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. The use of nanostructure materials marked the coming of the era of super-semiconductor solar cells, 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.

Nanotechnologies and nanomaterials are used in photovoltaic sun elements not as contribution to the fashion, but as effective, and it is possible, as the unique mean of creation of the energy effective photovoltaic modules, able to compete with the systems of traditional energy, based on incineration of fossil organic fuel.

Nanostructure photovoltaic sun elements are created from quantum geterostructures – quantum dots, nanowires, nanotubes and other formations, embedded into the electroconductive materials, in that number, polymers and mesoporous oxides of metals. Except for that, the thin layers of these formations, deposited on the surface of ordinary silicon sun panels, can multiply the amount of sun radiation that interacts with the atomic structures of semiconductor and thus, to multiply the total efficiency.

Quantum dots (nanoparticles) – semiconductor crystals, which size is a few nanometers. They own positive quantum optical properties, which are absent in bulk materials, thanks to the conduct of «electro-holes» pairs (excitons) in quantum dots. In particular, quantum dots give a possibility of tuning of band-gaps. Changing the size of quantum dots, it is possible to regulate the spectral range of work of semiconductor. In other words, nanostructures allow to regulate a wave-length, on which the photons are irradiated or absorbed. The greater is the width of the band-gap of a semiconductor, the more energetic the photons absorbed. On the other hand, a lower bandgap results in the capture of more photons including those in the red end of the solar spectrum, resulting in a higher output of current but at a lower output voltage. The optimum bandgap that provides the maximal energy effectiveness of solar-electric energy conversion is achieved with the use of mixture of quantum dots of different sizes.

Quantum points can be mounted both into a variety of different form, in sheets or three-dimensional arrays in combination with organic polymers, dyes or porous films. In the colloidal form suspended in solution, they can to create junctions on inexpensive substrates such as plastics, glass or metal sheets. Three-dimensional grates from quantum points continue the life of excitons, facilitating the collection of ‘hot carriers’ to generate a few excitons at absorption of only one photon (today three exciitons are got at absorption of only one photon according to the reports National Renewable Energy Laboratory), thus the effectiveness of sun-electric energy transformation may be increased to 42%.

The multiexcitonic generation is possible only in the case, when the energy of absorbed photon considerably exceeds the energy of the bandgap. In particular a firm Solexant works on the development of high effective quantum dots sun elements on the basis of multiexcitonic generation. For quantum dots made of lead selenide PbSe with a diameter of 2,9 nm the efficiency yield 300%, when the energy of the photon absorbed is four times that of the bandgap. Similar results are got for lead sulfide quantum dots. In an ideal, the quantum dots solar cells may provide an efficiency of transformation of energy of the Sun into electric energy more than 65%, practically doubling the energy effectiveness of sun modules (Los Alamos National Laboratory). 

The scientist of Delaware University works on the creation of energy effective photovoltaic devices on the basis of semiconductor inorganic colloid nanostructure quantum dots. Tandem devices are developed by vapor deposition of quantum dots on the upper surface of conventional well known sun elements. Quantum dots have the different bandgaps.

The scientists of Pennsylvania State University develop semiconductor multijunction photovoltaic elements with the use of radial single junction (a-Si/nc-Si) - nanowires deposited on inexpensive glass substrates.

High efficiency of sun energy into electric energy transformation is expected for hybrid organic-inorganic photovoltaic elements. Inorganic nanorods with the tuned bandgap are combined with organic polymeric material with holes conductivity (University of Florida).

Due to combining of nanotubes with amorphous silicon it is planned to separate the pathways of photons and charges and thus to attain efficiency of 25% (Solasta). Technologies of forming of interface layer are developed for the photovoltaic bulk devices on the basis of discrete organic nanostructure with a size of 10 nm, in which the electro-active self-assembler molecules improve energy effectiveness due to reduction of recombination of charges in an interface layer (Washington University). The use of nanostructure materials on the basis of ZnO or Ag – nanoconductors is planned also for making of inexpensive contacts for sun elements (Stanford University).

Organic and polymeric sun elements are created from thin film (usually 100 μm) of organic semiconductors such as polymers integrated with a little quantity of the molecular compounds: polyphenylene vinylene, copper phthalocyanine, carbon fullerenes and fullerene derivatives. The polymeric elements differ strongly from the inorganic analogues. They do not rely on the large built-in electric field of a p-n-junction to separate the electrons and holes. The active region of organic element consists of two materials: donor and acceptor of electrons. At transformation of photon into «electron – hole» pair (in material of «donor») the charges try to remain bound in the form of exciton and are separated, when the exciton diffuses to the donor-acceptor interface. The short length of diffusion of polymeric elements limits an efficiency of transformation of sun energy into electric one. Energy effectiveness of polymeric compounds is today lower (5.7% related to publications of the National Renewable Energy Laboratory) comparatively with inorganic compounds. But these photosensitive elements are quickly perfected.

The works in direction of improvement of quality of materials, improvement of architecture and study of physics of work of excitonic organic sun elements are executed. The industrial samples of inexpensive thin film sun elements and modules on organic polymers are developed with possibility of their integration into build constructions (Konarka). The scientists of the University of Michigan work on the development of organic molecular multijunction tandem photovoltaic sun elements. Photo-electric sun elements on the basis of a multiple polymeric layers are created with separated donor, acceptor and interface layers and additive protective layer on the basis of gel (California University).

Thermo-voltaic elements – another direction of application of nanostructure polymers. Such possibility is given by the photosensitive conjugated polymers with additional single and double («carbon-carbon or «carbon-nitrogen») bounds, which allow advancing their conductivity by a few orders. To improve work of the conjugated polymers in the infra-red region of spectrum, the researchers of University of Toronto wrapped the polymers MEH-PPV (absorbs radiation in the interval of waves-lengths of 400…600 nm) around lead sulphide quantum dots (maximal absorption of radiation in an interval of 800…2000 nm). The wrapping of quantum dots by a polymer allows displacing the spectrum of absorption of light of polymer into an infra-red end of optical spectrum. Although as a result of experiment the weak energy effectiveness of polymer is got (0.001%), but, as researchers consider, for a few years a situation will be strongly changed to the best.  

Dye-sensitized solar cells (DSSC) – another paradigm of improvement of nanostructure photovoltaic sun elements. Such structure may be created on the basis of ruthenium metalorganic dye that is used as a layer of light absorbing material. The parameters of dye-sensitized sun element strongly depend on the use of mesoporous layer of the titanium dioxide to considerably (20…30 times) amplify the photosensitive surface area. The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO₂, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid.

Very prospect is also the photochemical Graetzel element. The technology for the Graetzel cell has been described as artificial photosynthesis. It uses inexpensive microscopic particles of titanium (a pigment used in white paints and toothpaste) covered with ruthenium dye and a conductive electrolyte fluid, all sandwiched between glass. Light on the dye excites electrons that are absorbed by the pigment and become relatively strong low-voltage electricity. New techniques for producing dye-sensitized solar cells have made it possible to inject the pigment in kilometer-long strips between clear flexible plastic and thin foil that can be cut into any shape or size (Pennsylvania State University). The sandwich is only about 1/8-inch thick. Graetzel's patented technology is being used to make flexible solar energy collectors. The dye attracts the light and the pigment stores the energy until it is needed. A 4x4-inch cell stores about 2 watts of energy.

The DSSC materials are presently developed (sensitizers, nanostructure layers, electroconductive materials (phases), electroconductive substrates. Researches are working to arise the stabilities of parameters of elements to prevent the degradation of organic-inorganic hybrids, and also inorganic Graetzel dye-sensitized sun elements. The goal of these researches is to increase of the number of the issued electrons on every photon of light (University of Colorado). The electrodes and electrolytes for dye-sensitized sun elements are developed with the use of polymeric gel electrolyte (Pennsylvania State University).

The laboratory samples of dye-sensitized sun elements attained the energy effectiveness > 11% at energy effectiveness of the modules 6.7% (NREL).

The process of shock ionization in composite nanocristalline photoelectric devices is optimized. In an electroconductive polymeric element the «Janus» nanoparticles are implanted to increase of multiexcitonic generation and energy efficiency (Voxtel).

Super-semiconductor solar cells. Multiexcitonic technologies that became possible at the use of the nanostructure mounted into the photosensitive matrices create a real possibility to improve considerably the energy effectiveness of photovoltaic semiconductor sun elements and attain almost 100% transformation of sun radiation energy into electric energy. These achievements allow conducting the speech about a possible appearance of the ideal «super-semiconductive» photovoltaic elements, which own by the properties of «superconductivity» - the phenomenon of great increase of pairs «electron-hole» (excitons) at certain terms. The terms  of «super-semiconductivity» and «super-semiconductor» are used related to the set notions: «superconductivity» and «superconductor», which are used for description of the phenomenon of flow of electric current in a solid without the losses, and also of the materials, to which is peculiar such phenomenon. The transition to the superconductive state takes place at a certain temperature, which is named the critical temperature of superconductive transition.

In obedience to the Bardeen-Cooper-Schrieffer microscopic theory of superconductivity the electrons of conductivity at certain temperatures unite in «cooper» pairs. The bound in such pairs is strong enough, and pairs, when moving along the grate, help each other to avoid dispersion. The attraction between the negatively charged electrons the mentioned theory explains in such way: during motion of electron along crystalline grate there is an electrostatic pushing away between him and negative electronic shells of atoms. These shells become deformed, removed from the electron which moves freely. In other words, the atoms are polarized. Near the electron a positive charge is formed, that can accompany an excited electron. To the appeared positive spatial charge some other electron will be attracted, that also will move with a positive charge synchronously, and, consequently, synchronously with the first electron. The electronic gas at achievement of superconductivity grows into a «cooper liquid».

Scientific association is in expectation of the creation of high effective super-semiconductor photovoltaic elements. It is necessary to mark, the photovoltaic semiconductors of sun elements own already in today's kind by the properties of semiconductor and conductor.

The pure super-crystalline organic materials conduct an electric current otherwise than conventional solid structures. Super-crystalline semiconductors can form the splashes of current on the electric contacts. In super crystals an atomic or molecular patterns are not only repeated oneself, but the super-structure of flat traps for electrons is repeated also. The distance between these plane traps, which are yet sometimes named «soliton walls» with properties of soliton – wave excitations in a nonlinear environment. The distance between these soliton walls are typically hundreds to thousands times greater than the distances between the organic molecules. Another abolition of super-crystalline semiconductors is a possibility to change the period and electronic properties of structure by changing of the external factors, for example, magnetic field.

To assure the wide introduction of super-semiconductor photovoltaic nanostructure elements into the sun devices it is necessary yet to conduct a plenty of researches, every time insignificantly changing ingredients for the receipt of new materials with necessary parameters. Photovoltaic nanostructure materials, which work at a room temperature, in particular must be created that would allow their wide use.

Vasil Sidorov on April 29, 2010 from Technopark QUELTA

E-mail: sidorovvasil@gmail.com