Study Of The Opto-electronic Properties In The Organic Conjugated Polymers

Recently, conjugated polymers have attracted many interests because of a unique set of properties: the electronic properties of metals and semiconductors and the processing advantages and mechanical properties of polymers such as flexibility. As a new kind of functional material, conjugated polymers or small oligomers have been attracting much theoretical and experimental interest both because of the unique electric, magnetic and optical properties that occur in these materials and because of the technological potential of electronic devices fabricated from them. On the other hand, conjugated polymers have common characteristic of the soft condensed matters and can serve as a medium to understand other organic and even biological molecules.In the past twenty years, the conjugated polymers have offered promise for use in applications, particularly in the area of optoelectronics where semiconducting luminescent polymers can be used to fabricate large area, flexible devices such as light-emitting diodes, displays, integrated circuits, solar cells, and plastic lasers. In 2002, Dediu et al. observed spin injection into thin films of conjugated organic material sexithienyl (T6) in the sandwich structure of La_0.7Sr_0.3MnO₃/T₆/La_0.7Sr_0.3MnO₃ at room temperature. In 2004, Xiong et al. built a spin valve using the Alq3 as a spacer, where the Alq3 is sandwiched between layers of cobalt and half-metallic manganite La_0.7Sr_0.3MnO₃. Spin polarized injection and transport in organic semiconductor not only can broaden our understanding on the physical world of the organic materials but also can have a substantial impact on the applications of spintronics and bionomics.Compared with the conventional inorganic materials, in the organic materials the bond structures are sensitively related to the electronic occupation because of the strong electron-lattice interactions. Once the electronic occupation is changed, such as, through photoexcitation, the bond structure will undergo a distortion, which, vice versa, results in a reconstruction of the electronic states. The interdependence of electrons and lattice is an important characteristic of organic materials. Many peculiar phenomena of organic materials come of the strong electron-lattice interactions. Therefore, unlike inorganic materials, the primary carries in organic polymers are charged excitations, such as solitons, polarons or bipolarons, depending upon the degeneracy of the materials. In addition, in polymers, photoexcitations will lead the formation of a self-trapping exciton state, which has been widely considered to be responsible for the photoluminescence (PL). Studies on the charged carriers and the exciton have become to be the elements to understand the optoelectronics for the organic materials. In recent years, it has been found that the photoexcited formations of the charged carriers and the exciton are tightly related. How to see clearly the relation and control the translation between them has been the focus in these years in order to improve the performance of the optoelectronic devices.In this paper, in the framework of the extended SSH model, we have studied the excitonic formation in coupled polymeric molecules, and showed the factors that affect the luminescence efficiency for the polymers. It has been well known that the single photon excitation of an exciton will lead to the formation of a biexciton, which is found to be reversed polarized. We have investigated the effective methods to realize the reverse polarization from two points of view. In addition, we have studied the re-excitations of an exciton by a second photo-absorption and discussed the mechanism of exciton dissociation. It is found that the exciton dissociation is an effective method to create the charged carriers. The detailed research and main results are given as follows:1. Exciton formation with interchain couplings in conjugated polymersIt has been predicted in earlier studies that interchain coupling has a visible effect on the exciton binding energy in a solid-state sample. The binding energy is one of the key parameters that govern the physics of opto-electronic organic devices, and effect of the interchain couplings on it has become to be an attractive question in these years. Both the photoexcited electron-hole pair and the injected one are more easily to combine into an exciton, where the formed exciton has a stronger binding energy. Therefore, Based on the framework of the tight binding approach and the nonadiabatic dynamics, the formation of an exciton in inter-coupled polymer chains is studied.1.1 It is found that two possible excitonic states can form due to the photoexcitations: one is that an exciton is mainly localized in one single chain (a localized exciton), and another is that an exciton is spread averagely between chains (a spread exciton). By calculating the dependences of the creation energy and the binding energy on the interchain couplings, we obtain that (1) the localized exciton is energetically favorably produced to the spread one; (2) the localization of an exciton is stronger than that of a polaron; (3) the binding energy of a localized exciton is always larger than that of a spread one, and the interchain coupling can remarkably reduce the exciton binding energy. The result shows that one can get the required exciton binding energy by controlling the aggregation state of the polymer chains for different applications of the opto-electronic organic devices. In photovoltaic devices, one would like the binding energy to be as small as possible to produce charge carriers efficiently. While in light emitting devices, one would like to suppress the production of charge carriers and enhance the production of excitons, and in this case, a large binding energy should be expected.1.2 Furthermore, the excitonic formation dynamics in a two-chain system is studied. Here effect of the interchain couplings is considered in two ways: one is to freeze the neighboring chain and study the exciton dynamics in one single chain; the other is to treat the chains equivalently and allow the exciton to spread between chains. It is found that with the increasing of the interchain coupling, an exciton needs a long time to form. The present investigation shows that the response time of polymeric molecules in a dilute solution should be faster than that in a solid film.2. The reverse polarization phenomenon in conjugated polymersFemtosecond technology has provided a powerful tool to study an ultrafast process, especially to explore new properties of excited states. The transient and dynamical process in the excited states can be revealed in detail. By using femtosecond spectroscopy, it is found that in polymers the primary photoexcitations are intrachain excitons. And very recently, the biexciton, which is a sequential reexcitation of the intrachain exciton, is observed in PPV. In 2000, Sun et al. theoretically predicted a new photoinduced phenomenon called photoinduced polarization reversion (PPR) by studying the double photon excitation of a perfect Ï€-conjugated polymer chain. The polarization direction is opposite to the direction of the induced electric field when the polymer chain absorbs two photons to form a biexciton.When a polymeric molecule absorbs one photon, its electric dipole will be reversed. This phenomenon was called photoinduced polarization inversion (PIPI). Now PIPI has been found in several other materials. Such a switching effect is very important not only in the basic study, but also in the technology application.2.1 Effect of interchain coupling on the inversed polarization of a biexciton has been studied in the framework of a tight-binding approximation. It was found that a biexciton state is mainly confined in one chain even the interchain coupling is included. The inversed polarization will be enhanced remarkably with the interchain coupling in the case of a weak non-degenerate confinement. However, the inversed polarization will keep nearly unchanged for a strong non-degenerate confinement.2.2 Reverse polarization can be obtained by a double photon absorption, which is a peculiar property of organic Ï€-conjugated molecules. It has been predicated that a photon-switch device could be designed by the reverse polarization of a biexciton in high-conjugated polymers. However, it is not easy to get a double photon process in experiment. In present work, a little easy reachable method to get the reverse polarization was predicted by single photon absorption for a charged oligomer. It was found that strength of the reverse polarization is conjugation-length dependent. Reverse polarization only exists in small molecules and it reaches a maximum value at a suitable conjugation-length. Although the e-e interactions are included with a fixed parameter U=2.0eV and V=U/3, it was found that the reverse polarization does appear when the e-e interaction is not too strong. We could not give a clear picture about the effect of e-e interaction as the mean-field treatment is questionable in the case of large U. In addition, we indicated that the reverse polarization is a nonlinear behavior with the induced electric field. Effect of the interchain interaction as well as the non-degenerate confinement of oligomer molecules on the reverse polarization was studied. To verify the reverse polarization experimentally, it was suggested that the experiment should be carried out on solid-state samples and the oligomer molecules must be synthesized with a minimized confinement effect.3. The mechanism of charge carriers photogeneration in conjugated polymersIt has been predicted that polarons or bipolarons are the main charge carriers in organic polymers. The photogeneration mechanism of these carriers and their yields have previously been the main focus of study. As early as 1960s, investigations have been made on charge carrier photogeneration in some molecular crystals, such as anthracene crystals, which were explained in terms of the Onsager theory of geminate recombination. In recent years, different experimental data have been interpreted to support two very different concepts concerning the charge carrier photogeneration in polymers. Heeger et al. indicated that both charge carriers (polarons) and neutral excitons are independently generated via optical interband transitions, and the process is independent of the electric field. On the other hand, a number of experiments carried out on derivatives of poly(p-phenylene vinylene) and methyl substituted ladder-type poly(p-phenylene) (m-LPPP) considered charge carrier generation as a two-step process, in which neutral excitons are first created due to photoexcitation and then dissociate into charge carriers through re-excitation with electric field or temperature assisted. Although most of the experimental data favour the latter method, the mechanism of exciton dissociation is still a heavily disputed question. By means of electric field modulated femtosecond pump-probe absorption spectroscopy, Gulbinas et al. showed that two different exciton dissociation mechanisms take place depending on the excitation photon energy. When the excitation energy is relatively low, excitons slowly dissociate into charge pairs during the entire exciton lifetime, and the process is strongly electric field dependent. When the excitation energy is high, charge pairs appear fast and require less assistance of an electric field for the exciton dissociation. Unfortunately, it is impossible to distinguish the "charge pairs" between free charge carriers participating in photocurrent and Coulombically bound charge pairs not participating in photocurrent.3.1 Charge carrier photogeneration is important for photovoltaic cells or electrophotographic devices, while the opposite process (recombination of a charge pair into an exciton) determines the operation of light emitting diodes. According to the experimental data favoring charge carriers resulting from exciton dissociation, we theoretically studied the process of exciton dissociation by considering two kinds of optical transitions for an exciton. The calculation supports the conclusion that photocurrent in conducting polymers may result from a dissociation of an exciton. It was found that the two kinds of optical transitions have a different dependence upon the external electric field.3.2 In addition, the interchain coupling has a certain effect on the exciton dissociation, which makes the different quantum efficiency of charge carrier photogeneration in a solid-state sample from that in a solution. The theoretical calculation is qualitatively consistent with the experimental observations. Further investigations should be done in the future that include additional physical processes, such as the interchain charge transfer, strong external fields, and the thermal effect.3.3 From the nonadiabatic dynamics, we simulate the opto-electronic conversion in the polymeric materials, and the possible applications for that are also included.