No abstract

Monolayers of a series of 1-alkynes, from 1-dodecyne to 1-octadecyne, have been prepared on the hydrogen-terminated Si(100) surface via a thermal reaction of the organic compound with this Si surface. An efficient procedure is presented for the synthesis of 1-alkynes from the corresponding 1-alkenes. The resulting monolayers were characterized by water contact angle measurements, ATR infrared spectroscopy, and X-ray reflectivity. The results show that these 1-alkynes give well-ordered, covalently bonded monolayers, which are at least as ordered as those of the corresponding 1-alkenes. The exact binding geometry of the 1-alkyne to the Si surface was investigated. The results from IR spectroscopy and X-ray reflectivity measurements indicate that the 1-alkynes form two Si−C bonds to the surface per reacting molecule. Quantum mechanical calculations confirm that this formation of two Si−C bonds is not only chemically possible but also energetically much more favorable than formation of only one Si−C bond per reacting molecule.

In this study the performance differences of layered and bulk-heterojunction based organic solar cells composed of the prototypical p-and n-type organic semiconductors pentacene ͑PEN͒ and fullerene ͑C60͒ are correlated with the physical properties of the heterostructures. The electronic structure of layered and codeposited thin PEN and C60 films on the conducting polymer substrate poly͑ethylenedioxythiophene͒:poly͑styrenesulfonate͒ ͑PEDOT:PSS͒ was investigated with ultraviolet photoelectron spectroscopy. Layered structures of C60 on PEN precovered PEDOT:PSS exhibited an offset of the highest occupied molecular orbital ͑HOMO͒ levels of 1.45 eV. In contrast, codeposited films of PEN and C60 showed a reduced HOMO-level offset of 0.85 eV, which increased to 1.45 eV by precoverage of the substrate with a thin PEN layer. In this case, the PEN-HOMO level was Fermi-level pinned at 0.35 eV binding energy and charge transfer between PEN and PEDOT:PSS decreased the vacuum level by 0.75 eV. In addition, the morphology and crystal structure of the respective systems have been investigated by atomic force microscopy ͑AFM͒, x-ray diffraction ͑XRD͒ and Fourier-transform infrared spectroscopy, which indicated pronounced phase separation of PEN and C60 in the codeposited films. XRD revealed crystalline growth of PEN in all investigated cases forming crystallites that exceeded the nominal film thickness by an order of magnitude, whereas C60 was crystalline only if grown on the PEN precovered substrates. AFM investigations allowed to correlate morphology and structure revealing micro-and nanophase separation between PEN and C60.

Pentacene ͑P͒ and 6,13-pentacenequinone ͑PQ͒ have been vacuum codeposited onto SiO 2 in order to control phase separation in thin films for the application as bulk heterojunctions in organic photovoltaic devices. Structural investigations by means of scanning electron microscopy ͑SEM͒ and atomic force microscopy revealed pronounced phase separation of the two materials at length scales that turned out to be tunable by the variation of the deposition rate. X-ray diffraction provided evidence for polymorphism in pure films of P and PQ on SiO 2 . While pure films exhibited both the bulk and thin-film phase, the bulk phase is mainly suppressed within the co-deposited films ͑P+PQ͒. This was corroborated by Fourier-transform infrared spectroscopy results. SEM investigations of pure and codeposited films indicated that PQ bulk crystallites of up to 200 nm height form continuous paths to the substrate and grow within a matrix formed of P and PQ thin-film phases. The obtained heterojunction morphologies thus appear interesting for the application in organic-based photovoltaic cells.

The isothermal crystallization of a semicrystalline poly(ethylene oxide-b-ethylene/butylene) diblock copolymer (PEO-b-PBh) in uniform lamellar films has been investigated by X-ray reflectivity, optical microscopy, and atomic force microscopy. Crystallization of the PEO block leads to an increase in the lamellar thickness of both blocks. As the density of PEO increases upon crystallization, this effect is accompanied by a contraction in the lateral direction, which results in cracking of the film. The combination of the different techniques allows construction of a complete model with an integer or half-integer number of folds in the vertically oriented crystalline stems.