Dynamics Of Charge Transport In Organic Semiconductors

Organic semiconductors usually refer toπ-conjugated conducting materials with conductivity between that of metals and insulators.In recent years,much attention has been paid to organic semiconductors for their abundant properties in electronics, optics and magnetism,which lead to the rapid development of organic electronics as well as the new fields of organic spintronics and molecular electronics.On the basic research side,the concepts and theories uncovered from organic semiconductors have been the fundamental principles of related studies in physics,chemistry and material science,by which the basic researches have been greatly accelerated.On the applied research side,organic semiconductors have been widely applied as functional materials to flexible large-area displays,solid-state lighting and solar cells.The conventional industry of inorganic semiconductors will be greatly impacted and challenged by the promising commercial potentials of organic semiconductors for their advantages of low cost and easy possibility.The microscopic processes related to the charge-carriers in organic devices usually consist of charge injection,charge transport and charge-carriers recombinations etc.As the charge transport process plays a key role in the operation of organic devices,it is of fundamental importance to obtain a comprehensive understanding of the charge transport properties of organic semiconductors.Organic conjugated polymers consisting ofπ-conjugated macromolecules is one of the most important and the most commonly used organic semiconductors in organic optoelectronic devices.For the strong electron-phonon(e-ph) couplings,the charge carriers in conjugated polymers are not electrons and holes as in the conventional inorganic semiconductors but the self-trapped states such as solitons, polarons and bipolarons,revealing abundant information of the underlying physics. An important species of conjugated polymers that can be used to improve the efficiency of organic optoelectronic devices is copolymers,which with donor-acceptor electronic structures have unique advantages in the applications in OLED and OPVC. Conjugated polymers can be treated as one-dimensional systems for their quasi-one-dimensional characteristics in conduction.Theoretically,the most important microscopic model for conjugated polymers is the well-known SSH tight-binding model proposed by Su,Schrieffer and Heeger in 1979.It has been widely used in the study of charge transport in conjugated polymers.While a large number of studies have been performed to explore the static and dynamic properties of charge carriers, many open problems are still left in conjugated polymers for the diversity of the molecular structures of conjugated polymers.For example,what is the form in which the charge of a polaron exists after its dissociation under high electric fields? What about its dynamic behavior? What are the main factors in affecting the motion of a polaron and the geminate combination of two oppositely charged polarons in donor-acceptor copolymers? The answers to these questions are of importance to the applications of conjugated polymers.Another kind of important organic semiconductors is organic molecular crystals (OMCs) made up of n'-conjugated oligomers.The crystalline structure of OMCs makes them the best organic semiconductors in conductivity and strong e-ph couplings also exist in OMCs.As the conductivity of OMCs shows clear characteristic of anisotropy,they can be treated as low- or even one-dimensional systems.OFET made of single-crystal OMCs can be employed to effectively study the intrinsic charge transport properties of organic semiconductors.While the charge transport in OMCs has been an issue of investigation for several decades,the understanding of its intrinsic mechanism is still unsatisfying and even under debating. The most commonly used microscopic models for OMCs are those based on the well-known Holstein polaron theory.However,it has been realized that the polaron theory is inadequate in describing the charge transport in OMCs for the inconsistency of results between theories and related experiments.Recent studies revealed that the nonlocal e-ph couplings are much more important than previously expected and the effects of thermal fluctuations in transfer integrals may be the important ingredients omitted in the polaron theory,and it is referred to as the thermal disorder theory.It is needed to systematically study,especially in the dynamic aspect,on the charge transport in OMCs based on the thermal disorder theory.In this thesis,we will perform dynamic studies with respect to the charge transport properties in conjugated polymers,copolymers and OMCs.The outline and the main conclusions of the studies are as follows.1.Intrachain charge-carrier dynamics in organic conjugated polymers under high electric fields.The studies to date on the dynamics of charge carriers,especially polarons,in conjugated polymers are mainly focused on the processes such as the charge injection and relaxation into polarons under weak electric fields,the properties of polaron motion under moderate electric fields,and the polaron stability and its dissociation under high electric fields.However,it is unclear that how the charge carrier behaves after the dissociation of a polaron under high electric fields.By using the SSH tight-binding model and a nonadiabatic dynamic method,we study the dynamic behaviors of charge carriers in conjugated polymers under high electric fields.1.1 It is found that a polaron will dissociate under high electric fields to propagate in terms of a free electron by decoupling with the trap of the lattice.A periodic oscillation of the electron under the electric fields is obtained and identified as Bloch oscillations(BOs) in the organic lattice.1.2 Different from the case in a rigid lattice,the BOs in the organic lattice have a drift behavior in the direction of the electric fields.A transient polaron state as a result of the strong e-ph coupling in the organic lattice is obtained at the end of each period of BO's,which plays an important role in the drift the BOs.The presence of transient polaron state indicates that it is possible that a polaron can be reproduced by some means or other after its dissociation under high electric fields.1.3 The effects of electron-electron(e-e) interactions and bond disorder are briefly discussed.It is found that the e-e interactions described by the extended Hubbard model can hardly affect the behaviors of the BOs,but the bond disorder has negative influence on the BOs by destroying the periodicity of the organic lattice. 2.Intrachain polaron motion and geminate combination in donor-acceptor copolymers.Copolymers with donor-acceptor electronic structures have wide applications in organic optoelectronic devices.On one hand,they can serve as organic layers for charge-carriers recombination in OLED;On the other hand,they can also be used for charge-carriers separation in OPVC.However,the operations of the two devices are completely opposite to each other:Oppositely charged polarons need to recombinate to form excitons for photon radiation in OLED,while excitons need to be dissociated to create oppositely charged polarons for charge conservations in OPVC.It is thus of importance to effectively choose donor-acceptor copolymers to meet the different needs.To this end,we study the intrachain polaron motion and geminate combination in donor-acceptor copolymers by using an extended version of the SSH tight-binding model and a nonadiabatic dynamic method.2.1 Two effects are mainly concerned:Level offset and interfacial coupling.A general rule associated with the ratio of the level offset to the binding energy of a polaron or exciton is obtained to optimize the optoelectronic applications of donor-acceptor copolymers.2.2 According to this rule,we identify two cases for the polaron motion and four cases for the geminate combination of oppositely charged polarons.2.3 It is found that an interface with weak coupling serves as an energy barrier and that with strong coupling as a well.Interfacial coupling can be as important as level offset in affecting the polaron motion and geminate combination in donor-acceptor copolymers.2.4 The effect of e-e interactions is also briefly discussed by employing the Hubbard model.It is found that a triplet exciton is much more energetically stable than a singlet one.This can be explained by the exciton binding energy and accords well with related experiments.3.Charge transport in organic molecular crystals under the influence of thermal disorder. The understanding of the intrinsic mechanism of charge transport in OMCs is currently still unsatisfying.Besides the polaron theory based on the Holstein polaron model,the thermal disorder theory focusing on the effects of thermal fluctuations in transfer integrals is recently proposed.It actually refers to controversy with respect to the relative importance of the local and nonlocal e-ph couplings in OMCs.By mainly focusing on the effects of thermal disorder induced by molecular motions,we study the spreading of a wave packet in a one-dimensional lattice with off-diagonal dynamic disorder to explore the charge transport mechanism in OMCs.3.1 We give a unified description of charge transport in OMCs with repect to tempratures and electric fields.Three distinct regimes of charge transport are identified and accord well with related experiments.3.2 The charge transport behavior described by the polaron theory as the transition from bandlike transport to small polaron hopping is reproduced in our results,but identified instead as a crossover between distinct regimes of electronic hopping among localized states.3.3 Due to the fact that a unified description of charge transport in OMCs can be obtained beyond the polaron theory,it is suggested that the role of polaronic effects on the intrinsic charge transport in OMCs is not as important as previously expected.