The working principle of photovoltaic cells, the process of doping and photovoltaic conversion
Photovoltaic cells and applications

The working principle of photovoltaic cells, the process of doping and photovoltaic conversion

  1. Photovoltaic cells

Photovoltaic cells are made of semiconductor materials, and the two commonly used semiconductor materials are silicon and zirconium. Both silicon and wire are poor conductors of electricity because they don’t have enough free electrons to sustain a continuous current flow. In order to improve its conductivity to obtain greater current, people have adopted the method of “doping”, that is, adding impurities to the pure crystal to enhance its conductivity. The thermal stability of silicon is better than that of thread, so it is often used in photovoltaic cells.

  1. Doping

A silicon atom in its pure crystalline state has 4 electrons in its valence band (the outer electron band). To improve conductivity, silicon crystals can be doped with atoms containing more or less than 4 valence electrons. This doping process creates a new semiconductor material. If an atom containing 3 valence electrons is doped into a silicon crystal, a P-type or positive-type material is formed. In P-type materials, there are atoms with fewer valence electrons than the base silicon material, i.e., electrons-deficient. If an atom containing 5 valence electrons is doped into a silicon crystal, an N-type or negative-type material is formed. There are surplus electrons in N-type materials. Want to learn more about battery semiconductor materials? go now.

After doping, two different materials, N-type and P-type, are obtained. Further processing of these two materials in a special furnace results in a PN junction device. Figure 1 shows the cross-sectional structure of this PN junction device. P-type materials lack electrons, and N-type materials are rich in electrons.

Figure 1 - Cross-sectional structure of a PN junction device
Figure 1 – Cross-sectional structure of a PN junction device
  1. Semiconductor theory

By doping, a larger electron flow than pure silicon can be obtained. As mentioned above, the doped silicon material is referred to as P-type silicon or N-type silicon. Phosphorus gas is often used to make N-type materials. Phosphorus has 5 electrons in its valence electron, and a small amount of phosphorus atoms is infiltrated into the silicon material, and these extra electrons are free to move, which increases the free electrons of the material. Boron is commonly used to make P-type materials. Boron has 3 electrons in its valence electron. By doping, a small amount of boron atoms can be infiltrated into the silicon material. The lack of electrons in p-type materials creates positively charged holes that attract electrons. Holes are also formed when electrons leave the valence shell of an atom, when other electrons flow to fill those holes (see Figure 2). In P-type materials, holes move in the opposite direction to electrons. The concept of hole movement exists only in semiconductors. In the outer circuit, there are no flowing holes, only flowing electrons.

Figure 2 - Movement of electrons between atoms
Figure 2 – Movement of electrons between atoms

Doping operations are performed in a dust-free environment. Two materials are made by placing a pure silicon material in a furnace and infiltrating boron or phosphorus gas (P or N depending on the material type) into the silicon crystals. Further processing in a dedicated furnace produces a PN junction, resulting in a photovoltaic cell. In the process of generating a PN junction, an electric field is formed at both ends of the junction.

In the production of such devices, the doping process is critical. The electric field across the PN junction causes current to flow through the cell only in one direction, not the other way around. This unique property of semiconductor devices is called diode characteristics. Semiconductor devices called diodes have many uses. Diodes are made like photovoltaic cells, but they work differently and are used differently. Current that flows in only one direction is called direct current. Photovoltaic cells allow direct current to pass in one direction, and a direct voltage can be generated between the two terminals of the battery cell.

Photovoltaic cells are sheet-like devices with conductive contact electrodes and printed circuits connected to their upper and lower surfaces. These contact electrodes connect the upper and lower surfaces of the cell together by an external load. At the same time, the contact electrode can also connect many cells in series to form an assembly (the cells can be connected in parallel, but usually they are connected in series first, and then in series and parallel at the assembly level).

  1. Photovoltaic effect

Solar cells are technically called photovoltaic cells. Among them, “light” represents light, “volt” represents electricity, and “photovoltaic” is the electricity generated by light. Photovoltaic cells are sensitive to optical radiation from the sun, which contains photon energy. Photons are packets of energy that, when absorbed by the battery, increase the mobility of electrons in the semiconductor material. The electric field repels the free electrons from the P-type material into the N-type material and does not allow the free electrons to return to the P-type material, and at the same time, the holes move towards the surface of the P-type material. This process creates a potential difference (DC voltage) across the PN junction. When the external circuit is connected, the electrons return from the N-type material to the P-type material through the load. The positively charged holes help improve the conductivity of the silicon material, but the holes do not travel to the external circuit.

The above process is often referred to as the “photovoltaic effect” or “photovoltaic action”. This effect can be summed up as: When a material absorbs photon energy, the movement of electrons in that material generates a voltage. When the solar radiation stops hitting the surface of the cell, the generation of voltage also stops. Photovoltaic action continues to occur as long as sunlight is shining on the cell. The main factors affecting the life of photovoltaic cells are external damage and the aging of surface contact electrodes.

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