Low fill factors can be caused by two things; high series resistance, or low shunt resistance. Given your high Voc, it is likely to be series resistance causing charge build-up. Do they have an s-shape characteristic?

http://scitation.aip.org/content/aip/journal/jap/105/9/10.1063/1.3117513

http://pubs.acs.org/doi/abs/10.1021/jp9082163

What is your electron transport layer? If you are using organic/fullerene electron transport layers then I would suggest using a buffer layer such as BCP or LiF to assist in the extraction of your electrons, I believe this is a common issue.

http://www.sciencedirect.com/science/article/pii/S0040609006006146

http://pubs.rsc.org/en/Content/ArticleLanding/2015/NR/c4nr05975a#!divAbstract

If you're using different charge extraction layers it could be that there aren't enough intrinsic dopants (either in your electron or hole transport layers), which is impeding extraction, so you may need to modify your deposition/ post-deposition procedure. I don't know how sensitive Spiro is to deposition/ post-deposition procedure.

Lastly, it could be caused by impurities in your materials or incomplete perovskite formation, but I think this is less likely.

Low fill factors can be caused by two things; high series resistance, or low shunt resistance. Given your high Voc, it is likely to be series resistance causing charge build-up. Do they have an s-shape characteristic?

http://scitation.aip.org/content/aip/journal/jap/105/9/10.1063/1.3117513

http://pubs.acs.org/doi/abs/10.1021/jp9082163

What is your electron transport layer? If you are using organic/fullerene electron transport layers then I would suggest using a buffer layer such as BCP or LiF to assist in the extraction of your electrons, I believe this is a common issue.

http://www.sciencedirect.com/science/article/pii/S0040609006006146

http://pubs.rsc.org/en/Content/ArticleLanding/2015/NR/c4nr05975a#!divAbstract

If you're using different charge extraction layers it could be that there aren't enough intrinsic dopants (either in your electron or hole transport layers), which is impeding extraction, so you may need to modify your deposition/ post-deposition procedure. I don't know how sensitive Spiro is to deposition/ post-deposition procedure.

Lastly, it could be caused by impurities in your materials or incomplete perovskite formation, but I think this is less likely.

Dear Kenneth, thanks for the feedback. Electron transport layer was based on TiO2, and in order to complete the doping of spiro it was left in dry oxygen for overnight (well reported method now). The devices IV curve do have S-Shape curve which can be seen in attached file. I did made the ETL in different ways, though Voc was in the range of 0.85-0.9V for all devices with, Jsc in the range of 14-17mA/cm2, however FF was in the range of 20-25, i am thinking to do metal evaporation in a different chamber just to see if that is causing it.

For TiO2, it could be that it needs to be annealed at a higher temperature (I think 500 C is commonly done), I think light soaking is sometimes needed as well, or your charge extraction layers could be too thick.

You could try testing some of your old devices under constant illumination and see if they improve over time.

I am annealing it at high temp with recommended spin coating conditions. However, i guess i should check the thickness for these layers. 

Thanks for the suggestion to do measurements under constant illumination, however e.g. if i have an improvement what will that tell us?

What are the thicknesses of your perovskite and spiro-OMeTAD layers, and how is the morphology of perovskite layer? Did you check with XRD that your perovskite film formation is okay, i.e. it does not have the PbI2 peak?

Dear Sanjib, your questions are very valid, i recently started working on perovskites and definitely looking forward to analyze my samples with XRD, and thickness measurements.

However, by following the two step spin coating deposition process i was getting the UV-Vis for perovskite films on FTO just like published work (attached). However, XRD should also be done along with thickness measurements.  

I assume you mean a fill factor of 0.20 to 0.25. Such a low fill factor in the case of a high open circuit voltage, signifies an unacceptably high series resistance, as many have mentioned. 

Series resistance can originate from the bulk substrate and the transparent electrode. The bulk substrate resistance can be reduced by effective doping and by improving mobility (impurity scattering) and the transparent electrode resistance can be improved by a thicker layer. 

Dear hammad,

It seems that i came very late to the discussion. I saw your I-V curve of  your solar cells.

It  has a typical S-shape. please Look at it again. Such low fill factor is the consequence of an s-shaped iv curve. The S-curve is normally caused by nonlinear metal semiconductor contacts of the solar cells. Such MS contacts act as a schotky contact instead of functioning as an ohmic contact. Such phenomena was observed since long time and analysed. For more details please see the link:https://www.researchgate.net/publication/232710923_A._Zekry_and_G._El-Dllal__effect_of_MS_contact_on_the_electrical_behavior_of_solar_cells_Journal_of_solid-state_Electronics_Vol.31_No.1_1988

You can benefit from this paper to enhance your cells.

wish you success

Article A. Zekry and G. El-Dllal, “ effect of MS contact on the elec...

I think the low fill factor of 0.25 is due to low shunt resistance due to some impurities in the perovskite material used.

Dear All, 

Thanks for such a constructive feedback, i am working on it now. Hopefully i will have a better update on this work soon.

After doing XRD, it is quite clear that the method i was using was not reacting all of the PbI2, which might be the reason for low FF, looking forward to make cells with modified method.....

Dear Hammad

what did you do for converting hole PbI2, I myself have the exact problem. voltage is high enough, yet FF. It seems just the surface of the PbI2 has converted to perovskite.

Dear Cheema,

You may take help from the article attached.....

Best wishes

 Dear Hammad,

Hope you are well and the problem of low fill factor has been solved. The paper of given  link studied the effect of excess and deficiency of PbI2 in perovskite on the performance of the solar cell. It may be of interest to you. According to the data in this paper and the relatively high fill factor obtained, i would expect that that the spiro ome tad layer may be the cause of high nonlinear series resistance and consequently the low fill factor. It would be great that you to tell us about your state of the art.

http://pubs.acs.org/doi/pdf/10.1021/jacs.6b06320

best wishes

Dear Asaduzzaman and respected Mr. Zekry, 

Thanks for sharing the revlevant info, which I am sure will be helpful for people having similar issues. Unfortunately, I was not able to pursue the work to solve it, I moved to another group wit a focus on dye-sensitized solar cells. 

Regards,

Hammad

check your perovskite crystallinity/purity and thickness. Your perovskite meso layer should be less than total of 1000 nm (capping layer + mesoporous TiO2). Thicker TiO2 could result to thinner perovskite capping layer. XRD should reveal how complete is your perovskite's formation. Lower conversion is very common when using sequential step. This is from my experience. Good luck!