Research On Carrier Relaxation Mechanisms In Organic Semiconductors

From the beginning of the discovery of organic semiconductor materials to the commercial application,they have always brought people a new understanding of physics.The first is the study of the conduction mechanism of organic semiconductor materials.The carriers of organic semiconductor materials are quasi-particles such as soliton and polaron.The electronic energy band structure of the material in the ground state and the excited state is completely different,and doping can drastically change the conductivity of the material.The decoration of organic semiconductor materials has promoted their application in light-emitting devices.The characteristics of low cost and easy processing of organic materials have promoted their continuous research in the field of solar cells.The highest power conversion efficiency of organic solar cell has exceeded 18% at present.In addition to the progress of organic electronics,the field of organic spintronics also has rapid development.Organic spin valves have physical phenomena completely different from traditional spin valves.In addition,the new properties of organic semiconductor materials or devices under multi-field regulation are also emerging in an endless stream,such as organic magnetic field effects,organic multi-iron,organic pure spin currents,etc.These studies revealed that organic semiconductor materials have complex interactions in terms of charge,spin,and phonon.Based on the recent theoretical and experimental research progress of organic electronics,this paper studied the two physical processes of carrier relaxation in organic materials.They are the spontaneous localization mechanism of carriers in organic semiconductor materials and the phenomenon of organic long-persistent luminescence in organic light-emitting devices,respectively.Due to the strong electron-lattice interaction in organic semiconductor materials,the coupling of the charges injected into the material and the lattice will form a polaron,which makes the overall energy of the system lower.These results have been confirmed in in experiment and theory.About the cause of the polaron,it is generally considered to be the symmetry breaking factors in organic materials,such as defects,impurities,etc.Recent first-principles calculation results show that for fully polyacetylene molecules,after injection of electrons,the electrons can spontaneously form polarons.The authors concluded that the spontaneous formation of polaron is caused by the effect of electron association.Here,we use the quantum dynamics method of tight-binding approximation to study the dynamic process of polaron generation in polyacetylene molecules under different initial conditions.By analyzing the changes of its molecular configuration,charge density and energy,we analyzed the physical mechanism of this spontaneous localization,providing a new idea for a better understanding of the localization of electronic states in materials.Our research has found that even if there is no symmetry breaking factor,polarons can be spontaneously generated in fully ordered polyacetylene molecules,and the spontaneous generation of polarons is related to the boundary effect of finite-length molecules.Organic long-persistent luminesce is generally a characteristic of certain inorganic materials(also known as night pearls),and the organic long-persistent luminesce time of organic materials generally cannot reach the order of seconds.The Adachi research group has discovered organic long-persistent materials that can luminous in the order of tens of seconds or even hours.We constructed a model of an organic long-persistent luminesce system composed of a donor and an acceptor,and used Marcus transition theory and quantum random walking method to simulate the transition of carriers between molecules,the formation excitons through attraction between positive and negative polarons,the physical process of composite luminescence in the end,and compared with the experimental results.Then by changing the system temperature,donor molecule concentration and other parameters to study how external conditions affect the luminous efficiency and luminous time.The study found that the light-emitting time will be more shorter with the increasing of temperature and donor concentration.