What are solar photovoltaic (PV) panels made of? How do they work?
The majority of solar photovoltaic panels are made of the second most abundant element found on Earth. The vast availability of this element in form of different compounds makes it difficult to obtain. But before getting to know what it is and how we obtain it, let us know why we want it.
We want it to replace fossil fuels and secure the energy supply. We want it to become energy independent and cut the costs of our everyday energy needs. Solar power is a major clean energy contributor to other renewable energy sources. The discovery of the photovoltaic effect is an opportunity to use energy from the sun easily everywhere.
What does solar photovoltaic mean?
Photovoltaic (PV) is an effect that occurs when photons of light strike a semiconducting material and this generates a current. We call it solar photovoltaic when that striking light is from the sun. To explain in simple words, the light of any nature has photons and these photons of light have energy. This energy of the photon is transformed into electrical energy via a semiconductor material. Semiconductors are the elements or compounds with electrical conductivity less than conductors (i.e. metals like Copper, Iron, aluminum, etc) but more than insulators or nonconductors (i.e. ceramics, glass, wood).
As we now know photovoltaic technology works with semiconductors. Let us understand the one semiconductor which is mostly used to make a solar panel.
What are solar photovoltaic panels made of?
Solar panels are made of solar cells and these solar cells are made of semiconducting material. Where silicon (Si) is the most used semiconducting element. The availability, associated cost, efficiency and durability of silicon make it an ideal choice to make a solar panel. Now, silicon is used in different ways to make different types of solar panels. The two most common types of solar panels are mono-crystalline Si and poly-crystalline Si.
Mono-crystalline silicon solar panels
These solar panels are made of single crystal silicon solar cells. Several solar cells are connected to form a solar panel. These mono-crystalline silicon solar cells are made with Czochralski crystal growth method. Through fractional distillation and zone refining of sand (SiO2), raw silicon is obtained. This raw Si which is also impure and in a poly-crystalline state is heated to 1415 deg C. A pure single silicon crystal is dipped in this raw Si and then polled up with rotation. An ingot or boule of mono-crystalline Si is obtained from the process. From this ingot of single crystal silicon, wafers are cut to form the base of the solar cell. These are black in colour and solar panels made from these cells appear black or blackish grey.
Poly-crystalline silicon solar panels
As the name suggests, these solar panels are made from multi-crystalline solar cells. Raw silicon obtained from sand after fractional distillation and zone refining is directly poured into a square mold or container. This square mold is also called an ingot after it cools down. Wafers are cut from this square mould which forms the base of a poly-crystalline solar cell. These are blue in color and panels made from these give a blue tint.
However, mono-crystalline solar cells are more efficient than poly ones, they are costlier too. With the ease of manufacturing and low costs, poly-crystalline silicon solar panels are being brought more into use.
Still obtaining wafers in form of mono or poly-crystalline solar cells does not complete the process. Making these solar cells impure by adding impurities is the most important step in solar cell manufacturing. Let us understand this better ahead.
Doping of solar cells
In pure form, any semiconductor element acts like an insulator and is undoped. Also, sometimes silicon wafers obtained are a negative type or n-type and sometimes a positive type or p-type. This is because impurities are coated or added to molten silicon mixture to make it impure. This addition of impurities makes silicon a true semiconductor. To make these solar cells a generator of electricity, negative-positive junctions are made. For instance, for a p-type silicon wafer, an n-type coating is done. Phosphorous is used for n-type coating and Boron for p-type coating.
N-type silicon has extra electrons in them and p-type silicon has extra holes. The junction of n-type and p-type silicon completes the solar cell making. Each such solar cell can generate a current when light falls on it. Several solar cells are connected in series and parallel to form a solar panel.
How do solar photovoltaic panels work?
Till now we understood what photovoltaic is and what semiconductor is mostly used and how it is made to use. Now, what happens in the solar panel when light strikes it? How is electricity generated from light in a cell made from sand (SiO2)?
When sunlight strikes the solar panel, it transfers its energy to the valence (free) electrons in the n-type silicon of the solar cell. The energized electron then moves towards free holes in the p-type silicon layer of the cell via an external circuit. Thus, completing the circuit of the cell which generates the current. The n-type layer facing the sun is thin with a micron-wide depletion region with a p-type layer that is thick. Different types of p-n junction models are used in different type of photovoltaic panels. But, the basic of electricity generation from the solar cell is the same.
Each cell in the panel generates current and has voltage to make it flow. The solar cells are connected in parallel connection for desired voltage of the solar panel and connected in series for the desired current. Connection is like that of batteries. Where solar cells and also solar panels form a solar array, connections are done in series and parallel for desired voltage and current rating of the power plant.
The solar panel generates direct current (DC) which can be directly used to power a DC load or store it directly in a battery. Otherwise, DC is converted to AC or alternating current with an inverter to be used and transmitted like grid electricity.
There are many types of solar panels which are made from different types of elements and compounds. We have discussed the most used silicon-based solar photovoltaic panels. How they are made and how they work is a result of decades of research and findings. With improvement in efficiency and reduction in costs, solar panels are ought to be the primary energy supply of the future.