**1. Introduction**

356 Solar Cells – New Aspects and Solutions

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> Models of solar cells are constructed using the concepts of band theory and thermodynamic principles. The former have been most extensively used in calculations of the efficiency of solar cells (Luque & Marti, 2003; Badesku et al., 2001; De Vos et al., 1993, 1985; Landsberg & Tonge, 1989, 1980; Leff, 1987). Thermodynamic description is performed by two methods. In one of these, balance equations for energy and entropy fluxes are used, whereas the second (the method of cycles) comes to solutions of balance equations (Landsberg & Leff, 1989; Novikov, 1958; Rubin, 1979; De Vos, 1992).Conditions are sought under which energy exchange between radiation and substance produces as much work as possible. Work is maximum when the process is quasi-static. No equilibrium between substance and radiation is, however, attained in solar cells. We therefore believe that the search for continuous sequences of equilibrium states in solar energy conversion, which is not quasi-static on the whole, and an analysis of these states as separate processes aimed at improving the efficiency of solar cells is a problem of current interest. Examples of such use of the maximum work principle have not been found in the literature on radiant energy conversion (Luque & Marti, 2003; Badesku et al., 2001; De Vos et al., 1993, 1992, 1985; Landsberg & Tonge, 1989, 1980; Leff, 1987; Novikov, 1958; Rubin, 1979).
