Use linear or quasi-linear regime for Org-FET, keeping low Vds and measure step by step the current Id versus gate voltages in many point Id-Vg. Each Id(Vgs) corresponds to a specific effective channel thickness.
@Cristtian Ravariu, sorry for late reply, Thank you for great answer. But I would like to know what is behind theory, if you know any research article or book or any kind of, please let me know
Generally in typical OFETs, the thickness of semiconductors in the channel does not affect much the characteristics of the device. Charge transport happens in ~2nm right at the interface of semiconductors and dielectrics. The below paper may help:
The thickness become important when you use very thin dielectric layer; meaning the potential drop from gate to source no longer mainly occurs in the dielectric.