In the modern age, solar energy has been able to grasp the top spot among renewable sources of energy as alternative to conventional sources. Its contribution to the world's total electric supply has grown significantly.
In this post we will be providing the working principle of solar cells or photovoltaic cells.
The Sun is the ultimate source of energy for all biotic components of an ecosystem. It is abundant and absolutely freely available energy for planet Earth. We get energy from the sun in the form of radiations. The solar radiation mainly consists of three components namely - Visible, Thermal and Ultraviolet. Most of the UV radiations are captured by the ozone layer. Visible rays make things visible to us. They are reflected from objects which are then sensed by our eyes. The thermal radiation is what keeps things warm and we utilise this thermal energy carried by these radiations for the purpose of producing electricity. The Sun provides the same amount of energy to the earth in a single hour as humans use in a span of 365 days. Harvesting even 1/1000 th of 1% of the solar energy that hits the earth would tremendously counterbalance the use of non-renewable resources. Humans have been using solar energy for thousands of years. Energy from sunlight helps plants to grow and make food, and when they get fossilised we get our fuels such as coal, natural gas etc.
Let's dive a little into the history of using solar energy for generating electricity. Edmund Becquerel discovered photovoltaic effect. While experimenting with a cell made of electrodes and a conducting solution, he observed that value of current was enhanced when exposed to light. Charles Fritts developed the first solar panel using solar cells made up of selenium on a thin layer of gold to form a device giving less than 1% efficiency. Russell Shoemaker Ohl, an American engineer is credited for inventing the modern semiconductor based solar cell in 1939. He was a semiconductor researcher who also explained the working of solar cell which is discussed in this post .
Modern solar cells are based on single junction semiconductor with crystalline silicon developed around 1960s. Standard rating for poly-crystalline photovoltaic or thin-film solar cells is 37%.
How can we produce electricity from solar radiation?
For that we have to take help from the second most abundant element on earth-Silicon (comprising of 27.7% of the total mass of Earth's crust). From where can we get Silicon then? The answer is from Sand. The most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica (silicon dioxide, or SiO2), usually in the form of quartz. The sand has to be converted into 99.999% pure silicon crystals for usage in solar cells.
To achieve this the sand is put through a complicated purification process. For this the sand is heated with carbon in an electric arc furnace at 2000 degrees Celsius. The carbon reacts with oxygen to form carbon-dioxide leaving behind raw silicon at the bottom of the furnace. Then the raw silicon is mixed with hydrogen gas (from HCl) to get purified polycrystalline silicon. This is then made into wafers which form the main component of any solar or photovoltaic cell. This wafers are poly-crystalline as it is easy to synthesis.
Semiconductors and P-N junction:-
The seed of development of modern solid-state semiconductors goes back to 1930's, when it was realised that some solid state semiconductors and their junctions offer the possibility of achieving conditions where we can control the number and direction of charge carriers through them. Excitation by light, heat or applied-voltage can alter the mobile charge carriers in semiconductor devices.
As we study the wafers of silicon we will find in the lattice structure the silicon atoms are bonded to each other. So, the electrons are not mobile enough for carrying charge. Let us dope the Si with a pentavalent element like phosphorus, when the atom of +5 valency element occupies the position of an atom in the crystal lattice of silicon, the one extra electron remains weakly bonded with the parent atom and is free to move. This type of doping is known as n-type doping. In this situation if the electrons get sufficient energy they will move freely (under external applied E. field, radiation or heat). However this movement of electron is random and cannot be used to generate a potential difference or simply will not result in any current through the load.
In order to produce current we need unidirectional flow of electrons and for that a driving force is used. And this driving force or potential is practically achieved by a P-N junction. Let's discuss how can a P-N junction create the driving force. Similar to n-type doping if we dope the Si with Boron having 3-valence electrons, then there will one hole (deficient of electron) for each atom. This is called p-type doping.
If a p-type and a n-type semiconductors are joined together a P-N junction is formed which is governed by diffusion and drift of electrons and holes. We know that in the n-type semiconductor, the concentration of electrons is more than the concentration of holes. Similarly, for p-type, the concentration of holes is more than that of the electrons. During the formation of P-N junction some of the electrons migrate towards the p-side and fill the holes available there , this way a Depletion Region will be formed. In the depleted region there are no free electrons and holes. Due to this movement of electrons and holes, the n-side is left with an ionised donor (positive charge) and the p-side is left with ionised acceptor (negative charge). This results in an electric field across the depletion region directed from the n-side to p-side.
Fig :- Silicon solar cell (PERC) front and back.
Solar cell:-
A solar cell is basically a p-n junction, which produces electric current when solar radiation excites the atoms in the depletion region. No external voltage is applied and the solar panel is designed such that it provides maximum surface area for absorption of radiations since we are interested in more power.
When light strikes the p-n junction something fascinating occurs. The n-side is basically taken as thin as possible so that the radiations can penetrate up to the depletion region.
The whole process of generating an emf using a solar cell consists of 3 main steps:-
1)The energy from the solar radiation (hv>Eg) transfers the energy to the atoms in the depletion region which excites them. In turn this produces electron-hole pairs close to the junction.
2)As mentioned above there is an electric field present in the depletion region. Due to this the e-h pairs are driven out of the region. The electrons will be swept to the n-side and holes to the p-side.
3)The concentration of electrons and holes in the n-side and p-side becomes very high respectively. This results in the development of a potential difference.
Thus, the induced emf will drive the electrons from the n-side to the p-side through an externally applied load. The electrons will recombine with the holes in the p-side, in this way a solar cell continuously produces DC current.
In a practical solar cells, the top wafer (n-side) is very thin and heavily doped whereas the bottom wafer (p-side) is thick and lightly doped. This enhances the performance and efficiency of the cell. In this case the thickness of depletion region is much higher to normal p-n junction. It results in production of more e-h pairs. Hence, more current is generated.
As shown in the figure each solar cells are connected in series, the negative side (top) side is connected with the positive (back) side of the next solar cell. Then a parallel connection of these several series-connected units is made, for maximising current output and increasing efficiency. It increases the current and voltage value to a usable range. One such unit with combined series and parallel connected solar cells is known as a Solar module (fig above). Stacking several such modules makes up a typical solar panel.
Both top and bottom side of a solar cell are covered by EVA sheets (Ethylene Vinyl Acetate). It protects the solar cells from shock, dirt and humidity. It prevents the reaction of oxygen and other gases from oxidising the cells. Thus, provides a longer running lifetime.
Semiconductors with band gap close to 1.5 eV are ideal materials for solar cell fabrication. Solar cells are made with Semiconductors like Si (Eg=1.1eV), GaAs (Eg= 1.43 eV), CdTe(Eg=1.45 eV) etc.
Important criteria for selection of a material for solar cell fabrication are :-
- Band gap (~1.0 to 1.8 eV)
- High optical absorption (~10^4 1/cm)
- High electrical conductivity
- Availability of raw materials and cost
- Inert surface
Solar cells are used in satellites, space vehicles and calculators. Development in solar cell technology is a topic of rigorous research.
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