Recent Development of Carbon Nanotube Transparent Conductive Films

Mechanical Durability and Flexibility in Perovskite Photovoltaics: Advancements and Applications

The remarkable progress in perovskite solar cell (PSC) technology has witnessed a remarkable leap in efficiency within the past decade. As this technology continues to mature, flexible PSCs (F-PSCs) are emerging as pivotal components for a wide array of applications, spanning from powering portable electronics and wearable devices to integrating seamlessly into electronic textiles and large-scale industrial roofing. F-PSCs characterized by their lightweight, mechanical flexibility, and adaptability for cost-effective roll-to-roll manufacturing, hold immense commercial potential. However, the persistent concerns regarding the overall stability and mechanical robustness of these devices loom large. This comprehensive review delves into recent strides made in enhancing the mechanical stability of F-PSCs. It covers a spectrum of crucial aspects, encompassing perovskite material optimization, precise crystal grain regulation, film quality enhancement, strategic interface engineering, innovational developed flexible transparent electrodes, judicious substrate selection, and the integration of various functional layers. By collating and analyzing these dedicated research endeavors, this review illuminates the current landscape of progress in addressing the challenges surrounding mechanical stability. Furthermore, it provides valuable insights into the persistent obstacles and bottlenecks that demand attention and innovative solutions in the field of F-PSCs.

Interface Reactive Sputtering of Transparent Electrode for High-Performance Monolithic and Stacked Perovskite Tandem Solar Cells

Sputtered indium tin oxide (ITO) fulfills the requirements of top transparent electrodes (TTEs) in semitransparent perovskite solar cells (PSCs) and stacked tandem solar cells (TSCs), as well as of the recombination layers in monolithic TSCs. However, the high-energy ITO particles will cause damage to the devices. Herein, the interface reactive sputtering strategy is proposed to construct cost-effective TTEs with high transmittance and excellent carrier transporting ability. Polyethylenimine (PEI) is chosen as the interface reactant that can react with sputtered ITO nanoparticles, so that, coordination compounds can be formed during the deposition process, facilitating the carrier transport at the interface of C60/PEI/ITO. Besides, the impact force of energetic ITO particles is greatly alleviated, and the intactness of the underlying C60 layer and perovskite layer is guaranteed. Thus, the prepared semitransparent subcells achieve a significantly enhanced power conversion efficiency (PCE) of 19.17%, surpassing those based on C60/ITO (11.64%). Moreover, the PEI-based devices demonstrate excellent storage stability, which maintains 98% of their original PCEs after 2000 h. On the strength of the interface reactive sputtering ITO electrode, a stacked all-perovskite TSC with a PCE of 26.89% and a monolithic perovskite-organic TSC with a PCE of 24.33% are successfully fabricated.

Large-Area Flexible Carbon Nanofilms with Synergistically Enhanced Transmittance and Conductivity Prepared by Reorganizing Single-Walled Carbon Nanotube Networks

Large-area flexible transparent conductive films (TCFs) are highly desired for future electronic devices. Nanocarbon TCFs are one of the most promising candidates, but some of their properties are mutually restricted. Here, a novel carbon nanotube network reorganization (CNNR) strategy, that is, the facet-driven CNNR (FD-CNNR) technique, is presented to overcome this intractable contradiction. The FD-CNNR technique introduces an interaction between single-walled carbon nanotube (SWNT) and Cu─-O. Based on the unique FD-CNNR mechanism, large-area flexible reorganized carbon nanofilms (RNC-TCFs) are designed and fabricated with A3-size and even meter-length, including reorganized SWNT (RSWNT) films and graphene and RSWNT (G-RSWNT) hybrid films. Synergistic improvement in strength, transmittance, and conductivity of flexible RNC-TCFs is achieved. The G-RSWNT TCF shows sheet resistance as low as 69 Ω sq-1 at 86% transmittance, FOM value of 35, and Young's modulus of ≈45 MPa. The high strength enables RNC-TCFs to be freestanding on water and easily transferred to any target substrate without contamination. A4-size flexible smart window is fabricated, which manifests controllable dimming and fog removal. The FD-CNNR technique can be extended to large-area or even large-scale fabrication of TCFs and can provide new insights into the design of TCFs and other functional films.

Machine Learning-Assisted Exploration and Identification of Aqueous Dispersants in the Vast Diversity of Organic Chemicals

Dispersion represents a central processing method in the organization of nanomaterials; however, the strong interparticle interaction represents a significant obstacle to fabricating homogeneous and stable dispersions. While dispersants can greatly assist in overcoming this obstacle, the appropriate type is dependent on such factors as nanomaterial, solvent, experimental conditions, etc., and there is no general guide to assist in the selection from the vast number of possibilities. We report a strategy and successful demonstration of the machine-learning-based "Dispersant Explorer", which surveys and identifies suitable dispersants from open databases. Through the combined use of experimental and molecular descriptors derived from SMILES databases, the model showed exceptional predictive accuracy in surveying about ∼1000 chemical compounds and identifying those that could be applied as dispersants. Furthermore, fabrication of transparent conducting films using the predicted and previously unknown dispersant exhibited the highest sheet resistance and transmittance compared with those of other reported undoped films. This result highlights that, in addition to opening new avenues for novel dispersant discovery, machine learning has a potential to elucidate the chemical structures essential for optimal dispersion performance to assist in the advancement of the complex topic of nanomaterial processing.

Flexible, Transparent and Conductive Metal Mesh Films with Ultra-High FoM for Stretchable Heating and Electromagnetic Interference Shielding

Despite the growing demand for transparent conductive films in smart and wearable electronics for electromagnetic interference (EMI) shielding, achieving a flexible EMI shielding film, while maintaining a high transmittance remains a significant challenge. Herein, a flexible, transparent, and conductive copper (Cu) metal mesh film for EMI shielding is fabricated by self-forming crackle template method and electroplating technique. The Cu mesh film shows an ultra-low sheet resistance (0.18 Ω □-1), high transmittance (85.8%@550 nm), and ultra-high figure of merit (> 13,000). It also has satisfactory stretchability and mechanical stability, with a resistance increases of only 1.3% after 1,000 bending cycles. As a stretchable heater (ε > 30%), the saturation temperature of the film can reach over 110 °C within 60 s at 1.00 V applied voltage. Moreover, the metal mesh film exhibits outstanding average EMI shielding effectiveness of 40.4 dB in the X-band at the thickness of 2.5 μm. As a demonstration, it is used as a transparent window for shielding the wireless communication electromagnetic waves. Therefore, the flexible and transparent conductive Cu mesh film proposed in this work provides a promising candidate for the next-generation EMI shielding applications.

Molecular Doping Modulation and Applications of Structure-Sorted Single-Walled Carbon Nanotubes: A Review

Single-walled carbon nanotubes (SWCNTs) that have a reproducible distribution of chiralities or single chirality are among the most competitive materials for realizing post-silicon electronics. Molecular doping, with its non-destructive and fine-tunable characteristics, is emerging as the primary doping approach for the structure-controlled SWCNTs, enabling their eventual use in various functional devices. This review provides an overview of important advances in the area of molecular doping of structure-controlled SWCNTs and their applications. The first part introduces the underlying physical process of molecular doping, followed by a comprehensive survey of the commonly used dopants for SWCNTs to date. Then, it highlights how the convergence of molecular doping and structure-sorting strategies leads to significantly improved functionality of SWCNT-based field-effect transistor arrays, transparent electrodes in optoelectronics, thermoelectrics, and many emerging devices. At last, several challenges and opportunities in this field are discussed, with the hope of shedding light on promoting the practical application of SWCNTs in future electronics.

Highly Flexible and Acid-Alkali Resistant TiN Nanomesh Transparent Electrodes for Next-Generation Optoelectronic Devices

Transparent electrodes are vital for optoelectronic devices, but their development has been constrained by the limitations of existing materials such as indium tin oxide (ITO) and newer alternatives. All face issues of robustness, flexibility, conductivity, and stability in harsh environments. Addressing this challenge, we developed a flexible, low-cost titanium nitride (TiN) nanomesh transparent electrode showcasing exceptional acid-alkali resistance. The TiN nanomesh electrode, created by depositing a TiN coating on a naturally cracked gel film substrate via a sputtering method, maintains a stable electrical performance through thousands of bending cycles. It exhibits outstanding chemical stability, resisting strong acid and alkali corrosion, which is a key hurdle for current electrodes when in contact with acidic/alkaline materials and solvents during device fabrication. This, coupled with superior light transmission and conductivity (88% at 550 nm with a sheet resistance of ∼200 Ω/sq), challenges the reliance on conventional materials. Our TiN nanomesh electrode, successfully applied in electric heaters and electrically controlled thermochromic devices, offers broad potential beyond harsh environment applications. It enables alternative possibilities for the design and fabrication of future optoelectronics for advancements in this pivotal field.

Review on design strategies and applications of flexible cellulose‑carbon nanotube functional composites

Combining the excellent biocompatibility and mechanical flexibility of cellulose with the outstanding electrical, mechanical, optical and stability properties of carbon nanotubes (CNTs), cellulose-CNT composites have been extensively studied and applied to many flexible functional materials. In this review, we present advances in structural design strategies and various applications of cellulose-CNT composites. Firstly, the structural characteristics and corresponding treatments of cellulose and CNTs are analyzed, as are the potential interactions between the two to facilitate the formation of cellulose-CNT composites. Then, the design strategies and processing techniques of cellulose-CNT composites are discussed from the perspectives of cellulose fibers at the macroscopic scale (natural cotton, hemp, and other fibers; recycled cellulose fibers); nanocellulose at the micron scale (nanofibers, nanocrystals, etc.); and macromolecular chains at the molecular scale (cellulose solutions). Further, the applications of cellulose-CNT composites in various fields, such as flexible energy harvesting and storage devices, strain and humidity sensors, electrothermal devices, magnetic shielding, and photothermal conversion, are introduced. This review will help readers understand the design strategies of cellulose-CNT composites and develop potential high-performance applications.

Fabrication of Flexible and Transparent Metal Mesh Electrodes Using Surface Energy-Directed Assembly Process for Touch Screen Panels and Heaters

Transparent conductive electrodes (TCEs) are indispensable components of various optoelectronic devices such as displays, touch screen panels, solar cells, and smart windows. To date, the fabrication processes for metal mesh-based TCEs are either costly or having limited resolution and throughput. Here, a two-step surface energy-directed assembly (SEDA) process to efficiently fabricate high resolution silver meshes is introduced. The two-step SEDA process turns from assembly on a functionalized substrate with hydrophilic mesh patterns into assembly on a functionalized substrate with stripe patterns. During the SEDA process, a three-phase contact line pins on the hydrophilic pattern regions while recedes on the hydrophobic non-pattern regions, ensuring that the assembly process can be achieved with excellent selectivity. The necessity of using the two-step SEDA process rather than a one-step SEDA process is demonstrated by both experimental results and theoretical analysis. Utilizing the two-step SEDA process, silver meshes with a line width down to 2 µm are assembled on both rigid and flexible substrates. The thickness of the silver meshes can be tuned by varying the withdraw speed and the assembly times. The assembled silver meshes exhibit excellent optoelectronic properties (sheet resistance of 1.79 Ω/□, optical transmittance of ≈92%, and a FoM value of 2465) as well as excellent mechanical stability. The applications of the assembled silver meshes in touch screen panels and thermal heaters are demonstrated, implying the potential of using the two-step SEDA process for the fabrication of TCEs for optoelectronic applications.

Metallic Micro-Nano Network-Based Soft Transparent Electrodes: Materials, Processes, and Applications

Soft transparent electrodes (TEs) have received tremendous interest from academia and industry due to the rapid development of lightweight, transparent soft electronics. Metallic micro-nano networks (MMNNs) are a class of promising soft TEs that exhibit excellent optical and electrical properties, including low sheet resistance and high optical transmittance, as well as superior mechanical properties such as softness, robustness, and desirable stability. They are genuinely interesting alternatives to conventional conductive metal oxides, which are expensive to fabricate and have limited flexibility on soft surfaces. This review summarizes state-of-the-art research developments in MMNN-based soft TEs in terms of performance specifications, fabrication methods, and application areas. The review describes the implementation of MMNN-based soft TEs in optoelectronics, bioelectronics, tactile sensors, energy storage devices, and other applications. Finally, it presents a perspective on the technical difficulties and potential future possibilities for MMNN-based TE development.

Molecular Carbons: How Far Can We Go?

The creation and development of carbon nanomaterials promoted material science significantly. Bottom-up synthesis has emerged as an efficient strategy to synthesize atomically precise carbon nanomaterials, namely, molecular carbons, with various sizes and topologies. Different from the properties of the feasibly obtained mixture of carbon nanomaterials, numerous properties of single-component molecular carbons have been discovered owing to their well-defined structures as well as potential applications in various fields. This Perspective introduces recent advances in molecular carbons derived from fullerene, graphene, carbon nanotube, carbyne, graphyne, and Schwarzite carbon acquired with different synthesis strategies. By selecting a variety of representative examples, we elaborate on the relationship between molecular carbons and carbon nanomaterials. We hope these multiple points of view presented may facilitate further advancement in this field.

Flexible Quantum-Dot Light-Emitting Diodes Using Embedded Silver Mesh Transparent Electrodes Manufactured by an Ultraprecise Deposition Method

Transparent conductive electrodes (TCEs) fabricated onto flexible substrates are crucial parts of organic-light-emitting diodes (OLEDs), which are vastly utilized for display and lightning applications. Indium tin oxide (ITO), which is so far the most popular material for transparent and conductive electrodes, is found to be an unsuitable candidate for flexible devices mostly due to its brittleness. Here, we present a novel approach for the fabrication of transparent, conductive, and flexible electrodes for optoelectronic applications made of silver metal mesh by an ultraprecise deposition (UPD) method. The fabricated mesh exhibits an 80% (λ = 550 nm) optical transmittance and a sheet resistance of 11 Ω/sq. The Ag-mesh embedded into the polymer is implemented as an anode for a quantum-dot light-emitting diode (QLED) in order to assess its performance. The fabricated QLED is characterized by the maximum external quantum efficiency (EQE) of 2% and a current efficiency (CE) of 6 cd/A, reaching the maximum luminance (L) of 3200 cd/m2 at a current density of 100 mA/cm2. This method shows a fast and relatively simple approach to fabricate optoelectronic devices without the need for special treatment and sophisticated equipment.

Flexible Transparent Conductive Electrodes: Unveiling Growth Mechanisms, Material Dimensions, Fabrication Methods, and Design Strategies

Flexible transparent conductive electrodes (FTCEs) constitute an indispensable component in state-of-the-art electronic devices, such as wearable flexible sensors, flexible displays, artificial skin, and biomedical devices, etc. This review paper offers a comprehensive overview of the fabrication techniques, growth modes, material dimensions, design, and their impacts on FTCEs fabrication. The growth modes, such as the "Stranski-Krastanov growth," "Frank-van der Merwe growth," and "Volmer-Weber growth" modes provide flexibility in fabricating FTCEs. Application of different materials including 0D, 1D, 2D, polymer composites, conductive oxides, and hybrid materials in FTCE fabrication, emphasizing their suitability in flexible devices are discussed. This review also delves into the design strategies of FTCEs, including microgrids, nanotroughs, nanomesh, nanowires network, and "kirigami"-inspired patterns, etc. The pros and cons associated with these materials and designs are also addressed appropriately. Considerations such as trade-offs between electrical conductivity and optical transparency or "figure of merit (FoM)," "strain engineering," "work function," and "haze" are also discussed briefly. Finally, this review outlines the challenges and opportunities in the current and future development of FTCEs for flexible electronics, including the improved trade-offs between optoelectronic parameters, novel materials development, mechanical stability, reproducibility, scalability, and durability enhancement, safety, biocompatibility, etc.

Multi-functionalized carbon nanotubes towards green fabrication of heterogeneous catalyst platforms with enhanced catalytic properties under NIR light irradiation

Metal/carbon nanotubes (CNTs) have been attractive hybrid systems due to their high specific surface area and exceptional catalytic activity, but their challenging synthesis and dispersion impede their extensive applications. Herein, we report a facile and green approach towards the fabrication of metal/CNT composites, which utilizes a versatile glycopeptide (GP) both as a stabilizer for CNTs in water and as a reducing agent for noble metal ions. The abundant hydrogen bonds in GP endow the formed GP-CNTs with excellent plasticity, enabling the availability of polymorphic CNT species from dispersion to viscous paste, gel, and even to dough by increasing their concentration. The GP molecules can reduce metal precursors at room temperature without additional reducing agents, enabling the in situ immobilization of metal nanoparticles (e.g. Au, Ag, Pt, and Pd) on the CNT surface. The combination of the excellent catalytic properties of Pd particles with photothermal conversion capability of CNTs makes the Pd/CNT composite a promising catalyst for the fast degradation of organic pollutants, as demonstrated by a model catalytic reaction using 4-nitrophenol (4-NP). The conversion of 4-NP using the Pd/CNT composite as the catalyst has increased by 1.6-fold under near infrared light illumination, benefiting from the strong light-to-heat conversion effect of CNTs. Our proposed strategy opens a new avenue for the synthesis of CNT composites as a sustainable and versatile catalyst platform.

2D Materials in the Display Industry: Status and Prospects

With advances in flexible electronics, innovative foldable, rollable, and stretchable displays have been developed to maintain their performance under various deformations. These flexible devices can develop more innovative designs than conventional devices due to their light weight, high space efficiency, and practical convenience. However, developing flexible devices requires material innovation because the devices must be flexible and exhibit desirable electrical insulating/semiconducting/metallic properties. Recently, emerging 2D materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted considerable research attention because of their outstanding electrical, optical, and mechanical properties, which are ideal for flexible electronics. The recent progress and challenges of 2D material growth and display applications are reviewed and perspectives for exploring 2D materials for display applications are discussed.

Robust method for uniform coating of carbon nanotubes with V2O5 for next-generation transparent electrodes and Li-ion batteries

Composites comprising vanadium-pentoxide (V2O5) and single-walled carbon nanotubes (SWCNTs) are promising components for emerging applications in optoelectronics, solar cells, chemical and electrochemical sensors, etc. We propose a novel, simple, and facile approach for SWCNT covering with V2O5 by spin coating under ambient conditions. With the hydrolysis-polycondensation of the precursor (vanadyl triisopropoxide) directly on the surface of SWCNTs, the nm-thick layer of oxide is amorphous with a work function of 4.8 eV. The material recrystallizes after thermal treatment at 600 °C, achieving the work function of 5.8 eV. The key advantages of the method are that the obtained coating is uniform with a tunable thickness and does not require vacuuming or heating during processing. We demonstrate the groundbreaking results for two V2O5/SWCNT applications: transparent electrode and cathode for Li-ion batteries. As a transparent electrode, the composite shows stable sheet resistance of 160 Ω sq-1 at a 90% transmittance (550 nm) - the best performance reported for SWCNTs doped by metal oxides. As a cathode material, the obtained specific capacity (330 mA h g-1) is the highest among all the other V2O5/SWCNT cathodes reported so far. This approach opens new horizons for the creation of the next generation of metal oxide composites for various applications, including optoelectronics and electrochemistry.

Interfacial Super-Assembly of Intertwined Nanofibers toward Hybrid Nanochannels for Synergistic Salinity Gradient Power Conversion

Capturing the abundant salinity gradient power into electric power by nanofluidic systems has attracted increasing attention and has shown huge potential to alleviate the energy crisis and environmental pollution problems. However, not only the imbalance between permeability and selectivity but also the poor stability and high cost of traditional membranes limit their scale-up realistic applications. Here, intertwined "soft-hard" nanofibers/tubes are densely super-assembled on the surface of anodic aluminum oxide (AAO) to construct a heterogeneous nanochannel membrane, which exhibits smart ion transport and improved salinity gradient power conversion. In this process, one-dimensional (1D) "soft" TEMPO-oxidized cellulose nanofibers (CNFs) are wrapped around "hard" carbon nanotubes (CNTs) to form three-dimensional (3D) dense nanochannel networks, subsequently forming a CNF-CNT/AAO hybrid membrane. The 3D nanochannel networks constructed by this intertwined "soft-hard" nanofiber/tube method can significantly enhance the membrane stability while maintaining the ion selectivity and permeability. Furthermore, benefiting from the asymmetric structure and charge polarity, the hybrid nanofluidic membrane displays a low membrane inner resistance, directional ionic rectification characteristics, outstanding cation selectivity, and excellent salinity gradient power conversion performance with an output power density of 3.3 W/m². Besides, a pH sensitive property of the hybrid membrane is exhibited, and a higher power density of 4.2 W/m² can be achieved at a pH of 11, which is approximately 2 times more compared to that of pure 1D nanomaterial based homogeneous membranes. These results indicate that this interfacial super-assembly strategy can provide a way for large-scale production of nanofluidic devices for various fields including salinity gradient energy harvesting.

A novel CNTs/QCS/BiOBr composite membrane with electron-ion transfer channel for Br- recovery in ESIX process

Based on the superior selectivity of bismuth oxybromide (BiOBr) for Br^-, the excellent electrical conductivity of carbon nanotubes (CNTs), and the ion exchange capacity of quaternized chitosan (QCS), a three-dimensional network composite membrane electrode CNTs/QCS/BiOBr was constructed, in which BiOBr served as the storage space for Br^-, CNTs provided the electron transfer pathway, and QCS cross-linked by glutaraldehyde (GA) was used for ion transfer. The CNTs/QCS/BiOBr composite membrane exhibits superior conductivity after the introduction of the polymer electrolyte, which is seven orders of magnitude higher than that of conventional ion-exchange membranes. Furthermore, the addition of the electroactive material BiOBr improved the adsorption capacity for Br^- by a factor of 2.7 in electrochemically switched ion exchange (ESIX) system. Meanwhile, the CNTs/QCS/BiOBr composite membrane displays excellent Br^- selectivity in mixed solutions of Br^-, Cl^-, SO₄^2- and NO₃^-. Therein, the covalent bond cross-linking within the CNTs/QCS/BiOBr composite membrane endows it great electrochemical stability. The synergistic adsorption mechanism of the CNTs/QCS/BiOBr composite membrane provides a new direction for achieving more efficient ion separation.

A review on accelerated development of skin-like MXene electrodes: from experimental to machine learning

Foreshadowing future needs has catapulted the progress of skin-like electronic devices for human-machine interactions. These devices possess human skin-like properties such as stretchability, self-healability, transparency, biocompatibility, and wearability. This review highlights the recent progress in a promising material, MXenes, to realize soft, deformable, skin-like electrodes. Various structural designs, fabrication strategies, and rational guidelines adopted to realize MXene-based skin-like electrodes are outlined. We explicitly discussed machine learning-based material informatics to understand and predict the properties of MXenes. Finally, an outlook on the existing challenges and the future roadmap to realize soft skin-like MXene electrodes to facilitate technological advances in the next-generation human-machine interactions has been described.

Silver Nanoparticle/Carbon Nanotube Hybrid Nanocomposites: One-Step Green Synthesis, Properties, and Applications

Hybrid nanocomposites of silver nanoparticles and multiwalled carbon nanotubes (AgNPs/MWCNTs) were successfully synthesized by a green one-step method without using any organic solvent. The synthesis and attachment of AgNPs onto the surface of MWCNTs were performed simultaneously by chemical reduction. In addition to their synthesis, the sintering of AgNPs/MWCNTs can be carried out at room temperature. The proposed fabrication process is rapid, cost efficient, and ecofriendly compared with multistep conventional approaches. The prepared AgNPs/MWCNTs were characterized using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The transmittance and electrical properties of the transparent conductive films (TCF_Ag/CNT) fabricated using the prepared AgNPs/MWCNTs were characterized. The results showed that the TCF_Ag/CNT film has excellent properties, such as high flexible strength, good high transparency, and high conductivity, and could therefore be an effective substitute for conventional indium tin oxide (ITO) films with poor flexibility.

Architecting Lignin/Poly(vinyl alcohol) Hydrogel with Carbon Nanotubes for Photothermal Antibacterial Therapy

With the development of antimicrobial resistance, rapid and effective killing of bacteria is required for infected wound healing after skin trauma. Herein, we reported a one-pot reaction strategy to prepare a composite hydrogel with antibacterial activity through high-efficiency photothermal therapy. We take poly(vinyl alcohol) as the matrix, and lignin-derived from biomass was introduced into the hydrogel to increase the tensile strength of the prepared hydrogel to 108.58 kPa, and the elongation at break reaches 200.8%. The electrostatic interaction between lignin and chitosan enhanced the reactivity of lignin. Carbon nanotubes endow the hydrogel with photothermal antibacterial activity that can kill more than 97% of either Escherichia coli or Staphylococcus aureus within 5 min, avoiding the problem of bacterial resistance. Experimental evaluation on mice showed that the hydrogel could effectively promote wound healing of full-thickness skin defects. The hydrogels with good mechanical properties, antioxidant activity, and excellent photothermal antibacterial ability show good potential to repair the damaged tissue and are expected to be used in the clinical transformation of wound dressing.

High Temperature-Resistant Transparent Conductive Films for Photoelectrochemical Devices Based on W/Ag Composite Nanonetworks

The traditional Ag nanowire preparation means that it cannot meet the demanding requirements of photoelectrochemical devices due to the undesirable conductivity, difficulty in compounding, and poor heat resistance. Here, we prepared an Ag nanonetwork with superior properties using a special template method based on electrospinning technology. The transparent conductive films based on Ag nanonetworks have good transmittance in a wide range from ultraviolet to visible. It is important that the films have high operability and are easy to be compounded with other materials. After compounding with high-melting-point W metal, the heat-resistance temperature of the W/Ag composite transparent conductive films is increased by 100 °C to 460 °C, and the light transmission and electrical conductivity of the films are not significantly affected. All experimental phenomena in the study are analyzed theoretically. This research can provide an important idea for the metal nanowire electrode, which is difficult to be applied to the photoelectrochemical devices.

Impact of Graphene Monolayer on the Performance of Non-Conventional Silicon Heterojunction Solar Cells with MoOx Hole-Selective Contact

In this work, a new design of transparent conductive electrode based on a graphene monolayer is evaluated. This hybrid electrode is incorporated into non-standard, high-efficiency crystalline silicon solar cells, where the conventional emitter is replaced by a MoO_x selective contact. The device characterization reveals a clear electrical improvement when the graphene monolayer is placed as part of the electrode. The current-voltage characteristic of the solar cell with graphene shows an improved FF and V_oc provided by the front electrode modification. Improved conductance values up to 5.5 mS are achieved for the graphene-based electrode, in comparison with 3 mS for bare ITO. In addition, the device efficiency improves by around 1.6% when graphene is incorporated on top. These results so far open the possibility of noticeably improving the contact technology of non-conventional photovoltaic technologies and further enhancing their performance.

Fast-speed, Highly Sensitive, Flexible Humidity Sensors Based on a Printable Composite of Carbon Nanotubes and Hydrophilic Polymers

Carbon nanotubes (CNTs) are a promising material for humidity sensors and wearable electronics due to their solution capability, good flexibility, and high conductivity. However, the performance of CNT-based humidity sensors is limited by their low sensitivity and slow response. Herein CNTs and hydrophilic polymers were mixed to form a composite. The hydrophilicity of the polymers and the network structure of the CNTs empowered the humidity sensors with a high response of 171% and a fast response/recovery time of 23 s/10 s. Owing to the sticky and flexible polymers, the humidity sensors showed strong adhesion to the PET substrate and exhibited outstanding bending durability. Furthermore, the flexible humidity sensor was applied to monitor human breathing and detect finger movements and handshaking.

Electrochemical Nanosensors for Sensitization of Sweat Metabolites: From Concept Mapping to Personalized Health Monitoring

Sweat contains a broad range of important biomarkers, which may be beneficial for acquiring non-invasive biochemical information on human health status. Therefore, highly selective and sensitive electrochemical nanosensors for the non-invasive detection of sweat metabolites have turned into a flourishing contender in the frontier of disease diagnosis. A large surface area, excellent electrocatalytic behavior and conductive properties make nanomaterials promising sensor materials for target-specific detection. Carbon-based nanomaterials (e.g., CNT, carbon quantum dots, and graphene), noble metals (e.g., Au and Pt), and metal oxide nanomaterials (e.g., ZnO, MnO2, and NiO) are widely used for modifying the working electrodes of electrochemical sensors, which may then be further functionalized with requisite enzymes for targeted detection. In the present review, recent developments (2018-2022) of electrochemical nanosensors by both enzymatic as well as non-enzymatic sensors for the effectual detection of sweat metabolites (e.g., glucose, ascorbic acid, lactate, urea/uric acid, ethanol and drug metabolites) have been comprehensively reviewed. Along with this, electrochemical sensing principles, including potentiometry, amperometry, CV, DPV, SWV and EIS have been briefly presented in the present review for a conceptual understanding of the sensing mechanisms. The detection thresholds (in the range of mM-nM), sensitivities, linear dynamic ranges and sensing modalities have also been properly addressed for a systematic understanding of the judicious design of more effective sensors. One step ahead, in the present review, current trends of flexible wearable electrochemical sensors in the form of eyeglasses, tattoos, gloves, patches, headbands, wrist bands, etc., have also been briefly summarized, which are beneficial for on-body in situ measurement of the targeted sweat metabolites. On-body monitoring of sweat metabolites via wireless data transmission has also been addressed. Finally, the gaps in the ongoing research endeavors, unmet challenges, outlooks and future prospects have also been discussed for the development of advanced non-invasive self-health-care-monitoring devices in the near future.

Highly Sensitive MoS2 Photodetectors Enabled with a Dry-Transferred Transparent Carbon Nanotube Electrode

Fabricating electronic and optoelectronic devices by transferring pre-deposited metal electrodes has attracted considerable attention, owing to the improved device performance. However, the pre-deposited metal electrode typically involves complex fabrication procedures. Here, we introduce our facile electrode fabrication process which is free of lithography, lift-off, and reactive ion etching by directly press-transferring a single-walled carbon nanotube (SWCNT) film. We fabricated Schottky diodes for photodetector applications using dry-transferred SWCNT films as the transparent electrode to increase light absorption in photoactive MoS₂ channels. The MoS₂ flake vertically stacked with an SWCNT electrode can exhibit excellent photodetection performance with a responsivity of ∼2.01 × 10³ A/W and a detectivity of ∼3.2 × 10¹² Jones. Additionally, we carried out temperature-dependent current-voltage measurement and Fowler-Nordheim (FN) plot analysis to explore the dominant charge transport mechanism. The enhanced photodetection in the vertical configuration is found to be attributed to the FN tunneling and internal photoemission of charge carriers excited from indium tin oxide across the MoS₂ layer. Our study provides a novel concept of using a photoactive MoS₂ layer as a tunneling layer itself with a dry-transferred transparent SWCNT electrode for high-performance and energy-efficient optoelectronic devices.

Fully Transparent, Ultrathin Flexible Organic Electrochemical Transistors with Additive Integration for Bioelectronic Applications

Optical transparency is highly desirable in bioelectronic sensors because it enables multimodal optical assessment during electronic sensing. Ultrathin (<5 µm) organic electrochemical transistors (OECTs) can be potentially used as a highly efficient bioelectronic transducer because they demonstrate high transconductance during low-voltage operation and close conformability to biological tissues. However, the fabrication of fully transparent ultrathin OECTs remains a challenge owing to the harsh etching processes of nanomaterials. In this study, fully transparent, ultrathin, and flexible OECTs are developed using additive integration processes of selective-wetting deposition and thermally bonded lamination. These processes are compatible with Ag nanowire electrodes and conducting polymer channels and realize unprecedented flexible OECTs with high visible transmittance (>90%) and high transconductance (≈1 mS) in low-voltage operations (<0.6 V). Further, electroencephalogram acquisition and nitrate ion sensing are demonstrated in addition to the compatibility of simultaneous assessments of optical blood flowmetry when the transparent OECTs are worn, owing to the transparency. These feasibility demonstrations show promise in contributing to human stress monitoring in bioelectronics.

Carbon nanotube-graphene hybrids for soft electronics, sensors, and actuators

Soft devices that are mechanically flexible and stretchable are considered as the building blocks for various applications ranging from wearable devices to robotics. Among the many candidate materials for constructing soft devices, carbon nanomaterials such as carbon nanotubes (CNTs) and graphene have been actively investigated owing to their outstanding characteristics, including their intrinsic flexibility, tunable conductivity, and potential for large-area processing. In particular, hybrids of CNTs and graphene can improve the performance of soft devices and provide them with novel capabilities. In this review, the advances in CNT-graphene hybrid-based soft electrodes, transistors, pressure and strain sensors, and actuators are discussed, highlighting the performance improvements of these devices originating from the synergistic effects of the hybrids of CNT and graphene. The integration of multidimensional heterogeneous carbon nanomaterials is expected to be a promising approach for accelerating the development of high-performance soft devices. Finally, current challenges and future opportunities are summarized, from the processing of hybrid materials to the system-level integration of multiple components.

Spatially modulated femtosecond laser direct ablation-based preparation of ultra-flexible multifunctional copper mesh electrodes and its application

Multifunctional electrodes possess superior properties such as high photoelectric properties and high stability. Laser manufacturing process is one of the widely used method for electrode fabrication. However, the current multifunctional electrode laser manufacturing process suffers from low fabrication speed. Here, we report a high-efficiency laser digital patterning process to fabricate copper-based flexible transparent conducting electrodes. By using a spatially modulated, one single laser spot is modulated into an array of spots with equal intensity, and the fabrication speed can be improved by more than 20 times over the traditional single pulse processing. In addition, copper mesh electrodes with a high photoelectric property have been fabricated. A transparent touch screen panel and multifunctional windows are fabricated with transparent electrodes to demonstrate their use in vehicle defogging, portable heating, and wearable devices.

Tiny NaOH Assisted Facile Preparation of Silk Nanofibers and Their Nanotube-Compositing Strong, Flexible, and Conductive Films

Natural silk nanofibers (SNFs) can not only be used as good building blocks for two- or three-dimensional biomaterials but also provide a clue for understanding the theory of structure-function relationships. Nevertheless, it is still difficult to directly extract SNFs from natural silk fibers due to their high crystallinity and recalcitrant complex structures. In the present study, a dilute alkali-assisted separation of high-yield SNFs is proposed. The degummed silk was first treated with a tiny amount of alkali at a mild temperature, followed by high-pressure homogenization. Under the optimized conditions (2% sodium hydroxide, 0 °C, 48 h), SNFs with diameters of 8-42 nm and lengths of 0.9 ± 0.3 μm were prepared with yields higher than 75%, which retained the natural structures at the nanoscale and some inherent properties of silk fibers. Interestingly, SNFs can be used as a stabilizing matrix to assist carbon nanotubes (CNTs) to disperse, aiming to form a uniform and stable CNT/SNF dispersion. Thereafter, a strong and flexible conductive composite film was fabricated with good mechanical properties. The composite film showed good piezoelectric properties and electric thermal response, which has promising application prospects for SNFs, such as in optical devices, nanoelectronics, and biosensors.

Material Design and Device Fabrication Strategies for Stretchable Organic Solar Cells

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

Transparent Conducting Films Based on Carbon Nanotubes: Rational Design toward the Theoretical Limit

Electrically conductive thin-film materials possessing high transparency are essential components for many optoelectronic devices. The advancement in the transparent conductor applications requires a replacement of indium tin oxide (ITO), one of the key materials in electronics. ITO and other transparent conductive metal oxides have several drawbacks, including poor flexibility, high refractive index and haze, limited chemical stability, and depleted raw material supply. Single-walled carbon nanotubes (SWCNTs) are a promising alternative for transparent conducting films (TCFs) because of their unique and excellent chemical and physical properties. Here, the latest achievements in the optoelectronic performance of TCFs based on SWCNTs are analyzed. Various approaches to evaluate the performance of transparent electrodes are briefly reviewed. A roadmap for further research and development of the transparent conductors using "rational design," which breaks the deadlock for obtaining the TCFs with a performance close to the theoretical limit, is also described.

Electrochemical Redox In-Situ Welding of Silver Nanowire Films with High Transparency and Conductivity

Silver nanowire (AgNW) networks with high transparency and conductivity are crucial to developing transparent conductive films (TCFs) for flexible optoelectronic devices. However, AgNW-based TCFs still suffer from the high contact resistance of AgNW junctions with both the in-plane and out-of-plane charge transport barrier. Herein, we report a rapid and green electrochemical redox strategy to in-situ weld AgNW networks for the enhanced conductivity and mechanical durability of TCFs with constant transparency. The welded TCFs show a marked decrease of the sheet resistance (reduced to 45.5% of initial values on average) with high transmittance of 97.02% at 550 nm (deducting the background of substrates). The electrochemical welding treatment enables the removal of the residual polyvinylpyrrolidone layer and the in-situ formation of Ag solder in the oxidation and reduction processes, respectively. Furthermore, local conductivity studies confirm the improvement of both the in-plane and the out-of-plane charge transport by conductive atomic force microscopy. This proposed electrochemical redox method provides new insights on the welding of AgNW-based TCFs with high transparency and low resistance for the development of next-generation flexible optoelectronic devices. Furthermore, such conductive films based on the interconnected AgNW networks can be acted as an ideal supporter to construct heterogeneous structures with other functional materials for wide applications in photocatalysis and electrocatalysis.

Structural Integrity Preserving and Residue-Free Transfer of Large-Area Wrinkled Graphene onto Polymeric Substrates

Wrinkled graphene offers many advantageous features resulting from modifying the structural and physical properties as well as the chemical reactivity of graphene. However, its inadequate transferability to other substrates has limited its usability. This paper reports a roll-based clean transfer approach that enables the damage-free and contamination-free transfer of large-area wrinkled graphene onto polymeric substrates without compromising the integrity of wrinkle structures. The method implements the simultaneous imidazole-assisted etching and doping of chemical vapor-deposited graphene to fabricate multilayer graphene on a thermoplastic polystyrene (PS) substrate coated with a water-soluble poly(4-styrenesulfonic acid) (PSS) sacrificial layer via a roll-based transfer process. The compliant PSS layer affords the conformal contact between the PS substrate and graphene during the wrinkle formation process, enabling the controllable fabrication of graphene wrinkle structures on a large area. The water-soluble properties of PSS simplify the typically difficult separation of wrinkled graphene from the PS substrate after its transfer onto a target substrate. This improves the transferability of wrinkled graphene, rendering the transfer process solvent-free and residue-free. This work demonstrates the feasibility of the formulated method by transferring centimeter-scale wrinkled graphene onto currently used transparent flexible substrates (i.e., polyethylene terephthalate and polydimethylsiloxane). The results indicate that the transferred wrinkled graphene possesses the desirable combination of superior stretchability, optical transmittance, sheet resistance, and electromechanical stability, rendering its suitable application to transparent flexible and stretchable electronics.

Highly Transparent and Flexible Zn-Ti3C2Tx MXene Hybrid Capacitors

With the development of transparent and wearable electronic devices, energy supply units with high transmittance and flexibility, long cycle life, and high power and energy density are urgently needed. Zinc ion hybrid capacitors (ZIHCs) combined with the advantages of both supercapacitors and zinc ion batteries are promising energy supply components in the abovementioned devices. In addition, the preparation of multifunctional devices has become a trend for the need of space- and resource-saving. Therefore, obtaining ZIHCs with high transmittance and exploring their potential applications are meaningful challenges. Herein, a transparent and flexible ZIHC composed of a patterned zinc foil anode, transparent MXene cathode, and ZnSO₄-polyacrylamide (PAM) hydrogel electrolyte is designed and realized. The ZIHC exhibits a superior capacitance of 318 μF cm^-2 (5 mV s^-1) with 94% transmittance and retains 76% of the initial capacitance after 10,000 charge-discharge cycles. It also shows excellent flexibility, i.e., its capacitance has no obvious attenuation under different bending states. Interestingly, the leakage current of the ZIHC is highly sensitive to electric fields, which shows potential application in electric field detection. This work presents a method to realize the multifunctional ZIHC with electric field sensing function for transparent and flexible wearable devices in the future.

Recent Advances in Structure Separation of Single-Wall Carbon Nanotubes and Their Application in Optics, Electronics, and Optoelectronics

Structural control of single-wall carbon nanotubes (SWCNTs) with uniform properties is critical not only for their property modulation and functional design but also for applications in electronics, optics, and optoelectronics. To achieve this goal, various separation techniques have been developed in the past 20 years through which separation of high-purity semiconducting/metallic SWCNTs, single-chirality species, and even their enantiomers have been achieved. This progress has promoted the property modulation of SWCNTs and the development of SWCNT-based optoelectronic devices. Here, the recent advances in the structure separation of SWCNTs are reviewed, from metallic/semiconducting SWCNTs, to single-chirality species, and to enantiomers by several typical separation techniques and the application of the corresponding sorted SWCNTs. Based on the separation procedure, efficiency, and scalability, as well as, the separable SWCNT species, purity, and quantity, the advantages and disadvantages of various separation techniques are compared. Combined with the requirements of SWCNT application, the challenges, prospects, and development direction of structure separation are further discussed.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Electrode materials for stretchable triboelectric nanogenerator in wearable electronics

Stretchable Triboelectric Nanogenerators (TENGs) for wearable electronics are in significant demand in the area of self-powered energy harvesting and storage devices. Designing a suitable electrode is one of the major challenges in developing a fully wearable TENG device and requires research aimed at exploring new materials and methods to develop stretchable electrodes. This review article is dedicated to presenting recent developments in exploring new materials for flexible TENGs with special emphasis on electrode components for wearable devices. In addition, materials that can potentially deliver properties such as transparency, self-healability and water-resistance are also reviewed. Inherently stretchable materials and a combination of soft and rigid materials including polymers and their composites, inorganic and ceramic materials, 2D materials and carbonaceous nanomaterials are also addressed. Additionally, various fabrication strategies and geometrical patterning techniques employed for designing highly stretchable electrodes for wearable TENG devices are also explored. The challenges reflected in the present approaches as well as feasible suggestions for future advancements are discussed.

Schematic illustration of the general requirements of components of a wearable TENG.

Flexible Metal Electrodes by Femtosecond Laser-Activated Deposition for Human-Machine Interfaces

Flexible metal electrodes are essential for flexible electronics, where the main challenge is to obtain mask-free patterned metals directly on substrates such as poly(dimethylsiloxane) (PDMS) at low cost. This work highlights a feasible strategy named femtosecond laser-activated metal deposition for electroless deposition of metals (Cu, Ni, Ag, and Au) on PDMS, which is suitable for maskless and low-cost fabrication of metal layers on PDMS and even on other materials of different natures including polyethylene terephthalate, paper, Si, and glass. The electrical conductivity of the PDMS/Cu electrode is comparable to that of bulk Cu. Moreover, robust bonding at the PDMS/Cu interface is evidenced by a scotch tape test and bending test of more than 20,000 cycles. Compared with previous studies using a nanosecond laser, the restriction on absorbing sensitizers could be alleviated, and catalysts could originate from precursors without polymer substrates under a femtosecond laser, which may be attributed to nonlinear absorption and ultrashort heating time with the femtosecond laser. Implementing a human-machine interface task is demonstrated by recognizing hand gestures via a multichannel electrode array with high fidelity to control a robot hand.

Photosensitive-Stamp-Inspired Scalable Fabrication Strategy of Wearable Sensing Arrays for Noninvasive Real-Time Sweat Analysis

Wearable sweat sensing is essential to the development of personalized health monitoring in a noninvasive manner with molecular-level insight. Hence, there is an increasing demand for convenient, facile, and efficient fabrication of wearable sensing arrays. Inspired by a photosensitive stamp (PS), we present herein a simple, low-cost, and eco-friendly vacuum filtration-transfer printing method (termed PS-VFTP) for the scalable preparation of single-walled carbon nanotube (SWCNT) based flexible electrode arrays. This method can economically yield customized flexible SWCNT arrays with praiseworthy performance, such as high reproducibility, precision, uniformity, conductivity, and mechanical stability. In addition, the flexible SWCNT arrays can be easily functionalized into high-performance electrochemical sensors for the simultaneous monitoring of sweat metabolites (glucose, lactate) and electrolytes (Na⁺, K⁺). The integration of wearable sensing arrays with a signal acquisition and processing circuit system in the intelligent wearable sensors empowers them to realize noninvasive, real-time, and in situ sweat analysis during exercise. More meaningfully, such a PS-VFTP strategy can be easily expanded to the economical manufacturing of other flexible electronic devices.

Bioinspired Toughening Mechanisms in a Multilayer Transparent Conductor Structure

With increasing demands and interest in flexible and foldable devices, much effort has been devoted to the development of flexible transparent electrodes. An in-depth understanding of failure mechanisms in nanoscale structure is crucial in developing stable, flexible electronics with long-term durability. The present work investigated the mechanoelectric characteristics of transparent conductive electrodes in the form of dielectric/metal/dielectric (DMD) sandwich structures under bending, including one time and repeated cyclic bending test, and provides an explanation of their failure mechanism. We demonstrate how a thin metallic layer helps to enhance the mechanical robustness of the DMD as compared with that without, tune the mechanical properties of the cohesive layer, and improve the electrode fracture resistance. Abnormal crack propagation and toughening of multilayer DMD structures are analyzed, and its underlying mechanisms are explained. We consider the knowledge of the failure mechanisms of transparent conductive electrodes gained from the present study as a foundation for future design improvements.

Hydrothermal Unzipping of Multiwalled Carbon Nanotubes and Cutting of Graphene by Potassium Superoxide

The dual use of potassium superoxide (KO₂) to unzip multiwalled carbon nanotubes (MWCNTs) and cut graphene under hydrothermal conditions is described in this work. The KO₂-assisted hydrothermal treatment was proven to be a high-yield method for forming graphene nanoribbons and dots or sub-micro-sized graphene nanosheets. Starting with functionalized MWCNTs, the method produces water-dispersible graphene nanoribbons with characteristic photoluminescence depending on their width. Using pristine graphene, the hydrothermal treatment with KO₂ produces nanosized graphene sheets and graphene quantum dots with diameters of less than 10 nm. The latter showed a bright white photoluminescence. The effective hydrothermal unzipping of MWNTs and the cutting of large graphene nanosheets is a valuable top-down approach for the preparation of graphene nanoribbons and small nanographenes. Both products with limited dimensions have interesting applications in nanoelectronics and bionanotechnology.

Kinetics-Controlled Growth of Metallic Single-Wall Carbon Nanotubes from CoRex Nanoparticles

The controlled growth of metallic single-wall carbon nanotubes (m-SWCNTs) is very important for the fabrication of high-performance interconnecting wires, transparent conductive electrodes, light and conductive fibers, etc. However, it has been extremely difficult to synthesize m-SWCNTs due to their lower abundance and higher chemical reactivity than semiconducting SWCNTs (s-SWCNTs). Here, we report the kinetically controlled growth of m-SWCNTs by manipulating their binding energy with the catalyst and promoting their growth rate. We prepared CoRe₄ nanoparticles with a hexagonal close-packed structure and an average size of ∼2.3 nm, which have a lower binding energy with m-SWCNTs than with s-SWCNTs. The selective growth of m-SWCNTs from the CoRe₄ catalyst was achieved by using a low concentration of carbon source feed at a relative low temperature of 760 °C. The m-SWCNTs had a narrow diameter distribution of 1.1 ± 0.3 nm, and their content was over 80%.

Stable Doping of Single-Walled Carbon Nanotubes for Flexible Transparent Conductive Films

Possessing excellent electronic and mechanical properties and great stability, single-walled carbon nanotubes (SWCNTs) are exceptionally attractive in fabricating flexible transparent conductive films. Doping is a key step to further enhance the conductivity of the SWCNT films and the reliable doping is highly needed. We developed a feasible strategy that uses solid acids such as phosphotungstic acid (PTA) to dope the SWCNT films stably relying on the nonvolatility of the dopants. The sheet resistance of the films was reduced to around a half of the original value meanwhile with no obvious change in transmittance. The doping effect maintained during a 700 days' observation. The excellent flexibility of the PTA-doped films was demonstrated by a bending test of 1000 cycles, during which the sheet resistance and transmittance was basically unaffected. The blue shifts of G band in the Raman spectra and the increase of work function measured by the Kelvin probe force microscopy both reveal the p-type doping of the films by PTA. The strong acidity of PTA plays a key role in the doping effect by increasing the redox potential of the ambient O₂ and thus the Fermi level of the SWCNTs is brought down. The great feasibility and robustness of our doping strategy are desirable in the practical application of SWCNT-based flexible transparent conductive films. This strategy can be extended to the p-type doping of various CNT-based assemblies (such as sponges and forests) as well as other material families, expanding the application spectrum of polyacids.