The Interface Physics And Device Applications Of Two-dimensional Organic Semiconducting Crystals

Since the discovery of highly doped conductive polymers,the organic semiconductors have developed rapidly.Organic semiconductors are assembled through weak van der Waals forces,which make their physical and chemical properties(band gap,optical absorption,and molecular arrangement)highly tunable.In the past two decades,by optimizing the molecular structure,a variety of functional organic semiconducting materials have been synthesized.The device performance and stability have been significantly improved,which has promoted the application of organic field-effect transistors(OFETs),organic light emitting diodes(OLEDs),and organic solar cells.With the further development of organic electronics and optoelectronic devices,it is particularly important to understand the growth mechanism of organic semiconductors,device physics,and transport mechanisms.However,the research on these key issues is limited by the crystallinity of organic semiconductors(grain boundaries and defects,etc.)and the non-ideal interfaces of the devices(interface traps and impurities,etc.).These difficulties have increased the difficulty of studying the intrinsic properties of carrier transports in organic semiconductors.In recent years,organic single crystalline semiconductors have become an excellent platform for studying the intrinsic microscopic behavior of charge carriers in organic semiconductor devices,because organic single crystals can significantly reduce the effects of grain boundaries,traps,and defects and have long-range molecular order.Although the organic bulk crystals have the excellent mobility and stability,the carrier distribution and charge transport at the semiconductor/insulator interface cannot be accurately and directly studied.Besides,carrier injection at the metal/semiconductor interface will have a large contact resistance,and photo-generated carriers in the organic layer need a long diffusion distance,which will hinder the intrinsic research of the interfacial problems in organic devices.In this work,two-dimensional(2D)organic semiconducting crystals are utilized for fabricating the OFETs and four-probe devices to study the contact resistance,charge carrier distribution,and transports.This research not only solves the problem of the growth of large-area organic crystalline thin films with controllable number of layers,but also applies ultrathin crystals to the new four-probe device,providing guidance approach for accurately extracting carrier mobility of advanced electronic devices.We believe that an in-depth understanding of the microscopic charge transport mechanism in organic semiconductors will promote the development of organic electronics and optoelectronics.The innovative research results of this paper mainly include the following aspects:(I)We first study the contact problem of metal/semiconductor interfaces in organic field-effect transistors with organic polycrystalline thin films.We find that the access resistance through the bulk part of organic films severely influences the estimation of Schottky barriers and affects charge carrier transport.The Schottky barrier determines the carrier injection at the metal/semiconductor interface,while the access resistance dominates the contact resistance.The estimation of the Schottky barrier depends on the access resistance and the gate voltage.After the access resistance is eliminated,the intrinsic Schottky barrier exhibits a weak dependence on the work function of the metal.This work comprehensively studies the relationship between Schottky barrier and carrier transport in organic transistors,and provides strategies for further optimizing device performance and applications.(II)In order to eliminate the contact resistances in organic thin film transistors,we fabricate 2D organic crystalline films over a millimeter-sized area by thermally induced self-assembly methods,which based on molecule–substrate van der Waals(vd W)interactions.By controlling the annealing temperature,the number of layers can be precisely controlled.The bilayer OFETs exhibits excellent electrical performance,and its maximum mobility reaches 12.8 cm2 V-1 s-1.In addition,we find that single-layer crystalline films can serve as interfacial molecular template layers to fabricate heterojunctions with well-balanced bipolar transport behavior.This work shows the significances of high-quality 2D organic crystalline films for studying the microscopic behavior of intrinsic carriers,and open up a way for realizing complicated electronic applications,such as lateral heterojunctions and superlattices.(III)An effective approach to accurately control the carrier distribution with molecular-layer precision is essential to examine the interfacial carrier distribution and the correlation with the charge transport in molecular crystalline semiconductors.Here,we find that the carrier accumulation is strictly modulated in highly ordered,few-layer molecular crystalline semiconducting films by tuning the polaronic coupling between the charge carriers and dielectric.The admittance method reveals that the carriers are mainly concentrated in the first molecular layer on a high-? dielectric,while on the low-? dielectric,the carriers can extend to the second layer and show stronger delocalization.Therefore,dimensional transition of the charge transport in different molecular layers was observed,revealing the intrinsic correlation between the carrier distribution and motion dynamics.Furthermore,such molecular layer-defined charge accumulation and transport achieved using highly ordered,few-layer molecular crystalline films may reveal new quantum phenomena and yield new device applications.(IV)Due to the spatial confinement effect of carriers in high-quality molecular crystalline films,the carrier concentration at the semiconductor/dielectric interface increases,so the Coulomb interaction among carriers at the interface of the polarized oxide cannot be ignored.We further study the carrier transport in highly ordered,few-layer organic crystalline semiconductors by a geometryindependent gated van der Pauw(G-VDP)method,confining the charge carrier distribution at the precision of molecular layers and minimizing the effects of extrinsic factors.We find that the electrical performance of the device exhibits non-ideal characteristics.The Fr?hlich polaron model can explain this feature well,and shows that at high carrier concentration,the Coulomb interaction among carriers will increase the hopping barrier height by the characteristic parameter ???/2.This understanding of the physical process between Coulomb interactions and polaron transport in organic crystalline semiconductors can enable promising strategies for achieving more ideal transistor performance and new device physics.In this thesis,we explore the growth mechanism of high-quality organic crystalline thin films,the contact effect at the semiconductor/metal interface,and the intrinsic relationship between the carrier distribution at the semiconductor/insulator interface and the charge transport.It provides a comprehensive understanding for studying the physical mechanism of organic electronics and improving the performance and stability of organic devices.