According to the materials used to prepare solar cells, in addition to the multi-element compound thin film solar cells, organic semiconductor thin film solar cells, and amorphous silicon thin film solar cells, there are two types of thin-film solar cells described in this article.
Dye-sensitized nano-thin film solar cell
The dye-sensitized nano-thin film battery is a battery invented by Dr. Michel Graetzel of the Swiss Federal Institute of Technology. Nanocrystalline chemical solar cells (NPC cells for short) are modified by a narrow band gap semiconductor material and assembled on another large energy gap semiconductor material. The narrow band gap semiconductor material uses transition metal Ru and organic compound sensitizing dyes, and the large energy gap semiconductor material is nano-polycrystalline TiO2 and made into electrodes. In addition, NPC batteries also use appropriate oxidation-reduction electrolytes. The working principle of nano-polycrystalline TiO2: the dye molecules absorb sunlight energy to transition to an excited state, the excited state is unstable, and electrons are injected into the adjacent conduction band of TiO2 quickly; the electrons lost in the dye are quickly compensated from the electrolyte, and the electrons that enter the conduction band of TiO2 eventually enter the conductive film, and then generate a photocurrent through the external circuit. It is a new type of battery in which the nanometer titania porous film is sensitized by photosensitive dye, which greatly improves the efficiency of the photoelectrochemical cell. This kind of battery has a stable efficiency outdoors. In 1998, the efficiency of the small-area battery of the Swiss Federal Academy of Sciences was 12%. Some countries have conducted pilot tests, and the specific battery efficiency is: Germany INAP’s 30cmx30cm is 6%; Australia STI’s 10cmx20cm is 5%. China’s large-area dye-sensitized nano-thin film solar cell research project with the Institute of Plasma Physics of the Chinese Academy of Sciences as the main undertaking unit has built a 500W array-scale small demonstration power station, making China a world leader in certain aspects of this research field.
Polycrystalline silicon thin film solar cell
Research work on polycrystalline silicon thin film batteries began in the 1970s, earlier than amorphous silicon thin film batteries. However, people’s attention was mainly focused on amorphous silicon thin film batteries. After the research work of amorphous silicon thin film batteries encountered difficult problems, people naturally began to pay attention to polysilicon thin film batteries. Since polycrystalline silicon thin film batteries use far fewer silicon materials than single crystal silicon batteries, there is no light-induced attenuation problem of amorphous silicon thin film batteries, and it is possible to prepare them on a cheap substrate, and the expected cost is much lower than that of single crystal silicon batteries, so people hope to reduce the cost of solar cell modules to about $1/W. The polycrystalline silicon thin film battery can also be used as the bottom cell of the amorphous silicon tandem cell, which can improve the spectral response and life of the battery. Therefore, the development has been relatively rapid since 1987. Now the photoelectric performance of polycrystalline silicon thin film batteries is stable, and the highest laboratory efficiency of Astro Power has reached 16%. At present, chemical vapor deposition methods are mostly used to prepare polysilicon thin film batteries, including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) processes. In addition, liquid phase epitaxy (LPE) and sputtering deposition methods can also be used to prepare polycrystalline silicon thin film batteries. LPE growth technology has been widely used in high-quality and compound semiconductor heterostructures, such as GaAs, AIGaAs, Si, Ge and SiGe, etc. The principle is to deposit silicon film by melting silicon in the matrix and lowering the temperature. U.S. Astro Power’s battery efficiency using LPE can reach 12.2%. Chen Zheliang of China Optoelectronics Development Technology Center used liquid phase epitaxy to grow silicon grains on metallurgical silicon wafers, and designed a new type of solar cell similar to crystalline silicon thin-film solar cells, called “silicon grain” solar cells
At present, the so-called third-generation solar cell research center of the University of New South Wales, led by Professor Martin Green, is actively carrying out theoretical research and scientific experimental work of ultra-efficient (>50%) solar cells. The focus of the research is how to fully collect the carriers that transition from the valence band to the high-level conduction band. The current research and experiment batteries mainly include superlattice batteries, “hot carrier” batteries, new types of “stacked” batteries, and “thermal photovoltaic” batteries.