有机半导体电荷输运的动力学研究
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摘要
有机半导体通常是指导电性介于金属和绝缘体之间、由具有π-共轭结构的有机分子组成的导电材料。近年来,有机半导体丰富的电学、光学和磁学性质引起了人们广泛的关注和极大的兴趣,使有机电子学以及新兴的有机自旋电子学得以迅速发展,并且对分子电子学等相关领域也产生了很大的推动作用。在基础研究方面,有机半导体的概念和理论已经成为物理、化学和材料等学科相关研究的基础,极大地推动了基础科学研究的发展;在应用研究方面,有机半导体作为一类重要的有机功能材料,在大面积柔性显示、固态发光以及太阳能电池等方面具有广泛的应用,还兼具低成本和易加工等特点,因此商业前景巨大,对传统的无机半导体产业将形成强有力的冲击和挑战。有机光电子器件中涉及载流子的微观物理过程包括电荷的注入、输运和复合等行为,其中电荷输运对器件性能起着至关重要的作用,因此全面理解有机半导体中的电荷输运性质具有十分重要的意义,一直以来是人们研究的重点。
由π-共轭有机高分子组成的有机共轭聚合物是最为重要的有机半导体之一,也是目前有机光电子器件的主要材料。由于存在很强的电子-声子(e-ph)耦合作用,共轭聚合物的载流子不再是传统的电子和空穴,而是孤子(soliton)、极化子(polaron)和双极化子(bipolaron)等由电荷与晶格耦合在一起的自陷态,包含了非常丰富的物理内容。共轭聚合物中有一类称为共聚物的材料,可有效提高有机光电子器件的性能,其中具有给体-受体结构的共聚物在有机发光二极管(OLED)和有机光伏电池(OPVC)的应用中具有独特的优势。共轭聚合物在导电性上具有准一维的特征,通常可简化为一维体系来处理。理论上,用于描述共轭聚合物最为重要的微观模型是1979年由Su、Schrieffer和Heeger提出的著名的SSH紧束缚模型,被广泛地应用于共轭聚合物的电荷输运研究中。虽然人们已经对载流子的静态及其动力学性质进行了大量的研究,但是共轭聚合物结构的多样性使得还有许多问题并没有清晰的物理图像,例如强电场下共轭聚合物中的极化子在解离之后以何种形态存在,其动力学行为如何;给体-受体共聚物中的极化子运动以及正负极化子之间的复合受哪些因素的影响等。这些问题的解决都将对共轭聚合物的应用具有积极的指导意义。
由π-共轭有机小分子组成的有机分子晶体则是另一类非常重要的有机半导体,有序的晶体结构使其成为导电性最好的有机半导体材料。有机分子晶体中也存在很强的e-ph耦合作用,并且在导电性方面具有明显各向异性的特点,从而可作为低维甚至一维系统来处理。以单晶形式的有机分子晶体制成的有机场效应晶体管(OFET)可有效用于有机半导体本征电荷输运性质的研究。人们虽然对有机分子晶体导电性的研究已经有了几十年的历史,但是目前对其电荷输运机制的理解依然不是很清晰和全面,并且在某些问题上存在很大的争议。理论上,用于研究有机分子晶体电荷输运性质最为常用的微观理论为极化子理论,这是在著名的Holstein极化子模型基础上建立起来的。然而由于得到的一些结论与许多实验观测是矛盾的,因此人们逐渐意识到仅采用极化子理论来描述有机分子晶体的电荷输运性质可能是不充分的,需要考虑其它可能因素的影响。最近的研究认为,由于分子之间的范德瓦尔斯力比较弱,有机分子晶体中的非局域e-ph耦合可能比人们以前认为的要重要得多,并且发现对跃迁积分热涨落效应的忽略可能是导致极化子理论在某些情况下失效的主要原因,我们称这种理论为有机分子晶体中的热无序理论。目前,基于热无序理论研究有机分子晶体中电荷输运机制的工作还非常少,需要系统地尤其是从动力学角度开展相关研究,这对全面理解有机分子晶体中的电荷输运性质具有重要的意义。
在本论文的研究中,我们将针对有机半导体电荷输运中所存在的上述问题,从动力学角度对均聚物、共聚物和有机分子晶体三种有机半导体结构中的电荷输运性质进行研究。研究内容和主要结论有:
1.有机共轭聚合物链中强电场下的载流子动力学
目前关于共轭聚合物中载流子尤其是极化子的动力学研究主要集中在以下几个方面:弱电场下电荷的注入以及弛豫形成极化子的过程;中等强度电场下极化子的运动性质;强电场下极化子的稳定性及其解离等。而人们对于极化子在强电场下解离之后的行为和物理图像却并不清楚。我们采用SSH紧束缚模型和非绝热动力学方法,研究了共轭聚合物分子链中强电场下载流子的动力学行为。
1.1发现极化子在强电场下将发生解离,电荷脱离晶格束缚后以自由电子的形式进行运动。电子在电场作用下发生周期性的振荡,经分析确定为有机晶格中的Bloch振荡。
1.2与刚性晶格中的Bloch振荡不同,有机晶格中的振荡电子整体上存在定向漂移。由于有机晶格中强的e-ph耦合作用,在每一个Bloch振荡周期末可出现瞬时极化子态,这对于振荡电子的定向漂移具有重要作用。瞬时极化子态的存在表明,极化子在强电场下发生解离之后可以通过某种方式再产生。
1.3简单分析了电子-电子(e-e)相互作用和键无序对上述过程的影响。发现采用扩展的Hubbard模型描述的e-e相互作用对Bloch振荡几乎没有影响;键无序由于破坏了晶格的周期性,对Bloch振荡的形成具有非常不利的影响。
2.给体-受体共聚物链中极化子的运动和成对复合
具有给体-受体电子结构的共聚物在有机光电子器件中具有广泛的应用,它们既可以作为电荷复合层用于OLED,又可以作为电荷分离层用于OPVC。由于上述两种器件具有完全相反的物理过程,即OLED中正负极化子需要复合形成激子进而发光,而OPVC中形成的激子需要解离成正负极化子从而进行电荷储存,因此如何合理地选取给体-受体共聚物使之分别满足上述不同的需要是非常重要的。为此,我们基于一个扩展形式的SSH紧束缚模型,利用非绝热动力学方法对给体-受体共聚物链中极化子在均匀外电场下的运动和成对复合过程进行了研究。
2.1重点考虑两种因素的影响:能级偏移和界面耦合。我们得到了一个与能级偏移和极化子或激子束缚能的比率有关的基本准则,用于优化给体-受体共聚物在有机光电子器件中的应用。
2.2依据该准则,我们得到了极化子运动的两种不同情形和带相反电荷极化子成对复合的四种不同情况。
2.3对界面耦合的研究发现,界面对于弱耦合具有能垒的作用,而对于强耦合则具有势阱的作用,界面耦合在极化子运动和成对复合过程中可以起到与能级偏移同等重要的作用。
2.4利用Hubbard模型简单讨论了e-e相互作用的影响,发现极化子复合形成的三重态激子比单态激子要稳定得多,这可以通过激子束缚能来进行解释并与相关实验符合较好。
3.有机分子晶体中热无序影响下的电荷输运
目前,人们对有机分子晶体中电荷输运机制的理解依然不是很清楚,并存在很大争议。除了在Holstein极化子模型基础上建立起来的极化子理论外,人们新近又提出了关注跃迁积分热涨落效应的热无序理论,这实际是关于有机分子晶体中局域e-ph耦合和非局域e-ph耦合各自重要性的问题。我们通过重点考虑分子热运动引起的热无序效应,从动力学角度研究了一个波包在非对角动态无序的一维晶格中的扩散行为,从而对有机分子晶体中的电荷输运机制进行研究。
3.1我们对有机分子晶体中的电荷输运给出了一个关于温度和电场的统一描述,并且得到了与实验符合很好的三种不同的电荷输运模式;
3.2我们的结果中也得到了极化子理论所描述的由带输运到小极化子跳跃输运机制转变的电荷输运行为,但分析发现这实际对应于两种不同的电子跳跃输运机制之间的转变;
3.3鉴于我们对有机分子晶体中电荷输运的统一描述是在极化子理论之外给出的,因此本研究间接表明极化子效应在有机分子晶体本征电荷输运中所起的作用并没有人们以前认为的那样重要。
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.
由π-共轭有机高分子组成的有机共轭聚合物是最为重要的有机半导体之一,也是目前有机光电子器件的主要材料。由于存在很强的电子-声子(e-ph)耦合作用,共轭聚合物的载流子不再是传统的电子和空穴,而是孤子(soliton)、极化子(polaron)和双极化子(bipolaron)等由电荷与晶格耦合在一起的自陷态,包含了非常丰富的物理内容。共轭聚合物中有一类称为共聚物的材料,可有效提高有机光电子器件的性能,其中具有给体-受体结构的共聚物在有机发光二极管(OLED)和有机光伏电池(OPVC)的应用中具有独特的优势。共轭聚合物在导电性上具有准一维的特征,通常可简化为一维体系来处理。理论上,用于描述共轭聚合物最为重要的微观模型是1979年由Su、Schrieffer和Heeger提出的著名的SSH紧束缚模型,被广泛地应用于共轭聚合物的电荷输运研究中。虽然人们已经对载流子的静态及其动力学性质进行了大量的研究,但是共轭聚合物结构的多样性使得还有许多问题并没有清晰的物理图像,例如强电场下共轭聚合物中的极化子在解离之后以何种形态存在,其动力学行为如何;给体-受体共聚物中的极化子运动以及正负极化子之间的复合受哪些因素的影响等。这些问题的解决都将对共轭聚合物的应用具有积极的指导意义。
由π-共轭有机小分子组成的有机分子晶体则是另一类非常重要的有机半导体,有序的晶体结构使其成为导电性最好的有机半导体材料。有机分子晶体中也存在很强的e-ph耦合作用,并且在导电性方面具有明显各向异性的特点,从而可作为低维甚至一维系统来处理。以单晶形式的有机分子晶体制成的有机场效应晶体管(OFET)可有效用于有机半导体本征电荷输运性质的研究。人们虽然对有机分子晶体导电性的研究已经有了几十年的历史,但是目前对其电荷输运机制的理解依然不是很清晰和全面,并且在某些问题上存在很大的争议。理论上,用于研究有机分子晶体电荷输运性质最为常用的微观理论为极化子理论,这是在著名的Holstein极化子模型基础上建立起来的。然而由于得到的一些结论与许多实验观测是矛盾的,因此人们逐渐意识到仅采用极化子理论来描述有机分子晶体的电荷输运性质可能是不充分的,需要考虑其它可能因素的影响。最近的研究认为,由于分子之间的范德瓦尔斯力比较弱,有机分子晶体中的非局域e-ph耦合可能比人们以前认为的要重要得多,并且发现对跃迁积分热涨落效应的忽略可能是导致极化子理论在某些情况下失效的主要原因,我们称这种理论为有机分子晶体中的热无序理论。目前,基于热无序理论研究有机分子晶体中电荷输运机制的工作还非常少,需要系统地尤其是从动力学角度开展相关研究,这对全面理解有机分子晶体中的电荷输运性质具有重要的意义。
在本论文的研究中,我们将针对有机半导体电荷输运中所存在的上述问题,从动力学角度对均聚物、共聚物和有机分子晶体三种有机半导体结构中的电荷输运性质进行研究。研究内容和主要结论有:
1.有机共轭聚合物链中强电场下的载流子动力学
目前关于共轭聚合物中载流子尤其是极化子的动力学研究主要集中在以下几个方面:弱电场下电荷的注入以及弛豫形成极化子的过程;中等强度电场下极化子的运动性质;强电场下极化子的稳定性及其解离等。而人们对于极化子在强电场下解离之后的行为和物理图像却并不清楚。我们采用SSH紧束缚模型和非绝热动力学方法,研究了共轭聚合物分子链中强电场下载流子的动力学行为。
1.1发现极化子在强电场下将发生解离,电荷脱离晶格束缚后以自由电子的形式进行运动。电子在电场作用下发生周期性的振荡,经分析确定为有机晶格中的Bloch振荡。
1.2与刚性晶格中的Bloch振荡不同,有机晶格中的振荡电子整体上存在定向漂移。由于有机晶格中强的e-ph耦合作用,在每一个Bloch振荡周期末可出现瞬时极化子态,这对于振荡电子的定向漂移具有重要作用。瞬时极化子态的存在表明,极化子在强电场下发生解离之后可以通过某种方式再产生。
1.3简单分析了电子-电子(e-e)相互作用和键无序对上述过程的影响。发现采用扩展的Hubbard模型描述的e-e相互作用对Bloch振荡几乎没有影响;键无序由于破坏了晶格的周期性,对Bloch振荡的形成具有非常不利的影响。
2.给体-受体共聚物链中极化子的运动和成对复合
具有给体-受体电子结构的共聚物在有机光电子器件中具有广泛的应用,它们既可以作为电荷复合层用于OLED,又可以作为电荷分离层用于OPVC。由于上述两种器件具有完全相反的物理过程,即OLED中正负极化子需要复合形成激子进而发光,而OPVC中形成的激子需要解离成正负极化子从而进行电荷储存,因此如何合理地选取给体-受体共聚物使之分别满足上述不同的需要是非常重要的。为此,我们基于一个扩展形式的SSH紧束缚模型,利用非绝热动力学方法对给体-受体共聚物链中极化子在均匀外电场下的运动和成对复合过程进行了研究。
2.1重点考虑两种因素的影响:能级偏移和界面耦合。我们得到了一个与能级偏移和极化子或激子束缚能的比率有关的基本准则,用于优化给体-受体共聚物在有机光电子器件中的应用。
2.2依据该准则,我们得到了极化子运动的两种不同情形和带相反电荷极化子成对复合的四种不同情况。
2.3对界面耦合的研究发现,界面对于弱耦合具有能垒的作用,而对于强耦合则具有势阱的作用,界面耦合在极化子运动和成对复合过程中可以起到与能级偏移同等重要的作用。
2.4利用Hubbard模型简单讨论了e-e相互作用的影响,发现极化子复合形成的三重态激子比单态激子要稳定得多,这可以通过激子束缚能来进行解释并与相关实验符合较好。
3.有机分子晶体中热无序影响下的电荷输运
目前,人们对有机分子晶体中电荷输运机制的理解依然不是很清楚,并存在很大争议。除了在Holstein极化子模型基础上建立起来的极化子理论外,人们新近又提出了关注跃迁积分热涨落效应的热无序理论,这实际是关于有机分子晶体中局域e-ph耦合和非局域e-ph耦合各自重要性的问题。我们通过重点考虑分子热运动引起的热无序效应,从动力学角度研究了一个波包在非对角动态无序的一维晶格中的扩散行为,从而对有机分子晶体中的电荷输运机制进行研究。
3.1我们对有机分子晶体中的电荷输运给出了一个关于温度和电场的统一描述,并且得到了与实验符合很好的三种不同的电荷输运模式;
3.2我们的结果中也得到了极化子理论所描述的由带输运到小极化子跳跃输运机制转变的电荷输运行为,但分析发现这实际对应于两种不同的电子跳跃输运机制之间的转变;
3.3鉴于我们对有机分子晶体中电荷输运的统一描述是在极化子理论之外给出的,因此本研究间接表明极化子效应在有机分子晶体本征电荷输运中所起的作用并没有人们以前认为的那样重要。
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.
引文
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