The Effects Of Temperature On The Dynamic Properties Of Bipolaron In Organic Polymers

With the in-depth study of organic polymers,people have more understanding of the dynamic properties of the carriers(such as polaron,bipolaron and other nonlinear elementary excitations).The conductive and luminescent properties of these nonlinear elementary excitations which play an important role in organic devices(LED,FET)have attracted widespread attention.As a common self-localized excitation,the bipolaron plays an important role in organic polymers just like polaron.It is worth mentioning that the study of the dynamic properties of bipolaron provides more channels for improving the electroluminescence efficiency in organic polymers.At present,people not only analyze the formation and stability of bipolaron and its dynamic characteristic under the action of external field,but also analyze the collision and composite dynamic processes between the bipolaron and other elementary excitations.It is worth noting that the important factor of temperature has been ignored in the continuous research on the dynamic properties of bipolaron under the action of external field.In this paper,based on the tight-binding SSH model and nonadiabatic molecular dynamics method,and by using the Langevin equation of motion to introduce the effect of temperature,we investigated the effect of temperature on the stability of bipolaron and on the collision and combination of oppositely charged bipolarons in electric field effect.It is found that these processes affect the electroluminescence efficiency in organic polymers.The main contents are as follows:1.The effect of temperature on the stability of bipolaron.(1)The friction constant has a certain effect on the system to reach thermal equilibrium.The higher the friction constant is,the easier the system reach thermal equilibrium.When the friction constant has a fixed value,the temperature will not change the time of system reaching thermal equilibrium.(2)As the temperature increases,the random motion of bipolaron becomes drastic,and the bipolaron still maintains obvious lattice defect,but the polaron is dissociated under the same condition.(3)In this chapter,thermal effects on energy of system are also discussed.At lower temperatures,the electrons lose energy,and the energy shifts to the lattice.At higher temperatures,the lattice loses energy,and the energy shifts to the electrons.2.The effect of electric field on the composite dynamic properties of oppositely charged bipolarons.Under the action of electric field,two relatively independent bipolarons with opposite charges move towards each other and collide,forming the elementary excitations,such as exciton,biexciton and excited polaron.The results show that the yields of these elementary excitations depend on the electric field strength.At weaker electric fields,the total yield of luminescent products in this process is relatively low.With the increase of the electric field strength,the total yield of luminescent products gradually increases,the maximum yield of luminescent products appears when the electric field is in the range of 1.4mV/?~1.7mV/?.At stronger electric fields,the total yield of luminescent products decreases gradually.Continue to increase the electric field,the stability of the oppositely charged bipolarons will be affected.Under strong electric fields,the oppositely charged bipolarons will dissociate after collision.3.The effect of temperature on the composite dynamic properties of oppositely charged bipolarons.(1)When only the temperature is applied,two bipolarons move randomly.With the increase of temperature,the random motions of two bipolarons become more drastic,but they are not combine.(2)Considering the effect of temperature,the combination process of the oppositely charged bipolarons changes greatly under the electric field.The temperature promotes the formation of luminescent products under lower or higher electric fields,and the yield of luminescent products varies greatly.In a range of mediate electric fields,the temperature makes the yield of various luminescent products fluctuate but the change is not obvious,and the electric field still plays a big role in the formation of luminescent products.The results of this paper show that the electric field and temperature affect the yields of exciton,biexciton and excited polaron.Therefore,the selection of electric field and temperature parameters to increase the yield of luminescent products is also a way to improve the electroluminescence efficiency.