Electronic Structure at the Interface between Rubrene and Perylenediimide Single Crystals: Impact of Interfacial Charge Transfer and its Modulation

Abstract

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 ]