衬底对开路电压的影响.doc

Substrate effects on the electronic properties of an organic/organic heterojunction有机/有机异质结中衬底对电子性质的影响The electronic structures of copper-phthalocyanine/tris8-hydroxyquinoline aluminum CuPc/Alq3 heterojunctions on Mg and indium tin oxide ITO substrates have been studied by photoemission spectroscopy. While the typical vacuum energy level lineup occurred at the CuPc/Alq3 junction on ITO, the same junction formed on Mg displayed vastly different electronic structures, showing a 0.5 eV band bending associated with the formation of a space charge layer. The substrate effects were explained by the proximity of the Mgs Fermi level to the lowest unoccupied molecular orbital of CuPc, resulting in spontaneous charge transfer. The results show the feasibility of tuning the electronic properties of an organic heterojunction via the Fermi level of the substrate. 人们已经通过光激发光谱研究了Cu/Alq3异质结在Mg和ITO衬底上的电子结构。然而典型的能级lineup只发生在ITO衬底上的Cu/Alq3异质结，同样的异质结在Mg衬底上会有完全不同的电子结构，表现出0.5V能带弯曲，形成一个空间电荷层。这个衬底作用是因为Mg的费米能级接近CuPc的LUMO能级，使电荷自发转移。结果表明可以通过调整衬底的费米能级来改变有机异质结的电子性质具有可行性。 In recent years, considerable efforts have been directed at developing high-efficiency organic electronic and optoelectronic devices.13 In a multilayer device structure, junctions formed between different organic semiconductors play a crucial role in various electronic processes including charge injection, electron-hole recombination, and dissociation.35 These processes are influenced by the discontinuity in the local energy levels at the interface. Thus, an accurate picture of the relative positions of molecular energy levels at organic/organic interfaces is essential for the optimization of device performance. 最近几年，研发高效率的有机电子和光电子器件方面取得了很大的进步。在多层器件结构中，有机半导体之间的结起到关键作用，不同有机半导体之间的结具有不同的电子活动如电荷注入、电子-空穴复合和激子解离。这些电子活动受到界面处能级不连续性的影响。因此，为了优化器件性能，有必要画出有机/有机界面处分子能级的相对位置。 Studies in the past show that the assumption of vacuum level alignment in the Schottky-Mott model does not give an accurate account of the electronic structures of metal/organic interfaces.69 In fact, an interfacial dipole of about 0.51 eV exists at most metal/organic junctions.69 However, except for some specific interfaces, energetics for the majority of undoped organic/organic heterojunctions are seemingly dominated by a near vacuum level alignment i.e., 0.1 eV, which is attributed to the weak intermolecular van der Waals interaction between organic materials.6,1012As a result, molecular energy levels in the contacting organic semiconductors remain flat away from the interface, and the interfacial energy offsets are commonly estimated by lining up the separately measured electronic energy levels of two organic materials. For example, the energy offsets of the highest occupied molecular orbitals HOMOs and the lowest unoccupied molecular orbitals LUMOs at a heterojunctionare respectively obtained from the energy differences between the ionization potentials IPs and electron affinities EAs of the two contacting organic semiconductors. The energy offsets across organic heterojunctions are therefore generally considered to be independent of substrates and the formation sequence.以往的研究表明，在肖特基-莫特模型中真空能级对齐的假设并不能解释金属/有机界面的电子结构。实际上，在大多数金属/有机结界面处存在大约0.5ev界面偶极子极化能。然而，出来一些特殊的界面外，大多数未掺杂的有机/有机异质结能级主要受到接近真空能级的对齐的控制。真空能级对齐来源于有机材料间的弱分子作用力，即范德瓦尔斯力。所以，有机半导体中远离界面处的能级依然是平的，界面处的能级差可以通过测量两种有机材料的电子能级得来。例如，异质结中HOMO和LUMO能级差是有两种接触有机半导体的电离势与电子亲合势之差得到的，所以有机异质结的能级差不受衬底和制造流程影响。 Nevertheless, the mechanisms of the variation of the Fermi level EF position in the organic layers and “band bending” in the energy level alignment at the organic heterojunctions are yet unclear. In particular, when the Fermi level an organic heterojunction is close to the HOMO or LUMO, the validity of the vacuum level alignment concept remains to be examined. To this end, we perform a direct measurement of the electronic structure at the interface between copper phthalocyanine CuPc and tris8- hydroxyquinoline aluminum Alq3 on Mg and indium tin oxide ITO substrates using ultraviolet photoemission spectroscopy UPS and x-ray photoemission spectroscopyXPS. Possible effects of the substrate-induced EF variation in the organic energy gap on the energy level alignment are explored. We find that when the EF is far away from the LUMO edges of organic materials, the heterojunction shows typical flat band vacuum level alignment. On the contrary, when the EF is close to the LUMO edge, band bending in the molecular levels is induced through a space charge region at the interface, which consistently changes the energy offsets, leading to substantial deviation from vacuum level alignment. 然而，对于有机层中费米能级位置为什么会变化和有机异质结中能级对齐的“能带弯曲”的机理还不是很清楚。特别地，当异质结中费米能级接近HOMO能级或者LUMO能级时，真空能级对齐这个观点的有效性还需要检验。因此，我们利用UPS和XPS直接测量CuPc和Alq3在Mg和ITO界面间的电子能级结构。我们探讨了有机能级带隙中由衬底产生的费米能级的变化对能级对齐的影响。我们发现当费米能级远离有机材料的LUMO能级边缘时，异质结显示了典型的平带真空能级对齐。相反当费米能级接近有机材料的LUMO能级边缘时，界面的空间电荷区域会使分子能级发生弯曲，正是由于能级不断弯曲，使得分子能级大幅度偏离真空能级。 Experiments were carried out in a VG ESCALAB 220i-XL ultrahigh vacuum UHV surface analysis system, which consists of two chambers for sample analysis and preparation, respectively. ITO 30 /sq coated glass substrates were ex situ treated by UV-ozone exposure and immediately loaded into the UHV system. Mg films were in situ deposited on ITO-coated glass substrates in the preparation chamber. Organic heterojunctions were formed by depositing CuPc in steps on a 100-thick Alq3 layer deposited in situ on ITO or Mg substrates. The samples were transferredto the analysis chamber for measurements after each deposition step. UPS analysis using a He I radiation 21.22 eV source was performed to measure the valence states and the vacuum level position with an energy resolutionof 90 meV. For the collection of secondary electrons, samples were negatively biased at 4 V. XPS measurements using a monochromatic aluminum K source 1486.6 eV were used to study the interfacial chemical reactions and the development of possible molecular level bending across the interface. All measurements were done at room temperature. 实验在UHV表面分析系统中进行，该系统具有两个腔室，分别用于样品分析和样品准备。淀积在玻璃衬底上的ITO（30/sq）需要在紫外臭氧环境下处理，之后立刻放入UHV系统中。在样品准备腔室内将镁薄膜淀积在ITO上。把CuPc和Alq3依次淀积在ITO和Mg衬底上制成有机异质结。制备完成后，将样品送到分析腔室进行测量，配有21.22ev的He光源的UPS分析系统测量价带和真空能级位置，分辨率达到90mv。为了吸收二次电子，需要给样品加上-4V的电压。XPS测量用于研究界面的化学反应和界面中可能的分子能级弯曲。本实验中所有的测量都是在室温下完成的。 Figure 1 shows the UPS spectra of the CuPc/Alq3 interface on Mg substrate as a function of CuPc thickness ranging from a monolayer 34 to 100 . The bottom specatrum was measured from a 100-thick Alq3 layer deposited on an Mg substrate. Upon increasing CuPc coverage, characteristic emission features of CuPc and Alq3 show progressive shifts towards the lower binding energy BE region. This is accompanied by a 0.7 eV rising of the vacuum level. When the nominal thickness of CuPc is larger than 12 , all the valence features of the underlying Alq3 layer are fully attenuated. Since the inelastic mean free path of photoemitted electrons in UPS is 1015 , the disappearance of Alq3 valence features at 12 CuPc implies that CuPc has a high sticking probability close to unity to the Alq3 surface, and can form an almost complete layer even at the initial deposition stage. The IP values obtained here are 5.8 eV for Alq3 and 4.8 eV for CuPc, in good agreement with previous results. 图1显示的是CuPc/Alq3异质结在镁衬底上的UPS光谱图，其中CuPc的厚度是变化的：从单层（3-4）到100。最下面是100厚度的Alq3淀积在镁衬底上的光谱图。随着CuPc厚度的增加，CuPc和Alq3的发射特性显示了向更低的能级移动的趋势，并使得真空能级提高0.7ev。理论上，当CuPc的厚度超过12时，底层的Alq3的价带就被减弱了。由于在UPS内光生电子的刚性平均自由程是10-15，CuPc厚度为12时Alq3的价带消失表明CuPc在Alq3表面具有较高的电子粘附概率，在刚开始淀积是能形成几乎完整的膜。测量Alq3的电离势为5.8ev，CuPc的电离势为4.8ev，与先前的结果吻合良好。 The HOMO peak offset at the interface is estimated by spectral decomposition using a Gaussian function after background subtraction see the inset of Fig. 1. The HOMO peak of Alq3 is located at 3.92 eV with a full width at half maximumFWHM of about 0.85 eV. The HOMO edge of Alq3 EHOMO is thus estimated to be 3.20 eV by linear extrapolation at a low BE of FWHM/2 ln 2 from the peak maximum. The EHOMO of a 100-thick CuPc film is at 1.50 eV with a FWHM of 0.53 eV. For the CuPc 6 /Alq3 interface, features from both CuPc and Alq3 can be observed andthey show a large energy level shift relative to the bulk films. In order to accurately determine the true HOMO offset directly at the interface, additional XPS measurements were carried out. Evolution of XPS C 1s, N 1s, O 1s, Al 2p, and Cu 2p core level spectra shows no new components or broadening except for the intensity attenuation of the Alq3 components as the CuPc coverage increases. The results indicate the absence of a chemical reaction at the interface, which is consistent with the UPS results, which show no sign of formation of interface states in the organic gap （Fig. 1） 为了准确确定界面处HOMO能级差，还需要XPS测量。这个结果表面界面没有发生化学反应，这与UPS的结果相符合，因为没有迹象表面有机带隙界面太形成。 Figure 2 shows BE shifts, with respect to the bulk Alq3Fig. 2a and CuPc films Fig. 2b, in the HOMO, the vacuum level VL, and the XPS core levels all extracted from the UPS and XPS spectra as functions of CuPc coverage for the CuPc/Alq3 junction on Mg. Due to the superposition of the C 1s core level spectra, it is somewhat difficult to determine the exact amount of the C 1s core level shift in the Alq3 layer. As shown in Fig. 2, similar shifts are found for the valence band features and the core levels, which are associated with the energy level bending at the interface. With increasing CuPc coverage, electronic features of Alq3 at the interface show a 0.5 eV shift toward the lower BE region Fig. 2a. These rule out the contribution from charging in Alq3 and reflects the energy level bending in organic semiconductors at the interface. Spectral features of CuPc show a rigid downward shift of about 0.5 eV toward the interface. 图2显示的是结合能转移，分别是Alq3膜和CuPc膜的HOMO能级、真空能级和XPS芯能级（都是从UPS光谱和XPS光谱中得来的），这些值是在镁衬底上CuPc/Alq3结中CuPc含量的函数。由于最上面的是C的芯能级光谱，所以确定Alq3芯能级的偏移有点困难。如图2所示，在价带和芯能级处也有相似的偏移，与能级在界面处弯曲相一致。随着CuPc 含量增加，界面处Alq3的电子能级向更低的结合能区域偏移0.5ev。所有的这些都排除了Alq3电荷的贡献和有机半导体在界面处能级偏移的作用。 The same UPS and XPS experiments were repeated by depositing the CuPc/Alq3 junction onto a UV-ozone-treated ITO instead of a Mg substrate individual spectrum is not shown here. In this case, except for the intensity attenuation of the features of the Alq3 underlayer by the CuPc overlayer, all the molecular level positions remain constant with a negligible vacuum level shift at the interface, indicating the absenceof band bending in either layer. 这次把衬底换成ITO,取代镁，把CuPc/Alq3淀积在紫外臭氧环境内处理过的ITO衬底上，之后重复UPS和XPS实验。在这次实验中，除了CuPc和Alq3重叠部分有变弱的特征外，在界面处其他所有的分子能级位置基本保持不变。 Combining the results from the UPS and XPS measurements, energy diagrams of the CuPc/Alq3 heterojunction on ITO and Mg substrates are depicted in Fig. 3. The Fermi levels in the two materials are aligned in a thermodynamic equilibrium state. In the schematic energy diagram, the LUMO edge ELUMO is derived via the charge transport gaps of 4.2 and 1.7 eV for Alq3 and CuPc, respectively.13 ITO and Mg substrates shift the EF position in Alq3 film from 1.5 to 3.2 eV above the HOMO edge. The energy level lineup at the CuPc/Alq3 interface on ITO is consistent with the traditional concept of vacuum level alignment. However, the same heterojunction formed on the Mg substrate shows remarkably different electronic structures. The HOMO offset in the bulk of the two organics on the Mg substrate is estimated to be 1.7 eV, which is different from the value of 0.9 eV in the same junction formed on ITO. As shown in Fig. 3b, the built-in potential barriers in Alq3 and CuPceVbiA and eVbiC represent the energy level bending in the space charge regions. Taking into account the total energy level bending, the HOMO offset directly at the interfaceEHOMO for CuPc/Alq3 on Mg is, however, reduced to 0.7 eV, while the LUMO offset ELUMO is increased to 1.8 eV. 结合UPS和XPS的测量结果，我们画出了CuPc/Alq3异质结淀积在ITO和镁衬底上的能级图，如图3所示。这两种材料的费米能级在热力学平衡状态下回达到平衡。在能级结构图中，LUMO能级边缘（ELUMO）。在Alq3薄膜中ITO和镁衬底使得费米能级的位置从1.5ev移到3.2ev，超过了HOMO能级边缘。在CuPc/Alq3界面处能级平衡，与传统的真空能级对齐观点相符合。然而在镁衬底上相同的异质结却表现了截然不同的电子结构。镁衬底上两种有机材料的HOMO能级差估计在1.7ev，而在ITO衬底上确实0.9ev。如图3（b）所示，在CuPc和Alq3内的内建电势代表了空间电荷区域的能级弯曲。考虑所有的能级弯曲，CuPc/Alq3异质结界面处HOMO能级偏差减小到0.7ev，LUMO能级偏差增加到1.8ev。 Analogous to conventional inorganic semiconductor heterojunctions,the final energy level offsets are determined by a balance of charges in the space charge regions on both sides of the heterojunction, resulting in band bending. Once the metal/organic interface is formed, the EF position in the organic energy gap or the injection barrier height is dependent on the substrates by means of charge transfer between metals and organic semiconductors driven by the difference in electronegativity.6,7,14 It provides the probability to manipulate the EF position or the work function of the organic semiconductors by changing the substrates. Before the formation of the CuPc/Alq3 heterojunction, the EA of CuPc is larger than that of the work function of Alq3 when Mg is used as the substrate see Fig. 3b. In order to achieve thermal equilibrium with Fermi level alignment, charge carriers will spontaneously flow across the interface, giving rise to the formation of an accumulation layer of electrons in CuPc and a depletion layer in Alq3 adjacent to the interface. The space charges are stored in states generated by the extension of occupied and unoccupied orbitals in the energy gap. Therefore, built-in potential barriers are formed on both sides of the heterojunction. Since the LUMO of CuPc for the CuPc/Alq3 junction on Mg is close to the Fermi level, we expect a large number of energy states for electrons and thus a narrow space charge width 50 in the CuPc layer. On the contrary, when ITO is used as the substrate, the above conditions are not satisfied and thus the vacuum level alignment occurs Fig. 3a. For such a heterojunction, the LUMO offset induces a large barrier for electron injection from CuPc into Alq3, which has been reported in the organic light-emitting devices. 与传统的无机半导体异质结类似，最终的能级偏差是由异质结两边的空间电荷区电荷的平衡决定的，电荷平衡导师能带弯曲。一旦金属/有机界面形成，有机能隙（或者注入势垒高度）的费米能级位置依赖于衬底，金属和有机半导体之间通过电负性差实现电荷转移。本文提供了通过改变衬底来控制费米能级位置（或者说有机半导体功函数）的可能性。当镁作为衬底时在CuPc/Alq3异质结形成之前，CuPc的电子亲合势比Alq3的功函数大。为了在热平衡条件下使费米能级达到平衡，电荷载流子会自发向界面处移动，使得在CuPc层产生电荷积累，Alq3毗邻界面处形成耗尽层。空间电荷存储在能隙的占据轨道和非占据轨道。因此，在异质结两侧都会形成内建电势。由于在镁衬底上CuPc/Alq3异质结中CuPc的LUMO能级接近费米能级，我们预计在CuPc层内有大量的电子能级态。相反，当使用ITO作衬底时，上面所给的条件都不满足，所以产生了真空能级对齐，如图3所示。对于这样的异质结，LUMO能级差使得从CuPc层到Alq3层电子注入有更大的势垒，在有机光致发光器件中以有过报道。 The results observed here indicate that the energy level alignment at organic heterojunction is affected by the EF position in the energy gap, and the applicability of the traditional concept of vacuum level alignment at an organic heterojunction is limited to some specific conditions. This is in sharp contrast to the previous studies, in which the organic heterojunctions were commonly formed either on other substratese.g., Au, or between two wide-energy-gap organic materials.11,16 For example, the energy offsets at the N,N-bis-1-naphthyl-N,N-dphenyl-1,1-biphenyl-4,4-di