Structural and Optical Properties of Mixed Diindenoperylene–Perfluoropentacene Thin Films

Abstract

Mixed films of perfluorinated pentacene and diindenoperylene are studied for their structural and optical properties on SiO 2 and glass substrates. Grazing incidence Xray diffraction shows that the compounds mix on the molecular level. Two structures are identified. Their out-ofplane lattice spacing is determined by X-ray reflectivity, and their growth morphology is characterized using atomic force microscopy. It is found that one structure consists of molecules in a standing-up and the other in a lying-down orientation. The uniaxial dielectric function of mixed films is determined with spectroscopic ellipsometry, and the in-plane optical extinction coefficient is measured by real-time differential reflectance spectroscopy during growth. The question of intermolecular coupling between perfluorinated pentacene and diindenoperylene is discussed.

Structure formation in perfluoropentacene:diindenoperylene blends and its impact on transient effects in the optical properties studied in real-time during growth

“…32. For the equimolar mixture no Bragg-peaks at q || -positions corresponding to the pure films were observed, but Bragg-peaks significantly different in q || -position indicate the formation of a mixed crystal phase.…”

“…32. There, also the shift of the first observable peak in 2, xy (E) of the equimolar PFP:DIP mixture with film thickness was discussed.…”

“…The data are in agreement with post growth ellipsometry data. 25,32 Although the assignment for most of the peaks is not unique, the peaks at 1.8 eV and 2.0 eV can be attributed to PFP, since DIP shows no absorption in this region. For photon energies of 2.2 eV and higher, both DIP and PFP contribute to 2, xy (E).…”

“…[37][38][39][40][41][42][43][44] It is very tempting to study blends of PFP and DIP as the two compounds exhibit favourable intermolecular interactions, inter alia due to their quadrupole moments of opposite sign and are known to form an intermixed crystal phase. 32 Yet, the structure formation during the growth of mixed thin films is possibly influenced by the competition between the favourable intermolecular interaction and the steric incompatibility due to the size mismatch between PFP and DIP. As there is a tight connection of structural and optical properties, the latter may be significantly affected by changes in the film structure or the crystalline order during the growth of the samples.…”

Organic–Organic Heterostructures: Concepts and Applications

“…Recently, it was shown that a molecular complex is also formed in mixed PFP:DIP thin films. [123] Importantly, upon mixing PEN and PFP, in optical absorption spectra a new transition arises at 1.6 eV (Figure 7 b), which is not present in either of the spectra of the pure materials and indeed also below both of the respective gaps. [121] Detailed analysis shows that the absorption spectrum of PEN:PFP cannot be explained by a superposition of the spectra of the pure substances.…”