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You need a very sensible apparatus to perform accurate measurements, as suggested by Sadeq H. Lafta, But most important is the sample structure and the measurement configuration. It is too long to explain all here, but I suggest you to search for Van der Paw method/configuration, which takes in the proper account the influence of the contact resistance of electrodes with your sample surface. Moreover, just take into the due account surface contact geometry because dealing with low-conductive films a large current crowding effect at contacts may be expected (which is detrimental to your measurements), and consider the possibility of rectifying behaviour at contacts as well (light may also play a role)