G. Hernandez-Sosa

Para-sexiphenyl-CdSe/ZnS nanocrystal hybrid light emitting diodes

CdSe/ZnS core/shell nanocrystals (NCs) are integrated into para-sexiphenyl (p-6P) based hybrid light emitting diodes, to obtain green and red emission in addition to blue emission originated from p-6P. For the active region of the devices, ultrathin layers of p-6P and NCs are deposited by hot wall epitaxy and spin casting, respectively, resulting in current-voltage characteristics with small leakage currents and low onset voltages. The achieved electroluminescence exhibits narrow emission line widths and thus high color purity, as required for color display applications.

Alternately deposited heterostructures of α-sexithiophene–para-hexaphenyl on muscovite mica(001) surfaces: crystallographic structure and morphology

Multi-component systems of para-hexaphenyl (p-6P) and α-sexithiophene (α-6T) molecules show great promise for tuning the fluorescence colour of optically active films. As the opto-electronic properties of rod-like molecules in thin films strongly rely on their anisotropic orientation, a technique for preparation of well-defined, anisotropic multicomponent systems is required. We demonstrate that a p-6P film of less than two nanometer thickness grown on muscovite mica(001) substrates acts as an efficient alignment layer for epitaxial growth of α-6T crystallites. On top of such a p-6P alignment layer, multilayer heterostructures of alternately deposited p-6P and α-6T molecules were grown. Combined X-ray diffraction and transmission electron microscopy studies show that molecules forming α-6T crystallites align parallel to those in the p-6P crystallites leading to the perfect adoption of their herring-bone structures. This alignment is desirable for optical applications and we show that it is preserved for heterostructures composed of up to 120 alternately deposited p-6P (0.8 nm) and α-6T (3.4 nm) nominal layers (120 cycles). Although for co-evaporated α-6T–p-6P molecules formation of a mixed crystal polymorph is reported, we show that in periodically deposited α-6T–p-6P heterostructures phase separation occurs and both molecules crystallize in their well-known equilibrium structures.

Growth and optical properties of α-sexithiopene doped para-sexiphenyl nanofibers

The deposition of highly crystalline self-assembled para-sexiphenyl (PSP) nanofibers doped with α-sexithiophene (6T) is reported. It is demonstrated that the morphology and orientation of the nanofibers are not altered by the presence of 6T, while the photoluminescence (PL) emission can be modified by the doping concentration, suggesting effective incorporation of the dopant into the nanofibers. Moreover, strong dependence of the PL intensity with the excitation polarization underlines the high molecular order within the crystalline matrix. Due to the expected dipole-dipole interaction provided by the highly ordered structures the observed results can be explained by a Forster energy transfer.

Photo-Fries-based photosensitive polymeric interlayers for patterned organic devices

This work reports on the investigation of the photosensitive polymer poly(diphenyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) (PPNB), which undergoes the photo-Fries rearrangement upon illumination with UV-light, used as interfacial layers in organic electronic devices. Two cases were investigated: the use of a blend of PPNB with poly-vinylcarbazole (PVK) as an interlayer in para-sexiphenyl (PSP) based organic light emitting diodes (OLEDs) and the use of PPNB as gate dielectric layer in organic field effect transistors (OFETs). The photo-Fries rearrangement reaction causes a change of the polymer chemical structure resulting in a change of its physical and chemical properties. The electroluminescence spectra and emission of the PSP OLEDs are not affected when fabricated with a non-UV-illuminated PPNB:PVK blend. However, the electroluminescence is totally quenched in those OLEDs fabricated with UV-illuminated PPNB:PVK blend. Although the dielectric constant of PPNB increases upon UV-treatment, it is demonstrated that those OFETs built with UV-treated PPNB as gate dielectric have lower performance than those OFETs built with non-UV-treated PPNB. Furthermore, the effect of the UV-illumination of PPNB and PPNB:PVK blend on the growth of the small molecules C60 and PSP has been studied by atomic force microscopy. Using photolithography, this kind of photochemistry can be performed to spatially control and tune the optical and electrical performance of organic electronic devices.

Para-Sexiphenyl Layers Grown On Light Sensitive Polymer Substrates

In this contribution the deposition of Para-sexiphenyl (PSP) layers on poly (diphenyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) (PPNB) by Hot Wall Epitaxy (HWE) is reported. It is demonstrated that pre-treating the substrate by UV-illumination induces a clear change in the morphology of the grown PSP films due to the polarity modification of the substrate surface. PPNB surface polarity increases when illuminated by UV via photo-Fries rearrangement. By detailed atomic force microscopy analysis the influence on the growth kinetics by the substrate temperature, deposition time and particularly by the UVtreatment of the substrate was investigated. A high crystalline order of the films is underlined by the observation of growth spirals and terraced islands, providing mono-layer step heights of standing PSP-molecules.