SOLAR CELLS

SOLAR CELLS

In this article I will try to explain in a simple way, which are the solar cells , how they are manufactured, their principles, how they work and their parameters, that if without entering into technicalities, that confuse you even more.

As you can see the physical principles of a photovoltaic cell are very basic.

What are solar cells?

SOLAR CELLS

The  solar cells , also called cells are devices that convert solar energy into electricity, either through the photovoltaic effect or by prior conversion of that energy into heat or chemical energy.

In these cells, light falls on a two-layer semiconductor device that produces a difference in photovoltage or potential between the layers, which is capable of conducting the current through an external circuit.

The solitary photovoltaic cells is basically a device that converts solar energy into electricity, this process, called photoelectric effect, takes place when the solar radiation hits the two-layer semiconductor material, of which the cell is composed.

The semiconductor material of which the photovoltaic cell is composed is silicon, this semiconductor is used thanks to the abundance that already exists and the low price involved in its optimization, for the conversion of solar energy into electricity.

Who invented the solar cells?

Let’s do some history. The first photovoltaic cell in the world was built by Edmond Becquerel, in 1839 when he was only 19 years old; in those times it was not common for precocious geniuses to make their great creations in a garage, like today, but Edmond developed his invention in his father’s laboratory, as you see the circumstances do not change much in time.

Since then the knowledge of materials and technology has evolved for its manipulation to achieve today a super efficient photovoltaic cell at affordable prices for everyone.

The photovoltaic cell became popular after being an important part of the artificial satellite Vanguard I in 1958, and its later incorporation into more advanced satellites, during the 1960s.

Well enough stories and let’s cut to the chase:

What dimensions do solar cells have?

Generally the photovoltaic cells that are commercialized today have a thickness between 0.25 and 0.35 mm and consist of mono or polycrystalline silicon, depending on their purity, they are usually square and flattened in their vertices, with an area between 100 and 225 m².

How many volts do solar cells generate?

With special conditions (a radiation of 1 Kw / m² at a temperature of 25º C) these devices can produce a current between 3 and 4 A (amperes), a voltage of 0.5 V (volts) and a power of 1 , 5 to 2 Wp (watts point);

As you can see this production is very small but grouping several of them, we can increase these values. Shaping the  photovoltaic solar panels .

Every day you experiment with new materials to increase the efficiency of the photovoltaic cell, this efficiency is usually known as conversion efficiency, which is the percentage of energy contained in the solar radiation that affects the cell transformed into electrical energy.

How efficient are the solar cells?

We currently find silicon cells in the market with a conversion efficiency between 13% and 17%, also more efficient solar panels have been developed, reaching in laboratory experiments up to 35% thanks to nanotechnology.

What characteristics does a solar cell have?

An important characteristic of  solar cells  is that their voltage does not depend on their size and is very constant with the change in light intensity, unlike the current that circulates through a device and that is proportional to the intensity of light and its size.

To compare the various cells, they are classified by current density or by amps per square centimeter in their area.

How does a solar cell work?

As I explained earlier, a modern photovoltaic cell is composed of a semiconductor material, which has specific electronic properties.

The semiconductor material used is silicon which when exposed to solar radiation absorbs photons of light with enough energy to cause the “jump of electrons”, moving them from their original position to the illuminated surface.

When releasing these electrons with their negative charge (n) causes the appearance of gaps or lagoons which receive a special name because they behave as if they were particles with positive charges.

You should also know that in a semiconductor like the one I am explaining to you, the number of electrons is equal to the number of holes.

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Being more precise when an electron gains enough energy (provided by the solar radiation) it is released from the atom to which it was linked, creating a hole in it, that is when it is said that an electron-hole pair has been generated (pair eh) .

In order to obtain current extraction, it is necessary to fabricate a pn  junction  that consists in manufacturing a semiconductor in which one zone is of type  n  and the other of type .

This fabrication does not consist of gluing a semiconductor  to a  n,  but it must be done in such a way that the crystalline lattice is not interrupted when passing from one region to another. It is necessary then, the use of special technologies.

The existence of the pn junction   makes possible the appearance of an  electric field  in the photovoltaic cell (with direction from  to  p)  that separates the pairs eh: the gaps, positive charges, directs them towards the contact on the p side which causes the extraction of an electron from the metal that constitutes the contact; the electrons, negative charges, direct them towards the n-side contact by injecting them into the metal.

This makes it possible to maintain an electric current through the external circuit and consequently the operation of the cell as a photovoltaic generator.

How are solar cells manufactured?

The  solar cells  are made using polycrystalline plates that are made in a molding process in which the silicon that is melted is poured into a mold and allowed to settle. When it is, it is sliced ​​into plates and they are ready.

The polycrystalline solar cells are inexpensive to be easier to produce, but they are not as efficient as monocrystalline plates, which are more expensive but their performance is much higher. In the process almost half of the silicon is lost as dust during cutting.

The amount of  energy  delivered by each solar cell is determined by the type and area of ​​the material, the intensity of the sunlight and by the length of the wave of sunlight.

The monocrystalline solar cells made of monocrystalline silicon can not convert more than 25% of solar energy into electricity, although in the future it is expected that this percentage is higher.

The manufacture of a photovoltaic cell begins with the production of monocrystalline plates (wafers), polycrystalline plates or thin sheets.

How are mono and polycrystalline solar cells manufactured?

The monocrystalline plates (approximately 0.30 to 0.50 millimeters thick) are cut from a large mono crystalline ingot that has been developed at approximately 1400 ° C, this is a very expensive process.

The silicon must be of a very high purity and have a crystal structure almost perfect.

The polycrystalline plates are made by a molding process in which the molten silicon is poured into a mold and allowed to settle.

Then it is sliced ​​into plates. As the polycrystalline plates are made by molding they are appreciably cheaper to produce, but not as efficient as the crystalline mono cells.

The lowest performance is due to imperfections in the crystalline structure, resulting from the molding process. It could be explained as a grouping of many silicon crystals that give a non-uniform appearance to the surface.

In the two processes mentioned above, almost half of the silicon is lost as dust during cutting.

Amorphous silicon, one of the thin-film technologies, is created by depositing silicon on a glass substrate of a reactive gas such as silane (SiH4).

Amorphous silicon is one of a group of thin-film technologies. This type of solar cell can be applied as a film to low cost substrates such as glass or plastic.

What other solar cell technologies are there?

Other thin-film technologies include thin sheet of multicrystalline silicon, copper selenide and indium / cadmium sulfide cells, cadmium telluride / cadmium sulfide cells and gallium arsenide cells.

Thin-film photovoltaic cells have many advantages including easier deposition and assembly, the ability to be deposited on cheap substrates or building materials, ease of mass production, and great convenience for large applications.

In the production of a silicon photovoltaic cell, atoms of impurities (doped) are introduced to create a p-type region and a n-type region in order to produce a pn junction.

The doping can be done by diffusion at high temperature, where the plates are placed in an oven with the dopant introduced in the form of steam.

There are many other methods of doping silicon. In the manufacture of some thin-film devices the introduction of dopants can occur during the deposition of the sheets or layers.

What are the types of solar cells?

Types of Photovoltaic Cells and their structure

The variety of materials used in the manufacture of photovoltaic cells and the different technologies of manufacturing and applications of photovoltaic energy is of such amplitude at present, that it is necessary to classify them in some way, we establish the following:

    • For the type of materials.
    • By internal structure of the materials.
    • By device structure.
    • For its applications.

By the type of materials:

Within this classification we highlight 3 types:

  • Of simple material: the most used material is silicon (Si), although germanium (Ge) and selenium (Se) are also used.
  • Of binary compounds: the binary compounds that have been investigated have been many, although the most usual have been: CdTe, AsGa, InP, CdS, …
  • Of ternary compounds: among others it is worth mentioning some compounds such as AlGaAs, and the compounds of chalcopyrite structure based on Cu, such as CuInSe2, …

This classification could be almost endless, since the number of elements present in an alloy of semiconducting and metallic materials can be as large as you want.

For the internal structure of the material:

As for the internal crystalline structure in which these materials can be manufactured and obtained, the following classification can be made:

  • Monocrystalline:  the cell is grown or processed as a single crystal. The most typical are Si, AsGa, CdTe … Cells with this crystalline structure usually show good efficiencies but with high manufacturing costs.

Examples of monocrystalline cells used in solar panels could be, for example, the  SolarWorld Sunmodule Plus SW290 Mono solar panel . The internal structure of a monocrystalline photovoltaic solar cell would be like the one shown in the figure.

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  • Multicrystalline: these cells partially sacrifice the final performance of the cell in exchange for a decrease in its cost. The internal structure is formed by a multitude of large grains or monocrystals. The crystalline orientation of these grains is totally random.
  • Polycrystalline:  although with a structure also based on small crystals or grains, the grain size in these materials is much lower than that of multicrystalline materials. Examples of polycrystalline cells used in solar panels could be found in our section on  solar panels . The internal structure of a polycrystalline photovoltaic solar cell would be like the one shown in the figure.

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  • Hybrid Devices: many modern cells are manufactured from monocrystalline layers or substrates on which a second material with polycrystalline structure is deposited by means of thin-film techniques.
  • Amorphous: the only material currently used in this form is Silicon, normally with hydrogen incorporated in the manufacturing process. One of the problems presented is the degradation that occurs in its performance after the first months of operation. The internal structure of an amorphous photovoltaic solar cell would be like that shown in the figure.

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Parameters of a photovoltaic cell

Generally in the data sheets of the photovoltaic modules usually appear some electrical data that come to determine the electric behavior of the module under standard conditions of measurement (STC) that are usually, by international agreement, of 1000w / m2 and T = 25ºC. Some of these technical specifications are usually:

  • The short-circuit current or current (Icc)  is produced at zero voltage and is measured by connecting a simple ammeter to the output of the cell or panel. Their values ​​are usually between 3 and 7 Amperes. We can also find it with the nomenclature Isc for Short Circuit, short circuit, in English.
  • The open circuit voltage (Vca)  is really the maximum voltage that a cell or panel can give and is measured directly between terminals of the cell or panel with a voltmeter. We can also find it with the Vsc nomenclature in English.
  • The peak power or Wp  is simply the maximum actual product of the current and the voltage produced. Obviously the theoretical Wp is superior to the real Wp, that is explained below with the Form Factor.

Now imagine that the product of Icc x Vca is represented by a square, the lower left corner represents the origin 0,0 of a coordinate axis and the upper right represents the theoretical maximum power Wpt = Icc x Vca. Well, we know that the real Wp power will always be less than the theoretical Wp and consequently the real power Wpr = Ip x Vp will be less than Wpt = Icc x Vca.

For the form factor is the quotient of FF = Wpr / Wpt and its result, obviously, will always be less than one. This data gives us an idea of ​​the quality of the photovoltaic cell or panel.

The amount of energy delivered by a photovoltaic cell is determined by:

  • The wavelength of sunlight
  • The area of ​​the photovoltaic cell
  • The type of material
  • The intensity of solar radiation

For example, monocrystalline silicon photovoltaic cells currently can not convert more than 25% of solar energy into electricity, because radiation in the infrared region of the electromagnetic spectrum does not have enough energy to separate the positive and negative charges in the material .

The polycrystalline silicon photovoltaic cells currently have an efficiency of less than 20% and amorphous silicon cells have an efficiency close to 10%, due to internal energy losses higher than those of monocrystalline silicon.

A 100 cm2 monocrystalline silicon photovoltaic cell will produce about 1.5 watts of power at 0.5 volts of DC and 3 amps under sunlight in the middle of summer (1000Wm-2).

What does the output power of the solar cells depend on?

The output power of the cell is almost directly proportional to the intensity of sunlight. (For example, if the intensity of sunlight is halved, the energy output will also be halved).

An important characteristic of photovoltaic cells is that the voltage of the photovoltaic cell does not depend on its size, and it remains fairly constant with the change in light intensity.

The current in a device, however, is almost directly proportional to the intensity of light and size. To compare different cells, they are classified by current density, or amperes per square centimeter of the area of ​​the cell.

The power delivered by a photovoltaic cell can be increased quite effectively by employing a tracking mechanism to keep the photovoltaic device directly in front of the sun, or by concentrating the sunlight using lenses or mirrors.

However, there are limits to this process, due to the complexity of the mechanisms, and the need to refresh the cells. The current is relatively stable at high temperatures, but the voltage is reduced, leading to a drop in power due to the increase in the temperature of the photovoltaic cell.

What are organic solar cells?

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In 1953 scientists from the Bell laboratories successfully developed a photovoltaic cell that produced only 5mW of electrical power, subsequent investigations have improved the quality of said cell and can now operate at an efficiency of 20%.

In the beginning, applying this type of technology was very expensive; it was only used in space shuttles, now we can see that photovoltaic cells are used in various devices such as calculators, in the supply of emergency signals on the roads, in decorative garden lamps, and so on.

Photovoltaic technology based on semiconductors, such as silicon, requires very specialized manufacturing conditions, which implies a high cost and is not profitable for certain applications.

With the discovery of organic semiconductors, an alternative has been envisaged to reduce manufacturing costs in photovoltaic cells.

Principle of operation of the organic photovoltaic cell

The organic photovoltaic cells (OPVs), are usually based on a mixture of bulk heterojunction (BHJ), which is obtained by mixing in an organic solvent, an electron-rich organic semiconductor polymer with a fullerene, which is easily reducible .

This electron donor-acceptor mixture already placed in the solar cell is photoactive, upon receiving the solar radiation generates an excited state known as an exciton, which is formed by a hollow pair-electron.

This in the presence of the electric field generated by the electrodes is separated in the electron and the gap generating the electric current.

Efficiency and life time

The efficiency in the OPV cells has increased considerably since the 70’s until today: in 1975 it was 0.001%; in 1986 of 1%; in 2006 it was 5.5%; in 2009 it was 6.1% and currently (2011-2015) efficiencies greater than 10% have been reported (this under laboratory conditions).

Another important aspect to consider in the PVO is the useful life at the beginning it was only from weeks to months now this life time is approaching the year and this is very little if we compare it with the lifetime of the conventional silicon cells that they can have a life time of 20 to 30 years, this is clear with their respective maintenance.

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The prototypes of organic cells seek efficiency with low costs and greater environmental protection.