Dicyanovinyl–Substituted Oligothiophenes: Structure‐Property Relationships and Application in Vacuum‐Processed Small Molecule Organic Solar Cells

Abstract

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...

Interrelation between Crystal Packing and Small‐Molecule Organic Solar Cell Performance

“…The use of small molecules in OSCs provides in contrast to polymeric materials another advantage: single crystals suitable for X-ray structure analysis become available providing detailed insight into intermolecular interactions and non-bonding atomic contacts responsible for the observed packing motifs. [14] In this contribution, synthesis, thermal, and optoelectronic properties of a series of terminally acceptor-substituted quaterthiophenes will be presented. A correlation between X-ray data and photovoltaic performance of corresponding BHJ-SMOSCs will be established.…”

“…We have synthesized a series of dicyanovinyl (DCV)-substituted quaterthiophenes DCV4T 1, [14] DCV4T-Me 2, and DCV4T-Et 3 by systematical variation of substituents at the terminal thiophene units from hydrogen to methyl and ethyl (Scheme 1). As in the case of parent compound DCV4T 1, [14] for the synthesis of quaterthiophenes DCV4T-Me 2 and DCV4T-Et 3 a convergent route was chosen.…”

π-Linkage effect of push-pull-structure organic small molecules for photovoltaic application

“…Among thiophene and its derivatives, oligothiophene architecture is very effective to tailor molecular D and A moieties, producing molecules where the HOMO level and the lowest unoccupied orbital (LUMO) level can be controlled for the design of molecule with a low E g [36][37][38][39]. Besides, molecules using oligothiophene as a highly extended π-conjugated bridge not only broaden the absorption region but also promote strong π-π stacking of conjugated backbones [40][41][42][43][44][45].…”

Recent progress of bulk heterojunction solar cells based on small-molecular donors

“…Bäuerle and co-workers [53] synthesized a series of terminally dicyanovinyl (DCV)-substituted oligothiophenes. The structural fine-tuning of these A-D-A-type oligothiophenes led to highly efficient materials with sufficiently (24) and C 60 in a 2 : 1-ratio resulted in a conversion efficiency of 5.2%, with a J sc of 11.1 mA/cm 2 , a V oc of 0.97 V and a FF of 49%.…”