Only a small part of the light energy projected on the entire illuminated surface of the solar cell can be turned into electrical energy, because it is affected by many factors. The radiation of sunlight in outer space is basically constant, but after being absorbed by the atmosphere with different composition and thickness (It includes the selective absorption of large and variable water vapor), the solar spectrum (Distribution of irradiance in different wavelength ranges) reaching the ground is changing anytime and anywhere. In general, the solar spectrum reaching the ground is in the wavelength range of 0.3~4μm, and its total energy is about 100mW/cm2. Because part of the sunlight is absorbed by the atmosphere, the solar spectrum reaching the ground is less than the solar constant.
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 constant, and the distance between the sun and the earth is different every day of the year. The radiation intensity at a certain point is inversely proportional to the square of the distance from the radiation source, which means that the solar radiation intensity above the earth’s atmosphere will vary 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 1.5×108km), 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.
Although the ground spectral irradiance varies, its variation has certain rules. In fact, the efficiency of solar cells is much lower than the theoretical calculation value. Because solar cells have a lot of losses during the conversion process, the losses can be summarized as follows:
(1) Part of the light projected on the surface of the solar cell is reflected off and does not enter the solar cell. This reflection loss is a considerable loss. The reflectivity of the pure silicon surface is about 30% in the 0.4~1μm wavelength range, and the reflectivity of other materials is also quite high. In order to reduce the loss, a thin layer of silicon oxide, or titanium oxide, cerium oxide, etc., is coated on the surface of pure silicon during the manufacture of solar cells to reduce reflection. These films are transparent in the spectral range.
(2) After light enters the solar cell, photons with energy greater than the “forbidden band” width (that is, those with a wavelength less than the cut-off wavelength) are absorbed by the solar cell, generating electron-hole pairs. The light that does not generate hole pairs is lost in the solar cell, and after the electron-hole pairs are generated, a part of the remaining energy is transferred to the semiconductor crystal lattice in the form of heat in a short period of time, which also causes loss. For silicon solar cells, it accounts for 53% of the total energy of light emitted by a person.
(3) Some of the minority carriers generated by light excitation flow to the PN junction (the formation of PN junction) in a diffusion manner, which is a part that contributes to the current output. The other part is far away from the junction and recombined on the surface and inside of the solar cell.
(4) The open circuit voltage of the solar cell is less than its band gap, 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 power curve, 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. What needs to be pointed out here is that when solar cells are used, multiple solar cells are combined, and they are combined in series and parallel. Because the voltage and current between them are difficult to be completely consistent, so they can not achieve the best working condition, so the solar cell module is less efficient than the monolithic solar cell.
Read more about: “PN junction photoelectric effect”.