【作者】 张文华；

【导师】 徐法强；

【作者基本信息】 中国科学技术大学 ， 同步辐射及应用， 2008， 博士

【摘要】 随着有机半导体工业的飞速发展,人们对有机微电子器件中基础物理、化学问题的研究越来越深入。在以有机电致发光二极管（OLED）、有机场效应晶体管（OFET）及有机光伏电池（OPVC）等典型膜型结构器件中,提高界面载流子注入效率以及器件寿命一直是人们努力的重点。界面载流子的注入效率最主要的是受注入势垒的影响。在Schottky-Mott模型下,金属/有机界面电子（空穴）注入势垒取决于金属费米能级到LUMO（HOMO）的能量大小,但实际的多数金属/有机界面和部分有机/有机界面,由于界面偶极的存在而偏离该模型。因此,研究有机半导体界面的电子能级结构和界面偶极对有机光电器件的优化具有基础指导意义。光电子能谱技术（UPS/XPS）已经被证实是研究有机半导体表面与界面能级排列及界面化学的一种行之有效的实验方法。本文利用同步辐射光电子能谱技术,以苯并咪唑苝（BZP）和F代苯并咪唑苝（FBZP）作为有机探针分子,在超高真空环境中采取分步沉积的原位薄膜制备方法,对有机/金属、有机/有机、有机/无机半导体等一系列界面的形成过程与电子结构进行了较为系统的研究,其中特别考察了有机分子的F原子修饰对有机/金属界面形成的影响。另外,本文还研究了活泼金属Rb掺杂的Alq薄膜电子结构及O₂和温度等环境因素对表面与界面电子结构的影响。具体研究内容和取得的结果包括以下几个方面:1.BZP/Ag、Ag╱BZP和BZP/AlOx等界面BZP分子吸附到多晶Ag表面时,“pillow”效应引起金属表面功函降低,形成大小约0.3eV界面偶极。界面相互作用仅局限在一个分子单层以内。在费米能级以下0.8eV和1.8eV出现极化子型和双极化子型界面带隙态。该带隙态在250℃以下可稳定存在,但在室温暴露少量O₂后,带隙态迅速消失。可见环境O₂对BZP/Ag界面电子结构影响显著。热的Ag原子沉积到BZP薄膜上引起明显界面扩散,在结合能0.6eV和1.6eV形成两个明显带隙态。带隙态结合能整体较BZP/Ag界面中带隙态降低0.2eV,归因于Ag对光电发射中形成的空穴更强的终态屏蔽效应。BZP与惰性Ag原子的界面化学和沉积顺序关系不大,两种界面都是非反应型界面。BZP分子吸附到原位沉积的新鲜Al膜（有部分氧化）上,界面相互作用明显,界面层厚度超过30A;由于存在AlOx和BZP分子之间明显的界面化学反应,BZP在AlOx表面的吸附引起金属表面功函仅降低0.05eV。2.FBZP/Ag,Ag/FBZP,FBZP/AlOx等界面F原子取代BZP分子中的一个H后,分子原有的平面共轭结构被破坏,产生一定的分子扭曲。强电负性F原子的修饰,还可能对周围分子或原子产生诱导极化作用。对于FBZP/Ag界面,由于同时存在的“pillow”效应和F的诱导作用两个相反的因素,导致Ag表面功函增大约0.2eV。与BZP/Ag相比,FBZP/Ag界面的两个极化子型带隙电子态结合能稍小,为0.6eV和1.7eV。FBZP在AlOx表面的吸附,界面作用更复杂,包括“pillow”效应、F原子的诱导极化以及界面反应等作用,几个因素综合作用结果引起AlOx表面功函增大约0.3eV。3.BZP/NPB,BZP/ITO,BZP/SiO₂BZP/NPB界面形成中NPB分子能级发生弯曲,导致能级排列偏离Schottky-Mott模型,界面空穴由NPB向BZP的注入势垒明显小于反向电子由BZP向NPB的注入势垒,NPB表现出良好的空穴传输特性。BZP/ITO界面没有观察到明显能带弯曲和界面带隙态,界面偶极的形成归因于BZP分子的极化。BZP在SiO₂绝缘层上的吸附,界面相互作用非常弱,界面能级排列符合Schottky-Mott模型,形成典型的惰性突变界面。4.Rb-Alq少量活泼碱金属Rb原子沉积到Alq薄膜表面,具有电子掺杂作用,导致费米能级向上移动约0.2eV,并且在结合能1.2eV位置观察到明显带隙态。该带隙态在110℃低温具有一定稳定性,但暴露少量O₂后,由于Rb的氧化而迅速消失。更多还原

【Abstract】 With up-growing development in the organic semiconductor industry, more and more insights have been put into the fundamental physics and chemistry about the operations of the organic microelectronic devices. In those typical layer-based organic devices, including organic light emission diode （OLED）, organic field effect transistor （OFET） and organic photovoltaic cell （OPVC）, the most focused work is to improve the injection efficiency through the interfaces and the lifetime of the devices. Mostly, the major factor to control carrier injection is the interface barrier height. At metal/organic semiconductor interface, the electron （hole） injection barrier depends on the position of the LUMO （HOMO） with respect to the metal Fermi level under the Schottky-Mott rule. Most of the actual metal/organic or some of the organic/organic interfaces, however, deviate from the rule due to the existence of interface dipoles. Therefore, the studies of energy level alignment and dipoles at organic interfaces would play key role on the optimization of organic electronic devices. It has been demonstrated that photoemission （UPS/XPS） is one of the most successful experimental methods to research the interface energy alignment and chemistry.In this thesis, 3,4,9,10-perylenetetracarboxylic bisimidazole （BZP） and fluorinated BZP （FBZP） were employed as probe molecules. A series of interfaces, including organic/metal, organic/organic and organic/inorganic semiconductors, were investigated systematically to elucidate the interface formation and electronic structures by synchrotron radiation photoemission （SRPES） method. All the molecules or metal atoms were deposited step-wised in situ to substrate under ultra high vacuum system. Particularly, the influence of fluorination on the electronic structures of organic/metal interfaces was discussed. In addition, the surface and interface structure of Rb-doped Alq film, as well as the effects of oxygen and temperature on it, were also assessed.1. Organic/Metal interfaces: BZP/Ag, Ag/BZP, BZP/AlOx The deposition of BZP on polycrystalline Ag surface decreased the work function of the metal by about 0.3eV due to "pillow effect". Interfacial interaction was found to be localized mainly within one monolayer adsorption. At binding energy of 0.8eV and 1.8eV, a polaron and a bi-polaron type gap states were detected respectively, which could be stable at 250℃and disappeared promptly after exposure to O₂. The deposition of hot Ag atoms on BZP film caused distinct interface diffusion and also the formation of two gap states at 0.6 and 1.6eV, which were 0.2eV lower than the binding energy of those at BZP/Ag interface owing to the larger screening effect from Ag atoms to the core hole during the photoemission process. Both BZP/Ag and Ag/BZP interfaces showed nonreactive characteristic regardless of the deposition sequence. As for the adsorption of BZP molecules on the fresh AlOx surface （partially oxidized）, obvious chemical interaction was observed between BZP and AlOx. The thickness of the interface was expanded to more than 30A. The surface work function decreased finally only approximate 0.05eV as a consequence of both "pillow effect" and chemical reaction.2. Effect of fluorination: FBZP/Ag, Ag/FBZP, FBZP/AlOxThe substitution of a hydrogen atom by fluorine breaks the original planar structure of BZP and causes molecular distortion to some extent. The strong electrophilic effect of F atom most probably polarizes the surrounding molecules or atoms. At FBZP/Ag interface, the work function of Ag surface ultimately increased about 0.2eV as a result of the co-existence of the electrophilic effect and "pillow effect", which have reverse influence on the work function. Compared with BZP/Ag, the gap states at FBZP/Ag interface had lower binding energies at 0.6eV and 1.7eV respectively, and the influence of fluorination on electron transport had been discussed in 4^th chapter. The interface interaction between FBZP and AlOx was more complex since all factors mentioned above might happen simultaneously resulting in the work function increased by about 0.3eV.3. BZP/NPB, BZP/ITO, BZP/SiO₂During the interface formation of BZP with NPB, band bending from NPB was inferred and caused interface energy alignment to deviate from Schottky-Mott rule. The hole injection barrier from NPB to BZP was found smaller than the electron injection barrier from BZP to NPB, which was consistent with the fact that NPB was a normal hole transfer material. The interface dipole formation at BZP/ITO interface was interpreted as BZP polarization on ITO since no gap state and band bending were detected. The interaction was very weak as BZP was deposited on SiO₂ surface. The energy level alignment at BZP/SiO₂ was in accordance with Schottky-Mott rule showing a typical abrupt interface formed.4. Rb doped Alq:When a small amount of Rb was deposited on Alq film, the active Rb atoms exhibit obvious electron doping effect and arise Fermi level up by 0.2eV with an advent of gap state at 1.2eV. The gap state could be tolerant with the annealing at 110℃while faded away immediately after exposure to O₂, which was ascribed to the oxidation of Rb adsorbate.更多还原