Study On The Design,Preparation And Performance Of Polyurethane Dielectric Materials For Organic Thin Film Transistors

Organic thin-film transistors(OTFTs)are logic switch transistors devices using organic molecules or polymers as semiconductor materials.Because of their wide material sources,simple production process,and low-cost,OTFTs have great potential application value in the electronic paper and the active matrix display backplane circuit etc.Since the birth of the first thin-film transistor in 1986,a large number of experimental researches have been put into it.After decades of efforts,many progresses have been made in the improvement of device structure and device material.At present,the perfect performance of organic thin-film transistors has already surpassed the level of monocrystalline silicon,and has attracted wide attention in both academic and commercial value.As an important part of the organic thin-film transistor,the insulation layer not only affects the overall level of the threshold voltage of the device,but also the interfacial properties and the organic structure will further affect the organic semiconductor materials,and then affecting the mobility of the device.Therefore,the development of new and excellent insulation material has become the focus of the research.In the first chapter,we briefly reviewed the history of the development of organic thin-film transistors,introduced the device structure and basic working principle of organic thin-film transistor,expounded the evaluated parameters and the working curve of organic thin-film transistor,and summarized the types and development of organic thin-film transistor semiconductor materials and insulating materials.In the second chapter,in order to explore the influence of the internal structure of insulating materials on the crystallization of semiconductors,we designed and synthesized two monomers that one was contained double bond structure and the another one contained phenyl ring and cyano group.The insulating material was obtained by polyurethane polymerization with hydroxyethyl terephthalate and dicyclohexyl methane-4,4 '-diisocyanate.The prepared polyurethane insulation films had good thermodynamic stability and smooth surfaces,no pinhole structures and perfect photosensitivity.And with the increased of benzene content in the polymer,the selectable crystal nucleus of the p-6P semiconductor molecule also increased accordingly.This resulted in the change of crystal size and crystal number of semiconductor molecules.And the results demonstrate that the dielectric constant of the device can reach 7.3 due to the existence of benzene and cyano group.In the third chapter,in order to optimize the threshold voltage of the device,one monomer that contained cyano and biphenyl groups had been designed and synthesized.Then we got three insulating materials with different monomer proportions that could be cured by UV light.The surfaces of the thin-films were very smooth,no pinholes,and hydrophobic,which had better compatibility with semiconductor molecules.The p-6P/VOPc organic thin-film transistors prepared with these polyurethane insulating materials had revealed good performance,and with the increased of the content of biphenyl in the polyurethane polymers,the threshold voltage of the devices were getting lower and lower,the optimal threshold voltage of-0.5 V.In the fourth chapter,in order to explore the influence of the difference between the main chain and the side chain of the insulating material on the performance of OTFTs,we have designed and synthesized three different monomers proportions of main chain biphenyl type polyurethane insulating material.These polyurethane polymer thin-films could be prepared by spin-coating,and the prepared polyurethane insulating thin-films had good thermal properties.The p-6P/VOPc organic thin-film transistors prepared with these polyurethane insulating materials had better performance.But comparing with side chain biphenyl materials,we found that the threshold voltage of the device prepared by the main chain biphenyl polyurethane material is higher,this may be due to the enhancement of the overall symmetry of the material after the introduction of the biphenyl group in the main chain,the occurrence of the chain segment reversal embed the dipole.However,the overall performance of the device was excellent.We could see that with the increase of biphenyl content in polyurethane materials,the threshold voltage of devices is decreasing.The optimal threshold voltage is-5.5 V,which is much lower than most commercial polymer materials.