Principle of solar cell power generation

1. Valence band, forbidden band, conduction band

Conduction is due to the transfer of electrons. There are three kinds of energy bands in the outer layer of the atom. My understanding is that the valence band is the region close to the nucleus, and the electrons in the valence band will be strongly bound. The conduction band is a region far from the nucleus, where the electrons are relatively free and can be transferred under the applied electric field, that is, can conduct electricity; The band gap is the region between the valence band and the conduction band, where no electrons exist, and electrons can absorb energy from the valence band to cross the band gap to the conduction band, this kind of electron derelict is called "transition".

As can be seen from the figure below, the band gap of different substances is different, and the band gap of the conductor is very narrow or even no band gap, so the transition can be easily completed at room temperature, and electrons can be transferred from the valence band to the conduction band. However, the specific price width of the band-gap area of the insulator is generally greater than 5eV, and conduction will not occur at normal room temperature, but it is heard that it is also possible to guide haha under strong stimulation; The band gap of the semiconductor is less than 3eV, and the electrons in the valence band of the point stimulus (light, heat) may cross the band gap and reach the conduction band to complete the transition.

2. Classification of semiconductors

Semiconductors are divided into pure semiconductors (intrinsic semiconductors) and doped semiconductors. The intrinsic semiconductor is relatively pure, it is generally composed of quadrivalent element silicon, germanium, in the external stimulus (thermal movement), a few electrons in the covalent bond may break free, complete the transition, will produce a free electron and a hole, free electrons can be directional movement under the external electric field, the formation of electronic current.

Doped semiconductors are other elements produced in intrinsic semiconductors, which are generally divided into two types, that is, P-type semiconductors and N-type semiconductors that we say every day; Among them, P-type semiconductors are doped with trivalent elements, such as boron, gallium, indium, etc. After the silicon atoms in the original arrangement are replaced by boron atoms, some electrons will be replaced by holes of boron atoms (figure left); N-type semiconductors are doped with pentavalent elements, phosphorus, antimony, arsenic, etc., and the silicon atoms in the original arrangement are replaced by phosphorus atoms, which will have some more electrons.

But two things to note:

1) N-type and P-type semiconductors are electrically neutral ha (very simple question I thought for a long time), because the doped B or P atom itself is neutral, the number of nuclei and electrons are the same, it should be noted here that the doped atom nucleus is not the +4 of the original silicon atom Ha.

2) Intrinsic semiconductors can excite electron hole pairs due to thermal motion at room temperature, but relatively few, of course, doped semiconductors can also intrinsically excite electron hole pairs, these negatively charged free electrons and positively charged holes are called carriers; P-type semiconductors because of doping trivalent elements, so the holes in the majority of carriers, so the holes in P-type semiconductors are called many, free electrons are called few; N-type semiconductors are doped with pentavalent elements, so the free electrons account for the majority of carriers, which is called many, and N-type holes are called few.

3. Diffusion and drift

Electrons or holes move to form an electric current, they move in two ways, one is diffusion, the other is drift; The movement of many children is called diffusion, in fact, just like adding some ink to white water, the ink in the area with more ink will spread to less place; The movement of the minority is called drift, which is generally the movement of electrons or holes under the action of electric field force under the electromotive force. In the PN junction we will talk about later, drift and diffusion are two kinds of motion in opposite directions, and eventually drift and diffusion reach equilibrium and form PN junction.

4.PN knot

This name should be familiar to everyone, from the first day I look at photovoltaic, I know PN junction, but have not been clear it. We talked about P-type semiconductors and N-type semiconductors, what happens when we put the two together?

Firstly, due to the different concentration of electrons and holes on both sides, the diffusion of electrons and holes will occur. There are more holes in P-type semiconductor, which will spread to N-type; N-type semiconductors have more free electrons, which diffuse into P-type semiconductors.

After the diffusion movement of free electrons and holes, the holes and electrons will be combined and disappear, but the carriers of the whole P-type semiconductor and N-type semiconductor will not be uniformly mixed, and the carriers on both sides of the line near the middle of PN will preferentially compound, but the electrons and holes away from the middle line will not compound, which can be understood as the back is blocked by the front ions and can not be passed. So we can't get back together.

In the P-type semiconductor, some atoms close to the PN intermediate line lose their holes and become negative ions. Some of the atoms near the PN intermediate line in the N-type semiconductor lose electrons and become positive ions; The contact between the P-type semiconductor and the N-type semiconductor forms a spatial load zone.

In the space loading region, the negative ions in the left P region and the positive ions in the right N region will form a self-built electric field and generate electric field force. The direction of the current is the green arrow, and the hole will move from right to left due to the electric field force, and the electron will move from left to right due to the electric field force, and yes that's the drift of the electron and the hole that we said above; It can be found that the drift is equivalent to the electrons and holes that have just diffused past drifting back, so eventually when the drift and diffusion motion reach equilibrium, the middle space current carrying region will tend to be stable, forming a PN junction.

PN junction is actually a diode, with one-way conductivity, in its external positive power supply (positive N, negative P), the same direction as the self-built electric field, electron and hole drift will increase, PN width is reduced, the current can flow smoothly, it should be the meaning of small resistance; When the negative power supply is connected externally (positive P, negative N), in the opposite direction of the self-built electric field, the diffusion of electrons and holes will increase, the PN junction region will increase, the current can not pass smoothly, and the resistance will eventually tend to be infinite. (That is, positive connection can be powered on, reverse connection can not be powered on)

5. Photovoltaic effect

Finally to the end, recall that under the external stimulation of the semiconductor, the electrons bound by the nucleus can cross the forbidden band from the valence band into the conduction band, becoming free electrons, flying themselves.

Our sunlight is this stimulus, when the sun shines on the above large semiconductor composed of P-type and N-type semiconductors, there are three regions of atoms will be affected by photons, produce a transition reaction, forming electron hole pairs, respectively: The PN junction region, the P-type and N-type semiconductor region other than the PN junction, is that every place in the whole block may produce electron hole pairs. As shown below:

The electrons generated by light on the PN junction will be thrown into the N-type region due to the influence of electric field force, and the holes generated on the PN junction will be thrown into the P-type region due to the influence of electric field force. There are a few electron holes on the edge of the PN junction may also pass through the PN junction, but it should be unlikely; At this point, the solar cell has been formed, and a circuit is connected to both ends of the semiconductor to generate an electric current. This is solar cell power generation!

To conclude:

1. First, trivalent and pentavalent elements are doped in the intrinsic semiconductor to form a P-type/N-type semiconductor. (That is, when making silicon wafers, doping to form P-type/N-type silicon wafers)

2. P-type and N-type semiconductors are put together to form PN junctions due to the diffusion of many sons and the drift of few sons. (When making a battery chip, it will be further doped on the silicon chip to form a PN junction)

3. When light is irradiated in PN junction, electrons transition from valence band to conduction band, forming free electrons and holes, and move to N and P under the influence of electric field force, the external loop can form a current.