60w Led Street Light Science:

In a solar cell, four electrons are required to replace an atom with a silicon atom to form a covalent bond, so an electron must be borrowed from the circumference to stabilize the compound. This causes holes to appear in the crystal. Although the silicon nucleus has four positive charges, the outermost electron number is also four, so the silicon crystal is electrically neutral. At the position of the silicon atom, if only three positively charged atoms are replaced, it is as if a negative charge is vibrating at the atomic position, and capturing a hole forms an electron orbit like the Bohr model of the hydrogen atom.