Highly Crystalline Organic Semiconductor Thin Films By Dip-coating And Their Field-effect Transistors

In recent years,a variety of solution processing methods have been developed for the preparation of large area,continuous and highly crystalline organic semiconductor thin films.Among these methods,dip-coating is the first commercial film preparation technology.Dip-coating shows high tolerance for the types and shapes of substrate,requires simple equipment and capable of rapid large-area preparation.As a result,it is widely used in solar energy,electronics,automobile,machinery and other industries.A lot of researches have focused on the preparation of organic semiconductor films by dip-coating in recent years,but there are still some challenges.For example,it is impossible to prepare a large area organic semiconductor thin films with full coverage,and the film thickness is relatively thick,which restrict the development of dip-coating in the field of organic electronics.In view of the above challenges,the main research contents of this paper are as follows:(1)Meniscus angle(MA)was proposed for the first time to measure the dynamic interaction between solution and substrate in dip-coating and large area,continuous,and crystalline organic semiconductor thin films were successfully prepared by controlling MA.Previous reports only focused on single factors(such as substrate,solvent,concentration,etc.)or the combination of these factors on the dip-coating results,neglecting the dynamic interaction between solution and substrate.Therefore,the depositions obtained by previous dip-coating were stripes or treelike crystals with low coverage,which was not favorable for device preparation.By adjusting the meniscus angle with different substrates,large area,continuous and crystalline thin films of various organic semiconductors were successfully prepared in this work.The average mobility of organic field-effect transistors(OFETs)based on continuous thin film was 82% higher than that of ribbon array devices.This work revealed the key role of solution-substrate dynamic interaction during dip-coating and provides theoretical and technical support for the preparation of large area,continuous and crystalline organic semiconductor films by dip-coating.(2)Two-phase dip-coating was proposed and moleculay thin organic semiconductor single crystals were successfully obtained.The traditional single-phase dip-coating of organic semiconductor films were thick(tens of nanometers),and the thick films had large injection resistance,which was not conducive to the construction of OFETs and other devices.In this work,the idea of preparing ultra-thin organic semiconductor crystalline films by two-phase dip-coating method was proposed.Water was used as the base phase and organic semiconductor solution was spread on the water phase as the second phase.The influence of aging time,solvent type,solution concentration and withdrawal speed on the morphology,structure and thickness of the films in the process of two-phase dip-coating were systematically studied.The charge transport performance of the ultra-thin films was studied,and OFETs with excellent performance were obtained.The water-based two-phase dip-coating provided a simple and efficient method for the preparation of large area,continuous organic semiconductor ultra-thin single crystals for many optoelectronic applications.