Density of States Engineering of n-Doped Conjugated Polymers for High Charge Transport Performances

Side Chain Engineering toward Chemical Doping of Conjugated Polymers

ConspectusSolution-processable conjugated polymers are typically composed of two distinct structural components: rigid conjugated backbones and flexible side chains, each with unique roles and properties. The conjugated backbone forms the core framework of the polymer and is directly responsible for its optoelectronic properties, such as light absorption, emission, and charge transport. Meanwhile, the conjugated backbone can undergo chemical doping, where molecular dopants introduce charge carriers to modulate the carrier density and electrical conductivity. Therefore, the conjugated backbone is the critical determinant of the resulting optoelectronic performance. However, on the other hand, the flexible side chains, originally introduced to improve solution processability, were long considered chemically inert to the doping reaction. Recent advances have shown that the role of side chains is more than just improving solubility, demonstrating the significant impact of side chains on the packing of the conjugated backbone, film morphology, and electronic properties of conjugated polymers. Side chain engineering has become an essential design strategy for creating high-performance conjugated polymers in various applications.In this Account, we aim to emphasize the importance of side chain engineering toward controllable chemical doping of conjugated polymers, where side chain engineering allows us to tune the molecular packing, doping efficiency, and film morphology, thereby enhancing charge transport and optoelectronic performance. Specifically, the length, branching structures, and functional groups of the side chains can be systematically varied to control the solubility, miscibility, and interactions of conjugated polymers with dopants. For example, longer or branched side chains can improve solubility but may disrupt the π-π stacking between the conjugated backbones, thereby reducing the charge transport efficiency of the polymer. Shorter or linear side chains may enhance backbone packing and electronic coupling, though at the expense of reduced solubility. The impact of side chains on the doping process is particularly noteworthy. Although side chains are chemically inert to doping reactions, their design influences all three critical steps of the doping process: mixing, ionization, and carrierization. Side chains affect the spatial distribution of dopants during mixing, modulate the local environment to facilitate charge transfer during ionization, and influence the dissociation of ion pairs into free charge carriers during carrier generation. Functional side chains with polar groups, for example, can enhance dopant-polymer compatibility, while those with functional groups can modulate the dielectric environment to weaken ion pairing and promote free carrier generation. The interplay between side chains and the conjugated backbone is critical to achieving optimal optoelectronic performance in applications such as organic photovoltaics, field-effect transistors, and thermoelectrics. Rational side chain engineering provides a powerful tool to address these challenges in doping, morphology control, and charge transport, bringing more opportunities to design advanced conjugated polymers and chemical dopants tailored to specific applications.

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.

π-Conjugated Polymers for High-Performance Organic Electrochemical Transistors: Molecular Design Strategies, Applications and Perspectives

The last decade has witnessed significant progress in organic electrochemical transistors (OECTs) due to their enormous potential applications in various bioelectronic devices, such as artificial synapses, biological interfaces, and biosensors. The remarkable advance in this field is highly powered by the development of novel organic mixed ionic/electronic conductors (OMIECs). π-Conjugated polymers (CPs), which are widely used in various optoelectronics, are emerging as key channel materials for OECTs. In this review, after briefly introducing OECT, we then mainly focus on the latest progress in CPs for high-performance OECTs. The correlations of their structure, basic physicochemical properties, and device performance are elucidated by evaluating their electronic characters, optoelectronic properties, and OECT performance. Then, the applications of CP-based OECTs are briefly presented. Finally, we discuss several remaining issues or challenges in this field and give our insights into advancing CPs for enhanced OECT performance.

A Facile Electrochemical Strategy for Achieving a High-Conductivity Polypyrrole Derivative with Intrinsic Metallic Transport as a High-Performance Electrochromic Conducting Polymer Film

Conjugated polymer electrochromic materials (PECMs) with tailored optical and electrical properties are applied in smart windows, electronic displays, and adaptive camouflage. The limitation in the electrical conductivity results in slow and monotonous color switching. We present a polypyrrole film incorporated with a toluene-p-sulfonic group (PPy-TSO-F), via a one-step electrodeposition technique. The PPy-TSO-F thin film (110 nm) achieves an impressive electrical conductivity of 1011 S cm-1, a high carrier mobility of 82 cm2 V-1 s-1, and intrinsic metallic electronic behavior. It demonstrates exceptionally reversible multicolor switching, transitioning from emerald green (-1.5 V), to bluish green (-1.4 V), bright yellow (-1.2 V), greenish yellow (-0.6 V), reddish brown (0.1 V), dark brown (0.3 V), and atrovirens (0.6 V). The fast charge transport and high carrier mobility render the film with an ultrafast electrochromic switching speed of 0.01 s/0.02 s. This research provides a new route to designing ultrafast multicolor switching PECMs with metallic charge transport.

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.

Tailoring the Density of State of n-Type Conjugated Polymers through Solvent Engineering for Organic Electrochemical Transistors

Conjugated polymers with ethylene glycol-type side chains are commonly used as channel materials in organic electrochemical transistors (OECTs). To improve the performance of these materials, new chemical structures are often created through synthetic routines. Herein, we demonstrate that the OECT performance of these polymers can also be improved by changing their density-of-state (DOS) profile through solvent engineering. Depending on the solvent polarity, it solvates the backbone and side chains of the conjugated polymer differently, leading to differences in molecule orientation, π-stacking paracrystallinity, and film defects, such as grain boundaries and pinholes. This then results in a change in the DOS profile of the polymer. A more intense and narrow-width DOS distribution is usually observed in organic films with an "edge on" orientation and fewer film defects, while films with a "face on" orientation and apparent defects show a broadened DOS profile. The OECT devices that use the polymer film with a more intense and narrow-width DOS profile exhibit a better-normalized transconductance and figure-of-merit μC* than those with a broadened DOS profile (0.74 to 4.29 S cm-1 and 3.5 to 14.3 F cm-1 V-1 s-1). This study provides useful insights into how the DOS profile affects the mixed ionic-electronic conduction performance and presents a new avenue for improving n-type OECT materials.

Band-like transport in solution-processed perylene diimide dianion films with high Hall mobility

It is crucial to prepare high-mobility organic polycrystalline film through solution processing. However, the delocalized carrier transport of polycrystalline films in organic semiconductors has rarely been investigated through Hall-effect measurement. This study presents a strategy for building strong intermolecular interactions to fabricate solution-crystallized p-type perylene diimide (PDI) dianion films with a closer intermolecular π-π stacking distance of 3.25 Å. The highly delocalized carriers enable a competitive Hall mobility of 3 cm2 V-1 s-1, comparable to that of the reported high-mobility organic single crystals. The PDI dianion films exhibit a high electrical conductivity of 17 S cm-1 and typical band-like transport, as evidenced by the negative temperature linear coefficient of mobility proportional to T-3/2. This work demonstrates that, as the intermolecular π-π interactions become strong enough, they will display high mobility and conductivity, providing a new approach to developing high-mobility organic semiconductor materials.

A Bifunctional Luminescent Whitening and Sensing Material Based on Photoluminescence and Mechanoluminescence

A bifunctional luminescent whitening and luminescent sensing composite material, BaMgAl12O17:Eu2+/polydimethylsiloxane (BAM/PDMS), that utilizes natural sunlight and mechanical energy is presented. By increasing the Eu2+ content, the photoluminescence (PL) excitation spectrum of the material shows a maximum redshift of 23 nm due to 5d level splitting of Eu2+, resulting in more spectral overlap with sunlight and an excellent PL whitening effect. Meanwhile, the self-recoverable mechanoluminescence (ML) of the material can be easily excited under mechanical stimuli due to contact electrification, exhibiting a unique stress sensing effect. Based on the unique features of PL whitening and ML sensing, the material is applied to model cars through a spray process, and the results demonstrate that the bifunctional BAM/PDMS material shows promising applications in automobile decoration.

Ground-state electron transfer in all-polymer donor:acceptor blends enables aqueous processing of water-insoluble conjugated polymers

Water-based conductive inks are vital for the sustainable manufacturing and widespread adoption of organic electronic devices. Traditional methods to produce waterborne conductive polymers involve modifying their backbone with hydrophilic side chains or using surfactants to form and stabilize aqueous nanoparticle dispersions. However, these chemical approaches are not always feasible and can lead to poor material/device performance. Here, we demonstrate that ground-state electron transfer (GSET) between donor and acceptor polymers allows the processing of water-insoluble polymers from water. This approach enables macromolecular charge-transfer salts with 10,000× higher electrical conductivities than pristine polymers, low work function, and excellent thermal/solvent stability. These waterborne conductive films have technological implications for realizing high-performance organic solar cells, with efficiency and stability superior to conventional metal oxide electron transport layers, and organic electrochemical neurons with biorealistic firing frequency. Our findings demonstrate that GSET offers a promising avenue to develop water-based conductive inks for various applications in organic electronics.

The iron group transition-metal (Fe, Ru, Os) coordination of Se-doped graphitic carbon (Se@g-C3N4) nanostructures for the smart therapeutic delivery of zidovudine (ZVD) as an antiretroviral drug: a theoretical calculation perspective

This study employed density functional theory (DFT) computational techniques at the ωB97XD/def2svp level of theory to comprehensively explore the electronic behavior of Fe-group transition metal (Fe, Ru, Os) coordination of Se-doped graphitic carbon (Se@g-C3N4) nanosystems in the smart delivery of zidovudine (ZVD), an antiretroviral drug. The HOMO-LUMO results of the interactions show a general reduction in energy gap values across all complexes in the following order: ZVD_Se@C3N4 < ZVD_Ru_Se@C3N4 < ZVD_Fe_Se@C3N4 < ZVD_Os_Se@C3N4. ZVD_Se@C3N4 exhibits the smallest post-interaction band gap of 3.783 eV, while ZVD_Os_Se@C3N4 presents the highest energy band gap of 5.438 eV. Results from the corrected adsorption energy (BSSE) revealed that Os_Se@C3N4 and Ru_Se@C3N4 demonstrated more negative adsorption energies of -2.67 and -2.701 eV, respectively, pointing to a more favorable interaction between ZVD and these systems, thus potentially enhancing the drug delivery efficiency. The investigation into the drug release mechanism from the adsorbents involved a comprehensive examination of the dipole moment and the influence of pH, shedding light on the controlled release of ZVD. Additionally, investigating the energy decomposition analysis (EDA) revealed that ZVD_Ru_Se@C3N4 and ZVD_Fe_Se@C3N4 exhibited the same total energy of -787.7 kJ mol-1. This intriguing similarity in their total energy levels suggested that their stability was governed by factors beyond reactivity, possibly due to intricate orbital interactions. Furthermore, analyzing the bond dissociation energies showed that all systems exhibited negative enthalpy values, indicating that these systems were exothermic at both surface and interaction levels, thus suggesting that these processes emitted heat, contributing to the surrounding thermal energy.