In accordance with the transversal sizes (thickness) of semiconductor structure the sun elements can be de classified onto the bulk and thin film structures. In an ideal a phase-morphological structure is identical for both types of sun elements and differs only by errors (defects) that are determined by the features of making technologies of bulk and thin film elements.

Bulk solar cells include a wafer of semiconductor material with a thickness of 180…240 µm. Such a wafer is got by sawing of thick purveyance into the row of thin elements with the subsequent tooling to achieve the necessary sizes and quality of surfaces.

Today for creation of bulk solar cells the «sun silicon» in all his varieties is mainly used, because of relatively low cost of making, in particular, monocrystalline silicon c-Si, often made using the Czochralsky method, polycrystalline silicon or multicrystalline silicon (poly-Si or mc-Si) made from square ingots – large blocks of molten silicon. Ribbon silicon is the type of multicrystalline silicon. It is formed by drawing flat thin film from molten silicon. These cells have lower efficiency than polycrystalline silicon, but in that time smaller production cost as this approach does not require of mechanical sawing from ingot.

The photovoltaics market is still dominated by wafer-based solar cells. The sun elements on the basis of monocrystalline silicon showed maximal efficiency (22%) of transformation of solar energy into electrical one. The laboratory monocrystalline silicon samples (university of New South Wales, Suntech power) demonstrated the record efficiency 25%. To the wide introduction of the monocrystalline silicon the very high cost of making interferes.

The sun elements on the basis of polycrystalline silicon provide efficiency of energy transformation near 14…16%. It should be noted that in sufficient measure the high cost of these elements is determined by the high cost of polycrystalline silicon.  Therefore the producers try to decrease the thickness of wafers made from polycrystalline silicon or try to make the wafers from cheap metallurgical « dirty » silicon.

The power efficiency of amorphous silicon structures today arrives 6.8%.

The improvement of bulk silicon solar elements is conducted in the direction of development of new methods of growing of crystals, development of architecture (charts) of silicon elements, and also modernization of technological processes of making, attestation and diagnostics of sun silicon elements (NREL). The mathematical (computer) design allows optimizing the technological processes of production, beginning from technological processes of silicon manufacture and concluding by setting of equipment, and also allows defining the prospects of increasing of energy effectiveness and of decreasing of the production cost of solar cells. In an eventual kind, all these measures are directed on the increase of output of electric energy on a gram of silicon.

With the purpose of diversification of the ways of receipt of silicon and of removal of Siemens-process, even, exotic technologies of Si-synthesis of polycrystalline silicon from wastes of agrotechnical production are explored (Mayaterials).

Thin film technologies allow to decrease the mass and sizes of semiconductor material and to create sun photovoltaic solar cells with a less cost is comparative with the bulk element.

According to the type of chemical elements and compounds, accountable for a photo-electric effect, the thin film solar elements are distinguished into the monocomponent elements (silicon Si, germanium Ge) and multicomponent sun elements on the basis of compounds: cadmium telluride CdTe, lead telluride PbTe, lead selenide PbSe, lead sulfide PbS, gallium arsenide GaAs,  copper-indium selenide CuInSe₂ (CIS), copper-indium-germanium di-selenide CuInxGaSe₂ (mixture of semiconductors CuInSe₂ and CuGaSe₂), which is briefly named CIGS, gallium-indium phosphate GaInP₂, indium phosphide/gallium arsenide/germanium (GaInP/GaAs/Ge), zinc-selenium phosphate ZnSnP₂, zinc-germanium arsenide ZnGeAs₂.

Greater part of market of thin film sun elements (85%) is occupied by thin film elements on the basis of polycrystalline silicon (thin film c-Si). The thin polycrystalline silicon layers are deposited on a substrate by chemical dispersion from gaseous silane and gaseous hydrogen. Depending on the parameters of causing it is possible to get different pellicle structures, in particular, thin film amorphous silicon (a-Si or a-Si:H), thin film polycrystalline silicon and nanocrystalline silicon (nc-Si or nc-Si:H), which sometimes named microcrystalline silicon. The efficiency of transformation of sun energy into electrical energy for thin film silicon elements makes 7…10%.

Combination of protocrystalline silicon with the small quantity of nanocristalline silicon allows to create the effective sun elements, although they has lower energy effectiveness in relation to the bulk silicon samples but prevail them in relation to cost indexes.

Combination of amorphous silicon a-Si (width of band-gap 1,7 eV) with the nanocristalline silicon (width of band-gap 1,1eV) enables to create so called the tandem two-layers structures. The up layer made of amorphous silicon adsorbs visible rays and transmits infra-red rays to bottom layer made of nanocristalline silicon.

Recently developed technologies allow to get the multiple transmission of long-wave infra-red radiation through the layers and to increase the possibility of his absorption by the atoms of thin film polycrystalline silicon. 

United Solar Ovonic, LLC creates the manufacturing of the multiple spectral thin film solar modules on the base of silicon structures to power the dwelling-houses.

For creation of thin film elements the telluride cadmium CdTe is widely used (approximately 15% of market of thin film elements). He is characterized by simplicity of depositing of compound onto a substrate and allows providing easily a mass production in comparison with other compounds. Energy effectiveness of thin film elements on the basis of CdTe is approximately 11%. Today the efforts of developers are directed onto the mastering of process of depositing of layers on a substrate made of metallic foil instead of glass substrate (South Florida University).

Very promising for use are the compounds of copper-indium selenide CuInSe₂ (CIS), copper-indium-germanium di-selenide CuInxGaSe₂ (mixture of semiconductors CuInSe₂ and CuGaSe₂), which is briefly named CIGS which consist of elements of I, III and IY groups of the periodic system. Maximal energy effectiveness of thin film elements (19%) is got with the use of the layers on the basis of copper-indium-germanium di-selenide CuInxGaSe₂. Yet higher efficiency of transformation (near 30%) can be received with the use of optical concentrators in an aggregate with tandem geterostructures. 

Presently the thin film photovoltaic elements and modules on CIGS–compounds are created to equip the industrial photovoltaic solar power plants (Dow Chemical Company) and to power the dwelling-houses (Nanosolar, DayStar Technologies, Inc.). The scientists of Stanford University explore the inorganic thin film solar elements on the basis of CIGS-nanowires with a diameter less than 200 nm. The researchers of MIT work on the creation of high effective multiple spectral polycrystalline tandem photovoltaic elements on the basis of CuInSe₂ - compounds.

Record efficiency of thin film sun elements is attained in geterostructures on the basis of gallium arsenide GaAs. These elements consist of a few layers of semiconductors, formed by epitaxy from a steam phase. For example, a three-transitional structure consists of semiconductors GaAs, Ge and GaInP₂.

Every type of semiconductor is characterized by the certain band-gap width and, accordingly, effectively takes in the light of certain range of light-spectrum. Semiconductors are selecting in a way, to overleap practically all spectrum of sun radiation and provide the maximally possible generation of electric current. The geterostructure sun elements on the basis of GaAs attained record efficiency near 40,7% with 500-time concentration of sun radiation. The maximal energy effectiveness of monostructure thin film elements on the basis of gallium arsenide GaAs attained today is equal 25.8% (Radboud University, Nijmegen, Denmark).

The number of publications of results of researches of tandem geterostructure sun elements on the basis of gallium-indium phosphide GaInP, gallium arsenide GaAs and germanium Ge is multiplied. The researchers are promising to promote energy effectiveness of tandem thin film photovoltaic sun semiconductors ZnSnP₂ and ZnGeAs2 on the chemical elements of the II-IV-V groups of the periodic system to 20% (Arizona State University).

Conclusions. The analysis of achievements that was conducted in the branch of bulk and thin film sun elements showed that only the bulk sun elements on the basis of monocrystalline, polycrystalline and ribbon silicon (a type of microcrystalline silicon) attained of the level of industrial production. The sun elements on the basis of amorphous silicon have less power efficiency in comparison with the classical polycrystalline silicon, but their cost is considerably less.

Greater part of market of thin film sun elements (85%) is occupied by the elements on the basis of polycrystalline silicon (thin film c-Si) and cadmium telluride CdTe (approximately 15% of market of thin film elements). The record efficiency for thin film sun elements is attained in geterostructures on the basis of gallium arsenide GaAs (40,7% under suns concentration), in the compounds of copper-indium selenide CuInSe₂ and copper-indium-gallium di-selenide CuInxGaSe₂ (19% in ordinary terms and 30% - at combination of tandem elements with optical concentrator). 

Without regard to the progress attained in the recent years, sun technologies of first and second generations are found only on a way to confession and conquest of power market. To improve the energy effectiveness of thin film solar elements the main efforts of researchers above all are directed onto the development of exitonic and tandem structures that allow to extend the spectral range of work of devices. But indeed a substantial breakthrough in introduction of thin film sun technologies is expected subject to the condition of development of a principally new architecture of the sun elements of future generation, based on the use of photovoltaic nanostructure materials, in particular on the basis of organic (polymeric) matrices, which, in conformity with the results of the published researches, provide high energy effectiveness and reduction of cost of the sun modules.

Vasil Sidorov on April 25, 2010 from Technopark QUELTA

E-mail: sidorovvasil@gmail.com