The principle of operation of solar cells. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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Although many of us are unaware of it, the way to generate electricity from sunlight has been known for over 100 years. The phenomenon of photoelectricity was first observed by Edmond Becquerel in 1839. In one of his many experiments with electricity, he placed two metal plates in a conductive solution and illuminated the installation with sunlight. To his great amazement, he found that an electromotive force (EMF) was generated in this process.

This accidental discovery went unnoticed until 1873, when Willoughby Smith discovered a similar effect when a selenium plate was irradiated with light. And although his first experiments were imperfect, they marked the beginning of the history of semiconductor solar cells.

In search of new energy sources, Bell Labs invented the silicon solar cell, which became the forerunner of today's photovoltaic converters.

Only in the early 50s. the solar cell has reached a relatively high degree of perfection.

Fundamentals of the theory of semiconductors

Silicon is the main semiconductor material in modern electronics. Most modern solar cells are also made from silicon.

A semiconductor is a substance that is neither a good conductor nor a good insulator. For example, copper is an excellent conductor, its scope is very wide. Wherever it is required to transfer electrical energy from one place to another, copper is an indispensable assistant. The same can be said about aluminum.

On the other hand, glass has negligible electrical conductivity but is a good dielectric. If you need to block the path of electric current, a glass insulator will successfully solve this problem. By the way, the insulators of the pole pieces in the first telephones were made of glass.

Ris.1

The electrical conductivity of semiconductors lies between these two limiting cases. In some applications, semiconductors can serve as conductors, in others they can serve as insulators. However, pure silicon is still closer to insulators and conducts electricity very poorly. The reason for this is due to the peculiarity of its crystal structure.

Silicon atoms are connected to each other with the help of so-called valence electrons. It is best to think of these connections as "hands". Each silicon atom has four arms.

Silicon atoms are very "sociable", they do not like loneliness. Therefore, they try to hold "hands" with the atoms surrounding them.

Since each atom has four "hands" with which it takes the "hands" of its neighbors, together they form the lattice shown in Fig. 1. As a result, all four "arms" of the atom are occupied. Consequently, in such a structure there are no free electrons ("hands"), and without free electrons, an electric current is hardly possible.

For the needs of electronics, this state of affairs is unacceptable. For current to flow, the crystal must have free electrons. This is achieved by introducing impurities into the original substance. This process is called doping.

Semiconductor doping

Let's assume that we took and replaced one silicon atom in our crystal structure with an atom having a valency equal to five (in other words, having five "arms". For example, such an atom is a boron atom. Once among its "new neighbors" and taking them "by the hands", this atom will soon discover that one "hand" of it is free. (The author is mistaken - phosphorus atoms with a valence of five are used as a donor (source of free electrons), and as acceptors, allowing you to enter into silicon crystal positive charges (holes), boron atoms are used, which are characterized by a valency of three. - Approx. ed.)

Ris.2

This unrelated "hand" is nothing but a free electron. Since the boron atom is more or less satisfied that four of its five "arms" - electrons are occupied, it is not particularly worried about the fate of the fifth. At the slightest perturbation, the electron will "break off".

This is the essence of doping. The more impurities we introduce into the crystal, the more free electrons will be in it and the better the silicon will conduct electric current.

During doping, the reverse process can also occur. If the silicon atom is replaced by a trivalent atom, such as phosphorus, a so-called hole will appear in our structure. Consequently, there is a shortage of electrons in the crystal, and it will readily accept them into its lattice.

Due to the fact that in such a structure the atoms try to capture electrons, the resulting holes will move through the structure lacking electrons. In fact, electrons move from hole to hole and thus conduct electricity.

Solar cell manufacturing

Now you might think that if you take a doped silicon crystal with a lack of electrons and a doped crystal with an excess of electrons and put them together, something must happen.

Ris.3

With close mechanical contact between two crystals, the atoms in the near-surface regions approach each other so much that the phosphorus atoms easily donate their extra electrons, and the boron atoms readily accept them.

As a result, the electrical equilibrium of the crystal is restored. But remember that crystals have a very rigid structure, so the exchange will only occur between atoms that are in closest contact with each other. The thickness of the area of ​​this contact does not exceed the size of several atoms, and the volume of the semiconductor remains unchanged.

Of course, it takes more than just joining two pieces of silicon together to get this effect. Silicon is most commonly doped using a high-temperature diffusion process. As a result, at the boundary between the regions in the depth of the semiconductor, doped with different impurities, a hyperthin interface region, called the pn junction, is formed.

It is within this region that the conversion of light into electricity takes place.

When a particle of light, called a photon, strikes a pn junction with sufficient energy, it knocks out an electron, making it free, that is, capable of moving. The energy of the photon is then transferred to the electron. In this case, a hole is formed in the crystal lattice. It must be borne in mind that the transition region tends to maintain equilibrium. This process, called photoionization, occurs not only in the region of the pn junction, but also in any other part of the crystal, into which sunlight penetrates, having a sufficiently large energy necessary to create free charge carriers - an electron and a hole.

Due to the fact that there is a lack of holes in the n-type material, and a lack of electrons in the p-type material, the hole and electron are separated and migrate in different directions.

But now the balance is off. An electron that has received the energy of a photon seeks to reconnect with its antipode (hole) and is ready to spend its energy on this. Unfortunately, the pn junction is a potential barrier that the electron cannot overcome.

However, if we connect the regions with p- and n-type conductors with a conductor, then this obstacle will be successfully overcome and the electron will "get through" to its hole through the "back door". In this case, the electron spends its energy along the way, which we use.

Solar cell characteristics

The pn junction is a formidable obstacle to the movement of electrons. But it cannot be called irresistible. The energy that an electron receives from a photon is usually not enough for it to overcome this barrier and connect with a hole, but this is not always the case.

Fig. 4: 1 - upper collector grating; 2 - n-type diffusion layer; 3 - np-junction, 4 p-type base layer; 5 - bottom contact.

The potential barrier height of the pn junction is about 600 mV (0,6 V). Electrons with energies over 600 mV can "climb" this wall and be absorbed. Therefore, the maximum voltage that a solar cell can develop is 600 mV. However, the actual value depends on the type of semiconductor material and the design of the solar cell.

Ris.5

Connecting a load to a solar cell reduces the energy of some electrons, including the more energetic ones. As a result, the total voltage of the solar cell and the number of electrons that can overcome the pn-junction barrier are reduced.

As the load resistance increases, an increasing number of electrons will be “pumped out” through it, and the voltage will decrease even more. However, at some point a strange thing happens. At 450 mV (0,45 V), the current (electron flux) stops increasing even though the voltage continues to decrease. The "plateau" of the current is reached.

This phenomenon is due to the finite number of photons incident on the pn junction. It is known that the more photons reach the pn junction, the more electrons are released. More photons - more current.

However, there comes a time when literally every photon that has entered the pn junction is used and the number of free electrons, and hence the current, no longer increase. This corresponds to the appearance of a "plateau" in the characteristic of the solar cell.

Of course, the number of free electrons also depends on the surface area and light intensity. Obviously, as the cell area increases, more photons are captured and the current increases. Similarly, as light intensity increases, the concentration of photons in a given area increases, which also increases the current.

Solar Cell Efficiency

Usually, the average intensity of sunlight reaching the earth's surface is taken to be 100 mW/cm2. In other words, a 10x10 cm2 solar cell should theoretically generate 10 watts of power.

Unfortunately, no solar cell can and will not generate such power: there will always be losses. The highest efficiency (efficiency factor) achieved so far (and even then with cascade photocells in the experimental laboratory) is about 30%. The efficiency of a conventional silicon solar cell ranges from 10-13%. An element with an area of ​​100 cm2 can generate about 1 watt of power.

Of course, the efficiency of a solar cell depends on many factors, among which the change in ambient temperature is the most significant. As the temperature increases, the crystal lattice is excited and its atoms vibrate more intensely. This, in turn, leads to an increase in the energy level of electrons inside the structure. Over time, when the energy level of electrons rises so much that most of them are able to overcome the potential barrier of the pn junction, recombination sharply increases in the semiconductor. This leads to a decrease in the number of electrons reaching the grid collectors, and the electric current in the load decreases. On the other hand, low temperature contributes to the actual enhancement of the photoelectric effect.

The main reason for the decrease in the efficiency of solar cells with increasing temperature is the decrease in the value of the potential barrier of the pn-junction, which leads to a drop in the voltage generated by the cell.

Author: Byers T.

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