2. Photovoltaic Energy Conversion

This chapter provides an introduction to the basic principles of solar energy conversion including its thermodynamic limits. We discuss the optical and electrical requirements for an ideal photovoltaic device and show examples of possible realizations based on semiconductors. To recall the basics, a brief review on semiconductor physics with emphasis on the p-n junction is given. We discuss the role of the electrochemical potential as driving force for the conversion of sunlight into electricity. We conclude with estimations on the maximum power-conversion efficiency for a single band-edge absorber and introduce approaches for achieving it or even going beyond it. Readers without any background in solid state physics might consider consulting an introductory textbook while reading this chapter. If the reader comes to the conclusion that his/her background in physics is not sufficient, he might consider to skip this chapter and directly start with Chap. 3, as a detailed understanding of thermodynamics is not required to follow most of the elaborations on the working principle of organic solar cells in subsequent chapters. The reader interested in the fundamental laws of solar energy conversion should follow this chapter and be able to answer the following questions afterwards: (a) What is the thermodynamic limit of solar-thermal energy conversion with a device located on the earth surface? What is the role of entropy? (b) Why is the power-conversion efficiency of a solar cell based on a single semiconductor limited to 33 %? What tradeoffs have to be made? (c) Where is the “maximum” of the solar spectrum located? What are possibilities of expressing spectra (e.g. from the sun) considering energy versus wavelength or photon fluxes versus intensity fluxes (irradiance)? (d) What are the main optical and electrical properties of semiconductors and how can they be derived? (e) What are the relations between Fermi levels and charge carrier densities? (f) What are the driving forces for the movement of charge carriers? What is the concept of quasi-Fermi levels? (g) What is the effect of recombination on the photovoltage of a solar cell? Which loss processes are unavoidable? (h) How does a p-n junction solar cell work? Are there alternative architectures? (i) What are the basic requirements for a solar cell? Consider the role of selective contacts and the built-in electric field. (j) Why should a good solar cell show a high electroluminescence quantum yield, i.e. large emission? (k) What are the main concepts for overcoming the so-called Shockley-Queisser limit?