G. Smestad / Solar Energy Materials and Solar Cells 33 (1994) 391-395 393

Of fundamental importance in solar energy conversion is the transfer of electrons and protons. In electrochromic devices, for instance, a shift of protons from one material to another can have a profound effect on the color of the material. In solar cell devices and in photochemistry, it is the electron which is excited by light and transferred from the light absorber to another site where photoconversion can take place. In the biological sciences, it is becoming clear how electron and proton transfer are coupled. This should serve as a guide to those seeking to devise efficient man-made electronic or "protonic" devices. In this well formulated book, both these types of transfer are compared to one another. Forty six well known authors contributed to this union of topics. In the first chapter, the basic mechanisms and equations found in electron transfer are described. This is done via the theory of Rudolf Marcus (Nobel Prize in Chemistry 1992) and includes examples such as gas phase and metal organic interactions. The Inverted region, where the electron transfer rate decreases with increases driving force is described theoretically and experimentally. Methods are discussed for the mea- surement of rate constants, ket , as high as 101°-1012 s -1 such as found in photosynthesis. In the next chapters, electron transfer in metal organic com- pounds, light induced electron transfer and electron hopping in metal oxide clusters are discussed. A review article by Dr. Michael Graetzel, on electron transfer in semiconducting colloids and membranes, is especially important for those in solar conversion research since the "artificial photosynthetic" systems which are described allow man- made systems to approach the configurations found in nature and described in later chapters. Solar ceils of 7% efficiency are described which involve the electron transfer from organometallic compounds to inexpensive semiconducting oxides. The next chapters detail the electron and proton transfer (and their interrelationship) in the photosynthetic reaction centers in terms of the framework of Chapter 1. The relationship between the energetics and the spatial arrangements involved in the transfer are described. Proton transfer in hydrogen bonded systems is also covered in several chapters. Although the book covers a multitude of converging fields, this book is supplemented by excellent illustrations, figures and references which make this book useful for those who want to see the future of molecular electronics, protonics and photonics.

Greg Smestad

The Photosystems: Structure, Function and Molecular Biology, Edited by J. Barber (Elsevier Science B.V., Amsterdam, 1992) ISBN 0-444-89440-3 (hbk.); 558 pages; price US$243.00.

Much recent research in electronics is now focused on what is being called nanotechnology. In reading the 11th volume of Topics in Photosynthesis, scientists and engineers working in the field of solar energy would gain an appreciation for some of the most successful solar powered molecular machines on Earth. The book begins with an introduction, by R. Cogdell and R. Malkin, to plant and

394 G. Smestad / Solar Energy Materials and Solar Cells 33 (1994) 391-395

bacterial photosystems. Of particular interest is the illustration and description of the bacterial photosynthetic reaction center. It illustrates the marvellous orienta- tional relationship between the bacterial chlorophyll molecule, carotenoids and the pheophytin electron acceptors within the cell membrane. Another aspect of interest to those in solar conversion is the speed of this electron transfer, which on the order of 1-200 × 10 -12 s. The next chapter postulates that even though photosynthesis is a non-equilibrium system, the maximum conversion efficiencies must be described by the first and second laws of thermodynamics. Following in the tradition of researchers such as P.T. Landsberg, the approach is to consider the entropy associated with the absorption of light, and to define an "effective" temperature for use in efficiency equations. My critique is that little mention is made to more recent methods pioneered by Ross, Haught and Bolton, which bypass the direct calculation of the entropy involved, but instead form a detailed balance between solar input and luminescence output. What this chapter has made clear though is that the absorbance found in photosynthesis, and the input spectrum need to be carefully defined before a adequate assessment of the thermodynamics can be made. Energy transfer and trapping in Photosystem II, PSII, is described in the next chapter. An extremely helpful diagram outlines the molecular energy levels and associated optical absorption spectrum for Chlorophyll a. Presented is that the long lived triplet state of CI a is important role for in vitro reactions, while the lowest excited singlet state is important for redox reactions within the living photosynthetic reaction center. Other important topics covered in this book include, hole burning, a quantum mechanical treatment of electron and exciton transfer, the Mn oxygen evolution center of PS II, thermoluminescence, photoinhibition, and the molecular biology and structural engineering of PSI and PSII. Although this book was written for the molecular plant biologist working in this field, I recommend it to those interested in solar energy materials who wish an in- depth look at the current understanding of photosynthesis.

Greg Smestad

Handbook on Semiconductors, Volume 1: Basic Properties of Semiconductors (Else- vier Science B.V., Amsterdam, 1992) ISBN 0-444-888551

The book provides the promised information about the basic aspects of crystal structure and energy bands of semiconductors as well as basic transport properties. It gives sufficient information to aspects of band modelling in semiconductors, which is treated in several chapters within the book. Some aspects, like the method of pseudopotentials, used to gather accurate information about the electronic structure of semiconducting materials, are treated more than once, permitting insights into the topic from several points of view. The chapters are written by experts in their fields, allowing the reader to gain insights from the mathematical and experimental proofs outlined. The text of each chapter starts with an introduc- tory paragraph, helping even the novice in the field to move quickly into the