Solar cells are devices that use the principle of photoelectric conversion to convert solar radiation light into electrical energy through semiconductor materials. This photoelectric conversion process is usually called the “photovoltaic effect.” The photovoltaic effect is abbreviated as the photovoltaic effect, which refers to the phenomenon that light causes a potential difference between different parts of a non-uniform semiconductor or a combination of semiconductor and metal.
All substances are composed of atoms. Atoms are composed of nuclei and electrons rotating around the nucleus. In the normal state of semiconductor materials, the nucleus and electrons are tightly combined (in a non-conductor state), but under the stimulation of some external factors, the nucleus and electrons The binding force of electrons decreases, and the electrons get rid of the bondage of the nucleus and become free electrons, as shown in Figure 1. When sunlight shines on the semiconductor, the photon provides energy to the electron, and the electron will transition to a higher energy state. Among these electrons, the available electrons in the actual use of optoelectronic devices are:
(1) Valence band electronics.
(2) Free electrons or holes.
(3) Electrons present in the impurity energy level.
The electrons available in solar cells are mainly valence band electrons. The process by which valence band electrons obtain light energy and transition to the conduction band determines the absorption of light, which is usually called intrinsic or intrinsic absorption.
The solar cell is a combination of P-type semiconductor and N-type semiconductor. P-type semiconductor (P refers to positive) is composed of single crystal silicon mixed with a small amount of trivalent elements through a special process, which will form a band inside the semiconductor. Positive holes. N-type semiconductor (N refers to negative, negatively charged) is composed of single crystal silicon mixed with a small amount of pentavalent elements through a special process, which will form negatively charged free electrons in the semiconductor. When N-type and P-type are two different types After the semiconductor material is in contact, due to diffusion and drift, a built-in electric field from P-type to N-type is formed at the interface. When light shines on the surface of the solar cell, photons with energy greater than the band gap excite electron and hole pairs. These unbalanced minority carriers are separated under the action of the internal electric field and accumulate at the two poles of the solar cell. The solar cell can then provide current to the external load, as shown in Figure 2. This is the working principle of the photovoltaic effect solar cell.
The formation of PN junction
In a single crystal semiconductor, part of the doped acceptor impurity is a P-type semiconductor, and the other part doped with a donor impurity is an N-type semiconductor, the transition area near the interface between the P-type semiconductor and the N-type semiconductor is called a PN junction. There are two types of PN junctions: homojunction and heterojunction. PN junctions made of the same semiconductor material are called homojunctions, and PN junctions made of two semiconductor materials with different band gaps are called heterojunctions. There are alloying methods, diffusion methods, ion implantation methods, and epitaxial growth methods for manufacturing PN junctions. The epitaxial growth method is usually used to manufacture heterojunctions.
There are many positively charged holes and negatively charged ionized impurities in P-type semiconductors. Under the action of an electric field, the holes can move, while the ionized impurities (ions) are immobile. There are many movable negative electrons and fixed positive ions in N-type semiconductors. When the P-type and N-type semiconductors are in contact, holes near the interface diffuse from the P-type semiconductor to the N-type semiconductor, and electrons diffuse from the N-type semiconductor to the P-type semiconductor. Holes and electrons meet and recombine, and the carriers disappear. Therefore, in the junction region near the interface, there is a distance lack of carriers, but there are charged fixed ions distributed in the space, which is called the space charge region. The space charge on the side of the P-type semiconductor is negative ions, and the space charge on the side of the N-type semiconductor is positive ions. The positive and negative ions generate an electric field near the interface, which prevents the carriers from further diffusing and reaches equilibrium.
The homojunction can be doped with a piece of semiconductor to form the P region and the N region. Because the activation energy ΔE of the impurity is very small, almost all the impurities are ionized into acceptor ions NA－ and donor ions ND＋ at room temperature. Because there is a difference in the concentration of carriers at the interface of the PN region, they must diffuse toward each other. Imagine that at the moment when the junction is formed, the electrons in the N area are multitons, and the electrons in the P area are minority electrons. The electrons flow from the N area to the P area. When the electrons and holes meet, they will recombine, so that the original N area will be recombined. There are very few electrons near the junction, leaving unneutralized donor ions ND＋, forming positive space charges. Similarly, after holes diffuse from the P area to the N area, negative space charges are formed by the immobile acceptor ion NA－. Immovable ion regions (also called depletion regions, space charge regions, and barrier layers) are generated on both sides of the interface between the P and N regions, and a space galvanic layer appears, forming an internal electric field (called a built-in electric field). The diffusion of multiple sons in the two regions has the effect of resisting and helping the drift of minority sons. Until the diffusion flow equals to the drift flow, the equilibrium is reached, and a stable built-in electric field is established on both sides of the interface.
Read more: PN junction energy band and contact electromotive force difference and photoelectric effect