Energy conversion in solar cells based on the photovoltaic effect, which occurs in inhomogeneous semiconductor structures when exposed to solar radiation.
The nature of this phenomenon.
The heterogeneity of the structure of solar cells can be obtained by doping the same semiconductor with different impurities (the creation of p - n-junctions), or by combining different semiconductors with varying bandgap-energy electron detachment from the atom (the creation of heterojunctions), or by changing the chemical composition semiconductor, which leads to the appearance of the gradient of the band gap (the creation of graded-gap structures). There are also various combinations of the following methods.The conversion efficiency depends on the electrophysical characteristics of inhomogeneous semiconductor structure and optical properties of the solar panels, among which the most important role is played by the photoconductivity due to the internal phenomena in semiconductors irradiated by sunlight.The principle of solar cells can be explained by the example of converters with pn-junction, which are widely used in modern solar and cosmic energy. A pn junction is created by doping monocrystalline semiconductor material of the plate with a certain type of conductivity (ie, either p-or n-type) impurity, which provides the creation of a surface layer with a conductivity opposite tipa.Kontsentratsiya dopant in the layer must be much higher than the impurity concentration in the base (the original single crystal) material to neutralize the core there is available free charge carriers, and a conductivity of opposite sign. At the boundary of n-and p-layers as a result of charges peretechki depleted zones are formed with an uncompensated positive charge in the bulk n-layer and three-dimensional negative charge in the p-layer. These areas together to form a pn-junction.Arising in the transition potential barrier (contact potential) prevents the passage of the main charge carriers, ie electrons from the p-layer, but free to skip minority carriers in opposite directions. This property of the pn-junctions and determines the possibility of photo-emf FEP irradiated with sunlight.Created by light in both layers of FEP nonequilibrium charge carriers (electron-hole pairs) are separated by the pn-junction: minority carriers (t.e.elektrony) pass freely through the junction, and the main (holes) are delayed. Thus, under the influence of solar radiation through the pn-junction in both directions will leak current equilibrium minority carrier-photoelectrons and photo-holes that just need to work with FEP. If we now close the external circuit, the electrons from the n-layer, completing work on a load, will be back in the p-layer and recombine there (combined) with the holes moving inside the solar cells in the opposite direction. For the collection and removal of electrons to an external circuit on the surface of the semiconductor structure of FEP is the contact system. On the front, contact the illuminated surface of the converter are carried out in a grid or a comb, and on the back may be solid.
Major irreversible losses of energy in solar cells are associated with:- A reflection of sunlight from the surface proebrazovatelya,- The passage of radiation through the absorption of solar cells, without it,- Scattering on thermal lattice vibrations of excess photon energy,- Recombination fotopar formed on surfaces and in the amount of solar cells,- Internal resistance of the transducer,- And some other physical processes.
To reduce all forms of energy loss in solar cells are developed and successfully used a variety of activities. Among them are:- The use of semiconductors with the optimum for solar energy gap;- Improving the directional properties of the semiconductor structure by its optimal doping, and a built-in electric fields;- The transition from homogeneous to heterogeneous and graded gap semiconductor structures;- Optimization of the design parameters of solar cells (the depth of the pn-junction, the thickness of the base layer, the frequency of the contact grid, etc.);- The use of multi-functional optical coatings that provide illumination, temperature control and protection from cosmic radiation, solar cells;- Development of solar cells, transparent in the wavelength region of the solar spectrum beyond the edge of fundamental absorption band;- Creation of cascade solar cells of specially selected by the band gap semiconductors, allowing to convert in each stage of radiation passing through the previous stage, and so on;
Also, a significant increase in efficiency of solar cells has been achieved through the creation of probes with two-way sensitivity (up to 80% efficiency to an existing one hand), the use of luminescent reradiating structures, pre-expansion of the solar spectrum into two or more spectral regions with multilayer thin-film beamsplitters (dichroic mirrors), followed by conversion of each plot range of individual solar cells and t.d.5In the energy conversion systems SES (solar power) in principle can be used by any and currently being developed types of solar cells with different structures on the basis of a variety of semiconductor materials, but not all of them meet the complex requirements for these systems:- High reliability with long-term (ten years) of the resource;- Availability of raw materials to manufacture sufficient to convert the number of elements of the system and the ability to organize their mass production;- Acceptable in terms of energy payback period for a system of transformation;- Minimum expenditure of energy and mass associated with the management system conversion and energy transfer (space), including the orientation and stabilization of the plant as a whole;- Ease of maintenance.For example, some of the promising materials readily available in necessary to create a number of SES Islands because of the limited natural resources of raw materials and the complexity of its pererabotki.Otdelnye methods to improve energy and operational characteristics of solar cells, for example through the creation of complex structures, poorly compatible with the capabilities organization of mass production at low cost, etc.High performance can be achieved only if the organization is fully automated production of solar cells, for example, based on tape technology and creating a network of specialized companies developed the appropriate profile, ie in fact, an entire industry, comparable in scale to the modern electronic industry. Production of solar cells and assembly of solar panels on automated lines will provide cost savings in battery module 2-2.5.The most likely materials for photovoltaic conversion of solar energy in the SPP is being considered silicon and gallium arsenide (GaAs), and in the latter case we are talking about гетерофотопреобразователях (HFP) with the structure of AlGaAs-GaAs.
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