Structure and workings of solar cells

Solar cells or also often called photovoltaics are devices that are able to convert sunlight directly into electricity.

Solar cells can be referred to as the main actors to maximize the enormous potential of solar energy that reaches earth, although, in addition to being used to produce electricity, energy from the sun can also be maximized by its thermal energy through a solar thermal system.

Solar cells can be analogous to devices with two terminals or connections, where when the conditions are dark or not enough light functions as a diode, and when illuminated with sunlight can produce voltage.

When irradiated, generally one commercial solar cell produces dc voltage of 0.5 to 1 volt, and short-circuit current in the milliampere scale per cm2.

This voltage and current are not enough for various applications, so generally, a number of solar cells are arranged in series to form a solar module. One solar module usually consists of 28-36 solar cells, and the total produces a dc voltage of 12 V under standard radiation conditions (Air Mass 1.5).

These solar modules can be combined in parallel or in series to increase the total voltage and output current according to the power needed for a particular application.

Solar modules usually consist of 28-36 solar cells arranged in series to increase the total output power.

Solar Cell Structure

In accordance with the development of science & technology, the types of solar cell technology have developed with various innovations.

There are so-called one, two, three and four generation solar cells, with different structures or constituent cells (Types of solar technology will be discussed in the words "Solar Cells: Types of technology").

In this paper, we will discuss the structure and workings of solar cells that are common in the market today, namely solar cells based on silicon material which also generally includes the structure and workings of first-generation solar cells (silicon solar cells) and second (thin films). )

The structure of a commercial solar cell that uses silicon material as a semiconductor.

In general Solar Cells consist of:

1. Substrate / Metal Backing

The substrate is a material that supports all components of solar cells.

Substrate material must also have good electrical conductivity because it also functions as a positive terminal contact for solar cells, so metal or metal materials such as aluminum or molybdenum are generally used.

For dye-sensitized solar cells (DSSC) and organic solar cells, the substrate also functions as a place of entry of light so that the material used is conductive but also transparent materials such as tin oxide (ITO) and fluorine doped tin oxide (FTO).

2. Semiconductor material

Semiconductor materials are a core part of solar cells that typically have thicknesses of up to several hundred micrometers for first-generation solar cells (silicon), and 1-3 micrometers for thin layer solar cells. This semiconductor material serves to absorb light from sunlight.

Semiconductors used are silicon material, which is commonly applied in the electronics industry.

Whereas for thin layer solar cells, semiconductor materials are commonly used and have entered the market, for example Cu (In, Ga) (S, Se) 2 (CIGS), CdTe (cadmium telluride), and amorphous silicon materials, in addition to semiconductor materials other potential in intensive research such as Cu2ZnSn (S, Se) 4 (CZTS) and Cu2O (copper oxide).

The semiconductor part consists of a junction or a combination of two semiconductor materials namely p-type semiconductors (the materials mentioned above) and n-type (n-type silicon, CdS, etc.) that form a p-n junction. This P-n junction is the key to the working principle of solar cells.

The definition of p-type semiconductors, n-types, and also the principle of p-n junctions and solar cells will be discussed in the "how solar cells work" section.

3. Contact metal / contact grid

In addition to the substrate as positive contact, above some semiconductor material is usually overlaid metal material or transparent conductive material as negative contact.

4. Anti-reflective coating

Reflection of light must be minimized in order to optimize the light absorbed by the semiconductor. Therefore usually solar cells are coated with an anti-reflection layer.

This anti-reflection material is a thin layer of material with a large optical refractive index between the semiconductor and air that causes light to be turned towards the semiconductor to minimize the reflected light back.

5. Encapsulation/glass cover

This section functions as an encapsulation to protect solar modules from rain or dirt.

How solar cells work

Conventional solar cells work using the principle of the p-n junction, the i.e. junction between p-type and n-type semiconductors. This semiconductor consists of atomic bonds which have electrons as basic constituents.

N-type semiconductors have excess electrons (negative charges) while p-type semiconductors have excess holes in their atomic structure. The condition of the excess electrons and holes can occur by doping the material with dopant atoms.

For example, to get p-type silicon material, silicon is doped by boron atoms, while to get n-type silicon material, silicon is doped by phosphorus atoms.

 The role of the p-n junction is to form an electric field so that electrons (and holes) can be extracted by contact material to produce electricity.

When p-type and n-type semiconductors are contacted, the excess electrons will move from n-type semiconductors to p-type to form positive poles on n-type semiconductors, and vice versa negative poles in p-type semiconductors.

As a result of the flow of electrons and holes, an electric field is formed which, when sunlight hits the juncture of the PN junction, will push electrons to move from the semiconductor to negative contact, which is then used as electricity, and instead the hole moves towards positive contact waiting for electrons to come, such as illustrated in the figure below.

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