Astrid Vogt

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).

Fused Thiophene-Pyrrole-Containing Ring Systems up to a Heterodecacene†

A new class of π-conjugated polycyclic hydrocarbons that promises interesting electronic properties is presented. The synthesis and extension of the S,N-heteroacene series consisting of only five-membered heterocyclic rings up to a very long, stable, and still soluble decacene SN10 is realized by multiple Pd-catalyzed aminations of halogenated thiophene precursors as key reactions. These novel heteroacenes were characterized by optical spectroscopy and electrochemistry providing interesting structure-property relationships. Nearly complete bond-length equalization in the inner part of the conjugated backbone and an unusual herringbone packing in the solid state underline the structural features of these novel systems.

The influence of alkyl side chains on molecular packing and solar cell performance of dithienopyrrole-based oligothiophenes

The synthesis of a series of dithieno[3,2-b:2′,3′-d]pyrrole (DTP) containing A–D–A oligomers for use in solution-processed bulk heterojunction solar cells is presented. This series allows investigation of the effect of alkyl chains attached to the thiophene moieties or the nitrogen of the DTP unit on the thermal and physical properties of the oligomers. The photoactive layers were probed using absorption spectroscopy, grazing incident X-ray diffraction, and atomic force microscopy. From these experiments, it was found that the hexyl chains attached to the thiophene units are instrumental in obtaining good packing and high fill factors in the solar cell devices. For the fabrication of solar cells, both PEDOT:PSS and V2O5 were investigated as hole transport layers and PDMS was employed as a solvent additive. High open-circuit voltages of up to 1.1 V, moderate short-circuit current densities, and high fill factors of up to 0.63 were obtained yielding power conversion efficiencies as high as 5.3%.

Thiophene–pyrrole containing S,N-heteroheptacenes : synthesis, and optical and electrochemical characterisation

A novel family of S,N-heteroheptacenes SN7a–d with a variable thiophene–pyrrole ratio and a heteroring fusion sequence is presented. All SN7-derivatives can be synthesized in efficient multi-step synthetic routes with good overall yields. The crucial cyclization step to the stable and soluble fused systems is achieved by multiple Pd-catalyzed Buchwald–Hartwig aminations or C–S coupling reaction in high yields. The comparison of the optoelectronic properties provides interesting structure–property relationships and gives valuable insights into the role of nitrogen atoms within the series of thiophene–pyrrole containing S,N-heteroheptacenes. Remarkable differences in emission behaviour and noticeable correlations of the oxidation potentials of the S,N-heteroheptacenes offer helpful information for the construction of other heteroacenes. Additionally, UV-Vis-NIR absorption spectra of the corresponding radical cations generated by chemical oxidation were monitored and compared.

Spectroscopic Study of Thiophene–Pyrrole-Containing S,N-Heteroheptacenes Compared to Acenes and Phenacenes

In this study, we report a detailed spectroscopic study concerning the energy levels and vibrational structure of thiophene-pyrrole-containing S,N-heteroacenes. The aim of the study is first, to understand the differences in the photoluminescence (PL) efficiencies in this structurally similar series and second, to compare the electronic structure of S,N-heteroacenes to that of linear acenes and phenacenes, with a view to derive guidelines for the design of singlet fission materials. For S,N-heteroacenes comprising seven fused heterocyclic rings, we observe a higher PL quantum yield for derivatives with terminal thienothiophene units than for thienopyrrole-capped ones. This is assigned to a stronger tendency of the thienopyrrole-capped derivatives to form nonemissive associates in dilute solution, producing emissive excimers at higher concentration. By conducting time-resolved PL studies at 77 K, we further determine the lowest singlet and triplet energies for the S,N-heteroacenes with three, five, and seven fused rings. We show that their energies evolve with oligomer length analogously to those of phenacenes, yet in a fundamentally different way from that of linear acenes. This difference in evolution is attributed to the increasingly biradical character in acenes with increasing chain length in contrast to the S,N-heteroacenes and phenacenes.

Influence of alkyl chain length in S,N-heteropentacenes on the performance of organic solar cells

A novel family of alkyl-substituted S,N-heteropentacenes SN5 4–6 with a systematic variation of the alkyl side chains from methyl to hexyl is presented. The parent heteroacene series 4 and the acceptor end-capped series 5 and 6 were synthesized and characterized with respect to their structure–property relationships. The dicyanovinyl (DCV) end-capped derivatives 6a–f were implemented as donors in vacuum-processed bulk-heterojunction solar cells with fullerene C60 as acceptor, their photovoltaic behaviour studied, and structure–device relationships discussed. The devices exhibited power conversion efficiencies (PCE) in the range of 4–6% and interestingly a correlation between the length of the alkyl side chains and the photovoltaic performance of the resulting devices was discovered: an odd–even effect with higher values for the even-numbered alkyl chains. Atomic force microscopy (AFM) studies on the active layer surfaces gave hints for the explanation of this effect.