Working Principle of Solar Cell or Photovoltaic Cell

Conversion of light energy in electrical energy is based on a phenomenon called photovoltaic effect. When semiconductor materials are exposed to light, the some of the photons of light ray are absorbed by the semiconductor crystal which causes a significant number of free electrons in the crystal. This is the basic reason for producing electricity due to photovoltaic effect. Photovoltaic cell is the basic unit of the system where the photovoltaic effect is utilised to produce electricity from light energy. Silicon is the most widely used semiconductor material for constructing the photovoltaic cell. The silicon atom has four valence electrons. In a solid crystal, each silicon atom shares each of its four valence electrons with another nearest silicon atom hence creating covalent bonds between them. In this way, silicon crystal gets a tetrahedral lattice structure. While light ray strikes on any materials some portion of the light is reflected, some portion is transmitted through the materials and rest is absorbed by the materials.

The same thing happens when light falls on a silicon crystal. If the intensity of incident light is high enough, sufficient numbers of photons are absorbed by the crystal and these photons, in turn, excite some of the electrons of covalent bonds. These excited electrons then get sufficient energy to migrate from valence band to conduction band. As the energy level of these electrons is in the conduction band, they leave from the covalent bond leaving a hole in the bond behind each removed electron. These are called free electrons move randomly inside the crystal structure of the silicon. These free electrons and holes have a vital role in creating electricity in photovoltaic cell. These electrons and holes are hence called light-generated electrons and holes respectively. These light generated electrons and holes cannot produce electricity in the silicon crystal alone. There should be some additional mechanism to do that.

Adding a pentavalent impurity like phosphorus to silicon leads to covalent bonding of four out of the five valence electrons of each phosphorus atom with neighboring silicon atoms. The fifth electron remains unbound and free to conduct electricity.

This fifth electron is only loosely bound to its parent phosphorus atom, and even at room temperature, the available thermal energy can free it. Once disassociated, this electron becomes mobile within the crystal, aiding in conductivity without being tied to any specific bond or hole. These free electrons come from pentavalent impurity are always ready to conduct current in the semiconductor. Although there are numbers of free electrons, still the substance is electrically neutral as the number of positive phosphorous ions locked inside the crystal structure is exactly equal to the number of the free electrons come out from them. The process of inserting impurities in the semiconductor is known as doping, and the impurities are doped are known as dopants. The pentavalent dopants which donate their fifth free electron to the semiconductor crystal are known as donors. The semiconductors doped by donor impurities are known as n-type or negative type semiconductor as there are plenty of free electrons which are negatively charged by nature.

Adding trivalent impurities like boron, instead of pentavalent ones, results in a different type of semiconductor. Boron replaces some silicon atoms, pairing its three valence electrons with those of neighboring silicon atoms, leaving one electron unpaired. This creates a ‘hole’ or a vacancy that seeks an electron to complete the bond, essential for p-type semiconductor behavior.

This vacancy, known as a positive hole, allows for the movement of electrons within the crystal. As covalent bonds are broken and reformed, holes appear and disappear, facilitating electrical conduction similar to free electrons. These dynamic holes make trivalent-doped semiconductor, or p-type semiconductors, crucial as they accept electrons to maintain electrical balance.

In n-type semiconductor mainly the free electrons carry negative charge and in p-type semiconductor mainly the holes in turn carry positive charge therefore free electrons in n-type semiconductor and free holes in p-type semiconductor are called majority carrier in n-type semiconductor and p-type semiconductor respectively.

There is always a potential barrier between n-type and p-type material. This potential barrier is essential for working of a photovoltaic or solar cell. While n-type semiconductor and p-type semiconductor contact each other, the free electrons near to the contact surface of n-type semiconductor get plenty of adjacent holes of p-type material. Hence free electrons in n-type semiconductor near to its contact surface jump to the adjacent holes of p-type material to recombine. Not only free electrons, but valence electrons of n-type material near the contact surface also come out from the covalent bond and recombine with more nearby holes in the p-type semiconductor. As the covalent bonds are broken, there will be a number of holes created in the n-type material near the contact surface. Hence, near contact zone, the holes in the p-type materials disappear due to recombination on the other hand holes appear in the n-type material near same contact zone. This is as such equivalent to the migration of holes from p-type to the n-type semiconductor. So as soon as one n-type semiconductor and one p-type semiconductor come into contact the electrons from n-type will transfer to p-type and holes from p-type will transfer to n-type. The process is very fast but does not continue forever. After some instant, there will be a layer of negative charge (excess electrons) in the p-type semiconductor adjacent to the contact along the contact surface. Similarly, there will be a layer of positive charge (positive ions) in the n-type semiconductor adjacent to contact along the contact surface. The thickness of these negative and positive charge layer increases up to a certain extent, but after that, no more electrons will migrate from n-type semiconductor to p-type semiconductor. This is because, while any electron of n-type semiconductor tries to migrate over p-type semiconductor it faces a sufficiently thick layer of positive ions in n-type semiconductor itself where it will drop without crossing it. Similarly, holes will no more migrate to n-type semiconductor from p-type. The holes when trying to cross the negative layer in p-type semiconductor these will recombine with electrons and no more movement toward n-type region.

In other words, negative charge layer in the p-type side and positive charge layer in n-type side together form a barrier which opposes migration of charge carriers from its one side to other. Similarly, holes in the p-type region are held back from entering the n-type region. Due to positive and negative charged layer, there will be an electric field across the region and this region is called depletion layer.

Now let us come to the silicon crystal. When light ray strikes on the crystal, some portion of the light is absorbed by the crystal, and consequently, some of the valence electrons are excited and come out from the covalent bond resulting free electron-hole pairs.

If light strikes on n-type semiconductor the electrons from such light-generated electron-hole pairs are unable to migrate to the p-region since they are not able to cross the potential barrier due to the repulsion of an electric field across depletion layer. At the same time, the light-generated holes cross the depletion region due to the attraction of electric field of depletion layer where they recombine with electrons, and then the lack of electrons here is compensated by valence electrons of p-region, and this makes as many numbers of holes in the p-region. As such light generated holes are shifted to the p-region where they are trapped because once they come to the p-region cannot be able to come back to n-type region due to the repulsion of potential barrier.

As the negative charge (light generated electrons) is trapped in one side and positive charge (light generated holes) is trapped in opposite side of a cell, there will be a potential difference between these two sides of the cell. This potential difference is typically 0.5 V. This is how a photovoltaic cells or solar cells produce potential difference.