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photovoltaic cells

Photovoltaic cells: what are they and how do they work?

Photovoltaic cells are the basic element for the production of electricity. Find out what the features are and how they work

A photovoltaic system is characterized by a set of solar panels, placed in series or in parallel; at the base of the panels are the photovoltaic cells.

Let’s find out together what they are and how the basic elements of a photovoltaic system work.

The design of a photovoltaic system is a very delicate operation, to be carried out with particular care and attention. To avoid making mistakes that could cost you dearly after the construction of the plant, I recommend you rely on an effective Solar Design Software to design systems in a safe and guided way.

What are photovoltaic cells?

Photovoltaic cells are the basic element in the production of electricity through solar energy. The cells can be made of various materials; the most used and high-performance semiconductor is silicon.

Silicon has an important characteristic: a minimum amount of energy is enough for valence electrons to be used to generate an electric current.

To exploit the passage of electrons from the valence band to the conduction band, doping is applied, the phenomenon for which impurities are inserted inside the semiconductor: boron for the negative charge and phosphorus for the positive charge.

Tetravalent silicon doping schema

Tetravalent silicon doping

After solidification, these systems are placed next to each other by means of a neutral junction, positive part with the negative one, and, at the point where the connection is, there is a null charge. This mechanism creates a positively charged system on one side and negatively charged on another side, resulting in a stack.

Types of photovoltaic cells

The solar cells vary depending on the crystal that characterizes them:

  • monocrystalline cells;
  • polycrystalline cells;
  • amorphous cells.
Types of photoltaic cells

Types of photovoltaic cells

Monocrystalline photovoltaic cells

Monocrystalline cells consist of a single crystal, oriented in the same direction. This feature allows you to make the most of the solar energy that the cell can capture. Cells coloration is typically black.

The system adopted to produce the monocrystalline cells is the Czochralski system. It is a process in which a crystal seed is inserted into a silicon melt, inside which the seed rotates vertically in an anti-clockwise direction and, by dipping very slowly, causes the melt itself to crystallize in an orderly manner on the seed being dipped.

Monocrystalline photovoltaic cells

Monocrystalline photovoltaic cells

At the end of the process, all the crystals are homogeneously oriented so as to materialize a single macro crystal and give rise to a silicon ingot.

We are talking about the basic structure; in fact, this crystal can be subsequently drugged positively or negatively with Boron or Phosphorus.

The obtained ingot must be cut into very thin wafer slices of the thickness of a few hundred micrometers, i.e. one thousandth of a millimeter, a fundamental value to consider, since the lower the thickness, the greater the efficiency of the cell.

Monocrystalline silicon bar

Monocrystalline silicon bar

Once the wafers are obtained, positive or negative, they will be laid on the top, forming the photovoltaic cell.

Polycrystalline photovoltaic cells

Polycrystalline cells are obtained from the waste of the electronics industry; they are made up of several crystals and their typical colour is that of iridescent blue.

Polycrystalline cells

Polycrystalline cells

In this case, cooling takes place in a thermostatic bath, in a unidirectional manner from one end to the other, through a technical gradient. The process takes place slowly, but faster than the monocrystalline cell method.

The speed in the realization has energy and economic advantages: the ingot made has a lower cost of production, however the orientation of the crystal is more disordered and does not allow to make the most of the incident solar energy.

The last process involves the deposit of metal filaments to collect the electrons on the negatively charged part, from which the various modules will then be created.

Detail of polycrystalline cell

Detail of polycrystalline cell

Amorphous photovoltaic cells

Amorphous photovoltaic cells have a more chaotic structure than silicon which, in this configuration, no longer has a crystalline shape. The structure of amorphous silicon can be exploited for the realization of the thin film, that is, for the realization of elements with a very thin size, adapted to surfaces that are not perfectly flat.

They are used in the field of architectural integration, urban furnishing or structural elements of buildings.

The process of creating an amorphous cell is very simple: on a rigid substrate (glass or metal) several layers of material are deposited with suitable technologies (sputtering or evaporation), two of these layers (the outermost) become conduction electrodes, while the inner layer becomes junction of the photovoltaic cell.

In this process, the technologies mainly used are the triple junction technology and the and cadmium-telluride/cadmium-sulfide (CTS).

Amorphous photovoltaic cell

Amorphous photovoltaic cell

How photovoltaic cells work?

In order to make a photovoltaic cell energy productive, it is necessary to connect it to two measuring instruments, namely the amperometer, which measures the flow of electric current and the voltmeter, which measures the voltage between two nodes.

In an electrical circuit, the two instruments are placed in parallel to prevent the ammeter from measuring the electrical resistance present inside the voltmeter itself and to ensure the measurement of the performance of the cell regardless of external systems.

Within the circuit, an electrical load is also placed that influences the performance of the cell: depending on the load, both the intensity and the voltage of the electrical current produced vary.

Photovoltaic cell - electrical measurements

Photovoltaic cell – electrical measurements

Depending on the load entered, it is also possible to distinguish between closed circuit and open circuit. These two conditions are associated with two reference values that are used to understand the performance of the cell.

For the open circuit, a null current is measured in the ammeter, since there is no electrical contact, while the voltage will present the maximum value that the cell can produce. That value is called open circuit voltage (Voc).

In the closed circuit there is the minimum load and the current is maximum, being inversely proportional to the electrical load. The maximum current that can be produced in these cases is defined as closed circuit current (Isc); this current is associated with a zero voltage.

By reporting these values on a diagram called “V-I CURVE”, characteristic curve of a photovoltaic cell or voltamperometric curve, it is possible to understand the efficiency of a photovoltaic cell and, therefore, of a solar panel, through 4 fundamental parameters:

  • peak intensity;
  • peak voltage;
  • peak power;
  • fill factor.

More precisely, the voltage values are arranged on the abscissae and the electric current values on the ordinates.

Distinguishing factors of a photovoltaic cell

Distinguishing factors of a photovoltaic cell

How photovoltaic cells work according to arrangements

Photovoltaic cells are diodes and act like batteries. They can be connected in series or in parallel. The series connection, obtained by connecting the positive side of a cell with the negative side of another cell, generates a double voltage (sum of the two), while the electrical intensity remains constant.

The same happens on the amperometric curve: by connecting the cells in series, the curve has an increase in abscissae (voltage).

To vary the current intensity, however, it is necessary to connect the cells in parallel; in this way, by connecting the positive electrodes to the negative ones, a constant voltage is obtained, while the current intensity increases and the amperometric curve will move upwards.

Photovoltaic cells are also affected by temperature, which can compromise their performance. Specifically, if the temperature reaches certain values, there is a sudden drop in voltage. High temperatures can compromise bonding efficiency and decrease performance.

Even when the incident solar radiation varies, there is a curve variation. Specifically, as incident solar radiation decreases, the electrical intensity decreases significantly and the curve tends to crush, while the voltage values do not change much.

Therefore, in the analyses prior to the installation of photovoltaic panels, it is essential to refer to the concrete climate data found from solar atlases or field measurements, possibly using a specific Solar Design Software which provides you with the actual solar radiation data taken from the main reference climate databases.

Solar cells in photovoltaic panels

The photovoltaic cells are connected in modules, giving rise to photovoltaic panels, which, in turn, connect to each other constituting thephotovoltaic system. In such a connection, the panels may be arranged in different strings and modules and, depending on how they are connected to each other, may increase in intensity or voltage.

PV cells: configuration of 2 strings of 3 modules

PV cells: configuration of 2 strings of 3 modules

PV cells: configuration of 3 strings of 2 modules

PV cells: configuration of 3 strings of 2 modules

Once the power to be installed has been established, it can be decided to produce it with a variable number of panels. The choice of arrangement and power can be determined according to the solar radiation value of the location and exposure of the panel.

Once again, a Solar Design Software is useful able to support you in choosing the correct number of modules according to the design criteria defined to maximize annual production or maximum power.

In addition, such software has specific libraries of modules, inverters, batteries and photovoltaic components, essential for the correct design.