Short circuited illuminated PN junction

[Axe199]

I understand why the potential difference across the pn junction decrease in case of the open circuit, but why does it decrease in the short circuit,
And why in the J-V equation we use V = Voc ( where qVoc is the energy between the quasi fermi levels) and what if the quasi fermi levels aren't continuous across the whole material?

 

[eljose79]

Thank you for your questions. The decrease in potential difference across the pn junction in both open circuit and short circuit conditions is due to the flow of current. In the open circuit case, there is no external load connected to the junction, so all the current generated within the junction is due to the diffusion of charge carriers. This creates a potential barrier at the junction, resulting in a decrease in potential difference.

In the short circuit case, there is a direct connection between the p and n regions, allowing for a large flow of current. This current flow reduces the potential barrier at the junction, leading to a decrease in potential difference. Essentially, in both cases, the potential difference decreases due to the flow of current within the junction.

As for your question about the J-V equation, it is used to describe the relationship between the current and voltage in a pn junction. The Voc in this equation represents the open circuit voltage, which is the maximum voltage that can be obtained from the junction in the absence of an external load. This is because, in the open circuit condition, there is no current flow, so the voltage measured across the junction is equal to the potential difference between the quasi fermi levels.

If the quasi fermi levels are not continuous across the whole material, it means that there is a potential difference within the material itself. This can happen in cases where there are different doping levels or temperature gradients within the material. In this case, the J-V equation may not be accurate, and other factors would need to be considered to accurately describe the behavior of the junction. I hope this helps to clarify your questions.