Solar panels are manufactured by accurately assembling the six principal components. The focus is on panels made using silicon-based crystalline solar cells, which are the most commonly available and deployed solar panel technology.
[Also read: Are solar panels really eco-friendly?]
The six principal components of a solar panel are:
1. Solar photovoltaic cells
2. Toughened glass – 3 to 4mm thick
3. Extruded Aluminium frame
4. Encapsulation – EVA film layers
5. Polymer rear back-sheet
6. Junction box – diodes and connectors
Solar photovoltaic cells
What are Solar Cells?
Silicon semiconductors are building blocks of solar cells. The cells absorb sunlight and convert it into electricity. They work on the principle of Photovoltaic Effect, hence the alternative name photovoltaic cells. The cells are connected electrically and put together into a large frame that makes a solar panel.
How are Solar Cells made?
[Watch Video: Solar City shows how solar panels are made]
Stage One: Purifying the silicon
Silicon dioxide goes into an electric arc furnace. A carbon arc is applied to release oxygen. This yields silicon of 99% purity. It is further purified until the silicon is deemed to be completely pure.
Stage Two: Making single crystal silicon
Solar cells are made from silicon boules by the Czochralski method. A silicon seed crystal gets dipped into melted poly-crystalline silicon. While withdrawing the seed crystal, it is rotated, forming a pure boule leaving behind the impurities in the liquid.
Stage Three: Making silicon wafers
Silicon wafers are sliced individually from the boule using a circular saw. The silicon wafers are polished to remove any saw marks, although some manufacturers choose to leave these marks as rougher cells absorb light effectively.
Stage Four: Doping
Phosphorus or small quantities of Boron dope the silicon boul.
Stage Five: Placing electrical contacts
Electrical contacts are used to connect the solar cells and the receiver of the produced current. Thin strips of tin-coated copper are placed between the cells, once the electrical contacts are in position.
Stage Six: The anti-reflective coating
Pure silicon reflects up to 35% of the sunlight. An anti-reflective coating is put onto the silicon wafer to reduce the loss in sunlight absorption. Titanium Dioxide and Silicon Oxide are generally used to coat the silicon wafers.
Stage Seven: Encapsulating the cell
Silicon rubber or ethylene-vinyl acetate are employed to seal finished solar cells. The encapsulated solar cells get placed into an aluminium frame and a Mylar or Tedlar back-sheet. The encapsulation and framing, along with a few other steps, produce the final product: the solar panel.
Small residential solar panels contain 60 solar cells linked together to generate a voltage between 30-40 volts depending on the type of solar cell used. Larger solar panels used for residential, commercial systems and utility-scale solar farms contain 72 or 96 cells and operate at a higher voltage. The solar cells connect via electrical contacts known as busbars, which allow the current to flow through all the cells in a circuit.
Now, let’s understand more about solar panels manufacturing.
The front glass protects the solar cells from weather and impact from hail or airborne debris. High strength tempered glass, which is 3 to 4mm thick, is used. It is designed to resist mechanical loads and extreme temperatures. In the event of an accident or severe impact, tempered glass is much safer than standard glass as it shatters into tiny fragments rather than sharp jagged sections.
Highly transmissive glass is used by manufacturers to enhance the efficiency and performance of solar panels. The highly transmissive glass has low iron content and an anti-reflective coating on the rear side which reduces losses and improves light transmission.
The aluminium frame protects the edge of the laminate section housing the cells and provides a solid structure to mount the solar panel in position. The corner sections can either be screwed, pressed or clamped together providing distinct levels of strength and stiffness.
Ethylene-vinyl acetate (EVA) is a specially designed polymer highly transparent (plastic) layer used to encapsulate the cells and hold them in position during manufacture. The film must be extremely durable and tolerant of extreme temperature and humidity. It plays a significant part in the long term performance by preventing moisture and dirt ingress.
The lamination on either side of the PV cells provides some shock absorption and helps protect the cells and interconnecting wires from vibrations and sudden impact.
The back-sheet is the rearmost layer of solar panels which acts as a moisture barrier and provides both mechanical strength and electrical insulation. The back sheet material comprises of various polymers including PP, PET, and PVF. It offers different levels of protection, thermal stability, and long term UV resistance.
Junction Box and Connectors
The junction box is a small weatherproof enclosure located behind the panel. It securely attaches the cables required to interconnect the panels. The junction box is necessary as it is the central point where all the cells sets interconnect, and hence must be protected from moisture and dirt. The junction box also houses the bypass diodes which are needed to prevent back current which occurs when some cells are shaded or dirty.
Solar MC4 Connectors
MC4 connectors special weather-resistant plugs and sockets. The MC4 stands for a multi-contact 4mm diameter connector. The connectors must be very robust, secure, UV resistant and maintain a good connection with minimal resistance at both low and high voltages up to 1000V.
The design of the connectors is for use with the standard 4mm or 6mm double-insulated solar DC cable, with tinned copper multi-strand core for minimum resistance.
Based on the purity of silicon, solar cells are classifiable as mono-crystalline and poly-crystalline. The base of the silicon cell can be built using various additives. The result is either positive (p-type) silicon or negative (n-type) silicon.
The Big Picture
Today, solar panels used for domestic purposes can absorb only 20% of the sunlight that they receive and turn it into electricity. The metric is known as solar panel efficiency. There are several other forms of solar cells available used for commercial and industrial purposes. These solar cells yield efficiencies of up to 40%.
Solar panel manufacturers today offer a range of models with various features and efficiencies. Some of the widely used technologies in India include:
1. PERC (Passivated Emitter Rear Cell)
2. HIT (Heterojunction with Intrinsic Thin-layer)
3. Split modules with half-cut cells
4. BIPV (Building-Integrated Photovoltaics)
5. Bifacial solar panels
6. Duplex solar panels
For detailed information on the different cells and solar panel types, refer to our complete guide.