At first  - read about it:

http://www.solarfreaks.com/what-is-the-fill-factor-of-a-solar-panel-t138.html

Then measure IV dependence, find higher power and multiply your Isc with Voc (receive another power. FF=one power divede enother power.

thank you sir

Adding to the above, from the principle point of view there is no difference  for measuring  the I-V characteristic of any solar cell. They are two terminal devices. They have an anode and a cathode and you measure their terminal characteristics. The solar cells operate in the Fourth quadrant of the I-V diagram when they are illuminated. To get the fill factor you  have to scan the whole I-V curve from the short circuit to open circuit conditions by connecting variable resistive load between the anode and he cathode.

In DSSC as a photoelectrochemical cell, it has a working electrode , which is the photo electrochemical electrode, a counter electrode and may have reference electrode. The reference electrode i used when determining the characteristics of the electrodes. An electrochemical electrode constitutes a half cell. The cell is composed of  the working and the counter electrode.

These are just basic definitions to put things into order as we follow the concept: you measure as you define.

Then you may also use the method in the paper:

https://www.researchgate.net/publication/233902875_Experimental_Performance_Characterization_of_Photovoltaic_Modules_Using_DAQ

wish you success

Conference Paper Experimental Performance Characterization of Photovoltaic Mo...

There is some softwares in compliance with kiethley which can calculate it for you.

regards