The thickness of the silicon wafer play a crucial rule in the final photocurrent which drops dramatically below 70 nm. Silicon is weak adsorber and need a thick layer to adsorb the light. Decreasing the thickness results in a lower cost but in a lower yield in the production line process.One good reason to go thinner is that the lifetime does not become an issue ( the thinner the layer the less the chance the carriers recombine). The open circuit voltage of a silicon solar cells is a crucial parameter and the highest value achieved so far (750 mV) is in HIT structure by Panasonic which have an impressive 24.7% at 98 micron thickness. The 25% mark is a limit for silicon solar cells and new concept such has light trapping and plasmonic will be required.

Thank you Gianfranco for your useful contribution. Yes, as the thickness of silicon decreases and becomes smaller than certain thickness, the photocurrent decreases because the longer wavelength photons will not be absorbed as the absorption efficient of silicon at longer wavelengths is relatively small. Therefore, the conversion efficiency decreases as the thickness is reduced.

As the minority carrier lifetime decreases the diffusion length of minority carrier decreases and the collected photocurrent is reduced. At the same time the reverse saturation current decreases as the minority carrier lifetime gets smaller. Both effects lead to reduction in the conversion efficiency with decreased lifetime.Therefore the the minority carrier lifetime must not be smaller than certain value such that the minority carrier diffusion length is equal or greater than the substrate thickness.

Dear colleague, there are two other remaining parameters would you further share?

Dear Researchgate colleagues: In this comment i would like to complete the effect of the remaining parameters of the p-substrate on the conversion efficiency of the solar cell.

The doping concentration affects largely the conversion efficiency of the solar cells.As the doping density increases the reverse saturation current decreases leading to an increase in the open circuit voltage and consequently the conversion efficiency. This increase is sustained until the high doping effects begins to appear. The high doping effects are the reduction of the bangap and the the minority carrier life time. Both causes the reverse saturation current to decrease again after reaching a peak with the doping concentration in the substrate. The optimum substrate doping concentration amounts to about 10^17/cm^3.

The back surface field is achieved by producing a p+ layer under the back metallization. This layer reflects the minority carrier electrons and prevent them from reaching the highly recombining metal- silicon contact. Decreasing minority carrier recombination basically enhances the conversion efficiency.

This was an outline of the effect of the p-substrate parameters on the conversion efficiency of the solar cells.

Any other comments are welcomed.

A colleague of mine has made a modification of the well-known simulation program PC1D, in which you can fast and easily play around with the parameters you mention here and see how it affects different aspects of the solar cell performance. The modified program is free and can be downloaded from our website.

Download page for the program:

http://www.ife.no/en/ife/departments/solar_energy/downloads

Here is an article with some examples of use: http://www.ife.no/en/ife/departments/solar_energy/downloads/Preprint-%20SiPV2013.pdf

To Trygve Mongstad:

Sir both the links you have given are not working. It's saying that the files has moven. can you please give the new links.

Hi all,

Thank you for the generous knowledge sharing. I would like to ask, how about the effects of the grain size of the silicon wafers? As far as I know ( I may be wrong), smaller grain size (larger boundaries) leads to increased re-combination of carriers. Larger grain size would also lead to rougher surface.

Other than that, would grain size of Si have effect on the reflective index, ARC growth rate, etc? 

Dear Muhammad,

You excited another important point concerning the substrate material properties which is the grain size effect. The grain size has a profound effect on the performance parameters of the solar cells. It is so that the resistivity of the material increase with decreasing the grain size. This is because the grain boundaries acts as potential barriers for the motion of the carries across the grain boundaries. The other effect is that the carrier lifetime decreases with the decrease in the grain size.. Both effects lead to the reduction of the conversion sufficiency with the grain size.

Building on this information one synthesized two new materials for solar cells, the silicon polycrystalline columenar structure which is called also poly crystalline semi crystalline silicon. Such material behaves nearly as a single crystal. 

The other material is amorphous silicon which is used as a pin solar cells. Only few micrometers of the this material is requires to absorb an appreciable portion of the incident solar radiation contrary to about 200 um of single crystal silicon. But the solar cell efficiency ar appreciably less than that of the single crystal.

So, the degree of crstallinty affects much the opto electronic properties of the materials. which is exploited to same material in fabricating the solar cells.

The refractive index may be less affected by the presence of the grains and consequently the anti reflective coating.

wish you success 

Why the surface passivation and the quality of the substrate is not discussed. It influences the lifetime of the minority carrier. when we are concerned about junction type substrate then the parameters above cannot be forgotten.

For a silicon wafer of 300micrometer thickness illuminated by strong sunlight(irradiance of 100mW/m^2), spectral distribution AM1.5G) electrons and holes are generated at a rate of G=9*10^18/cm3S. In steady state, G=∆n/τ .It is important to determine the electron population in solar cell. It is possible if the lifetime is known. For common value of lifetime of τ = 80microsec we obtain ∆n = 7.2*10^14/cm^3 . The value of carrier density is directly related to voltage of the solar cell. The relationship is logarithmic and it is necessary to increase the lifetime by factor of 10 to obtain higher voltage, which approximately represent 10% relative improvement in the device performance. Also cells output current is related to the lifetime. This is because electrons take sometime to travel across the silicon wafer and they reach the pn junction only if lifetime is long enough to complete the journey. 

Electrons and holes can recombine at the surface of the wafer given by parameter "surface recombination velocity". The recombination rate is directly proportional to this parameter and to the excess carrier concentration ∆n. In general, the effective lifetime would be zero when the surface recombination velocity is very high. But in reality, there is a limit on how low the effective lifetime can be because electrons and holes have to travel towards the surfaces by relatively slow diffusion mechanism in order to recombine.

Regarding doping concentration (for bulk region of the wafer specially) usually two recombination are held responsible SRH(Shockley-Red-Hall) model and Auger model .SRH is usually seen at low temperature and low doping and Auger at high temperature and high doping.Both predicts lifetime is a function of excess carrier density ∆n, dopant density, Na for p-type, density of recombination centers, defect energy level(usually material quality).Usually in room temperature this models are not taken into consideration.

Temperature conditions are very critical factors in determining lifetime.

Emitter diffusion length , i mean emitter thickness or selection of interface and emitter layer doping profile also significant to efficiency progression. In addition, passivation effect that could minimize front surface recombination is also very effective for efficiency progression. Base diffusion length and rear surface passivation affect on longer wavelength (NIR) spectral response.

Dear Colleagues,

welcome,

We studied intensively the possibility of making solar cells from heavily doped silicosis material as a cheaper materials.We found it good possible by changing structure of the solar cells to achieve efficient collection of the photocurrent. To see the study please follow the paper in the link: Article Performance Enhancement of a Proposed Solar Cell Microstruct...

Best wishes

Dear Prof. Abdelhalim,

Very interesting work to be able to fabricate micro-solar cell. you mentioned in the manuscript that the width of the device have to be less than the electron deffusion length to decrease recombination of electron-hole pairs. I would like to know from you, if the device is larger than the chosed dimentions, is it will work or no?

Thanks in advance,

Best Regards,

Rehab.

Is it width or thickness of active layer? Diffusion length is one parameters the other difficulties for p type is active layer is B-O complex defects that has lot of light induced thermal degradation effect.

Dear Rehab,

welcome,

The solar cell will make out the whole wafer irrespective of its dimension say 6 inch. This large area is subdivided into parallel micrometer wide sub cells similar to the structure given in the paper in my last comment.

The difference between the planar type conventional solar cell and this type is the deep trench isolation between the subcells. Such trench isolation is existing only in our proposed structure. If you have fabrication facility in your lab i can send you complete drawing for the whole cell.

Have a nice day

Best wishes

Dear Bablu,

welcome

In the simulation we used the real measured parameters of the p-silicon substrate. The other issue to make the junction vertical parallel to the incident solar radiation and so the collection of the generated photgenerated electrons and holes will be lateral. So the device width can be controlled according to the diffusion length of the material. It is so that the distance from the trench isolation to the next one must be smaller than the double the diffusion length.

This proposed structure is suitable for producing efficient solar cells from relatively low quality materials.

Best wishes

Abdelhalim Zekry

I appreciate your kindness to send me a draw for the whole structure of the device.

Thanks in advance,

Best Regards,

Rehab.

Following..

Dear Rehab,

I made for you a sketch of the proposed solar cell made from heavily doped p silicon. If this sketch is sufficient please tell me.

Best wishes