Bis-Silicon-Bridged Stilbene: A Core for Small-Molecule Electron Acceptor for High-Performance Organic Solar Cells

Bis-Silicon-Bridged Stilbene: A Core for Small-Molecule Electron Acceptor for High-Performance Organic Solar Cell

High-Performance Inverted Polymer Solar Cells with Zirconium Acetylacetonate Buffer Layers

Inverted polymer solar cells incorporating solution-processed zirconium acetylacetonate (ZrAcac) buffer layers were demonstrated. The optimal device delivered a power conversion efficiency up to 9.2%, displaying ∼20% improvement compared with the device of conventional configuration. The performance improvement by adopting ZrAcac as the cathode buffer layer is attributed to the enhanced light-harvesting, facilitated electron transport, and reduced bimolecular recombination loss. The morphology of ZrAcac buffer layer was found to be critical in achieving high performance, which was tunable through the selection of processing solvents. A flat and uniform ZrAcac film consisting of ∼20 nm nanoscale aggregates deposited from a chloroform solution was proved to be highly effective, which only requires a short light-soaking time

Low-Bandgap Small-Molecule Donor Material Containing Thieno[3,4‑<i>b</i>]thiophene Moiety for High-Performance Solar Cells

By replacing the central thiophene of <b>STDR</b>, a sepithiophene terminated with two 3-ethylrhodanine moieties, with 2-ethylhexyl 3-fluorothieno­[3,4-<i>b</i>]­thiophene-2-carboxylate, an A–D–Q–D–A-type small molecule has been developed for high-performance organic solar cells with improved photocurrent. <b>STDR</b>-<b>T</b><i><b>b</b></i><b>T</b> exhibits a significant bathochromic shift with a low optical bandgap of approximately 1.60 eV in the thin film. Accordingly, <b>STDR</b>-<b>T</b><i><b>b</b></i><b>T</b> shows broad external quantum efficiency spectral response up to 800 nm. A high short circuit current (<i>J</i>_sc) of 10.90 mA cm^–2 was achieved for <b>STDR</b>-<b>T</b><i><b>b</b></i><b>T</b>:PC₇₁BM-based devices; this is significantly higher than that of <b>STDR</b>:PC₇₁BM-based devices, <i>J</i>_sc: 5.61 mA cm^–2, with a power-conversion efficiency (PCE) of 5.05%. Compared with <b>STDR</b>-based devices, <b>STDR</b>-<b>T</b><i><b>b</b></i><b>T</b>-based devices show balanced charge carrier transport, better thin-film morphology, and favorable charge separation/collection

Developing Quinoidal Fluorophores with Unusually Strong Red/Near-Infrared Emission

Despite the dominant position of aromatic fluorophores, we report herein the design and synthesis of quinoidal fluorophores based on rarely emissive quinoidal bithiophene. Quinoidal bitheno­[3,4-<i>b</i>]­thiophene, <b>QBTT-C6</b>, consisting of cruciform-fused (<i>E</i>)-1,2-bis­(5-hexylthiophen-2-yl)­ethene and quinoidal bithiophene, shows a fluorescence quantum yield of 8.5%, 25-fold higher than that of the parent quinoidal <b>QBT</b> chromophore, but its maximum emission is at similar wavelengths. <b>QBTT-Ar</b>’s featuring intramolecular charge transfer can further shift the maximum emission into the near-infrared region. The intramolecular charge transfer is programmably enhanced by tuning the substituents on the aryl groups from the electron-withdrawing trifluoromethyl to the electron-donating methoxy groups. Unexpectedly, a positive relationship between intramolecular charge transfer and fluorescence quantum yield is observed; as a result, <b>QBTT-FL</b> gives an unprecedentedly high fluorescence quantum yield of up to 53.1% for quinoidal oligothiophenes. With detailed photophysical and theoretical investigations, we demonstrate that the nonradiative intersystem crossing (S₁ → T₂) is significantly restrained in <b>QBTT-Ar</b>’s, which can be attributed to the faster reverse intersystem crossing (T₂ → S₁) characteristic of a small activation energy. This work reveals the possibility for developing red/near-infrared fluorophores from the less explored quinoidal molecules because of their intrinsically narrow bandgaps

Carbon-Bridged Oligo(phenylenevinylene)s: Stable π‑Systems with High Responsiveness to Doping and Excitation

The high responsiveness of π-conjugated materials to external stimuli, such as electrons and photons, accounts for both their utility in optoelectronic applications and their chemical instability. Extensive studies on heteroatom-stabilized π-conjugated systems notwithstanding, it is still difficult to combine high performance and stability. We report here that carbon-bridged oligo­(<i>p</i>-phenylenevinylene)­s (<b>COPV-</b><i><b>n</b></i>) are not only more responsive to doping and photoexcitation but also more stable than the conventional <i>p</i>-phenylenevinylenes and poly­(3-hexylthiophene), surviving photolysis very well in air, suggesting that they could serve as building blocks for optoelectronic applications. Activation of the ground state by installation of bond angle strain toward the doped or photoexcited state and the flat, rigid, and hindered structure endows <b>COPV</b>s with stimuli-responsiveness and stability without recourse to heteroatoms. For example, <b>COPV-6</b> can be doped with an extremely small reorganization energy and form a bipolaron delocalized over the entire π-conjugated system. Applications to bulk and molecular optoelectronic devices are foreseen

Two-Dimensional π‑Expanded Quinoidal Terthiophenes Terminated with Dicyanomethylenes as n‑Type Semiconductors for High-Performance Organic Thin-Film Transistors

Quinoidal oligothiophenes (<b>QOT</b>), as classical n-type semiconductors, have been well-known for a long time but with non-optimal semiconducting properties. We report here the design and selective synthesis of new two-dimensional (2D) π-expanded quinoidal terthiophenes, <b>2DQTT</b>s, with proximal (<b>2DQTT-i</b>) and distal (<b>2DQTT-o</b>) regiochemistry for high-performance n-channel organic thin-film transistors (n-OTFTs) featuring high electron mobility, solution processability, and ambient stability. The elegant combination of thieno­[3,4-<i>b</i>]­thiophene [TT, donor (D)] and 5-alkyl-4<i>H</i>-thieno­[3,4-<i>c</i>]­pyrrole-4,6­(5<i>H</i>)-dione [TPD, acceptor (A)] units with relatively large π-surface endows these <b>2DQTT</b>s with distinctive 2D structural characteristics and flat configuration stabilized by weak intramolecular S–O/S weak interactions. Furthermore, the A–D–A–D–A electronic structure maintains an adequately low LUMO energy level. These <b>2DQTT</b>s are shown to exhibit outstanding semiconducting properties with electron mobilities of up to 3.0 cm² V^–1 s^–1 and on/off ratios of up to 10⁶ (<b>2DQTT-o</b>) in ambient- and solution-processed OTFTs. Investigations on thin-film morphology reveal that the microstructure of <b>2DQTT</b>s is highly dependent on the orientation of the fused thiophene subunits, leading to differences in electron mobilities of 1 order of magnitude. X-ray diffraction studies in particular reveal increased crystallinity, crystalline coherence, and orientational order in <b>2DQTT-o</b> compared to <b>2DQTT-i</b>, which accounts for the superior electron transport property of <b>2DQTT-o</b>

A Thieno[3,4‑<i>b</i>]thiophene-Based Non-fullerene Electron Acceptor for High-Performance Bulk-Heterojunction Organic Solar Cells

A thieno­[3,4-<i>b</i>]­thiophene-based electron acceptor, ATT-1, is designed and synthesized. ATT-1 exhibits a planar conjugated framework, broad absorption with a large absorption coefficient, and a slightly high LUMO energy level. Bulk-hetero­junction (BHJ) solar cells based on PTB7-Th electron donor and ATT-1 electron acceptor delivered power conversion efficiencies of up to 10.07%, which is among the best performances reported for non-fullerene BHJ solar cells using PTB7-Th as the electron donor

Efficient Solution-Processed n‑Type Small-Molecule Thermoelectric Materials Achieved by Precisely Regulating Energy Level of Organic Dopants

To achieve efficient n-type doping, three dopants, 2-Cyc-DMBI-H, (2-Cyc-DMBI)₂, and (2-Cyc-DMBI-Me)₂, with precisely regulated electron-donating ability were designed and synthesized. By doping with a small-molecule 2DQTT-<i>o</i>-OD with high electron mobility, an unexpectedly high power factor of 33.3 μW m^–1 K^–2 was obtained with the new dopant (2-Cyc-DMBI-Me)₂. Notably, with the intrinsically low lateral thermal conductivity of 0.28 W m^–1 K^–1, the figure of merit was determined to be 0.02 at room temperature. Thus, we have demonstrated that small molecules with high electron mobility and low-lying LUMO energy levels can achieve high doping efficiency and excellent thermoelectric properties by doping with n-type dopants featuring highly matched energy levels and excellent miscibility

1,3-Bis(thieno[3,4‑<i>b</i>]thiophen-6-yl)‑4<i>H</i>‑thieno[3,4‑<i>c</i>]pyrrole-4,6(5<i>H</i>)‑dione-Based Small-Molecule Donor for Efficient Solution-Processed Solar Cells

A small molecule <b>TBTT-1</b> with 5-(2-ethylhexyl)-1,3-bis­(2-(2-ethyl­hexyl)­thieno­[3,4-<i>b</i>]­thiophen-6-yl)-4<i>H</i>-thieno­[3,4-<i>c</i>]­pyrrole-4,6­(5<i>H</i>)-dione (<b>TBTT</b>) as the central moiety was designed and synthesized for solution-processed bulk-heterojunction solar cells. <b>TBTT-1</b> exhibits a broad absorption with a low optical band gap of approximately 1.53 eV in the thin film. An optimized power conversion efficiency (PCE) of 7.47% with a high short-circuit current of 14.95 mA cm^–2 was achieved with diphenyl ether (DPE) as additive, which is the highest PCE for TPD-based small-molecule solar cells. According to the detailed morphology investigations, we found that DPE processing helped to enhance π–π stacking and reduce the scales of phase separation, which led to improved exciton splitting and charge transport in BHJ thin film, and thus enhanced device performance

Ullmann-Type Intramolecular C–O Reaction Toward Thieno[3,2‑<i>b</i>]furan Derivatives with up to Six Fused Rings

A new strategy for the efficient synthesis of thieno­[3,2-<i>b</i>]­benzofuran derivatives (15 examples) was achieved on the basis of successive regioselective intermolecular Suzuki and newly developed intramolecular Ullmann C–O reactions in up to a 70% overall yield. The fast intramolecular C–O reaction can be realized by an efficient catalytic combination of CuI/1,10-phenanthroline in up to a 97% yield. This method is suitable for the construction of highly fused thieno­[3,2-<i>b</i>]­furan-containing heterocycles including <b>DTBDF</b> and <b>TTDBF</b>. The π–π and hydrogen-bonding interactions observed for the <b>C</b>_<b>8</b><b>-DTBDF</b> single crystal suggest its great potential for OFET applications in the near future