Orientation Structure Control And Electrical Properties Of Thiophene Polymer Semiconductor Thin Films

Polymer semiconductor materials have a wide range of application prospects in the construction of large-area and flexible electronic devices due to their various types,wide sources,low cost,good flexibility and solution processability.As an important part of organic electronic devices,the structure of polymer chain and structure of condensed state for conjugated polymers will immediately affect the transfer of charges,and thus affect the performance of the device.In addition to devising and regulating the chemical structure and composition of molecules,precise control of the molecular chain orientation and crystalline structure morphology of conjugated polymers is also of great significance to the optimization of device performance.This thesis takes organic semiconductor materials such as poly-3-hexylthiophene(P3HT)and its derivatives as the research object.The oriented polymer substrate and crystallizable small molecules are used as the host materials to induce the orderly alignment of the molecular chains of the guest materials such as P3HT by epitaxial crystallization.And the controllable solvent vapor treatment method and dip-coating method are used in combination to optimize the molecular chain orientation structure and film crystalline morphology,and the effects of the orientation structure on the charge transport properties of the film are further explored.This thesis has reference significance for the preparation of large-area oriented conjugated polymer films with anisotropic electrical properties and the development of oriented stretchable flexible electronic technology.The main work contents are as follows:1.The synergistic effect of epitaxial crystallization and controlled solvent vapor treatment has been used to precisely control the orientation structure of P3HT film.Firstly,oriented polyethylene(PE)prepared by melt-drawn method was used as the substrate,and the epitaxial crystallization behavior of the P3HT film prepared by spin-coating of chloroform and o-dichlorobenzene(o-DCB)was investigated.The results show that the molecular chains of P3HT are aligned parallel to the drawing direction of the PE film.At the same time,the films prepared with the high boiling point solvent(o-DCB)occured more ordered "side-on" molecular chain orientation structures.Secondly,the synergistic effect of carbon disulfide(CS2)solvent vapor on the oriented P3HT film on the PE substrate was studied.It was shown that the orientation structure of P3HT strongly depends on the solvent vapor pressure and the vapor treatment time.There is an optimal vapor pressure value and an optimal treatment time,which makes the UV-vis absorption anisotropy ratio of the P3HT film reach 7.1.Furthermore,the test results of an organic field-effect transistor(OFET)prepared based on this P3HT orientation structure showed that the carrier mobility measured parallel to the PE drawing direction was 7.5 times that in the perpendicular direction.2.The epitaxial crystallization behavior of P3HT-b-polycaprolactone(PCL)block copolymer on oriented PE substrate was studied.The highly epitaxial crystallization effect of the soft segment PCL in the P3HT-b-PCL block copolymer on the oriented PE substrate was used to promote the arrangement of the hard segment P3HT molecular chains.The experiment proved that the molecular chains of the two components of the P3HT-b-PCL block copolymer are aligned orderly on the oriented PE substrate,which was confirmed by the use of polarization spectroscopy and transmission electron microscopy(TEM).And in situ atomic force microscopy(AFM)was used to observe the nano phase separation of the block copolymer on PE substrate.It was shown that PCL formed a highly oriented layered structure perpendicular to the PE stretching direction,which was less affected by the hard P3HT segments.The P3HT chain segment was short,forming a granular crystal structure.3.The epitaxial crystallization behavior of thienothiophene(PBTTT-C14)polymer on oriented polybutene-1(iPBu)film was further investigated.The influences of the solution temperature on the orientation structure and crystal morphology of PBTTT-C14 was clarified.It was shown that after spin-coating the PBTTT-C14 solution at high temperature on the oriented iPBu substrate,films with an oriented crystal structures could be obtained,in which the molecular chains of PBTTT-C14 were aligned parallel to the drawing direction of the iPBu film.At the same time,the OFET device prepared based on the orientation structure indicated that the carrier mobility tested parallel to the iPBu drawing direction was three times that of the perpendicular direction.However,after spin-coating PBTTT-C14 soultion at low temperature with pre-aggregated structure on oriented iPBu substrate,a large number of disordered long nanofiber crystals and a small amount of oriented crystal structure induced by iPBu substrate were formed in the film.This phenomenon indicates the further ordering of the aggregated segment in the low temperature.The stronger inter-chain interactions were derived,which severely weakened the epitaxial crystallization ability of the oriented substrate,resulting in the formation of PBTTT-C14 nanowires without forming in-plane orientation crystals.The OFET device based on this structure has no anisotropy,and its carrier mobility is 0.36 cm2 V-1 s-1.4.Flexible P3HT/polydimethylsiloxane(PDMS)composite film was utilized to systematically investigate the conductive properties and morphology evolution of the orientation of the P3HT film under tensile strain.First,a P3HT/1,3,5-TCB/chloroform mixed solution was used to prepare a large-area oriented P3HT film by dip-coating.Characterization results such as polarizing microscopy(POM),UV-vis,TEM indicated that the P3HT molecular chains are highly arranged along the dip-coating direction,forming micron-long fiber bundles.Then,P3HT/PDMS composite film prepared by the combination of oriented P3HT and PDMS was used to explore the conductive properties under different stretching strain.It was found that when the film was stretched in different directions,the electrically response showed a significant anisotropy.When stretching in the direction of the fiber bundles,the electrical conductivity of the film was greatly reduced after the strain exceeds 30%.When stretched perpendicular to the fiber direction,the conductivity began to decrease when the strain exceeds 50%and greatly reduced after a strain exceeds 80%.The P3HT film was further characterized to evolve the surface structure of the P3HT film under different strain.The results showed that when parallel stretching,the fiber bundles appeared cracks under smaller strain.While perpendicular stretching,the fiber bundles were not easily broken and the network interleaving strip and distance between the fiber bundles occured.Therefore,the charge could still be transferred.In addition,the microscopic deformation mechanism of the oriented organic semiconductor film was further discussed.