Fluorinated Dithienylethene–Naphthalenediimide Copolymers for High-Mobility n-Channel Field-Effect Transistors

Strong Acceptors Based on Derivatives of Benzothiadiazoloimidazole

Despite the rapid progression of organic semiconductors, developing high-air-stability n-type organic semiconductors are still challenging. Herein, novel strong acceptors based on benzothiadiazoloimidazole units are reported. The results reveal that the strong acceptor BTI-NDI-BTI-a has good solubility and high electron affinity (3.94 eV), accompanied by 1D slipped-stacking crystals. Notably, the material presents promising potential for developing into air-stable n-type organic semiconductor materials.

Isomerically Pure Oxindole-Terminated Quinoids for n-Type Organic Thin-Film Transistors Enabled by the Chlorination of Quinoidal Core

Quinoidal compounds have great potential utility as high-performance organic semiconducting materials because of their rigid planar structures and extended π-conjugation. However, the existence of E and Z isomers adversely affects the charge-transport properties of quinoidal compounds. In this study, three isomerically pure oxindole-terminated quinoids were developed by introducing chlorine atoms in the quinoidal core. The synthesized quinoids were confirmed to have a Z,Z configuration by means of ¹ H NMR spectroscopy, density functional theory calculations, and single-crystal X-ray analysis. Importantly, the strategy of chlorination allowed to maintain low-lying frontier molecular orbital energy levels and ensure favorable intermolecular packing. Consequently, all three quinoidal compounds showed n-type transport characteristics in organic thin-film transistors, with electron mobilities up to 0.35 cm²  V^-1  s^-1 , which is the highest value reported to date for oxindole-terminated quinoids. Our study can provide new guidelines for the design of isomerically pure quinoids with high electron mobilities.

Recent Structural Engineering of Polymer Semiconductors Incorporating Hydrogen Bonds

Highly planar, extended π-electron organic conjugated polymers have been increasingly attractive for achieving high-mobility organic semiconductors. In addition to the conventional strategy to construct rigid backbone by covalent bonds, hydrogen bond has been employed extensively to increase the planarity and rigidity of polymer via intramolecular noncovalent interactions. This review provides a general summary of high-mobility semiconducting polymers incorporating hydrogen bonds in field-effect transistors over recent years. The structural engineering of the hydrogen bond-containing building blocks and the discussion of theoretical simulation, microstructural characterization, and device performance are covered. Additionally, the effects of the introduction of hydrogen bond on self-healing, stretchability, chemical sensitivity, and mechanical properties are also discussed. The review aims to help and inspire design of new high-mobility conjugated polymers with superiority of mechanical flexibility by incorporation of hydrogen bond for the application in flexible electronics.

Acceptor Modulation Strategies for Improving the Electron Transport in High-Performance Organic Field-Effect Transistors

High-performance ambipolar and electronic type semiconducting polymers are essential for fabricating various organic optoelectronic devices and complementary circuits. This review summarizes the strategies of improving the electron transport of semiconducting polymers via acceptor modulation strategies, which include the use of single, dual, triple, multiple, and all acceptors as well as the fusion of multiple identical acceptors to obtain new heterocyclic acceptors. To further improve the electron transport of semiconducting polymers, the introduction of strong electron-withdrawing groups can enhance the electron-withdrawing ability of donors and acceptors, thereby facilitating electron injection and suppressing hole accumulation. In addition, the relationships between the molecular structure, frontier molecular orbital energy levels, thin film morphology, microstructure, processing conditions, and device performances are also comprehensively discussed. Finally, the challenges encountered in this research area are proposed and the future outlook is presented.

High Miscibility Compatible with Ordered Molecular Packing Enables an Excellent Efficiency of 16.2% in All-Small-Molecule Organic Solar Cells

In all-small-molecule organic solar cells (ASM-OSCs), a high short-circuit current (J_sc ) usually needs a small phase separation, while a high fill factor (FF) is generally realized in a highly ordered packing system. However, small domain and ordered packing always conflicted each other in ASM-OSCs, leading to a mutually restricted J_sc and FF. In this study, alleviation of the previous dilemma by the strategy of obtaining simultaneous good miscibility and ordered packing through modulating homo- and heteromolecular interactions is proposed. By moving the alkyl-thiolation side chains from the para- to the meta-position in the small-molecule donor, the surface tension and molecular planarity are synchronously enhanced, resulting in compatible properties of good miscibility with acceptor BTP-eC9 and strong self-assembly ability. As a result, an optimized morphology with multi-length-scale domains and highly ordered packing is realized. The device exhibits a long carrier lifetime (39.8 μs) and fast charge collection (15.5 ns). A record efficiency of 16.2% with a high FF of 75.6% and a J_sc of 25.4 mA cm^-2 in the ASM-OSCs is obtained. These results demonstrate that the strategy of simultaneously obtaining good miscibility with high crystallinity could be an efficient photovoltaic material design principle for high-performance ASM-OSCs.

The aging effect on the enhancement of thermal stability, mechanical stiffness and fluorescence properties of histidine-appended naphthalenediimide based two-component hydrogels

A histidine attached naphthalenediimide (NDI)-containing amphiphilic molecule (NDIP) self-assembles into nanotubes in aqueous solution at pH 6.6 as revealed by high-resolution transmission electron microscopy studies. This histidine-appended NDI forms a two-component hydrogel in the presence of tartaric acid at a molar ratio of 1 : 2. A morphological transformation was observed from a nanotube structure in the non-gel aggregated state of histidine appended NDI to interconnected cross-linked nanofibers of the two-component hydrogel in the presence of tartaric acid. Interestingly, the gel exhibits an unusual behavior upon aging compared to the fresh gel. It is found that the thermal stability and gel stiffness increase very significantly upon aging. Another important feature noted is that the very weak fluorescence of the fresh gel is transformed into bright greenish fluorescence upon aging. These results suggest that intermolecular interactions among the gelator molecules and tartaric acid in the gel phase slowly increase with time to form a mechanically very stiff and thermally robust gel.

Azaisoindigo-Based Polymers with a Linear Hybrid Siloxane-Based Side Chain for High-Performance Semiconductors Processable with Nonchlorinated Solvents

Developing nonchlorinated solvent-processed polymeric semiconductors to avoid environmental concerns and health hazards caused by chlorinated solvents is especially urgent. Here, a molecular design strategy, composed of backbone fluorination and side chain optimization, is used for preparing high-solubility and high-performance azaisoindigo-based polymers. The effects of different backbones and side chains on the solubility, film crystallinity, molecular stacking, and charge transport properties are mainly investigated. A long linear hybrid siloxane-based chain (C₆-Si₇) is chosen to improve the solubility, while the incorporation of fluorine (F) is used to enhance the film crystallinity and charge mobility. By optimizing the backbone and side chain, both solubility and charge mobility of the azaisoindigo-based polymer are significantly improved. As a result, PAIIDBFT-Si films processed with toluene, tetrahydrofuran, ether, and alkanes, achieved charge mobilities of 4.14, 3.78, 2.14, and 2.34 cm² V^-1 s^-1, respectively. The current study provides an effective strategy for the design and synthesis of high-performance polymeric semiconductors processed with nonchlorinated solvents.

Ultraviolet Photodetecting and Plasmon-to-Electric Conversion of Controlled Inkjet-Printing Thin-Film Transistors

Direct ink-jet printing of a zinc-oxide-based thin-film transistor (ZnO-based TFT) with a three-dimensional (3-D) channel structure was demonstrated for ultraviolet light (UV) and visible light photodetection. Here, we demonstrated the channel structures by which temperature-induced Marangoni flow can be used to narrow the channel width from 318.9 ± 44.1 μm to 180.1 ± 13.9 μm via a temperature gradient. Furthermore, a simple and efficient oxygen plasma treatment was used to enhance the electrical characteristics of switching I_ON/I_OFF ratio of approximately 10⁵. Therefore, the stable and excellent gate bias-controlled photo-transistors were fabricated and characterized in detail for ultraviolet (UV) and visible light sensing. The photodetector exhibited a superior photoresponse with a significant increase of more than 2 orders of magnitude larger drain current generated upon UV illumination. The results could be useful for the development of UV photodetectors by the direct-patterning ink-jet printing technique. Additionally, we also have successfully demonstrated that a metal-semiconductor junction structure that enables plasmon energy detection by using the plasmonic effects is an efficient conversion of plasmon energy to an electrical signal. The device showed a significant variations negative shift of threshold voltage under different light power density with exposure of visible light. With the ZnO-based TFTs, only ultraviolet light detection extends to the visible light wavelength.

Molecular engineering of (E)-1,2-bis(3-cyanothiophene-2-yl)ethene-based polymeric semiconductors for unipolar n-channel field-effect transistors

The unipolar n-type polymeric semiconductors based on (E)-1,2-bis(3-cyanothiophene-2-yl)ethene were developed and their structure–property relationships were investigated.

Evaluating the Impact of Fluorination on the Electro-optical Properties of Cross-Conjugated Benzobisoxazoles

Six 2,4,6,8-tetrarylbenzo[1,2- d:4,5- d']bisoxazoles (BBOs) were synthesized: three bearing phenyl substituents at the 2- and 6-positions and three bearing perfluorophenyl groups at those positions. The influence of perfluoro-aryl group substitution on the physical, optical, and electronic properties of 2,4,6,8-tetrarylbenzo[1,2- d:4,5- d']bisoxazoles (BBO) was evaluated using both experimental and theoretical methods. The density functional theory (DFT) model was found to be well-matched to the experimental optical data, as evidenced by the UV-vis spectra. Both cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS) were used to determine the position of the HOMO with varying results. The values obtained by CV were deeper than those obtained via UPS and correlated well with the theoretical calculations. However, the UPS values were more consistent with the expected outcomes for a system with segregated frontier molecular orbitals (FMOs). The UPS results are also supported by the electrostatic potential maps, which indicate that the electron density within the LUMO and HOMO is nearly completely localized along the 2,6- or 4,8-axis, respectively. The summation of the results indicates that strongly electron-withdrawing groups can be used to selectively tune the LUMO level with minor perturbation of the HOMO, something that is challenging to accomplish in typical donor-acceptor systems.

Concurrent Optical Gain Optimization and Electrical Tuning in Novel Oligomer:Polymer Blends with Yellow-Green Laser Emission

Electrically pumped organic lasing requires the integration of electrodes contact into the laser cavity in an organic light-emitting diode (OLED) or organic field effect transistor configuration to enable charge injection. Efficient and balanced carrier injection requires in turn alignment of the energy levels of the organic active layers with the Fermi levels of the cathode and anode. This can be achieved through chemical substitution with specific aromatic functional groups, although paying the price for a substantial (and often detrimental) change in the emission and light amplifying properties of the organic gain medium. Here, using host-guest energy transfer mixtures with hosts bearing a systematic and gradual shift in molecular orbitals is proposed, which reduces the amplified spontaneous emission (ASE) threshold of the organic gain medium significantly while leaving the peak emission unaffected. By virtue of the low guest doping required for complete host-to-guest energy transfer, the injection levels in the blends are attributed to the host whereas the gain properties solely depend on the guest. It is demonstrated that the ASE peak and thresholds of blends with different hosts do not differ while the current efficiency of OLEDs devices is deeply influenced by molecular orbital tuning of the hosts.

Regio-regular alternating diketopyrrolopyrrole-based D1–A–D2–A terpolymers for the enhanced performance of polymer solar cells

We designed and synthesized regio-regular alternating diketopyrrolopyrrole (DPP)-based D₁–A–D₂–A terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) using a primary donor (D₁) [3,3′-difluoro-2,2′-bithiophene (F2T2)] and a secondary donor (D₂) [2,2′-bithiophene (T2), (E)-1,2-di(thiophen-2-yl)ethene (TVT), or dithieno[3,2-b:2′,3′-d]thiophene (DTT)]. A PDPP2DT-F2T2 D–A polymer was synthesized as well to compare optical, electronic, and photovoltaic properties. The absorption peaks of the terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) were longer (λ_max = 801–810 nm) than the peak of the PDPP2DT-F2T2 polymer (λ_max = 799 nm), which is associated with the high-lying HOMO levels of the terpolymers (−5.08 to −5.13 eV) compared with the level of the PDPP2DT-F2T2 polymer (−5.38 eV). The photovoltaic properties of these DPP-based polymers were investigated under simulated AM 1.5G sunlight (100 mW cm⁻²) with a conventional structure (ITO/PEDOT:PSS/polymer:PC₇₁BM/Al). The open-circuit voltages (V_oc) of photovoltaic devices containing the terpolymers were slightly lower (0.68–0.70 V) than the V_oc of the device containing the PDPP2DT-F2T2 polymer (0.79 V). The short-circuit current (J_sc) of the PDPPF2T2DPP-DTT device was significantly improved (14.14 mA cm⁻²) compared with that of the PDPP2DT-F2T2 device (8.29 mA cm⁻²). As a result, the power conversion efficiency (PCE) of the PDPPF2T2DPP-DTT device (6.35%) was increased by 33% compared with that of the simple D–A-type PDPP2DT-F2T2 device (4.78%). The highest J_sc and PCE values (the PDPPF2T2DPP-DTT device) were attributed to an optimal nanoscopically mixed morphology and strong interchain packing with a high face-on orientation in the blend film state. The study demonstrated that our strategy of using multiple donors in a regio-regular alternating fashion could fine-tune the optical, electronic, and morphological properties of D–A-type polymers, enhancing the performance of polymer solar cells.

We designed and synthesized regio-regular alternating diketopyrrolopyrrole (DPP)-based D₁–A–D₂–A terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) for use in polymer solar cells.

Modulating Surface Morphology and Thin-Film Transistor Performance of Bi-thieno[3,4-c]pyrrole-4,6-dione-Based Polymer Semiconductor by Altering Preaggregation in Solution

Due to their strong intermolecular interactions, polymer semiconductors aggregate in solution even at elevated temperature. With the aim to study the effect of this kind preaggregation on the order of thin films and further transistor performance, bi-thieno[3,4-c]pyrrole-4,6-dione and fluorinated oligothiophene copolymerized polymer semiconductor P1, which shows strong temperature-dependent aggregation behavior in solution, is synthesized. Its films are deposited through a temperature-controlled dip-coating technique. X-ray diffraction and atomic force microscopy results reveal that the aggregation behavior of P1 in solution affects the microstructures and order of P1 films. The charge transport properties of P1 films are investigated with bottom-gate top-contacted thin-film transistors. The variation of device performance (from 0.014  to 1.03 cm² V^-1 s^-1) demonstrates the importance of optimizing preaggregation degree. The correlation between preaggregation degree and transistor performance of P1 films is explored.

Novel Conjugated Polymers Prepared by Direct (Hetero) arylation: An Eco-Friendly Tool for Organic Electronics

The phthalimide (PhI) moiety has been attracting more attention as an excellent acceptor building block in donor-acceptor (D-A) conjugated polymers. In this paper; three D-A conjugated polymers with or without thiocarbonyl moieties are successfully prepared by the direct (hetero)-arylation polymerization (DHAP), which is an atom efficient and facile synthetic strategy to obtain polymer materials. Compared with the traditional carbon-carbon coupling reactions, this method possesses more advantages, including: fewer synthetic steps, avoidance of the preparation of the organometallic reagents, higher atom economy and fewer toxic byproducts, better compatibility with chemically sensitive functional groups and so on. All three of these designed PhI-based polymers exhibited favourable optoelectronic and thermal performance. The optical, thermodynamic and electrochemical properties of the synthesized polymers were systematically investigated using ultraviolet-visible (UV-vis) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and cyclic voltammetry (CV). The results of these three polymers indicated that thionation of the carbonyl was a highly effective methods to improve the properties of PhI-based polymers; and provided impetus for the development of thionated PhI derivatives for organic electronic applications.

Vinylene and benzo[c][1,2,5]thiadiazole: effect of the π-spacer unit on the properties of bis(2-oxoindolin-3-ylidene)-benzodifuran-dione containing polymers for n-channel organic field-effect transistors

Two polymers based on (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5-b′]difuran-2,6(3H,7H)-dione (BIBDF) coupled with (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) or dithienylbenzothiadiazole (TBT), namely PBIBDF-TVT and PBIBDF-TBT were synthesized via the Stille coupling reaction. The effect of benzothiadiazole or vinylene-π spacer of the copolymers on optical properties, energy levels, electronic device performance and microstructure were studied. It was found that PBIBDF-TBT based OFET devices, annealed at 180 °C, showed better performance with the highest electron mobility of 2.9 × 10⁻² cm² V s⁻¹ whereas PBIBDF-TVT polymer exhibited 5.0 × 10⁻⁴ cm² V s⁻¹. The two orders of magnitude higher electron mobility of PBIBDF-TBT over PBIBDT-TVT is a clear indicator of the better charge transport ability of this polymer semiconductor arising from its higher crystallinity and better donor–acceptor interaction.

Bottom-gate-top-contact OFET device structure using PBIBDF-TVT and PBIBDF-TBT based polymer semiconductors.