I am working on Si thin film solar cells. I got voc of near 500 mV and Isc of about 25 mA/cm2 but with the fill factor of about 19 % only. Aluminum was base electrode and TCO was the front electrode. Surface roughness was the order of 85 nm with some porous film structure. The thickness of the device was 20 micron with the thickness of both the electrodes are about 100 nm.
Your fill factor can be low due to numerous reasons. The thickness of your electrodes is on the same order as your absorber surface roughness, so bad contacts would be my first guess. The shape of your JV curve can give important clues about the loss mechanisms that are at play. I would recommend looking at series and shunt resistances first.
There is a lot of great literature on the topic (e.g. Chalcogenide photovoltaics by HW Schock and R Scheer).
pveducation.org is also a good resource, where you can see the qualitative effects of different loss mechanisms on your JV curve.
I hope this helps.
Thank you Dr. Ling for detailed analysis of my problem and sharing valuable literature with me.
The film is nc-Si with 100% crystallinity. Yes, It is nearly triangular shape. Even in second attempt, I find it the same but with the Jsc near to 55 mA/cm2 and Voc of 540 mV.
Regards
Ankit
Ankit Goyal, do you have the possibility to measure SunsVoc?
If you have a well behaved SunsVoc curve with a pseudo-FF >> 0.2 it would be an indication that you lose FF because of insufficient transport.
Also, what happens, if you measure the cell in reverse bias (negative voltages)? Does the JV curve saturate?
A Jsc of 55mA/cm2 is impossible to achieve with a nc-Si absorber. Are you sure about the correct size of your solar cell? Are you measuring with a mask? You might be collecting from outside the cell area or typed in the wrong area...?
Good luck!
Dr. Haschke,
I do not have access to Suns Voc, Though I agree that 55 mA Jsc is much higher for nc-Si and it may be because of collection from outside the cell area. The top electrode was TCO so no mask was used.
I didn't attempted the negative bias yet. I will do this.
I am preparing my samples again and will do the measurements again. Then I will let you know the results and will need more suggestions.
Thanks
Ankit
Instead of Suns-Voc you could measure at different illumination intensities, using neutral-density filters to reduce your illumination. Than plot Jsc-Voc, it is in principle the same that you would obtain from SunsVoc.
Additionally, to me it is very likely that you are collecting from outside of your solar cell area, which can, depeding on your cell design already lead to very low FF.
Dear Dr Haschke,
Thank you for your suggestions. It helped a lot. Now I got FF of about .5 with Jsc of 20 mA/cm2 and Voc of 0.4 V. But the results are only for the area of 1 mm2. When I tried the same on the area of 1 cm2 and 7 cm2, I got FF of .25 , Jsc of 4 mA/cm2 and Voc of about 3 mV.
The efficiency is the first case is 3.5% and in second about 0.3 %. I think it is due to improper charge collection at the larger areas.
Both reverse and forward curve was saturating. But I got a well behaved Voc curve and a proper diodic curve for both dark and light measurements
Would you like to suggest anything else that should I try ?.
I am not sure that should I report the efficiency for 1 mm2 or for the larger areas or both.
Thanks and regards
Ankit
Dear Ankit,
i came late to your question. I hope that you solved the problem.
In order to give you an advice , one would see the layer structure of your solar cells. Is It a heterojunction or homo pn junction structure. The other information required is the I-v curves dark and illuminated.
But there are some remarks:
You have high recombination losses because the additional recombination at the surfaces of the silicon as your silicon thickness is too small.
You have also lower absorption efficeincy as you silicon is not sufficient to absorb longer wavelengths.
When you provide the required info more comments will be given.
Best wishes
Dear Prof. Zekry,
It is a homo P-N junction. The structure of the solar cell is Ag/N type nc-Si/ P-type nc-Si/Al and thickness of the cell is about 30 micron.
One more question is that I am getting reverse leakage current and couldn't reason it properly.
My supervisor think that adding an intrinsic layer would probably eliminate the reverse leakage current and improve the Jsc and Voc. But I feel the other wise that this layer is not necessary as thickness of the cell is already good and it may actually reduce the overall Voc if I add an intrinsic layer of about 20 micron.
Please help.
Regards
Ankit
It is 100% nanocrystalline without any amorphous phase and roughness is about 80 no with nearly 15% porosity.
I think the best way to enhance your SC performance, is to find a compromise between FF, Voc and Isc
FF is like energy or 2D space that cover with voltage and current. Maximum voltage and current density, Jsc attainment at the same time is naturally difficult. You need to optimized that may compare with maximum power transfer theory rule as analogy. Nano crystalline materials attain high effective radiation efficiency (ERE) usually shows high current density but at the same time absorption related electrical potential development is minimized. That`s why nc simply not suitable for SPV.
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