This is a non-trivial problem...I believe you will need to both know the thickness of your film to a high accuracy, do conductivity measurements on a very specific area, and know factors like bulk mobility for the material. Are you looking to do this for illuminated or dark samples?

Thank you Susan, I knew the film thickness and mobility, i need to do the conductivity. I am looking for illuminated and dark samples also, If you know or have any article regarding please let me know

May be you could try Kelvin probe measurements to determine the surface potential and then the charge. There is another version of KPM called the surface photovoltage (SPV) measurement that you could do on illuminated samples. Here is a paper that might be useful http://pubs.acs.org/doi/abs/10.1021/nl048176c

Thank you Vini, I am going through that article, I will contact you if i have any doubt

I agree with Vini. The surface potential of the film is in darkness and under irradiation may be measured by dynamic probe (Kelvin method). I measure the surface potential of films by dynamic probe connected to a storage oscilloscope.

Would it be possible to determine the electrostatic potential with the help of TEM?