The majority of solar photovoltaic cells, or PV cells, are made using silicon crystalline wafers. The wafers can be one of two main types, monocrystalline (mono), or polycrystalline (poly) also known as multi-crystalline. The most efficient type is monocrystalline which are manufactured using the well known Czochralski process. However, more recently heterojunction or HJT cells have become more popular due to the increased efficiency and improved high temperature performance as explained below.
How are silicon PV cells made?
Silicon is first extracted from a specific type of sand known as silica sand, or silicon dioxide, which is usually made from crushed quartz rock.
A number of different manufacturing processes are required to crystalline silicon solar cells starting from a raw material called Quartzite, which is a form of quartz sandstone rock. First, Quartzite or silica sand is converted into metallurgical grade silicon by combining Carbon and Quartzite in an arc furnace. This process occurs at very high temperatures and results in 99% pure silicon. Next, the metallurgical grade silicon is converted into Polysilicon using either a chemical purification process called the Siemens process, or upgraded metallurgical-grade silicon (UMG-Si) using a number of more economical metallurgical processes being developed.
At this stage, we have a form of pure polycrystalline silicon which can be doped with trace amounts of either boron or phosphorous to become either P-type or N-type polysilicon. To make polycrystalline wafers, the doped silicon is melted and cast into large rectangular blocks before being thinly sliced using a diamond wire cutter to produce the polycrystalline or multi-crystalline wafers. The wafers can then be coated with a very thin layer of either P or N-type to form the PN-junction (photovoltaic cell).
To manufacture the more efficient monocrystalline wafers, the doped silicon can be drawn into a single solid crystal ingot using the Czochralski process. This process involves melting the polycrystalline silicon under high pressure and temperature to slowly grow a single monocrystalline crystal known as an ingot.
Basic steps to produce monocrystalline PV cells
Silica sand is purified in an arc furnace to create 99% pure silicon
The 99% silicon is further refined to become almost 100% pure silicon
The silicon is doped with boron or phosphorous (P-type or N-type)
The doped silicon is drawn into a solid crystal ingot using the Czochralski process.
The solid round ingot is diamond wire-cut into thin square wafers
The thin base wafer is coated with an ultra-thin layer of either P-type or N-type silicon to form the PN-junction.
The rear side Aluminium surface field or PERC layers are added.
Metallic fingers and anti-reflective coatings are added.
Flat ribbon busbars (as shown) or thin wire (MBB) busbars are added.
P-Type Vs n-Type silicon Cells
All crystalline solar cells (mono and poly) are made using a very thin wafer of base silicon with the two main types being p-type and n-type. These are made when the silicon is ‘doped’ with specific chemical elements to create a positive (p-type) or negative (n-type) charge.
N-type silicon cells are more expensive to manufacture but offer higher performance and a lower rate of light induced degradation, plus an improved temperature coefficient.
The chemical elements used for doping are phosphorous which creates a positive charge and boron which results in a negative charge. Depending on the type of cell construction either n-type or p-type doped silicon is used as the base or 'substrate' of the cell. The vast majority of both mono and multi crystalline cells used today use p-type which has a base of boron doped silicon. At present only a few manufacturers such as LG, Panasonic and SunPower use the more efficient n-type silicon wafers although more companies are starting to develop n-type cells.
N-Type – Negatively charged Silicon doped with Phosphorous
P-Type – Positively charged Silicon doped with Boron
Both cell types use a combination of p and n-type silicon which together form the p-n junction which is fundamental to the function of a solar cell. The difference is p-type cells use the Boron doped silicon as the base together with a ultra-thin layer of n-type silicon, while n-type cells are the opposite and use an n-type silicon base with a thin layer of p-type.
As explained the p-type and n-type silicon are brought together and form what’s known as a p-n junction. The junction creates an electric field which enables the flow of electrons when solar radiation passes through the cell. The photovoltaic effect is when light photons (energy) free the electrons from the silicon creating a flow of electricity.
Advantages of N-Type
Due the very nature and material composition n-type cells offer higher performance through having a greater tolerance to impurities and lower defects which increases overall efficiency. In addition n-type cells have greater temperature tolerance compared to both mono and multi p-type cells. More importantly n-type cells do not suffer from the issues of LID (light induced degradation) due to the boron-oxygen defects which are a common issue with p-type cells doped with Boron.
Lower impurities in N-type substrate
Improved high temperature performance
Lower degradation (LID)
Cost Vs Efficiency
N-type cell construction is more expensive as it uses what's known as a boron diffusion process to add the thin p-type 'emitter' layer. This diffusion process is more complex and requires higher temperatures compared to the p-type cell phosphorous diffusion process. Although the n-type cells are more expensive to manufacture the base n-type silicon is of a much higher purity which enables higher efficiency, lower losses and much lower degradation over time, this results in higher generation and performance which improves payback.
Heterojunction - HJT cells
Heterojunction or HJT solar cells generaly use a base of high-purity N-type crystalline silicon with additional thin film layers of amorphous silicon on either side of the cell forming what is known as the heterojunction. The two different photovoltaic materials absorb slightly different wavelengths of light and thus boost overall cell efficiency. Current HJT panels on the market achieve panel efficiencies as high as 21.7%.
One of the the most impressive characteristics of HJT cells is the incredibly low temperature coefficient which is close to 40% lower compared to multi and mono silicon crystalline cells. Solar panel power output is rated at a cell temperature of 25°C or STC (Standard Test Conditions), every degree above this slightly reduces power output. In common multi and mono cells, the temperature coefficient is around 0.38% per °C which can reduce total power output by up to 18% during very hot windless days. In comparison HJT cells have a much lower temperature coefficient of around 0.26% /°C which drops cell losses down to approximately 10% on very hot days.
Sources & References
Scientific World Journal - Advancements in N-Type Base Crystalline Silicon Solar Cells and Their Emergence in the Photovoltaic Industry - Atteq ur Rehman and Soo Hong Lee*
Electrical Engineering and Technology - www.electrical4u.com
PVeducation.org - https://www.pveducation.org/pvcdrom/pn-junctions/formation-of-a-pn-junction
How are solar panels made?
Here in our detailed article we describe how solar panels are manufactured and recycled. Solar panels are made using six main components in advanced manufacturing facilities using precise optical sensors to position each component along with specialised testing and quality control equipment.