Study On The Effect Of Electrostatic Field In Organic Polymers On Singlet And Triplet Excitons

After more than 40 years of development,organic semiconductor materials,with their excellent photoelectric and spin characteristics,have opened up a broad prospect in scientific research and applications.Compared with inorganic semiconductor materials,organic semiconductor materials have unique advantages such as flexibility,light weight,low price and structure modulability,which make up for the disadvantages of inorganic materials in the development of electronics.Organic electronic devices such as organic light-emitting diodes,organic solar cells,organic lasers,organic spin valves and so on have been continuously developed,showing great potential in applications.However,organic semiconductor materials also have some disadvantages,such as short life,poor electrical conductivity and so on,which restrict the development of organic electronics.So the research on organic semiconductor materials has a long way to go.At present,there are many research methods for organic semiconductor materials,such as quantum dynamics method,TO model of organic small molecules,density matrix theory,first-principles calculations and so on.The research method used in this paper is a tight-binding SSH model,and in order to study the spin-dependent characteristics of organic semiconductors,spin correlation is added based on the original model.Because of the strong electro-acoustic coupling interaction inside the organic semiconductor material,the charge will exist in the form of localized state,forming quasi-particles such as soliton,polaron and exciton.Exciton plays an important role in electronic devices such as organic light-emitting diodes and organic solar cells.Organic light-emitting diode(OLED)is a kind of device with electroluminescent process as its main principle.The working process in the device is all carried out in the external electric field.The charge is injected from positive and negative electrodes,and forms a positive and negative polaron in the organic layer,and the positive and negative polaron meet to form an exciton.According to the different spin configurations,excitons can be divided into singlet exciton and triplet exciton.According to exciton statistics,the ratio of both generation is 1:3.Because of the spin-exciton exclusion of triplet exciton,only singlet can transition to ground state and emit photons.It is very meaningful to discuss the proportion of single-state excitons generated in the system.Nowadays,there are many physical mechanisms to explain the phenomenon that the singlet exciton yield in the system does not accord with the exciton statistics,including TTA mechanism,TPI mechanism,RISC mechanism and so on.Meng et al.also found that spin-orbit coupling can increase the ratio of singlet exciton generation.In organic semiconductors,spin polarization is weakened and spin mixing occurs due to spin-orbit coupling and other spin-related actions.In this paper,the effect of external electric field on exciton and exciton yield in spin-mixed organic semiconductor system is discussed.The application of external weak electric field results in charge polarization of excitons in the spin mixing system.At this point,the proportion of excitons generated by different spin configurations can be obtained by projection method.We find that the external electric field will lead to the decrease of singlet exciton yield,which is more obvious in the system with strong electron-electron interaction and electro-acoustic coupling interaction.Moreover,the spin mixing in organic semiconductor system leads to the change of exciton dissociation critical electric field,in addition.In addition,we discuss the effect of spin flip effect induced by hyperfine interaction on lattice points on exciton yield in spin mixing systems.We found that hyperfine interactions lead to a decrease in the proportion of singlet exciton generation.Especially in the weaker superfine field,the yield is more sensitive with the change of interaction intensity.