Device Engineering For High Mobility N-Type Organic Thin Film Transistors

The development of high-performance n-type organic thin film transistors(OTFTs)via solution processing remains challenging,while high mobility n-type OTFTs are essential for the construction of organic circuits.Apart from the structural design of semiconducting materials,device engineering is an alternative way to effectively enhance the performance of OTFTs.The processing of the semiconductor films and interfacial control are both important amongst these engineering strategies.Highly oriented thin films can be obtained by bar coating,which can significantly increase the mobilities.Besides,the interface engineering can not only tune the work function of the electrodes and reduce the injection barrier,but also improve the interfacial tension to optimize the microstructure of the thin films.In the current thesis,we have used a diketopyrrolopyrrole-based(DPP)ambipolar conjugated polymer,P4FTVT-C32,as the semiconductor material,and combined bar coating processing with interface engineering for high-quality thin films and high-performance n-type OTFTs.The relationships of the processing technique/interface modification-thin film morphology-electrical properties of OTFTs were studied in detail.The main results are:(1)Highly ordered P4FTVT-C32 thin films have been obtained by bar coating processing.The polymer backbones were uniaxially aligned along the coating direction and the thin films displayed uniformly oriented fiber-like morphology.All the bar-coated OTFTs exhibited ambipolar transport behavior and showed evident anisotropic charge transport properties.When the source to drain electric field is parallel to the coating direction,highest saturation electron mobilities of 13.03 cm² V^-1 s^-1 were reached,significantly higher than those of spin-coated devices.(2)The morphology of the bar-coated P4FTVT-C32 thin films and the electrical properties of the OTFTs have been effectively controlled by using the amino group-containing polymer,?-poly-L-lysine(?-PL),as the interface modification layer.The size of the crystalline domain in the thin films could be increased on substrates modified with?-PL.Compared with unmodified devices,the?-PL layer could completely suppress hole injection and reduce the electron injection barrier,resulting in a threshold voltage shift of 40 V.The OTFTs presented unipolar n-type transport behavior,with maximum saturation mobility of 6.63 cm² V^-1 s^-1.(3)Polyethylenimine ethoxylated(PEIE)with higher amino group density can further enhance electron injection,and improve the surface tension of the substrate at the same time.The crystalline domains of the polymer films were significantly increased in size,and highly oriented films featuring these large crystalline domains were obtained.OTFTs with PEIE modification exhibited pure n-type transport behavior.Saturation and linear mobilities up to 9.38 cm² V^-1 s^-1and 8.35 cm² V^-1 s^-1 were achieved when the electric field is parallel to the coating direction,with reliability factor>80%.To the best of our knowledge,these are the record high reliable electron mobilities of conjugated polymer-based OTFTs.We fabricated fully organic inverters with gain value as high as 214 based on these high mobility n-type OTFTs,which display potential for organic logic circuits.