It All Starts with a Solar Cell

Posted on Posted in Knowledge Center

Working knowledge of the basics behind PV construction and manufacturing at the cell level.

The basic building block for all PV modules is the solar cell, which is wired within a PV module to produce the voltage and current desired by the manufacturer. (Current is the flow of electrons, and voltage is the pressure that makes electrons move. Solar cells are manufactured in such a way that when they’re placed in sunlight, the photons in the light excite the electrons in the cells. When the module is connected to an electrical circuit, useful work, such as turning a fan or powering a refrigerator, can be done.

Let us understand the two parts of the cell-manufacturing process that help cells do this useful work: 

  • Doping solar cells to create semiconductors
  • Creating a one-way electron path with a PN junction

Doping solar cells to create semiconductors

When a solar cell is completely manufactured, it becomes a semiconductor, a material that acts as both an electrical conductor and insulator. Solar cells become conductive when exposed to light, which makes them able to pass current. However, silicon — the primary ingredient of solar cells — is naturally a much better insulator than a conductor. Insulators inhibit the flow of electrical current, which isn’t a desired feature for solar cells. In order to enable the flow of electrons (and therefore become semiconductors), the cells are doped during manufacturing. Typically two elements, boron and phosphorous, are used in the doping process. 

Unlike in sports, doping is an acceptable and highly encouraged activity in the manufacturing of solar cells. Because the silicon won’t readily produce an electrical current in its natural state, the addition of the dopants allows the current to flow. Typically, boron is introduced to the silicon during the first stages of cell manufacturing, and phosphorous is introduced to the silicon by diffusing a vapour directly onto the manufactured cell. 

The addition of these dopants adds electrons and electron holes to each side of a solar cell. The phosphorous atoms have extra electrons within them, and the boron has extra electron holes, waiting to be filled with the electrons. The phosphorous-doped side becomes known as the N type, or the negative side of the cell (the side facing the sun), and the boron-doped side becomes the P type, or positive side (the side facing away from the sun). 

Creating a one-way electron path with a PN junction 

When sunlight hits the phosphorous-doped (N type) side of a solar cell,
the electrons in the cell become excited. They’re so anxious to get moving that they’ll gladly go to the boron-doped (P type) side of the cell if given the proper path. 

That path involves a junction between the positive and negative side of the cell. This positive-negative junction (or PN junction) acts as a diode, allowing the electrons to pass from the positive (bottom) side to the negative (front) side of the cell but not in the reverse direction. This means the electrons flow from the negative side of the cell through the circuit and to the positive side of the cell. As more electrons move from the negative side to the positive side, the electrons on the positive side are pushed up through the PN junction to the negative side of the cell, and the process continues as long as sun- light is present. The PN junction ensures that the electrons move through the circuit. 

Solar Cell with a PN Junction

Figure(a) : A solar cell with a PN junction.

Connecting cell construction to the photovoltaic effect 

The phrase photovoltaic effect describes solar cells’ ability to produce voltage and current when exposed to sunlight. Here’s a step-by-step breakdown of how a cell’s construction allows that to happen.

  1. Energy from the sunlight’s photons excites the electrons located on the solar cell’s N type, giving them the potential (voltage) to move.
  2. When the solar cells are connected to a load, the excited electrons start moving (current flow) from the N type to the P type, performing useful work along the way.
  3. The electrons go to the cell’s P type and combine with the electron holes.
  4. As sunlight continues to strike the cell and more electrons are sent through the circuit, the electrons are forced from the P type back to the N type through the PN junction to continue the process.  

Electrons Through the PN Junction

Figure(b) : The movement of electrons through the PN junction

Source : PV Design for Dummies by Ryan Mayfield, President, Renewable Energy Associates

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