Theoretical Study On Trions In Conjugated Polymers

Organic conjugated polymers are a new type of materials with advantages forlarge-area applications. Compared with the traditional inorganic materials, organicsubstances have many virtues such as low-cost, easy processing, and so on. In recentyears, many optoelectronic devices based on conjugated polymers have been transferredfrom experiments in lab into industrial products. Recent developments in the organiclight-emitting diode, photovoltaic cell, and single-crystal field-effect transistor, andstudies of magnetic action on these devices, have promoted effectively the progress ofoptoelectronics. Especially, the organic electroluminescence now has became the mostactive field of the fastest growing technologies for flat-panel and/or large area flexibledisplay, data information storage and processing. However, due to the complexity oftheir structure and properties in conjugated polymers, there are still many physicalmechanisms in this field are opened. Such as the physical mechanism of nolinearexcitations, the mobility ratio of charged carriers, the operating principle andlight-emitting efficiency of light-emitting devices, etc.Due to its low-dimension characteristic, additional electrons or holes in theconjugated polymer chain will induce self-localized excitations, such as polarons andbipolarons. It has been generally accepted that the charge carriers in conjugatedpolymers are comprised of these excitations which include both charges and latticedistortions. The theoretical research on the mobility and the light-emitting of theseexcitations in polymers can help us to understand some novel phenomenon in lowdimension system, and provide theoretical guidance for improving the light-emittingefficiency in organic light-emitting devices and designing new type of organic functionalmaterialsThe dynamic motion and recombination process of these excitations are believed tobe of fundamental importance in understanding the operation of optoelectronic devicesbased on conjugated polymers. Several theoretical studies have recently pointed out thatscattering and/or recombination processes between polarons, biploarons, excitons, andother excitations, may form a new type of self-localized excitations, trion. The trion can not only conduct, but also light, so it must play an important role in determining thecharge motion and electroluminescence efficiency in conjugated polymers. It has beenwell accepted that the decaying of trion by radiation luminescence is the primarymechanism for some physical processes accompanied by light-emitting. The discoveringof trion provides a new channel to improve the light-emitting efficiency in conjugatedpolymers. Therefore, study on the formation mechanism and photoelectric property oftrion is valuable in both the theoretical and the practical fields.The mobility ratio of carriers is a key factor that determines the operation efficiencyand response time in organic devices. It is accepted that the van der Waals force betweenmolecules induces the decrease of mobility ratio of carriers. One of the principal tasks inboth the theoretical and the practical fields is to find the proper organic materials withhigh carrier mobility ratio in the room temperature. Most of the previous works focus onthe dynamic motion of the polaron while the contributions to the charge/spin motionfrom the bipolaron and the trion are ignored. On the other hand, the Coulombinteractions between these carriers are also often ignored in many theoretical works. Thephysical processes in conjugated polymers are mainly determined by two interactions:the electron-lattice interactions and the electron-electron interactions. Ignoring the latermay lead to some physical features being erased in these processes. Hubbard proposedthe Hubbard model to inscribe the electron-electron interactions in order to discuss theelectron correlation in the d-narrowband of transition metal in1963. Using thehartree-fork approximation, i.e., translating the multi-particle problem into the singleparticle one, the issue of coulomb interactions between electrons can be simplified.In our paper, based on the tight-binding Su-Schrieffer-Heeger (SSH) model and byusing a nonadiabatic molecular dynamic method, we investigated in details the staticproperty, formation mechanism and dynamic motion of charged trion, and especially theinfluence of electron-electron interactions on the dynamics of the charge trion inconjugated polymers. The conclusions of this paper include three parts.1. The static properties of charged trionsBased on the SSH model to describe the electron-lattice interactions, and theHubbard model to describe the electron-electron interactions, we can obtain parameters that represent the static property of charged trion, such as the lattice configurations, theelectronic states, the distribution of charge, and the wave function, by self-consistentlysolving two combined equations. As a three-particle self-localized excitation, the bindingenergy, the stability and the effective mass of the charged trion are large than those ofboth the polaron and the bipolaron. Both the electron-lattice interaction and theelectron-electron interaction markedly influence the static property of charged trion: theelectron-lattice interaction is in favor of the trion formation and enhances its stability; theonsite Coulomb interactions and the nearest-neighbor interactions all enhance thelocalizability of the lattice configuration defect and the charge distribution of trion,which results in the stability and the effective mass of charged trion increasing.We also consider the (hyber)polarizabilities of trion in conjugated polymers. Thecharged trion induces reverse polarization, as the biexciton does. The onsite Coulombinteractions enhance both the reverse polarization from the charged trion and the positivepolarization from the ground state; the nearest-neighbor interactions enhance the reversepolarization from the charged trion but restrain the positive ones from the ground state.On the other hand, the onsite Coulomb interactions enhance and the nearest-neighborinteractions restrain the second order hyberpolarizability of trion.2. The dynamic formation of trions in conjugated polymersThe dynamic formation process of a trion by recombination of an on-chainbipolaron with an off-chain counterion trapped in a radical molecule, under the influenceof an external electric field, has been investigated using a nonadiabatic evolution method.We demonstrate that the dynamic formation of the trion depends sensitively on thestrength of the applied electric field, the radical potential, and the coupling between thepolymer chain and radical. The trion can be formed efficiently via the bipolaron-trappedcounterion reaction under the appropriate conditions. Our simulation provides theoreticalevidence for the trion formation channel proposed by Kadashchuk et al. Thebipolaron-trapped counterion reaction also makes an additional contribution toelectroluminescence in conjugated polymers that lies beyond the set of processesinvolving polaron recombination. 3. The dynamic motion of trions in conjugated polymersThe dynamic processes of trions under an external electric field have beeninvestigated using a nonadiabatic evolution method. We demonstrate that the saturationvelocity of trions is lower than that of polarons and bipolarons due to its greater stabilityand effective mass. As in the cases of the polaron and the bipolaron, the dissociation oftrions due to the electric field also involves two steps. It is found that the trion cansustain an electric field of15.0mV/.Both the onsite Coulomb interactions U and the nearest-neighbor interactions Vrestrain the trion motion under the external electric field. It should be noted that, byconsidering the electron-electron interactions, the trion involves three typical velocitieswith the parameters of the electron-electron interactions being changed, Such as the highsupersonic velocity VⅢ, the supersonic velocity VⅡ, and the sonic velocity VⅠ. In order toexplain above results, we have traced motions and distributions of charged particlescontained in trion by their charge density centers under an external electric field. Wefound that the different electron-electron interactions will induce different structure oftrion. The trion tends to be a compound of an exciton (bipolaron) and a positive polaronwhen the onsite Coulomb interactions are strong (weak). But in the condition of thenearest-neighbor interactions being strong (weak), the trion tends to be a compound of abipolaron (exciton) and a positive polaron. It is obvious that there are competitionsbetween above two interactions. The typical structures of trion include the exciton+polaron mode, the bipolaron+polaron mode, and the intermediate mode between them.The three typical velocities of trion are result from the variety of its structure and closelyrelated with above structure mode.At last, we found that the carrier, such as the trion, velocity is dependent on itsdipole moment under the same external electric field: the larger the dipole moment is,the higher the velocity is.