Humans have always used energy from the sun. From sun-drying foods to propelling spacecrafts using sunlight, we have come a long way. And yet, innovation in the solar industry has no intention of slowing down.
The industry began with a simple, inefficient, and ridiculously expensive solar cell, and has reached a point where we have a plethora of options and high efficiencies. This article takes a look at the available solar panel and cell technologies, with a small peek into the future.
1. Monocrystalline (and Polycrystalline) Silicon Solar Panels
Monocrystalline silicon (mono-Si) solar panels are so common that if you throw a rock at a gathering of solar panels (which you shouldn’t!), it will probably hit a mono-Si panel. Polycrystalline solar panels, on the other hand, are like the floppy discs of the solar world - nobody really uses them anymore.
As the name suggests, monocrystalline panels use solar cells made from a single crystal of silicon. High-tech machinery in solar factories cuts super-thin wafers from a single ingot of crystalline silicon. These ingots are then used to create solar cells, which are connected together and compacted between layers of EVA sheets, a back sheet, and a glass in the front, together creating a mono-Si solar panel.
Monocrystalline cell and panel (source - ResearchGate)
Unlike mono-Si, polycrystalline solar panels are made from cells that use multiple small crystals of silicon. This results in an irregular lattice structure, which results in lower efficiencies. Polycrystalline solar panels struggle to achieve efficiencies of more than 18%, while monocrystalline easily soars past 20-22%. Despite its slightly higher cost, mono-Si is now the most widely used solar panel technology, laying claim to a staggering 80% market share.
Although newer, fancier technologies are promising higher production and novel features, monocrystalline panels offer such value for money that they are here to stay for a long while.
2. Thin Film Solar Panels
There is a significantly large group of solar experts who believe that thin film is the future of the solar industry, and they may not be wrong about it. Thin film solar panels can be mono- or polycrystalline, but their biggest feature is their flexibility. All thin film panels show some degree of bendability, allowing them to be installed on uneven surfaces.
That’s not all, these panels also come without a glass or frame, which makes them a lot lighter and more durable than traditional solar panels. They also create less waste and are much easier to manufacture. Moreover, they are also cheaper, thanks to fewer materials and easier production.
So why are we still using regular monocrystalline solar panels?
Because thin film solar panels are yet to catch up with traditional solar panels when it comes to efficiencies. They also have a shorter lifespan than regular solar panels.
Thin film solar panel (source - PowerFilm Solar)
Thin film solar panels are typically made of three components:
- Photovoltaic material: this layer generates all the power in the panel, and is made from crystalline silicon, amorphous silicon or newer technologies such as cadmium telluride (CdTe) or copper indium gallium selenide (CIGS).
- Conductive sheet: This is a layer that prevents electrical loss and helps transport electricity properly throughout the panel. It is generally made using conducting metals such as aluminium.
- Protective layer: A protective layer guards the solar cells and conductive sheet against the elements as well as shocks.
3. Passivated Emitter Rear Cell (PERC) Solar Panels
Despite being relatively new, PERC is a solar panel technology that is gaining rapid popularity. PERC solar cells are very similar to traditional solar cells. The only difference is an extra layer on its back (as you can guess from the name). This extra layer is reflective in nature, and reflects light back into the cell instead of allowing it to pass through.
This reflected light knocks off even more electrons from the solar cell, which means greater current generation. In practical language, PERC solar panels offer higher efficiencies than traditional solar panels.
Compared to the efficiency of traditional solar panels, which clocks in at 18-21%, PERC panels boast efficiencies of 20 - 23%. PERC solar panels can be monocrystalline or polycrystalline, but most companies manufacture mono PERC panels, thanks to their higher efficiency.
Since the construction of PERC solar cells is similar in some ways to that of traditional solar cells, PERC panels can be easily manufactured in existing facilities with some modification to the production machinery.
4. N-Type Solar Panels
Traditional solar panels have a large, positive layer at its base, known as the p-type layer, and a thin, negatively charged n-type layer on the top. The p-type layer forms the bulk of the cell, giving it the name of a ‘p-type panel’. P-type panels use boron to establish a positive charge in the silicon in that layer, and this is where the problem lies.
Boron reacts with oxygen impurities in the cell, creating defects which slow down or block a number of electrons, therefore reducing power generation. This effect is called light induced degradation (LID). P-type panels are notorious for having a seriously low power output during mornings and late afternoons.
Enter N-type solar panels. This technology reverses the two main layers in the cell, making the negatively charged n-type layer the bulk of the cell and also the lower layer. This layer uses phosphorus instead of boron, eliminating the LID effect and producing significantly more energy over the panel’s lifespan.
N-type vs p-type base layer (source: Luxor Solar)
TOPCon Solar Panels
A new entry in N-type solar panels that is making waves in the industry is the TOPcon solar panel technology. TOPCon stands for Tunnel Oxide Passivated Contact. It uses an N-type construction, but also inserts a delicate tunnel oxide layer between the p-doped layer and a transparent conductive oxide (TCO) layer.
5. Half-Cell Solar Panels
Half-cut cell solar panels, typically also known as half-cell solar panels, are a perfect example of a simple solution to a complex problem. Unlike traditional solar panels, half-cell panels use solar cells cut in half. Instead of having one panel of, say, 60 cells, a half-cell panel will have two sections of 30 half cells each.
Image - typical half-cell panel (source - REC)
This means a half-cell panel has the same power rating as a regular solar panel, but the current rating of each cell is halved. This has two important benefits.
Firstly, the panel has lower losses, since most losses are primarily a function of current and not voltage. In other words, half-cell panels have better efficiency. Secondly, the layout of half-cell panels allows them to perform optimally even when they are partially shaded.
6. Heterojunction Solar Panels
Heterojunction (HJT) solar panels are a modern hybrid of crystalline silicon panels and amorphous, thin film panels. Heterojunction solar panels are a sandwich of a layer of crystalline silicon cells, typically n-type, monocrystalline cells and a thin film cell layer. In addition, they also have a TCO layer (transparent conductive oxide), such as Indium Tin Oxide (ITO).
HJT cell arrangement (source: Sinovoltaics)
This arrangement offers important benefits, such as reduced recombination losses. In other words, a greater number of electrons travel through the circuit and therefore offer greater power generation.
Further Innovations
The above list tells us about the new but commercialised solar technologies available today. But aside from that, thousands of labs and universities around the globe are engaged in solar research, regularly claiming a new advancement in another exciting, futuristic tech. Here are a few examples:
Transparent Solar Cells
If you have seen gigantic buildings in Auckland or other big cities, you probably know how much sunlight is wasted as it falls on the glass or wall surfaces of the buildings. But of course, replacing vertical walls with traditional solar panels would pose multiple problems, including that of poor aesthetics.
Scientists have targeted this problem and developed transparent solar cells. Now, instead of regular glass, buildings can use photovoltaic glass and generate power and to the onlooker, the building will still look as pleasant as any other building.
Transparent solar cell developed at MIT
Since a lot of light can pass through transparent solar cells, it is obvious that they won’t be as efficient as traditional solar panels. But this is not about efficiency. Their transparency makes them wonderfully useful in an endless number of applications, including car windshields, mobile phone screens, urban buildings, and more.
Biohybrid Solar Cells
Plants have been using energy in the sunlight way before humans even set foot on this planet. So it makes sense to learn a thing or two from trees and mimic some of their ability to use sunshine for energy.
Scientists have successfully extracted and used protein complexes in trees to create solar cells that generate solar power. It is called a ‘biohybrid’ cell as it combines organic (bio) and inorganic materials.
Solar Paint
Let us admit - solar photovoltaics is a highly complex technology, with micro-level cell layers and soldering and heat treatments and what not. But researchers have found an incredibly simple way to go solar - ‘paint’! Multiple technologies of solar paint have made their way to labs worldwide. Some examples include perovskite paint, quantum dots and hydrogen solar paint.
Each technology is significantly different from the others, but they all achieve a common goal - generating power through a painted surface.
Schematic showing how solar paint generates power (source: Republic of Solar)
It may still be several years before solar paint becomes commercially available. Optimistically, we might see early commercial products within the next 5 to 10 years, assuming significant progress in efficiency, stability, and cost-effectiveness.
Wrapping Up
Within just a decade, the available technology options in solar panels have grown impressively. Buyers willing to install a straightforward, budget-friendly system can choose the highest selling monocrystalline option. Similarly, those looking to maximize their power generation in a limited space or a less-than-ideal roof layout can choose other options such as N-type or PERC.
And for fans of science fiction or of futuristic tech in general, there are always more exciting technologies on the horizon, like solar paint and transparent cells. All-in-all, the journey of solar panel tech so far has been exciting, and the future looks even more so!