TCO/metal/TCO structures for energy and flexible electronics

There is increasing attention paid to improving transparent conductive electrodes for applications in large area photovoltaic devices and displays that are being developed for energy and electronics. To date, transparent and conductive oxides (TCO) based on In₂O₃, ZnO, or SnO₂ are commonly used, but advanced devices require new electrodes with lower resistivities than previously achieved and with optical properties superior to those of the present generation. TCO/metal/TCO multilayer structures have emerged as an interesting alternative because they provide optical and electrical characteristics globally superior to those attainable with a single-layer TCO or metal electrode and can be deposited at low temperatures onto inexpensive plastic substrates. Indeed, the fabrication of thin film devices on flexible substrates has substantial interest for application to lightweight products and implementation of roll-to-roll deposition processes that can significantly reduce production costs. In this sense, organic electronics that require low deposition temperatures have the best chance to be the first transferred from conventional glass to inexpensive plastic substrates. The present critical review summarizes current TCO/metal/TCO research results, first analyzed for materials and thickness selection as a function of the optical transmittance and electrical resistance parameters, and then analyzed according to other important properties such as mechanical reliability and thermal and humidity stability. The review concludes with a brief discussion of the results obtained for TCO/metal/TCO structures applied as electrodes in several organic electronic devices.     

Field emission properties of RuO2 thin film coated on carbon nanotubes

Carbon nanotubes (CNTs) have been widely applied in field emission (FE) due to their high geometric aspect ratio and low work function. More recently, researchers have introduced ruthenium dioxide (RuO₂) as a field emitter because of its excellent chemical and thermal stability due to its oxide nature. This study used the surface morphology of CNTs and the field emission (FE) stability of RuO₂ to improve FE characteristics. Since the work functions of CNTs and RuO₂ are very close, this study combined these two elements by applying a thin film of RuO₂ on the CNT surface. In the process of covering the tips of CNTs with a thin film of RuO₂ eventually obtained the best matching of these two elements. The study not only enhanced the FE performance of CNTs but also extended FE lifetime by applying a thin film of RuO₂ on the CNT tips.     

Transparentes leitfähiges TiO2:Nb – ein TCO‐Material der Zukunft?

Transparent conductive TiO₂:Nb – a TCO material of the future? Niobium doped titania is a widely unknown TCO material. We present the results of TiO₂:Nb thin films deposited by DC and pulse DC sputtering from a ceramic titania target with a content of approximately 6 wt.‐percent Nb in a pilot scale in‐line sputtering plant. With DC sputtering on Borofloat 33 substrate a 100 nm thin film show after annealing at 450 °C a resistivity of 7.2 × 10^?4 Ωcm with a low extinction coefficient of 0.02 and a transmittance in the visible range of 74.8%.     

Negative ions in reactive magnetron sputtering

Negative ions are present in magnetron sputtering if electronegative elements are involved. The majority of the negative ions impinging on transparent conductive oxide (TCO) films during growth is O^? produced at the oxidised target surface while it is rather unimportant whether it is bulk oxide or a surface oxide formed in reactive sputtering. O^? bombards the film with energies equivalent to the target voltage and by far exceed 100 eV. It is hence apt to cause radiation damage in sensitive TCO films. This is shown by the lateral distribution of the highenergy O^? flux in planar magnetron sputtering that exhibits the same pattern imaging the erosion groove as the resistivity of TCO films. This lateral inhomogeneity strongly depends on the erosion groove depth. The emission of O^? further depends on the sputtered material, for TiO₂ deposition it is much less than for other TCO materials. The emission probability correlates to the secondary electron emission coefficient of the oxide.     

Aligning Solution‐Derived Carbon Nanotube Film with Full Surface Coverage for High‐Performance Electronics Applications

The main challenge for application of solution‐derived carbon nanotubes (CNTs) in high performance field‐effect transistor (FET) is how to align CNTs into an array with high density and full surface coverage. A directional shrinking transfer method is developed to realize high density aligned array based on randomly orientated CNT network film. Through transferring a solution‐derived CNT network film onto a stretched retractable film followed by a shrinking process, alignment degree and density of CNT film increase with the shrinking multiple. The quadruply shrunk CNT films present well alignment, which is identified by the polarized Raman spectroscopy and electrical transport measurements. Based on the high quality and high density aligned CNT array, the fabricated FETs with channel length of 300 nm present ultrahigh performance including on‐state current I_on of 290 µA µm^?1 (V_ds = ?1.5 V and V_gs = ?2 V) and peak transconductance g_m of 150 µS µm^?1, which are, respectively, among the highest corresponding values in the reported CNT array FETs. High quality and high semiconducting purity CNT arrays with high density and full coverage obtained through this method promote the development of high performance CNT‐based electronics.     

Effects of Conformal Nanoscale Coatings on Thermal Performance of Vertically Aligned Carbon Nanotubes

The high aspect ratio and the porous nature of spatially oriented forest‐like carbon nanotube (CNT) structures represent a unique opportunity to engineer a novel class of nanoscale assemblies. By combining CNTs and conformal coatings, a 3D lightweight scaffold with tailored behavior can be achieved. The effect of nanoscale coatings, aluminum oxide (Al₂O₃) and nonstoichiometric amorphous silicon carbide (a‐SiC), on the thermal transport efficiency of high aspect ratio vertically aligned CNTs, is reported herein. The thermal performance of the CNT‐based nanostructure strongly depends on the achieved porosity, the coating material and its infiltration within the nanotube network. An unprecedented enhancement in terms of effective thermal conductivity in a‐SiC coated CNTs has been obtained: 181% compared to the as‐grown CNTs and Al₂O₃ coated CNTs. Furthermore, the integration of coated high aspect ratio CNTs in an epoxy molding compound demonstrates that, next to the required thermal conductivity, the mechanical compliance for thermal interface applications can also be achieved through coating infiltration into foam‐like CNT forests.     

Field emission properties of RuO2 thin film coated on carbon nanotubes

Carbon nanotubes (CNTs) have been widely applied in field emission (FE) due to their high geometric aspect ratio and low work function. More recently, researchers have introduced ruthenium dioxide (RuO₂) as a field emitter because of its excellent chemical and thermal stability due to its oxide nature. This study used the surface morphology of CNTs and the field emission (FE) stability of RuO₂ to improve FE characteristics. Since the work functions of CNTs and RuO₂ are very close, this study combined these two elements by applying a thin film of RuO₂ on the CNT surface. In the process of covering the tips of CNTs with a thin film of RuO₂ eventually obtained the best matching of these two elements. The study not only enhanced the FE performance of CNTs but also extended FE lifetime by applying a thin film of RuO₂ on the CNT tips.     

Golden Bristlegrass‐Like Hierarchical Graphene Nanofibers Entangled with N‐Doped CNTs Containing CoSe2 Nanocrystals at Each Node as Anodes for High‐Rate Sodium‐Ion Batteries

Golden bristlegrass‐like unique nanostructures comprising reduced graphene oxide (rGO) matrixed nanofibers entangled with bamboo‐like N‐doped carbon nanotubes (CNTs) containing CoSe₂ nanocrystals at each node (denoted as N‐CNT/rGO/CoSe₂ NF) are designed as anodes for high‐rate sodium‐ion batteries (SIBs). Bamboo‐like N‐doped CNTs (N‐CNTs) are successfully generated on the rGO matrixed nanofiber surface, between rGO sheets and mesopores, and interconnected chemically with homogeneously distributed rGO sheets. The defects in the N‐CNTs formed by a simple etching process allow the complete phase conversion of Co into CoSe₂ through the efficient penetration of H₂Se gas inside the CNT walls. The N‐CNTs bridge the vertical defects for electron transfer in the rGO sheet layers and increase the distance between the rGO sheets during cycles. The discharge capacity of N‐CNT/rGO/CoSe₂ NF after the 10 000th cycle at an extremely high current density of 10 A g^?1 is 264 mA h g^?1, and the capacity retention measured at the 100th cycle is 89%. N‐CNT/rGO/CoSe₂ NF has final discharge capacities of 395, 363, 328, 304, 283, 263, 246, 223, 197, 171, and 151 mA h g^?1 at current densities of 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 A g^?1, respectively.     

Electrochemical investigation of TiO2/carbon nanotubes nanocomposite as anode materials for lithium-ion batteries

Mechanically blended composite of nanosized TiO₂ and carbon nanotubes (CNTs) was investigated as potential anode materials for Li-ion batteries. It was found that the TiO₂/CNTs nanocomposite exhibits an improved cycling stability and higher reversible capacity than CNTs. The reversible capacity of the TiO₂/CNTs composite reaches 168 mAh g^− 1 at the first cycle and remains almost constant during long-term cycling. The electrochemical results show that the TiO₂ nanoparticles in the composite not only restrain the formation of surface film, but also make a contribution to the overall reversible capacity.     

Flexible Graphene‐Wrapped Carbon Nanotube/Graphene@MnO2 3D Multilevel Porous Film for High‐Performance Lithium‐Ion Batteries

The ingenious design of a freestanding flexible electrode brings the possibility for power sources in emerging wearable electronic devices. Here, reduced graphene oxide (rGO) wraps carbon nanotubes (CNTs) and rGO tightly surrounded by MnO₂ nanosheets, forming a 3D multilevel porous conductive structure via vacuum freeze‐drying. The sandwich‐like architecture possesses multiple functions as a flexible anode for lithium‐ion batteries. Micrometer‐sized pores among the continuously waved rGO layers could extraordinarily improve ion diffusion. Nano‐sized pores among the MnO₂ nanosheets and CNT/rGO@MnO₂ particles could provide vast accessible active sites and alleviate volume change. The tight connection between MnO₂ and carbon skeleton could facilitate electron transportation and enhance structural stability. Due to the special structure, the rGO‐wrapped CNT/rGO@MnO₂ porous film as an anode shows a high capacity, excellent rate performance, and superior cycling stability (1344.2 mAh g⁻¹ over 630 cycles at 2 A g⁻¹, 608.5 mAh g⁻¹ over 1000 cycles at 7.5 A g⁻¹). Furthermore, the evolutions of microstructure and chemical valence occurring inside the electrode after cycling are investigated to illuminate the structural superiority for energy storage. The excellent electrochemical performance of this freestanding flexible electrode makes it an attractive candidate for practical application in flexible energy storage.     

Oxygen Evolution Assisted Fabrication of Highly Loaded Carbon Nanotube/MnO2 Hybrid Films for High‐Performance Flexible Pseudosupercapacitors

To date, it has been a great challenge to design high‐performance flexible energy storage devices for sufficient loading of redox species in the electrode assemblies, with well‐maintained mechanical robustness and enhanced electron/ionic transport during charge/discharge cycles. An electrochemical activation strategy is demonstrated for the facile regeneration of carbon nanotube (CNT) film prepared via floating catalyst chemical vapor deposition strategy into a flexible, robust, and highly conductive hydrogel‐like film, which is promising as electrode matrix for efficient loading of redox species and the fabrication of high‐performance flexible pseudosupercapacitors. The strong and conductive CNT films can be effectively expanded and activated by electrochemical anodic oxygen evolution reaction, presenting greatly enhanced internal space and surface wettability with well‐maintained strength, flexibility, and conductivity. The as‐formed hydrogel‐like film is quite favorable for electrochemical deposition of manganese dioxide (MnO₂) with loading mass up to 93 wt% and electrode capacitance kept around 300 F g^?1 (areal capacitance of 1.2 F cm^?2). This hybrid film was further used to assemble a flexible symmetric pseudosupercapacitor without using any other current collectors and conductive additives. The assembled flexible supercapacitors exhibited good rate performance, with the areal capacitance of more than 300 mF cm^?2, much superior to other reported MnO₂ based flexible thin‐film supercapacitors.     

NixMnyCozO Nanowire/CNT Composite Microspheres with 3D Interconnected Conductive Network Structure via Spray‐Drying Method: A High‐Capacity and Long‐Cycle‐Life Anode Material for Lithium‐Ion Batteries

The combination of high‐capacity and long‐term cycling stability is an important factor for practical application of anode materials for lithium‐ion batteries. Herein, Ni_xMn_yCo_zO nanowire (x + y + z = 1)/carbon nanotube (CNT) composite microspheres with a 3D interconnected conductive network structure (3DICN‐NCS) are prepared via a spray‐drying method. The 3D interconnected conductive network structure can facilitate the penetration of electrolyte into the microspheres and provide excellent connectivity for rapid Li⁺ ion/electron transfer in the microspheres, thus greatly reducing the concentration polarization in the electrode. Additionally, the empty spaces among the nanowires in the network accommodate microsphere volume expansion associated with Li⁺ intercalation during the cycling process, which improves the cycling stability of the electrode. The CNTs distribute uniformly in the microspheres, which act as conductive frameworks to greatly improve the electrical conductivity of the microspheres. As expected, the prepared 3DICN‐NCS demonstrates excellent electrochemical performance, showing a high capacity of 1277 mAh g^?1 at 1 A g^?1 after 2000 cycles and 790 mAh g^?1 at 5 A g^?1 after 1000 cycles. This work demonstrates a universal method to construct a 3D interconnected conductive network structure for anode materials     

Effects of atomic layer deposited HfO2 compact layer on the performance of dye-sensitized solar cells

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the photoelectrode back into liquid electrolyte. An atomic layer deposited (ALD) hafnium oxide (HfO₂) ultra thin compact layer was grown on the surface of the transparent conducting oxide (TCO) and its effects on the DSSC performance were studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy measurements. It was found that this compact layer was effectively blocking the back-reaction of electrons from TCO to the liquid electrolyte, resulting in the overall photoconversion efficiency being enhanced by 66% compared to a DSSC with a conventional sol-gel processed TiO₂ compact layer. Reasons for the improved photovoltaic performance were attributed to passivation of the TCO surface, better electronic quality of the compact layer material and the higher compactness, shown by atomic force microscopic images, obtained from gas-based deposition methods. Also, an increased short-circuit current density suggests that the interfacial resistance for the injection of electrons from the porous nanoparticle network to TCO was reduced. Further, the theory of electron recombination at the TCO/compact layer/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO₂ compact layer.     

Advantages of transparent conducting oxide thin films with controlled permittivity for thin film photovoltaic solar cells

Our recent investigations have identified a pathway to produce transparent conducting oxide (TCO) films that demonstrate higher infrared transparency. The technique involves controlling the dielectric permittivity of the TCO film such that the electrical properties are maintained, but the plasma frequency (ω_p) is shifted to longer wavelength. This has the effect of reducing free-carrier absorption in the visible and near-infrared spectral region, thus producing a TCO film with higher optical transmission. The technique has been demonstrated for sputtered films of indium tin oxide by adding small amounts of ZrO₂ to a ceramic sputtering target, and for SnO₂:F films deposited by chemical vapor deposition using a metalorganic Zr source.     

Transparente und leitfähige Veredelung von Kunststoffoberflächen

Deposition Techniques for Transparent Conducting Thin‐Films on Glass and Polymer Substrates We report on thin films deposited at atmospheric pressures on glass and polymer substrates with various techniques. The introduced thin‐film materials show intrinsic properties being suitable for different applications while maintaining the principle properties of the substrates themselves (e. g. shape. rigidity/flexibility, transparency). With the main focus on optical and electronic applications the properties of the deposited films can be adjusted by the choice of coating material (e. g. metal oxide, CNT), the film's shape (compact, particulate) and the deposition process itself. We compare deposition and properties of different TCO‐materials with CNT‐based thin film techniques and demonstrate approaches for the integration of these processes in production lines.     

Bioactive ceramic coatings containing carbon nanotubes on metallic substrates by electrophoretic deposition

A range of potentially bioactive ceramic coatings, based on combinations of either hydroxyapatite (HA) or titanium oxide nanoparticles with carbon nanotubes (CNTs), have been deposited on metallic substrates, using electrophoretic deposition (EPD). Sol–gel derived, ultrafine HA powders (10–70 nm) were dispersed in multi-wall nanotube-containing ethanol suspensions maintained at pH = ∼3.5 and successfully coated onto Ti alloy wires at 20 V for 1–3 min For TiO₂/CNT coatings, commercially available titania nanopowders and surface-treated CNTs in aqueous suspensions were co-deposited on stainless steel planar substrates. A field strength of 20 V/cm and deposition time of 4 min were used working at pH = 5. Although the co-deposition mechanism was not investigated in detail, the evidence suggests that co-deposition occurs due to the opposite signs of the surface charges (zeta potentials) of the particles, at the working pH. Electrostatic attraction between CNTs and TiO₂ particles leads to the creation of composite particles in suspension, consisting of TiO₂ particles homogenously attached onto the surface of individual CNTs. Under the applied electric field, these net negatively charged “composite TiO₂/CNT” elements migrate to and deposit on the anode (working electrode). The process of EPD at constant voltage conditions was optimised in both systems to achieve homogeneous and reasonably adhered deposits of varying thicknesses on the metallic substrates_.     

One‐Step Transfer and Integration of Multifunctionality in CVD Graphene by TiO2/Graphene Oxide Hybrid Layer

We present a straightforward method for simultaneously enhancing the electrical conductivity, environmental stability, and photocatalytic properties of graphene films through one‐step transfer of CVD graphene and integration by introducing TiO₂/graphene oxide layer. A highly durable and flexible TiO₂ layer is successfully used as a supporting layer for graphene transfer instead of the commonly used PMMA. Transferred graphene/TiO₂ film is directly used for measuring the carrier transport and optoelectronic properties without an extra TiO₂ removal and following deposition steps for multifunctional integration into devices because the thin TiO₂ layer is optically transparent and electrically semiconducting. Moreover, the TiO₂ layer induces charge screening by electrostatically interacting with the residual oxygen moieties on graphene, which are charge scattering centers, resulting in a reduced current hysteresis. Adsorption of water and other chemical molecules onto the graphene surface is also prevented by the passivating TiO₂ layer, resulting in the long term environmental stability of the graphene under high temperature and humidity. In addition, the graphene/TiO₂ film shows effectively enhanced photocatalytic properties because of the increase in the transport efficiency of the photogenerated electrons due to the decrease in the injection barrier formed at the interface between the F‐doped tin oxide and TiO₂ layers.     

Ultrafast Flame Annealing of TiO2 Paste for Fabricating Dye‐Sensitized and Perovskite Solar Cells with Enhanced Efficiency

Mesoporous TiO₂ nanoparticle (NP) films are broadly used as electrodes in photoelectrochemical cells, dye‐sensitized solar cells (DSSCs), and perovskite solar cells (PSCs). State‐of‐the‐art mesoporous TiO₂ NP films for these solar cells are fabricated by annealing TiO₂ paste‐coated fluorine‐doped tin oxide glass in a box furnace at 500 °C for ≈30 min. Here, the use of a nontraditional reactor, i.e., flame, is reported for the high throughput and ultrafast annealing of TiO₂ paste (≈1 min). This flame‐annealing method, compared to conventional furnace annealing, exhibits three distinct benefits. First, flame removes polymeric binders in the initial TiO₂ paste more completely because of its high temperature (≈1000 °C). Second, flame induces strong interconnections between TiO₂ nanoparticles without affecting the underlying transparent conducting oxide substrate. Third, the flame‐induced carbothermic reduction on the TiO₂ surface facilitates charge injection from the dye/perovskite to TiO₂. Consequently, when the flame‐annealed mesoporous TiO₂ film is used to fabricate DSSCs and PSCs, both exhibit enhanced charge transport and higher power conversion efficiencies than those fabricated using furnace‐annealed TiO₂ films. Finally, when the ultrafast flame‐annealing method is combined with a fast dye‐coating method to fabricate DSSC devices, its total fabrication time is reduced from over 3 h to ≈10 min.     

TiO2 thin films using organic liquid materials prepared by Hot-Wire CVD method

TiO₂ thin films prepared by Hot-Wire CVD method have been studied as a protecting material of transparent conducting oxide (TCO) against atomic hydrogen exposures for the fabrications of Si thin film solar cells. It was found that electrical conductivity of the films at room temperature reached a value of 0.4 S/cm. This value is 2-3 orders of magnitude higher than that of TiO₂ films prepared by RF magnetron sputtering and electron-beam evaporation methods in our previous works. The conductivity improvement seems to be partly due to the enlargement of TiO₂ crystallites.     

The effect of temperature on the growth of carbon nanotubes on copper foil using a nickel thin film as catalyst

Growth of carbon nanotubes (CNTs) on bulk copper foil substrates has been achieved by sputtering a nickel thin film on Cu substrates followed by thermal chemical vapor deposition. The characteristics of the nanotubes are strongly dependent on the Ni film thickness and reaction temperature. Specifically, a correlation between the thin film nickel catalyst thickness and the CNT diameter was found. Two hydrocarbon sources investigated were methane and acetylene to determine the best conditions for growth of CNTs on copper. These results demonstrate the effectiveness of this simple method of directly integrating CNTs with highly conductive substrates for use in applications where a conductive CNT network is desirable.