**1. Introduction**

Provision of energy is one of the most pressing problems facing humanity in the 21st century. Without energy, it is impossible to overcome the critical issues of our time. Industrial world suggests continuous growth in energy consumption in the future.

According to the U.S. Department of Energy, the world's generating capacity is now close to 18 TW. The main source of energy even in highly developed countries is fossil fuel, i.e. coal, oil and natural gas. However, resources of fossil fuel are limited, and its production and consumption irreversibly affect the environmental conditions with the threat of catastrophic climate change on Earth. Other energy sources, particularly *nuclear* energy, are also used that would fully meet *in principle* the energy needs of mankind. Capacity of existing nuclear reactors (nearly 450 in the world) is 370 GW. However, increasing their capacity up to 18 TW or about 50 times (!), is quite problematic (to provide humanity with *electric* energy, the capacity of nuclear power should be increased about 10 times). Resources of hydroelectric, geothermal, wind energy, energy from biofuels are also limited. At the same time, the power of solar radiation of the Earth's surface exceeds the world's generating capacity by more than 1000 times. It remains only to master this accessible, inexhaustible, gratuitous and nonhazardous source of energy in an environmentally friendly way.

Solar energy can be converted into heat and electricity. Different ways of converting sunlight into electricity have found practical application. The power plants, in which water is heated by sunlight concentrating devices resulting in a high-temperature steam and operation of an electric generator, are widespread. However, solar cells are much more attractive due to the *direct* conversion of solar radiation into electricity. This is the so-called *photovoltaics.* Under the conditions of the growing problems of global warming, photovoltaics is the most likely candidate to replace fossil fuels and nuclear reactors.
