Thienoisoindigo-Based Conjugated Polymers Synthesized by Direct Arylation Polycondensation

Thiazole-Fused Naphthalene Diimide-Based n-Type Conjugated Polymer: Synthesis, Properties, and Applications in Organic Electronics

Significant progress has been achieved in the development of p-type polymers with high hole mobilities. However, the advancement of n-type polymers has been comparatively slower, and the lack of high-performance electron-deficient materials continues to hinder the progress and practical application of various electronic devices. In this study, a new n-type conjugated polymer, PNDTzI, is synthesized based on a thiazole-fused naphthalene diimide unit. The incorporation of two thiazole rings at the β-position of the naphthalene ring effectively reduced steric hindrance between the naphthalene and thiophene units, resulting in an optimized backbone conformation. Density functional theory (DFT) calculations and cyclic voltammetry measurements show that PNDTzI exhibits a highly planar structure and a relatively deep lowest unoccupied molecular orbital (LUMO) energy level of -3.95 eV, facilitating efficient electron injection and transport. Field-effect transistors (FETs) using PNDTzI as the semiconducting layer exhibited the typical n-type transport characteristics with an electron mobility of up to 0.12 cm2V-1s-1. Additionally, the thermoelectric performance of the n-doped PNDTzI films is evaluated, and the doped thin film shows a Seebeck coefficient of -354 µV K-1, and a power factor (PF) of 0.04 µWm-1K-2.

Recent developments in polymer semiconductors with excellent electron transport performances

Benefiting from molecular design and device innovation, electronic devices based on polymer semiconductors have achieved significant developments and gradual commercialization over the past few decades. Most of high-performance polymer semiconductors that have been prepared exhibit p-type performances, and records of their carrier mobilities are constantly being broken through. Although ambipolar and n-type polymers are necessary for constructing p-n heterojunctions and logic circuits, only a few materials show outstanding device performances, which leads to their developments lagging far behind that of p-type analogues. As a consequence, it is extremely significant to summarize polymer semiconductors with excellent electron transport performances. This review focuses on the design considerations and bonding modes between monomers of polymer semiconductors with high electron mobilities. To enhance electron transport performances of polymer semiconductors, the structural modification strategies are described in detail. Subsequently, the electron transport, thermoelectric, mixed ionic-electronic conduction, intrinsically stretchable, photodetection, and spin transport performances of high-electron mobility polymers are discussed from the perspective of molecular engineering. In the end, the challenges and prospects in this research field are presented, which provide valuable guidance for the design of polymer semiconductors with excellent electron transport performances and the exploration of more advanced applications in the future.

Anisole Processible n-Type Conjugated Polymers Synthesized via C─H/C─H Oxidative Direct Arylation Polycondensation for Organic Electrochemical Transistors

The development of n-type polymers for organic electrochemical transistors (OECTs) has lagged significantly behind their p-type counterparts. Moreover, these polymers are often synthesized via Stille polycondensation. Herein, three polymers with thiazole-flanked diketopyrrolopyrrole is synthesized as the monomer through C─H/C─H oxidative direct arylation polycondensation (Oxi-DArP). This protocol employs unfunctionalized (C─H terminated) monomers, and the generated byproducts are environmentally benign. The electron-deficient polymer backbone confers these polymers with LUMO energy levels below -4.20 eV, enabling all of them to exhibit n-type behavior in OECTs. Additionally, the resulting polymers are soluble in the green solvent anisole. With an optimized alkyl spacer between oligo(ethylene glycol) side chain and conjugated backbone, the polymer gTzDPP-C8 showed the best device performance with geometry normalized transconductance (gm,norm) up to 6.31 S cm-1 and µC* up to 23.1 F V-1 cm-1 s-1. This work has successfully proved that C─H/C─H Oxi-DArP is a promising method for synthesizing n-type OMIECs to fulfill high-performance OECTs.

Conjugated Polymers from Direct Arylation Polycondensation of 3,4-Difluorothiophene-Substituted Aryls: Synthesis and Properties

3,4-Difluorothiophene-substituted aryls, i.e., 1,4-bis(3,4-difluorothiophen-2-yl)-benzene (Ph-2FTh), 1,4-bis(3,4-difluorothiophen-2-yl)-2,5-difluorobenzene (2FPh-2FTh), and 4,7-bis(3,4-difluorothiophen-2-yl)-2,1,3-benzothiadiazole (BTz-2FTh), are synthesized as C─H monomers for the synthesis of conjugated polymers (CPs) via direct arylation polycondensation (DArP) with diketopyrrolopyrrole (DPP) and isoindigo (IID) derivatives as C─Br monomers. The Gibbs free energies of activation for direct arylation (ΔG298 K , kcal mol-1 ) for α─C─H bonds of thiophene moieties as calculated by density functional theory (DFT) are 14.3, 16.5, and 16.4 kcal mol-1 for Ph-2FTh, 2FPh-2FTh and BTz-2FTh, respectively, meaning that inserting an electron-deficient unit in 3,3',4,4'-tetrafluoro-2,2'-bithiophene (4FBT, ΔG298K : 14.6 kcal mol-1 ) may cause a reactivity decrease of the C─H monomers. Photophysical and semiconducting properties of the resulting six CPs (i.e., DPP-Ph, DPP-2FPh, DPP-BTz, 2FIID-Ph, 2FIID-2FPh, and 2FIID-BTz) are characterized in detail. DPP-based CPs show ambipolar transport properties while IID-based ones exhibited n-type behavior owing to the deeper frontier molecular orbital energy levels of IID-based CPs. With source/drain electrodes modified with polyethylenimine ethoxylated, n-channel organic thin-film transistors with maximum electron mobility of 0.40, 0.54, 0.29, 0.05, 0.16, and 0.01 cm2 V-1 s-1 for DPP-Ph, DPP-2FPh, DPP-BTz, 2FIID-Ph, 2FIID-2FPh, and 2FIID-BTz, respectively, are fabricated. DPP-2FPh exhibits the best device performance due to the good film morphology and the highest intermolecular packing order.