Solution-Processed Two-Dimensional Organic Semiconducting Crystals And Their Applications In High-Performance Field-Effect Transistors

The conducting and semiconducting properties of organic materials were found in 1980's and applied in optoeletronics,such as organic field-effect transistors(OFETs).In the past several decades,many multifunctional organic materials have been synthesized and device perfoemance of OFETs has achieved great improvement.Compared with traditional inorganic semiconducting materials,organic materials possess unique characteristics,such as light weight,high flexibility and stretchability.Meanwhile,the fabrication of large-scale organic thin films is compatible with inorganic manufacturing and completed by low-temperature solution process,which can dramatically reduce the equipment requirement and cost.Besides,organic materials with specific optical and electrical properties can be produced by molecular design,synthesis and modification and device performance of OFETs can be improved by structure optimization.However,the carrier moboility of OFETs remains to be realtively low compared with inorganic FETs.Thus,organic devices can act as a complement and expand to satisfy the realistic demands for their unique properties instead of replacing the inorganic devices.Here,we systematically study the deposition of two-dimensional(2D)organic molecular films and the charge transport at the semiconductor/dielectric interface in OFETs,and realize high-performance OFETs.This work overcomes the long-standing critical problems in producing large-area,2D organic films and contributes to investigate the fundamental device physics in organic electronics.Moreover,solution-processed 2D organic films can integrate with other materials,such as 2D atomic crystals,showing great potentials in realizing novel optoelectronics,such as heterojunctions and flexible electronics.The innovations in our work mainly contains the following aspects.First,we propose a new strategy for 2D molecular crystalline films utilizing a "floating-coffee-ring driven assembly" and optimize the deposition by controlling the solvent,solute concentration and pump velocity.The 2D films exhibit a typical single-crystalline feature,atomic smoothness,and large-area high morphologic uniformity.The carrier mobility of OFETs based on 2D films yields average and maximum values of 4.8 and 13.0 cm2 V-1 s-1.In particular,to reduce the operation voltage and power consumption of OFETs,we successfully utilize a high-k dielectric to deposit 2D organic semiconducting films and fabricate OFET device,exhibiting high average and maximum carrier mobility of 4.7 and 9.4 cm2 V-1 s-1.Second,we propose an antisolvent-assisted spin-coating method to deposit large-area monolayers and optimize their surface morphology by controlling solute concentration,antisolvent amount and spin-coating speed.Supplementary semiconducting films are subsequently thermal evaporated on the monolayers and exhibit better molecular packing and crystallinity.OFETs based on the hybrid-deposited organic films yield a high carrier mobility up to 11.3 cm2 V-1 s-1,showing much improved device performance compared to that without this templating monolayer.Besides,we further apply this method in integrated fabrication and realize high-performance transistor arrays.Third,we successfully deposit a flat-lying monolayer at the semiconductor/dielectric interface and the resulting OFETs exhibit markedly improved device performance.This ultrathin flat-lying monolayer can effectively suppress the interfacial polarization effect,reduce the broadening the density of trap states and smooth the charge transport,acting as a buffering layer.Besides,we further utilize the ultrathin PMMA film as buffering layer,which also displays similar function and promote the charge injection and reduce the contact resistance.