The universal role of high-k fluorinated dielectrics in assisting the carrier transport in transistors for a broad range of printable semiconductors is explored. These results present general rules for how to design dielectric materials and achieve devices with a high carrier concentration, low disorder, reliable operation, and robust properties.

R ecent advances in solution-processable conjugated polymers have been promoted in the field of organic fieldeffect transistors (OFETs) due to their applicability to flexible devices through low-cost manufacturing processes such as the printing technique. 1 Various types of semiconductors are being introduced as promising candidates for high-performance OFETs that exceed a field-effect mobility (μ FET ) of 1 cm 2 / (V•s) by tailoring their structural characteristics. 2,3 Most of these polymers require a high-level planarity in their conjugated backbones in an effort to achieve high charge-carrier mobility, 4 and such planarity is commonly accomplished by the acquisition of two main structural features. First, the monomeric units should have rigid and flat structures to inhibit bond rotation. 5−9 In addition, avoiding steric hindrance between monomeric units is also important for maintaining the planarity of backbones. 10−12 Recently, many researchers have demonstrated research results concerning high-mobility conjugated polymers with planar backbones and found it obvious that the restriction of chemical bond torsion is indeed important in obtaining excellent charge-transport properties. 4,13 The second important structural feature for high-mobility conjugated polymers could be the donor−acceptor (D−A) configuration. Intrinsically different polarities of electron rich and electron deficient units allow this type of copolymers to have strong intermolecular interactions. 2−4,14 Some of the more popular types of units that are composed of the D−A conformation include 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) 2 and isoindigo, 12,15 which have exhibited an excellent μ FET of more than 1−10 cm 2 /(V•s). On the other hand, there also are conjugated polymers that are comprised of electron-donating units only, which also have potential for use in high-mobility OFETs. 6,16 In this paper, we classify these polymers as donor− donor (D−D) types, as opposed to the D−A types. These polymers are usually composed of a fused aromatic structure and are designed to have long-range intermolecular side chain interdigitations, leading to the formation of a three-dimensional lamellar π-stacking structure. By avoiding D−A type which is already a widely used conformational trend in conjugated polymers, we can further enlarge the diversity of monomeric combinations. However, most of the D−D-type polymers have shown a relatively lower degree of mobility compared with the D−A types because of the inevitable angular torsion 6 or a lack of structural stiffness 16 that diminishes the overall planarity in a conjugated system, resulting in a relatively lower μ FET (10 −5 ∼ 1 cm 2 /(V•s)) than D−A type polymers. To obtain high-mobility

Since quinoidal molecules have double-bond linkages between aromatic rings, they have many advantages for efficient charge transport resulting from high planarity and extended π-conjugation length. However, they unavoidably generate some isomers, which cause difficulty in purification and characterization. In this study, sulfur–oxygen conformation locking and steric repulsion approach is introduced to manipulate syn- and anti-isomerization of a quinoidal building block (bis-QEDOT). As a result, isomer-free bis-QEDOT is synthesized by introducing the 3,4-ethylenedioxy group, and the geometrical structure of bis-QEDOT is identified by thin-layer chromatography, 1H NMR, and density functional theory calculation. Furthermore, thiophene (T), bithiophene (2T), and thienylene vinylene (TV) as π-conjugated building blocks are polymerized with bis-QEDOT. Due to the quinoid structure, PQEDOT-T, PQEDOT-2T, and PQEDOT-TV show an intensified near-IR absorption and a low band gap around ∼1.16 eV. Grazing incidence wide-angle X-ray diffraction reveals that three quinoidal polymers show in the (h00) diffraction peaks up to third order after thermal annealing at 250 °C, demonstrating high crystallinity of the films. Finally, the electrical properties of the three polymers are investigated as an active layer in organic field-effect transistors showing hole mobilities of 4.3 × 10–2 (PQEDOT-T), 1.8 × 10–2 (PQEDOT-2T), and 7.8 × 10–3 cm2 V–1 s–1 (PQEDOT-TV).

Four donor–acceptor (D–A) type conjugated polymers were synthesized for organic photovoltaics and organic field effect transistors.

Fluorine (F) substitution on conjugated polymers in polymer solar cells (PSCs) has a diverse effect on molecular properties and device performance. We present a series of three D-A type conjugated polymers (PBT, PFBT, and PDFBT) based on dithienothiophene and benzothiadiazole units with different numbers of F atoms to explain the influence of F substitution by comparing the molecular interactions of the polymers and the recombination kinetics in PSCs. The preaggregation behavior of PFBT and PDFBT in o-DCB at the UV-vis absorption spectra proves that both polymers have strong intermolecular interactions. Besides, more closely packed structures and change into face-on orientation of fluorinated polymers are observed in polymer:PCBM blends by GIXD which is beneficial for charge transport and, ultimately, for current density in PSCs (4.3, 13.0, and 14.5 mA cm for PBT, PFBT, and PDFBT, respectively). Also, the introduction of F atoms on conjugated backbones affects the recombination kinetics by suppressing bimolecular recombination, thereby improving the fill factor (0.41, 0.68, and 0.69 for PBT, PFBT, and PDFBT, respectively). Consequently, the PCE of PSCs reached 7.3% without any additional treatment (annealing, solvent additive, etc.) in the polymer containing difluorinated BT (PDFBT) that is much higher than nonfluorinated BT (PBT ∼ 1%) and monofluorinated BT (PFBT ∼ 6%).

Organic photosensitizers have been investigated as effective light-sensing elements that can promote strong absorption with high field-effect mobility in organic phototransistors (OPTs). In this study, a novel organic photosensitizer is synthesized to demonstrate broad-band photoresponse with enhanced electrical performance. An unsymmetrical small molecule of a solubilizing donor (Dsol)–acceptor (A)–dye donor (Ddye) type connected with a twisted conjugation system is designed for broad-band detection (ranging from 250 to 700 nm). This molecule has high solubility, thereby facilitating the formation of uniformly dispersed nanoparticles in an insulating polymer matrix, which is deposited on top of OPT semiconductors by a simple solution process. The broad-band photodetection shown by the organic photosensitizer is realized with improved mobility close to an order of magnitude and high on/off current ratio (∼105) of the organic semiconductor. Furthermore, p-type charge transport behavior in the channel of the OPT is enhanced through the intrinsic electron-accepting ability of the organic photosensitizer caused by the unique molecular configuration. These structural properties of organic photosensitizers contribute to an improvement in broad-band photosensing systems with new optoelectronic properties and functionalities.

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