The development of modern industry, on the one hand, increase the demand for energy, causing the energy crisis; the other hand, the use of conventional energy, releasing large amounts of carbon dioxide gas, resulting in a global "greenhouse effect". To this end, countries trying to get rid of dependence on conventional energy sources, to accelerate the development of renewable energy. "Inexhaustible," as the best renewable energy, solar features, use of solar power has environmental advantages, but do not have to consider the security issues. In developed countries attach great importance to the EU countries are planning to power solar photovoltaic conversion in 2010 accounted for 1.5% of all electricity, the United States launched a "million roofs" program. Energy shortages, environmental protection is becoming increasingly serious in China, cost-effective use of solar energy is especially important Needs.
The solar cell is the use of the photovoltaic effect of solar energy is directly converted into electrical energy a device. Simple device of conventional solar cells shown in Figure 1. When the N-type and P-type semiconductor material of two different types of contact, due to diffusion and drift effects, built-in electric field at the interface formed by the P-type point to the N-type. When the photons of light in the surface of solar cells, energy is larger than the band gap will inspire the electron and hole under the action of the electric field, including the non-equilibrium minority carrier separated from the upper and lower poles of the battery cumulative, so that The battery can be provided to the outside load current.
Since the start of terrestrial solar cell commercialization from the mid-1970s, the crystalline silicon as a basic cell materials occupy a dominant position, and can be sure that this situation does not occur in the next 20 years a fundamental shift. Preparation of crystalline silicon material, solar cell including: monocrystalline silicon solar cells, cast polycrystalline silicon solar cells, amorphous silicon solar cells and thin-film crystalline silicon cells. Monocrystalline silicon cells with high cell conversion efficiency, good stability, but the cost is high; amorphous silicon solar cells with high efficiency, low cost, but low conversion efficiency, and efficient attenuation of the more powerful; cast polycrystalline silicon solar cells stabilized conversion efficiency, and the highest cost performance; thin-film crystalline silicon solar cells is now only in the R & D stage. At present, the cast polycrystalline silicon solar cell has been replaced by the Czochralski silicon photovoltaic materials become the most important. Cast polycrystalline silicon solar cell conversion efficiency slightly lower than the Czochralski monocrystalline silicon solar cells, various defects in the material, such as grain boundaries, dislocations, micro-defects, and carbon and oxygen impurities in the material, and process tarnished of the transition metal is considered to be a key reason for lower cell conversion efficiency, research on defects and impurities in the casting polysilicon law, as well as the process in the use of appropriate gettering and passivation process is the key to further enhance the casting of polycrystalline silicon cells. In addition, searching for casting polysilicon surface texture of wet chemical etching method is currently the low-cost preparation of high-efficiency battery technology.
PV materials from the solid-state physics in terms of silicon material is not the best, mainly because silicon is an indirect band semiconductor materials, the optical absorption coefficient is low, so the study of other photovoltaic materials become a trend. Among them, cadmium telluride (CdTe) and copper indium selenide (of CuInSe2) recognized two very promising photovoltaic material, and some progress has been made, but the distance from the large-scale production, and compete with crystalline silicon solar needs a lot of work to do.
