When it comes to solar panels, one of the most frequently asked questions is which type of solar cell is better: monocrystalline or polycrystalline?
Well, if you’re looking for a detailed answer, you’ve come to the right place.
In this article, we will do an in-depth comparison between monocrystalline and polycrystalline solar panels, including:
How are they made?
How do you look?
How effective are they?
How do they react to heat?
What is their expected lifespan?
Are they recyclable?
How much do they cost?
But first, let’s see how solar PV works
Solar photovoltaic (PV) is the direct conversion into electricity at the junction of two substances exposed to solar energy. It does this through a process known as the photovoltaic effect, in which photons are absorbed and electrons are lost. Solar energy is made up of photons, which are small packets of electromagnetic energy. Materials that exhibit this photovoltaic effect are referred to as PV or solar cells.
Solar cells are made from semiconductor materials such as silicon, which are used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field that is positive on one side and negative on the other. When light energy hits the solar cell, electrons are knocked out of the atoms of the semiconductor material. If electrical conductors are attached to the plus and minus sides to form a circuit, electrons can be captured in the form of electric current, i.e. electricity. This current can then be used to power a load such as a lamp or tool.
The first photovoltaic module was built by Bell Laboratories in 1954.
So, without further ado, let’s get to the manufacture of the solar panels.
How are monocrystalline solar modules manufactured?
In 1918, Polish scientist Jan Czochralski discovered a brilliant process for making monocrystalline silicon and dubbed it the Czochralski process, and later, in 1941, the first cell was built.
There are 8 main steps in the manufacture of monocrystalline solar cells and in this section we will quickly go through each of them.
Making Metallurgical Silicon
The main component of monocrystalline solar modules is silicon, also known as quartz sand, quartzite or SiO2.
The first step in manufacturing monocrystalline cells is to extract pure silicon from quartzite to produce metallurgical silicon.
To create metallurgical silicon, special furnaces are used to melt SiO2 and carbon at temperatures in excess of 2,552 degrees Fahrenheit, leaving 98% to 99% pure silicon.
Despite the high purity of metallurgical silicon, it is not pure enough to be used in photovoltaic modules.
Therefore, further cleaning must be carried out.
Clean metallurgical silicon
The next step is to clean this metallurgical silicon using the Siemens process.
First, we expose the metallurgical Si silicon powder in a reactor with HCl at high temperatures, resulting in SiHCl3 gas.
The gas is then cooled and liquefied for distillation.
Distillation is the process of evaporating and then condensing the liquid to remove unwanted impurities.
For example, you can boil seawater (salt water) and then condense the steam to get pure water since the salt stays at the bottom of the pan.
Using the same concept, liquefied SiHCl3 is heated and then cooled to remove higher and lower boiling point impurities such as calcium and aluminum.
After distillation, the liquified SiHCl3 is fed into another insulated reactor with hot rod, then mixed with hydrogen gas and re-vaporized at temperatures up to 2732 degrees Fahrenheit.
With the heat and the presence of H2 gas, the Cl atoms dissolve, leaving approximately 99.9999% pure silicon.
Create silicon ingots
What distinguishes monocrystalline cells from polycrystalline cells is that monocrystalline panels consist of a single ingot of pure silicon.
Making a single ingot of pure silicon was really difficult until Czochralski discovered this brilliant way.
First, a seed crystal, a small rod of pure single-crystal silicon, is immersed in the molten silicon.
After dipping the rod, it is now time to slowly pull the seed crystal up while rotating it to minimize the convection effect in the melt.
When the seed crystal is mined, the liquid silicon slowly solidifies over 4 days, creating a large, homogeneous, cylindrical, single crystal silicon, also known as a silicon ingot.
The size of the silicon block depends on 3 factors: the temperature gradient, the cooling speed and the rotation speed.
Create silicon wafers
So far you have a giant monocrystalline silicon ingot, but how can you make solar panels out of it?
Well the answer is very simple, wire saw.
The third step is to cut the silicon block into very thin slices using a very sharp wire saw, resulting in 1mm or 0.0393 inch thick silicon slices.
After cutting the pads, it’s time to polish and wash the pads to get rid of dust, dirt, and scratches.
Because the surface of the wafer is very flat, a lot of light rays are reflected, and of course you don’t want that because it reduces the efficiency of the solar panel.
For this reason, the manufacturers roughen and etch the surface of the panes so that the light can be refracted several times, which improves the efficiency of the panel and largely prevents light reflections.
Silicon wafers are positively charged. In other words, they act like a p-type material.
To conduct electricity you need a pn junction and to create a pn junction a layer of negatively charged phosphorus is added to each wafer and then the wafers are placed in special ovens at 1652 degrees Fahrenheit to mix the phosphorus with it to inject nitrogen.
The mixture of nitrogen and phosphorus creates a strong n-type layer, resulting in a highly efficient p-n junction wafer, which of course increases the panel’s efficiency.
In order to reduce power losses, a highly conductive silver alloy is pressed onto the front of the wafer, which guarantees perfect power transport and further improves the conductivity of the monocrystalline cell.
Finally, the final step in building monocrystalline modules is assembly.
Each monocrystalline solar module consists of 32 to 96 pure crystal wafers arranged in rows and columns.
The number of cells in each panel determines the total power output of the cell.
How are polycrystalline solar modules manufactured?
Polycrystalline solar modules, also called multicrystalline or multicrystalline, are also made from pure silicon.
However, unlike monocrystalline ones, they are made from many different silicon fragments rather than a single pure ingot.
The difference between the production of mono and poly solar cells is that after the silicon is cleaned, the molten silicon is cooled and fragmented instead of slowly drawing the ingot to form a homogeneous cylindrical crystal (Czochralski process).
These fragments are then melted in furnaces and poured into cube-shaped growth crucibles.
After the molten silicon solidifies, the ingots are sliced thinly, then polished, enhanced, diffused, and assembled like monocrystalline slabs.
B. Appearance of monocrystalline vs. polycrystalline solar panels
What do monocrystalline modules look like?
Because pure silicon block is round, cutting results in square discs with rounded edges that create small gaps between cells when assembled.
And because they’re made of pure silicon, they appear uniformly dark due to the way light interacts with pure silicon.
Therefore, you can easily identify monocrystalline solar cells by their uniform dark appearance and rounded squares with small gaps between each cell.
í Don’t worry, although the monocrystalline solar cell is dark, there are many colors and patterns for the backs and frames to suit your preferences.
What do polycrystalline solar modules look like?
Unlike monocrystalline solar cells, which appear uniformly dark, polycrystalline cells tend to have a blue cast due to the way sunlight interacts with the multicrystalline.
Since polycrystalline wafers are not cut from cylinders like monocrystalline wafers, they also do not have rounded edges.
You can easily recognize them by their bluish tint and the lack of rounded edges.
Polycrystalline cells also have many colored backsheets and frame designs that will definitely match your roof.
C. Efficiency of monocrystalline vs. polycrystalline solar modules
Solar panel efficiency is an indicator of the cell’s ability to convert sunlight into electricity.
For example if we bring 2 different solar panels, one with 10% efficiency and the other with 20% and we illuminate the same amount of light for the same duration.
The latter will produce almost twice as much electricity as the former.
How efficient are monocrystalline solar modules?
Of the different types of solar panels, monocrystalline cells have the highest efficiency, typically in the 15-20% range, and are expected to be even higher.
Fun Fact: In 2019, the National Renewable Energy Laboratory successfully developed a six-junction solar cell with an efficiency of 47.1%, setting two new world records.
How efficient are polycrystalline solar modules?
Because each polycrystalline cell is made up of too many crystals, the electrons have less room to move, reducing the efficiency of power generation.
Although mono-crystalline cells have higher efficiencies, the difference between mono- and poly-crystalline cells is not that big.
Most polycrystalline PV cells have efficiencies between 13% and 16%, which is still a very good ratio and is expected to increase in the future.
D. Temperature Coefficient of Mono-Si vs. Poly-Si?
Another big factor that is widely overlooked is the temperature coefficient.
The temperature coefficient is a measure of how well the solar cell performs as the temperature increases.
In other words, it gave the loss of efficiency for each degree of increase in temperature.
How does temperature affect the efficiency of monocrystalline solar panels?
Most monocrystalline solar cells have a temperature coefficient of around -0.3%/C to -0.5%/C.
So if the temperature rises by 1 degree Celsius or 32 degrees Fahrenheit, the monocrystalline solar cell temporarily loses 0.3% to 0.5% of its efficiency.
How does temperature affect the efficiency of polycrystalline solar panels?
Polycrystalline PV cells have a higher temperature coefficient than monocrystalline cells.
This means that polycrystalline modules lose more efficiency as the temperature rises, making them less than optimal for use in hot areas.
E. Expected Life
The lifespan of the solar cell is indicated by the rate of degradation or loss of annual energy production.
Most solar panels have a degradation rate of 0.3% to 1%.
This means that the overall performance of your system decreases by 0.3% to 1% every year.
How long do monocrystalline solar panels last?
Most monocrystalline photovoltaic modules have an annual efficiency loss of 0.3% to 0.8%.
Suppose we have a monocrystalline solar panel with a degradation rate of 0.5%.
In 10 years the system is running at 95% efficiency, in 20 years the system is running at 90% efficiency and so on until it loses a significant portion of its power generation capacity that it becomes inefficient.
Most monocrystalline solar panels come with a 25 or 30 year warranty. However, you can expect your system to last up to 40 years or more.
How long do polycrystalline solar panels last?
Polycrystalline PV cells have a slightly higher rate of degradation, causing them to lose efficiency a little faster than monocrystalline cells.
Don’t get me wrong, they still have a lifespan of 20 to 35 years and sometimes even longer.
Are monocrystalline solar panels recyclable?
The short answer is yes, monocrystalline solar cells can be recycled.
Monocrystalline solar modules consist of 3 main components:
Monocrystalline cells: About 85% of silicon wafers are recycled
Glass: Almost 95% of glass can be reused
Metal: 100% of the metal parts are recyclable
Are polycrystalline solar panels recyclable?
As with monocrystalline cells, approximately 90% of all materials used to manufacture polycrystalline cells are recyclable.
And by 2030, nearly 45 million new modules will be made from recycled materials, equivalent to $380 million.
How Much Do Mono Si Solar Panels Cost?
Monocrystalline solar panels have many advantages, but one of their main disadvantages is their high initial cost.
Among all types of PV solar panels, monocrystalline is certainly the most expensive to manufacture.
Because the manufacturing process of monocrystalline solar cells is very energy-intensive and produces a lot of silicon waste.
What is the price of polycrystalline solar panels?
Compared to their efficiency, polycrystalline solar panels have a lower cost per watt, making them cheaper than monocrystalline.
This is because the manufacturing process creates less waste and uses less energy, which reduces production costs.
Fun Fact: Sometimes Poly-Si panels are made from leftovers from Mono-Si production, reducing the amount of silicon waste.
It is important to mention that while poly-Si cells are cheaper, they take up more space than monocrystalline cells to produce the same amount of energy, making them less space efficient.