Renewable energy refers to the energy that can be reused within a certain range. In other words, as long as the earth and the sun are not destroyed, renewable energy sources such as wind, geothermal, and solar energy will be inexhaustible. Take solar energy as an example. The solar energy absorbed by the earth is 173,000 terawatts, which is 10,000 times the total energy used by human beings on the earth.
We can’t help thinking, one day, human society can completely rely on solar energy to operate?
In fact, people have been eyeing solar energy for a long time and tried to convert it into electricity that can be used directly. The most stupid way is to use sunlight to provide heat to boil water, and then use the steam of boiling water to generate electricity. However, each energy conversion must be accompanied by consumption, and the efficiency of boiling water is not high. So people fell into meditation: how to turn solar energy directly into electricity?
The person who made this idea a reality for the first time was Edmund Becquerel.
One day in 1839, Edmund, who was studying phosphorescence, discovered something extraordinary. He put silver chloride in an acid solution, connected two platinum electrodes, and exposed them to the sun. The result was that he found the voltage in the middle of the two electrodes.
At that time, people did not know the principle of this phenomenon, only that light can generate electric potential, so this phenomenon is called the photovoltaic effect. Nowadays, solar cells basically use the photovoltaic effect, so solar cells are also called solar photovoltaic cells.
At present, the most widely used photovoltaic cells are mainly made of semiconductor materials such as silicon. So how do people use semiconductors and photovoltaic effects to make solar cells?
Part.1 Atomic Band Structure
Simply put, the energy band refers to the different regions that we divide the electron into according to its energy. We all know that the nucleus is positively charged, it will attract negatively charged electrons. The closer the electrons to the nucleus are, the stronger the bondage. Now we split the atom, the nucleus sinks down, and the electrons are placed on it.
In this case, we can divide the electrons into two active regions: one is the region closer to the nucleus, where the electrons are tightly attracted, which we call the valence band. The second is the area far away from the nucleus, where the electrons are not supervised and are relatively free. If there is an external electric field to make these electrons run, the material will conduct electricity. We call this area the conduction band.
In addition to these two areas, there is another area above the valence band and below the conduction band, where electrons are not allowed to exist, which we call the bandgap.
Band structure of atoms
The basic atomic energy band structure is like this, but there are some details we need to pay attention to: first, the energy band can also be subdivided into different energy levels, and due to the Pauli incompatibility principle, each energy level can only accommodate two electronic. Secondly, most atoms don’t have so many electrons, and even the valence band is not full, and there are no electrons in the conduction band. In addition, the electrons in the valence band are not honest, they may “derail”, that is, cross the forbidden band and rush to the conduction band. Of course, this process is called a transition, and the transition is to absorb energy.
Considering these three details, some readers may have guessed that there are two completely different materials in the self-heating world: one kind of bandgap is very narrow, or there is no band gap at all. At room temperature, the outer electrons of its valence band can easily jump to the conduction band, this is the conductor. On the contrary, if the forbidden band of the material is very wide, generally greater than three electron volts (3eV), and the electrons stay in the valence band honestly at room temperature, then it cannot conduct electricity, and this is an insulator.
Band structure of different solids
Part.2 “Fixical” Semiconductor
Is there any material whose energy gap between the valence band and the conduction band is less than 3eV? Yes, it is a semiconductor, which in the usual sense refers to a substance whose electrical capacity is between a conductor and an insulator.
However, the value of a semiconductor is not manifested in its electrical conductivity, but in its “horizontal” electrical conductivity. The conductivity of semiconductors is easily changed by external factors. Later we will see how the photovoltaic effect changes the conductivity of semiconductors. Next, we will take the silicon atom as an example to explore the mysteries inside semiconductors.
1. Intrinsic semiconductor structure
Semiconductors without any impurities, such as pure silicon and pure germanium, are called intrinsic semiconductors. Let’s take a look at the silicon atom. It has 14 electrons, the electron arrangement is 2-8-4, and the outermost layer has 4 electrons. The nature of the element is mainly determined by the outermost electrons. The outermost electrons of silicon have a tendency to either find four more electrons and make up four pairs or throw away all four electrons.
In silicon crystals, each silicon atom is adjacent to the top, bottom, left, and right. There are four electrons in the outermost layer of silicon. It will share these electrons with adjacent silicon atoms so that the outermost layer of each silicon atom Just collected 8 electrons. Perfect!
Silicon crystal covalent bond
2. Structure of impurity semiconductor
What is the difference if we dope intrinsic semiconductors with some impurities? For example, if one of the silicon atoms is replaced with a phosphorus atom, the phosphorus atom has 15 electrons, the arrangement is 2-8-5, the outermost layer has 5 electrons, and the adjacent silicon atom has 8 electrons, which is more. Come out an electron. In this way, every time a phosphorus atom is doped, there will be an electron that is nowhere to be placed, and if there is too much doping, a “single electron army” will be formed. We call such semiconductors N-type semiconductors. N (Negative) means that the electrons are negatively charged.
On the contrary, if we dope with the boron atom, it has 5 electrons, and the outermost layer has 3 electrons. The boron atom and the surrounding silicon atoms can only make up 7 electrons. These 7 electrons are still one electron to form a stable structure, so a “hole” is generated here. We call it a P-type semiconductor. P (Positive) means that holes can be equivalent to positively charged particles.
3. Why do semiconductors conduct electricity?
According to the previous statement, impurity semiconductors have freely moving charges and can naturally conduct electricity. Where does the free charge of intrinsic semiconductors come from?
In fact, under ideal conditions (ie, absolute zero), intrinsic semiconductors do not conduct electricity, and all valence electrons are bound to covalent bonds. However, general semiconductor applications are carried out at room temperature. At this time, due to thermal movement, the semiconductor will inherently excite a pair of holes and electrons.
In the two impurity semiconductors, of course, there are intrinsic excitations. That is to say, in N-type semiconductors, there are holes, but the number is less than free electrons. Of these two types of carriers, the more numerous are called more carriers, and the lesser ones are called minority carriers. The opposite is true in P-type semiconductors.
Part.3 Combination of N-type and P-type semiconductors
What happens if we connect two impurity semiconductors?
N-type semiconductors have more electrons and fewer holes, and P-type semiconductors have more holes and fewer electrons. This is a bit like a mixture between two different solutions. Most of the electrons here want to go to the other side, and most of the holes there want to go here. This behavior is called multi-child diffusion, but this diffusion starts at the beginning. There is a problem. I don’t know if you have played the game of “sticking bark”. The two need to “stick” together within the specified time. Once the time comes, the person who has not posted will be eliminated.
The same is true for electrons and holes. They can’t afford to stay close and far away, so often the multiple electrons at the junction of the two impurity semiconductors are directly “attached”. Remember that both of our semiconductors are doped with atoms, and the whole is electrically neutral. We just drew the free electrons and holes in the conduction band, and there are atomic nuclei and inner electrons below. Now that the electrons run away, or the holes are filled, the two areas will show electrical properties. N-type semiconductors that have lost electrons are positive, and P-type semiconductors that have lost holes are negative. This structure is called a PN junction.
Does it sound dizzy? The following schematic diagram can help you understand the formation process of the PN junction intuitively.
After the PN junction is formed, its two ends show different electrical properties, and then an electric field from N to P is formed. This electric field is formed spontaneously, and we can call it a self-built electric field. At this time, let’s take a look at the situation of a few births. The electrical properties of few births and many births are opposite. Since the self-built electric field hinders the diffusion of many births, it promotes the movement of the minority births to the opposite side. This process is called the drift of the minority births. When the multi-carrier diffusion and the minority-carrier drift reach a dynamic equilibrium, the PN junction is formed stably at this time.
After layering, we know why silicon is called a semiconductor, and why two kinds of semiconductors can be spliced to get a structure with its own electric field-PN junction. The paving is over, and it’s time for the photovoltaic effect to come out!
Part.4: hit the light on the PN junction
What happens when sunlight hits the PN junction? Yes, it is the photovoltaic effect. The role of the photovoltaic effect is to make those valence band electrons that have been paired again “tempted” and form electron-hole pairs again. The essence is that the valence band electrons we mentioned earlier absorb the energy of light, the energy becomes higher, and it jumps to the conduction band.
These electron holes are thrown to both sides under the influence of the self-built electric field, forming an electric field from P to N. This is the photo-generated electric field, and the direction is opposite to the self-built electric field. At this time, an external loop is connected. Due to the existence of the potential difference, a current is generated in the loop! At this point, our work of converting light energy into electrical energy through the photovoltaic effect and semiconductors is complete.
Photovoltaic cells have undergone nearly a hundred years of development. The inorganic semiconductor photovoltaic cells exemplified in this article are the most mature of them. In addition, there are some photovoltaic cells based on organic semiconductor materials, such as dye-sensitized solar cells and some perovskite solar cells. Regardless of organic or inorganic, the basic principles of these photovoltaic cells are inseparable from the various semiconductor-related theories we have introduced.
Although photovoltaic cells based on these theories and materials have not reached their limit, the overall theoretical conversion efficiency is only 30%, and the true conversion efficiency is difficult to reach the theoretical value. Now some researchers have begun to explore photovoltaic cells based on new working principles, such as carrier solar cells, impurity photovoltaic cells, etc. They hope to increase the photoelectric conversion efficiency to 60% or even higher. For the photovoltaic industry, which is still in its infancy, we always have great confidence that it may be an important option for mankind to solve energy problems in the future.