Matthias Stolte

Efficient Solution-Processed Bulk Heterojunction Solar Cells by Antiparallel Supramolecular Arrangement of Dipolar Donor–Acceptor Dyes†

Research on small-molecule-based organic semiconductors has undoubtedly been strongly influenced by xerographic photoconductors like triarylamines, the first important organic electronic materials in market products. Their development was strongly influenced by the B ssler model, which provided a rationale for the design of amorphous organic photoand semiconductors. According to this model, only compounds that lack dipole moments are considered promising for charge-carrier transport because the increased energetic disorder associated with dipole moments is thought to impede charge hopping. Recently, we questioned this paradigm in the field of organic photovoltaics (OPV) and successfully implemented highly dipolar merocyanine dyes as active components for light harvesting as well as exciton and hole transport in solution-cast bulk heterojunction (BHJ) solar cells. The rationale behind our concept was that highly dipolar donor–acceptor (D–A) substituted p systems (also called push-pull dyes) self-assemble into centrosymmetric dimers, thus effectively eliminating molecular dipole moments on the supramolecular and material levels. Two drawbacks of our BHJ materials, however, limited the acceptance of our concept so far. Firstly, the best solar cells were obtained for merocyanine dyes whose molecular scaffolds were equipped with rather bulky substituents that interfere with close face-to-face antiparallel dimerization. Secondly, the power-conversion efficiencies (h) under standard AM1.5, 100 mW cm 2 simulated solar illumination conditions for solution-cast BHJ cells with fullerenes—although significantly advanced by more sophisticated vacuum processing—could not be improved beyond 2.6%, which is significantly lower than the best solutionprocessed small-molecule-based BHJ devices fabricated with A–D–A and D–A–D chromophores, for example, acceptorsubstituted oligothiophenes (up to 3.7%) and triarylamines (up to 4.3%), diketopyrrolopyrroles (up to 4.4%), and squaraines (up to 5.2 %). Herein, we introduce dipolar D– A dyes with flat structures that undoubtedly form centrosymmetric dimers with perfectly cancelled dipole moments in the solid state. Solution-processed BHJ solar cells derived thereof exhibit power-conversion efficiencies up to 4.5–5.1% (dependent on light intensity), clearly placing D–A dyes now among the top-performing small molecules in the field of organic photovoltaics. Scheme 1 outlines the synthetic route that follows our earlier work on merocyanine dyes for photorefractive materials and the simple access to 5-dialkylamino-thiophene-2carbaldehydes by Hartmann. Detailed synthetic procedures and characterization data are described in the Supporting Information.

Organic Semiconductors based on Dyes and Color Pigments

Organic dyes and pigments constitute a large class of industrial products. The utilization of these compounds in the field of organic electronics is reviewed with particular emphasis on organic field-effect transistors. It is shown that for most major classes of industrial dyes and pigments, i.e., phthalocyanines, perylene and naphthalene diimides, diketopyrrolopyrroles, indigos and isoindigos, squaraines, and merocyanines, charge-carrier mobilities exceeding 1 cm(2) V(-1) s(-1) have been achieved. The most widely investigated molecules due to their n-channel operation are perylene and naphthalene diimides, for which even values close to 10 cm(2) V(-1) s(-1) have been demonstrated. The fact that all of these π-conjugated colorants contain polar substituents leading to strongly quadrupolar or even dipolar molecules suggests that indeed a much larger structural space shows promise for the design of organic semiconductor molecules than was considered in this field traditionally. In particular, because many of these dye and pigment chromophores demonstrate excellent thermal and (photo-)chemical stability in their original applications in dyeing and printing, and are accessible by straightforward synthetic protocols, they bear a particularly high potential for commercial applications in the area of organic electronics.

Tailored merocyanine dyes for solution-processed BHJ solar cells

A solution-cast photovoltaic film of a newly designed merocyanine chromophore blended with PCBM exhibited an appreciably high power conversion efficiency of 2.59%.

Air‐Stable n‐Channel Organic Single Crystal Field‐Effect Transistors Based on Microribbons of Core‐Chlorinated Naphthalene Diimide

Ribbon-shaped single crystal transistors based on naphthalene diimide Cl2-NDI exhibit excellent n-channel performance with the mobility as high as 8.6 cm(2) V(-1) s(-1) in air. The combination of ambient stability and high mobility n-channel transport closes the gap between p- and n-channel SCFETs and opens the door for the manufacture of high performance complementary organic circuits.

Impact of Molecular Flexibility on Binding Strength and Self-Sorting of Chiral π-Surfaces

In this work, we have explored for the first time the influence of conformational flexibility of π-core on chiral self-sorting properties of perylene bisimides (PBIs) that are currently one of the most prominent classes of functional dyes. For this purpose, two series of chiral macrocyclic PBIs 3a-c and 4a-c comprising oligoethylene glycol bridges of different lengths at the 1,7 bay positions were synthesized and their atropo-enantiomers (P and M enantiomers) were resolved. Single crystal analysis of atropo-enantiomerically pure (P)-3a not only confirmed the structural integrity of the ethylene glycol bridged macrocycle but also illustrated the formation of π-stacked dimers with left-handed supramolecular helicity. Our detailed studies with the series of highly soluble chiral PBIs 4a-c by 1- and 2-D (1)H NMR techniques, and temperature- and concentration-dependent UV/vis absorption and circular dichroism (CD) spectroscopy revealed that in π-π-stacking dimerization of these PBIs chiral self-recognition (i.e., PP and MM homodimer formation) prevails over self-discrimination (i.e., PM heterodimer formation). Our studies clearly showed that with increasing conformational flexibility of PBI cores imparted by longer bridging units, the binding strength for the dimerization process increases, however, the efficiency for chiral self-recognition decreases. These results are rationalized in terms of an induced-fit mechanism facilitating more planarized π-scaffolds of PBIs containing longer bridging units upon π-π-stacking.

Single-crystal field-effect transistors of new Cl 2 -NDI polymorph processed by sublimation in air

Physical properties of active materials built up from small molecules are dictated by their molecular packing in the solid state. Here we demonstrate for the first time the growth of n-channel single-crystal field-effect transistors and organic thin-film transistors by sublimation of 2,6-dichloro-naphthalene diimide in air. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode (β-phase) is obtained that is distinguished from the previously reported herringbone packing motif obtained from solution (α-phase). We are able to fabricate single-crystal field-effect transistors with electron mobilities in air of up to 8.6 cm(2) V(-1) s(-1) (α-phase) and up to 3.5 cm(2) V(-1) s(-1) (β-phase) on n-octadecyltriethoxysilane-modified substrates. On silicon dioxide, thin-film devices based on β-phase can be manufactured in air giving rise to electron mobilities of 0.37 cm(2) V(-1) s(-1). The simple crystal and thin-film growth procedures by sublimation under ambient conditions avoid elaborate substrate modifications and costly vacuum equipment-based fabrication steps.

Perylene Bisimide Dimer Aggregates: Fundamental Insights into Self‐Assembly by NMR and UV/Vis Spectroscopy

A novel perylene bisimide (PBI) dye bearing one solubilizing dialkoxybenzyl and one bulky 2,5-di-tert-butylphenyl substituent was synthesized and its aggregation behavior was analyzed by NMR and UV/Vis spectroscopy in various chloroform/methylcyclohexane (MCH) solvent mixtures. In the presence of no less than 10 vol % chloroform, exclusive self-assembly of this PBI dye into π-stacked dimers was unambiguously confirmed by means of both concentration-dependent (1) H NMR and UV/Vis spectroscopic experiments. Based on ROESY NMR, a well-defined π-stacked dimer structure was determined and further corroborated by molecular modeling studies. By varying the solvent composition of chloroform and MCH, the solvent effects on the Gibbs free energy of PBI dimerization were elucidated and showed a pronounced nonlinearity between lower and higher MCH contents. This observation could be related to a further growth process of dimers into larger aggregates that occurs in the absence of chloroform, which is required to solvate the aromatic π surfaces. With the help of a single-crystal structure analysis for a related PBI dye, a structural model could be derived for the extended aggregates that are still composed of defined π-π-stacked PBI dimer entities.

Synthesis and Molecular Properties of Acceptor-Substituted Squaraine Dyes

A broad series of more than 20 acceptor-substituted squaraines was synthesized that feature different acceptor functionalities at the central squaraine four-membered ring. The influence of these acceptor units on the reactivity of semisquaraine precursors and stability of the respective squaraines were explored. Thereby the dicyanovinyl group was found to be the most versatile acceptor group that enabled various modifications at the donor moiety of the squaraine scaffold, leading to an extended series of dicyanovinyl-functionalized squaraines. The variation of donor units afforded a set of NIR fluorophores that cover a wavelength region from the visible at about 650 nm far into the NIR up to 920 nm with fluorescence quantum yields between 0.93 and 0.11 and outstanding optical brightness. This excellent optical property is related to a rigid molecular scaffold that is fixed in an all-cis configuration by the additional dicyanovinyl acceptor unit. The change of the molecular symmetry from C(2h) to C(2v) upon functionalization of the squaraine core with dicyanovinyl acceptor group has been confirmed in solution by electro-optical absorption (EOA) spectroscopy, revealing permanent ground-state dipole moments μ(g) in the range between 4.3 and 6.4 D. These dipole moments direct an antiparallel packing of the molecules in the solid state according to single-crystal X-ray analyses achieved for four dicyanovinyl-functionalized squaraines. The structural properties, the EOA results, as well as the band shapes of the optical spectra indicate that these polymethine dyes are cyanine-type chromophores. It is worth noting that the orientation of the dipole moment vectors is orthogonal to the orientation of the transition dipole moment vectors, which is an uncommon but characteristic feature of this rather novel class of polymethine dyes. With regard to applications of these dyes in organic solar cells, their redox properties were also studied by cyclic voltammetry.

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).

Influence of Solid-State Packing of Dipolar Merocyanine Dyes on Transistor and Solar Cell Performances

A series of nine dipolar merocyanine dyes has been studied as organic semiconductors in transistors and solar cells. These dyes exhibited single-crystal packing motifs with different dimensional ordering, which can be correlated to the performance of the studied devices. Hereby, the long-range ordering of the dyes in staircase-like slipped stacks with J-type excitonic coupling favors charge transport and improves solar cell performance. The different morphologies of transistor thin films and solar cell active layers were investigated by UV-vis, AFM, and XRD experiments. Selenium-containing donor-acceptor (D-A) dimethine dye 4 showed the highest hole mobility of 0.08 cm(2) V(-1) s(-1). BHJ solar cells based on dye 4 were optimized by taking advantage of the high crystallinity of the donor material and afforded a PCE of up to 6.2%.