What To Expect from Conducting Polymers on the Playground of Thermoelectricity: Lessons Learned from Four High-Mobility Polymeric Semiconductors

Modification of the electronic structures of two benchmark donor–acceptor (D–A) copolymers poly­[(4,4′-bis­(2-ethylhexyl)­dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­silole)-2,6-diyl-<i>alt</i>-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT) and poly­[{2,5-bis­(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-diyl}-<i>alt</i>-{[2,2′:5′,2″-terthiophene]-5,5″-diyl}] (PDPP3T) by chemical doping is reported. Simply by dipping polymer films into dopant solution, high electrical conductivity is achieved and thermoelectric property of the films is optimized. Despite their deep HOMO levels, optical absorption extending continuously to 2000 nm is observed in PSBTBT, and a high power factor around 25 μW m^–1 K^–2 is obtained in PDPP3T. Furthermore, temperature-dependent measurement of electrical conductivity and Seebeck coefficients is carried out to understand transport mechanisms and energetic distribution of carrier density of states (DOS). In parallel, doping treatment and corresponding characterizations are performed on donor polymers poly­(3-hexylthiophene) (P3HT) and poly­(2,5-bis­(3-dodecylthiophen-2-yl)­thieno­[3,2-<i>b</i>]­thiophene) (PBTTT-C12) for comparison. Ultimately, based on comprehensive characterizations and comparisons of the four polymers in terms of bulk mobility, DOS, film microstructures, and molecular structures, etc., a primitive correlation between solution-processable polymeric semiconductors and thermoelectric properties of their doped products is established

Five-Ring Fused Tetracyanothienoquinoids as High-Performance and Solution-Processable n‑Channel Organic Semiconductors: Effect of the Branching Position of Alkyl Chains

Dicyanomethylene-substituted quinoidal dithieno­[2,3-<i>d</i>;2′,3′-<i>d</i>′]­benzo­[1,2-<i>b</i>;4,5-<i>b</i>′]­dithiophene compounds (<b>QDTBDT</b>s) with alkyl chains branched at different positions were synthesized. Thin-film transistor characteristics showed that the type of charge carriers in <b>QDTBDT</b>s could be tuned by changing the branching position of the alkyl chains. <b>QDTBDT-2C</b> exhibited n-channel behavior, and the observed electron mobility was 0.57 cm² V^–1 s^–1 without post-treatment, one of the highest values reported for spin-coated thin-film transistors with no annealing under ambient conditions. <b>QDTBDT-4C</b>-based transistors displayed electron-dominated ambipolar transport behavior, with electron mobilities reaching 0.2 cm² V^–1 s^–1 and hole mobilities in the range of 10^–3–10^–4 cm² V^–1 s^–1. <b>QDTBDT-3C</b> showed solution-concentration-dependent carrier transport characteristics, exhibiting n-type behavior at low solution concentrations and ambipolar performance at high solution concentrations with an electron mobility of 0.22 cm² V^–1 s^–1 and a hole mobility of 0.034 cm² V^–1 s^–1. CMOS-like inverters fabricated from <b>QDTBDT-2C</b> displayed high gain and high noise margins

Interface Engineering of Semiconductor/Dielectric Heterojunctions toward Functional Organic Thin-Film Transistors

Interface modification is an effective and promising route for developing functional organic field-effect transistors (OFETs). In this context, however, researchers have not created a reliable method of functionalizing the interfaces existing in OFETs, although this has been crucial for the technological development of high-performance CMOS circuits. Here, we demonstrate a novel approach that enables us to reversibly photocontrol the carrier density at the interface by using photochromic spiropyran (SP) self-assembled monolayers (SAMs) sandwiched between active semiconductors and gate insulators. Reversible changes in dipole moment of SPs in SAMs triggered by lights with different wavelengths produce two distinct built-in electric fields on the OFET that can modulate the channel conductance and consequently threshold voltage values, thus leading to a low-cost noninvasive memory device. This concept of interface functionalization offers attractive new prospects for the development of organic electronic devices with tailored electronic and other properties

Conductive Copper Benzenehexathiol Coordination Polymer as a Hydrogen Evolution Catalyst

A graphene-like coordination polymer based on copper­(II) benzenehexathiol (Cu-BHT, <b>1</b>) with high electric conductivity (10³ S·cm^–1) was prepared recently. The high conductivity makes this material a good candidate for electrocatalysis, and here its catalytic activity toward hydrogen evolution reaction (HER) was evaluated. Cu-BHT shows good activity and stability for HER in acidic solutions under high current densities. By changing the preparation conditions, the morphology of Cu-BHT materials was controlled at the mesoscale, which allows the preparation of a thin film (TF-<b>1</b>), nanocrystal (NC-<b>1</b>), and amorphous nanoparticle (NP-<b>1</b>) of Cu-BHT. The overpotential of Cu-BHT toward HER shows an improved activity from 760 mV (NC-<b>1</b>) to 450 mV (NP-<b>1</b>) at a current density of 10 mA·cm^–2. A Tafel slope of ∼95 mV·dec^–1 and an exchange current density of 10^–3 mA·cm^–2 were achieved under optimized conditions. Density functional theory calculations suggest that the “Cu-edge site” on the (100) surface plays an important role in improving the HER catalytic performance of Cu-BHT nanoparticles

Acceptor–Donor–Acceptor Small Molecules Based on Indacenodithiophene for Efficient Organic Solar Cells

Four A-D-A type small molecules using 4,4,9,9-tetrakis­(4-hexylphenyl)- indaceno­[1,2-b:5,6-b′]­dithiophene as central building block, bithiophene or terthiophene as π-bridges, alkyl cyanoacetate or rhodanine as end acceptor groups were synthesized and investigated as electron donors in solution-processed organic solar cells (OSCs). These molecules showed excellent thermal stability with decomposition temperatures over 360 °C, relatively low HOMO levels of −5.18 to −5.22 eV, and strong optical absorption from 350 to 670 nm with high molar extinction coefficient of 1.1 × 10⁵ to 1.6 × 10⁵ M^–1 cm^–1 in chloroform solution. OSCs based on blends of these molecules and PC₇₁BM achieved average power conversion efficiencies of 2.32 to 5.09% (the best 5.32%) after thermal annealing. The effects of thiophene bridge length and end acceptor groups on absorption, energy level, charge transport, morphology, and photovoltaic properties of the molecules were investigated

Thiophene-Diketopyrrolopyrrole-Based Quinoidal Small Molecules as Solution-Processable and Air-Stable Organic Semiconductors: Tuning of the Length and Branching Position of the Alkyl Side Chain toward a High-Performance n‑Channel Organic Field-Effect Transistor

A series of thiophene-diketopyrrolopyrrole-based quinoidal small molecules (<b>TDPPQ-2</b>–<b>TDPPQ-5</b>) bearing branched alkyl chains with different side-chain lengths and varied branching positions are synthesized. Field-effect transistor (FET) measurement combined with thin-film characterization is utilized to systematically probe the influence of the side-chain length and branching position on the film microstructure, molecular packing, and, hence, charge-transport property. All of these <b>TDPPQ</b> derivatives show air-stable n-channel transporting behavior in spin-coated FET devices, which exhibit no significant decrease in mobility even after being stored in air for 2 months. Most notably, <b>TDPPQ-3</b> exhibits an outstanding n-channel semiconducting property with electron mobilities up to 0.72 cm² V^–1 s^–1, which is an unprecedented value for spin-coated DPP-based n-type semiconducting small molecules. A balance of high crystallinity, satisfactory thickness uniformity and continuity, and strong intermolecular interaction accounts for the superior charge-transport characteristics of <b>TDPPQ-3</b> films. Our study demonstrates that tuning the length and branching position of alkyl side chains of semiconducting molecules is a powerful strategy for achieving high FET performance

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

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>

Tetracyanodibenzotetrathiafulvalene Diimides: Design, Synthesis, and Property Study

Tetracyanodibenzotetrathiafulvalene diimide (TCDBTTF-DI), an isomer of core-expanded naphthalene diimides bearing two 2-(1,3-dithiol-2-ylidene)­malononitrile moieties (NDI-DTYM2), has been designed and synthesized to explore the effect of its isomeric structure on the optical and electrochemical properties of the materials. UV–vis spectra show that TCDBTTF-DI exhibits variation in its absorption peaks while maintaining a similar optical band gap to NDI-DTYM2. Electrochemical studies indicate that TCDBTTF-DI can not only accept but also lose electrons, in contrast to the solely electron-accepting behavior of NDI-DTYM2

Diketopyrrolopyrrole-Containing Quinoidal Small Molecules for High-Performance, Air-Stable, and Solution-Processable n-Channel Organic Field-Effect Transistors

We report the synthesis, characterization, and application of a novel series of diketopyrrolopyrrole (DPP)-containing quinoidal small molecules as highly efficient n-type organic semiconductors in thin film transistors (TFTs). The first two representatives of these species exhibit maximum electron mobility up to 0.55 cm² V^–1 s^–1 with current on/current off (<i>I</i>_on/<i>I</i>_off) values of 10⁶ for <b>1</b> by vapor evaporation, and 0.35 cm² V^–1 s^–1 with <i>I</i>_on/<i>I</i>_off values of 10⁵–10⁶ for <b>2</b> by solution process in air, which is the first demonstration of DPP-based small molecules offering only electron transport characteristics in TFT devices. The results indicate that incorporation of a DPP moiety to construct quinoidal architecture is an effective approach to enhance the charge-transport capability