Improving the Electronic Transporting Property for Flexible Field-Effect Transistors with Naphthalene Diimide-Based Conjugated Polymer through Branching/Linear Side-Chain Engineering Strategy

Nonhalogenated Solvent Processable and High-Density Photopatternable Polymer Semiconductors Enabled by Incorporating Hydroxyl Groups in the Side Chains

Polymer semiconductors hold tremendous potential for applications in flexible devices, which is however hindered by the fact that they are usually processed by halogenated solvents rather than environmentally more friendly solvents. An effective strategy to boost the solubility of high-performance polymer semiconductors in nonhalogenated solvents such as tetrahydrofuran (THF) by appending hydroxyl groups in the side chains is herein presented. The results show that hydroxyl groups, which can be easily incorporated into the side chains, can significantly improve the solubility of typical p- and n-types as well as ambipolar polymer semiconductors in THF. Meanwhile, the thin films of these polymer semiconductors from the respective THF solutions show high charge mobilities. With THF as the processing and developing solvents these polymer semiconductors with hydroxyl groups in the side chains can be well photopatterned in the presence of the photo-crosslinker, and the charge mobilities of the patterned thin films are mostly maintained by comparing with those of the respective pristine thin films. Notably, THF is successfully utilized as the processing and developing solvent to achieve high-density photopatterning with ≈82 000 device arrays cm-2 for polymer semiconductors in which hydroxyl groups are appended in the side chains.

Self-Doping Naphthalene Diimide Conjugated Polymers for Flexible Unipolar n-Type OTFTs

The development of high-performance organic thin-film transistor (OTFT) materials is vital for flexible electronics. Numerous OTFTs are so far reported but obtaining high-performance and reliable OTFTs simultaneously for flexible electronics is still challenging. Herein, it is reported that self-doping in conjugated polymer enables high unipolar n-type charge mobility in flexible OTFTs, as well as good operational/ambient stability and bending resistance. New naphthalene diimide (NDI)-conjugated polymers PNDI2T-NM17 and PNDI2T-NM50 with different contents of self-doping groups on their side chains are designed and synthesized. The effects of self-doping on the electronic properties of resulting flexible OTFTs are investigated. The results reveal that the flexible OTFTs based on self-doped PNDI2T-NM17 exhibit unipolar n-type charge-carrier properties and good operational/ambient stability thanks to the appropriate doping level and intermolecular interactions. The charge mobility and on/off ratio are fourfold and four orders of magnitude higher than those of undoped model polymer, respectively. Overall, the proposed self-doping strategy is useful for rationally designing OTFT materials with high semiconducting performance and reliability.

Boosting the Thermoelectric Performance of the Doped DPP-EDOT Conjugated Polymer by Incorporating an Ionic Additive

The thermoelectric (TE) performance of organic materials is limited by the coupling of Seebeck coefficient and electrical conductivity. Herein a new strategy is reported to boost the Seebeck coefficient of conjugated polymer without significantly reducing the electrical conductivity by incorporation of an ionic additive DPPNMe₃ Br. The doped polymer PDPP-EDOT thin film exhibits high electrical conductivity up to 1377 ± 109 S cm^-1 but low Seebeck coefficient below 30 µV K^-1 and a maximum power factor of 59 ± 10 µW m^-1 K^-2 . Interestingly, incorporation of small amount (at a molar ratio of 1:30) of DPPNMe₃ Br into PDPP-EDOT results in the significant enhancement of Seebeck coefficient along with the slight decrease of electrical conductivity after doping. Consequently, the power factor (PF) is boosted to 571 ± 38 µW m^-1 K^-2 and ZT reaches 0.28 ± 0.02 at 130 °C, which is among the highest for the reported organic TE materials. Based on the theoretical calculation, it is assumed that the enhancement of TE performance for the doped PDPP-EDOT by DPPNMe₃ Br is mainly attributed to the increase of energetic disorder for PDPP-EDOT.

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.

Side-Chain Engineering of Conjugated Polymers for High-Performance Organic Field-Effect Transistors

Past decades have witnessed the rapid development of conjugated polymers because of their promising semiconducting properties and applications in organic field-effect transistors (OFETs). Recent studies have shown that side-chain engineering of conjugated polymers is an efficient strategy to increase semiconducting performance. This Perspective focuses on the side-chain modulation of conjugated polymers and evaluating their effects on the performance of OFETs. The challenges and potential applications of functional high-performance OFETs through side-chain engineering are also discussed.

Naphthalene diimides: perspectives and promise

In this review, we describe the developments in the field of naphthalene diimides (NDIs) from 2016 to the presentday. NDIs are shown to be an increasingly interesting class of molecules due to their electronic properties, large electron deficient aromatic cores and tendency to self-assemble into functional structures. Almost all NDIs possess high electron affinity, good charge carrier mobility, and excellent thermal and oxidative stability, making them promising candidates for applications in organic electronics, photovoltaic devices, and flexible displays. NDIs have also been extensively studied due to their potential real-world uses across a wide variety of applications including supramolecular chemistry, sensing, host-guest complexes for molecular switching devices, such as catenanes and rotaxanes, ion-channels, catalysis, and medicine and as non-fullerene accepters in solar cells. In recent years, NDI research with respect to supramolecular assemblies and mechanoluminescent properties has also gained considerable traction. Thus, this review will assist a wide range of readers and researchers including chemists, physicists, biologists, medicinal chemists and materials scientists in understanding the scope for development and applicability of NDI dyes in their respective fields through a discussion of the main properties of NDI derivatives and of the status of emerging applications.

An insight into the role of side chains in the microstructure and carrier mobility of high-performance conjugated polymers

The effect of linear-chain interdigitation on device performance was studied in detail by both experimental and theoretical methods.

Modulation of Binary Neuroplasticity in a Heterojunction-Based Ambipolar Transistor

To keep pace with the upcoming big-data era, the development of a device-level neuromorphic system with highly efficient computing paradigms is underway with numerous attempts. Synaptic transistors based on an all-solution processing method have received growing interest as building blocks for neuromorphic computing based on spikes. Here, we propose and experimentally demonstrated the dual operation mode in poly{2,2-(2,5-bis(2-octyldodecyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl)dithieno[3,2-b]thiophene-5,5-diyl-alt-thiophen-2,5-diyl}(PDPPBTT)/ZnO junction-based synaptic transistor from ambipolar charge-trapping mechanism to analog the spiking interfere with synaptic plasticity. The heterojunction formed by PDPPBTT and ZnO layers serves as the basis for hole-enhancement and electron-enhancement modes of the synaptic transistor. Distinctive synaptic responses of paired-pulse facilitation (PPF) and paired-pulse depression (PPD) were configured to achieve the training/recognition function for digit image patterns at the device-to-system level. The experimental results indicate the potential application of the ambipolar transistor in future neuromorphic intelligent systems.

Structural Transition and Interdigitation of Alkyl Side Chains in the Conjugated Polymer Poly(3-hexylthiophene) and Their Effects on the Device Performance of the Associated Organic Field-Effect Transistor

Direct grazing-angle X-ray scattering evidence of the order-disorder transition and interdigitation of side chains in a conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. The free methyl ends of the side chains exhibit closest packing, as in n-alkane crystallization, and cause a structural mismatch due to the difference between their packing density and the areal density of the attached ends. This mismatch is resolved by increases in the tilt angle of the side chains and local interdigitation. In situ X-ray scattering and electrical measurements show that the structural transition and interdigitation of these side chains strongly affect its surface morphology as well as the charge transport properties of the resulting P3HT-based organic field-effect transistor. Since most conjugated polymers have side chains, the results of this study provide a deeper understanding of the effects of side chains on the structural and electrical properties of conjugated backbones. These results also provide a new perspective on the formation of a metastable polymorph consisting of interdigitated P3HT.

Multisubstituted Azaisoindigo-Based Polymers for High-Mobility Ambipolar Thin-Film Transistors and Inverters

Ambipolar semiconducting materials have great potential in complementary-like organic logic circuits. Accessing such logic circuits demands balanced hole and electron mobilities. However, the lack of ambipolar high-mobility polymer semiconductors with balanced charge carrier-transporting properties precludes the rapid development of organic logic circuits. In this context, structural modification of semiconductor materials to enhance the electron/hole transport is of great urgency. Herein, a multifunctionalization strategy is used to achieve this goal. Combined electron-withdrawing moieties involving fluorine and pyridinic nitrogen atoms can not only reduce the frontier molecular orbital energies but also planarize the polymer backbone, demonstrating synergetic effects on the control over the carrier injection process at the metal-semiconductor interface and microstructure-sensitive charge transport in the channel. A balanced ambipolar behavior with electron/hole mobilities of 3.88/3.44 cm² V^-1 s^-1 was observed, and complementary-like inverters with high gains of greater than 200 were achieved. Microstructure and thin-film morphology were characterized to further reveal the relationship between device performances and macroscopic observables. This multifunctionalization strategy bodes well for developing new ambipolar semiconducting materials.