【摘要】：與量子耗散密切相關(guān)的開(kāi)放量子系統(tǒng)中的電輸運(yùn)問(wèn)題一直是科研人員關(guān)注的焦點(diǎn)。在開(kāi)放量子系統(tǒng)中,若中間傳輸層采用有機(jī)材料,電-聲相互作用對(duì)于體系中載流子輸運(yùn)性質(zhì)有著重要的影響。為了得到有機(jī)聚合物器件的實(shí)時(shí)輸運(yùn)性質(zhì),并且充分考慮有機(jī)材料中強(qiáng)的電-聲耦合,本文采用基于格林函數(shù)的hierarchical equations of motion(HEOM)與非絕熱分子動(dòng)力學(xué)相結(jié)合的方法,詳細(xì)討論了共軛聚合物系統(tǒng)中元激發(fā)動(dòng)力學(xué)以及載流子瞬態(tài)輸運(yùn)過(guò)程。對(duì)于中間體系電子部分哈密頓我們采用緊束縛的Su-Schrieffer-Heeger(SSH)模型來(lái)描述,源電極和漏電極則采用無(wú)相互作用的費(fèi)米子庫(kù)來(lái)描述。通過(guò)基于格林函數(shù)的HEOM方法可以得到描述體系電子部分的密度矩陣和一階伴隨密度矩陣的微分方程組;對(duì)晶格部分采用的是經(jīng)典處理,其滿足牛頓運(yùn)動(dòng)方程。利用Runge-Kutta方法對(duì)整個(gè)系統(tǒng)演化進(jìn)行數(shù)值模擬。我們首先運(yùn)用HEOM方法研究了開(kāi)放量子系統(tǒng)中不同強(qiáng)度電-聲耦合下體系載流子的輸運(yùn)性質(zhì)。計(jì)算結(jié)果表明:偏壓較小時(shí),電-聲相互作用阻礙體系中載流子輸運(yùn),但隨著偏壓的增加,電-聲耦合反而促進(jìn)載流子輸運(yùn)。這是由于偏壓的增加使得電極中的載流子能夠注入到體系中,電-聲耦合作用下,誘導(dǎo)晶格發(fā)生畸變,導(dǎo)致激子態(tài)的形成,激子態(tài)的出現(xiàn)促進(jìn)了載流子輸運(yùn)。并且電-聲相互作用越大,晶格弛豫能力越強(qiáng),能級(jí)越向帶隙中心偏移,使得偏壓窗內(nèi)部的態(tài)越多,電流越大。另外我們還研究了中間體系尺寸效應(yīng)的影響,發(fā)現(xiàn),隨著中間體系長(zhǎng)度的增加,能級(jí)分布變密,在相同的偏壓下,相對(duì)于小尺寸體系更容易形成新的激子態(tài),新激子態(tài)的出現(xiàn)會(huì)促進(jìn)電流的再次增加。另外,當(dāng)體系與電極的耦合減弱時(shí),電-聲相互作用越強(qiáng),VI-曲線中出現(xiàn)的電流平臺(tái)越明顯。這是因?yàn)轶w系與電極耦合減小時(shí),會(huì)使得體系中能級(jí)展寬越小,并且電-聲耦合越大,晶格弛豫能力越強(qiáng),所以出現(xiàn)的階梯狀平臺(tái)越明顯。我們還模擬了Sweep偏壓下,系統(tǒng)的動(dòng)力學(xué)輸運(yùn)行為。計(jì)算發(fā)現(xiàn):當(dāng)偏壓以遞增的形式加入,增加到最大值后再遞減,體系伏-安特性曲線中,電流會(huì)出現(xiàn)回滯現(xiàn)象。這是由于有機(jī)聚合物中存在較強(qiáng)的電-聲相互作用,運(yùn)動(dòng)的電子和空穴會(huì)誘導(dǎo)晶格發(fā)生畸變,畸變的晶格會(huì)產(chǎn)生一個(gè)局域的勢(shì)場(chǎng),電子和空穴會(huì)束縛到這個(gè)勢(shì)場(chǎng)中形成激子態(tài)。由于這種“自陷”效應(yīng)的存在,當(dāng)偏壓逐漸遞增時(shí),體系會(huì)形成激子態(tài);當(dāng)偏壓增加到最大值再逐漸遞減時(shí),體系的激子態(tài)會(huì)湮滅。并且激子態(tài)形成的偏壓值和激子態(tài)湮滅的偏壓值不同,進(jìn)而導(dǎo)致電流會(huì)出現(xiàn)回滯效應(yīng)。我們進(jìn)一步研究發(fā)現(xiàn):當(dāng)中間體系尺寸增加時(shí),在一定的偏壓范圍內(nèi),電流的回滯效應(yīng)會(huì)多次出現(xiàn)。增強(qiáng)電-聲耦合強(qiáng)度,使得體系帶隙變大,會(huì)導(dǎo)致電流回滯效應(yīng)越明顯;增強(qiáng)體系與電極的耦合,使得體系能級(jí)展寬變大,電流回滯效應(yīng)越微弱。
[Abstract]:The problem of electrical transport in open quantum systems, which is closely related to quantum dissipation, has always been the focus of attention of researchers. In an open quantum system, if organic materials are used in the intermediate transport layer, the electro-acoustic interaction plays an important role in the carrier transport properties in the system. In order to obtain the real time transport properties of organic polymer devices and to fully consider the strong electro acoustic coupling in organic materials, the method of combining hierarchical equations of motion (HEOM) based on Green's function with non adiabatic molecular dynamics is used in this paper. The excitation kinetics and carrier transient transport in conjugated polymer systems are discussed in detail. For the electronic part of the intermediate system Hamiltonian is described by a tight-binding Su-Schrieffer-Heeger (SSH) model and the source electrode and leakage electrode are described by a fermionic library without interaction. By using the HEOM method based on Green's function, the differential equations describing the electronic part and the first order adjoint density matrix of the system can be obtained, and the lattice part is treated by classical method, which satisfies the Newtonian equation of motion. The Runge-Kutta method is used to simulate the whole system evolution. We first use HEOM method to study the transport properties of carriers in open quantum systems with different intensities of electro-acoustic coupling. The results show that the electro acoustic interaction hinders the carrier transport when the bias voltage is small, but with the increase of the bias voltage, the electro acoustic coupling accelerates the carrier transport. This is due to the increase of the bias voltage, the carriers in the electrode can be injected into the system, and the lattice distortion is induced by the electro-acoustic coupling, which leads to the formation of the exciton state, and the emergence of the exciton state promotes the carrier transport. The larger the electric-acoustic interaction is, the stronger the lattice relaxation ability is, and the more the energy level shifts to the center of the band gap, the more the states in the bias window are and the larger the current is. In addition, we also study the effect of the size effect of the intermediate system. It is found that with the increase of the length of the intermediate system, the energy level distribution becomes denser, and at the same bias voltage, it is easier to form a new exciton state than the small size system. The appearance of the new exciton state will promote the increase of the current again. In addition, when the coupling between the system and the electrode is weakened, the stronger the electro-acoustic interaction is, the more obvious the current platform appears in the VI- curve. This is because when the coupling between the system and the electrode decreases, the energy level in the system becomes smaller, and the larger the electro-acoustic coupling is, the stronger the lattice relaxation ability is, and the more obvious the ladder platform is. We also simulate the dynamic transport behavior of the system under Sweep bias. It is found that when the bias voltage is added in the form of increasing voltage and then decreasing when the bias voltage is increased to the maximum value, the hysteresis of the current will occur in the volt-ampere characteristic curve of the system. This is due to the existence of strong electro-acoustic interaction in organic polymers, in which moving electrons and holes will induce lattice distortion, and distorted lattices will produce a localized potential field in which electrons and holes will be bound to form exciton states. Due to the existence of this "self-trapping" effect, the exciton state of the system will be formed when the bias voltage increases gradually, and the exciton state of the system will annihilate when the bias voltage increases to the maximum value and then decreases gradually. Moreover, the polarization voltage of exciton state is different from that of exciton state annihilation, which leads to hysteresis effect of current. It is found that the hysteresis effect of the current will occur many times in a certain range of bias voltage when the size of the intermediate system increases. The stronger the electro-acoustic coupling intensity, the larger the band gap, the more obvious the hysteresis effect is, and the stronger the coupling between the system and the electrode is, the wider the energy level is, and the weaker the current hysteresis effect is.
【學(xué)位授予單位】：河北師范大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：O469

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