Dear Mehrdad,

Welcome!

Thin films like the PEDOT:PSS are characterized by measuring their electrical , optical and opto electronic properties. Measuring the sheet resistance of the film Rs= roh/t , where roh is the resistivity of the material of the film and t is the film thicness and knowing the film thickness one can calculate the resistivity of the material. To measure the resistance one deposits the film on an insulating substrate such as glass or alumina. Then one deposit metal electrodes in form of strip line. One needs at least two such line which are spaced by certain specific distance which is much larger than the strip line width. One has to select the metal such that its makes a low resistance ohmic contact. Here you may use gold, silver or aluminum.

After depositing the metal you have to anneal the metal contacts are about 120 degree centigrade to affect good contact. 

You can use the four point probe which is fairly independent on the type metal contact. The four point probe is a standard method in semiconductor engineering.

For more information please refer to the Link: http://www.ep2-bayreuth.de/wp-content/uploads/2012/08/2012-JPolySciB-St%C3%B6cker.pdf

wish you success

Dear Abdelhalim Zekry, like always your answer was helpful, thanks!

As a general point, infinite resistance would not indicate a conducting film - it would indicate an insulating film (or one with a resistance too high for the multimeter to measure). As Abdelhalim points out, you need proper ohmic contacts to accurately measure film resistance.

Dear Keith Fraser, thanks a lot!