Recent structural evolution of lactam- and imide-functionalized polymers applied in organic field-effect transistors and organic solar cells

Rational Molecular Design of Diketopyrrolopyrrole-Based n-Type and Ambipolar Polymer Semiconductors

Diketopyrrolopyrrole (DPP)-based polymer semiconductors have drawn great attention in the field of organic electronics due to the planar structure, decent solubilizing capability, and high crystallinity. However, the electron-deficient capacity of DPP derivatives are not strong enough, leading to relatively high-lying lowest unoccupied molecular orbital (LUMO) energy levels of the corresponding polymers. As a result, n-type and ambipolar DPP-based polymers are rare and their electron mobilities also lag far behind the p-type counterparts, which limits the development of important p-n-junction-based electronic devices. Therefore, new design strategies have been proposed recent years to develop n-type/ambipolar DPP-based polymers with improved performances. In this view, these molecular design strategies are summarized, including copolymerization of DPP with different acceptors and weak donors, DPP flanked aromatic ring modification, DPP-core ring expansion and DPP dimerization. The relationship between the chemical structures and organic thin-film transistor performances is intensively discussed. Finally, a perspective on future trends in the molecular design of DPP-based n-type/ambipolar polymers is also proposed.

Recent Progress of Fluorinated Conjugated Polymers

In recent years, conjugated polymers have received widespread attention due to their characteristic advantages of light weight, favorable solution processability, and structural modifiability. Among various conjugated polymers, fluorinated ones have developed rapidly to achieve high-performance n-type or ambipolar polymeric semiconductors. The uniqueness of fluorinated conjugated polymers contains the high coplanarity of their structures, lower frontier molecular orbital energy levels, and strong nonbonding interactions. In this review, first the fluorinated building blocks, including fluorinated benzene and thiophene rings, fluorinated B←N bridged units, and fluoroalkyl side chains are summarized. Subsequently, different synthetic methods of fluorinated conjugated polymers are described, with a special focus on their respective advantages and disadvantages. Then, with these numerous fluorinated structures and appropriate synthetic methods bear in mind, the properties and applications of the fluorinated conjugated polymers, such as cyclopentadithiophene-, amide-, and imide-based polymers, and B←N embedded polymers, are systematically discussed. The introduction of fluorine atoms can further enhance the electron-deficiency of the backbone, influencing the charge carrier transport performance. The promising fluorinated conjugated polymers are applied widely in organic field-effect transistors, organic solar cells, organic thermoelectric devices, and other organic opto-electric devices. Finally, the outlook on the challenges and future development of fluorinated conjugated polymers is systematically discussed.

Functionalized Phenanthrene Imide-Based Polymers for n-Type Organic Thin-Film Transistors

High-performing n-type polymers are crucial for the advance of organic electronics field, however strong electron-deficient building blocks with optimized physicochemical properties for constructing them are still limited. The imide-functionalized polycyclic aromatic hydrocarbons (PAHs) with extended π-conjugated framework, high electron deficiency and good solubility serve as promising candidates for developing high-performance n-type polymers. Among the PAHs, phenanthrene (PhA) features a well-delocalized aromatic π-system with multiple modifiable active sites . However, the PhA-based imides are seldom studied, mainly attributed to the synthetic challenge. Herein, we report two functionalized PhAs, CPOI and CPCNI, by simultaneously incorporating imide with carbonyl or dicyanomethylene onto PhA. Notably, the dicyanomethylene-modified CPCNI exhibits a well stabilized LUMO energy level (-3.84 eV), attributed to the synergetic inductive effect from imide and cyano groups. Subsequently, based on CPOI and CPCNI, two polymers PCPOI-Tz and PCPCNI-Tz were developed. Applied to organic thin-film transistors, owing to the strong electron-deficiency of CPCNI, polymer PCPCNI-Tz shows an improved electron mobility and largely decreased threshold voltage compared with PCPOI-Tz. This work affords two structurally novel electron-deficient building blocks and highlights the effectiveness of dual functionalization of PhAs with strong electron-withdrawing groups for devising n-type polymers.

Organic Donor-Acceptor Systems for Photocatalysis

Organic semiconductor materials are considered to be promising photocatalysts due to their excellent light absorption by chromophores, easy molecular structure tuning, and solution-processable properties. In particular, donor-acceptor (D-A) type organic photocatalytic materials synthesized by introducing D and A units intra- or intermolecularly, have made great progress in photocatalytic studies. More and more studies have demonstrated that the D-A type organic photocatalytic materials combine effective carrier separation, tunable bandgap, and sensitive optoelectronic response, and are considered to be an effective strategy for enhancing light absorption, improving exciton dissociation, and optimizing carrier transport. This review provides a thorough overview of D-A strategies aimed at optimizing the photocatalytic performance of organic semiconductors. Initially, essential methods for modifying organic photocatalytic materials, such as interface engineering, crystal engineering, and interaction modulation, are briefly discussed. Subsequently, the review delves into various organic photocatalytic materials based on intramolecular and intermolecular D-A interactions, encompassing small molecules, conjugated polymers, crystalline polymers, supramolecules, and organic heterojunctions. Meanwhile, the energy band structures, exciton dynamics, and redox-active sites of D-A type organic photocatalytic materials under different bonding modes are discussed. Finally, the review highlights the advanced applications of organic photocatalystsand outlines prospective challenges and opportunities.

Real-time monitoring polymerization degree of organic photovoltaic materials toward no batch-to-batch variations in device performance

Polymerization degree plays a vital role in material properties. Previous methodologies of molecular weight control generally cannot suppress or alleviate batch-to-batch variations in device performance, especially in polymer solar cells. Herein, we develop an in-situ photoluminescence system in tandem with a set of analysis and processing procedures to track and estimate the polymerization degree of organic photovoltaic materials. To support the development of this protocol, we introduce polymer acceptor PYT constructed by near-infrared Y-series small molecule acceptors via Stille polymerization, and shed light on the correlations between molecular weight, spectral parameters, and device efficiencies that enable the design of the optical setup and confirm its feasibility. The universality is verified in PYT derivatives with stereoregularity and fluoro-substitution as well as benzo[1,2-b:4,5-b']dithiophene-based polymers. Overall, our result provides a tool to tailor suitable conjugated oligomers applied to polymer solar cells and other organic electronics for industrial scalability and desired cost reduction.

Design of Novel Functional Conductive Structures and Preparation of High-Hole-Mobility Polymer Transistors by Green Synthesis Using Acceptor-Donor-Acceptor Strategies

The design of novel acceptor molecular structures based on classical building blocks is regarded as one of the efficient ways to explore the application of organic conjugated materials in conductivity and electronics. Here, a novel acceptor moiety, thiophene-vinyl-diketopyrrolopyrrole (TVDPP), was envisioned and prepared with a longer conjugation length and a more rigid structure than thiophene-diketopyrrolopyrrole (TDPP). The brominated TVDPP can be sequentially bonded to trimethyltin-containing benzo[c][1,2,5]thiadiazole units via Suzuki polycondensation to efficiently prepare the polymer PTVDPP-BSz, which features high molecular weight and excellent thermal stability. The polymerization process takes only 24 h and eliminates the need for chlorinated organic solvents or toxic tin-based reagents. Density functional theory (DFT) simulations and film morphology analyses verify the planarity and high crystallinity of the material, respectively, which facilitates the achievement of high carrier mobility. Conductivity measurements of the polymeric material in the organic transistor device show a hole mobility of 0.34 cm2 V-1 s-1, which illustrates its potential for functionalized semiconductor applications.

Research Progress in Donor-Acceptor Type Covalent Organic Frameworks

Covalent organic frameworks (COFs) are new organic porous materials constructed by covalent bonds, with the advantages of pre-designable topology, adjustable pore size, and abundant active sites. Many research studies have shown that COFs exhibit great potential in gas adsorption, molecular separation, catalysis, drug delivery, energy storage, etc. However, the electrons and holes of intrinsic COF are prone to compounding in transport, and the carrier lifetime is short. The donor-acceptor (D-A) type COFs, which are synthesized by introducing D and A units into the COFs backbone, combine separated electron and hole migration pathway, tunable band gap and optoelectronic properties of D-A type polymers with the unique advantages of COFs and have made great progress in related research in recent years. Here, the synthetic strategies of D-A type COFs are first outlined, including the rational design of linkages and D-A units as well as functionalization approaches. Then the applications of D-A type COFs in catalytic reactions, photothermal therapy, and electronic materials are systematically summarized. In the final section, the current challenges, and new directions for the development of D-A type COFs are presented.

Synthesis and Derivatization of an Isomerized Bithiophene Imide (iBTI) Acceptor with a Controllably Twisted Backbone

A heptagonal isomerized bithiophene imide (iBTI) acceptor has been effectively synthesized on a gram scale. Its series of β-, α',β-, α,α'-, α,α',β-, and α,α',β,β'-substituted derivatives can be obtained by controlling brominated sites. Single-crystal analyses indicate that the torsion angle of the imide backbone depends on the number and rigidity of β-substituted groups. Furthermore, the helical chirality of tetrasubstituted and [7]helicene-like derivatives based on iBTI shows great promise for the construction of chiral semiconductor materials.

Solution-Processed Isoindigo- and Thienoisoindigo-Based Donor-Acceptor-Donor π-Conjugated Small Molecules: Synthesis, Morphology, Molecular Packing, and Field-Effect Transistor Characterization

Molecular design and precise control of thin-film morphology and crystallinity of solution-processed small molecules are important for enhancing charge transport mobility of organic field-effect transistors and gaining more insight into the structure-property relationship. Here, two donor-acceptor-donor (D-A-D) architecture small molecules TRA-IID-TRA and TRA-TIID-TRA comprising an electron-donating triarylamine (TRA) and two different electron-withdrawing cores, isoindigo (IID) and thienoisoindigo (TIID), respectively, were synthesized and characterized. Replacing the phenylene rings of central IID A with thiophene gives a TIID core, which reduces the optical band gap and upshifts the energy levels of frontier molecular orbitals. The single-crystal structures and grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that TRA-TIID-TRA exhibits the relatively tighter π-π stacking packing with preferential edge-on orientation, larger coherence length, and higher crystallinity due to the noncovalent S···O/S···π intermolecular interactions. The distinctly oriented and connected ribbon-like TRA-TIID-TRA crystalline film by the solution-shearing process achieved a superior hole mobility of 0.89 cm² V^-1 s^-1 in the organic field-effect transistor (OFET) device, which is at least five times higher than that (0.17 cm² V^-1 s^-1) of TRA-IID-TRA with clear cracks. Eventually, rational modulation of fused core in the π-conjugated D-A-D small molecule provides a new understanding of structural design for enhancing the performance of solution-processed organic semiconductors.

Precision Synthesis of Conjugated Polymers Using the Kumada Methodology

Since the discovery of conductive poly(acetylene), the study of conjugated polymers has remained an active and interdisciplinary frontier between polymer chemistry, polymer physics, computation, and device engineering. One of the ultimate goals of polymer science is to reliably synthesize structures, similar to small molecule synthesis. Kumada catalyst-transfer polymerization (KCTP) is a powerful tool for synthesizing conjugated polymers with predictable molecular weights, narrow dispersities, specific end groups, and complex backbone architectures. However, expanding the monomer scope beyond the well-studied 3-alkylthiophenes to include electron-deficient and complex heterocycles has been difficult. Revisiting the successful applications of KCTP can help us gain new insight into the CTP mechanisms and thus inspire breakthroughs in the controlled polymerization of challenging π-conjugated monomers.In this Account, we highlight our efforts over the past decade to achieve controlled synthesis of homopolymers (p-type and n-type), copolymers (diblock and statistical), and monodisperse high oligomers. We first give a brief introduction of the mechanism and state-of-the-art of KCTP. Since the extent of polymerization control is determined by steric and electronic effects of both the catalyst and monomer, the polymerization can be optimized by modifying monomer and catalyst structures, as well as finding a well-matched monomer-catalyst system. We discuss the effects of side-chain steric hindrance and halogens in the context of heavy atom substituted monomers. By moving the side-chain branch point one carbon atom away from the heterocycle to alleviate steric crowding and stabilize the catalyst resting state, we were able to successfully control the polymerization of new tellurophene monomers. Inspired by innocent role of the sterically encumbered 2-transmetalated 3-alkylthiophene monomer, we introduce the treatment of hygroscopic monomers with a bulky Grignard compound as a water-scavenger for the improved synthesis of water-soluble conjugated polymers. For challenging electron-deficient monomers, we discuss the design of new Ni(II)diimine catalysts with electron-donating character which enhance the stability of the association complex between the catalyst and the growing polymer chain, resulting in the quasi-living synthesis of n-type polymers. Beyond n-type homopolymers, the Ni(II)diimine catalysts are also capable of producing electron-rich and electron-deficient diblock and statistical copolymers. We discuss how density functional theory (DFT) calculations elucidate the role of catalyst steric and electronic effects in controlling the synthesis of π-conjugated polymers. Moreover, we demonstrate the synthesis of monodisperse high oligomers by temperature cycling, which takes full advantage of the unique character of KCTP in that it proceeds through distinct intermediates that are not reactive. The insight we gained thus far leads to the first example of isolated living conjugated polymer chains prepared by a standard KCTP procedure, with general applicability to different monomers and catalytic systems. In summarizing a decade of innovation in KCTP, we hope this Account will inspire future development in the field to overcome key challenges including the controlled synthesis of electron-deficient heterocycles, complex and high-performance systems, and degradable and recyclable materials as well as cutting-edge catalyst design.