Pathways For Charge Carriers Research Articles

Tailoring Sodium Carboxymethylcellulose Binders for High‐Voltage LiCoO2 via Thermal Pulse Sintering

Polyvinylidene fluoride (PVDF), as the commercial cathode binder for lithium‐ion batteries, presents several practical challenges, including insufficient conductivity, weak adhesion to active materials, and the use of toxic N‐methylpyrrolidone for slurry preparation. However, while most water‐soluble binders can address the aforementioned issues, they fail to meet the requirements of high‐voltage cathodes. In this work, we innovatively employed a thermal pulse sintering strategy to modify carboxymethyl cellulose sodium (CMC), enabling their application in 4.6 V LiCoO2 (93% capacity retention after 200 cycles). This strategy facilitates the decomposition of electrochemically active carboxyl groups, leading to ring opening reactions that generate numerous ether linkages (‐C‐O‐C‐) without introducing undesirable side effects on LiCoO2. The resulting components form additional charge carrier (i.e., Li+ and e‐) pathways on the cathode surface. Additionally, the heating process also promotes uniform coating of the binder on the surface of LiCoO2, creating a protective layer that inhibits interfacial side reactions. Through proposing a scalable and economic manufacturing technology of multifunctional binder, this work enlightens the avenues for practical high‐energy‐density batteries.

Tailoring Sodium Carboxymethylcellulose Binders for High-Voltage LiCoO2 via Thermal Pulse Sintering.

Polyvinylidene fluoride (PVDF), as the commercial cathode binder for lithium-ion batteries, presents several practical challenges, including insufficient conductivity, weak adhesion to active materials, and the use of toxic N-methylpyrrolidone for slurry preparation. However, while most water-soluble binders can address the aforementioned issues, they fail to meet the requirements of high-voltage cathodes. In this work, we innovatively employed a thermal pulse sintering strategy to modify carboxymethyl cellulose sodium (CMC), enabling their application in 4.6 V LiCoO2 (93% capacity retention after 200 cycles). This strategy facilitates the decomposition of electrochemically active carboxyl groups, leading to ring opening reactions that generate numerous ether linkages (-C-O-C-) without introducing undesirable side effects on LiCoO2. The resulting components form additional charge carrier (i.e., Li+ and e-) pathways on the cathode surface. Additionally, the heating process also promotes uniform coating of the binder on the surface of LiCoO2, creating a protective layer that inhibits interfacial side reactions. Through proposing a scalable and economic manufacturing technology of multifunctional binder, this work enlightens the avenues for practical high-energy-density batteries.

Long-Lived Charge Carrier Photogeneration in a Cooperative Supramolecular Double-Cable Polymer.

A newly designed C3-symmetric disc-shaped chromophore, BTT(NDI)3, features electron accepting naphthalene diimides linked to an electron donor BTT core. BTT(NDI)3 self-assembles in apolar solvents into highly ordered, chiral supramolecular fibers through π-π and 3-fold hydrogen-bonding interactions. This leads to a cooperative formation of plane-to-plane stacking of BTTs and J-aggregation of the outer NDIs. Such a structure ensures high charge mobility. Only photoexcitation of BTT in the BTT(NDI)3 polymers triggers a unidirectional electron transfer from BTT to NDI and results in (BTT•+-NDI•-) lifetimes that are by up to 3 orders of magnitude longer compared to (NDI•+-NDI•-) that is formed upon NDI photoexcitation. A multiphasic decay implies ambipolar pathways for charge carriers, that is, electron and hole delocalization along the respective BTT and NDI stacks. Our supramolecular approach offers potential for developing functional supramolecular polymers with continuous pathways for electrons and holes and, in turn, minimizing charge recombination losses in organic photovoltaic devices.

Bio-Inspired Photosynthesis Platform for Enhanced NADH Conversion and L-Glutamate Synthesis.

Inspired by the layered structure, light absorption, and charge carrier pathway of chloroplast thylakoids in natural photosynthesis, we propose a novel artificial photosynthesis platform, which is composed of layered structured vaterite as the scaffold with gold nanoparticles (AuNPs), photosensitizer eosin Y (EY), and redox enzyme L-glutamate dehydrogenase (GDH) as the functional components. The EY exhibited significantly enhanced light absorption and charge carrier generation due to the localized surface plasmon resonance (LSPR) around the AuNPs and light refraction within the layers. This artificial photosynthesis platform can regenerate reduced nicotinamide adenine dinucleotide (NADH) under visible light and promote the rapid conversion of α-ketoglutarate to L-glutamate (0.453 Mm/h). The excellent biocompatibility of layered vaterite significantly enhances the resistance of GDH to harsh conditions, including high pH (pH = 10) and elevated temperatures (37-57 °C).

Achieving High Efficiency and Stability in Organic Photovoltaics with a Nanometer-Scale Twin p-i-n Structured Active Layer.

In pursuing high stability and power conversion efficiency for organic photovoltaics (OPVs), a sequential deposition (SD) approach to fabricate active layers with p-i-n structures (where p, i, and n represent the electron donor, mixed donor:acceptor, and electron acceptor regions, respectively, distinctively different from the bulk heterojunction (BHJ) structure) has emerged. Here, we present a novel approach that by incorporating two polymer donors, PBDBT-DTBT and PTQ-2F, and one small-molecule acceptor, BTP-3-EH-4Cl, into the active layer with sequential deposition, we formed a device with nanometer-scale twin p-i-n structured active layer. The twin p-i-n PBDBT-DTBT:PTQ-2F/BTP-3-EH-4Cl device involved first depositing a PBDBT-DTBT:PTQ-2F blend under layer and then a BTP-3-EH-4Cl top layer and exhibited an improved power conversion efficiency (PCE) value of 18.6%, as compared to the 16.4% for the control BHJ PBDBT-DTBT:PTQ-2F:BTP-3-EH-4Cl device or 16.6% for the single p-i-n PBDBT-DTBT/BTP-3-EH-4Cl device. The PCE enhancement resulted mainly from the twin p-i-n active layer's multiple nanoscale charge carrier pathways that contributed to an improved fill factor and faster photocurrent generation based on transient absorption studies. The PBDBT-DTBT:PTQ-2F/BTP-3-EH-4Cl film possessed a vertical twin p-i-n morphology that was revealed through secondary ion mass spectrometry and synchrotron grazing-incidence small-angle X-ray scattering analyses. The thermal stability (T80) at 85 °C of the twin p-i-n PBDBT-DTBT:PTQ-2F/BTP-3-EH-4Cl device surpassed that of the single p-i-n PBDBT-DTBT/BTP-3-EH-4Cl devices (906 vs 196 h). This approach of providing a twin p-i-n structure in the active layer can lead to substantial enhancements in both the PCE and stability of organic photovoltaics, laying a solid foundation for future commercialization of the organic photovoltaics technology.

Significant Enhancement in Dielectric Properties of Polyimide Alloys Through a Two‐Phase Interlocking Structure

AbstractHigh temperature dielectric polymers are the favored materials for energy storage devices under harsh‐environment, e.g., electronic devices and power systems. It is widely acknowledged that the energy storage capabilities of dielectric polymers are markedly deteriorated at elevated temperature because of the exponential increased leakage current. Herein, all‐organic dielectric polymer alloys with two‐phase continuous hard‐soft structure have been firstly investigated via blending high glass transition temperature (Tg) fluorinated polyimide (FPI) and high bandgap aliphatic polyimide (API). The large energy band difference between FPI and API is conducive to trap energy and greatly inhibits conduction loss. In addition, the hard and soft molecular chains with two‐phase interlocking structures are more closely arranged, bringing torturous pathways for charge carriers and reducing free volume, which enhances the breakdown strength. FPI/API alloy with high Tg (296 °C) and concurrent large bandgap delivers an ultrahigh discharge energy density of 6.6 J cm−3 at 150 °C and 3.02 J cm−3 at 200 °C with 90% discharge efficiency, significantly surpassing those reported dielectrics. Moreover, the FPI/API alloy exhibits remarkable cyclability and dielectric stability up to 10000 charge‐discharge cycles even at elevated temperatures. This work provides an unprecedented opportunity on structure design of dielectric polymers to achieve high‐temperature energy storage capacitors.

The Fabrication and Characterization of Silicon Surface Grooving Using the CV Etching Technique for Front Deep Metallic Contact Solar Cells

This study experimentally investigated the use of the chemical vapor etching method for silicon surface grooving for regular front deep metallic contact solar cell applications. The thickness of silicon wafers is a crucial parameter in the production of solar cells with front and back buried contacts, because silicon surface grooves result in a larger contact area, which in turn improves carrier collection and increases the collection probability for minority carriers. A simple, low-cost HNO3/HF chemical vapor etching technique was used to create grooves on silicon wafers with the help of a highly effective anti-acid mask. The thick porous layer of powder that was produced was easily dissolved in water, leaving patterned grooved areas on the silicon substrate. A linear dependence was observed between the etched thickness and time, suggesting that the etching process followed a constant etch rate, something that is crucial for ensuring precise and reproducible etching results for the semiconductor and microfabrication industries. Moreover, by creating shorter pathways for charge carriers to travel to their respective contacts, front deep contacts minimize the overall distance they need to traverse and therefore reduce the chance of carrier recombination within the silicon material. As a result, the internal quantum efficiency of solar cells with front deep metallic contacts improved by 35% compared to mc-Si solar cells having planar contacts. The use of front deep contacts therefore represents a forward-looking strategy for improving the performance of silicon solar cells. Indeed, this innovative electrode configuration improves charge carrier collection, mitigates recombination losses, and ultimately leads to more efficient and effective solar energy conversion, which contributes to sustainable energy development in the areas of clean energy resources. Further work needs to be undertaken to develop energy sustainably and consider other clean energy resources.

Insights into Thermal Transport through Molecular π-Stacking.

π-Stacking, which is a ubiquitous structural motif in assemblies of aromatic compounds, is well-known to provide a transport pathway for charge carriers and excitons, while its contribution to thermal transport is still unclear. Herein, based on detailed experimental observations of the thermal diffusivity, thermal conductivity, and specific heat of a single-crystalline triphenylene featuring a one-dimensionally π-stacked structure, we describe the nature of thermal transport through the π-stacked columns. We reveal that acoustic phonons are responsible for thermal transport through the π-stacked columns, which exhibit crystal-like behavior. Importantly, the thermal energy stored as intramolecular vibrations can also be transported by coupling to the acoustic phonons. In contrast, in the direction perpendicular to the π-stacked columns, an amorphous-like thermal transport behavior dominates. The present finding offers deep insight into nanoscale thermal transport in organic materials, where the constituent molecules exist as discrete entities linked together by weak intermolecular interactions.

Numerical Simulation of Mixed Conducting Membranes for CO2 Capturing Using Synthetic Microstructures

A side effect of the industrial revolution is the significant increase in atmospheric concentration of CO2 and subsequent greenhouse effects, which necessitated the need for developing technologies for extracting CO2 from high temperature flue gases [1]. Using dense membranes to separate and transport CO2 is promising because of their high selectivity and ability to operate under high temperatures compared to other methods[2]. CO2 permeation flux through dense membranes can be improved significantly by using multiphase composite materials, one promising example of which is Molten Carbonate/Solid Oxide membranes [2, 3]. The topological characteristics of the composite material’s microstructure has a significant impact on the overall effective conductivity of the membrane as the volume fraction and tortuosity of each phase affect the charge carrier pathways. It is difficult to quantify the impact of different features of the microstructure using homogeneous simulations or experimental methods, which necessitates the use of heterogeneous simulations to provide a better understanding on how the different microstructural features affect the overall macroscopic performance [4, 5, 6].In this work, the effect of microstructure on the performance of Molten carbonate/Solid Oxide mixed oxide-ionic and carbonate-ionic conductor (MOCC) will be studied. Unlike previous work presented in the literature, which was limited to one microstructure obtained experimentally by focused ion beam and scanning electron microscope [7, 8], a range of microstructures will be generated synthetically using Dream3D software. The generated microstructures will have different particle sizes and volume fractions for each phase. In addition, the variations that can occur within the same volume fraction ratio will be also considered. COMSOL will be used to solve the Nernst-Plank equations using Finite Element Method for each phase. The detailed simulation results 1) will be compared to the conventional homogeneous simulations; 2) will be validated by experimental CO2 capturing flux; 3) will be used to assess the effect of the microstructure parameters on the performance of the membrane when paired with a Methane oxidative coupling reactor as shown in Fig. 1.

Superior Carrier Mobility Enabled by the Charge Channel Leads to Enhanced Thermoelectric Performance in BiCuSeO Composites.

BiCuSeO oxyselenides possess a highlighted thermoelectric performance among oxides, which originates from their intrinsically low thermal conductivity. However, intrinsic factors causing low thermal transport are also detrimental to carrier transport, leading to ultralow carrier mobility and relatively low electrical transport properties. Here, high-conductivity single-wall carbon nanotubes (SWCNTs) are adopted as the charge channels to be embedded in a BiCuSeO-based matrix, providing a transport pathway for charge carriers. The results show that carrier mobility is increased to 188 cm2 V-1 s-1 due to the SWCNTs composited, triggering an enhancement in electrical transport properties. Besides, the SWCNTs embedded in the matrix introduce abundant interfaces, suppressing phonon transport and depressing lattice thermal conductivity. With these achievements, a maximum zT of 0.84 at 818 K is realized in the composite with 0.1wt% SWCNTs. The mechanical property of the composites is strengthened as well because of the SWCNTs. The work indicates that the SWCNTs, as the charge channels, propose an effective approach for enhancing carrier mobility in BiCuSeO-based materials, finally optimizing the thermoelectric performance as well as the mechanical property.

Optimization of Gas-Sensing Properties in Poly(triarylamine) Field-Effect Transistors by Device and Interface Engineering.

In this study, we investigated the gas-sensing mechanism in bottom-gate organic field-effect transistors (OFETs) using poly(triarylamine) (PTAA). A comparison of different device architectures revealed that the top-contact structure exhibited superior gas-sensing performance in terms of field-effect mobility and sensitivity. The thickness of the active layer played a critical role in enhancing these parameters in the top-contact structure. Moreover, the distance and pathway for charge carriers to reach the active channel were found to significantly influence the gas response. Additionally, the surface treatment of the SiO2 dielectric with hydrophobic self-assembled mono-layers led to further improvement in the performance of the OFETs and gas sensors by effectively passivating the silanol groups. Under optimal conditions, our PTAA-based gas sensors achieved an exceptionally high response (>200%/ppm) towards NO2. These findings highlight the importance of device and interface engineering for optimizing gas-sensing properties in amorphous polymer semiconductors, offering valuable insights for the design of advanced gas sensors.

Construct Schottky interface containing energy-filtering effect: An efficient strategy to decouple thermopower and conductivity

Organic/inorganic thermoelectric hybrids demonstrate great potential for wearable applications. However, their scalability is hindered by an inferior power factor (S2σ). Nowadays, achieving deep optimization of S2σ necessitates a strategy to decouple the Seebeck coefficient (S) and electrical conductivity (σ). In this work, we propose a strategy to break the coupling between S and σ by constructing a Schottky interface that exhibits an energy-filtering effect. We validate the feasibility of this approach using a PANI/TiN–TiO2/carbon paper. The results demonstrate a 1.16-fold increase in σ and a 1.08-fold increase in S in PANI/TiN–TiO2/carbon paper achieved through the construction of a Schottky-type TiN/TiO2 interface. The separation of hole/electron at the TiN/TiO2 interface serves as the scattering center for ionized impurity scattering and facilitates the transport pathway for charge carriers. These factors are crucial in determining the simultaneous optimization of S and σ, respectively. Additionally, the energy-filtering effect of the TiN/TiO2 interface plays a positive role in the ionized impurity scattering mechanism by selectively filtering out low-energy carriers. This further strengthens decoupling of the thermoelectric properties. The 14.9% PANI/11.2% TiN–14.5% TiO2/59.44% carbon paper displays the highest S2σ and achieves a high ZT value of 223.6 μVm−1 K−2 and 0.31 at 300 K, highlighting the advantages of PANI-based thermoelectric hybrids. This work provides valuable guidance for the design of thermoelectric hybrids incorporating multi-interface morphology.

Organic Photovoltaic Stability: Understanding the Role of Engineering Exciton and Charge Carrier Dynamics from Recent Progress.

Benefiting from the synergistic development of material design, device engineering, and the mechanistic understanding of device physics, the certified power conversion efficiencies (PCEs) of single-junction non-fullerene organic solar cells (OSCs) have already reached a very high value of exceeding 19%. However, in addition to PCEs, the poor stability is now a challenging obstacle for commercial applications of organic photovoltaics (OPVs). Herein, recent progress made in exploring operational mechanisms, anomalous photoelectric behaviors, and improving long-term stability in non-fullerene OSCs are highlighted from a novel and previously largely undiscussed perspective of engineering exciton and charge carrier pathways. Considering the intrinsic connection among multiple temporal-scale photocarrier dynamics, multi-length scale morphologies, and photovoltaic performance in OPVs, this review delineates and establishes a comprehensive and in-depth property-function relationship for evaluating the actual device stability. Moreover, this review has also provided some valuable photophysical insights into employing the advanced characterization techniques such as transient absorption spectroscopy and time-resolved fluorescence imagings. Finally, some of the remaining major challenges related to this topic are proposed toward the further advances of enhancing long-term operational stability in non-fullerene OSCs.

Water Evaporation Triggered Self‐Assembly of MXene on Non‐Carbonized Wood with Well‐Aligned Channels as Size‐Customizable Free‐Standing Electrode for Supercapacitors

Herein, non‐carbonized wood‐based electrodes and separators with well‐aligned channels and excellent mechanical properties are developed for supercapacitors. To enhance the conductivity and boost the capacitance, Ti3C2 (MXene) nanosheets with high electrical conductivity and excellent electrochemical activity are loaded into the wood cells via self‐assembly triggered by fast evaporating water in Ti3C2 suspension. By the assistance of positive charged polydopamine microspheres with large surface area, the self‐restacking of Ti3C2 nanosheets can be avoided and the high mass loading (50 wt%) can be achieved due to the extra driving force for Ti3C2 absorption. Benefiting from the conductive Ti3C2 nanosheets with massive active sites and the multiple well‐aligned channels in wood with efficient transportation pathways for charge carriers, the as‐designed free‐standing electrode shows a large areal capacitance of 1060 mF cm−2 at 0.5 mA cm−2 and high capacitance retention of 67% at 10 mA cm−2. Also, this electrode is highly size‐customizable, showing a good ability to be industrially processed into various shapes and dimensions. Furthermore, an all‐wood based supercapacitor with Ti3C2/wood composites as two layers of electrodes and a wood slice as the separator is fabricated, presenting a high energy density of 10.5 μW h cm−2 at 389.9 μW cm−2.

Towards fast-charging high-energy lithium-ion batteries: From nano- to micro-structuring perspectives

Electric vehicles (EVs) have been playing an indispensable role in reducing greenhouse gas emissions for our modern society. However, current EVs are difficult to meet people’s diverse travel needs, especially in long endurance and fast-charging capacities. At the heart of this issue is the physicochemical limit of current lithium-ion batteries (LIBs), which are the core parts for powering the vehicles. Hence, LIBs with simultaneous high energy and power are critically required to further promote the development of EVs. In this review, we first summarize the key electrochemical processes in electrochemical reactions which lead to the corresponding overpotentials in or between multiple battery components. Furthermore, numerical simulations are employed to quantitatively analyze the effects of versatile electrode parameters on electrochemical properties in high-energy NMC811//graphite systems. On the basis of the in-depth understandings from simulation, recent experimental efforts on designing electroactive materials and electrode architectures across multiple length scales are discussed. Among them, nano-structuring can promote local mass transport and stabilize the interfaces at the particle level, while micro-structuring can establish efficient pathways for charge carriers at the electrode level. Finally, we conclude that a tight feedback loop among structure engineering, characterization and simulation should be followed to speed up the understanding of the deficiencies existing in current electrode designs as well as point out the possible electrode optimization routes for next-generation fast-charging LIBs.

Ternary-Oxides CuWO4/BiVO4/FeCoOx Films for Photoelectrochemical Water Oxidation: Insights into the Photoinduced Charge Transfer Pathway

Photoelectrochemical (PEC) water oxidation using semiconductor oxide films as a working electrode is an essential approach for investigating the effective utilization of sunlight and the production of green fuel. Herein, we report a ternary-oxides-based CuWO4/BiVO4/FeCoOx film deposited entirely by RF-magnetron sputtering using homemade ceramic targets. Our CuWO4/BiVO4 photoanode exhibits a significant photocurrent density of 0.82 mA/cm² at 1.23 V vs. RHE under AM 1.5G illumination, corresponding to a record > 380% increase to that of pure CuWO4 photoanode. To further boost the PEC performance, we deposited an ultrathin layer of amorphous FeCoOx cocatalyst, resulting in a triple CuWO4/BiVO4/FeCoOx heterojunction with a significant reduction in onset potential and a 500% increase in photocurrent density of pure CuWO4. Experimental studies and numeric computations were used to provide insights into the photoinduced charge carrier pathway across heterojunctions. Our results reveal noticeable interface potential barriers for charge carriers at the CuWO4/BiVO4 heterojunction, potentially lowering PEC efficiency without external potentials. Conversely, the deposition of the FeCoOx ultrathin layer over the CuWO4/BiVO4 heterojunction induces a - junction on the BiVO4/FeCoOx interface, which, when combined with the abundant FeCoOx oxygen vacancies, results in improved charge separation and transport, as well as enhanced photoelectrochemical stability. Our study provides a feasible strategy for producing photocatalytic heterojunctions systems and novel tools for investigating interface effects on photoinduced charge carrier pathways for PEC water splitting.

Facile fabrication of SnO2 / MoS2 / rGO ternary composite for solar light-mediated photocatalysis for water remediation

Tin oxide (SnO2) nanostructure has potential application for the removal of organic contaminants from industrial effluents. However, higher bandgap value (3.8 eV and fast recombination of charge carriers (e- - h+ pair)) have limited its applications. To overcome these limitations, we have proposed a double Z-scheme strategy, which not only reduced the rate of recombination but lso provides a new pathway for charge carriers to degrade the organic toxins. SnO2 surface sensitization with MoS2 nanofibers by hydrothermal method followed by composite formation with reduced graphene oxide (rGO) via ultrasonication was employed as photocatalyst for methylene blue (MB) degradation. The surface sensitized SnO2 with MoS2 and its composite with rGO were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-rays (EDX), and ultraviolet-visible (UV-Vis.) spectroscopy. XRD analysis revealed that SnO /MoS2 heterostructure has very small crystallite. From UV-visible studies, a redshift was observed for the SnO2 /MoS2 heterostructure as compared to SnO2, which further extends to the visible spectrum with the introduction of reduced graphene oxide (rGO). The photocatalytic studies revealed that SnO2 /MoS2 /rGO composite significantly improved the photocatalytic efficiency up to 90% under 75 min in the presence of solar light. EIS measurements exhibited that SnO2/MoS2/rGO has better charge transfer and mass transfer potential as compared to SnO2.

Recent advances in wide solar spectrum active W18O49-based photocatalysts for energy and environmental applications

ABSTRACT Developing a facile and proficient strategy for photocatalysis using abundant solar light will result in a clean and renewable process for solving energy and environmental pollution issues. Semiconductor (SC)-based transition metal oxides are used widely, especially, oxygen-deficient tungsten oxide (W18O49) is expected to play an important role in photocatalytic applications because of its abundance, low cost, nontoxicity, surface plasmon resonance (SPR) response, chemical stability, and efficient charge transfer nature. This current review highlights the significance of different modifications in morphological, structural, SC coupling strategies and their influence on photocatalytic applications such as; H2 evaluation, CO2 reduction, and degradation of organics/heavy metal ion treatments. Further, the charge carrier pathways in different heterojunction mechanisms and their role on the photocatalytic activity also discussed in detail. This review highlights new directions and future advancements for developing highly efficient wide-spectrum active W18O49-based SC photocatalysts.

DFT study of electronic structure and mobility of pristine and fluorinated methylammonium lead halide perovskites (CH 3 NH 3 PbX 3 , X = I, Br, Cl)

We present the electronic structure and mobility of pristine and fluorinated methylammonium lead halide perovskites (CH3NH3PbX3, X = I, Br, Cl) using DFT calculations. We find that both CH3NH3PbX3 and CF3NH3PbX3 exhibit direct bandgaps (1.5-2.8 eV) which lies within the suitable bandgap regime for solar cell applications. The carrier mobilities of fluorinated MAPbX3 are much lower than pristine MAPbX3 indicating poor solar cell performance in spite of possible gain in stability due to enhanced hydrophobicity. The fluorination does not affect the chemical stability as is evident from the negative formation and cohesive energies, but an extremely high dipole moment (6.88 Debye) of CH 3 NH 3 + may severely affect the charge carrier pathways.

Band-like Transport of Charge Carriers in Oriented Two-Dimensional Conjugated Covalent Organic Frameworks

A tunable topology and a porous network make π-conjugated covalent organic frameworks (COFs) a new class of organic semiconductors for optoelectronic, smart sensing, and catalytic applications. Although some of the COFs exhibit enhanced electric conductivity with a high charge carrier mobility, the nature and pathways of charge transport still remain elusive. In order to unveil the transport mechanism, herein, we have developed crystalline π-conjugated COFs using planar building blocks, and a wafer-scale self-supporting thin film was grown, which could be transferred onto any of the desired substrates. The COF film was found to be highly oriented and exhibited a high in-plane electronic conductivity. The conductivity was almost independent of temperature with an ultra-low activation energy of 14.3 meV, approaching a band-like transport of charge carriers within the crystalline domains. The COF films also showed a high photoresponsivity in electronic conduction against a complete visible range, demonstrated as a flexible photodetector device. This work represents a thorough investigation of the mechanism and direction of charge transport in crystalline π-conjugated COF semiconductors, which suggests their feasibility as key active materials in multi-functional organic electronics.