It is important to begin by mentioning that a solar panel is different in scale and applications from a solar cell. In fact, such a panel is an assembly of multiple photovoltaic cells. While smaller solar cells are used to power everyday objects like calculators, the bigger and rarer solar panels are used in almost exotic ways — in emergency road signs, buoys, parking lots — in order to provide power to the lights. The process by which solar cells convert the energy from the sun directly into power is interesting and helps explain why more research needs to be carried out before the process can become cost-effective.
Process of Conversion of Photons into Electrons
We will now explain the process through which light gets converted to electricity in a solar panel, by taking the solar cell as a unit of observation. Solar cells, also popularly known as photovoltaic cells, help convert the energy derived from the sun directly into charge. Photovoltaic cells are composed of a unique material known as silicon. Basically, what happens is when the light hits the cell, a specific part of it gets absorbed by the material of the semiconductor. Electrons get knocked loose due to the energy and this gives them the ability to drift freely.
All photovoltaic cells posses one or several electric fields that force the loosely flowing electrons to drift in a particular direction. This electron flow happens to be a current which can be drawn off for use externally, by inserting metal contacts both on the bottom and top of the cell. Together with the voltage of the cell, this current defines the amount of power that the photovoltaic cell is capable of emitting.
Use of Silicon in Solar Cells
Silicon possesses a few unique chemical traits, especially when it is still in the form of a crystal:
- A silicon atom contains 14 electrons which are set in three separate shells.
- The initial two remain completely filled.
- Only four electrons fill half of the outer shell.
- An atom of silicon is always trying to complete the last shell and it shares electrons from four adjacent atoms to achieve this.
- This leads to the development of a crystalline structure which is vital for such a solar cell.
However, pure silicon in crystalline form is a bad electricity conductor since the electrons are unable to move freely. Therefore, the silicon present in a photovoltaic cell contains impurities that allow the cell to work. Phosphorus, forming N-type (negative) silicon, and boron, forming P-type (positive) silicon, are commonly added as impurities to pure silicon.
Structure of a Photovoltaic Cell
Two individual pieces of silicon happen to be electrically neutral. But when two impure silicon pieces -– the P-type and N-type -– are brought in contact with each other, an electric field is formed. The free electrons present on the N side seeing all the holes on the P side rush to fill them fast. All free electrons are unable to fill the free openings. The entire arrangement would, in fact, be wasted if they had. But, exactly at the opening point, they mix successfully to form a kind of barrier which makes it increasingly tough for electrons located on the N side to make the move to side P. Gradually, the process reaches a point of equilibrium and there exists an electric field dividing the different sides. This field serves as a kind of diode, permitting and even forcing electrons to drift from the P to the N half, instead of the other way round.
When light in proton form hits the solar cell, the energy results in dividing the hole-electron couples. Each photon carrying sufficient amount of energy will usually be able to free just a single electron, leading to a free opening as well. If this occurs very close to the field of electricity, or if free opening and free electrons tend to flow into influence range, the field is likely to shift the electron to side N and the hole to side P. This results in even more disruption of neutrality of electricity, and if an external charged path is presented, electrons make use of the path to travel to the P side to combine with openings sent by the electric field, carrying on work in the process. The flow of electron results in the current and the electric field of the cell results in the formation of a voltage. Power is formed as a product of both voltage and current.
A few elements remain before the cell can be used. Silicon is a highly shiny substance that can push photons away prior to the completion of their job. In order to remedy this, it is normal to apply an anti-reflective coating which lessens the losses. The ultimate step is the installation of a cover plate made of glass or any other object that can offer protection to the cell from the elements. The amount of energy absorbed from sunlight by the solar cell is not very much.
Loss of Energy in Solar Cells
It is possible to divide light into many wavelengths. The light that affects the cell contains photons consisting of a large variety of energies. Not all of them have energy enough to modify the opening-electron pair. However, there are other electrons that contain lots of energy. Only a specific portion of this energy, measured in terms of eV or electron volts is necessary to knock an extra electron loose. This is commonly known as a material’s band gap energy. If a photon contains excess energy than what is required, the additional energy gets lost. However, if the incremental energy is equal to the required amount, there is possibility of the formation of more than one electron – hole couple. But, the latter effect does not seem to be of much significance. Just these two outcomes account for more than 70 percent loss of the incident concerning radiation energy on the cell.
There are more losses involved in the process. The electrons need to drift from one side to another of the cell using an outer circuit. The bottom can be covered using a metal, leading to effective conduction. But in case the top is completely covered, the photons are unable to pass through the opaque conductor and lose most of their current.
In order to reduce these losses, cells are normally covered using a contact grid made of metal that lessens the distance required to travel by the electrons, covering just a limited part of the surface of the cell. Even then, the grid blocks a few electrons.
Common Solar Power Issues
A portion of the power generated by the photovoltaic panels can be collected using chemical batteries. However, the setup suffers from a lack of additional power initially. The photon-producing sunlight is also responsible for infrared and harmful ultraviolet waves which result in the physical degradation of the panels. The panels need to be exposed to the harsh elements of nature, are also capable of affecting efficiency in a serious manner.
The necessity of light in the emission of electrical current can be deduced from the name of the photovoltaic cell. Future scientists will face the challenge of creating regular-sized and further developed solar panels that can be used to produce additional energy for the times when there is no sunlight.