C₈-BTBT Thin Films Fabricated By Dip-coating And The Corresponding Transistor Performance

As the most representative material of flexible electronic materials,organic electronic materials have gradually attracted significant attention for application in the fields of biology,wearable devices,energy and sensors due to their advantages of light-weight,high flexibility,and molecular design.To improve the performance of organic devices,it is not only important to design and synthesize organic molecules,but also important to prepare high-quality organic thin films.In recent years,with the progress of the research on soluble organic semiconductor molecules,the solution method has become the focus of research in the field owing to its advantages such as no vacuum operation and large area and low-cost preparation.The dip-coating method is a solution method that can prepare highly oriented and homogeneous thin films under specific conditions,but there is still a lack of research on the mechanism of film formation and the selection of its optimal conditions.Therefore,in this paper,benzothiophene molecule C₈-BTBT was used as the research object to study the film formation by the pull method.The main body of the paper is divided into three parts for research and discussion.In the first part,the morphologies of thin films at different pulling speeds are studied,and the formation mechanism of thin films is explained.As the liquid surface has two shapes:meniscus and entrainment,which are dominated by evaporation and liquid film,the thin film thickness decreases first and then increases with the pulling speed increasing.There is a transition speed between the two shapes,which is15mm/min.At this speed,because the entrainment is the thinest,the thin film is the thinest.At the same time,the crystal will grow almost in the same direction through space restriction,so that the final thin film morphology orientation and uniformity is the best.When pulling speed was higher than 15mm/min,the liquid level is dominated by liquid film.Then,the film thickness will be thickened with the increase of the pulling speed under this shape,and the molecular limit in the assembly process will be lost and the assembly time will be reduced,so the uniformity of the film will gradually become worse.When pulling speed is lower than 15mm/min,the meniscus is dominant.At this time,the film formed by pulling will have a large area of defects due to slip-stick phenomenon during dip-coating.The optimal solution concentration for dip-coating is3mg/ml,less than this concentration the striations of the thin films are incompleted.Beyond this concentration,organic molecules are overpacked and the films are not uniform.In the second part,The device performance of organic field effect transistor prepared by dip-coating is studied.The higher the solution concentration,the better the device performance.The transistor prepared from 15mg/ml and 15mm/min pulling speed showed a mobility of 1.03cm²/Vs.This is because that the thickness of the thin film is the thickerwhen the concentration is higher and the speed is faster,and the number of carriers that can be provided in the device is larger.However,the thin film with a concentration of 3mg/ml and a pulling speed of 15mm/min has a mobility of0.27cm²/Vs due to its insufficient thickness.Likewise,the transistor dip-coated from a solution of 3mg/ml concentration with pulling speed of 90mm/min has a mobility of0.54cm²/Vs because of the increase in thickness.In the third part,the problem of insufficient film thickness can be solved by the combination of vacuum evaporation and dip-coating method.On the basis of C₈-BTBT films fabricated by dip-coating,the C₈-BTBT films are grown by vacuum evaporation.During the evaporation process,the C₈-BTBT molecules will first make the stripes thicker and wider,and finally make the stripes fuse,increasing the thickness and maintaining the orientation of the original thin film based on dip-coating.The mobility of the device obtained by this method is1.53cm²/Vs,higher than the devices based on either dip-coating or vacuum evaporation.In summary,dip-coating could be usd as a method to obtain high orientation,uniform thin films;By utilizing the combination of dip-coating and vacuum evaporation,the thickness of the film can be increased without losing the orientation so that the performance of the device can be further improved.