importance.
This book focuses on inorganic semiconductors made of nontoxic and abundant materials. They contain copper and other elements. The elements chosen are zinc, cadmium, and mercury (under II); silicon, germanium, and tin (under IV); and sulfur, selenium, and tellurium (under VI), thus accounting for 27 semiconductors discussed comprehensively. These have a near-optimal direct-bandgap energy of ∼1.5 eV, a value at which the conversion efficiency is maximum. Their absorption coefficient is high so that thin materials can be used.
The introductory chapter defines, with sample calculations, parameters such as abundance values, spectral efficiency, effective cubic lattice constant (used in later chapters to correlate properties of these 27 semiconductors), the effective medium approximation, and interpolation schemes. This is followed by six chapters on structural, thermal, elastic, band structure, optical, and carrier transport properties. Chapter 2 summarizes data on crystal structure and includes comparisons with III–V and II–VI semiconductors. The next chapter presents phase diagrams and properties of practical importance such as specific heat, Debye temperature, thermal expansion, and thermal conductivity, again comparing other semiconductors. The data on elastic constants, hardness, and lattice dynamic properties, covered in chapter 4, are useful to have in one place. The next chapter on band structure combines theory with empirical correlations of energy gap with molecular weight and effective cubic lattice constant. The chapter on optical properties is relevant to solar cells and optoelectronic applications. The final chapter on carrier transport properties includes discussions on electron and hole Hall mobilities and conduction mechanisms. A special feature is the attention devoted to material parameters—stoichiometry, alloying, doping, grain boundaries, graded structures—and heat treatment.
There are 26 categories of solar cells, including those made of earth-abundant materials, ranging in efficiency from 10.6% to 46%, each with its own technical and economic challenges. A brief summary would have been helpful comparing them and placing the Cu2-II-IV-VI4 semiconductors in context. This book is an authoritative source of information due to the in-depth discussions and adequate references, figures, tables, and appendices.
Reviewer: N. Balasubramanian is an independent research scholar working on renewable energy and ultrafine-grain materials in Bangalore, India.
