Y-Type Non-Fullerene Acceptors with Outer Branched Side Chains and Inner Cyclohexane Side Chains for 19.36% Efficiency Polymer Solar Cells

Raising the lowest unoccupied molecular orbital (LUMO) energy level of Y-type non-fullerene acceptors can increase the open-circuit voltage (V_oc ) and thus the photovoltaic performance of the current top performing polymer solar cells (PSCs). One of the viable routes is demonstrated by the successful Y6 derivative of L8-BO with the branched alkyl chains at the outer side. This will introduce steric hindrance and reduce intermolecular aggregation, thus open up the bandgap and raise the LUMO energy level. To take further advantages of the steric hindrance influence on optoelectronic properties of Y6 derivatives, two Y-type non-fullerene acceptors of BTP-Cy-4F and BTP-Cy-4Cl are designed and synthesized by adopting outer branched side chains and inner cyclohexane side chains. An outstanding V_oc of 0.937 V is achieved in the D18:BTP-Cy-4F binary blend devices along with a power conversion efficiency (PCE) of 18.52%. With the addition of BTP-eC9 to extend the absorption spectral coverage, a remarkable PCE of 19.36% is realized finally in the related ternary blend devices, which is one of the highest values for single-junction PSCs at present. The results illustrate the great potential of cyclohexane side chains in constructing high-performance non-fullerene acceptors and their PSCs.

Ultrafast Dynamics and Photoresponse of a Fungi-Derived Pigment Xylindein from Solution to Thin Films

Organic semiconductor materials have recently gained momentum due to their non-toxicity, low cost, and sustainability. Xylindein is a remarkably photostable pigment secreted by fungi that grow on decaying wood, and its relatively strong electronic performance is enabled by π-π stacking and hydrogen-bonding network that promote charge transport. Herein, femtosecond transient absorption spectroscopy with a near-IR probe was used to unveil a rapid excited-state intramolecular proton transfer reaction. Conformational motions potentially lead to a conical intersection that quenches fluorescence in the monomeric state. In concentrated solutions, nascent aggregates exhibit a faster excited state lifetime due to excimer formation, confirmed by the excimer→charge-transfer excited-state absorption band of the xylindein thin film, thus limiting its optoelectronic performance. Therefore, extending the xylindein sidechains with branched alkyl groups may hinder the excimer formation and improve optoelectronic properties of naturally derived materials.

Columnar Stacking of Partially Fluorinated [4]Helicenes: C-H⋅⋅⋅F Interactions Change the Stacking Orientation

The partial fluorination of polycyclic aromatic hydrocarbons often produces a layered crystal packing, where fluorinated aromatic surfaces are stacked over nonfluorinated aromatic surfaces. Herein, we report the synthesis and crystal packing of partially fluorinated [4]helicenes with steric congestion resulting from H and F atoms in the fjord region. F₆ -[4]Helicene forms head-to-tail columnar stacks consisting of an alternate arrangement of perfluorinated and nonfluorinated naphthalene moieties. With decreasing fluorine content, aromatic stacking switched from arene-fluoroarene (Ar^H -Ar^F ) hetero-stacking to Ar^H -Ar^H /Ar^F -Ar^F homo-stacking with the help of intermolecular C-H⋅⋅⋅F contacts in the fjord region. As a result, head-to-head columnar stacks appear. Therefore, the conventional Ar^H -Ar^F stacking motif is not always applicable to F_n -[4]helicenes with twisted π-surfaces.

Tetrabenzononacene: "Butterfly Wings" Stabilize the Core

In combination with bulky substituents at the core, fourfold benzannulation at the cata‐positions stabilizes a nonacene sufficiently to allow its isolation and characterization by ¹H NMR and X‐ray analysis. The four benzo units blueshift the absorption spectrum in comparison to a solely linear nonacene, but significantly increase the stability in the solid state.

Acenes beyond organic electronics: sensing of singlet oxygen and stimuli-responsive materials

Although they are often detrimental in organic electronics, the cycloaddition reactions of acenes, especially with singlet oxygen, are useful in a range of responsive materials.

Synthesis and physical properties of brominated hexacene and hole-transfer properties of thin-film transistors

A halide-substituted higher acene, 2-bromohexacene, and its precursor with a carbonyl bridge moiety were synthesized. The precursor was synthesized through 7 steps in a total yield of 2.5%. The structure of precursor and thermally converted 2-bromohexacene were characterized by solid state NMR, IR, and absorption spectra, as well as by DFT computation analysis. It exhibited high stability in the solid state over 3 months, therefore can be utilized in the fabrication of opto-electronic devices. The organic thin-film transistors (OFETs) were fabricated by using 2-bromohexacene and parent hexacene through vaccum deposition method. The best film mobility of 2-bromohexacene was observed at 0.83 cm² V⁻¹ s⁻¹ with an on/off ratio of 5.0 × 10⁴ and a threshold of −52 V, while the best film mobility of hexacene was observed at 0.076 cm² V⁻¹ s⁻¹ with an on/off ratio of 2.4 × 10² and a threshold of −21 V. AFM measurement of 2-bromohexacene showed smooth film formation. The averaged mobility of 2-bromohexacene is 8 fold higher than the non-substituted hexacene.

A halide-substituted higher acene, 2-bromohexacene, and its precursor with a carbonyl bridge moiety were synthesized.

Strategies for the Synthesis of Higher Acenes

The outstanding performance of pentacene-based molecules in molecular electronics, as well as the predicted enhanced semiconducting properties of extended acenes, have stimulated the development of new synthetic methods and functionalization strategies for the preparation of stable and soluble acenes larger than tetracene with the aim of obtaining improved functional materials.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Indacenodibenzothiophenes: synthesis, optoelectronic properties and materials applications of molecules with strong antiaromatic character

Indeno[1,2-b]fluorenes (IFs), while containing 4n π-electrons, are best described as two aromatic benzene rings fused to a weakly paratropic s-indacene core. In this study, we find that replacement of the outer benzene rings of an IF with benzothiophenes allows the antiaromaticity of the central s-indacene to strongly reassert itself. Herein we report a combined synthetic, computational, structural, and materials study of anti- and syn-indacenodibenzothiophenes (IDBTs). We have developed an efficient and scalable synthesis for preparation of a series of aryl- and ethynyl-substituted IDBTs. NICS-XY scans and ACID calculations reveal an increasingly antiaromatic core from [1,2-b]IF to anti-IDBT, with syn-IDBT being nearly as antiaromatic as the parent s-indacene. As an initial evaluation, the intermolecular electronic couplings and electronic band structure of a diethynyl anti-IDBT derivative reveal the potential for hole and / or electron transport. OFETs constructed using this molecule show the highest hole mobilities yet achieved for a fully conjugated IF derivative.

Synthesis and Investigation of the Effect of Substitution on the Structure, Physical Properties, and Electrochemical Properties of Anthracenodifuran Derivatives

A series of syn/anti mixtures of anthradifuran (ADF) and substituent compounds were systematically synthesized, and the effect of substitution at the 5,11-positions on the neutral and radical states of ADF was investigated. All compounds were measured and analyzed by absorption and fluorescence spectroscopy, cyclic voltammetry, electrochemical absorption spectroscopy, and DFT calculations. The absorption spectra of 5,11-substituent compounds in their neutral state were red-shifted. In addition, the substituted compounds exhibited increased thermal stability with respect to the parent 1a because of elongation of the π-conjugation and an increased steric hindrance effect due to the bulky ethynyl substituent groups. The cyclic voltammograms of all of the compounds exhibited irreversible reduction potentials and irreversible oxidation potentials, except in the case of (trimethylsilyl)silylethynyl-substituted ADF. When the materials were subjected to oxidation/reduction potentials, the radical cation and anion species were generated. The absorption spectra of the radical-cation species of the compounds exhibited similar characteristics and similar absorption ranges (550-1400 nm), whereas the spectra of the radical anion species were blue-shifted (550-850 nm) compared than that of the parent 1a(•-) (550-1100 nm). The DFT computation results suggested that the radical states of lowest energy transitions occurred primarily from π to π(SOMO) or from π(SOMO) to π*.

Singlet exciton fission in a hexacene derivative

Hexacene, an acene with six benzene rings, is notable for its exceptionally small triplet energy, around one third of the singlet energy. Herein, singlet fission, i.e., conversion of a singlet exciton into two triplets, is demonstrated in a thin film of hexacene derivative, employing both transient absorption spectroscopy and magnetic field effects on photocurrent.

Beyond pentacenes: synthesis and properties of higher acenes

Acenes consist of linearly annulated benzene rings. Their reactivity increases quickly with increasing chain length. Therefore acenes longer than pentacene are very sensitive towards oxygen in the presence of light and thus these molecules have not been well studied or have remained elusive in spite of synthetic efforts dating back to the 1930s. This review gives an historical account of the development of the chemistry of acenes larger than pentacene and summarizes the recent progress in the field including strategies for stabilization of higher acenes up to nonacene.