Why does the semiconductor PN junction have electricity when it is illuminated by sunlight? Scientists call this phenomenon of electricity generation by light “photovoltaic effect”. The core principle of the photovoltaic effect is the “hole conduction” of the PN junction.
Holes represent positive charges, and the movement of positive charges forms a current. This current is called “photogenerated current”, which is related to factors such as the area of the photovoltaic panel, the illuminance, and the surface temperature of the photovoltaic panel.
Experiments have shown that the size of the photo-generated current is greatly affected by the deviation of the installation angle of the photovoltaic panel and changes rapidly. Under the same experimental conditions, the photovoltaic voltage responds slowly to the deviation of the installation angle and is little affected by it.
The size of the battery’s driving capacity, that is, the size of the electromotive force, is directly related to the current. Although the electromotive force is expressed in “volts”, its “strength” is determined by the strength of the internal current.
In practice, we often In such a situation, the voltage of a (block) battery is still very high, but the current is very small, and the voltage is imaginary no matter how high it is. So what is current? Current is the directional movement of electrons. The direction of current flow in a loop is always opposite to the direction of electron flow.
For solar cells, the direction of the photogenerated current is the direction in which the “holes” move, that is, the opposite direction of the electron flow. Photo-generated current determines the power generation efficiency of solar cells, so optoelectronic products and photovoltaic projects should pay special attention to the installation angle of photovoltaic module panels. The angle deviation is a little bit, and the photogenerated current will drop a lot.
The generation of photo-generated current appears to be formed by “hole conduction” on the surface, but it is actually formed by the “directional filling of holes” of electrons. So what about “hole movement” and electron “filling holes”?
Let’s take a look at the internal structure of monocrystalline silicon, the material for solar cells. The molecular structure inside the single crystal silicon is a tetravalent electron crystal form. Silicon atoms rely on these four valence electrons to form strong ionic bonds with each other, which attract and repel each other. All silicon atoms form a regular arrangement. magic. The spaces between atoms, also called lattices, are spaces where free electrons move.
P-type semiconductor is a beautiful tetravalent monocrystalline silicon doped with trivalent boron atoms. As a result, a certain boron atom replaces the silicon atom and is mixed in the lattice, but because there are only three electrons around the boron atom, there must be one. The ionic bond is “vacant” due to mismatch (lack of a valence electron to match), which has a tendency to be unstable or unbalanced. The visual metaphor of “hole” comes from this “hole”. It shows a tendency to stabilize the lattice from time to time. This is the characteristic of P-type semiconductors.
Let’s talk about N-type semiconductors. Similar to P-type semiconductors, pure silicon is doped with pentavalent phosphorus atoms before the single crystal silicon forms a crystalline state under high temperature and high pressure. After the crystallization is formed, a certain phosphorus atom occupies the position of the silicon atom and is mixed in the lattice, the result must be that a valence electron cannot find a pair, and it will become a destabilizing factor without attribution. An N-type semiconductor is formed.
A good thing to do now is to bond the P-type semiconductor with the N-type semiconductor. As a result, a very thin semiconductor film is formed on the contact surface. This film is called PN junction by scientists. The PN junction is a built-in electric field and has unidirectional conductivity, that is, it is turned on when a forward electric field is added. , and the reverse electric field is applied to cut off.
PN junction is the most critical basis for the development of semiconductor device technology and electronic science. PN junctions currently in the lab can be as fine as nanometers. This means that there will be a historic leap in the development and application of VLSI, which brings us the benefit that the highest-end PCs can be made very light, thin, and small. The outer surface of the solar cell—the sunny side is a hole-rich P-type semiconductor, and the electron-rich N-type semiconductor is just below.
Under the excitation of solar photons, the free electrons in the N region are extremely active, and finally, break through the barrier of the PN junction, and start to fill the holes in the P region one by one. Interestingly, these holes are filled with electrons one by one from bottom to top, as if the holes are quietly moving from top to bottom. This is the “movement of holes,” a virtual movement where each atom in the lattice remains motionless in its place, only the electrons paired with the positive charge move.
The electromotive force of the solar cell is thus generated. However, the virtual movement of holes and the actual directional movement of electrons are only the accumulation of charged carriers to the upper and lower sides of the battery, which can only form electromotive force, but not current. What else is needed to form a current? It is necessary to add wires to the ends of the battery to form a loop.
