Up to this point, we’ve learned the theory behind the working principle of PV cells and the limit to their performance, but we haven’t done much to consider what real PV cells look like. In this quiz, we’ll consider the structure of real PV cells and modules, and we’ll look at some important components in real PV cell design.
We know that the band gap in a semiconductor material allows incident photons with sufficient energy to be absorbed, which excites an electron into the conduction band, leaving behind a hole in the valence band.
However, these excited electrons and holes can’t be collected unless they are separated to different spots in the PV cell. What does a PV cell need in order for electron-hole pairs excited by photons to be spatially separated?
Silicon is the most commonly used material in PV cells today. Typically, the PN junction in silicon PV cells is formed by placing a layer of n-type silicon on top of a layer of p-type silicon. Sunlight incident from the n-type side excites electron-hole pairs, and the electrons and holes are then separated by the PN junction.[could also be an animation]
To collect the electrons and holes separated by the PN junction, we need contacts attached to the P and N layers. These contacts act like wires that collect the current generated by the PV cell. The p-side contact is usually a full layer of metal on the bottom of the cell, while the n-side contact is usually made up of metal “fingers” on top of the cell.
Why doesn’t the n-side contact fully cover the top of the cell?
With a PN junction and contacts which allow for the collection of current, we have a PV cell. When exposed to sunlight, this PV cell would convert that incident sunlight to electricity. However this cell doesn’t look a system you would put on your roof to generate electricity. What is the problem with using a PV cell as is?
To protect the a PV cell from the elements, it needs some protective layers. Typically it is covered with glass in front, and something durable and waterproof on the back (such as metal). PV cells connected in series and/or in parallel and covered by protective layers are called a PV module.
How does PV module efficiency compare to PV cell efficiency?
There are two main reasons that PV module efficiency is slightly lower than PV cell efficiency:
The glass cover of the module reflects a small amount of sunlight incident on the module, and that reflected sunlight can’t be absorbed and converted to electricity
The coverage area ratio of PV cells on the module is less than 100%, and sunlight that hits the spaces between PV cells can’t be converted to electricity
One of the reasons that PV module efficiency is slightly reduced is that PV cells don’t completely cover the module.
The shape of typical crystalline silicon PV cells might seem a little strange, but it comes from balancing two competing factors: maximizing area coverage (and therefore module efficiency) and minimizing material waste. Silicon wafers are cut from cylindrical ingots, which means that the wafers are circular. If you cut a square cell from a silicon wafer (such that you could achieve 100% area coverage), what would the material waste from the wafer be (in percent)? That is, what percentage of the area of the circular wafer would end up being thrown out since it is not part of the cell?
If you used a circular cell so that you had zero material waste, what would the area coverage of the module be (in percent)? That is, what percentage of the module area would be covered by PV cells? Assume that the cells are arranged in a grid pattern.
Real cells cut a square from the circular wafer, but the square has circular corners, since the square side length is less than the diameter of the wafer.For cells of this shape, what is the material waste as a function of the ratio of the cell side length to wafer diameter \(s/D\)?
Hint: the area of a circular segment is shown in the diagram below (note, angle must be measured in radians)
What is the area coverage ratio as a function of the ratio s/D?
The plots below show module area coverage ratio and material waste as a function of the ratio \(s/D\):By cutting a cell from a circular wafer with the corners missing, you can achieve area coverage close to 100% with minimal material waste. This is why silicon PV cells take the shape that they have.
We now have an idea of the structure of real PV cells and how they are assembled into modules. The next quiz will look at predicting the performance of real PV cells and why they don’t reach the theoretical limit. Throughout the rest of the chapter we will examine different parts of the PV cell in more detail and consider different ways to improve PV performance.