Solar (or photovoltaic) cells convert the sun’s energy into electricity. Whether they’re adorning your calculator or orbiting our planet on satellites, they rely on the the photoelectric effect: the ability of matter to emit electrons when a light is shone on it.
Silicon is what is known as a semi-conductor, meaning that it shares some of the properties of metals and some of those of an electrical insulator, making it a key ingredient in solar cells. Let’s take a closer look at what happens when the sun shines onto a solar cell.
Sunlight is composed of miniscule particles called photons, which radiate from the sun. As these hit the silicon atoms of the solar cell, they transfer their energy to loose electrons, knocking them clean off the atoms. The photons could be compared to the white ball in a game of pool, which passes on its energy to the coloured balls it strikes.
Freeing up electrons is however only half the work of a solar cell: it then needs to herd these stray electrons into an electric current. This involves creating an electrical imbalance within the cell, which acts a bit like a slope down which the electrons will flow in the same direction.
Creating this imbalance is made possible by the internal organisation of silicon. Silicon atoms are arranged together in a tightly bound structure. By squeezing small quantities of other elements into this structure, two different types of silicon are created: n-type, which has spare electrons, and p-type, which is missing electrons, leaving ‘holes’ in their place.
When these two materials are placed side by side inside a solar cell, the n-type silicon’s spare electrons jump over to fill the gaps in the p-type silicon. This means that the n-type silicon becomes positively charged, and the p-type silicon is negatively charged, creating an electric field across the cell. Because silicon is a semi-conductor, it can act like an insulator, maintaining this imbalance.
Commercially used solar cells belong to following different types:
1. Crystalline Solar Cells:
- Ribbon silicon
- Mono-like-multi silicon
|S.No.||Property||Mono Crystalline||Poly Crystalline|
|2||Material||Single crystal of ultra pure silicon||Multiple crystals – varaiable size|
|3||Cost||More expensive||Less expensive|
|4||Efficiency||Better||With time catching up|
|5||Warranty||More||In comparison less|
|6||Method of manufacture||Created by slowly pulling a monocrystalline silicon seed crystal out of melted monocrystalline silicon using the Czochralski method to form an ingot of silicon.||Made through a simpler method,molten silicon is just put into a cast and cooled with a seed crystal. By using the casting method, the crystal surrounding the seed isn’t uniform and branches into many, smaller crystals, thus “polycrystalline”|
String Ribbon Solar Cells
String Ribbon solar panels are also made out of polycrystalline silicon. String Ribbon is the name of a manufacturing technology that produces a form of polycrystalline silicon. Temperature-resistant wires are pulled through molten silicon, which results in very thin silicon ribbons. Solar panels made with this technology looks similar to traditional polycrystalline solar panels.
2. Thin-Film Solar Cells (TFSC)
Depositing one or several thin layers of photovoltaic material onto a substrate is the basic gist of how thin-film solar cells are manufactured. They are also known as thin-film photovoltaic cells (TFPV). The different types of thin-film solar cells can be categorized by which photovoltaic material is deposited onto the substrate:
- Amorphous silicon (a-Si)
- Cadmium telluride (CdTe)
- Copper indium gallium selenide (CIS/CIGS)
- Organic photovoltaic cells (OPC)
- Silicon thin film: Silicon thin-film cells are mainly deposited by chemical vapor deposition (typically plasma-enhanced, PE-CVD) from silane gas and hydrogen gas. Depending on the deposition parameters, this can yield:
- Amorphous silicon (a-Si or a-Si:H)
- Protocrystalline silicon or
- Nanocrystalline silicon (nc-Si or nc-Si:H), also called microcrystalline silicon.
|Typical module efficiency||15-20%||13-16%||6-8%||9-11%||10-12%|
|Best research cell efficiency||25%||20%||13%||18%||20%|
|Area required for 1 kWp||6-9 m2||8-9 m2||13-20 m2||11-13 m2||9-11 m2|
|Typical length of warranty||25 years||20-25 years||10-25 years|
|Price (app) $/W||0.75||0.62||0.69|
|Temperature resistance||Performance drops 10-15% at high temperatures||Less temperature resistant than monocrystalline||Tolerates extreme heat||Relatively low impact on performance|
|Additional details||Oldest cell technology and most widely used||Less silicon waste in the production process||Tend to degrade faster than crystalline-based solar panels|
|Low availability on the market|
Solar Panel:A solar panel is a set of solar photo-voltaic(solar) modules electrically connected and mounted on a supporting structure.
Solar module: A photo-voltaic(solar) module is a packaged, connected assembly of solar cells. Commercially solar modules used belong to two main categories:
1. Crystalline module:
A basic c-Si cell consists of essentially seven layers. A transparent adhesive holds a protective glass cover over the anti-reflective coating that ensures all of the light filters through to the silicon crystalline layers.Similar to semiconductor technology, an N layer sandwiches against a P layer and the entire package is held together with two electrical contacts: positive topside and negative below.
2. Thin film module:
Not too different than c-Si components, thin-film solar cells consist of about six layers. In this case, a transparent coating covers the antireflective layer. These are followed by the P- and N-type materials, followed by the contact plate and substrate. And, obviously, the operating principle (photovoltaic) is the same as c-Si cells.
|Module/Property||Crystalline Silicon||Thin Film|
|Types of Technology||Mono-crystalline silicon (c-Si)
Poly-crystalline silicon (pc-Si/ mc-Si)
|Amorphous silicon (a-Si)
Cadmium Telluride (CdTe)
Copper Indium Gallium Selenide (CIG/ CIGS)
Organic photovoltaic (OPV/ DSC/ DYSC)
|Initial Cost for same KW||Higher||Has comparative cost advantage.|
|Space Requirement (for same Kw)||Requires less space||Less Space efficient and thus require more space|
|I-V Curve Fill Factor
(Idealized PV cell is 100%)
|Module efficiency||13%-19%||4%- 12%|
|Temperature Coefficients||Higher||Lower (Lower is beneficial at high ambient temperatures) and so efficiency is higher where temperature are high.|
|Durability||Old technology, more experience, time-tested and hence more reliable||Would need time|
|Additional||—-||Light weight, flexible- can also be installed on curved surfaces and also easier to handle.|
|Environmental Concerns:||—-||Some thin film solar products uses cadmium telluride (CdTe). Cadmium is a heavy metal that accumulates in plant and animal tissues. Cadmium is a ‘probable carcinogen’ in humans and animals. While cadmium telluride doesn’t pose a threat while the panel is in service, disposal of this toxic waste has concerns.|
There seems to be a feeling in the market that thin film will not only catch up with c-Si components, but also will surpass them on all levels, which truthfully are just cost and efficiency. One way to cut cost in thin-film solar cells is to use an environmentally unfriendly material like cadmium. The makers claim it’s safe as long as it’s encased and in use. As of now, however, there are no recycling plans for these components.