Effects Of Disorder On Charge Transport In Semiconducting Polymers

Organic semiconducting polymers are a new type of functional materials. They not only have abundant properties in electronics, optics and magnetism, but also have many advantages such as low cost, easy processing, flexibility, and so on. As a new kind of irreplaceable fundamental functional materials, polymers have promising potential application in energy sources, information, sensing probe, optoelectronic device, molecular device, electromagnetic shielding and metal preservation. Up to now, many significant progresses have been made for semiconducting polymers in many aspects, such as molecule design, material synthesis, doping, solubility, processability, and conducing mechanism. At present, many polymer-based optoelectronic devices have been transferred from a simple experiment into a new kind of practical technology, including organic light-emitting diodes (OLEDs), field effect transistors (FETs) and photovoltaic cells, etc. These rapid developments on applied technology not only provide strong experimental basis for the development and design of new organic functional materials, but also put forward the urgent need to resolve the physical issues to theorists.The microscopic physic processes related to the charge carriers in organic devices usually include charge injection, charge transport, recombination and scattering. As the charge transport process plays a key role in device performance, it is of great importance to fully understand the charge transport properties in organic devices. Different from the traditional semiconductors, most polymer molecules have strong electric-phonon interactions due to the quasi-one-dimensional structure. The charge carriers in polymers are not electrons and holes as in the conventional inorganic semiconductors but the self-trapped states such as solitons, polarons and bipolarons. These qusi-particles are composed of charge and lattice, i.e., localized charges associated with a lattice distortion due to strong electron-phonon interactions. Under the driven of applied electric field, the charge gets energy and moves firstly, and it will drag the lattice to move at the same time, reflecting abundant information of the underling physics. Theoretically, the most important microscopic model for semiconducting polymers is the one-dimensional SSH tight-binding model proposed by Su, Schrieffer and Heeger in1979. The SSH model has been successfully used in the study of electronic structure and optical properties of polyacetylene. In the later years, Bishop, Conwell, Stafstrom and Sun et al. have extended the SSH Hamiltonian to research both the dynamic transport of carriers and their microscopic mechanisms in conjugated polymers.Notwithstanding many studies have been performed to explore the static and dynamic properties of charge carriers, many open problems are still left in semiconducting polymers for the diversity of the molecular structures. Fully understanding the underlying transport mechanisms is a great challenging task. It is now clear that charge transport is fundamental to device performance, and one of the major bottlenecks that limit organic devices is the poor charge carrier mobility. This is because that the charge transport are directly influenced by many factors including temperature, electric field, charge-carrier density, pressure, impurities, morphology, disorder, polydispersity, etc. To date, existing theoretical studies mainly focused on the influence of temperature, electric field and charge-carrier densities on the charge carrier mobility. In spite of the importance of morphology and disorder, there are few theoretical studies that elucidate their effects on charge transport explicitly. The answers to these questions are of great importance for the applications of semiconducting polymers.In this work, polaron transport processes in semiconducting polymers with disordered structure and morphology are simulated using a nonadiabatic evolution method. The simulations are performed within the frame work of an extended version of the SSH model modified to include interchain coupling disorder, diagonal disorder and off-diagonal disorder. The outline and the main conclusions of the studies are given in the following.1. Effects of interchain coupling disorder on charge transportThe film morphology, structural disorder and polydispersity are complex in polymer materials. In order to avoid structural complexity and study the properties of polymers in an easy way, many theoretical studies have considered the interchain coupling as a fixed value. Thus the theoretical model can not fully describe the detailed features of real materials. As we all known, polymers are disordered materials, and structural disorder is an intrinsic factor which inevitably exists in polymers in terms of static defects. Therefore, it should play an important role in charge transport properties. Based on an extended SSH model, the disorder is introduced into the simulation by interchain hopping integrals which are considered as random variables chosen from a probability distribution. The dynamic processes of a polaron are simulated under an external electric field. There are many types of disordered distribution, and the one that is generally used is the Gaussian distribution. The mean value of Gaussian distribution denotes the strength of interchain interaction and the standard deviation describes the degree of interchain coupling disorder. Thus, the disorder can be introduced into the simulation in an easy and controlled way.During our numerical simulation, we find that the charge tends to form a polaron among the sites with fairly strong interchain interactions which can be regarded as traps for the charge carrier. When polaron propagates through the system with both disorder and electric field, the dynamics is determined by the competition between the electric field and the disordered interchain interactions. Two types of polaron dynamics are classified:the weak-coupling dynamics and the strong-coupling dynamics. We find that the strength of interchain interactions is dominant in weak-coupling dynamics and the effects of disorder are dominant in strong-coupling dynamics. The charge carriers tend to have higher mobility for stronger interchain coupling, and interchain coupling disorder can be more favorable for charge transport depending on the coupling strength and the electric field. In detail, in weak-coupling dynamics disorder depresses (enhances) the polaron velocity for weak (strong) electric field, whereas it can be neglected for moderate electric field or very weak interchain coupling strength. In strong-coupling dynamics, disorder reduces polaron velocity for weak electric field and can be neglected under strong electric field, whereas a weak (strong) disorder is in favor of (blocks) polaron motion for moderate electric field. 2. Effects of pure diagonal disorder and combined disorder on charge transportUsually, two kinds of disorder are distinguished:diagonal disorder (DD) and off-diagonal disorder (ODD). DD reflects the fluctuations in site energies of individual molecules or chain segments. ODD is related to fluctuations in the strength of interaction between adjacent molecules or chain segments. In polymers, torsion angles, chemical impurities, or chain backbone twists can result in DD, while interchain spacing, orientation, or relative position can cause ODD. Therefore, DD and ODD coexist in realistic polymer materials, the understanding and quantifying such disorder is of great importance to further the investigation of underlying transport mechanisms. We make use of an extended version of the Su-Schrieffer-Heeger (SSH) tight-binding model modified to include the DD, ODD and an external electric field. Thus, the effects of disordered structure and morphology on polaron transport in semiconducting polymers are introduced into the simulation in an easy and controlled way. In order to separate out the effects of DD and ODD, it is better to first consider the effects of only one type of disorder and then the combined effects of both types of disorder.For the case of pure DD, we consider two different cases:(i) a single chain and (ii) two coupled chains. The polaron transport mechanism is determined by the competition between DD and electric field in a single chain. For two coupled chains, there also exists the competition between DD and interchain coupling besides the competition between DD and electric field. The polaron transport undergoes a crossover from adiabatic polaron drift to nonadiabatic polaron hopping depending on the degree of disorder under a large scope of electric field, whereas the field-assisted tunneling dominates charge transport for a stronger electric field.For the effects of combined disorder, the competition effects and the synergetic effects between DD and ODD are equally important. It is demonstrated that the competition effect is dominant in weak coupling region, whereas the synergetic effect is dominant in strong coupling region. The competition effect tends to provide faster path for charge transport, while the synergetic effect tends to depress charge motion. On the whole, the DD depresses polaron motion in most cases, and ODD can open energetically easier pathways for charge transport.