The main reasons for the loss of solar cell conversion efficiency include electrical loss and optical loss.
Ⅰ. Electrical loss
The main causes of electrical loss are: carrier loss and ohmic loss.
(1) Carrier loss
Due to the defects of battery materials and other reasons, the generated electrons and holes and other carriers recombine, causing some of the carriers to disappear. Generally speaking, the longer the carrier lifetime, the higher the conversion efficiency η. If the junction area is too thick or the finger impurity is too high, it will cause the carriers to be neutralized, thereby reducing the life of the carrier. The carrier lifetime is proportional to the reciprocal of the recombination rate. There are four basic mechanisms of carrier recombination.
① Inter-band recombination; it is the inverse process of light absorption. The higher the concentration of majority carriers, the greater the possibility that electrons will meet and recombine with the empty six. The main influencing factors are the doping density and the type of gap semiconductor.
②Auger recombination: when the electron and the hole recombine, the excess energy is transferred to another electron in the conduction band or another hole in the valence band, and the energy carriers release their energy in the form of phonons. come out. The main influencing factor is the concentration of doping elements.
③Trap recombination: Impurities or defects form energy bands in the forbidden band, which act as steps for carrier generation and recombination. Such impurities and defects are called carrier generation-recombination centers (also called traps). The recombination mechanism is the recombination through the recombination center (trap). It is dominant in lightly doped Si.
④Surface recombination: The main surface recombination mechanisms include surface dangling bonds, surface damage caused by material processing, and impurities absorbed on the surface.
(2) Ohmic loss
During the transmission of photo-generated current in the battery, the loss caused by the conversion of the internal series resistance of the solar cell into heat is called ohmic loss. The series resistance of the solar cell is mainly composed of the silicon chip body resistance, the scattered square resistance, the grid line resistance and the contact resistance after sintering. Therefore, to improve the quality of silicon wafers, use silicon wafers with a substrate resistance that meets the requirements, narrow and thick metal gates, reduce light shielding, control diffusion temperature and time, and good ohmic contact of electrodes can reduce series resistance.
Ⅱ. Optical loss
For solar radiation, the semiconductor material with the best performance should have a cut-off wavelength above 0.8, including all visible light from red to violet. Each solar cell has its own spectral response curve to sunlight, which represents the sensitivity of the cell to light of different wavelengths (photoelectric conversion capability). For example, long-wavelength light is easily projected to the bottom electrode of the cell and is absorbed and converted by the bottom electrode. Lost as heat. Optical loss is mainly divided into three types: reflection loss, grid finger electrode shading loss, and transmission loss. Due to the light reflection on the surface of the battery, all sunlight cannot be incident on the solar battery. The metal bus main line and grid line will also occupy a certain light-receiving surface area because of their opacity, causing the grid finger electrode to shading loss. If the thickness of the battery is not enough, some photons that can be absorbed may pass through the back of the battery, which is the transmission loss. The indirect band gap semiconductor requires the thickness of the material to be thicker than that of the direct band gap.