I have simulated N type wafer based rear contact silicon solar cell, having P+ emitter and N+ bsf region on back side.. the hole concentration is obtained in the bulk of the device.. i m getting higher hole concentration on the top of bsf region compare to the top of emitter region. What is the reason for this ??. Also, recombination rate near bsf region is higher compared to emitter region. Since, minority carriers (hole) generated on top of emitter require lower diffusion length compared to the hole generated on top of bsf region..
Dear Rahul,
In order to interpret this distribution you have to give the the illumination condition of the solar cell together with its terminal voltage and current since the intended distribution depends on the operating condition of the soar cell. From your description to the hole distribution in the n-base region having a high low junction,the given distribution can occur under illumination and operating voltage smaller than the open circuit voltage.
Wish you success
Dear Abdelhalim,
Thank you for your response, the hole distribution is obtained under standard am1.5 illumination with short circuit condition...so what could be the reason for this kind of distribution..
Yes, i said the operating voltage is smaller than the open circuit voltage which means that the solar cell voltage may be zero , i.e. , short circuit.
This response is typical It is so when light is absorbed by the material it generates an excess charge through out the semiconductor material. This excess is the balance between consequence of photo generation and recombination. At the boundary of the semiconductor there is exist the boundary condition, where the junction space charge region acts as a collecting contact totally depleting the excess charges at that boundary. In the side of the the back surface field, there is extra surface recombination which leads to lowering the excess concentration at this boundary.
So, consequently the the hole concentration will be maximum in the middle decreasing the toward the ends of the n-substrate towards the junction and the high low junction.
The maximum value depends on the lifetime of minority carriers where as the lifetime increases the excess charge increases.
wish you success.
Dear Abdelhalim,
Thank you for your response, i appreciate your kind help. but still i m not getting what is happening to the device.. Sir, kindly have a look at my understanding for the same and than provide your feedback.
Looking at the structure, ideally we want all the holes that are generated in the device should reach n/p+ junction in order to be collected at the terminal... therefore, holes that are generated on the top of p+ region, in n type base will easily be collected at the p+ region if their diffusion length is appropriate.. Now the main point of discussion is the hole distribution on the top of n+ region, in n base, kindly refer to picture .. Since, as far as my understanding concludes that... It is difficult for the hole to travel toward n/n+ interface, since built-in potential for holes at n/n+ region... therefore, hole which are moving towards n+ region from n region is being reflected back to n region both in x as well as y direction.... also, their is probability that n+ region will inject holes to n region..this can be explain with the help of energy band diagram this will lead to piling up the hole in region near to n+.... so this can be the reason for higher recombination as well as higher hole concentration on top of the n+ region compared to the top of the p+ region...
Dear Rahul,
Than you for the displaying the distribution of holes along a middle horizontal line in the n-substrate. It is a typical distribution in case of a short circuiting the p+- n substrate. Under short-circuit condition a space charge region is formed around the junction having an associated electric field that collects the generated electron hole pairs and the the collection is complete in this region. However outside this region the substrate is neutral and the collection will be by diffusion of electrons and holes towards the field region where they will be collected and separated. So, the regions nearer to the junction will have lower hole concentration because its holes will diffuse to the collecting field regions. The electrons and holes generated in a region grater than the diffusion length will never reach the junction and they will be balanced by the recombination process. AS there is a lateral hole concentration gradient showing lateral hole flow to the junction. This concentration gradient decreases with the distance from the p-n junction region. This means that the diffusion current of the holes decreases as the distance increases from the junction. The effect of the boundary is negligibly small including the n+-n junction.
This behavior is typical.
In order to get more insight into the hole distribution i would propose to display the hole distribution along a vertical line passing in the middle of the P+ region from the 0 to 10 according to your units.
It seems me that you assumes that the recombination at the surface of the substrate is negligible.
I would like that you return to the link:https://www.researchgate.net/publication/236002879_A_course_on_photo-voltaic_array_systems
it may be useful to you.
wish you success
Book A course on photo-voltaic array systems
Dear Abdelhalim,
The answer is very useful. However, there must be some role of n+ region also, since i have also simulated the same device as shown in picture without n+ region. In that case doping level of n+ was set to be equal to the doping of n region.. Results shows 10 times higher hole concentration on top of n+ region compare to top of p+ region i.e. ~1012 cm-3 on the top of p+ and ~1013 cm-3 on the top of n+ region (actually replaced by n region), whereas in n+ region based device.. 100 times higher hole concentration is observed on the top of n+ region compare to the top of p+ region, i.e. ~1012cm-3 on the top of ~p+ and 1014 cm-3 on the top of n+ region. As shown in the attached picture in previous answer...So what is the actual role of N+ region in the hole distribution in N type region... Also, here i have attached the hole concentration from front surface interface to the p+ region i.e. in vertical direction. in this 0 is considered as SiO2/ n-Si interface and 10 is considered as the back surface (P+ region).
Thank you
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