Origin and role of gap states in organic semiconductor studied by UPS: as the nature of organic molecular crystals
Origin and role of gap states in organic semiconductor studied by UPS: as the nature of organic molecular crystals
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DOI：
10.1088/1361-6463/aa840f
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发表时间：
2017-09
期刊：
影响因子：
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通讯作者：
Jinpeng Yang;F. Bussolotti;S. Kera;N. Ueno
中科院分区：
文献类型：
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作者：
Jinpeng Yang;F. Bussolotti;S. Kera;N. Ueno
本文主要介绍了日本千叶大学利用紫外光电子能谱（UPS）及相关方法对有机半导体的桥接电子结构和电荷输运性质进行的实验研究，特别是对最高占据分子轨道带-最低未占据分子轨道带（HOMO-LUMO）带隙中电子态的起源和作用的研究。我们总结了实验观察，包括直接测量的“看不见的”间隙状态的灵敏度UPS，这表明在有机半导体中存在本征间隙状态。我们首先描述了有机分子固体的性质来理解有机半导体的特征，因为这种本征能隙态是这些特征相互作用的结果，这些特征给出了有机半导体和无机半导体之间的主要区别。然后，我们讨论了（i）的起源和作用的带隙状态与分子间相互作用/带色散和电子-声子耦合，（ii）费米能级钉扎问题在有机半导体，和（iii）的方法计算费米能级的HOMO-LUMO间隙内的实验组。当对有机体系施加弱扰动时，即通过与其它材料接触、通过注入电荷、通过升高温度和通过暴露于1个大气压的气体，有机半导体的差距态容易出现。我们最终发现，HOMO和LUMO的拖尾态总是存在的，它们的能量分布一定不是对称的;因此，它们产生了比以前认为的更大的费米能级移动。此外，计算结果表明，当系统温度不为零（T > 0）时，在没有带隙态的弱相互作用有机/导体系统中会发生费米能级钉扎现象，这是半导体器件领域中的一个著名现象。我们还描述了超高灵敏度UPS的有机系统，这是非常脆弱的紫外线照射后，我们以前发表的结果和一些更新的实验诀窍。
This article reviews experimental studies on ‘bridging electronic structure and charge transport property of organic semiconductors’ performed using ultraviolet photoelectron spectroscopy (UPS) and related methods mainly in Chiba University, Japan, in particular on the investigation of the origin and the role of electronic states existing in the highest occupied molecular orbital band–lowest unoccupied molecular orbital band (HOMO–LUMO) gap. We summarize experimental observations including direct measurements of ‘invisible’ gap states with ultrahigh sensitivity UPS, which demonstrate that there exist intrinsic gap states in organic semiconductors. We firstly describe the nature of organic molecular solids to understand features of organic semiconductors because such intrinsic gap states are a result of the interplay of these features, which give the principal difference between the organic semiconductor and inorganic counterpart. We then discuss (i) the origin and role of the band gap states in relation to intermolecular interaction/band dispersion and electron–phonon coupling, (ii) the Fermi level pinning issue in organic semiconductors, and (iii) the method of computing the Fermi level position within the HOMO–LUMO gap for experimental groups. The gap states of organic semiconductors appear easily when a weak perturbation is applied to the organic system, namely by contact with other material, by injecting a charge, by elevating temperature, and by exposure to 1 atm gas. What we finally found is that tailing states of HOMO and LUMO always exist, and their energy distributions must not be symmetric; they thus produce a larger Fermi level shift from the mid gap position than previously thought. Furthermore, as shown by computational work, Fermi level pinning, which is a well-known phenomena in semiconductor devices field, occurs in weakly interacting organic/conductor systems without any gap states if the system temperature is not zero (T > 0). We also describe the experimental knowhow of the ultrahigh-sensitivity UPS of organic systems, which are very fragile upon ultraviolet-light irradiation, and some updates on our previously published results.