Hello everyone this is the first blog among the many technical articles on Solar that we plan to bring out. Hope you will enjoy reading them and find the information useful!
This article is all about solar cell’s functioning. Almost everyone who is in the solar industry would know this much: that a solar photovoltaic (PV) module converts sunlight into electricity. But do you know how it can do this conversion?
The beauty of PV technology lies in the very nature of the universe – the way materials are made up of, the way energy interacts with it, and the way humans tailored the materials to result in this conversion. This blog tries to explain the basic physics behind the solar cell design even to an individual without a technical background.
Why should you bother with physics to understand how a solar cell works?
Current means moving charges. A solar cell first has to produce charges and then make them move to generate current. So you have to know how charge carriers are created.
To know that, you have to understand what “energy gap” is. And energy gap cannot even be defined unless you know what energy states of electrons are. But to know that, first you need to know what electrons are!
So let us start at the root of everything!
Light and Matter:
Everything in the universe is made of matter or energy. And they can transform from one form to another. To keep things simple, why don’t we begin by looking at them separately.
In solar cells, besides light there is another form of energy that comes into play – Thermal energy. We don’t really notice it, but the ambient temperature around us holds energy with the gas atoms and molecules in the air moving around and colliding with each other. The higher the temperature, the larger their velocity. At any temperature above 0 °K (-273 °C), there is a finite amount of thermal energy or heat.
The energy of the electrons is in their momentum – they orbit the nucleus like earth goes round the Sun. The farther they are from the nucleus, the faster they have to move. So the electrons in the outer orbitals have higher energies. If you plot the electron energies, they will be like step functions. i.e. they have distinct energy levels – these are called energy states.
In a crystal where there are many millions and millions of atoms, the billions and billions of electrons within interact. The energy states are no longer distinct and quantized, but they start overlapping and form continuous bands. In between the bands there will be gaps, which signify the forbidden energy states for the electrons.
The highest band filled with electrons is the valence band and these electrons will be bound to their respective atoms. The next band is called the conduction band. If electrons can get to these higher levels, they can break away from their mother atom and be free to move through the entire crystal. The gap between the two bands is the “Band gap” of the material – a crucial parameter of PV cells.
Types of matter: When the two bands, valence and conduction, overlap, all the conduction band electrons are free to be shared by the entire crystal. These materials are conductors – such as metals. When the bands are too far apart, the electrons are totally bound within their atoms and cannot contribute to transfer energy – either electrically or thermally. These are insulators (this is why typically heat proof materials like ceramics and plastics are also electrically insulating). When the energy gap is not too large, such as in the range of 0.6-4 eV, the materials are classified as semi conductors. This last type of matter – semi conductors – is of interest to us. Keep reading to know why.
Energy meets Matter!
When electrons are in valence band, they don’t have enough energy to get out of the atom. So they cannot contribute to current. But, they can be excited into conduction band by supplying the material with sufficient energy to overcome the gap. This energy can be either thermal or photon energy.
This is why semiconductors are of interest to us, because their gaps are not very large that naturally occurring sunlight and thermal energies can also excite some electrons.
Whenever an electron jumps from the valence band (the bound state of atom), it leaves behind a “hole” A hole means simply the absence of an electron – its electrical inverse. [What does this physically mean? The atom from which the electron left will have a net positive charge. This is embodied into a fictitious particle and it was called a hole]. It has similar mass as an electron but opposite charge and moves in valence band. So when energy (either light or heat) is supplied, an electron-hole pair is created.
“Doping” is good:
Photovoltaic application uses “doped” semiconductors, meaning the semiconductor is introduced with impurities of different valence. Then additional energy states are created in the band-gap. Now it takes a lot less energy to knock the dopant’s electrons up to conduction band.
The doped semiconductors have a much higher number of charge carriers than their pure counterparts. So, “doping” comes quite handy in solar cells.
How a junction creates electricity:
What happens when we bring differently doped semiconductors together? Let us make such a junction with P-semiconductor and N-semiconductor.
At any ambient temperature above 0 °K (-273 C), there is heat (thermal energy) and it creates free holes on P-side and free electrons on N-side. This is because of the band nature – in P side the electrons from valence band get excited to acceptor levels and get trapped there leaving only free holes. Similarly, on N side the electrons from donor levels get excited to conduction band and are free to move while the holes left behind are trapped in the donor level.
So, on the left there are more holes and on the right side more electrons. This difference in concentrations makes electrons and holes to diffuse across the interface, leaving behind charged anions (“-”charged ions that lost holes) on P side and cations (“+” charged ions from whom electrons left) on N-side.
The central region won’t have any free charges and is called depletion region. The ions in this region create an electric field that prevents further diffusion of electrons and holes across. You can visualize this field physically. Let us say a hole is trying to diffuse from P to N side. What does it see? The positively charged cations right at the junction. Since similar charges repel each other, the cations will repel this hole and it cannot enter N side.
Thus the electric field of the junction drives the charge carriers in opposite direction, holes from N to P and e-s. This is called “Thermal Generation Current”. And the charge flow caused by concentration difference, holes from P side and electrons from N side is called “Recombination Current”. When left alone, the junction will reach an equilibrium where thermal generation and recombination currents balance each other.
Suppose we don’t leave the junction and turn light on it. Now there will be thermal and photon energies. So the extra photon energy creates more electron-hole pairs. This is exactly like thermal generation except the source of energy is different. The resulting charge carrier flow (holes from N to P and electrons from P to N) is called “Photogenerated current”.
Therefore, when the P-N junction is under illumination, the net current that comes out of the cell is the sum of photo and thermal generation currents minus the recombination current.
I TOTAL = IPHOTO + ITHERMAL – IRECOMBINATION
Since the photo-generated and thermal-generated currents are originating from drift of charges along the junction’s electric field, they are together called “Drift Current”. On the other hand, the recombination current is called “Diffusion Current”, because it is driven by difference in concentrations of holes and electrons.
To extract power from such a P-N junction, we need to attach a load across it. Then some of the current goes through the load and the voltage potential across the junction drops. The recombination current becomes happy since its opposing force (the electric field of the junction) has just gotten smaller! So there will be a NET current from and a NET voltage across the junction. So the junction created electrical power!
The next blog will answer exactly how much current and voltage a solar cell can create, and elucidate how manufacturers write them up in a specification sheet. Keep with us till then!
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