20

February

10

How does a Solar Cell work?

by Sindhura

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?

 

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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.

  • Light is a form of energy. It is composed of mass-less particles that are pure energy, called photons. The natural sunlight that we can see, from violet to red colors, has photon energies ranging from 1.65 to 3.27 eV. (e means electron charge ,  eV means 1.6×10-19W-h). Quite small isn’t it? But then light has so many millions and millions of photons, their total energy is not insignificant.

     

    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.  

  • Matter on the other hand is primarily composed of particles with mass. Atoms make up matter. Atom’s core is the nucleus made of heavy and very dense particles called protons and neutrons. And then there are lighter particles called electrons which circle around the center in orbitals. Neutrons do not have any electric charge. Protons have positive charge while electrons carry negative charge. The charge of protons and electrons is exactly same. And their number within an atom is also exactly same; this is why atoms are electrically neutral. 

 

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.

  • Photon: When light hits a material, those photons with energies higher than the band gap get absorbed and their energy is used to excite the electrons from valence band to conduction band.  So each photon with energy > band gap, creates an electron-hole pair.
  • Thermal: At any temperature above 0 °K, there is a finite amount of heat. And there will be a finite, however small a fraction, of excited electrons even in semi conductors and insulators. Since the band gap is lower in semiconductors, they will have higher number of free charge carriers. But this is still insufficient for the material to be practically conducting. This fraction can be increased by doping the semiconductors with selected impurities.

 

 

“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.

  • If the added dopant has a higher valence than the semiconductor, it will have extra electrons available. This is an N-doped material.
  • In a P-doped semiconductor, the dopant has a lower valence so it will lack electrons and so has extra holes.

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!

10 thoughts on “How does a Solar Cell work?

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  2. Venkat Raju K

    Dear Ms.Sindhura:

    Thanks a Lot for explaining in so minute detail and as a layman I can understand what is the science behind this. Thanks a Lot . Hope you will continue with this type in future too.

    Reply
    1. Sindhura (Post author)

      Dear Mr. Venkat Raju,

      Thanks for your encouraging comments. I am glad to know the article has been useful to you. I have already made two more blogs on the working science of solar cells and shall keep doing so in the future.

      Regards,
      Sindhura.

      Reply
  3. Pingback: A technical blog on why PV modules' efficiencies are low | EfficientCarbon

  4. Ajay

    Hi Sindhura,
    Thanks for taking time to add more detail that I requested. It is much clear now. I will now go through your other blog on specification. Thanks for the great work.

    Reply
  5. RK

    Miss Sindura

    It is good to see a detailed description with technology involved, but frankly speaking, require some simple ways to understand for a non technical man or a layman like me with simple pictorial description.

    Rgds
    RK

    Reply
  6. Sindhura (Post author)

    Hi Ajay,

    Thank you very much for your encouraging comment.

    I totally agree with you that end seems a little abrupt. In fact I had given thought to making a pictorial representation, but could not perceive a way to avoid the traditional band diagram schematic. It gets complicated and will need things like Fermi-level, band bending, etc. explained.

    Anyway, after your comment I added another picture that depicts only the currents. I hope it is better!

    -Sindhura.

    Reply
  7. Ajay

    Hi Sindhura,
    Excellent blog. Found it very useful. Found the ending a little abrupt as I was expecting a schematic representation at the end after introducing light to the junction. Please keep the great work going.

    Thank you.

    Reply
  8. Pingback: Solar Module Specifications | EfficientCarbon

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