Solar Cells
Alternative and renewable sources of energy such as wind and sunlight, hydro and geothermal energy, all over the world attract more attention. The growing interest in them due to environmental considerations, on the one hand, and the limitations of traditional terrestrial resources - on the other. A special place among the alternative and renewable energy take photoelectric solar energy converters, the study of which has become a separate scientific field - Photovoltaic. However, the high cost of solar cells until recently closed its way into the area where you can get by without them. But times change, and economically advanced states in their national programs have stimulated massive use of solar panels. It - fashion, transnational lobbying someone's interests or stable trend whose time has come?
The source, which did not run out
The energy source of solar radiation, is a thermonuclear reaction - every second of the Sun ~ 6 * 1011 kg of hydrogen converted into helium. The mass defect at the same time is 4000 kg , which according to Einstein relation E = mc2 leads to the allocation of 4 * 1020 joules of energy. Most of this energy is emitted in the form of electromagnetic radiation in the range of 0,2-3 microns. Since the total mass of the Sun ~ 2 * 1030 kg , it should remain in fairly stable condition more than 10 billion years with a constant release of energy. The intensity of solar radiation in free space at a distance equal to the average distance between Earth and the Sun, called solar constant. Its value - 1353 W/m2. When passing through the atmosphere, sunlight is attenuated mainly due to absorption of infrared radiation by water vapor, ultraviolet radiation - the ozone and scattering of radiation by particles of atmospheric dust and aerosols. Rate of atmospheric influence on the intensity of solar radiation reaching the earth's surface, called the "air mass" (AM). AM is defined as the secant of the angle between the sun and the zenith. Figure 1 shows the spectral distribution of solar radiation intensity in different conditions. The upper curve (AM0) corresponds to the solar spectrum outside the Earth's atmosphere (for example, on board the spacecraft), i.e. zero air mass. It is approximated by the distribution of the intensity of blackbody radiation at a temperature of 5800 K. The curves of AM1 and AM2 illustrate the spectral distribution of solar radiation at Earth's surface, when the sun is at its zenith and when the angle between the sun and the zenith of 60 °, respectively. Thus the total radiated power - respectively about 925 and 691 W/m2. Average intensity of radiation on the Earth roughly coincides with the intensity of radiation at AM = 1.5 (Sun - 45 ° angle to the horizon). Thus, the use of efficient methods for converting solar energy can provide the rapidly growing demand for it almost forever.
Basic principles of solar
The simplest design of a solar cell (ASE) - device for converting solar radiation - on the basis of single-crystal silicon is shown in. At a small depth from the surface of a silicon wafer of p-type formed pn-junction with thin metal contacts. On the back side of the plate caused a continuous metallic contact. The simplest design of a solar cell (ASE) - device for converting solar radiation - on the basis of single-crystal silicon is shown in Fig. 2. At a small depth from the surface of a silicon wafer of p-type formed pn-junction with thin metal contacts. On the back side of the plate caused a continuous metallic contact. When solar cells illuminated, the absorbed photons generate no equilibrium electron-hole pairs. Electrons generated in the p-layer near the pn-junction, approaching pn-junction and its existing electric field are made in the n-region. Similarly, the excess holes created in the n-layer, partially transferred to the p-layer. As a result, n-layer acquires an additional negative charge, and p-layer - positive. Reduces initial contact potential difference between p-and n-type semiconductor layers, and in the external circuit voltage appears. The negative pole of the current source corresponds to the n-layer and p-layer - positive. The value of the usual photo-emf in the light of transition radiation of constant intensity is described by the current-voltage characteristics (VAC): U = (kT / q) ln ((Iph-I) Is / +1) where Is-saturation current, and Iph - photocurrent. CVC explains the equivalent circuit of the photocell, which includes a current source Iph = SqN0Q, where S - area of the photocell, but coefficient of collecting Q - dimensionless factor (<1), showing what percentage of all the created light of electron-hole pairs (SN0) pn-going transition. In parallel, a power supply included pn-junction, the current through which is equal to Is [eqU/kT-1]. Pn-junction shunts the load, and with increasing voltage current through it increases rapidly. In the load (resistance R) shown in the current I.
The problems of finding and using designs and materials for solar cells
For efficient operation of solar cells must be met certain conditions:
• optical absorption coefficient (a) an active semiconductor layer must be large enough to provide a substantial part of the energy absorption of sunlight within the thickness of the layer;
• generated upon illumination, electrons and holes must be effectively collected on the contact electrodes on both sides of the active layer;
• solar cell must have a large barrier height in semiconductor junction;
• the total resistance in series with the solar cell (excluding the load resistance) must be small in order to reduce power losses (Joule heat) in the process of work;
• the structure of the thin film should be uniform throughout the active region of the solar cell to prevent shorting and the influence of shunt resistance on the characteristics of the element.
Production of structures based on single-crystal silicon, satisfying these requirements - the process is technologically complex and expensive. Therefore, attention was drawn to materials such as alloys based on amorphous silicon (a-Si: H), gallium arsenide, and polycrystalline semiconductors. Amorphous silicon acted as a cheaper alternative to single-crystal. The first solar cells based on it were created in 1975. Optical absorption of amorphous silicon is 20 times higher than the crystalline. Therefore, for a substantial absorption of visible light enough to film a-Si: H thickness of 0,5- 1,0 mm , instead of expensive 300-mm silicon wafers. In addition, the existing technology of thin films of amorphous silicon large area of operations is not required cutting, grinding and polishing necessary for solar cells based on monocrystalline silicon. Compared with polycrystalline silicon cells, products based on a-Si: H produced at lower temperatures (300 ° C): You can use cheap glass substrates, which will reduce the consumption of silicon in 20 times. While the maximum efficiency of the experimental elements on the basis of a-Si: H - 12% - slightly lowers efficiency of crystalline silicon solar cells (~ 15%). However, it is possible that with the development of technology efficiency of cells based on a-Si: H reaches the theoretical ceiling - 16%. The simplest design of solar cells and-Si: H was based on the structure of metal - semiconductor (Schottky diode). Despite the apparent simplicity, their implementation is problematic enough - a metal electrode must be transparent and uniform in thickness and all states at the metal / a-Si: H - stable over time. Most solar cells based on a-Si: H formed on the tape, stainless steel or glass substrates coated with a conducting layer.