**1. Introduction**

High-effective solar energy conversion is impossible without deep examination of solar cell'components as physical materials. Studies of structure of matter, its optical and thermo‐ dynamic properties belong to the scope of several disciplines. The concept of generalized formula units is of particular importance. For example, a category of crystal lattice is the result of crystal's mathematical modeling, while the regularity of the crystal structure represents the basis of its zone structure, which, in turn, is the model for description of energy states of the electrons. It is clear that after all chemical bonds are broken and crystal is evaporated there will be no crystal lattice and its inherent energy bands. Only particles of the crystal remain. However, the band theory considers only energy states of the electrons. Therefore, studies of the relations between structural, thermodynamic and optical properties of material without addressing to the band theory of solids are of both theoretical and practical interest. Here we show that such relations can be found by presenting the sequence of ground and excited states of primitive and non-primitive crystal cells as a thermodynamic process. Calculation of frequencies at which maximal useful work can be produced by a crystal cell and radiation is presented by the example of magnesium. The number of such frequencies increases in the row diamond-graphite-soot. On the example of silicon it is suggested that objects can be compa‐ rable with ideal radiator, or black body, in terms of their property to absorb radiation energy if they are composed of nanoparticles, i.e. clusters with small number of atoms. Requirements formulated for such objects are to be used for maximally efficient conversion of solar radiation into useful work. At the same time, current-voltage characteristics of solar cells are of special interest. We report experimental results and data of their numerical simulation showing novel features of common silicon-based solar cells with metallic Cu/Ag-nanoclustered contacts.

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