Fabrication Of Highly Oriented Thin Films Based On NDI Derivatives And The Corresponding Transistor Performance

Organic semiconductor materials have the advantages of low cost,wide source,adjustable molecular structure,and can be used for large area preparation,which have good developments for prospect.In recent years,the mobility of organic semiconductor materials has been substantially improved.However,compared with p-type semiconductor materials,n-type semiconductor materials are relatively underdeveloped,and the development of n-type materials is very necessary for the construction of complementary logic circuits.To obtain high mobility n-type devices,in addition to the properties of the material itself,the preparation of high-quality films is also very important to improve the device performance.Thin films with high orientation and uniformity can be prepared by dip-coating.Therefore,in this thesis,the n-type organic small molecule semiconductor materials NDI-C6 and NDI-C18 were used to prepare organic thin films and devices by the dip-coating method.In order to further improve the coverage of the film,the film was modified by the combination of the dip-coating and the vacuum evaporation method.The main research of this paper is divided into the following three parts:In the first part,NDI-C6 thin films were prepared by dip-coating method.The effects of solvent type,solution concentration and pulling speed on the morphology of the films were investigated,and the film formation mechanism was explained.Firstly,we studied the effects of four solvents with different boiling points,chloroform,toluene,chlorobenzene and dichlorobenzene,on the morphology of the films.As the boiling point of the solvent decreases,the solvent volatilization rate increases,and the drawing speed of the films also needs to increase.When toluene and chlorobenzene are used as solvents,high quality pull-out films can be obtained.At different concentrations,the number of solute molecules involved in assembly is different.When the concentration is too low,the molecules involved in assembly are too few to form a continuous film.When the concentration is too high,there are too many molecules involved in assembly,and too dense accumulation will result in uneven film assembly and poor orientation.When toluene was used as the solvent,the film morphology was the best at 3 mg/m L and 5 mg/m L,while chlorobenzene was used as the solvent,the film morphology was the best at 5 mg/m L and 7 mg/m L.At different pulling speeds,there are two growth modes:meniscus mode and entrainment mode.The film thickness first decreases and then increases with the increase of pulling speed.There is a transition stage between the two modes,and the film has better orientation and uniformity at the transition speed.In the second part,to address the problems of enlarged grain boundary and thin thickness of films,we used a combination of the dip-coating method with vacuum evaporation method to prepared organic films to improve the device performance.On the basis of the dip-coating film,vacuum vapor deposition is further applied to increase the coverage of the film,reduce the grain boundary defects,and can increase the thickness of film quantitatively,so that the thickness of the film can be further increased while maintaining the orientation.After the evaporation and modification of 3-4molecular layers,the coverage of the film on the substrate increases obviously,and the substrate can be almost completely covered.Compared with the devices prepared by the dip-coating method alone,the mobility of the devices prepared by this method is obviously improved by about one order of magnitude,and the maximum mobility obtained reaching 4.02×10^-2cm²/Vs.In the third part,to further illustrate the effectiveness of vacuum evaporation method to modify the dip-coating films,we use NDI-C18 as the research object.Firstly,we investigate the influence of pulling speed and solution concentration on the film morphology,and then use the combination of dip-coating method and vacuum evaporation method to prepare highly oriented NDI-C18 films and devices.It is proved that this method can be applied to a variety of small molecule semiconductor materials.The NDI-C18 films prepared by dip-coating method also have meniscus mode and entrainment mode,and the best morphology is obtained at the transition stage between the two modes.After the modification of vacuum evaporation method,the best device performance was obtained at evaporation of 1.5 molecular layers,and the maximum mobility reached 1.54 cm²/Vs.In summary,we have prepared NDI-C6 and NDI-C18 films and devices using the dip-coating method,and the quality of films was further improved by the combination of dip-coating method and vacuum evaporation method,and the performance of devices was effectively improved.This method not only enables the preparation of high mobility n-type organic transistor devices,but also applies to other small organic molecules.It provides a new way for the preparation of high quality organic films and high performance organic transistors.