The Colliding Dynamics Of The Elementary Excitations In Quasi-one-dimensional Organic Semiconductors

In recent years, with improvement of experimental instruments and the development of theoretical methods, the research on organic semiconductors has been developed into a systematic and multidisciplinary field. Organic semiconductors can be broadly classified into two categories: small molecules or oligomers and polymers, in which polymers attract much attention and research. As a new kind of functional material, polymers have been the focus of the research work both because of the processing and performance advantages for low-cost and large-area application. In the past twenty years, many photoelectric devices based on conjugated polymers have offered promise for use in applications from experiment, such as light-emitting diodes (LEDs), field effect transistors photovoltaic cells, etc.. The work principle of these devices is mainly based on the physical processes involving charge injection, charge transport and electron-hole recombination.Contrast to the traditional semiconductor, organic semiconductor has its unique properties. First, most of polymers have qusi-one-dimensional structure due to the weak interaction force between the organic molecules. Second, owning to its soft properties, there are strong electron-phonon couplings in organic systems. It is generally believed that these excitations, such as solitons, polarons and bipolarons, are related to charge carriers in conjugated polymers. By photoexcitation or electron-hole combination, excitons can be formed in conjugated polymers. These elementary excitations are of fundamental importance for charge transport and photoluminescence of conjugated polymers. Studies on the dynamical and collision processes of these excitations are elements to understand the optoelectronics for the organic materials.In the paper, based on the one-dimensional tight-binding Su-Schrieffer-Heeger(SSH) model, by using a nonadiamatic molecular dynamic method, we simulate collision between polaron (bipolaron) and triplet exciton in conjugated polymers. The main results for our investigation in this paper are in the following:1. Investigation for collision between polaron and triplet excitonThe results of our simulations show that the lattice structure and the electronic state of the polaron and the exciton are changed very much in the collision process. Firstly, at lower electric field , the polaron will combine with the exciton and formed a charged exciton. Secondly, at mediate field strength, the polaron and the exciton will pass through each other. In the process of the collision, the lattice oscillated by large amplitude and at the same time an energy level appeared in energy gap. This means that there are new localized states are excited due to the collision. Thirdly, In stronger field strength, the polaron will be dissociated after the collision with the exciton. Thereare three channels for the polaron-triplet reaction: (1) P_↓+T→P_↑+S , (2)P + T→G + P~*, (3) P + T→T + e, where P denote a polaron and the arrowdenote the spin state of the polaron, T a triplet exciton, S a singlet exciton, G theground state and e a free charge. The probability of each channel depends on theexternal electric fields. Due to the radiatively decay of the singlet exciton and theexcited polaron, the polaron-triplet collision will contribute to the efficiency ofelectroluminescence.2. Investigation for collision between bipolaron and excitonIt is found that when a bipolaron collides with an exciton, the bipolaron transfers some charges to the exciton. By this way, two new qusi-excitations are formed: one is polaron and the other is excited polaron. The excited polaron can decay to the ground state through emitting a photon. Therefore, the collision between bipolarons and triplet excitons can enhance the efficiency of electroluminescence.