The answer is quite simple. There is not so many polymer dielectrics for the gate insulators which are enough compatible (i.e. the solvents in the meaning of the solubility or mechanical stress due to the surface tension) with the organic semiconductor layer. In other words, usually you will influence your organic semiconductor by spin-coating (or other wet method) of the gate insulator layer on its surface. Of course, there are also some polymer dielectrics (such as parylene) which can be deposited from the vapour phase, but this procedure is rather slow, complicated. and requires additional equipment.

Thank you Martin, But I would like to know more about the working principle and other parameters like contact resistance, mobility etc and how it will effect on the devices

Because it is not compatible with photolithograpy process. But you can realize it by shadow mask process, then you will not be able to fabrecate short channel ofet.

There are researchers using pentacenequinone deposited on pentacene crystal as dielectric , they get a mobility as high as 35, but for polymer dielectric, the solvent will damage the active layer

Actually, top gate structure is widely used in organic transistors, in particular when using solution processed organic materials, as TIPS (semiconductors) and Cytop (fluoropolymer gate dielectric), providing that semiconductors and dielectrics have orthogonal solvents. The advantages of this structure is that it also provides an incapsulation of the semiconductor thanks to dielectric and gate electrode overlayers. Current high mobility solution processed OTFTs use top gate structure. Concerning contact resistance, it is more influenced by the position of source and drain electrodes (staggered or coplanar devices) rather than by the gate electrode. On the other hand, coplanar OTFTs are usually bototm gate (BGBC- OTFTs) while staggered devices are often Top gate (TGBC OTFTs)

Thank you Luigic Mariucci