Transport in PV Cells (and pn diodes)

In summary: Your Name]In summary, we discussed the role of the emitter and depletion regions in a PV cell, with the emitter region being responsible for creating majority charge carriers and the depletion region creating the electric field for directing the charge carriers. While it is true that some charge carriers may recombine in the emitter region before reaching the depletion region, the majority of charge carriers are still collected and contribute to the overall current. The depletion region is designed to be larger and is the primary site for charge collection.
  • #1
maverick_starstrider
1,119
6
Hi,

I'm having some trouble wrapping my head around some of the concepts and language of charge transport in Photovoltaic cells (and thus pn-diodes). My biggest problem is understanding the role played by the emitter region vs. the depletion region.

In a typical PV cell the front emitter region is very small and heavily doped, where the back region is often a substrate and is thousands of times larger. It is often said that the emitter region may in fact be "dead" and thus not play a role in collecting charge (i.e. the recombination time is so fast that all electron-hole pairs excited there recombine before getting anywhere useful). However, at the same time it is often said that all charge that reaches the electric field in the depletion region is "collected".

What I don't understand is how this can be possible. All charge that reaches the depletion region MUST PASS THROUGH the emitter region to reach a contact. So how can an emitter region be dead but all charge that reaches the depletion region be collected? This seems a clear contradiction. If recombination is so high than shouldn't the depletion region just push charges to their "death" in the emitter region and none are collected?

Any insights are greatly appreciated. I'm a physicist so you can answer at any level of theory you like.
 
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  • #2


Hi there,

Thank you for your question about charge transport in Photovoltaic cells and pn-diodes. I can understand why this concept may be confusing, but let me try to clarify it for you.

First, let's talk about the role of the emitter and depletion regions in a PV cell. The emitter region is heavily doped and is responsible for creating majority charge carriers (electrons or holes) when sunlight hits the cell. The depletion region, on the other hand, is where the electric field is created due to the difference in doping between the p and n regions. This electric field is what separates the charge carriers and causes them to flow in a specific direction, creating a current.

Now, to address your concern about the emitter region being "dead" and the depletion region collecting all the charge. It is true that in some cases, the recombination rate in the emitter region may be higher than the collection rate, meaning that some of the charge carriers may recombine before reaching the depletion region. However, this does not mean that the emitter region is completely useless. It still plays a crucial role in creating the majority charge carriers that are necessary for the PV cell to function.

Think of it this way - the emitter region is like a factory that produces charge carriers, and the depletion region is like a conveyor belt that carries them to the contact where they can be collected. Even if some of the charge carriers are lost in the factory, the ones that do make it to the conveyor belt are still collected and contribute to the overall current.

Additionally, the depletion region is designed to be much larger than the emitter region so that it can collect the majority of the charge carriers. This is why it is often said that all charge that reaches the depletion region is collected - because the depletion region is designed to be the primary site for charge collection.

I hope this explanation helps to clear up any confusion. Please let me know if you have any further questions or if you would like me to elaborate on any specific concepts. As a physicist, I am happy to discuss this at any level of theory that you prefer.

 

Related to Transport in PV Cells (and pn diodes)

1. How do PV cells convert sunlight into electricity?

PV cells contain silicon layers that have been doped with different elements to create a pn junction. When sunlight hits the cell, photons from the sunlight are absorbed by the silicon atoms, causing electrons to be knocked loose. The pn junction then separates these electrons and creates a flow of electricity.

2. What is the role of transport in PV cells?

Transport refers to the movement of electrons within the PV cell. Once electrons are knocked loose by sunlight, they need to be transported to the external circuit to create electricity. This is done through a series of conductive materials within the cell, such as metal contacts and transparent conductive layers.

3. How does the pn junction in a PV cell contribute to transport?

The pn junction creates a built-in electric field that helps to separate the electrons and holes created by the absorbed photons. This electric field also assists in transporting the electrons to one side of the cell, while the holes are transported to the other side. This separation of charge creates the flow of electricity.

4. What factors affect the transport efficiency in PV cells?

The transport efficiency in PV cells can be affected by several factors, such as the material used in the cell, the thickness of the cell, and the design of the cell. Other external factors, such as temperature and shading, can also impact the transport efficiency. The overall goal is to minimize any barriers to electron flow within the cell to maximize efficiency.

5. What are the applications of pn diodes in PV cells?

Pn diodes are essential components of PV cells as they create the junction that separates electrons and holes. This allows for efficient transport of electrons to create electricity. Pn diodes are also used in other electronic devices, such as LED lights and transistors.

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