Preparation Of Molecular Crystal Arrays And P-n Junctions With Single Cell Thickness And Their Optoelectronic Property Study

Organic single crystals are well-known by their perfect molecular order,the absence of grain boundaries,and the minimized concentration of charge traps.Compared to bulky crystals,two dimensional crystals are superior to good flexibility,high transparency,and easy for fabricating integrated devices.Two dimensional crystals of organic semiconductors(2DCOSs)combine the advantages of organic single crystal and twodimensional materials.All these characteristics make 2DCOS as ideal candidates for studying structure-property relationships and the intrinsic properties of organic materials,investigating the mode of carrier transport,fabricating high-performance organic fieldeffect transistors(OFETs),and making large-scale flexible integrated circuits.When the thickness of organic semiconductor single crystals is down to the single-unit-cell thickness,the charge transport process is confined to the nanoscale space,which is likely to show unique optoelectronic characteristics that different from the traditional bulk organic semiconductor materials.There have been plenty of studies showing that the conducting channel of OFETs is located at the first few molecular layers or even the monolayer thickness close to the insulating layer.The organic semiconductor single crystal with thickness of single-unit-cell,due to its direct exposure of conductive channel,can be more convenient to study the intrinsic characteristics of organic semiconductor materials,such as the charge transport mechanism.In addition,compared to OFETs based on traditional bulk semiconductors,due to the reduced bulk resistance,the contact resistance of single-unitcell thick molecular crystal will be reduced accordingly,and high performance OFETs will be realized,which is of great significance for the development and applications of molecular crystals with single-unit-cell thickness.Here in this dissertation,based on the unique properties of single-unit-cell thick molecular crystals,a series of innovative achievements have been made in the preparation of single-unit-cell thick molecular crystal and seamless lateral monolayer molecule crystalline p-n heterojunctions as well as their applications in optoelectronic devices.The main results are as follows:(1)We demonstrated a low-cost,large-scale scalable inkjet printing process that enables the preparation of large-area single-unit-cell-thickness molecular crystal arrays by combining the advantages of horizontal solution shearing techniques and spatially restricted domains by choosing a molecular system driven by two-dimensional growth interactions,and showed generalizability to several organic semiconductor small molecules and various types of substrates.The identical thickness in single-unit-cell(single molecular layer)crystals is essential for obtaining uniform device arrays,which provides a potential method for the preparation of crystal arrays with uniform thickness and device arrays with uniform properties.The simplicity and low cost high-precision patterning of large-area single-unit-cell-thickness molecular crystals will help to investigate their basic scientific mechanisms and provide reliable samples for innovation of potential characterization techniques.(2)The transistor based on 2-hexy lthieno[4,5-b][1]benzothieno[3,2b][1]benzothiophene(BTBTT6-syn)single-unit-cell-thickness molecular crystal arrays exhibit textbook-like electrical properties.It is worth highlighting that the transfer curves of all OTFT arrays exhibit almost identical operation with almost no hysteresis,with switching ratio between 107 and 108,and a narrow mobility distrLbution(coefficient of variation(CV)of 4.5%)on a 2 cm×2 cm SiO2/Si substrate.This is a huge improvement over the wide mobility distribution based on large-area device arrays in previous literature results,and this has almost reached the performance requirements of integrated circuits in real practice.In addition,both p/p and p/n configuration inverters based on single-unit-cellthickness molecular crystals show relatively high gains,up to 134 and 261,respectively.Furthermore,it exhibits excellent performance in "OR gate" logic circuit.These findings demonstrate the enormous potential of our inkjet-printed prepared single-unit-cellthickness molecular crystal arrays for organic integrated circuits.(3)Herein,by performing one-step crystallization of a mixed solution containing two semiconductor materials by a general direct writing approach,seamless lateral organic monolayer molecule crystalline p-n heterojunctions with spatial control is demonstrated.Gate-/light-tunable electrionic properties were achiebed by the p-n heterojunctions with an ultrahigh rectifying ratio approaching 1.73×105 under light illumination,and the photosensitivity and on-off ratio up to 105 and 4×104 are obtained.Furthermore,we designed and fabricated tunable multi-value inverters,in which the five output logic states can be available.Compared with binary inverters,the introduction of quinary values enables us to design more complex logic circuits with fewer transistors.Finally,we made a preliminary exploration of the surface potential changes on p-n heterojunction interface of the seamless lateral monolayer molecular crystal under different wavelengths of light.When different light sources are applied,the p-n heterojunction interface shows obvious surface potential changes,which provides an ideal carrier and platform for further understanding and exploring the lateral charge transport and exciton physical mechanism of the p-n heterojunction.(4)We developed a simple and non-destructive top-electrode dry transferring method,which is highly desired.In this process,the prepatterned fan-shaped gold electrodes are covered by a～500 nm perfluoro(1-butenylvinylether)polymer(CYTOP)and then readily stripped and transferred to target monolayer molecular crystals by a tweezer.Under the assistance of this method,high performance devices are fabricated and charge transport anisotropy ratios of B 1.15 and 2.15 are obtained for 2,6-bis(4-hexylphenyl)anthracene(C6-DPA)and 1,4-bis((50-hexyl-2,0-bithiophen-5-yl)ethynyl)benzene(HTEB).The low anisotropic ratio of C6-DPA would facilitate the future applications in monolayer molecular crystal OFETs arrays without considering the influence of crystal orientation.