Dynamics Of Charge Carrier Transport In Metal/polymer/metal Structures

In recent years,with the improvement of experimental instruments and the development of theoretical methods,the research on organic conducting polymers has been developed into a systematic and multidisciplinary field.As a new kind of functional material,conjugated polymers or small oligomers have been the focus of the research work both because of 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.On the other hand,conjugated polymers have common characteristics 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 abundant function characteristics of conjugated polymers have been cognized.Many photoelectric devices based on conjugated polymers have offered promise for use in applications from experiment.Numerous high-performance photoelectric devices fabricated from organic polymers have been made including light-emitting diodes(LEDs),field effect transistors(FETs), photovoltaic cells,etc..The work principle of these devices is mainly based on physical processes involving the charge injection,the charge transport and the combination of electron-hole pairs.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,the lattice defect and energy band structure interact each other.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 charge transport and photoluminescence of conjugated polymers system.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 move.Studies on the dynamical processes of these excitations are elements to understand the optoelectronics for the organic materials.Within the tight-binding Su-Schrieffer-Heeger(SSH) model and a nonadiabatic dynamic evolution method,we simulate the injection and transport of charge carrier in metal/polythiophene(PT)/metal structure.In addition,we have also explored the dynamic process of charge carriers in metal/eopolymer structure.Finally,we discuss the dynamics of oppositely charged carriers in a metal/coupled poly(p-phenylene vinylene(PPV) chains/metal structure.The detailed study and main results are given below.1.Dynamics of charge carrier injection and transport in a metal/PT/metal structureIn the past twenty years,with the unique photoelectric properties,PT has been attached importance to technological applications,so it is important and significative to study the dynamic processes of injection and transport of the charge carriers in metal/PT/metal sandwich structure.1.1 The dynamical formation of the wave packet sensitively depends on the strength of applied voltage,the electric feid,and the contact between PT and the electrode.Each wave packet contains charges 0-2e depending upon the injection condition.On the other hand,we obtained that the wave packet is formed in the absence of the electric field and under weak or moderate electric fields.For a stronger electric field,the wave packet can be still formed while it moves rapidly to the right PT/metal interface,and the electronic charge can be ejected from PT into the right metal electrode.Furthermore,we compared the effect of the applied different electric fields on the charge ejection,and obtained that the strong electric field is in favor of the charges ejection,and the quantity of ejected charge also is uninteger.1.2 We have studied the injection and transport of two charges in metal/PT/metal system.The injected two charges form two wave packets in PT chain,the quantity of charge in each wave packet is 1e.The two wave packets are two single polarons,but not a bipolaron. 2.Study on transport of charge carrier in metal/eopolymer structureIncreasing attention has been paid to the research on organic optoelectronic devices based on copolymers,which is one kind of the most important polymers. Great deals of studies on the electronic structures of copolymers have been performed both theoretically and experimentally,and some valuable results,e.g.,the properties of organic quantum well and superlattice,the tunable band structures,etc.,have been obtained,which is significant for the further studies and applications of copolymers. For this reason,we systematically studied the dynamical transport properties of charge carriers driven by external electric fields in a metal/copolymer structure.2.1 With the different configuration of copolymer,the critical voltage that makes charges inject into copolymer is different,and the motion of charge carriers in copolymer is different too.The copolymer -(PPP)_m-(PT)_n-(PPP)_m-has barrier-well-barrier structure,the injected charges form wave packet in barrier and transport in chain.For weak electric field,the wave packet can transfer from barrier into well,but only localizes in well;for moderate electric fields,the wave packet gets through interface of two polymer and enters another barrier.2.2 For the copolymer-(PT)_m-(PPP)_n-(PT)_m- with well-barrier-well configuration, the injected charge localizes in PT well.Only under the stronger electric field,can the charge carrier jump over PT well and enter PPP chain.3.Dynamics of oppositely charged carriers in a metal/coupled PPV chains/metal structureRecent years,conjugated polymers including PPV and its derivant have been widely applied in optoelectronic devices.The conjugated polymers are used as the light-emitting and charge-transporting layers,in which electrons and/or holes are injected from the metal electrode and driven by the external field.In addition,the efficiency of LEDs also depends crucially on both intra- and interchain transport processes of the charge carriers.For the charge carrier picture to hold even for interchain charge transport,this process must involve hopping of charge from one chain to another followed by the creation of a geometrical defect in the charge accepting chain,and a corresponding annihilation of thus defect on the charge donating chain.To study the dynamics of injection and transport of the oppositely charged carriers is helpful to understand the microcosmic mechanism of luminescent device.3.1 It is shown that the oppositely charged carriers can be injected into PPV chains by adjusting the voltage biases of two metal electrodes.The behavior of the injected charge carriers depends closely on the electric field.For weak fields,the injected oppositely charged carriers form wave packets and move along different chains.For moderate electric fields,part quantity of charges of the two wave packets jumps over respectively to the neighboring chain and collides with the residual charge remained in the chain.The mixed state with excitons and polarons on each chain can be finally created due to the collision.For stronger electric fields,the injected oppositely charged carriers move too rapid to form stable wave packets.The extended carriers jump over respectively to the neighboring chains,collide and induce a lot of small lattice vibrations.Finally,the charges get across the interface of polymer/metal and ejected into the metal electrodes.3.2 In addition,the intensity of interchain coupling and the overlap length of the two PPV chains have important effect on the transfer of oppositely charged carriers. In the absence of interchain coupling,no matter how change the electric field,the formed wave packet transports only in each chain.For certain coupling intensity and overlap length of interchain,the key of interchain hopping of the oppositely charged carriers is the electric field strength.Even if the coupling intensity and overlap length are changed,the critical electric field that makes the oppositely charged carriers jump over to another chain remains is steady.