Photovoltaic effect refers to the phenomenon that a potential difference occurs between different parts when light is irradiated on an uneven semiconductor or a combination of semiconductor and metal materials. The photovoltaic effect is a phenomenon in which materials absorb light energy to generate electromotive force. It is a process in which the sun’s light radiation energy is converted into electrical energy through semiconductor materials.
The photovoltaic effect can occur in gas, liquid and solid materials, but only in solid materials can there be higher energy conversion efficiency. Especially in semiconductor materials, the photovoltaic effect has the highest efficiency.
Substances that can use the photovoltaic effect to generate electricity are called photovoltaic materials. Choose photovoltaic materials with high energy conversion efficiency to make photovoltaic cells, which can be used for photovoltaic power generation.
The general requirements of photovoltaic power generation for photovoltaic materials are: high photoelectric conversion efficiency; the material itself does not cause pollution to the environment: the material is convenient for industrial production, and the performance of the material must be stable; and so on.
There are more than a dozen kinds of semiconductor materials for manufacturing photovoltaic cells.
The first generation of photovoltaic cells are mainly based on silicon wafers, made of monocrystalline silicon and polycrystalline silicon materials, and are still the mainstream in the photovoltaic product market. The reserves of silicon in the earth’s crust are second only to oxygen, and the raw materials are quite abundant. However, the cost of crystalline silicon as a photovoltaic material in photovoltaic cells is relatively high (silicon materials used to make crystalline silicon cells account for more than 45% of the cell cost), and the size of silicon crystals cannot meet the requirements for large areas.
In addition to silicon (including monocrystalline silicon, polycrystalline silicon and amorphous silicon), the photovoltaic materials used include gallium arsenide, indium phosphide and other III-V compounds (gallium arsenide photovoltaic cells can withstand high temperatures, under the condition of 250 ℃ Good photoelectric conversion performance, suitable for high-concentration photovoltaic cells: but the cost is high, the main material gallium arsenide is difficult to prepare), cadmium sulfide and other group II-VI compounds, copper indium selenium and other multi-component compounds, and some functional polymer materials And the nanocrystalline material under development.
Photovoltaic cells are devices that use the photovoltaic effect to directly convert solar energy into electrical energy, also known as solar cells.
Common photovoltaic cells are composed of many single photovoltaic cells. Single photovoltaic cell refers to the smallest photovoltaic cell unit that has positive and negative electrodes and can convert light energy into electrical energy.
The structure of a typical single silicon photovoltaic cell is shown in Figure 1. From high-purity N-type or P-type single crystal silicon rods, a single wafer with a thickness of 0.25~0.5mm and a circular shape (30~100mm in diameter) or square (2cm×1cm, 1cm×2cm) is made. The battery base part (also called the substrate, 4 in Figure 1), diffuses some impurities different from the material on the surface to form a diffusion term area (3 in Figure 1) with a thickness of about 0.3μm, forming a P-N This is the core part of photovoltaic cells. The electrode drawn from the surface of the battery is the upper electrode, which is generally made of aluminum and silver materials to make a slender grid-shaped structure; the electrode drawn from the bottom of the battery is the lower electrode, and the lower electrode is generally made of nickel-tin material into a plate-shaped structure. The function of the upper and lower electrodes (1, 5 in Figure 1) is to draw out the photoelectromotive force; in order to increase the light energy absorption on the surface of the silicon wafer and reduce the light reflection loss, a layer of silicon dioxide and other materials should be plated on the surface of the battery The main function of the cover plate (2 in Figure 1) is to prevent moisture and dust.
There are two core steps in the manufacture of crystalline silicon photovoltaic cells: not the preparation of crystalline silicon wafers; the second is to manufacture photovoltaic cells on silicon wafers. Since the 21st century, crystalline silicon has been in short supply in the international market, and 80% of my country’s polycrystalline silicon raw materials have relied on imports.
The working principle of crystalline silicon solar photovoltaic cells is shown in Figure 2. When N-type silicon and P-type silicon are combined, electrons (with negative charges) in the N-type region diffuse to the P-type region, and holes (with positive charges) in the P-type region diffuse to the N-type region. Figure 2(b). At this time, the N-type zone is positively charged, and the P-type zone is negatively charged, and a built-in electric field is generated inside the silicon semiconductor. Figure 2(c), a voltage appears on both sides of the P-N junction. When sunlight shines on the semiconductor P-N junction, the photons in the solar radiation penetrate into the semiconductor, generating freely movable electrons and holes, forming new hole-electron pairs. Under the action of the P-N junction electric field, holes flow from the N-type area to the P-type area, and electrons flow from the P-type area to the N-type area. As a result, the contact electrodes at both ends of the P-N junction will be charged with positive and negative charges, respectively. If the circuits on both sides of the P-N junction are connected, a current will flow from the P-type area to the N-type area through the external circuit. If there is a load connected to the external circuit, it will output electric power to the load.
During this power generation process, the photovoltaic cell itself neither undergoes any chemical changes nor mechanical wear.
The polarity of the output voltage of the photovoltaic cell, with the P terminal as the positive electrode and the N terminal as the negative electrode. When the photovoltaic cell is used as an electro-diarrhea independently, it should be in a positive power supply state, that is, the current flows from the P terminal to the N terminal through the external circuit: when it is mixed with other power sources, the polarity of the photovoltaic cell is different, which determines Whether the battery is in a forward-biased or reverse-biased form. The photoelectric conversion efficiency of photovoltaic cells is mainly related to its structure, PN junction characteristics, material properties, battery operating temperature, radiation damage of radioactive particles, and environmental changes. Calculations show that when the air quality is AM 1.5, the current theoretical upper limit of the conversion efficiency of silicon solar cells is about 33%, and the actual efficiency of monocrystalline silicon photovoltaic cells is between 12% and 20%.
Photovoltaic cells are classified according to the structure (mainly the characteristics of the PN junction), and can be divided into homojunction photovoltaic cells, heterojunction photovoltaic cells, Schottky junction photovoltaic cells, composite junction photovoltaic cells, laminated photovoltaic cells, and thin films Photovoltaic cells (Figure 3) and wet photovoltaic cells.