Controlled Preparation And Optoelectronic Properties Of Two-Dimensional Organic Single Crystals

Two-dimensional(2D)organic single crystals contain minimal defects density and no grain boundaries,which are ideal carriers for studying the structure-property relationships and ideal materials for the preparation of a variety of organic optoelectronic devices.They have important scientific significance and application value in the fields of organic spin valves(OSVs),organic field effect transistors(OFETs),organic light-emitting diodes(OLEDs)and organic photovoltaic devices(OPVs).However,the realization of controlled growth of two-dimensional organic single crystals is still challenging,which limits the research on its optoelectronic properties and application to high-performance devices.In this thesis,controllable preparation strategies for two-dimensional organic single crystals have been developed,and its spin and charge transport properties have been explored.The main research results are as follows:(1)Large-area two-dimensional organic single crystals of6,13-Bis(triisopropylsilylethynyl)pentacene(TIPS-pentacene)with different thicknesses were grown on the surface of a liquid substrate by adjusting the concentration of the semiconductor solution.Subsequently,the first working single-crystalline organic spin valve was constructed and spin transport characteristics of organic semiconductor(OSC)were studied.Glycerol was chosen as a liquid substrate.The high surface tension together with the high viscosity of glycerol can promote the spreading and fix the position of the solution during evaporation,which was favorable for the production of thin and large-area single crystals.Single crystals of different thicknesses can be obtained by adjusting different solution concentrations.The magnetoresistance(MR)responses of the organic spin valves based on single crystals at different temperatures and thicknesses were investigated to study the spin injection and transport properties.Magnetoresistance value as large as 17% was probed with an intermediate layer thickness of 269 nm.More importantly,spin transport could still be observed in a single crystal of a thickness up to 457 nm,which was much larger than that of polycrystalline thin films.Our research provides a general method to construct single-crystalline organic spin valves and paves the way to probe the intrinsic spin transport properties of organic semiconductors based on single crystals.(2)A two-phase dip-coating strategy was proposed to controllably grow the large-area two-dimensional organic single crystals,and the optoelectronic properties of the two-dimensional organic single crystals were studied.Compared to traditional single-phase dip-coating method,two-phase dip-coating provided a larger surface to volume ratio of the solution,which led to a greater evaporation flux at the liquid–air interface.Therefore,heterogeneous nucleation was more likely to occur,thereby preparing large-area high-quality films.The effects of substrate type,solvent type,solution concentration,pulling speed,and drop amount of organic solution on the morphology of films were systematically studied.Several nanometer ultra-thin and centimeter-scale single crystals were obtained and used as the active layer of the OFETs,showing excellent optoelectronic performance.This work provides a fast and efficient new method for preparing large-area ultra-thin organic single crystals and provides a new idea for the further investigation of the optoelectric properties of the two-dimensional organic single crystals.