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See inside solar panels: there’s a world behind the glass
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See inside solar panels: there’s a world behind the glass

15 mins 25 Dec 2020
Don't disassemble a PV module. Never. Don't even give it a try. It will result in nothing but a destroyed solar panel. If you can't resist this temptation, well, keep on reading to learn what solar panels are made of and how they work.

Sunlight + silicon = energy

Edmond Becquerel's experimental setup
It all started back in 1839, when a young French physicist Edmond Becquerel discovered the photovoltaic effect. Becquerel believed that sunlight shining on electrodes submerged in a conductive solution would create an electric current. He conducted a simple experiment: he placed two platinum electrodes in a halide salt electrolyte solution and exposed them to light. As a result, it generated voltage and current.

A number of engineers and scientists worked in this area during the 19th century, trying to derive an ideal recipe of the solar cell. However, it wasn't until 1940 that Russell S. Ohl found out that electricity could be generated through the illumination of silicon. Working at Bell Laboratories in New Jersey, he was studying silicon crystals, when he came across a particularly curious one that had a crack down the middle.
Silicone crystal
Ohl was examining how much current flowed through the both sides of the crack and noticed that the amount of current was changing from time to time. Eventually, he realised it was light shining on the crystal that made some current trickle through it. To prove his assumption, Ohl attached that cracked crystal to a voltmeter and turned on a flashlight aimed at the silicon. The voltage instantly jumped up to half a volt.

Later, the phenomenon was explained by different levels of purity on either side of the crack. As there were some extra elements (impurities), one side had an excess of electrons, and the other side a deficit. When Russell S. Ohl directed light at the crystal, energy from the light kicked inactive electrons out and boosted them to travel around the crystal. Since opposites attract, the electrons from one side rushed to the other, creating a thin barrier of excess charges right at the central crack. Due to that barrier, there was only one way they could travel. Thus, all the electrons moving in one direction became an electric current.

How silicon solar cells are produced today

Ohl's crystal was the ancestor of today's solar cells. Over 90% of the current solar cell market is based on silicon. Here is how silicon solar cells are manufactured.

Stage 1.
Collecting raw material

Stage 1.
Collecting raw material

Silicon is the second most abundant element on Earth. However, it does not occur naturally in the pure form, meaning with less than one in a billion non-silicon atoms. High-purity silicon can be produced from silicon dioxide of either crushed quartz or quartzite gravel.

Stage 2.
Purifying the silicon

Stage 2.
Purifying the silicon

The raw material is placed into an electric arc furnace and processed there at very high temperatures. The silicon is refined to achieve almost 100% purity (even 99% purity isn't enough to produce solar cells).

Stage 3.
Making ingots

Stage 3.
Making ingots

Monocrystalline and polycrystalline ingots
Here is where the difference between monocrystalline and polycrystalline cells reveals. Monocrystalline silicon is grown as a large cylindrical ingot in a steel cylindrical furnace. On the other hand, polycrystalline silicon is molten right in a rectangular crucible and resolidified there as it cools down. At this stage, boron is added to give the silicone positive electrical polarity (P-silicon).

Stage 4.
Making silicon wafers

Stage 4.
Making silicon wafers

After the ingots have cooled down, they are ground and polished to achieve flat sides. The silicon ingots are then sliced into thin disks (wafers) using a wire saw. They are usually not thinner than a piece of paper. Monocrystalline wafers are usually hexagonal, while polycrystalline ones are rectangular.

Stage 5.
Putting the finishing touches

Stage 5.
Putting the finishing touches

The wafers are furnished with metal conductors added on their surface, which will ensure the conversion of solar energy into electricity. They are also covered by an anti-reflective coating (titanium dioxide, silicon oxide, etc.) to facilitate the absorption of sunlight. Phosphorus is diffused in a thin layer over the surface of the wafers to charge it with a negative electrical orientation. The combination of boron (added at Stage 3) and phosphorus will ensure the positive-negative (P/N) junction, which is crucial for the solar cell functioning.

If solar cells are wafers, then a solar panel is a sandwich

The main component of a solar panel is a set of solar cells. A typical residential PV panel consists of 60 or 72 cells to produce about 320W. However, this isn't the only layer of the 'solar sandwich'.

The soldered cells should be encapsulated. EVA (ethylene vinyl acetate) is typically used for this purpose. After that, a thin layer (6-7 mm) of glass is added on the front side, facing the sun. The backsheet is produced from a durable polymer-based material to prevent water, soil, and things like that from entering the panel from the back. There is also a junction box to enable connections inside the module.

It is all placed into an aluminum frame. The frame will provide protection against impact and weather. It will also allow the mounting of the panel in a variety of ways, for example with mounting clamps.

How solar panels turn sunlight into electricity

In a solar cell, crystalline silicon is sandwiched between conductive layers. Each silicon atom is connected to its neighbors by four strong bonds, which keep the electrons in place, so no current can flow. Here's the key: a silicon solar cell uses two different layers of silicon.

N-type silicon has extra electrons, and P-type silicon has extra spaces for electrons, called holes. Where the two types of silicon meet, electrons can wander across the P/N junction, leaving a positive charge on one side and creating negative charge on the other.

When one of these photons strikes the silicon cell with enough energy, it can knock an electron from its bond, leaving a hole. The negatively charged electron and location of the positively charged hole are now free to move around. However, they have only one way to go because of the electric field at the p/n junction. The electron is drawn to the N-side, while the hole is drawn to the P-side.

The mobile electrons are collected by thin metal fingers at the top of the cell. From there, they flow through an external circuit, doing electrical work, like powering a lightbulb, before returning through the conductive aluminum sheet on the back. Well, that's it – the physics behind solar magic.

Solar panels work all day, all night

The sun gets the ball rolling. Does it mean solar panels stop generating electricity as dusk falls, and get back to work at the crack of dawn? Well, it's true that the sun doesn't shine at night. However, some experts believe that moonlight can also power PV cells, so panels might produce around 10 W a night. It will be enough to boil the kettle or keep the light on in the bathroom all night. Even if those experts are wrong, you can always go for some cost-effective battery solutions to store the energy you have harvested during the day.

As for clouds, rain, snow and other precipitations, they do decrease the solar panel's efficiency, but only by around 10-25%. While they are capable of blocking direct sunlight, solar radiation in different ranges of wavelength still penetrates solar panels. It means electricity generation continues whatever the weather. Read How solar panels perform on cloudy days and why electricity prices matter more than the weather to find out how solar panels perform under different conditions.
Illustrations – Marina Fionova
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