Composition of the atomic layer of photovoltaic cells

Composition of the atomic layer of photovoltaic cells

The vast majority of commercial photovoltaic modules in history have been composed of silicon, and this is still the case today, although there are also other technologies and photovoltaic modules available or under development. Under certain conditions, electrons can move away from silicon atoms and become part of an electric current.

①Silicon atomic structure
Silicon atoms are made up of protons, neutrons and electrons. Positively charged protons and neutrons (no charge) are held together by strong nuclear energy to form the nucleus. Negatively charged electrons are much smaller than protons and neutrons. They move around the nucleus just like a planet orbits the sun, while maintaining a relatively stable position by electrostatic gravity (different electrical phase attraction, homoelectricity repulsion) .
Under stable conditions, silicon is composed of 14 protons, 14 neutrons, and 14 electrons, so it is electrically neutral. The atomic structure of silicon is shown in Figure 1.

Composition of the atomic layer of photovoltaic cells
Figure 1 Atomic structure of silicon

It can be seen from Figure 1 that electrons are arranged in orbitals around the nucleus (called the electron shell). The innermost electron shell can only hold 2 electrons, which tightly surround the nucleus. The second electron shell (which is also saturated) has 8 electrons, and they surround the nucleus very tightly. Both electron shells are saturated and cannot hold more electrons.
The third electron layer can hold up to 8 electrons, while only 4 electrons are located in the third electron layer of a neutral silicon atom. These electrons don’t surround the nucleus very closely. If these electrons obtain the energy (binding energy) equal to (or greater than) the nucleus bound to them, they will leave the electron layer and become free electrons.
Solar radiation can provide energy to electrons. When photons of solar radiation hit the outer electrons, energy conversion occurs. The incident photon loses the energy that allows an electron to escape from the electron layer. This phenomenon is called the photoelectric effect. If the energy of the incident photon is exactly equal to the energy of the electron leaving, then the photon will annihilate and produce a free electron. If the energy of the photon is greater than the energy of the bound electron, the electron will only get the energy that can be separated from the electron layer, and the remaining energy will be converted into heat of silicon. Therefore, not all solar radiation energy is used to generate free electrons, which limits the maximum conversion efficiency of silicon photovoltaic cells to below 40%. The photoelectric effect is shown in Figure 2.

Composition of the atomic layer of photovoltaic cells
Figure 2 Photoelectric effect

Free electrons will move freely at random and are quickly absorbed by silicon atoms that have lost one electron. This absorption is also accompanied by the emission of light, and the frequency of the emitted light is determined by the amount of energy released by the electron “falling” back into the electron layer. This frequency forms the characteristic atomic absorption spectrum, which can be used to identify different substances.

②Create PN junction
Silicon atoms are connected together in a crystalline form, and adjacent nuclei share outer electrons. At this time, silicon is electrically neutral, as shown in Figure 3.

Composition of the atomic layer of photovoltaic cells
Figure 3 The structure of crystalline silicon

Impurities can be added to crystalline silicon to change its properties, usually boron (B) and phosphorus (P) are used. The outer layer of boron has only 3 electrons (a total of 5 electrons in all electron layers), while the outer layer of phosphorus has 5 electrons (a total of 15 electrons in all electron layers). If boron or phosphorus atoms replace silicon atoms in the crystal structure, the situation shown in Figure 4 will appear.

Composition of the atomic layer of photovoltaic cells
Figure 4 Silicon with boron and phosphorus added

Boron-doped silicon forms a “P type” (positive type) due to the presence of holes. Phosphorus-doped silicon forms “N-type” (negative type) due to the presence of excess electrons. P-type silicon and N-type silicon are connected together to form a PN junction, as shown in Figure 5.

Composition of the atomic layer of photovoltaic cells
Figure 5 PN junction

Excess electrons near the PN junction in N-type silicon pass through the PN junction and enter the P-type region and fill in holes. This movement causes a positively charged region in the N-type region, because the positively charged protons are still “fixed” in the nucleus. Around; this movement also makes the P-type part become a negatively charged area due to the addition of extra electrons.
The separation of charges creates an electric field, as shown in Figure 6. The potential difference formed by the electric field is 0.5~0.6V. With the accumulation of electrons, the field strength increases, thereby preventing the inflow of more electrons and stopping the flow of electrons. By convention, the direction of the electric field is the direction in which the positive charge will move when placed in the field, and it is also the opposite direction of the electron flow.

Composition of the atomic layer of photovoltaic cells
Figure 6 Electric field near the PN junction

③Photoelectric effect and PN junction
If light with sufficient energy irradiates the silicon to produce a PN junction and passes through points near the PN junction, free electrons will be generated around the PN junction due to the photoelectric effect. These electrons move rapidly under the action of the electric field of the PN junction, and then continue to move to the surface of the battery. In the process of moving to the surface of the battery, some electrons are re-absorbed by the silicon atoms, while other electrons reach the surface of the battery. These electrons are gathered by the metal grid. If the grid is in contact with the other pole of the battery through an external circuit, a current will be generated, as shown in Figure 7.

Composition of the atomic layer of photovoltaic cells
Figure 7 Photoelectric effect and PN junction