The semiconductor solar cell is a semiconductor photodetector that has been optimized to absorb part of the sunlight and convert the voltage and current. However, it is different from general battery applications: the output voltage and current of the semiconductor solar cell will be affected by the load and change, unlike the general battery that can output a fixed voltage; when there is appropriate light, the semiconductor solar cell can output electrical energy, that is, Said that semiconductor solar cells do not have the ability to store electrical energy.
Semiconductor materials can absorb photons to generate electrons and holes. Through appropriate design, the semiconductor materials of different doping types are combined to form a diode. A semiconductor diode has a built-in electric field, which separates carriers (electrons and holes are collectively referred to as carriers) and forms a current in a specific direction. Therefore, a semiconductor solar cell is basically a designed semiconductor diode that can absorb light waves with energy greater than the energy gap of the semiconductor in the solar spectrum, and convert the energy of sunlight into electrical energy. Figure 1.1 is a schematic diagram of the semiconductor solar cell structure. Sunlight enters from the front of the battery, most of the light waves penetrate the anti-reflection layer and enter the semiconductor layer, and a small part of the light waves will be reflected back to the atmosphere by the metal mesh grid and the anti-reflection layer. The upper electrode contact of the diode is composed of a metal mesh grid. The design considers that the light wave is injected into the semiconductor by reducing the shading area, and the semiconductor absorbs the light energy and converts it into electrical energy. The anti-reflection layer between the mesh grids will increase the amount of light absorbed by the semiconductor. A semiconductor diode is composed of N-type semiconductor and P-type semiconductor. To make such a device, impurities need to be doped through diffusion, ion implantation or deposition to form a PN junction. The other electrode contacts of the diode are behind the solar cell. It is made by plating with a metal layer.
Figure 1.1 Schematic diagram of semiconductor solar cell structure
All electromagnetic radiation, including sunlight, is composed of photons, and they all carry a specific amount. Photons also have the properties of waves, so they have wavelength properties. The corresponding relationship between photon energy and light wave wavelength is
In the formula, h is Planck’s constant; c is the speed of light. Only photons with sufficient energy (greater than the energy gap of semiconductor materials) can generate electron-hole pairs, which is helpful for the generation of electric energy. Therefore, the solar spectrum is an important factor when designing effective solar cells.
The surface temperature of the sun is about 5762K, and its radiation energy spectrum is very close to blackbody radiation, covering the spectrum from ultraviolet to infrared (0.2~3um). Solar radiation is isotropic like all black body radiation. However, the distance between the sun and the earth is very far (approximately 1.5×108km), so only part of the photons can directly hit the earth. In practical applications, the sunlight incident on the surface of the earth is often regarded as a parallel beam. Outside the earth’s atmosphere, at the average distance of its orbit around the sun, the solar radiation intensity is defined as the solar constant, which is about 1366W/m2. When sunlight enters the atmosphere from outside the atmosphere, it will be scattered and absorbed by the clouds and the atmosphere. Its energy intensity decreases with the path length of the light through the atmosphere (or the air quality through which the light passes), so it is defined as “Air Mass” (Air Mass). ) To indicate how much of the solar radiation reaches the surface of the earth after the solar radiation passes through the atmosphere. Because the air quality through which sunlight passes is basically related to the angle between the sun’s azimuth and the vertical line of the earth’s surface, the air quality value is defined as
In the formula, θ is the angle of incidence (when the sun is directly above the head, θ=0). The air quality can be easily deduced from the height H of the object and its shadow length S
Since sunlight is scattered and reflected in the atmosphere, it absorbs the diffused part of the sunlight (indirect incidence) on the surface. This part of the light is about 20% of the direct incident light. Due to the diffuse part, for the sake of clarity, g (global) or d (direct) is often added to the air quality value to further define it. For example, the AM1.5g spectrum includes diffuse light, and the AM1.5d spectrum does not include diffuse light.
Figure 1.2 Black body radiation
Figure 1.2 shows the black body radiation (T=5762K), AM0 and AM1.5g solar radiation spectra, where the AM0 curve represents the zero air quality situation and represents the solar spectrum outside the earth’s atmosphere. The AMO spectrum is related to satellite and space exploration applications. Generally, on the surface of the earth, the solar radiation spectrum is represented by air quality 1.5 (AM1.5). This spectrum represents the spectrum of sunlight falling on the surface of the earth when the sunlight is at an angle of 48° to the vertical, and its total incidence The power density is about 963W/m2.