Scattering Processes Among Polaron, Bipolaron And Exciton In Conjugated Polymers

Conjugated polymer is a new kind of functional material.It has the electronic properties like metals and semiconducors, and also has the properties such as flexibility, low-cost and easy-shaping. So it attracts much attention in resent 20 years. At present, many photoelectric devices based on conjugated polymers have been developed and offer for practical use, including light-emitting diodes (LEDs), field effect transistors (FETs), photovoltaic cells, etc..Due to the strong electron-phonon coupling, carriers in conjugated polymers are not conventional electrons and holes in inorganic semiconductors but are solitons, polarons or bipolarons. These excitations are self-trapped states in which electrons (or holes) and lattice coupling together. Beside these excitations, there also exist other nonlinear excitations such as excitons and biexcitons. There are abundant transportation processes about these excitations moving in conjugated polymers.People have done much research on single soliton, polaron or bipolaron moving in polymer chains and obtained a number of findings.People also pay much attention to the process in which opposite charged polarons capture each other and form exciton.A series of theoretical and experimental method are developed to study this process.However, to date, the polaron-exciton, bipolaron-exciton and polaron-bipolaron scattering processes have not arouse much attention. Very little about that is known. Therefore, we hope that through studying these scattering processes we can address how the electronic state of the system evolve, what the products of the scattering process and how these scattering processes affect the transportation and luminescence properties of conjugated polymers.Molecular dynamics method has been widely used to studying the polaron dynamics and polaron pair recombination process and has been testified to be a valid method. In this paper, using non-adiabatic molecular dynamics we systematically investigate a series of scattering processes, including bipolaron-exciton, polaron-bipolaron and polaron-exciton, and obtained some valuable results.1.Scattering process between bipolaron and excitonThrough modifying the Su-Schrieffer-Heeger (SSH) Hamilton and making it include the electric field interaction, we simulate the scattering process between a negative bipolaron and an exciton using non-adiabatic molecular dynamics method. It was found that when a bipolaron colliding to an exciton, there are mainly two react channels:(1)the bipolaron transfers an electron to the exciton, so that the excton is converted into an excited polaron and the bipolaron changed to be a polaron;(2) the bipolaron transfers all its charge to the exciton, so the bipolaron and the excton convert into each other. Using a projection method, we calculated the probabilities of the two channels. In low external electric fields, the probability of the first channel is about 50%, while the second about 25%.In high external electric fields, the probability of the first channel is about 60%, while the second abut 20%.It is an important result that bipolaron and exciton can react and produce a polaron and an excited polaron.The excited polaron can decay to the ground sstate through emitting a photo, which indicated that the bipolaron-exciton scattering may have effects on the electroluminescence efficiency of conjugated polymers.2.Scattering process between a negative polaron and a positive bipolaronAlso based on the SSH Hamilton being modified to obtain the electric field and electron-electron interaction together, we simulated the scattering process between a negative polaron and a positive bipolaron in a single polymer chain using non-adiabatic molecular dynamics. It was shown that there exists a critical electric field.If the external electric field is lower than the critical electric field, there exist mainly three channels for the polaron-bipolaron reaction:(1)there is no electron transfer between polaron energy levels and bipolaron energy levels, the polaron and the bipolaron simply coalesce into a large lattice distortion named "trion"; (2) the polaron and the bipolaron recombine into a positive excited polaron;(3)the polaron is detrapped and forms free electron.If the external electric field is higher than the critical field, the polaron and the bipolaron will pass through each other but do not react.Like the bipolaron-exciton scattering process, the polaron-bipolaron scattering process also produces excited polaron.So this scattering process also may affect the efficiency of electroluminescence due to the excited polaron decaying to the ground state through emitting a photon.3.Scattering process between a negative polaron and a triplet excitonUsing the Hamilton as same as that we used in the polaron-bipolaron scattering process, we simulate the scattering process between a negative polaron and a triplet exciton.Calculations have shown that the results of this scattering process have close related to the spins of the poalron and the triplet exciton:(1)if the polaron has parallel spin to the triplet exciton, there exist very weak repulsion between them. Under the influence of the external electric field, the polaron can easily pass through the exciton.(2) if the polaron has opposite spin to the triplet exciton, the repulsion between them is very strong. In low external electric fields, the polaron is bounced back by the exciton when colliding to it. In high external electric fields, the polaron can pass through the exciton.If the electron-elecron interaction is enough strong the polaron dissociates when colliding to the exciton.In all kinds of conditions, the polaron always can not break the exciton through colliding because its binding energy is much smaller than the exciton binding energy.Our results show polaron is hard to react with exciton.This is mainly due to the large energy shift between the polaron energy levels and the exciton energy levels in the gap. Large energy shift makes charge transfer between polaron and bipolaron difficult, so that they are very hard to produce new products. In addition, large energy shift also indicate their binding energy difference is large. This induce the polaron always precede exciton to dissociate.These results need to be testified by the experiments.We first systematically studied the scattering processes among polaron, bipolaron and exciton.This deepened the understanding about the properties of conjugated polymers. By these works, we see that these scattering processes have abundant physical picture, and these results are valuable to understand the properties of conjugated polymers.Moreover, we have seen that these scattering processes do have important effects on the transportation and luminescence of conjugated polymers. We hope further investigations about these scattering processes in the future.