**1. Introduction**

380 Solar Cells – Thin-Film Technologies

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Solar cells (SC) are the most effective devices that allow direct one-stage conversion solar energy into electricity from the view of energy. The last yers tendency in traditional energetic forced to direct a significant part of research on the establishment of modern technology for production available and effective thin film SC that would not require the use of high temperature and pressure, a large number of rare and expensive materials. At the same time, to find ways for increase the conversion efficiency of solar energy it is necessary to understand the processes that occur in the elements. Therefore it is necessary to establish a correspondence between characteristics of elements and main structural, electronic and optical properties of initial semiconductor films. Therefore, the investigation of CdS thin films deposition process with desire photoelectric properties and fabrication on their base thin-film SC have great significance.

CdS is the main material for buffer layer in thin-film CdTe and Cu(In, Ga)Se2 solar cells. It has a high photosensitivity and absorption, favorable energy band gap (Eg) 2,4 eV and photoconductivity () 102 Om-1cm-1 and does not change the properties with SC surface temperature increase during the work. One more peculiarity of this material is absence of the hole conduction due the acceptor additives and point defects recombination. Effective lifetime of the main carriers is very large (10...100 ms), that causes a initial photocurrent increase up to 105 times (Hamakawa, 2002). An important advantage of CdS thin films use in SC is possibility of their synthesis by different methods, including chemical deposition from solution which has significant preference over others: 1) grown nanocrystalites with a form close to spherical, while the electrochemical deposition - nonspherical (Jager-Waldau, 2004); 2) CdS thin films deposited from solution have structural, optical and electrical parameters thet do not inferior parameters of films received by other methods, but used equipment is available, simple, does not require use of the high temperatures and pressures compared, for example, with the vacuum evaporation or ion (sputtering or pulverization, spraying) methods; 3) the method is not explosive and lowtoxic, compared with the vapor deposition methods; 4) enable control of the film growth and dynamically change the fabrication conditions for polycrystalline or smooth solid films.

Chemical Surface Deposition of CdS Ultra Thin Films from Aqueous Solutions 383

the sulfur vacancies such films always have n-type conductivity, and their resistance can vary widely, differing by the amount of eighth order. Epitaxial CdS films are characteristic due to carrier high mobility. With the increase of substrate temperature concentration of carriers grows by an exponential law. This increase the electron mobility. Optical properties of CdS films are strongly dependent on their microstructure and thus on the method and conditions of deposition. For example, evaporation of CdS results in smooth mirror reflective films, but increasing their thickness leads to a predominance of diffuse reflection. The CdS films, obtained by ion sputtering have the area with rapid change of transmission at 520 nm, corresponding CdS band gap. In the same time in the long-wave spectral range

Edmund Becquerel, a French experimental physicist, discovered the photo-voltaic efect in 1839 while experimenting with an electrolytic cell, made up of two metal electrodes placed in an electricity-conducting solution. He observed that current increased when the electrolytic cell was exposed to light (Becquerel, 1839). Then in 1873 Willoughby Smith discovered the photoconductivity of selenium. The rst selenium cell was made in 1877 (Adams, 1877), and ve years later Fritts (Fahrenbruch & Bube, 1983) described the rst solar cell made from selenium wafers. By 1914 solar conversion eficiencies of about 1 % were achieved with the selenium cell after it was nally realized that an energy barrier was

The modern era of photovoltaics started in 1954. In that year was reported a solar conversion efficiency of 6 % (Chapin at al., 1954) for a silicon single-crystal cell. In 1955 Western Electric began to sell commercial licenses for silicon PV technologies. Already in 1958 silicon cell efficiency under terrestrial sunlight had reached 14 %. At present, available in the market SC are mainly represented of monocrystalline silicon SC. Through hightemperature process of their formation, crystal (from ingots grown from melt by Czochralski method) and polycrystalline silicon solar cells have too high price, to be seen as a significant competitor to the formation of energy from solid fuels. Polycrystalline silicon provides lower expenses and increase production, rather than crystalline silicon. In 1998, approximately 30 % photovoltaic world production was based on the polycrystalline silicon wafers. Nowadays solar cells conversion efficiency based on monocrystalline silicon is 25 %,

In 1954 reported 6 % solar conversion efficiency (Reynolds at al., 1954) in what later came to be understood as the cuprous sulde/cadmium sulde heterojunction (HJ). This was the rst all-thin-lm photovoltaic system to receive signicant attention. In following years the efficiency of CuxS/CdS increased up to 10 % and a number of pilot production plants were installed, but after several years of research it was realized that these solar cells have unsolvable problems of stability owing to the diffusion of copper from CuxS to CdS layers. By taking advantage of new technology, work out on CuxS/CdS, researchers have rapidly raised the effciency of the gallium arsenide based cell with 4 % efficiency (Jenny at al., 1956)

However in the last 20 years other thin lms solar cells have taken the place of the cuprous sulde/cadmium sulde, and their eficiency have raised up to almost 20 %. The most predominant are two: copper indium gallium diselenide/cadmium sulde (Cu(In,Ga)Se2/CdS) and cadmium telluride/cadmium sulde (CdTe/CdS). The rst CdTe heterojunctions were constructed from a thin lm of n-type CdTe material and a layer of p-

films have high transparency.

**2.2 Use of the CdS films in photovoltaic cells** 

polycrystalline – 20 % (Green at al., 2011).

to present eficiencies exceeding 27 % (Green at al., 2011).

involved both in this cell and in the copper/copper oxide cell.
