The previous article described the types of crystalline silicon solar cells. This article will introduce the types of thin film solar cells
According to the materials used to prepare solar cells, thin-film solar cells can be divided into the following categories.
(1) Multicomponent compound thin film solar cells
Copper-in-selenium (CIS) thin-film solar cells: CuInSe2 has a band gap of 1.53eV, which is regarded as an ideal photovoltaic material. It can form high-conductivity Р-type and N-type only by introducing its own defects, which reduces the battery With regard to the requirements of grain size, impurity content, and defects, the battery efficiency has reached 15.4%. Doping an appropriate amount of Ga, Al or S can increase its band gap, which can be used to make high-efficiency single-junction or laminated batteries. CuInSe2 is a ternary Ⅰ-Ⅲ-VI2 compound semiconductor, and CuInSe2 is a direct band gap semiconductor material with an absorption rate of up to 105/cm. The electron affinity of CuInSe2 is 4.58eV, which is very small (0.08eV) from the electron affinity of CdS (4.50eV), which makes the heterojunction formed by them without conduction band spikes and reduces the potential of photogenerated carriers. base. The CuInSe2 thin film growth process is as follows: generally vacuum evaporation method, Cu-In alloy film selenization treatment method (including electrodeposition method and chemical thermal reduction method), closed space vapor transport method (CsCVT), spray pyrolysis method, Radio frequency sputtering method and so on. CIS solar cells are photovoltaic devices formed by depositing multiple layers of thin films on glass or other inexpensive substrates. The structure is: light→metal grid electrode/anti-reflection film/window layer (ZnO)/transition layer (CdS)/ Light absorbing layer (CIS)/metal back electrode (Mo)/substrate.
Cadmium telluride thin film solar cells: CdTe has a direct band gap of 1.5eV, and its spectral response is very consistent with the solar spectrum. It has a high absorption coefficient in the visible light range, and it can absorb 90% when it is 1um thick. CdTe is a group II-VI compound. Because the CdTe film has a direct band gap structure, its light absorption coefficient is extremely large, thus reducing the requirement for the diffusion length of the material. The thin-film semiconductor material with CdTe as the absorber and the window layer CdS form a heterojunction solar cell. Its structure is: light→anti-reflection film (MgF2)/glass substrate/transparent electrode (SnO2:F)/window layer (CdS) / Absorbing layer (CdTe) / Ohmic contact transition layer / Metal back electrode. The preparation methods include sublimation, MOCVD, CVD, electrodeposition, screen printing, vacuum evaporation and atomic layer epitaxy. Various methods have been used to produce CdTe thin-film solar cells with a conversion efficiency of more than 10%. Among them, the efficiency of the cell deposited with the CdS/CdTe junction reached 16.5%.
Nickel arsenide thin film solar cells: The battery material has a moderate band gap, and has better radiation resistance and high temperature performance than silicon. Solar cells can obtain higher efficiency. The maximum efficiency of the laboratory has reached more than 24%. The efficiency of solar cells for general aerospace use It is also between 18%~19.5%. The efficiency of the single junction cell grown on a single crystal substrate is 36% of the theoretical efficiency of the GalnP2/GaAs cascade cell. A laminated solar cell with an area of 4m2 and a conversion efficiency of 30.28% has been produced in the laboratory. Currently, gallium arsenide solar cells are mostly prepared by liquid phase epitaxy or metal organic chemical vapor deposition technology, so the cost is high, the output is limited, and cost reduction and production efficiency improvement have become the research focus. GaAs solar cells are currently mainly used in spacecraft.
(2) Organic semiconductor thin film solar cells
Organic semiconductors have many special properties and can be used to manufacture many thin-film semiconductor devices, such as field-effect transistors, field-effect electro-optic modulators, light emitting diodes, photovoltaic devices, etc. Organic semiconductors absorb photons to produce “electron-hole pairs” with a binding energy of about 0.2~1.0eV. The dissociation of “electron-hole pairs” at the interface between P-type semiconductor materials and N-type semiconductor materials leads to efficient charge separation, resulting in a normal The so-called heterojunction solar cell. Organic semiconductors used in photovoltaic devices are roughly divided into molecular organic semiconductors and polymer organic semiconductors. Later, double-layer organic semiconductor heterojunction solar cells appeared. Organic semiconductors can be classified into three categories: soluble, insoluble, and liquid crystal according to their chemical properties; sometimes they are divided into three categories: dyes, pigments, and polymers based on monomers. The doping of organic semiconductors adopts the introduction of other molecules and atoms, and electrochemical methods can also be used to oxidize them. Impurities that can make it into P type include Cl2, Br2, I2, NO2, TCNQ, CN-PPV and so on. Doping with alkali metals can make it N-type.
(3) Dye-sensitized nano-thin film solar cells
The dye-sensitized nano-thin film battery is a battery invented by Dr. Michel Graetzel of Switzerland. Nanocrystalline chemical solar cells (NPC cells for short) are modified by a narrow band gap semiconductor material and assembled on another large band gap semiconductor material. The narrow band gap semiconductor material uses transition metal Ru and organic compound sensitizing dyes. , The large energy gap semiconductor material is nano-polycrystalline TiO, which is made into electrodes. In addition, NPC batteries also use appropriate oxidation-reduction electrolytes. The working principle of nanocrystalline TiO2: the dye molecule absorbs sunlight and transitions to an excited state. The excited state is unstable. The electrons are quickly injected into the adjacent conduction band of TiO2. The electrons lost in the dye are quickly compensated from the electrolyte and enter the TiO2 conduction band. The electrons in the belt eventually enter the conductive film, and then generate a photocurrent through the outer loop. 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 small-area batteries in Switzerland was 12%. Some countries have carried out pilot tests. The specific battery efficiency is: Germany INAP’s 30cm×30cm is 6%; Australia STI’s 10cm× 20cm is 5%. The large-area dye-sensitized nano-thin film solar cell research project of China 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, enabling China to enter some aspects of this research field Ranks among the world’s leaders.
(4) Amorphous silicon thin film solar cells
Amorphous silicon is the earliest commercialized thin film battery. A typical amorphous silicon (α-Si) solar cell is to deposit a transparent conductive film (TCO) on a glass substrate, use plasma reaction to deposit P-type, 1-type, and N-type three layers of α-Si, and then evaporate metal on it The electrode Al/Ti, the light enters from the glass layer, and the battery current is drawn through the transparent conductive film and the metal electrode Al/Ti. Its structure is glass/TCO/P-I-N/AI/Ti. The substrate can also be made of plastic film, Stainless steel sheet and so on. After the introduction of a large amount of hydrogen (10%) in amorphous silicon, the forbidden band width increases from 1.leV to 1.7eV, which has strong light absorption. In addition, a thick “intrinsic layer” is added between the thinner P and N layers to form a P-I-N structure. The I layer with less impurity defects is used as the main absorption layer to form an electric field in the region where the photo-generated carriers are generated, which enhances the collection effect of carriers. In order to reduce the loss caused by the large lateral resistance of the top thin doped layer, the upper electrode of the battery adopts a transparent conductive film, and a texture is prepared on the transparent conductive film to enhance light transmission. At present, the most used transparent conductive materials are SnO2 and ITO (a mixture of In2O3 and SnO2), and ZAO (aluminum-doped zinc oxide) is considered a new type of excellent transparent conductive material. Due to the wide energy distribution of sunlight, semiconductor materials can only absorb photons with energy higher than its energy gap value, and the rest of the photons will be converted into heat energy, but cannot be converted into effective electrical energy through the photo-generated carriers transferred to the load. Therefore, For single-junction solar cells, even if they are made of crystalline materials, the theoretical limit of their conversion efficiency is only about 29%. In the past, amorphous silicon batteries were mostly in the form of single-junction batteries. Later, double-junction tandem batteries were developed to collect photo-generated carriers more effectively. BP Solar uses Si-Ge alloy as the bottom cell material. Because the band gap of Si-Ge alloy is relatively narrow, it enhances the spectral response of the cell as the bottom cell material. Beckaert uses amorphous silicon with different Ge content to make the three-junction series cell with two bottom cells, creating the highest stable efficiency of 6.3% for the amorphous silicon cell module. Among thin-film solar cells, amorphous silicon cells were the first to be commercialized. In 1980, Sanyo Electric Co., Ltd. used α-Si solar cells to make pocket calculators. In 1981, they realized industrial production. The annual sales of aSi cells accounted for To 40% of the world’s photovoltaic sales, with the continuous improvement of the performance of amorphous silicon cells, the cost continues to decline, and its application areas are also expanding. Expanded from calculators to a variety of consumer products and other fields. Such as solar radios, street lights, microwave relay stations, traffic crossing signal lights, weather monitoring and photovoltaic water pumps, household independent power supplies, and grid-connected power generation.
(5) Polycrystalline silicon thin film solar cells
The research work on polycrystalline silicon thin film batteries began in the 1970s, earlier than amorphous silicon thin film batteries, but people’s attention was mainly focused on amorphous silicon thin film batteries at that time, and the research work on amorphous silicon thin film batteries encountered difficulties. After solving the problems, people naturally pay more attention to polysilicon thin film batteries. Since polycrystalline silicon thin film batteries use far less silicon materials than single crystal silicon batteries, there is no light-induced attenuation problem of amorphous silicon thin film batteries, and it may be prepared on a cheap substrate. The expected cost is much lower than that of single crystal silicon batteries. It is hoped that the cost of solar cell modules will be reduced to about $1/W. Polycrystalline silicon thin film batteries can also be used as the bottom cell of amorphous silicon tandem cells, which can improve the spectral response and life of the battery. Therefore, it has developed rapidly since 1987. The photoelectric performance of polycrystalline silicon thin-film batteries is now stable, and Astropower’s highest laboratory efficiency has reached 16%. Currently prepared polysilicon thin film batteries mostly use chemical vapor deposition methods, 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, AlGaAs, Si, Ge and SiGe. The principle is to lower the temperature to deposit a silicon film by melting silicon in the matrix. The efficiency of cells prepared by Astropower in the United States 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 kind of crystalline silicon thin film solar energy. The new type of solar cell is called “silicon pellet” solar cell.
At present, the 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 experiment work on ultra-high-efficiency (>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, quantum dot batteries, new “stacked” batteries and “thermal photovoltaic” batteries.