PV cells technologies

Under this topic, we will discuss the different types of solar modules (conventionally called solar panels) available to choose from while designing a solar system. Hopefully, after reading through this, you will know enough about the different solar module technology to make an informed decision for your intended solar application.

Silicon based modules have always been produced since the 1950s, and for a long time, crystalline silicon was the only technology used. New technologies based on other semiconductors are now being deployed to compete with silicon.

Silicon has always been the preferred semiconductor because it is everywhere on earth, literally. It makes up 25% of the earth’s crust by mass. By making modifications to the silicon, it can be brought to a state where it is made to produce electricity (Check our topic on Photovoltaic effect). 

Here, we will talk about the silicon-based modules and other forms of modules that are based on other semiconductors.

SILICON BASED MODULES

To transform silicon into solar cells, it has to undergo some processes to become crystalline silicon. The two silicon-based modules are called monocrystalline and polycrystalline modules. The cells that make up both modules are made from crystalline silicon. They are popularly called 'mono' or 'poly' panels in the industry and they constitute over 90% of all solar modules deployed in solar projects.

Polycrystalline Technology

Polycrystalline modules, also known as polysilicon (p-Si) and multi-crystalline silicon (mc-Si), were introduced to the market in 1981 (newer technology as compared to monocrystalline). They are very easy to make when compared to making the mono crystalline module.

Overview of Polycrystalline module manufacturing.

Firstly, raw silicon is poured into a square-shaped mould. 

Raw Silicon

Then the raw silicon is then melted and allowed to cool therein. After cooling, they are cut into slices of perfectly square wafers.

Silicon wafer

It is the cooling that forms the distinctive edges and grains within the solar cell. Also, the multiple or separate crystals ‘poly’ are formed because the raw silicon doesn't completely melt, when the silicon cools, it forms multiple separate crystals, hence the polycrystalline.

 

Polycrystalline Silicon cell

This polycrystalline wafer is then doped and channels for electrons released by the photovoltaic effect are created. The different cells (usually of 60 or 72 cells) are wired together and encased, then sandwiched between layers of glass to make solar modules.

Polycrystalline module

1. Monocrystalline Technology

The manufacturing process of a monocrystalline cell demands greater resources than poly-crystalline cells and so generally cost more. Solar cells made of monocrystalline silicon (mono-Si) or single-crystalline silicon (single-crystal-Si) are the oldest and most developed of the technologies.

Overview of Monocrystalline module manufacturing.

 

A process called the Czochralski method is used to produce large cylindrical silicon ingots that has a distinct tube-like shape.

Chokhralski method

A silicon crystal ‘seed’ is placed in a vat of molten silicon. The seed is then slowly drawn up with the molten silicon, it cools around it, forming a solid crystal structure around the seed, called an ‘ingot’. 

The ingot of solid crystal silicon that is formed is then finely sliced into what is known as a silicon wafer. This is then made into monocrystalline solar cells that have nice rounded edges, as opposed to the sharp and unmistakable rectangular shape of polycrystalline solar cells.

Monocrystalline silicon ingots

The tubular ingots that make up the cells have their four corners cut out to create silicon wafers with rounded edges. A significant amount of the original silicon ends up as waste.

Cleaned ingots, ready for slicing

These solar cells are known as single-crystalline silicon because they are produced from a single continuous crystal structure and have a very even and uniform look, and this is mainly because only the purest silicon of the highest quality is used to make them.

Comparison between the perfectly square wafers of Polycrystalline and curved edges of monocrystalline.

1. THIN FILM TECHNOLOGY

Other than silicon, thin-film panels are made from a variety of materials, the technology can use materials including silicon, cadmium, copper, amongst others to create a solar cell. Thin film modules can be made to be rigid or flexible, allowing integration into buildings and products compared to crystalline silicon.

Thin Film solar  

 

Thin film solar cells are made by depositing one or several thin layers of photovoltaic material onto a substrate. The different types are categorised by the type of material deposited on the substrate. Such as:

• Cadmium telluride (CdTe)
• Amorphous silicon (a-Si)

• Copper indium gallium selenide (CIS/CIGS)

1. Cadmium Telluride 

The most popular thin-film solar panel is made from cadmium telluride (CdTe). 

Making this involves placing a layer of CdTe between transparent conducting layers that help capture sunlight. On the top is a glass layer for protection.

1. The amorphous silicon (a-Si) 

The amorphous silicon uses silicon, a similar composition of monocrystalline and polycrystalline panels. However, they are composed of non-crystalline silicon placed on top of glass, plastic, or metal. Not like silicon wafers in the case of mono and poly. 

1. Copper Indium Gallium Selenide (CIGS)

CIGS have all four elements placed between two layers (i.e. glass, plastic, aluminum, or steel), and electrodes are placed on the front and the back of the material to capture electrical currents.

 

1. QUICK COMPARISON OF ADVANTAGES AND DISADVANTAGES

 

Type	Advantage	Disadvantage
Polycrystalline	Easy to produce, cheaper option for homeowners. Cost less than Monocrystalline. Higher temperature coefficient, less sensitive to direct heat than monocrystalline panels Since the ingots aren’t cut to shape, poly panels produce much less silicon waste.	Efficiency is usually around 14-16% Needs more space to generate the same amount of power as mono panels, this is due to the lower conversion efficiency
Monocrystalline	Highest efficiency because one crystal structure is used to create the wafers, mono solar panels have the ‘purest’ type of silicon. Space efficient due to the efficiency. Longest lifespan and warranty Tend to be more efficient in warm weather.	Most expensive Due to the cutting process, mono panels generate the highest amount of silicon waste.
Thin Film	Can be made flexible for any type of installation  High temperature and shading have less effect on module performance uses much less material than traditional silicon solar manufacturing.	Lowest space efficiency Degrades faster than other types Shorter warranty

 References

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