The Dynamics Of The Polaron In Organic Conducting Polymers

The fact that the polymer polyacetylene reaches metallic levels of conductivity when chemically doped caused a stir when it was first reported in 1977. From then on, there has great interest in the studying of conductive conjugated polymers. As a new kind of functional material, conjugated polymers or small oligomers have been the focus of the research work both because the processing and performance advantages for low-cost and large-area application. The unique electric, magnetic and optical properties that occur in these materials have been realized and utilized. At present, numerous high-performance photoelectric devices fabricated from organic polymers have been made including light-emitting diodes, field effect transistors, photovoltaic cells, etc.Conducting polymers differ from saturated polymers in that each carbon of the main chain is bonded to only three other atoms. In conducting polymers, three of the electrons on each carbon reside in σ -bonding orbitals while the fourth electron resides in a delocalized p_z-orbital. The p_z-orbital of neighboring carbons overlap to form conjugated π bond, so conducting polymers can be called as conjugated polymers.Contrast to the traditional semiconductor, organic material has its unique properties. First, most of the conjugated polymers have the 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. Especially, the additional charge (electrons or holes) or photoexcitation in conjugated polymers will induce lattice deformation; on the contrary, the lattice defect will have the effect on the energy band structure of the organic system. So, it is generally believed that these self-trapping excitations, such as solitons (only in trans-polyacetylene), polarons and bipolarons, are related to the charge carriers in conjugated polymers. These elementary excitations are of fundamental importance for transport properties photoluminescence of conjugated polymers system. The qusi-particles are the compound states of charge and lattice, i.e., localized charges associated with a lattice distortion due to the strong electron-phonon (e-ph) interactions, which have different charge-spin characteristic to the traditional electron of hole. Under the driven of the external electric field, the charge gets energy and moves firstly, and at the same time it will drag the lattice atoms to motion. The dynamical processes of these excitations are one of the major and fundamental issues of interest in the organic electronic devices such as organic light-emitting diodes (OLED) and organic spintronics.Since 1970's, SSH Hamiltonian, the tight-binding semi-empirical calculation method that was found by Su, Schrieffer and Heeger has been demonstrated successfully for determining the electronic structures and optical properties in conjugated polymers. In the later years, Bishop, Sun and Xie et al have extended the SSH Hamiltonian to research the static and dynamic process of excitations. The further research in this field not only can broaden our understanding of the microcosmic physical world but also can have a substantial impact on the applications on organic polymer devices.With a tight-binding electron-phonon interacting model, we investigated the properties of the charged polaron in conjugated polymers. By using a nonadiabatic dynamic method, we have simulated the dynamic process of the formation, dissociation and recovery of a negative charged polaron and studied the physical mechanisms. In addition, we have also explored the effects of the electric mode on the dynamic process of a polaron. At last, the behavior of the polaron in multi-chain systems has been investigated. The detailed research and main results are given below:1. Polaron formation dynamics in conducting polymersThere are two ways for the formation of a polaron in conjugated polymer, one is via injection or doping, another is via photoexcitation. One hand, An et. al. studied the dynamic relaxation processes of various photoexcited states. They believe that photo-generated charge carriers (charged polaron) are the direct products after the photoexcitation. On the other hand, due to that the conjugated polymers are quasi-one-dimensional systems with strong electron-phonon interactions, it is generally believed that the injected electron or hole will induce lattice deformation and to form soliton, polaron. At present, the researches about the polaron formation are only confines to that the electron with a very low energy injection. However, the cases of the hot electrons with high energy injection are unknown. Moreover, under the strong electric field, a polaron will dissociate. When the field is decreased or turned off, whether the polaron can recovery from its dissociated state is need further research. Because the density of the polaron can affect the luminous efficiency and the conductivity of the photoelectric devices, it is significant to study the formation of the polaron. My investigations include,1.1 The electric-phonon (e-ph) coupling effect plays the crucial role in the formation process of a polaron. We analyze e-ph interaction force when an injected electron occupies various molecular orbitals in a dimerized polymer chain. It is found that when the electron distributes near the LUMO (the lowest unoccupied molecular orbital), the e-ph coupling effect is prominent. A polaronic state can be formed easily. With the electron occupying the orbitals with higher energy, the e-ph coupling gets weaker and delocalized. If the electron distributes around the HUMO (highest unoccupied molecular orbital), e-ph coupling effect is almost negligible, which means that the extra electron could not induce any lattice distortions.1.2 The formation of a polaron depends upon the energy of the hot electron. Usually, a low-energy injection is favorable for the formation of a polaron. But a high-energy injected electron is hardly to couple with the lattice to form a localized polaron.1.3 A dissociated polaron propagates in the form of a free-like electron and performs oscillations both in real and momentum spaces, which are the Bloch oscillations (BO's) in the organic system. The electron does not oscillate anymore when the electric field is turned off. It is found that whether a polaron recovers or not depends upon the time when the field is turned off (or where the electron distributes in the momentum space). The reason still traces back to the e-ph interactions stated above. If the electron occupies around the LUMO at the time when the field is turned off, the e-ph interaction is strong and the polaron is easy to recover. Otherwise, the e-ph interactions are weak and it is difficult for the recovery of a polaron.2. Effect of the electric field mode on the dynamic process of a polaronThe motion of the polaron is believed to be of fundamental importance for the transport properties of conjugated polymers for the use in, eg., polymer based LEDs. Up to now, many studies have focused on the field strength dependence of the charge carrier motion in the presence of a constant external electric field. In fact, a polaron is a coupled state of an electron (or hole) with the lattice. Since the transport process involves the response of the slow lattice deformation to the fast moving electron, the mode of the application of the field also becomes important.2.1 Field strength dependence of the polaron motion. Polaron dissociation will experience two sequent transitions under the high fields; one is the transition from the subsonic to the supersonic state, and the other from the supersonic to the dissociated state. The acoustic mode is decoupled from the charge when the polaron moves at a speed faster than the sound speed, and then the optical mode is decoupled at the second transition to make the polaron dissociate completely. The polaron have a saturated velocity under a fixed electric field, the maximum value is about four times of the sound one.2.2 Field applying rate dependence of the polaron motion. By raising the electric field from zero to a value linearly within various periods, we could control the applying rate. It is found that compared with a fast-applied field, a slow-applied field is favorable for the transportation of a polaron as an entity, which is because that the acoustic phonons can catch up with the electronic charges in a slowly-applied field, and the polaron moves with the velocity about the sound. Both the polaron dissociation and the transition of its velocity from the subsonic to supersonic value are found to be mode-dependent.3. Effect of the inter-chain coupling on the dynamics of the polaron In actual organic materials such as solid films of polymers used in OLEDs, the lengths of molecular chains are finite and the molecular chains are not isolated but interact with each other. It is certainly that a polaron will hop among chains during its transport process. Interchain interactions play an important role for high conductance and other electrical properties of organic polymers.3.1 We have studied the polaron dynamics in a system of coupled polymer chains in the presence of an external electric field. It is found that when the polaron reaches a chain end and whether it can be scattered to the surrounding chain depending on the strength of the field and the interchain transfer integral. The dependence of the electric field critical value and the strength of the interchain coupling are presented.3.2 The overlap length has effect on the dynamics of the polaron. The overlap length which is almost equal to the width of the polaron is favorable to the interchain migration of a polaron. When a geometrical defect exists in the coupling region, the polaron can not be scattered from one chain end to the adjacent chain even under a very strong electric field.