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

Abstract

We discuss the result of the competing effects of favourable intermolecular interactions and steric incompatibilities due to the size mismatch of perfluoropentacene (PFP) and diindenoperylene (DIP) on the structure formation and associated optical properties in mixed films. Using real-time grazing incidence X-ray diffraction we investigate the size of coherently scattering islands l(s) as a function of film thickness and mixing ratio. We find that for PFP:DIP 1:2 blends l(s) is by a factor of ~4 smaller than in pure DIP films, while l(s) of the PFP:DIP 2:1 blends is not significantly reduced compared with pure PFP. Yet, we observe an increase in l(s) with film thickness for all of the samples, independent on the mixing ratio. In parallel with the structural characterization we investigate the evolution of the absorption spectra in the visible spectral range and its dependence on l(s) in situ during film growth using differential reflectance spectroscopy. We observe a surprisingly strong effect of changes in the structural order on the shape of ε(2, xy)(E), evident by a pronounced evolution of characteristic peaks in the thickness range from 1.6 nm to 9.6 nm. The combined results of the real-time experiments allow to identify the thickness dependent crystal grain size as the origin of the observed transient effects in the absorption spectra.

Delayed phase separation in growth of organic semiconductor blends with limited intermixing

“…This effect is different from kinetically limited phase separation as observed for example in mixtures of diindenoperylene and buckminsterfullerenes where at the beginning of growth a non crystalline mixed phase forms . The data does not show a continuous increase of the coherent crystallite size as observed in other mixtures with coexisting phases . Blends of PEN with diindenoperylene show a very similar mixing behavior with the important difference that the long‐range order parallel to the substrate surface is reduced significantly.…”

Charge Separation at Nanostructured Molecular Donor–Acceptor Interfaces

“…• Phase separation: If the two materials are sterically incompatible [62] or the interaction energies within the single materials are significantly more attractive than the inter-species energy (χ > 2) usually a phase separation of the materials is observed. • Ordered complex: An ordered complex is usually formed if the interaction energy between the two different materials is higher than the interaction energies within one species and the materials are sterically compatible [61,63,64].…”

Non-equilibrium Roughness Evolution of Small Molecule Mixed Films Reflecting Equilibrium Phase Behavior

“…2a-f shows the roughness of six different types of molecular mixed films dependent on mixing ratio at a thickness of 20 nm. All of the studied binary compounds mix on the molecular level, but they can be distinguished by their tendency to form a co-crystal [28][29][30][32][33][34]. The roughness of all pure materials is in general relatively large.…”

“…A still more challenging case for complex materials are binary molecular systems, which are also important due to their electronic properties. Molecular mixed thin films are studied both with small mixing ratios (≈ 1:100) [23][24][25] for doping as well as large mixing ratios (≈ 1:1) for bulk heterojunctions and molecular complex formation [26][27][28][29][30][31][32][33][34]. Dependent on the effective interactions of the compounds, binary systems exhibit several different mixing behaviors in the bulk, such as solid solution, cocrystallization or phase separation [27,35].…”