Preparation And Optoelectronic Properties Of Monolayer Organic Semiconductors

Two-dimensional(2D)materials,such as graphene,MoS₂ and black phosphorus,have attracted extensive attention for their unique properties,such as ultra-thin thickness,regular structure and unique photoelectric properties.Different from inorganic materials,organic materials have many unrivalled advantages,such as light weight,low cost,flexibility,large-area processing,solution processing and their unique photoelectric performance,which make them promising functional materials for next generation electronics.Organic field-effect transistor(OFET)is one of the important elements to study the photoelectric properties of organic materials.In recent studies,it has been proved that the charge transport in OFETs only occurs in first or several molecular layers at the interface between dielectric and semiconductor.Such ultra-thin organic films have been proved in flexible,transparent and other photoelectric properties.In recent years,with the development of molecular engineering and device engineering,great progress has been made in the preparation and application of organic monolayer molecular films,in which the mobility of P-type and N-type semiconductors have reached the level of amorphous silicon.However,a series of problems still remain in the preparation and application of OFETs,such as long growth period,poor stability,and monotonic application,which makes the preparation of organic monolayer molecular film transistors remain in the labs.Considering these problems,this dissertation focused on the growth of monolayer molecular films and the application of their unique photoelectric properties.Several effective strategies have been summarized for the growth and application of monolayer molecular films.The main contents of this paper are introduced by following four parts:1.A two-dimensional space phase separation method has been implemented by spin coating the blend of polymer and small molecule,in which the controllable,simple,uniform,high quality and large area(millimeter level)monolayer molecular crystals(MMCs)have been prepared.The method has been found to be generally applicable to various small molecule semiconductors such as 2,6-bis-(4-hexyl-phenyl)-arthracene(C₆DPA),dioctyl-benzothieno-benzothiophene(C₈BTBT),1,4-bis((5'-hexyl-2,2'-bithiophen-5-yl)-ethynyl)benzene(HTEB)and naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups(NDI).With the reducing of semiconductor thickness,the ultra-thin MMCs show good flexibility and transparency,where the transmittance of the devices exceeds 93%(nearly 100%for some wave bands).Except that,the process has been implemented on various substrates,such as SiO₂,Si,quartz,Hf₂O₅ and plastic(PET),which is beneficial to the applications of flexible and transparent wearable devices.All MMC devices exhibit well-defined transfer characteristics.Among them,OFETs based on C₈BTBT MMCs exhibit a maximum mobility of 3 cm²V^-1s^-1.These results demonstrate that the method has a great potential in the preparation and application of MMCs.2.The MMCs P-N heterojunctions have been prepared by spin-coating the blend of the polymer and two organic small molecule semiconductors with similar lattice constants.After that,we have investigated the OFETs,organic circuits and OPV devices based on the MMCs P-N heterojunctions.Among them,the OFETs exhibit well-balanced bipolar characteristics,in which the hole mobility is 0.54 cm²V^-1s^-1 and the electron mobility is0.50 cm²V^-1s^-1.Based on the P-N junction,the inverter has also been prepared,with gain of?14.The photovoltaic properties of the heterojunctions have been also explored.The maximum value of open circuit voltage(V_oc)is 1.04±0.2 V.In addition,we have demonstrated that the performance of OPV devices could be adjusted by multiple parameters including gate bias,light intensity and wavelength.These results demonstrate the great potential of MMCs P-N heterojunction in OFET,organic circuit and OPV.Except that,it also provides a simple prototype for studying the transmission process and mechanism of organic solar cells,and offers a meaningful reference for next-generation optoelectronic devices at monolayer level.3.Multiple mechanisms in non-stable OFETs including charge trapping and contact resistance have been proposed indirectly by using mechanical transfer methods.Considering the multiple mechanisms,we have proposed a facile and effective method to obtain the stable and reliable devices.As described in this paper,a mass of island-like MMCs are formed on the surface and bottom of the polymers.The obtained MMCs play important roles in reducing contact resistance and interface trap density,which are beneficial to the injection and transmission of charges.This method has also been attested for other polymers with instable behaviors,such as DPP polymer series.And,the OFETs based on these polymers still maintain high mobilities,which expands their application to commercial devices.These results demonstrate the great potential of MMCs in the stabilization of devices.4.The organic porous MMCs have been prepared by a facile and simple strategy.The porous MMCs have been prepared on the surface of the dielectric layers through solvent etching and the rapid growth of crystals.We have investigated OFETs based on the porous MMCs and devices,which exhibit well-defined transfer characteristics.Benefit from the unique advantages of porous and ultra-thin properties,the gas sensors have been fabricated and investigated.The sensors based on the porous MMCs exhibit obvious selectivity.The detection limit of ammonia(NH₃)is down to 10 ppm and the response time is 700 ms.These results demonstrate that organic porous MMCs have great prospects in organic sensors.