To be able to know exactly how a silicon solar cell converts light into electricity we need to dive into the universe of the most basic building blocks of our existence. I’m talking about the minuscule world of the atoms, a place where forces rule and the laws of physics dominate.

Electron bonding makes the crystal

Silicone covalent bonding structure

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When the semimetal called silicon is in the form of a crystal its chemical properties are very special. The silicon atom has 14 electrons circulating around its core. The electrons are located at 3 different levels from the core or shells as the levels are also called. The two levels closest to the core are full containing 2 respectively 8 electrons each. The third layer located furthest out from the core is only half full containing but 4 electrons. An atom always strives to fill up its last shell and Silicone does this by dragging other silicon atoms towards itself and sharing one of their electrons while borrowing out its own in return. This way both the atoms will end up with an extra electron in their shell, they don’t really care that the extra electro also belongs in the shell of a neighbor. The Silicon atom does this bonding process with four other atoms closest to itself. This way it has now filled up its shell and sits there happy as a clam. This type of bonding is the reason why silicon will form a crystalline structure and it is this covalent bonding that basically enables the silicon to be used in the making of solar cells.

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Look… a lot of holes that needs to get filled!

When a beam of light strikes the silicon it has the possibility to hit one of the electrons in there. When it does it will knock that little sucker out of its shell leaving a “hole” in its wake. The electron will then aimlessly wander about until it stumbles upon another hole to fill. When you have many electrons wandering about you could direct them all towards one single point by luring the electrons with a lot of potential holes to fill. You will have them all rushing towards that place and thereby getting a current. Oh my! This sound awfully familiar to what happens in real life… lol

Purity is for dweebs

Since the silicon atom has all of its electrons in bindings with other electrons the amount of force it takes for the light to knock one out is relatively high. Meaning that there would be few electrons roaming and that means that the current and electricity from such a solar panel would be low. To amend this problem the silicon solar cell makers mix in another metal together with the silicon, so called impurities. For instance if we would take a couple of phosphorus atoms and mix them together with the silicon atoms we would get a much higher count of free electrons, why? Because Phosphorous is a metal with 5 electrons in its outer shell, the silicone will draw in and bond with the phosphorus atom just as they would with a fellow silicon atom. But since phosphorous has 5 electrons instead of 4 there will always be some sad electron without a pal. These lonely electrons are easier for the light to knock away, therefore there will be much more electros up and running through the material. This process of adding impurities to silicon is more famously known as doping. When silicon is doped with phosphorus the resulting silicon will have excess electrons running wild, a mixture like this is called n-type silicon or negative-silicon.

There is also a silicon mix named p-type silicon or positive-silicon. P-type is doped with a metal called boron. Remember that we said every atom strive to have a full outer shell, well the outer shell in this case has 8 free spaces or 8 holes where electrons could fit.  There are only 3 electrons in the outer shell of the boron atom meaning that even if all 3 of them are tied to a silicon buddy there will still be 2 more holes in the boron shell left to fill until the shell reaches its noble state. if there is a free electron close by it will rush towards the whole to place itself in it, there it will stay until it is freed again by some reason.

 

Hands up! Give over your electrons slowly!

Holes and electrons can also be stolen from a neighboring atom. If an atom has a hole that it wants filled, it can easily just take an extra electron from another atom close by to fill itself up. This will be resulting in the other atom to be the one with a hole instead. The same is done the other way around, if an atom has an extra electron it can push it away to be someone else’s burden. This way the positive and negative charges constantly move around in the silicon’s crystalline structure.

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Putting n type and p type together makes the silicon solar cell complete

ptype and ntype doped silicon solar cell

© Solar Panels Photovoltaic

So to conclude this, if n-type means that there are an excess of free negative electrons on the surface p-type means there will be an excess of positive openings or holes moving on the surface.

When you bring these two doped types together all the free electrons on one side see the available holes on the other side and there is a frantic electron race to try to fill them all up. Likewise the holes want to be filled and start moving towards the electrons. Unfortunately for the electrons they will not succeed in the venture to fill up all the positive spaces on the other side, if they did the silicon solar cell would be rendered useless. In the instant all the holes were full there would be nothing more to draw the electrons and therefore there would be no more electricity for us to harness.

 

Ion neutralization occurs at the P N Junction

Somewhere in the middle of the solar cell the negative electrons and the positive holes will meet. This point of the silicon solar cell is called a P N Junction. In this space both the charges will become neutral as they cancel each other out, meaning the positive and negative tension for each diffused pair will disappear.

 

A barrier will form

silicon solar cell barrier forming

© Solar Panels Photovoltaic

But since the diffused holes and electrons originally came from complete and neutral atoms, these atoms are now missing either a hole or an electron. Resulting in them having either too much electrons and being negatively charged or having too many holes therefore being positively charged. There will now be a layer of positive ions in the n-type material and a layer of negative ions in the p-type material. Both meeting at the silicon solar cells P N Junction creating an electrical field.

 

The large ions now take up the space where the small holes and electrons once used to meet to get neutralized. As you may know a similar charge on a magnet will repel the other magnet, the same thing happens here. By grouping a large number of similar charged ions together an electrical force field is created, repelling the smaller holes and electrons and keeping them from crossing over to the other side to be neutralized. Just like a big mountain or maybe a strong wind blowing them to the sides. But if you would give the small charges an alternative route to get to each other they will happily do so,powering devices on their way if they must.

Leading the current through our homes

This is when the sunlight comes into play. Every time a strong enough light beam hits an electron it will shoot it away from its place leaving a hole behind. Because of the electrical field acting as a bouncer the hole will be pushed to the p-type side to be with its buddies, likewise the electron will be pushed to the n-type side. Every time this happens the number of extra electrons and holes on either side of the barrier will get larger. There is nothing the tiny charged “particles” want more then get back to each other and into a neutral state. And as they grow in numbers they will push harder and harder on the barrier to get to each other. If there’s connected a wire to each doped side that leads around the barrier the electrons and holes will rush to use that path and reunite. If that wire is run through outlets in our home with obstacles on its way like TVs, computers, and ovens the flow of hole seeking electrons will power those on their journey to find their other half.

silicon solar cell current

© Solar Panels Photovoltaic

 

Watts, Ampère and Voltage

The strength of the barrier will decide how many holes and electrons it could keep separated from one another, or more exactly how many pairs it will be forcing to take the alternative route to get back into each other’s arms. This electric fields strength defines the voltage. How many electrons will be forced flowing through the wires leading around the barrier gives the current and is measured in amperes. When you have both voltage and current you will get electric power measured in Watts.

So this is how the basic electric cell works on an atomic level. But to be successful in converting sunlight into household energy it takes a bit more equipment then one silicon solar cell and some wire as the example states above. to find out exactly what items you need to run your basic home on solar power   continue to read >>>

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2 Responses to How a solar cell really works – lets take a look deep inside the silicon solar cell

  1. ali says:

    I am an electrical engineer and I want more information about solar energy
    Thank you

    • Solar Panels Photovoltaic says:

      Hi Ali and welcome to the site!
      Well since this site is dedicated to informing about solar power you certainly have come to the right place. Just browse around and you will find tons of useful solar information.

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