Classification and technological progress of solar cells

Classification and technological progress of solar cells
  1. Crystalline silicon solar cell technology
    Crystalline silicon cells are still the mainstream of current solar photovoltaic cells. Although technically speaking, crystalline silicon is not the best material, it is easy to obtain and its application technology is the same as that of the electronics industry. The highest theoretical efficiency of crystalline silicon single solar cells can reach 25%. In addition to efficiency, the thickness of the battery is also important. Thin silicon wafers mean less silicon material consumption, thereby reducing costs. In commercial production, the average thickness of silicon wafers has been reduced from 0.23mm in 2003 to 0.18mm in 2007, while the average efficiency has increased from 14% to 16%. By 2010, the thickness of the silicon wafer was reduced to 0.15mm, and the efficiency was increased to 17.5%.
  2. Thin-film solar cell technology
    Thin-film solar cells are made of cheap glass, stainless steel or plastic substrates coated with very thin photosensitive materials, which are cheaper than crystalline silicon solar cells that use more materials, and their price advantage can offset the current low efficiency problems. There are currently three types of thin-film photovoltaic cell materials that have been commercialized: amorphous silicon (a-Si), steel-selenium-copper/steel-selenium-nickel-copper (CIS/CIGS) and cadmium (CdTe). Their thickness is only a few microns, and can be automated when the output reaches a certain amount. At the same time, the process of producing components can also be optimized. The production process of thin-film battery modules uses less labor than the production of crystalline silicon battery modules, and the temporary shortage of silicon has also brought opportunities for thin-film technology to expand its market share. The European Energy Association predicts that thin-film photovoltaic cells will account for 20% of photovoltaic modules in 2010. Among the three commercialized thin-film photovoltaic technologies, the production and installation of amorphous silicon cells accounted for the largest proportion, accounting for 7.2% of the world’s total output in 2007. Multicrystalline thin film on glass (CSG) with glass substrate is a promising technology, and it has now entered the production stage. Microcrystalline silicon technology, especially the combination of amorphous silicon and microcrystalline silicon (a-Si/-Si), is another promising technology.
  3. Concentrating solar cell technology
    Concentrator cells use a variety of methods to focus sunlight to an effective area, so as to achieve the purpose of saving expensive semiconductor materials. The condensing method can use FRESNEL to focus the front sunlight transmission, or it can use reflection or refraction to focus. The condensing multiple of the concentrating solar cell can be from 2 times to more than 1000 times. Generally speaking, a focusing ratio of 20 times or less is a low-power focusing, 21 to 100 times is a medium-power focusing, and 100 times or more is a high-power focusing. At present, the efficiency of concentrating solar cells made of 11~V group compound (multijunction gallium arsenide type) semiconductors is the highest, reaching 40.7%. Concentrating solar cells also have their limitations in use: they have no effect on scattered light and cannot achieve the concentrating effect; a tracking system must be used to always aim the photovoltaic system at the sun.
  4. Dye-sensitized solar cells
    Dye-sensitized solar cells are a kind of photoelectrochemical cells, which are very different from physical cells based on the photovoltaic effect of semiconductor diodes. In recent years, photoelectrochemical cells composed of photoelectrodes, redox electrolytes and counter electrodes have received extensive attention and research. In this kind of photoelectrochemical cell, single crystal and polycrystalline forms of n-Si, p-Si, n-GaAs, p-GaAs, n-InP, p-InP and n-Cds and other semiconductor materials are used as photoelectric Extremely, in a suitable redox electrolyte, it can produce a photoelectric conversion efficiency of about 10%. However, the electrodes in the electrolyte often undergo severe light corrosion under irradiation conditions, and the battery stability is very poor. Therefore, dye-sensitized batteries with stable performance have become the focus of research and development of such batteries.

The oxide semiconductor material has good stability in the light solution, but its band gap is very wide and cannot absorb visible light. Wide band gap oxide semiconductor materials such as TiO2, ZnO and SnO2 sensitized with organic dye photosensitizers can absorb visible light, so they have been extensively studied in photographic technology since the 19th century.

After the photosensitive agent is adsorbed on the surface of the semiconductor, it can absorb visible light and excite electrons. The excited electrons are injected into the conduction band of the semiconductor electrode to improve the performance of the device. This kind of dye-sensitized semiconductor photoelectrode has been widely introduced in the research of solar cells in recent years. In order to improve battery efficiency, the dye photosensitizer should have the highest possible light yield and the widest possible absorption spectrum, while the semiconductor electrode should have a large surface area to absorb as much dye as possible.

In recent years, due to the development of high-performance nano-Ti02 film electrodes that can absorb a large amount of photosensitizers and the synthesis of a new type of rhodium (Rh) complex photosensitizer that can absorb a wide range of visible light and near infrared region of 400~800nm ​​or 900nm, the dye is sensitized The performance of solar cells (DSSC cells) has been greatly improved

Gratzel and his colleagues at the Lausanne Institute of Technology in Switzerland achieved a solar energy conversion efficiency of 7% to 10% under AMI.5 irradiation; they also developed a terpyridine complex photosensitizer with an absorption range of 900nm in the near-infrared region , The battery efficiency of 10.4% under AM1.5 conditions is obtained, the short-circuit current density Jsc=20.5mA/cm2, the open circuit voltage Voc=0.72v, and the fill factor FF=0.70.

There are many research institutions in my country engaged in the research of dye-sensitized Ti02 cells. The Hefei Institute of Plasma Physics, Chinese Academy of Sciences published in 2005 the cell efficiency of 8.95% (1 sun) and 9.18% (0.58 sun), the cell area is 0.21cm2, and the area The 1497.6cm2 pilot test battery has an efficiency of 5.7%, and an outdoor test array of 500W has been produced.

The material cost of DSSC battery is low, the packaging is simpler and easier, and the commercial manufacturing cost is expected to be quite low, so it is a promising solar cell.