In order to enhance our understanding of solar cells, this paper will introduce three kinds of solar cells: 1. Multi compound thin film solar cells, 2. Polymer multilayer modified electrode solar cells, 3. Nanocrystalline chemical solar cells. If you are interested in solar cells, you may as well read on.
1、 Multicomponent compound thin film solar cells
In order to find a substitute for monocrystalline silicon solar cells, people have developed not only polycrystalline silicon and amorphous silicon thin film solar cells, but also other materials of solar cells. They mainly include gallium arsenide III-V compounds, cadmium sulfide, cadmium sulfide and copper selenium thin film batteries. Among the above cells, although the efficiency of CDs and CdTe polycrystalline thin film solar cells is higher than that of amorphous silicon thin film solar cells, the cost is lower than that of monocrystalline silicon solar cells, and it is easy to be produced on a large scale, cadmium is highly toxic and will cause serious pollution to the environment, It is not the most ideal alternative for crystalline silicon solar cells. Gallium arsenide III-V compound and copper indium selenium thin film solar cells have received widespread attention due to their high conversion efficiency. GaAs belongs to III-V compound semiconductor materials, and its energy gap is 1.4ev, which is just the value of high absorption solar light, so it is an ideal battery material. MOVPE and LPE technologies are mainly used in the preparation of III-V compound thin film batteries such as GaAs. The preparation of GaAs thin film batteries by MOVPE method is affected by many parameters such as substrate dislocation, reaction pressure, III-V ratio and total flow rate. In addition to GaAs, other III-V compounds such as GaSb and GaInP have also been developed. In 1998, the conversion efficiency of GaAs solar cells made by the solar system research institute in Freiburg, Germany, was 24.2%, which was the European record. The conversion efficiency of GaInP battery prepared for the first time is 14.7%. See Table 2. In addition, the Institute also uses the stack structure to prepare GaAs, GaSb batteries, which stack two independent batteries together. GaAs is used as the upper battery, and GaSb is used as the lower battery. The efficiency of the battery reaches 31.1%.
CuInSe2 is called CIC for short. The energy of CIS material is reduced to 1.5 EV, which is suitable for the photoelectric conversion of sunlight. In addition, CIS thin film solar cells do not have the problem of light decay. Therefore, CIS as a high conversion efficiency thin film solar cell material has attracted people’s attention.
The preparation of CIS thin films mainly includes vacuum evaporation and selenidation. In vacuum evaporation method, copper, indium and selenium are evaporated from their respective evaporation sources, while in selenidation method, H2Se is used as the selenide. However, CIS with uniform composition is difficult to be obtained by this method. The conversion efficiency of CIS thin film battery has increased from 8% in 1980s to about 15% at present. The photoelectric conversion efficiency of the gallium doped CIS battery developed by Matsushita Electric Industry Company of Japan is 15.3% (area 1cm2). In 1995, the US Renewable Energy Research Institute developed CIS solar cells with a conversion efficiency of 17.1%, which is the highest conversion efficiency in the world so far. It is estimated that the conversion efficiency of CIS cells will reach 20% by 2000, which is equivalent to that of polysilicon solar cells.
CIS, as a semiconductor material of solar cells, has the advantages of low price, good performance and simple process, which will become an important direction for the development of solar cells in the future. The only problem is the source of materials. Because indium and selenium are relatively rare elements, the development of such batteries is bound to be limited.
2、 Polymer multilayer modified electrode solar cells
Polymer instead of inorganic materials in solar cells is a new research direction of solar cells. The principle of this device is to make use of different redox potentials of different redox polymers to make multilayer composite on the surface of conductive materials (electrodes) to make a unidirectional conductive device similar to inorganic p-n junction. The inner layer of one electrode is modified by a polymer with lower reduction potential, while the outer layer is modified by a polymer with higher reduction potential; The modification of the other electrode is just the opposite, and the reduction potentials of the two polymers on the first electrode are higher than those on the latter. When the two modified electrodes are put into the electrolytic wave containing photosensitizer. The electrons generated by the photosensitizer after absorbing light are transferred to the electrode with lower reduction potential. The electrons accumulated on the electrode with lower reduction potential can not be transferred to the outer layer polymer, and can only return to the electrolyte through the electrode with higher reduction potential through the external circuit, so there is photocurrent in the external circuit.
Due to the advantages of good flexibility, easy fabrication, wide sources of materials and low cost, organic materials are of great significance for the large-scale utilization of solar energy and the provision of cheap electric energy. However, the research on the preparation of solar cells with organic materials is just at the beginning, and neither the service life nor the cell efficiency can be compared with inorganic materials, especially silicon cells. Whether it can be developed into a practical product remains to be further explored.
3、 Nanocrystalline chemical solar cells
Among the solar cells, silicon solar cells are undoubtedly the most mature, but due to the high cost, it is far from meeting the requirements of large-scale application. For this reason, people have been exploring in the process, new materials, thin film of solar cells and so on. Among them, the newly developed nano-TiO2 crystal chemical energy solar cells have attracted the attention of scientists at home and abroad. Since Prof. Gratzel of Switzerland successfully developed nano-TiO2 chemical large solar cell, some domestic units are also carrying out research in this field. Nanocrystalline chemical solar cells (NPC cells) are formed by modifying and assembling a semiconductor material with a band gap to another semiconductor material with a large energy gap. The narrow band gap semiconductor material is sensitized by organic compounds such as transition metals Ru and OS. The large energy gap semiconducting material is nano polycrystalline TiO 2 and is made into electrodes, In addition, suitable redox electrolyte was selected for NPC battery. The working principle of Nanocrystalline TiO2: dye molecules absorb solar energy and jump to the excited state, the excited state is unstable, the electrons are injected into the adjacent TiO2 conduction band quickly, the lost electrons in the dye are quickly compensated from the electrolyte, the electricity in the TiO2 conduction band finally enters the conductive film, and then generates photocurrent through the external circuit.
The advantages of Nanocrystalline TiO2 solar cells lie in its low cost, simple process and stable performance. Its photoelectric efficiency is more than 10%, its cost is only 1 / 5 ~ 1 / 10 of that of silicon solar cell, and its lifetime can reach more than 20 years.