On the Origin of Gap States in Molecular Semiconductors—A Combined UPS, AFM, and X-ray Diffraction Study
On the Origin of Gap States in Molecular Semiconductors—A Combined UPS, AFM, and X-ray Diffraction Study
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DOI：
10.1021/acs.jpcc.1c03096
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发表时间：
2021-08
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通讯作者：
Jan Hagenlocher;Niels Scheffczyk;K. Broch;G. Duva;Nadine Rußegger;Lisa Egenberger;R. Banerjee;S. Kera;F. Schreiber;A. Hinderhofer
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作者：
Jan Hagenlocher;Niels Scheffczyk;K. Broch;G. Duva;Nadine Rußegger;Lisa Egenberger;R. Banerjee;S. Kera;F. Schreiber;A. Hinderhofer
有机半导体的HOMO-LUMO能隙内的电子态在衬底-有机和有机-有机界面的能级对准中起关键作用，因此是器件功能和效率的定义参数。它们被认为是由影响分子特定环境的结构缺陷引起的。通过改变分子束沉积生长的样品的衬底温度，我们能够控制它们的密度。使用原子力显微镜和X射线散射技术，我们可以区分缺陷，这取决于它们的长度尺度和有效方向。通过超低背景紫外光电子能谱法直接测量的相应缺陷密度与间隙态密度的比较，使得能够关联不同原型有机半导体的结构和电子性质。我们研究了能隙态对能级排列的影响，发现了结构缺陷和界面偶极子之间的直接联系。绝大多数基于有机半导体的（光）电子器件的特征在于具有形成异质结的至少两种不同化合物的架构。通常，这允许精确控制设备功能和性能。取决于所需的性质，化合物可以在分子水平（A/B）混合，导致共混物或形成平面异质结（A-on-B），在两个有机薄膜之间具有理想的尖锐界面。对于平坦界面观察到的现象是所谓的能级对准（ELA），其中分子前线轨道的相对位置可以移动，并且能量势垒可以比理论预期的更小或更大。已经提出了不同的机制来解释这种行为，但是，到目前为止，一个统一的模型仍然是难以捉摸的，取决于底物的反应性，不同的情况下必须加以区分。对于弱相互作用的惰性表面的情况，有人提出，ELA实际上是由有机化合物的能隙（间隙状态）内的小密度的电子状态决定的。它们被认为是由作为掺杂剂的结构或化学缺陷引起的，并且对间隙状态的存在进行建模的理论研究强调了ELA机制的重要性和影响。结果表明，它们的外观是一个共同的特点，在分子薄膜和它们的密度可以通过改变制备参数进行控制。到目前为止，这主要是通过使用不同的有机或无机掺杂剂的化学或分子掺杂来实现的。在本研究中，应用了一种替代掺杂的方法：通过在相应有机薄膜的生长过程中使用不同的衬底温度接收时间：2021年4月6日修订时间：2021年7月22日发布时间：2021年8月6日图1。(a)所研究化合物的示意图：并五苯（PEN，C22 H14），全氟并五苯（PFP，C22 F14），[6]非并苯（C26 H16）和C60。（B−e）不同结构缺陷的示意图，这些结构缺陷是电子能隙态的可能原因，每个结构缺陷代表不同的长度尺度或取向。相当大的缺陷源，如不同畴之间的边界（B）或微晶相对于表面法线的倾斜（d），可以与由微晶尺寸、失配、空位或杂质[红色块，（c，d）]在面内和面外方向上引入的微观不规则性区分开来。Article pubs.acs.org/JPCC © 2021 American Chemical Society 17929 https://doi.org/10.1021/acs.jpcc.1c03096 J. Phys. Chem. C 2021，125，17929−17938 D ow nl oa de d vi a U N IV T U E B IN G E N o n A ug us t 1 9，2 02 1 at 0 8：15：5 1（U T C）. See e e ht tp s：//p ub s. AC S。或g/ sh ar in gg ui de lines for or o pt io ns on how to le gi tim at el y sh ar e publ is he dartic le s.
Electronic states within the HOMO−LUMO gap of organic semiconductors play a key role in the energy level alignment of substrate−organic and organic−organic interfaces and therefore are a defining parameter for device functionality and efficiency. They are thought to result from structural defects influencing the specific environment of a molecule. Varying the substrate temperature for samples grown by molecular beam deposition, we are able to control their density. Using atomic force microscopy and X-ray scattering techniques, we can differentiate defects depending on their length scale and effective direction. Comparison of the respective defect density with the density of gap states, measured directly via ultra-low-background ultraviolet photoelectron spectroscopy, enables to correlate structural and electronic properties for different prototypical organic semiconductors. We investigate the impact of gap states on the energy level alignment and find a direct link between structural defects and the interface dipole. ■ INTRODUCTION The vast majority of (opto-)electronic devices based on organic semiconductors features an architecture with at least two different compounds forming a heterojunction. Generally, this allows a precise control of device functionality and performance. Depending on the desired properties, the compounds can be mixed at the molecular level (A/B) resulting in a blend or form a planar heterojunction (A-on-B) with an ideally sharp interface between the two organic thin films. A phenomenon observed for the flat interface is the so-called energy level alignment (ELA), where the relative positions of the molecular frontier orbitals can be shifted, and energy barriers can be smaller or larger than what is theoretically expected. Different mechanisms have been proposed to explain this behavior, but, until now, a unifying model is still elusive and depending on the reactivity of the substrate, different cases must be distinguished. For the case of weakly interacting, inert surfaces, it was proposed that ELA is in fact governed by a small density of electronic states within the energy gap of the organic compounds (gap states). They are thought to result from structural or chemical defects acting as dopants, and also, theoretical studies modelling the presence of gap states underline the importance and impact on the ELA mechanism. It was demonstrated that their appearance is a common feature in molecular thin films and their density can be controlled by changing preparation parameters. Until now, this was mainly done by chemical or molecular doping using different organic or inorganic dopants. In the present study, an approach alternative to doping is applied: by using different substrate temperatures during the growth of the respective organic thin Received: April 6, 2021 Revised: July 22, 2021 Published: August 6, 2021 Figure 1. (a) Schematics of the investigated compounds: pentacene (PEN, C22H14), perfluoropentacene (PFP, C22F14), [6]phenacene (C26H16), and C60. (b−e) Schematic representation of the different structural defects which are a possible cause of the electronic gap states, each of which represents a different length scale or orientation. Rather large defect sources such as boundaries between different domains (b) or the tilt of crystallites relative to the surface normal (d) can be distinguished from microscopic irregularities introduced by crystallite size, misfits, vacancies, or impurities [red block, (c,d] in the in-plane and out-of-plane directions. Article pubs.acs.org/JPCC © 2021 American Chemical Society 17929 https://doi.org/10.1021/acs.jpcc.1c03096 J. Phys. Chem. C 2021, 125, 17929−17938 D ow nl oa de d vi a U N IV T U E B IN G E N o n A ug us t 1 9, 2 02 1 at 0 8: 15 :5 1 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.