Elena Mena-Osteritz

Correlation of π-Conjugated Oligomer Structure with Film Morphology and Organic Solar Cell Performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.

Superstructures of self-organizing thiophenes

The self-assembling properties of π-conjugated oligo-, poly-, and cyclothiophenes (see Figure) have been studied with different techniques. Scanning tunneling microscopy on perfectly ordered two-dimensional monolayers at the solution–HOPG (highly oriented pyrolytic graphite) interface has successfully been applied. The submolecular resolution achieved in the STM images provides valuable lattice and molecular information. Supported by X-ray structure analyses of the 3D bulk material and by theoretical calculations, molecular conformations, molecule–molecule, and molecule–substrate interactions of the oligo-, poly-, and cyclomers have been analyzed and discussed.

Interrelation between Crystal Packing and Small-Molecule Organic Solar Cell Performance†

X-ray investigations on single crystals of a series of terminally dicyanovinyl-substituted quaterthiophenes and co-evaporated blend layers with C(60) give insight into molecular packing behavior and morphology, which are crucial parameters in the field of organic electronics. Structural characteristics on various levels and length scales are correlated with the photovoltaic performance of bulk heterojunction small-molecule organic solar cells.

A–D–A‐type S,N‐Heteropentacenes: Next‐Generation Molecular Donor Materials for Efficient Vacuum‐Processed Organic Solar Cells

A new class of acceptor-substituted S,N-heteropentacenes is developed for vacuum-processed organic solar cells, providing encouraging power conversion efficiencies of up to 6.5%. Atomic force microscopy (AFM) investigations give a direct correlation between the blend film morphology and the photovoltaic parameters, such as short-circuit current density (JSC ) and fill factor (FF).

Synthesis and characterization of structurally defined head-to-tail coupled oligo(3-alkylthiophenes)

A very efficient and selective synthesis to regioregular HT-coupled oligo(3-alkylthiophenes) has been developed. A series of benzyl ester-capped oligothiophenes up to the decamer has been synthesized using successive iodination/Suzuki-coupling reaction sequences. For the corresponding hexamer it was shown that saponification and decarboxylation to the HT-coupled oligo(3-alkylthiophene) proceeds in high yields. The dependence of the electronic properties on the chain length of the conjugated oligomers was investigated. The oligomers form very stable and well-ordered adsorbates at the solid–liquid interface and these were imaged by scanning tunneling techniques.

Synthesis and structural analysis of thiophene-pyrrole-based S,N-heteroacenes.

Fused S,N-heterohexacene 4 was synthesized by applying Pd-catalyzed tandem Buchwald-Hartwig coupling and further functionalized to corresponding acceptor-capped derivatives 5 and 6 showing bond length equalization in the π-conjugated backbone and intense optical transitions. Organic thin film transistors (OTFTs) based on a vacuum-deposited film of 6 exhibit p-channel charge-carrier mobilities as high as 0.021 cm(2) V(-1) s(-1) and current on/off ratios of 10(5).

Dithienopyrrole-based oligothiophenes for solution-processed organic solar cells

Isomeric dicyanovinylene-terminated oligothiophenes 1 and 2 comprising a central dithienopyrrole (DTP) unit have been developed for solution-processed small molecule organic solar cells (SMOSCs) giving the highest power conversion efficiency of 4.8% for DTP-based oligomeric materials.

Superstructure formation of large organic adsorbates on a metal surface: A systematic approach using oligothiophenes on Ag(111)

A series of ‘‘end-capped’’ oligothiophenes ECnT with different chain lengths n (n53 ‐ 6) were vapor deposited onto the Ag~111! surface. The adsorption and structural ordering was investigated by thermal desorption spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy. For all molecules we observe highly ordered monolayers with flat-lying molecules and long-range ordered domains of several hundred A diameter. For the two smaller oligomers~EC3T and EC4T!, two phases—a commensurate ~relaxed! and an incommensurate ~compressed!—are found, whereas the two longer molecules ~EC5T and EC6T! form only incommensurate phases. The six unit cells exhibit significantly different symmetries, containing 1, 2, or 4 molecules. The formation of the geometric structures is discussed under the aspect of molecular size, molecular symmetry, and interplay of the molecule-substrate and molecule-molecule interactions. The last aspect was also investigated by concomitant force-field calculations. @S0163-1829~98!02143-2#

The longest oligothiophene ever examined by X-ray structure analysis

The α-conjugated and structurally defined dodecithiophene 1 well crystallizes from solution into single crystals which were investigated by X-ray structure analysis. Molecular conformations and packing of the longest oligothiophene ever structurally characterized were determined. Surprisingly, rather rare syn-conformations at the terminal thiophene rings and partial twisting of alkyl side chains for compact packing were found, whereby the molecules form a layer-like arrangement with partial overlap of their heteroaromatic systems. In comparison, the molecular properties of the oligomer in a 2D self-assembled monolayer adsorbed at the liquid–solid interface were investigated by scanning tunnelling microscopy (STM). In comparison to the 3D bulk material, on graphite, a different molecular arrangement was found. A lamella-type ordering of molecularly resolved oligomers which exhibit an all-anti conformation could be imaged over wide areas. Larger lattice constants were determined which originate from molecule–substrate interactions.

Molecular and electronic structure of cyclo[10]thiophene in various oxidation states: polaron pair vs. bipolaron

The molecular structure of a cyclic oligothiophene, C10T, has been determined by single-crystal X-ray structure analysis. The exclusive syn-conformation of all thiophene units as confirmed in the solid state and the ring strain in this macrocycle result in its unusual and optoelectronic properties. This does not only apply to neutral C10T but also to its oxidized states, as demonstrated by absorption and ESR spectroscopy, supporting the formation of a polaron-pair structure upon oxidation of C10T to C10T2(·+) as has been discussed for linear oligothiophenes. To the best of our knowledge, C10T2(·+) represents an unambiguous example comprising a two-polaron structure (polaron-pair) of a thiophene-based conjugated macrocycle.