A Solar Cell or Photovoltaic Cell (PV cell) in the most basic definition absorbs the sun’s light and converts into electrical energy. In this post, we explore an in-depth description of the function of a PV cell. Understanding this mechanism is important as this field is rapidly advancing, producing much smaller and more efficient PV cells.
A PV cell is made of two layers of semiconductors, materials that have a conductivity (ability to carry an electrical current) level above that of insulators such as rubber and below that of conductors such as copper. The reason semiconductors are used in PV cells and in electronics in general is because in insulators there is very poor current flow and in conductors, it is harder to control current. Silicon is one of the most common semiconductors in electronics.
The two layers are the n-type silicon layer and the p-type silicon layer. The area where the two layers meet is the called the P-N Junction. To understand the solar cell mechanism, it is vital to understand how a circuit works. In the most basic simple circuit there is an energy source such as a battery and wire or a pathway for the flow of electrons. In circuits where there is a lightbulb, some of the energy held by the electron will be released as light energy and then the electrons will go to a lower energy state.
In the p-type silicon layer, atoms of Boron and Gallium are added. Both of these atoms have one less valence electron than silicon to make a bond with it. This is because silicon requires four more electrons to fill its valence shell and reach a stable state while Boron/Gallium only have 3 electrons to share. This means that the p-type layer has many “holes” for an extra electron. As a result, this layer has a positive charge.
In the n-type silicon layer, atoms which already have five valence electrons such as phosphorus are added. This means that there is an extra electron here when bonding to silicon happens. Hence, this layer has a negative charge.
The P-N junction is simply the name for the boundary between the n-type and the p-type silicon layers. Starting from the n-type side of the junction, the electrons move into the p-type side of the junction to fill the “holes.” This area where electrons near the junction move to fill holes is called the Depletion Zone. Now the p-type side of the Depletion Zone is negative while the n-type side is positive (opposite charge of their respective layers). This makes an electric field which stops additional electrons a bit further away from the junction to move.
Sunlight, made of light particles called photons have a lot of energy to transfer. When these particles (which can penetrate the silicon layers) reach the p-type side of the Depletion Zone where the holes were filled, it knocks the silicon atoms by giving them a lot of energy. This causes the electron to become excited and reach a higher energy state. Due to the effect of the electric field, these electrons will travel to the n-type layer.
At this point, if the n-type and the p-type layers are connected, an electric current or flow of electrons will happen. This is electrical energy which can be used by us.