I found from an article that nanostructured surface increases the blue response of the dead by creating a dead layer, i cannot understand how does it happen?
Another article suggests that sheet resistivity is lower due to reduction of dead layer .
I am confused about the use/formation of dead layer.
I am working on silicon solar cell with nanotextured surface with emitter layer formation by doping phosphorous to the tune of 10(19)/cm3?
Dear Srikanta,
This is an important question.
The conventional crystalline silicon solar cells have a p-substrate and an n+ emitter. It is so that the reverse saturation current decreases with the increase of doping concentration in the emitter. Therefore, the n+ emiitter is made as heavily doped as possible. Such emitter layer is produced by diffusion of phosphorus into the p substrate. In the diffusion process the surface layer has higher do[ping than the interior causing many crystallographic defects what act as effective recombination centers.
Therefore the carrier lifetime in such surface layers will be very small. Also Ohmic contacts to the n+ emitter layer favor such high doping concentration in the surface.
Unfortunately, this very low lifetime in addition to the surface recombination have a bad effect on the collection of the photogenerated electron hole pairs. It is so that the high energy photons of the incident solar spectrum will be absorbed at the surface layer as these photons have very short absorption length.The resulting e-h pairs generated in the surface layer will recombine very fast before reaching the field separation region of the pn junction. Therefore this region is termed dead layer because the generated e-h pairs will die and lost.
The remedy of this was in the development emitter layers which is sufficiently thin and has a optimum surface doping concentration of about 1x 10^19/cm^3 and passivated with dry silicon oxide to reduce the surface recombination.
Such cells are called blue cells because they respond well up to the blue light.
They are a class of high efficiency cells.
wish you success
Dear Dr. Ling,
WELCOME,
Sorry for coming very late to answer your interesting question.
As the doping increases and also the thickness of the layer increases its sheet resistance decreases as the flow of the current is lateral in this layer.
So, the low lateral series resistance is favored by higher doping and higher thickness. While the minority carrier recombination current favors relatively lower doping and surface passivation to reduce the volume and the surface recombination in the emitter layer.
The effect on the minority carrier current is more effective than lateral resistance. So, one optimizes the thickness and doping concentration to minimize the minority carrier current in the emitter while mitigating the effect of higher series resistance by better metallization pattern of the emitter. This concerns the first part of the question.
The lower doping with surface passivation and redesigned metal pattern has a lower recombination than the heavily doped emitter with dead layer. So, it will have better dark characteristics.
Best wishes
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