Transport in PV Cells (and pn diodes)

[maverick_starstrider]

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.

 

[AndyW]

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.