Study On The Organic Chiral Resistance

Organic optoelectronic materials are semiconducting in electrical,magnetic,and optical properties and soft,corrosion-resistance,and low-preparation-cost in chemical and mechanical properties.Therefore,organic optoelectronic materials play an important role in the new generation of functional devices such as organic light-emitting diodes and organic solar cells.The side-chain modification,doping,synthesis method,morphology,and structure of organic layer materials will have significant impacts on the device performance.In addition,the successive discoveries of spin injection and transport,organic spin-valve effect,and organic magnetoresistance effect in organic materials in the early 21th century gave birth to organic spintronics.At present,there has been a rapid development of experimental and theoretical research on the conductance of organic optoelectronic materials,the electrical.magnetic,and optical properties of organic heterojunctions,the organic magnetoresistance.and the spin selectivity in organic chiral materials.Existing theoretical models can explain experimental phenomena to a certain extent while some contradictions remain to be explained.In organic optoelectronic materials,there are two ways of electron transport:tunneling transport and hopping transport.Because of the existence of strong electron-phonon coupling,the carriers are polarons and bipolarons in localized states instead of free electrons or holes in extended states.These localized states will determine the transport properties of material and changes in lattice properties in the material will cause changes in transport properties.Organic optoelectronic materials can be divided into achiral materials and chiral materials.The chirality of a material has an important influence on its electrical,magnetic,and optical properties.At the end of the 20th century,the asymmetry of spin of electron transmission through chiral materials was found in experiments for the first time.With continuous deepening of the research on the chiral-related electrical,magnetic and optical properties exhibited by chiral materials,the research on organic chirality has become a frontier hotspot in the field of organic optoelectronic materials in recent years.In addition.spin-dependent transport properties of chiral materials make it to be expected to construct novel organic devices,such as chiral valves and chiral resistance.Considering the properties such as electron-phonon coupling and the richness of material types,this paper consists of the following two parts:the first part focuses on the regulation of electron hopping transition by manipulating lattice properties based on the one-dimensional tight-binding model;the second part presents the concept of chiral resistance and the regulation method of the chiral resistance in tunneling transport is studied.1.Isotope effect of carrier transport in organic materialsCarbon and hydrogen,which are the main constituent elements of organic materials,have isotopes in nature.In recent years,many experiments have found that isotopic substitution will affect the properties of materials such as their volume and their thermal conductivity.For devices,isotopic substitution will affect their optoelectronic properties,lifetime,and stability.For organic semiconductor devices,their performance closely relates to carrier mobility.Because of the strong electron-phonon coupling existing in organic materials,isotopic substitution will affect carrier mobility through(CH)groups.According to the experimental results of isotope effects in electrical,optical,and magnetic processes in organic semiconductors,in chapter 3,we adopt the tight-binding model with strong electron-phonon coupling to study the isotope effects on polaron transport in molecular crystals and polymers under diabatic approach.By introducing the effective mass of polaron,we revealed the relationship between mobility and the mass of lattice group.Our results show that the existence of deuterium and 13C element will reduce the mobility of organic materials,and the magnitude of the isotope effect related to the strength of the electron-phonon coupling.When isotopic substitution concentration remains constant,the distribution of isotopic elements does not affect the averaged mobility of the whole device.Our results provide theoretical support for manipulating device performance using isotopic substitution and a theoretical basis for material selection in device fabrication.2.Organic chiral resistanceMany organic molecules are chiral.In recent years.experiments have found that the properties of chiral molecules in electrical transport,spin-dependent transport,magnetism,optics,and electrochemistry are all chirality-related.These studies show that chirality,as one of the important properties of organic molecules,has significant application potential.Heterojunctions composed of layers of dissimilar materials are widely used because their resistance can be easily regulated by external fields.Because of the different spin preference between chiral molecules and their enantiomers,chiral molecules can be used to construct heterojunction structures in which resistance can be manipulated by the chiral configuration relationship of the molecules.In chapter 4,the model of the heterojunction structure composed of two chiral molecules is constructed.Considering the spin asymmetry in chiral molecules,the chiral-induced spin-orbit coupling is used to characterize the molecular chirality,and the electron transmission spectra and current-voltage curves in the heterojunction are calculated.The results show that the current of the heterostructure with the same chirality is greater than that of the opposite chirality.We defined the relative difference between the two as chiral resistance and explained it using the two-current model.Further calculations show that increasing molecular length,applying extensive strain,changing interface coupling,and changing chiral inversion ratio can increase the chiral resistance.This study contributes to the understanding of transport properties in chiral heterojunctions and the design of novel spintronic devices without magnetic elements.