Synthesis,Characterization And Field-Effect Transistors Properties Of D-A Conjugated Polymers With Fluorine-Containing Branched Side Chains

Polymer semiconductor materials and organic optoelectronic devices have received much attention due to the advantages of simple device preparation process,low cost,and large area preparation.Polymer semiconductor materials consist of a conjugated backbone that provides charge transport and a side chain that provides solubility.As research progressed,it was found that side chain engineering of polymers has a pivotal role.Side chain modification not only provides solubility and solution processability of polymers,but also influences chain segment stacking and film morphology of polymers,directs the self-assembly behavior of molecules,and thus affects the device properties of polymer semiconductors.In this thesis,a series of novel fluorinated branched alkyl side chain modified copolymers of naphthalenediimide and thiophene derivatives were designed and synthesized.In term of the molecular design,the branched side chains were far away from the conjugated backbone,which effectively reduces the steric effect of the side chains,and the introduction of heptafluoropropyl(-C₃F₇)at the end of the branched chains improves the physical and chemical properties of the material.The fluorine-containing side-chain modified NDI monomer was copolymerized with different donor units(thiophene(T),biphthiophene(BT),thiophene-vinyl-thiophene(TVT))by Stille coupling reaction to obtain three conjugated polymers PBFNDI-T,PBFNDI-BT,and PBFNDI-TVT,respectively.All three polymers exhibited better thermal stability(T_d>440°C)than the respective non-fluorinated alkyl side chain modified NDI-based polymers.The UV-visible absorption spectra of the three polymers exhibit typical D-A polymer dual-frequency absorption peaks and the absorption wavelengths cover the entire visible region,which bodes well for the potential application of this series of materials in organic solar cells.Electrochemical tests show that the LUMO energy level of the three polymers is around-4.0 e V,which indicates that the series can be used as n-type semiconductor materials,and the energy band gap gradually decreases with the expansion of the conjugated structure of the donor unit.The bottom-gate/bottom-contact(BGBC)structured organic thin-film transistors(OFETs)based on three polymers were prepared by solution-processed spin-coating,all of them exhibit n-type carrier transport characteristics in the glove box,with the highest mobilities of PBFNDI-T,PBFNDI-BT,and PBFNDI-TVT of the?_e=2.4×10^-4 cm~2V^-1s^-1,1.83×10^-2 cm~2V^-1s^-1,and 2.89×10^-3 cm~2V^-1s^-1,with current ratios I_on/I_off of 10~3?10~5.Moreover,the carrier mobility remained above 90%when the devices were placed in air at 30?70%humidity for three months,indicating that the introduction of fluorine-containing side chains imparts good environmental stability to the polymers.The GIWAXS diffractograms of the polymer films before and after annealing showed out-of-plane diffraction peaks(h00)and no in-plane diffraction peaks(0h0),indicating that the three polymers stacked mainly in the lamellar direction rather than the?-?direction.The AFM height maps shows that all three polymers exhibit lower root-mean-square roughness after annealing and for PBFNDI-BT and PBFNDI-TV appear rod-like nanostructures,which contribute to carrier transport.