Only a small part of the light energy projected on the entire illuminating surface of the solar cell can be converted into electricity, because it is affected by many factors. The radiation of sunlight in outer space is basically constant, but after absorption by the atmosphere with different compositions and thicknesses, including the selective absorption of water vapor with a large and variable content, the solar spectrum reaching the ground (irradiance in different distribution in the wavelength range) is changing anytime, anywhere. In general, the spectrum of sunlight reaching the ground is 0.3~4μm, and its total energy is about 100mW/cm², which is less than the solar constant due to absorption.
Solar radiation reaches the Earth through interstellar space, but because the Earth orbits the Sun in an elliptical orbit, the distance between the Sun and the Earth is not a constant, and the distance between the Sun and the Earth is different every day of the year. The intensity of radiation at a point is inversely proportional to the square of the distance from the radiation source, which means that the intensity of solar radiation above the Earth’s atmosphere varies with the distance between the sun and the earth. However, due to the large distance between the sun and the earth (the average distance is 1.5 x 108km), the intensity of solar radiation outside the earth’s atmosphere is almost constant. Therefore, people use the “solar constant” to describe the intensity of solar radiation above the earth’s atmosphere.
Solar cells In practical applications, the efficiency of solar cells is much lower than the theoretical calculation value. Because solar cells have many losses in the photoelectric conversion process, the losses include the following:
(1) Part of the light projected on the surface of the solar cell is reflected off without entering the solar cell. This reflection loss is a considerable loss. The reflectivity of the pure silicon surface is about 30% in the wavelength range of 0.4~1μm, while the reflectivity of other materials is also quite high. In order to reduce the reflection loss, a thin layer of silicon oxide or titanium oxide, ceria, etc. is coated on the surface of pure silicon to reduce reflection during the manufacture of solar cells. These thin layers are transparent in the spectral range.
(2) After the light enters the solar cell, the photons with energy greater than the “forbidden band” width (that is, the wavelength is smaller than the cut-off wavelength) are absorbed by the solar cell, and electron-hole pairs are generated, and the wavelength of electron-hole pairs is not generated in the solar cell. After the electron-hole pair is generated, a part of the remaining energy is transferred to the semiconductor lattice in the form of heat in a short time, resulting in loss. For silicon solar cells, it accounts for the total energy of incident light. 53%.
(3) Some of the minority carriers generated by photoexcitation flow to the PN junction by diffusion, which is a part that contributes to the current output. The other part moves away from the junction and recombines on and inside the solar cell.
(4) The open circuit voltage of the solar cell is less than its forbidden band width, and this loss is a voltage factor loss.
(5) The ratio of the maximum power output of a solar cell to the product of its open-circuit voltage and short-circuit current is called the curve factor (also known as the fill factor), and the loss of the solar cell during open-circuit and short-circuit is called power loss.
(6) The series resistance, contact resistance and film layer resistance of solar cells also cause losses. It should be pointed out here that when solar cells are used, they are combined with multiple solar cells, and they are combined in series and parallel. Because the voltage and current between them are difficult to be completely consistent, they cannot achieve the best working state. , so solar modules are less efficient than monolithic solar cells.