Study On The Mechanism Of Folaron And Exciton Transport Regulation In Organic Semiconductors

Science and technology are the primary productive forces and are the driving force for the advancement and development of society and civilization.The depletion of non-renewable energy on earth cannot meet the energy requirement of a large number of people.At the same time,as people's living standards continue increasing,the demand for physical and spiritual life gets more diverse and more demanding.Therefore,environmental protection and renewable energy are urgently needed.The emerging of organic semiconductors is a breakthrough in solving the above problems,which begins with the discovery of the first conductive polymer polyacetylene in the 1970s.During the following decades,investigations on it gradually developed into a subject with development potential and application value.Organic semiconductors are expected to replace inorganic semiconductors and occupy the mature inorganic material market where still exists many drawbacks.Compared with inorganic ones,organic semiconductors have strong electro-acoustic coupling,or in other words,the soft characteristics.This feature brings unique magneto,optical,and electronic properties to organic semiconductors on the one hand,and leads to a wide range of application prospects.Advantages including light weight,easy synthesis,simple preparation conditions,extensive material extraction,flexibility,and optical performance adjustment,just make up for the drawbacks of inorganic semiconductors.While it also brings disadvantages and restrictions,such as low mobility and short lifetime.The development of organic semiconductors is still in its infancy,there are still many controversies about the internal theoretical physical mechanism.Extensive theoretical and experimental studies are needed to deepen understanding and provide more possibilities for future practical applications.There are many methods for dealing with organic semiconductor systems,such as the TO model of organic small molecules,the TLM model of continuous media,the PPP model emphasizing the interaction between electrons,the Holstein model with full quantization,the surface hopping theory with multiple configurations,the quantum dynamics method,the Liouville equation for open system,density matrix theory,and first-principles calculations.With the maturation of organic electronics and the rapid development of organic spintronics,the framework of spin-related interaction in organic semiconductors has been gradually constructed.It draws lessons from the treatment of inorganic semiconductors,such as the Ising model,the Heisenberg model,and so on.Each model emphasizes different types of interactions,depending on the focused physical problems.In this dissertation,the tight-binding SSH model is adopted to highlight the most important property of organic semiconductors—strong electro-acoustic coupling.In view of the focused physics problems in organic spintronics,such as organic magnetoresistance,the organic multiferroic,the original SSH model isextended to spin space.On the one hand,it ensures the local state characteristics,and also cover the lack of the spin degree of freedom in the original model.In addition,considering the dynamical processes that exist in the organic,such as ultra-fast charge generation and ultrafast exciton dissociation,carrier coherent transport,etc.,the method in this dissertation combines tight-binding model with Ehrenfest non-adiabatic dynamics to study the electrical,magnetic,and optical properties of organic semiconductors.The evolutional state wave function of the time-dependentSchrodinger equation can truly reflect the dynamic evolution of electrons and lattices.Due to the strong electro-acoustic coupling,quasi-particles,such as solitons,polarons,bipolarons,excitons,and double excitons,exist in the form of localized states.In terms of transport efficiency,large effective mass and low mobility are indeed the disadvantages.While it also brings potential application value.For example,intrinsic semiconductors produce localized states after being excited.The change of the band structure causes a mismatch of the transient absorption spectrum and emission spectrum,named red-shift.On the one hand,this result comes from strong electro-acoustic coupling.On the other hand,it also comes from the complex micro-morphology,which inorganic semiconductors do not have.There are many novel physical phenomena,of which some have been reasonably explained,and some have not yet been reasonablly answered.Based on the hot topics in the fields of organic electronics,optoelectronics,and spintronics,this dissertation takes organic solar cells,organic light-emitting diodes,and organic magnetic resistance device as examples to investigate their internal physical processes.This article has studied and solved the following issues:1.Whether the ratio of exciton generation in organic semiconductors subject to exciton statistics.In organic light-emitting diodes are electroluminescent processes.Positive and negative charges are injected from the two electrodes.Carriers meet in the organic layer and form excitons.Exciton quench with light emitting.Since the injected charge has no spin polarization,the singlet excitons and the three triplet excitons are generated with the same probability.This ideal assumption suppresses the efficiency of electroluminescence in organic semiconductors,and is difficult to explain the luminous efficiency exceeding exciton statistics.Therefore,the effects of spin-related interaction on the ratio of exciton formation are studied.It is found that spin flip would render the originally spin-polarized state into spin-mixed one.Excitons are a mixing of singlets and triplets.By projecting to the exciton spin eigenstates,it is found that the hyperfine interactions can only change the ratio between the triplet substates and has no effect on the ratio of single states.The spin-orbital coupling influences the ratio between the four excitons manifolds.Especially,it can increase the generation ratio of singlet excitons,which may be an important breakthrough in the luminescence efficiency of OLEDs.2.Once an exciton is formed in an organic material,it will participate in and influence a series of physical processes in the organic semiconductor.Of particular importance is the transport process of excitons.It relates to the ultrafast transport of charge carrier and energy that has been discovered recently.In view of the important role of exciton transport in polymer-based D/A photovoltaic devices,we theoretically proposed two new kinds of exciton transport mechanism.One is induced by the interchain packing configuration in conjugated polymers.We find that there exists a distribution of driving force for exciton transport,which stems from the gradient of the exciton creation energy along the chains.The other one is induced by the ununiformed electric field from trapping charges in the amorphous distribution of polymers.We simulated the exciton transport under the non-uniform electric field and found that under strong non-uniform electric field,the photoexcitation can be dissociated into free charge instantaneously;in the weak non-uniform electric field area,the exciton firstly shows polarization.Due to the unbalanced electric field force,the polarized excitons move to the strong field region and undergoes partial charge separation.In both cases,the charge of the excitons dissociate will happen within 1 ps after its generation,so we believe that these mechanisms are the reasons for the ultrafast phenomenon in organic semiconductor materials.3.The charge and energy ultrafast transport in organic semiconductors involves another important physical factor,namely,coherence.The transition from coherence to incoherence transport determines the energy and carrier transfer efficiency.An important physical factors to keeping coherence is the phase.Therefore,we summaries the physical factors affecting the phase of the wave function and study the influence on the transport of organic semiconductors.We take the polaron as an example and introduce the phase perturbation.Especially,focusing here on methodologies,two creative approaches are applied:the modification of transfer integral and the phase-breaking addition to the wave-function.Both approaches prove that the disorder phase-break:ing of electron wave-function is the main factor that degrades the coherent transport and destroys carrier stability.This work highlights the importance of electron wave-function phase in organic semiconductors,which is often neglected and hardly measured directly in experiment.Our idea would complete the understanding of electron wa-ve-function on both the probability magnitude and the phase.The novel physical phenomena and physical mechanisms in the field of organic semiconductors are far more than the above discussed.There are still many problems in organic electronics.organic spintronics,and organic optoelectronics field that need to be discovered,solved and explored.The in-depth research has accelerated the pace of organic semiconductor applications,and the advent of the organic era is just around the cormer.