Electronic Structure at the Interface between Rubrene and Perylenediimide Single Crystals: Impact of Interfacial Charge Transfer and its Modulation
Electronic Structure at the Interface between Rubrene and Perylenediimide Single Crystals: Impact of Interfacial Charge Transfer and its Modulation
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红荧烯和苝二酰亚胺单晶界面的电子结构：界面电荷转移的影响及其调节
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发表时间：
2014
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通讯作者：
J. Brédas
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作者：
Xian;Yao Fu;Hong Li;J. Brédas
DOI：10.1002/adm.201400362密度近似（LDA）和带隙的半经验校正。根据TTF-TCNQ块体晶体结构，假设TTF和TCNQ层堆叠的界面几何结构; [ 14 ]然而，这种方法忽略了TTF和TCNQ的单个单晶显示出明显不同的晶格参数的事实。因此，两个单晶之间的界面可能与相应的共晶的界面非常不同。在这里，我们通过在半局域和杂化水平上进行有机D-A界面的分子动力学（MD）模拟和DFT计算，大大超越了这些假设。在裸或改性艾德SiO2/Si（100）衬底上的红荧烯和PDIFCN 2结晶膜的X射线衍射（XRD）实验分别显示红荧烯和PDIF-CN 2表面的优先（h 00）[ 15 ]和（00 l）[ 16 ]暴露。基于这些实验数据，我们假定红荧烯和PDIF-CN 2结晶薄膜在界面处的取向是沿着各自晶体结构的[100]和[001]方向。(Note必须在红荧烯晶体的原始坐标系上执行旋转操作，以在描述红荧烯/ PDIF-CN 2界面的新坐标系中将其（100）表面重新定向到（001），参见图1）。然后，我们对由三层红荧烯原材料（100）与三层PDIF-CN 2（001）（分别使用红荧烯和PDIF-CN 2晶体结构定义取向）接触组成的多层系统进行了MD模拟（详见“计算方法”部分和支持信息）。我们考虑了具有2.96 × 2.97 × 20 nm 3的超晶胞的周期性边界条件，包括沿着z方向（垂直于界面）的12 nm真空空间，以防止多层板之间的虚假相互作用。以10 ps的时间间隔拍摄界面的四个平衡快照。分子动力学结果表明，两种结晶薄膜之间的相互作用主要局限于最顶层的红荧烯和最底层的PDIF-CN 2。这进一步通过DFT水平的电子结构计算得到证实，其中我们考虑了由每个分子组分的两层组成的四层界面系统（参见支持信息中的C节和图S4）。因此，从平衡快照中，提取了四个独立的红荧烯/PDIFCN 2双层构型，并作为DFT计算的输入（尺寸：基于红荧烯晶体的周期性，在x-y平面内为0.74 × 1.485nm 2）。DFT计算的每个表面单胞包含两个红荧烯和两个PDIF-CN2分子（具有红荧烯（100）表面沿着x和y方向的2-D周期性）和真空空间有机供体-受体（D-A）界面处的电荷转移是有机（光）电子器件中的基本过程之一，例如有机太阳能电池-其中有效的激子解离和电荷分离仅发生在D-A界面处或附近-或者有机发光二极管-其中关键步骤是电子-空穴复合。[ 1-4 ]几年前，Morpurgo及其同事在四硫富瓦烯（TTF）和7，7，8，8-四氰基醌二甲烷（TCNQ）单晶之间形成的有机D-A界面处发现了一种新现象;由于相邻层之间从供体TTF到受体TCNQ的显著电子转移，证明了界面处的金属导电。[ 5 ]在这项工作之后，许多研究强调了具有良好界面的其他有机D-A系统的电传输特性和光电导响应。[ 6-11 ]最近，Morpurgo等报道了红荧烯与N，N′ -1 H，1 H -全氟丁基二氰基二萘甲酰二亚胺（PDIF-CN 2）单晶之间形成的界面处的奇特电输运特性。在这种情况下，他们发现电子载流子密度与温度呈线性关系，指向一个非常小的带隙，载流子迁移率在室温附近表现出带状行为，并在30 K时保持高达1 cm 2 V −1 s −1。[ 6 ]美国
DOI: 10.1002/admi.201400362 Density Approximation (LDA) and a semi-empirical correction to the bandgap. An interface geometry was assumed where TTF and TCNQ layers stack according to the TTF-TCNQ bulk crystal structure; [ 14 ] such an approach, however, neglects the fact that the individual single crystals of TTF and TCNQ display markedly different lattice parameters. As such, the interface between two single crystals can turn out to be very different from that of the corresponding co-crystal. Here, we go signifi cantly beyond these assumptions by carrying out molecular dynamics (MD) simulations of the organic D-A interface and DFT calculations at the semi-local and hybrid levels. X-ray diffraction (XRD) experiments on rubrene and PDIFCN 2 crystalline fi lms on bare or modifi ed SiO 2 /Si(100) substrates have shown preferential ( h 00) [ 15 ] and (00 l ) [ 16 ] exposures for the surfaces of rubrene and PDIF-CN 2 , respectively. Based on these experimental data, we assumed as a starting point that the orientations of the rubrene and PDIF-CN 2 crystalline thin fi lms at their interface are along the [100] and [001] directions of the respective crystal structures. (Note that a rotation operation has to be performed on the original coordinate system of the rubrene crystal to reorient its (100) surface to (001) in the new coordinate system describing the rubrene/ PDIF-CN 2 interface, see Figure 1 ). We then carried out MD simulations (see the “Computational Methodology” section and the Supporting Information for details) on a multilayer system consisting of three layers of rubrene original (100) in contact with three layers of PDIF-CN 2 (001) (with the orientations defi ned using the rubrene and PDIF-CN 2 crystal structures, respectively). We considered periodic boundary conditions with a supercell of 2.96 × 2.97 × 20 nm 3 , including a vacuum space of 12 nm along the z-direction (normal to the interface) to prevent spurious interactions between the multilayer slabs. Four equilibrated snapshots of the interface were taken with time intervals of 10 ps. The MD results point out that the interactions between the two crystalline thin fi lms are mainly limited to the very top layer of rubrene and the bottom layer of PDIF-CN 2 . This is further confi rmed by electronic-structure calculations at the DFT level where we consider a four-layer interface system consisting of two layers of each molecular component (see Section C and Figure S4 in the Supporting Information). Therefore, out of the equilibrated snapshots, four independent rubrene/PDIFCN 2 bilayer confi gurations were extracted and served as input for the DFT calculations (size: 0.74 × 1.485 nm 2 within the x-y plane based on the periodicity of the rubrene crystal). Each surface unit cell for the DFT calculations contains two rubrene and two PDIF-CN 2 molecules (with 2-D periodicity of the rubrene (100) surface along the xand y-directions) and a vacuum space Charge transfer at organic donor-acceptor (D-A) interfaces is one of the fundamental processes in organic (opto)electronic devices, such as organic solar cells –where effi cient exciton dissociation and charge separation only occurs at or near the D-A interface– or organic light-emitting diodes –where a critical step is electron-hole recombination. [ 1–4 ] A few years ago, Morpurgo and co-workers uncovered a new phenomenon at the organic D-A interface formed between tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) single crystals; a metallic conduction at the interface was demonstrated due to signifi cant electron transfer between adjacent layers from the donor TTF to the acceptor TCNQ. [ 5 ] Following this work, a number of studies have highlighted the electrical transport properties and photoconductive response of other organic D-A systems with well-defi ned interfaces. [ 6–11 ] Recently, Morpurgo and co-workers have reported peculiar electrical-transport characteristics at the interface formed between rubrene and N , N′ -1 H ,1 H -perfl uorobutyldicyanoperylenecarboxydiimide (PDIF-CN 2 ) single crystals. In that instance, they found that the electron carrier density has a linear dependence with temperature, pointing to a very-small-to-vanishing bandgap, with the carrier mobility exhibiting band-like behavior around room temperature and remaining as high as 1 cm 2 V −1 s −1 at 30 K. [ 6 ]