wileyonlinelibrary.comAdv. Funct. Mater. 2011, 21, 897-910 Photovoltaic properties of DCV n T-Bu (n = 4, 5 and 6) with various lengths of the oligothiophene unit were also investigated. The V OC decreased from 1.13 V for DCV4T-Bu to 1.0 V for DCV5T-Bu and to 0.93 V for DCV6T-Bu, which is ascribed to the decrease in their IP and an increase in energy of the highest occupied molecular orbital (HOMO). On the other hand, no clear trend was observed for short-circuit current density ( J SC ) and fi ll factor ( FF ) values. The power conversion effi ciency of DCV4T-Bu was lower compared to DCV5T-Bu because of its lower J SC and FF, while the small differences in J SC and FF values between DCV5T-Bu and DCV6T-Bu do not represent a general trend. [ 34 ] One advantage of the vacuum-processable SMOSCs is that tandem cells can easily be fabricated. Very recently, by combining in a tandem device two bulk heterojunctions of C 60 in blend layers with an optimized DCV n T-derivative and a novel pigment with absorption range between 600 and 750 nm developed by BASF, respectively, a certifi ed effi ciency of 6.1% on 2 cm 2 active area [ 36 ] and shortly after a certifi ed record effi ciency of 8.3% on 1.1 cm 2 device have been released. [ 37 ] Herein, we investigate structure-property relationships in a series of DCV n Ts with conjugated chain lengths of one to six thiophene units (n = 1-6) and without solubilizing alkyl side chains, well being aware that synthesis and characterization of longer derivatives might be problematic due to low solubility. However, we expect that these oligomers may show good packing and thermal stability which might infl uence the solar cell performance. Synthesis of Dicyanovinyl-FunctionalizedOligothiophenes DCV n T Linear SynthesesSynthesis of the smaller members of the series, DCV1T-DCV3T 1-3 , were already described in literature and were prepared for non-linear optical properties. [ 38 , 39 ] Based on our experience with alkylated DCV n Ts, we fi rstly synthesized DCV1T-DCV4T 1-4 by a linear approach ( Scheme 1 ). Thus, bi-, ter-, and quaterthiophene dialdehydes 8-10 , [40][41][42][43] which were prepared by Vilsmeier-Haack formylation of the parent oligothiophenes [44][45][46] were reacted with malononitrile and ß-alanine as catalyst to form target DCV n Ts 2 and 3 (n = 2, 3) in nearly quantitative yield and in high purity. DCV1T 1 was obtained from commercially available thiophene-2,5-dicarbaldehyde 7 and malononitrile without the use of catalyst. However, due to a dramatic decrease in solubility, reaction of quaterthiophene-dialdehyde 10 resulted in an inseparable mixture of target DCV4T 4 and the singly reacted intermediate with a DCV-group at one chain end and an unreacted formyl group at the other. Convergent SynthesesDue to solubility problems of the longer homologues, only a series up to the trimer could be reliably synthesized by the usual linear synthetic approach. Thus, we developed a convergent route to DCV-capped oligothiophenes DCV3T 3 to DCV6T 6 ( Scheme 2 ). Terminal buildin...

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.

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.

By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes, we find that the internal acceptor-donor-acceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields, substantial energetic disorder, and pronounced Poole-Frenkel behavior, which is unexpected for crystalline compounds. We show that the presence of static molecular dipoles causes large energetic disorder, which is mostly reduced not by compensation of dipole moments in a unit cell but by molecular polarizabilities. In addition, the presence of a well-defined π-stacking direction with strong electronic couplings and short intermolecular distances turns out to be disadvantageous for efficient charge transport since it inhibits other transport directions and is prone to charge trapping.

Dicyanovinyl (DCV)-substituted oligothiophenes are promising donor materials in vacuum-processed small-molecule organic solar cells. Here, we studied the structural and the electronic properties of DCV-dimethyl-pentathiophene (DCV5T-Me2) adsorbed on Au(111) from submonolayer to multilayer coverages. Using a multi-technique experimental approach (low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and two-photon photoemission (2PPE) spectroscopy), we determined the energetic position of several affinity levels as well as ionization potentials originating from the lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO), evidencing a transport gap of 1.4 eV. Proof of an excitonic state was found to be a spectroscopic feature located at 0.6 eV below the LUMO affinity level. With increasing coverage photoemission from excitonic states gains importance. We were able to track the dynamics of several electronically excited states of multilayers by means of femtosecond time-resolved 2PPE. We resolved an intriguing relaxation dynamics involving four processes, ranging from sub-picosecond (ps) to several hundred ps time spans. These show a tendency to increase with increasing coverage. The present study provides important parameters such as energetic positions of transport levels as well as lifetimes of electronically excited states, which are essential for designing organic-molecule-based optoelectronic devices.