As we all know, countless atoms are made up of nuclei and electrons. The nucleus is positively charged and the electrons are negatively charged. Electrons revolve around atomic nuclei in certain orbits, just like planets revolve around the sun. The atoms of single crystal silicon are arranged according to certain rules, and there are 4 electrons in the outermost electron shell of the silicon atom, as shown in Figure 1. The outer electrons of each atom have fixed positions and are bound by the nucleus. Under the excitation of external energy, such as solar radiation, they will get rid of the shackles of the nucleus and become free electrons, and at the same time leave a vacancy in its original place, which is the so-called “hole” in semiconductor physics.
When sunlight hits the PN junction, the atoms in the semiconductor release electrons due to the acquisition of light energy, and correspondingly, electron-hole pairs are generated, and under the action of the barrier electric field, the electrons are driven to the N-type region, and the holes are driven to the P-type region, so that the N-type region has excess electrons and the P-type region has excess holes. As a result, a photo-generated electric field opposite to the direction of the barrier electric field is formed near the PN junction, as shown in Figure 2.
The basis of the working principle of solar cells is the photovoltaic effect of the semiconductor PN junction. As shown in Figure 3, under the action of the built-in electric field of the PN junction, the holes in the N area move to the P area, while the electrons in the P area move to the N area, resulting in the accumulation of a large amount of negative charges (electrons) on the light-receiving surface of the solar cell and the accumulation of a large amount of positive charges (holes) on the backlight side of the battery. For example, metal electrodes are made on the upper and lower surfaces of the battery, and the load is connected with a wire. As long as the sun shines continuously, there will always be current flowing through the load.
