I want to fabricate organic solar cell. The literature says to etch away a portion of FTO substrate. I want to know why should I do that.
Dear Brishty,
welcome,
From the principle point of view, in any multilayered structure such as the organic solar cells one stacks the layers to perform their function without affecting the overall performance of the solar cells. Also, the lateral extension of each layer is subjected to the previous constraints.
May be the most important constraint is that the upper metal layer and the lower semimetal layer which is here the FTO must not touch. So, one has to remove the FTO in case of depositing the other electrode on the gals substrate.
However i can give you specific answer if you display the resulting solar cell structure from your literature.
Best wishes
You shouldn't, actually.
I did make and measured many small size solar cells without etching. The tricky part is to make connections to both electrodes and avoid short circuit. The transparent electrode and organic layers usually cover all glass, while the other electrode is to be made with a mask. Contacting to the transparent electrode is trivial. And a contact to the second electrode can be made using a small drop of liquid InGa alloy or similar. The drop should be slowly brought to contact, because any fast movement can scratch the layers and lead to a short circuit.
The method demands that the solar cell is used in horizontal position with illumination from above.
The very drop can be used as the other electrode, but it's difficult to control it's size, it's covered by oxides and adsorbed stuff...
Abdelhalim abdelnaby Zekry Thanks for your reply. I have attached the structure below. Also, in all the videos on solar cell fabrication they say to etch away FTO substrate.
Vladimir I Chukharev Thank you for your reply. Why should I use a mask during cathode deposition?
Brishty, you need a mask to reach the transparent electrode through a place not covered by the upper electrode. I do not know another way to reliably remove the upper electrode and organic layers not causing short-circuiting.
Abdelhalim abdelnaby Zekry why the upper metal layer and FTO must not touch?
Vladimir I Chukharev Okay, now I understand your explanation. But still I am curious to know why some video on solar cell fabrication says to etch away a portion of FTO substrate. Can you enlighten me please? Actually I am a bit confused with so many videos and literature.
@Brishty Deb
One of common designs is following. Etching partly removes FTO, leaving one half completely untouched and a few, e.g. 5, small spots of FTO on the other half of the plate. There should be no conductivity between any two FTO spots. Organic layers are deposited onto the prepared substrates. Then the organic layers are removed from the small FTO spots and some corners of the big FTO spot. Now, the metallic electrodes are deposited through a mask. Each metallic electrode covers a small FTO spot and a part of the big FTO spot. Since the small spots were cleaned, metal is in electrical contact with FTO here. Above the big FTO spot you get a solar cell with FTO and metallic electrodes. Actually, 5 solar cells of the same structure in this case. It's quite easy to connect a measuring instrument to both electrodes of any of the cells, even with allegator clips. Just place the connectors on a cleaned corner of the big FTO spot and on the corresponding small FTO spot.
Surely, the sizes and shapes can vary at your convenience.
Dear Brichty,
welcome,
The colleague Vladimir introduced some answers on your queries.
If the upper metal layer, here Aluminum touches the lower FTO conducting electrodes a short circuit will be formed between the two electrodes rendering the whole solar solar cell not usable. The upper electrode serves as a cathode of the solar cell and the lower electrode as the anode of the solar cell. they must not be permanently shorted to connect the load across them.
So, while depositing the aluminum metal cathode layer you can mask aluminum for preventing it to deposit on the bare FTO as Vladimir explained.
The partly etching the FTO layer as in the figure is not necessary.
After my opinion, this is done to access the upper electrode from the side above the part free from FTO. It may be because of practical consideration such that when boding a wire on the upper electrode it may penetrate to the lower electrode. But bonding on the part free from FTO will not cause short between the two electrodes. I myself do not tried this process but these expectations are based on my technological experience.
For time being i could not see any other cause. May be the colleagues who worked with this technology share their experience.
Best wishes
Vladimir I Chukharev thank you very much for the nice explanation. so, the patterning should be something like this?
You need some distance between each two of the FTO spots, in your drawing all small spots are touching the big one. Separate them by 1 ... 3 mm, so that you can easily clean only small spots from organic layers, e.g. by wiping them with a piece of cotton, wetted with a solvent. Otherwise correct.
Why do you shape the FTO to have such fingers? Any process must be justified!!
Valdimir spoke from making some partial solar cells because of the fabrication defects can render some part of the solar cell inoperative. So instead of making one large cell that has some defects one can get multiple cells some of them are good free of defects. I made my previous comment and it seems you overlooked it.
Best wishes
Abdelhalim abdelnaby Zekry Yeah, I somehow overlooked your previous comment. I sincerely apologize and thank you very much for your nice explanation. Vladimir I Chukharev Thank you again for your comment. Thank you both for clarifying things to me. It was very helpful.
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