Energy-level alignment at model interfaces of organic electroluminescent devices studied by UV photoemission: Trend in the deviation from the traditional way of estimating the interfacial electronic structures
Energy-level alignment at model interfaces of organic electroluminescent devices studied by UV photoemission: Trend in the deviation from the traditional way of estimating the interfacial electronic structures
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DOI：
10.1109/2944.669459
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发表时间：
1998-01-01
影响因子：
4.9
通讯作者：
Seki, K
中科院分区：
文献类型：
--
作者：
Ishii, H;Sugiyama, K;Seki, K
用紫外光电子能谱（UPS）研究了有机电致发光（EL）器件模型界面的电子结构。TTN（四硫杂萘并蒽）和TCNQ（四氰基醌二甲烷）的界面也分别作为空穴传输和电子传输材料的极端情况进行了研究。对于所研究的所有有机/金属界面，金属电极的功函数通过有机层的沉积而改变，即，真空能级在界面处移动，表明传统的能级排列模型的无效性，其中在有机/金属界面处假设共同的真空能级。在TCNQ/Au、DP-NTCI/Al受主/金属界面处，观察到有机层的真空度相对于金属的真空度向上移动，表明由于电子从金属转移到受主而形成界面偶极子。在其他有机/金属界面，TPD（N，N ′-二苯基-N，N ′-（3-甲基苯基）-1，1 ′-联苯-4，4 ′-二胺）/Au或ITO（氧化铟锡），ALq（3）（三（8-羟基喹啉）铝）/Al，DP-NTCI（N，N ′-二苯基-1，4，5，8-萘基四羧基酰亚胺）/Al或Au，观察到真空能级向下移动，在我们以前的卟啉/金属界面的研究中也观察到了这种向下的位移，并且似乎是有机/金属界面的趋势，在该界面处没有从金属到有机层的电子转移发生。这一趋势表明，传统模型往往低估（高估）空穴（电子）注入的势垒高度。另一方面，ALq（3）/TPD界面处的真空能级位移小于0.1eV，这表明传统模型具有明显的适用性。然而，有机/有机界面的情况并非总是如此：在TTN/TCNQ界面处观察到0.2eV的有限位移，这是由于电子从TTN到TCNQ的转移。还讨论了有机/金属界面处真空能级位移的可能来源。
The electronic structures of model interfaces of organic electroluminescent (EL) devices were investigated with UV photoemission spectroscopy (UPS). Interfaces of TTN (tetrathianaphthacence) and TCNQ (tetracyanoquinodimethane) were also studied as extreme cases for hole transport and electron transport material, respectively, For all organic/metal interfaces studied, the work function of metal electrode was changed by deposition of organic layer, i.e., the vacuum level was shifted at the interface, indicating the invalidity of the traditional energy level alignment model where a common vacuum level was assumed at organic/metal interface. At TCNQ/Au, DP-NTCI/AI, which are acceptor/metal interfaces, upward shift of the vacuum level of organic layer relative to that of metal was observed, suggesting the formation of interfacial dipole due to electron-transfer from metal to acceptor. At other organic/metal interfaces, TPD(N, N'-diphenyl-N, N'-(3-methylphenyl)-1, 1'-biphenyl-4, 4'-diamine)/Au or ITO (indium tin oxide), ALq(3) (tris(8-hydroxyquinolino) aluminum)/Al, DP-NTCI(N, N'-diphenyl-1,4,5,8- naphtyltetracarboxylimide)/Al or au, downward shift of the vacuum level was observed, Such downward shift has been also observed in our previous study for porphyrin/metal interfaces, and seems to be a trend for organic/metal interfaces at which no electron-transfer from metal to organic layer occurs. This trend suggets that the traditional model tends to underestimate (overestimate) the barrier height for hole (electron) injection, On the other hand, the vacuum level shift at ALq(3)/TPD interface was less than 0.1 eV, leading to an apparent applicability of the traditional model. However, it is not always the case for organic/organic interfaces: finite shift of 0.2 eV was observed at TTN/TCNQ interface due to electron-transfer from TTN to TCNQ, Possible origins of vacuum level shift at organic/metal interfaces were also discussed.