【作者】 刘凌云；

【导师】 徐法强；

【作者基本信息】 中国科学技术大学 ， 核科学与技术， 2015， 博士

【摘要】 近些年,随着有机半导体器件(OSC)的发展,有机/无机界面的研究越来越受到人们的关注,因为界面相互作用可以直接决定OSC器件的电荷传输特性,影响器件的效率。过渡金属氧化物(TMO)由于其自身的高功函、较深的导带能级等特性而经常被用作OSC器件中的缓冲层材料以辅助载流子的输运。此外,由于MPc/TMO界面在有机发光二极管(OLED)、有机光伏器件(OPV)、有机自旋阀(OSV)、有机场效应晶体管(OFET)等OSC器件中被广泛地应用,所以MPc/TMO界面相关的详尽、具体的研究工作也变得尤为重要,这对OSC器件的设计与结构优化等都有很重要的指导意义。本文中我们主要使用了同步辐射光电子能谱(SRPES)、近边X射线吸收精细结构(NEXAFS)谱等手段,并以激光拉曼光谱、原子力显微镜(AFM)等探测技术为辅助,详细、系统地研究了FePc/MoO3/ITO界面的电子结构及能级排列,对FePc/MoOx界面相互作用、电荷传输过程等进行了深入的探索并从机理上对界面反应进行了解释。具体的研究工作及获取的成果可概括为如下几点：第一,M003的功函数与其薄膜自身厚度有关。在一定的薄膜厚度范围内,M003的功函数会随着薄膜厚度的增加而增大,当薄膜厚度达到3nm时,其功函数达到最大值6.85eV,并且随着薄膜厚度继续增加,功函数始终维持在6.85eV,因此3nm是MoO3作为缓冲层的最佳厚度；另外,MoO3缓冲层在经过热退火处理后,其薄膜表面产生了大量的O空位,并在带隙中出现了一些带隙态,致使其电子结构发生了变化。能级结构图显示,FePc的最高占据分子轨道(HOMO)能级与MoOx的导带底(CBM)之间的能级差缩小了,意味着FePc/MoO3界面电荷传输势垒降低了,使电子更容易从FePc的HOMO能级进入到MoOx的CBM能级。另外MoOx中带隙态的产生,开辟了一条新的从ITO阳极经MoOx到FePc HOMO能级的空穴注入途径,使得MoOx从理论上更有利于提高空穴传输效率。第二,鉴于O空位的产生会辅助界面载流子的传输,因此我们详细探讨了MoO3在不同的退火温度下的还原程度。X-射线光电子能谱(XPs)数据结果显示MoO3热退火后的还原程度在一定温度范围内随着退火温度的升高而增大,当温度达到400°C时,其还原程度趋于稳定,不再随着温度的升高而变化。退火后的MoOx的薄膜与M003薄膜的AFM测试结果显示,相同厚度的MoOx薄膜表面颗粒尺寸要比MoO3的小得多,这也因此为FePc分子的沉积提供了更多的吸附点,从几何形貌上为FePc、MoOx的充分作用提供了有利条件。然而,XPS结果显示,吸附1ML FePc的MoOx的还原程度比单纯的MoOx的还原程度增强了很多,说明FePc的吸附促进了MoOx的还原,FePc与MoOx之间存在较强的相互作用,且这种相互作用主要是通过MoOx中的O原子与FePc分子结合发生的。NEXAFS结果表明,与MoOx的终端氧和反对称的桥位氧相关的谱峰强度降低了且谱峰间距变大,充分说明界面O原子周围的配位场发生了较大的改变。为此我们得出结论：FePc/MoOx界面的相互作用主要是通过Fe、O之间所形成的Fe-O配位键实现的。而Fe-O配位键的形成,为FePc与MoOx之间的电荷输运提供了便利的途径,这也解释了FePc与MoOx之间强相互作用的原因,并且也为其它MPc与TMO之间相互作用的研究提供了一定的参考价值。Ⅰ-Ⅴ曲线数据显示,在相同的施加电压下,FePc/MoOx/ITO样品中产生的电流比FePc/MoO3/ITO样品中的高,这意味着FePc/MoOx/ITO样品界面载流子传输效率比FePc/MoO3/ITO界面的高,即MoOx更有利于载流子的传输。第三,为了调变含磁性原子的有机分子与衬底的界面电子结构,我们采用了强磁场来对分子取向进行调控。通过在强磁场中生长FePc薄膜的一系列对比实验,我们发现当衬底平行放置时,在加磁场与不加磁场条件下生长在Si衬底上的两种有机膜的平均取向角度发生了10°的改变,而Cu衬底上的发生了14°的变化；当改变衬底与水平面的夹角为～70°时,Si衬底上的样品发生了～13。的变化,而Cu衬底上的也有-7°的变化,因此可以看出磁场对蒸发过程中的FePc分子是有明显的调控作用。而当将生长后的样品继续置于磁场中磁化一段时间后,其分子的取向角度又发生了很明显的改变,所以,磁场对成膜后的分子也有调控能力。更多还原

【Abstract】 Recently, the studies related to the organic/inorganic interfaces have attracted large interests since the interfacial interactions play a vital role in determining the charge transfer properties and the performance of the organic semiconductors (OSC) devices. Transition metal oxides (TMO) with high work functions (WF) and deep conduction band (CB) levels are usually used as buffer layers to assistant the charge transfer in the OSC devices. Moreover, MPc/TMO interfaces have wide range applications in organic light emitting diodes (OLED), organic photovoltaic devices (OPV), organic spin valves (OSV), organic field effect transistors (OFET) etc. Accordingly, a detailed and thorough study on the MPc/TMO interface becomes particularly important, which can give a useful guide in optimizing the OSC devices.In this work, we systematically investigated the interfacial electronic structure and energy alignment of the FePc/Mo03/ITO sample using the synchrotron radiation photoemission spectroscopy (SRPES), near edge X ray absorption fine structure (NEXAFS) spectra combined with conventional techniques like laser Raman spectroscopy, atomic force microscopy (AFM) etc. The interface interaction and charge transfer about the FePc/MoOx interface have been explained unambiguously and the main results are summarized as follows:First, the WF of MoO3depends on the thickness of the MoO3films. It was found the WF of MoO3increased with the gradual deposition of MoO3until the films reached a thickness of3nm at which the WF realized the maximum of6.85eV and didn’t vary with the further deposition of MoO3. So the3nm MoO3films is recognized as the favorable hole buffer layer with an optimum thickness. After the thermal annealing of the3nm MoO3film (200℃), a large amount of oxygen vacancies appeared at the surface of MoO3, and some gap states arose in the band gap, which changed the electronic structure of MoO3. The energy level diagram showed that the energetic distance between the highest occupied molecular orbitals (HOMO) of FePc and the CB minimum (CBM) of MoOx was decreased. This means the charge transfer from the HOMO levels of FePc to the CBM of MoOx becomes much easier. Principally, the gap states open up a new path for the hole transfer from the ITO anode to the HOMO levels of FePc via MoOx, and this is theoretically favorable for the improvement of the charge transfer efficiency. Second, we studied the reduction of MoO3at different annealing temperatures in detail. The X-ray photoemission spectroscopy (XPS) suggested that the reduction of MoO3was strengthened gradually with the annealing temperature increasing within400℃, over which the MoO3film remained stable and no further reduction was detected any more upon temperature. The AFM results showed that the MoOx particles had much smaller size than MoO3, which would supply more adsorption sites for FePc.The XPS spectra showed the MoOx adsorbed with1ML FePc had much higher reduction extent than the bare MoOx, which suggests the FePc promotes the reduction of MoOx, and some strong interactions exist at the FePc/MoOx interface that are caused by the O atoms of MoOx binding with FePc molecules. The NEXAFS spectra revealed that the intensity of the peak associated with the terminal oxygen and the asymmetrical bridging oxygen species was declined, and the energy separation was enhanced, implying a large variation of the coordination field around oxygen atoms at the FePc/MoOx interface. So we can conclude that the strong interaction at the FePc/MoOx interface is mainly caused by the Fe-0coordination, which provides a convenient route for charge transfer from FePc to MoOx.The Ⅰ-Ⅴ curves suggests there is a much larger electric current in the FePc/MoOx/ITO sample than in the FePc/MoO3/ITO sample at the same voltage. This means the MoOx is more favorable for charge transfer than the stoichiometric MoO3.Third, a high magnetic field of8.5T was used to modulate the organic molecular orientation at the substrates in order to change the interfacial electronic structure. It was found the molecules grown on the Si substrate had10°orientation change after applied to the magnetic field than without the magnetic field when the substrates were placed vertically to the magnetic field. However, for the Cu substrate, the change was14°. When the Si substrate has an angle of20°with the direction of the magnetic field, the molecular orientation changed13°relative to the case of the substrate vertical to the direction of the magnetic field, while for the Cu substrate, the change was7°. So we conclude that the magnetic field is effective to modify the molecular orientation during the FePc deposition. When the sample was put in the magnetic field for longer time after the growth, the FePc orientation angle also changed indicating the magnetic field effect on the molecular orientation of the deposited FePc films.更多还原