The interface of organic heterojunctions plays a crucial role for charge separation in photovoltaic cells, for charge generation layers in tandem devices, and the charge carrier density in bilayer field-effect transistors. Foremost, the structural and electronic properties are of paramount importance. Here two prototypical interfaces for planar organic heterojunctions are discussed: the combination of diindenoperylene (DIP) with fullerene (C60) and the combination of copper-phthalocyanine (H16CuPc) with its perfluorinated analog (F16CuPc). Angle dependent near edge X-ray absorption fine structure (NEXFAS) measurements were performed to determine the molecular orientation at the interface. These results are compared to measurements of the frontier level alignment by ultraviolet photoelectron spectroscopy (UPS).
The orientation of the rod-like DIP molecules is unaffected upon deposition of spherical C60 molecules on top. The highest occupied molecular orbital (HOMO) energy levels do not shift upon interface formation. In contrast, cofacial “lying” interface layers with π-orbital stacking of the two phthalocyanines is observed by UPS and NEXAFS. Here energy level bending of the HOMO levels is found. These results can be related to the solar cell performance of these two material combinations. Whereas DIP/C60 solar cells have an open circuit voltage of up to 0.9 V [1], the band bending in planar structures of H16CuPc/F16CuPc leads to a charge generation layer [2]. The absence of a photovoltaic effect at the planar heterojunction of the phthalocyanines is thus explained by these findings.
The combined experimental approach results in a comprehensive model for the electronic and morphological characteristics of the interface between the two investigated organic semiconductors.The presence of a π orbital stacking between different molecules at a heterojunction is also of interest for photovoltaic active interfaces or for ground-state charge-transfer. In all cases the performance of the interface strongly depends on the relative orientation of the π orbitals of the involved materials.
[1] J. Wagner, M. Gruber, A. Hinderhofer, A. Wilke, B. Bröker, J. Frisch,
P. Amsalem, A. Vollmer, A. Opitz, N. Koch, F. Schreiber, W. Brütting, Adv. Funct. Mater. 20 (2010) 4295. [2] A. Opitz, B. Ecker, J. Wagner, A. Hinderhofer, F. Schreiber, J. Manara,
J. Pflaum, W. Brütting, Org. Electron. 10 (2009) 1259.