Structure Formation during Organic Molecular Beam Deposition

Pure and blended thin films of copper phthalocyanine (CuPc), Buckminster fullerene (C60) and coronene (Cor) molecules were deposited in vacuum onto standard silicon wafers and served as model systems for organic layers, as they are applied in organic photovoltaics (OPVs), organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). The blends were prepared by co-deposition, i.e. simultaneous evaporation of two molecular species. The influence of substrate temperature, deposition rate and mixing on the formation of crystal structures and surface profiles was investigated by various x-ray scattering techniques, as well as by atomic force and scanning electron microscopy. The non-linear formation of surface roughness was observed in real-time during the growth by means of in-situ x-ray reflectivity. Depending on the molecular species, three different growth modes were found: The growth of distinct islands, wetting of the substrate and layer plus island growth. Higher substrate temperatures resulted in larger islands and larger crystalline domains at lower island densities. A similar effect was observed when reducing the deposition rate. Faster diffusion and a lower flux of impinging molecules accounts for the improved molecular self-assembly. Mixing of two molecular species lead to smooth CuPc-C60 blends at room temperature and extremely rough CuPc-C60 blends at 400 K. The domain sizes were significantly reduced in blends and long CuPc needles protruding from the thin film appeared. Although a theoretical description of the structure formation is challenging, the studies have shown that a systematic analysis enables to tailor the physical properties of organic thin films by a suitable choice of growth parameters, which is advantageous for technical applications

Interrupted growth to manipulate phase-separation in DIP:C60 organic semiconductor blends

by Rupak Banerjee, Alexander Hinderhofer, Michael Weinmann, Berthold Reisz, Christopher Lorch, Alexander Gerlach, Martin Oettel and Frank Schreibe

Thin film growth of phase-separating phthalocyanine-fullerene blends: A combined experimental and computational study

Blended organic thin films have been studied during the last decades due to their applicability in organic solar cells. Although their optical and electronic features have been examined intensively, there is still lack of detailed knowledge about their growth processes and resulting morphologies, which play a key role for the efficiency of optoelectronic devices such as organic solar cells. In this study, pure and blended thin films of copper phthalocyanine (CuPc) and the Buckminster fullerene (C60) were grown by vacuum deposition onto a native silicon oxide substrate at two different substrate temperatures, 310 K and 400 K. The evolution of roughness was followed by in-situ real-time X-ray reflectivity. Crystal orientation, island densities and morphology were examined after the growth by X-ray diffraction experiments and microscopy techniques. The formation of a smooth wetting layer followed by rapid roughening was found in pure CuPc thin films, whereas C60 shows a fast formation of distinct islands at a very early stage of growth. The growth of needle-like CuPc crystals loosing their alignment with the substrate was identified in co-deposited thin films. Furthermore, the data demonstrates that structural features become larger and more pronounced and that the island density decreases by a factor of four when going from 310 K to 400 K. Finally, the key parameters roughness and island density were well reproduced on a smaller scale by kinetic Monte-Carlo simulations of a generic, binary lattice model with simple nearest-neighbor interaction energies.Comment: 30 pages, 11 figure

Structural, optical, and electronic characterization of perfluorinated sexithiophene films and mixed films with sexithiophene

We report on the growth and characterization of molecular mixed thin films of α-sexithiophene (6T), a well-known organic p-type semiconductor with high hole mobility, together with its perfluorinated counterpart, the so far rarely studied tetradecafluoro-α-sexithiophene (PF6T). Pure and blended thin films of these two molecules with different mixing ratios were grown on silicon oxide in ultrahigh vacuum by coevaporation. The effect of perfluorination and mixing on crystal structure, morphology, electronic, and optical properties was examined. The evolution of the PF6T crystal structure was followed in situ in real time by X-ray scattering. We found a new thin film structure different from the reported bulk phase with molecules either standing-up or lying-down depending on the growth temperature. The different morphologies of pure films and blends were investigated with atomic force microscopy. The impact of mixing on the core-levels and on the highest occupied molecular orbitals of 6T and PF6T is discussed.by Berthold Reisz, Simon Weimer, Rupak Banerjee, Clemens Zeiser, Christopher Lorch, Giuliano Duva, Johannes Dieterle, Keiichirou Yonezawa, Jin-Peng Yang, NobuoUeno, Satoshi Kera, Alexander Hinderhofer, Alexander Gerlach and Frank Schreibe

Real-Time Structural and Optical Study of Growth and Packing Behavior of Perylene Diimide Derivative Thin Films: Influence of Side-Chain Modification

We study the growth of two n-type small-molecule organic semiconductors from the perylene diimide family: <i>N</i>,<i>N</i>′-bis-(2-ethylhexyl)-dicyanoperylene-3,4:9,10-bis­(dicarboximide) (PDIR-CN₂) and <i>N</i>,<i>N</i>′-1<i>H</i>,1<i>H</i>-perfluorobutyl-dicyanoperylene-3,4:9,10-bis­(dicarboximide) (PDIF-CN₂) whose chemical structures differ only in the imide substituents, branched alkyl chains −C₈H₁₆ and linear fluoroalkyl chains −C₄F₇H₂, respectively. Both types of substituents introduce some degree of steric hindrance for intermolecular interactions, affecting solid-state packing during thin film formation, and thus induce specific structure-dependent optoelectronic properties in thin films. The transition from an amorphous structure to crystalline domains with strong intermolecular coupling was followed in situ and in real time during growth. We investigated the structural and morphological properties by X-ray diffraction and atomic force microscopy as a function of the substrate temperature and chemical structure. We examined the relationship between the structural properties and thin film optical signatures probed via differential reflectance spectroscopy, ellipsometry, and temperature-dependent photoluminescence. A new crystalline PDIR-CN₂ polymorph at high temperatures emerges. In addition, we observed in PDIF-CN₂ that the fluorinated chains contribute to crystallization inhibition because of the higher overall steric hindrance compared to the alkyl chains

Real-Time Monitoring of Growth and Orientational Alignment of Pentacene on Epitaxial Graphene for Organic Electronics

The interaction between a graphene layer and pentacene (PEN) molecules leads to the formation of a lying-down phase, which can improve charge transport for organic vertical field effect transistors and enhance the optical absorption for increased light harvesting in organic solar cells. Here, we present a comprehensive study of PEN growth on epitaxial graphene on silicon carbide (SiC). Simultaneous grazing-incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) were used in situ for real-time monitoring of the PEN crystal growth with millisecond time resolution to identify two distinct anisotropic growth stages after the nucleation of the first monolayer (ML). In the first stage up to 1.5 nm, we observe rapid growth of pentacene domains along the (010) and (001) facets. This growth behavior is saturating after 1.5 nm. In a second stage, this is followed by continuous lateral crystal growth in only one in-plane direction (100) forming needle-shaped domains. In the second stage, an uninterrupted linear growth of the lying-down PEN phase is found based on the (001) diffraction up to 15 nm. Ex situ atomic force microscopy and polarized confocal Raman microscopy were used to further support the real-time observations of aligned PEN films on graphene

Real-Time Monitoring of Growth and Orientational Alignment of Pentacene on Epitaxial Graphene for Organic Electronics

The interaction between a graphene layer and pentacene (PEN) molecules leads to the formation of a <i>lying-down</i> phase, which can improve charge transport for organic vertical field effect transistors and enhance the optical absorption for increased light harvesting in organic solar cells. Here, we present a comprehensive study of PEN growth on epitaxial graphene on silicon carbide (SiC). Simultaneous grazing-incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) were used in situ for real-time monitoring of the PEN crystal growth with millisecond time resolution to identify <i>two</i> distinct anisotropic growth stages after the nucleation of the first monolayer (ML). In the first stage up to 1.5 nm, we observe rapid growth of pentacene domains along the (010) and (001) facets. This growth behavior is saturating after 1.5 nm. In a second stage, this is followed by continuous lateral crystal growth in only one in-plane direction (100) forming needle-shaped domains. In the second stage, an uninterrupted linear growth of the <i>lying-down</i> PEN phase is found based on the (001) diffraction up to 15 nm. Ex situ atomic force microscopy and polarized confocal Raman microscopy were used to further support the real-time observations of aligned PEN films on graphene

Thin-Film Texture and Optical Properties of Donor/Acceptor Complexes. Diindenoperylene/F6TCNNQ vs Alpha-Sexithiophene/F6TCNNQ

In this work, two novel donor/acceptor (D/A) complexes, namely, diindenoperylene (DIP)/1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane (F6TCNNQ) and alpha-sexithiophene (6T)/F6TCNNQ, are studied. The D/A complexes segregate in form of π–π stacked D/A cocrystals and can be observed by X-ray scattering. The different conformational degrees of freedom of the donor molecules, respectively, seem to affect the thin-film crystalline texture and composition of the D/A mixtures significantly. In equimolar mixtures, for DIP/F6TCNNQ, the crystallites are mostly uniaxially oriented and homogeneous, whereas for 6T/F6TCNNQ, a mostly 3D (isotropic) orientation of the crystallites and coexistence of domains of pristine compounds and D/A complex, respectively, are observed. Using optical absorption spectroscopy, we observe for each of the two mixed systems a set of new, strong transitions located in the near-IR range below the gap of the pristine compounds: such transitions are related to charge-transfer (CT) interactions between donor and acceptor. The optical anisotropy of domains of the D/A complexes with associated new electronic states is studied by ellipsometry. We infer that the CT-related transition dipole moment is perpendicular to the respective π-conjugated planes in the D/A complex