Principles of semiconductor solar photovoltaic cell

Semiconductor P-N junction of solar photovoltaic cell

admin on August 10, 2021 0 Comments • Tags: #electronconcentration #ntypesemiconductormaterial #ptypesemiconductormaterial

Intrinsic semiconductor materials are doped with group V impurity elements (phosphorus, arsenic, etc.), and the impurities provide electrons, making the electron concentration in them greater than the hole concentration, forming an N-type semiconductor (Figure 1) material, and the impurity is called “donor” “. At this time, the electron concentration is greater than the hole concentration, which is the majority carrier: while the hole concentration is lower, it is the minority carrier. Similarly, the doping of group III impurity elements (boron, etc.) in semiconductor materials makes the concentration of holes greater than the concentration of electrons, and crystalline silicon becomes a P-type semiconductor (Figure 2). For example, take silicon as an example. Doping a little boron, aluminum, gallium and other impurities into high-purity silicon is a P-type semiconductor; doping a little phosphorus, arsenic, antimony and other impurities is an N-type semiconductor. In N-type semiconductors, unbalanced electrons are called unbalanced majority carriers, and unbalanced holes are called unbalanced minority carriers. The opposite is true for P-type semiconductors. In semiconductor devices, unbalanced minority carriers often play an important role.

Figure 1 N-type semiconductor
Figure 2 P-type semiconductor

Whether it is an N-type semiconductor material or a P-type semiconductor material, when they exist independently, they are all electrically neutral, and the charge amount of ionized impurities is equal to the total charge number of carriers. When two semiconductor materials are connected together, for N-type semiconductor materials, electrons are majority carriers and have a high concentration: while in P-type semiconductors, electrons are therefore minority carriers and have a low concentration. Due to the existence of the concentration gradient, the diffusion of electrons is bound to occur, that is, electrons diffuse from the high-concentration N-type semiconductor material to the low-concentration P-type semiconductor material, forming a PN junction at the interface of the N-type semiconductor and the P-type semiconductor. Near the PN junction interface, the electron concentration in the N-type semiconductor gradually decreases, and the electrons diffused into the P-type semiconductor and the majority carrier holes in it recombine and disappear. Therefore, near the interface of the N-type semiconductor, due to the majority As the carrier electron concentration decreases, the positive charge number of ionized impurities is higher than the remaining electron concentration, and a positive charge region appears. Similarly, in P-type semiconductors, since holes diffuse from P-type semiconductors to N-type semiconductors, near the interface, the number of negative charges of ionized impurities is higher than the concentration of remaining holes, and negative charge regions appear. This area is called the space charge area of ​​the P-N junction. The positive and negative charge areas form an electric field from the N-type semiconductor to the P-type semiconductor, which is called the built-in electric field, also known as the barrier electric field. Because the resistance here is particularly high, it is also called a barrier layer. This electric field has a counteracting effect on the diffusion of multiple sons in the two regions, and has a helpful effect on the drift of minority sons, until the diffusion flow equals the drift flow to reach equilibrium, and a stable built-in electric field is established on both sides of the interface. The so-called diffusion means that under the influence of an external electric field, a free electron that moves randomly has an accelerated motion in the direction opposite to the electric field, and its speed increases continuously with time. In addition to drifting motion, carriers in semiconductors can also flow due to diffusion. When any particles like gas molecules are concentrated too much, if they are not restricted, they will disperse by themselves. The basic reason for this phenomenon is the irregular thermal velocity of these particles. As the diffusion progresses, the space charge region widens and the internal electric field increases. Because the function of the internal electric field is to hinder the diffusion of multiple carriers and promote the drift of minority carriers, when the diffusion motion and drift motion reach a dynamic balance, a stable PN junction will be formed. (Figure 3). The P-N junction is very thin. There are few electrons and holes in the junction, but there are positively charged ions on the side close to the N-type and negatively charged ions on the side close to the P-type. Due to the lack of carriers in the space charge region, the P-N junction is also called the depletion region.

Figure 3 Principle of semiconductor P-N junction formation

When a semiconductor with a PN junction is exposed to light, the number of electrons and holes increases. Under the action of the local electric field of the junction, the electrons in the P area move to the N area, and the holes in the N area move to the P area. There is charge accumulation at both ends, forming a potential difference.

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