Thin‐Film Electrodes: Microscopic and Nanoscopic Three‐Phase‐Boundaries of Platinum Thin Film Electrodes on YSZ Electrolyte (Adv. Funct. Mater. 3/2011)

Agglomerated Pt thin films have been proposed as electrodes for electrochemical devices like micro‐solid oxide fuel cells (μ‐SOFCs) operating at low temperatures. However, comprehensive studies elucidating the interplay between agglomeration state and electrochemical properties are lacking. In this contribution the electrochemical performance of agglomerated and “dense” Pt thin film electrodes on yttria‐stabilized‐zirconia (YSZ) is correlated with their microstructural characteristics. Besides the microscopically measurable triple‐phase‐boundary (tpb) where Pt, YSZ and air are in contact, a considerable contribution of “nanoscopic” tpbs to the electrode conductivity resulting from oxygen permeable grain boundaries is identified. It is demonstrated that “dense” Pt thin films are excellent electrodes provided their grain size and thickness are in the nanometer range. The results disprove the prevailing idea that the performance of Pt thin film electrodes results from microscopic and geometrically measurable tpbs only.     

Interconnection between Trait,Structure, and Composition of Grain Boundaries in Cu(In,Ga)Se2 Thin‐Film Solar Cells

Grain boundaries (GBs) are crucial for solar cells incorporating polycrystalline absorbers and particularly for those characterized by small grain sizes (≈2 µm). For example, random GBs in Si solar cells are found to have a detrimental effect on the cell performance being characterized by an increased recombination activity relative to grains. Yet, their role in Cu(In,Ga)Se₂ (CIGS) solar cells still remains controversial. The recent electron‐beam‐induced current (EBIC) study shows that 58% of the GBs in CIGS exhibit enhanced electrical properties considered to be benign (for the device performance). Yet, they coexist with 16% detrimental GBs (reduced electrical properties) and 27% neutral ones (no change in electrical property when compared with the bulk). In the present study, these different GBs are investigated by combining EBIC with electron backscattered diffraction and atom probe tomography techniques on identical GBs. For the first time, a successful correlation is shown (for any device) that interconnects the GB characteristics to its composition. Sufficient statistics demonstrate that the collective fluctuations of all elements at GBs determine its trait. In general, benign (detrimental) GBs are characterized by Cu depletion (enrichment) that favored the formation of donor (acceptor) defects.     

Fabrication of Organic Thin‐Film Transistors on Three‐Dimensional Substrates Using Free‐Standing Polymeric Masks Based on Soft Lithography

Here, a novel fabrication technique for integrated organic devices on substrates with complex structure is presented. For this work, free‐standing polymeric masks with stencil‐patterns are fabricated using an ultra‐violet (UV) curable polyurethaneacrylate (PUA) mixture, and used as shadow masks for thermal evaporation. High flexibility and adhesive properties of the free‐standing PUA masks ensure conformal contact with various materials such as glass, silicon (Si), and polymer, and thus can also be utilized as patterning masks for solution‐based deposition methods, such as spin‐coating and drop‐casting. Based on this technique, a number of integrated organic transistors are fabricated simultaneously on a cylindrical glass bottle with high curvature, as well as on a flat silicon wafer. It is anticipated that these results will be applied to the development of various integrated organic devices on complex‐structured substrates, which can lead to further applications.     

Self‐Adhesive and Ultra‐Conformable,Sub‐300 nm Dry Thin‐Film Electrodes for Surface Monitoring of Biopotentials

Accurate and unobtrusive monitoring of surface biopotentials is of paramount importance for physiological studies and wearable healthcare applications. Thin, light‐weight, and conformal bioelectrodes are highly desirable for biopotential monitoring. This report demonstrates the fabrication of sub‐300 nm thin, dry electrodes that are self‐adhesive and conformable to complex 3D biological surfaces and thus capable of excellent quality of biopotential (surface electromyogram and surface electrocardiogram) recordings. Measurements reveal single‐day stability of up to 10 h. In addition, the bending stiffness of the sensor is calculated to be ≈0.33 pN m², which is comparable to stratum corneum, the uppermost layer of human skin, and this stiffness is over two orders of magnitude lower than the bending stiffness of a 3.0 µm thin sensor. Laminated on a prestretched elastomer, when relaxed, the sensor forms wrinkles with a period and amplitude equal to 17 and 4 µm, respectively, which these values agree with theoretical calculations. Finally, with skin vibrations of up to ≈15 µm, the sensor exhibits motion artifact‐less monitoring of surface biopotentials, in contrast to a wet adhesive electrode that shows much greater influence.     

Resolving the Three‐Dimensional Microstructure of Polymer Electrolyte Fuel Cell Electrodes using Nanometer‐Scale X‐ray Computed Tomography

The electrodes of a polymer electrolyte fuel cell (PEFC) are composite porous layers consisting of carbon and platinum nanoparticles and a polymer electrolyte binder. The proper composition and arrangement of these materials for fast reactant transport and high electrochemical activity is crucial to achieving high performance, long lifetimes, and low costs. Here, the microstructure of a PEFC electrode using nanometer‐scale X‐ray computed tomography (nano‐CT) with a resolution of 50 nm is investigated. The nano‐CT instrument obtains this resolution for the low‐atomic‐number catalyst support and binder using a combination of a Fresnel zone plate objective and Zernike phase contrast imaging. High‐resolution, non‐destructive imaging of the three‐dimensional (3D) microstructures provides important new information on the size and form of the catalyst particle agglomerates and pore spaces. Transmission electron microscopy (TEM) and mercury intrusion porosimetry (MIP) is applied to evaluate the limits of the resolution and to verify the 3D reconstructions. The computational reconstructions and size distributions obtained with nano‐CT can be used for evaluating electrode preparation, performing pore‐scale simulations, and extracting effective morphological parameters for large‐scale computational models.     

All‐Solution‐Processed Indium‐Free Transparent Composite Electrodes based on Ag Nanowire and Metal Oxide for Thin‐Film Solar Cells

Fully solution‐processed Al‐doped ZnO/silver nanowire (AgNW)/Al‐doped ZnO/ZnO multi‐stacked composite electrodes are introduced as a transparent, conductive window layer for thin‐film solar cells. Unlike conventional sol–gel synthetic pathways, a newly developed combustion reaction‐based sol–gel chemical approach allows dense and uniform composite electrodes at temperatures as low as 200 °C. The resulting composite layer exhibits high transmittance (93.4% at 550 nm) and low sheet resistance (11.3 Ω sq^‐1), which are far superior to those of other solution‐processed transparent electrodes and are comparable to their sputtered counterparts. Conductive atomic force microscopy reveals that the multi‐stacked metal‐oxide layers embedded with the AgNWs enhance the photocarrier collection efficiency by broadening the lateral conduction range. This as‐developed composite electrode is successfully applied in Cu(In_1‐x,Ga_x)S₂ (CIGS) thin‐film solar cells and exhibits a power conversion efficiency of 11.03%. The fully solution‐processed indium‐free composite films demonstrate not only good performance as transparent electrodes but also the potential for applications in various optoelectronic and photovoltaic devices as a cost‐effective and sustainable alternative electrode.     

High‐Performance and Stable Organic Thin‐Film Transistors Based on Fused Thiophenes

A series of new organic semiconductors for organic thin‐film transistors (OTFTs) using dithieno[3,2‐b:2′,3′‐d]thiophene as the core are synthesized. Their electronic and optical properties are investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV‐vis and photoluminescence spectroscopies, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The compounds exhibit an excellent field‐effect performance with a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. XRD patterns reveal these films, grown by vacuum deposition, to be highly crystalline, and SEM reveals well‐interconnected, microcrystalline domains in these films at room temperature. TGA and DSC demonstrate that the phenyl‐substituted compounds possess excellent thermal stability. Furthermore, weekly shelf‐life tests (under ambient conditions) of the OTFTs based on the phenyl‐substituted compounds show that the mobility for the bis(diphenyl)‐substituted thiophene was almost unchanged for more than two months, indicating a high environmental stability.     

Functionalization of Nanostructured Hematite Thin‐Film Electrodes with the Light‐Harvesting Membrane Protein C‐Phycocyanin Yields an Enhanced Photocurrent

The integration of light‐harvesting proteins and other photosynthetic molecular machinery with semiconductor surfaces plays an important role in improving their performance as solar‐cell materials. Phycocyanin is one such protein that can be employed for this purpose. Phycocyanins have light‐harvesting properties and belong to the phycobilisome protein family. They are present in cyanobacteria, which capture light energy and funnel it to reaction centers during photosynthesis. Here, a way of increasing the photocurrent of hematite by covalent cross‐coupling with phycocyanin is reported. For this, a hematite–phycocyanin integrated system is assembled by consecutive adsorption and cross‐coupling of protein molecules, separated by an agarose layer and a linker molecule, on the top of a mesoporous hematite film. The hematite–phycocyanin assembly shows a two‐fold increased photocurrent in comparison with pristine hematite film. The increase in the photocurrent is attributed to the enhanced light absorption of the hematite film after integration with the protein, as is evident from the UV–vis spectra and from the photocurrent‐action spectrum. The assembly shows long‐term stability and thus constitutes a promising hybrid photoanode for photo‐electrochemical applications.     

n‐Type Quinoidal Oligothiophene‐Based Semiconductors for Thin‐Film Transistors and Thermoelectrics

Organic electronic devices have gained immense popularity in the last 30 years owing to their increasing performance. Organic thin‐film transistors (OTFTs) are one of the basic organic electronic devices with potential industrial applications. Another class of devices called organic thermoelectric (OTE) materials can directly transform waste heat into usable electrical power without causing any pollution. p‐Type transistors outperform n‐type transistors because the latter requires a lower orbital energy level for efficient electron injection and stable electron transport under ambient conditions. Aromatic building blocks can be utilized in constructing n‐type semiconductors. Quinoidal compounds are another promising platform for optoelectronic applications because of their unique properties. Since their discovery in 1970s, quinoidal oligothiophene‐based n‐type semiconductors have drawn considerable attention as candidates for high‐performance n‐type semiconductors in OTFTs and OTEs. Herein, the development history of quinoidal oligothiophene‐based semiconductors is summarized, with a focus on the molecular design and the influence of structural modification on molecular packing and thus the device performance of the corresponding quinoidal oligothiophene‐based semiconductors. Insights on the potential of quinoidal oligothiophenes for high‐performance n‐type OTFTs and OTEs are also provided.     

Insights into Nanoscale Electrochemical Reduction in a Memristive Oxide: the Role of Three‐Phase Boundaries

The nanoscale electro‐reduction in a memristive oxide is a highly relevant field for future non‐volatile memory materials. Photoemission electron microscopy is used to identify the conducting filaments and correlate them to structural features of the top electrode that indicate a critical role of the three phase boundary (electrode‐oxide‐ambient) for the electro‐chemical reduction. Based on simulated temperature profiles, the essential role of Joule heating through localized currents for electro‐reduction and morphology changes is demonstrated.     

Vertically Segregated Structure and Properties of Small Molecule–Polymer Blend Semiconductors for Organic Thin‐Film Transistors

A comprehensive structure and performance study of thin blend films of the small‐molecule semiconductor, 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene (diF‐TESADT), with various insulating binder polymers in organic thin‐film transistors is reported. The vertically segregated composition profile and nanostructure in the blend films are characterized by a combination of complementary experimental methods including grazing incidence X‐ray diffraction, neutron reflectivity, variable angle spectroscopic ellipsometry, and near edge X‐ray absorption fine structure spectroscopy. Three polymer binders are considered: atactic poly(α‐methylstyrene), atactic poly(methylmethacrylate), and syndiotactic polystyrene. The choice of polymer can strongly affect the vertical composition profile and the extent of crystalline order in blend films due to the competing effects of confinement entropy, interaction energy with substrate surfaces, and solidification kinetics. The variations in the vertically segregated composition profile and crystalline order in thin blend films explain the significant impacts of binder polymer choice on the charge carrier mobility of these films in the solution‐processed bottom‐gate/bottom‐contact thin‐film transistors.     

Controlled Release and Absorption Resonance of Fluorescent Silica‐Coated Platinum Nanoparticles

Aggregation‐driven seeded growth of uniform platinum nanoparticles and exclusive silica‐coating of the as‐grown platinum nanoparticles have been achieved successfully. Fluorescent Pt@SiO₂ nanoparticles have also been reproducibly prepared via effective co‐condensation of silanized dye molecules with tetraethylorthosilicate. The dye‐loaded Pt@SiO₂ nanoparticles have been exploited as model carriers for thermal‐responsive controlled release of guest molecules via slow hydrolysis/dissolution of silica shells; importantly, the encapsulated platinum cores have also been used as nanoprobes to simultaneously investigate their controlled release process. Meanwhile, unique size‐sensitive absorbance resonance in the dye‐loaded Pt@SiO₂ nanoparticles has been demonstrated for the first time, and it is expected to find novel application.     

Nanostructure/Swelling Relationships of Bulk and Thin‐Film PFSA Ionomers

Perfluorinated sulfonic acid (PFSA) ionomers are the most widely used solid electrolyte in electrochemical technologies due to their remarkable ionic conductivity with simultanous mechanical stability, imparted by their phase‐separated morphology. In this work, the morphology and swelling of PFSA ionomers (Nafion and 3M) as bulk membranes (>10 μm) and dispersion‐cast thin films (<100 nm) are investigated to identify the roles of equivalent weight (EW) and side‐chain length across lengthscales. Humidity‐dependent structural changes as well as different PFSA chemistries are explored in the thin‐film regime, allowing for the development of thickness‐EW phase diagrams. The ratio of macroscopic (thickness) to nanoscopic (domain spacing) swelling during hydration is found to be affine (1:1) in thin films, but increases as the thickness approaches bulk values, revealing the existence of a mesoscale organization governing the multiscale swelling in PFSAs. Ionomer chemistry, in particular EW, is found to play a key role in altering the confinement‐driven structural changes, including thin‐film anisotropy, with phase separation becoming weaker as the film thickness is reduced below 25 nm or as EW is increased. For the lower‐EW 3M PFSA ionomers, confinement appears to induce even stronger phase separation accompanied by domain alignment parallel to the substrate.     

Design of Novel Dielectric Surface Modifications for Perylene Thin‐Film Transistors

Dielectric surface modifications (DSMs) can improve the performance of organic thin‐film transistors (OTFTs) significantly. In order to gain a deeper understanding of this performance enhancement and to facilitate high‐mobility transistors, perylene based devices utilizing novel dielectric surface modifications have been produced. Novel DSMs, based on derivates of tridecyltrichlorosilane (TTS) with different functional end‐groups as well as polymeric dielectrics have been applied to tailor the adhesion energy of perylene. The resulting samples were characterized by electronic transport measurements, scanning probe microscopy, and X‐ray diffraction (XRD). Measurements of the surface free energy of the modified dielectric enabled the calculation of the adhesion energy of perylene upon these novel DSMs by the equation‐of‐state approach. These calculations demonstrate the successful tailoring of the adhesion energy. With these novel DSMs, perylene thin‐films with a superior film quality were produced, which enabled high‐performance perylene‐based OTFTs with high charge‐carrier mobility.     

Role of Side‐Chain Branching on Thin‐Film Structure and Electronic Properties of Polythiophenes

Side‐chain engineering is increasingly being utilized as a technique to impact the structural order and enhance the electronic properties of semiconducting polymers. However, the correlations drawn between structural changes and the resulting charge transport properties are typically indirect and qualitative in nature. In the present work, a combination of grazing incidence X‐ray diffraction and crystallographic refinement calculations is used to determine the precise molecular packing structure of two thiophene‐based semiconducting polymers to study the impact of side‐chain modifications. The optimized structures provide high‐quality fits to the experimental data and demonstrate that in addition to a large difference in interchain spacing between the two materials, there exists a significant disparity in backbone orientation as well. The calculated structures are utilized in density functional theory calculations to determine the band structure of the two materials and are shown to exhibit a dramatic disparity in interchain dispersion which accounts for the large observed difference in charge carrier mobility. The techniques presented here are meant to be general and are therefore applicable to many other highly diffracting semicrystalline polymers.     

Effect of Oxygen Binding Energy on the Stability of Indium‐Gallium‐Zinc‐Oxide Thin‐Film Transistors

From a practical viewpoint, the topic of electrical stability in oxide thin‐film transistors (TFTs) has attracted strong interest from researchers. Positive bias stress and constant current stress tests on indium‐gallium‐zinc‐oxide (IGZO)‐TFTs have revealed that an IGZO‐TFT with a larger Ga portion has stronger stability, which is closely related with the strong binding of O atoms, as determined from an X‐ray photoelectron spectroscopy analysis.     

Progress of Thin‐Film Silicon Photovoltaic Technologies in SANYO

Methods to achieve a good balance among a high conversion efficiency, a large panel size and a high deposition rate of µc‐Si:H for mass production are shown here. For this purpose, an original technology called the Localized Plasma Confinement CVD (LPC‐CVD) method is investigated. Using know‐how from this method, an amorphous silicon/microcrystalline silicon (µc‐Si:H) solar panel, whose size is Gen. 5.5 (1100 mm × 1400 mm) and whose µc‐Si:H deposition rate is 2.4 nm/s, with a conversion efficiency of 11.1% (V_oc = 161.7 V, I_sc = 1.46 A, FF = 72.4%, P_max = 171 W) is obtained. It is also experimentally confirmed that the value is equivalent to 10.0% of stabilized efficiency. Various reliability tests that conform to IEC standards have been performed for solar modules. It has been shown that the solar modules adapt to the major categories of IEC standards. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of Thiol Self‐Assembled Monolayer Processing on Bottom‐Contact Thin‐Film Transistors Based on n‐Type Organic Semiconductors

The performance of bottom‐contact thin‐film transistor (TFT) structures lags behind that of top‐contact structures owing to the far greater contact resistance. The major sources of the contact resistance in bottom‐contact TFTs are believed to reflect a combination of non‐optimal semiconductor growth morphology on the metallic contact surface and the limited available charge injection area versus top‐contact geometries. As a part of an effort to understand the sources of high charge injection barriers in n‐channel TFTs, the influence of thiol metal contact treatment on the molecular‐level structures of such interfaces is investigated using hexamethyldisilazane (HMDS)‐treated SiO₂ gate dielectrics. The focus is on the self‐assembled monolayer (SAM) contact surface treatment methods for bottom‐contact TFTs based on two archetypical n‐type semiconductors, α,ω‐diperfluorohexylquarterthiophene (DFH‐4T) and N,N′bis(n‐octyl)‐dicyanoperylene‐3,4:9,10‐bis(dicarboximide) (PDI‐8CN₂). TFT performance can be greatly enhanced, to the level of the top contact device performance in terms of mobility, on/off ratio, and contact resistance. To analyze the molecular‐level film structural changes arising from the contact surface treatment, surface morphologies are characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The high‐resolution STM images show that the growth orientation of the semiconductor molecules at the gold/SAM/semiconductor interface preserves the molecular long axis orientation along the substrate normal. As a result, the film microstructure is well‐organized for charge transport in the interfacial region.     

Printed Neuromorphic Devices Based on Printed Carbon Nanotube Thin‐Film Transistors

Hardware implementation of artificial synapse/neuron by electronic/ionic hybrid devices is of great interest for brain‐inspired neuromorphic systems. At the same time, printed electronics have received considerable interest in recent years. Here, printed dual‐gate carbon‐nanotube thin‐film transistors with very high saturation field‐effect mobility (≈269 cm² V^?1 s^–1) are proposed for artificial synapse application. Some important synaptic behaviors including paired‐pulse facilitation (PPF), and signal filtering characteristics are successfully emulated in such printed artificial synapses. The PPF index can be modulated by spike width and spike interval of presynaptic impulse voltages. The results present a printable approach to fabricate artificial synaptic devices for neuromorphic systems.     

Low‐Temperature,Nontoxic Water‐Induced Metal‐Oxide Thin Films and Their Application in Thin‐Film Transistors

Here, a simple, nontoxic, and inexpensive “water‐inducement” technique for the fabrication of oxide thin films at low annealing temperatures is reported. For water‐induced (WI) precursor solution, the solvent is composed of water without additional organic additives and catalysts. The thermogravimetric analysis indicates that the annealing temperature can be lowered by prolonging the annealing time. A systematic study is carried out to reveal the annealing condition dependence on the performance of the thin‐film transistors (TFTs). The WI indium‐zinc oxide (IZO) TFT integrated on SiO₂ dielectric, annealed at 300 °C for 2 h, exhibits a saturation mobility of 3.35 cm² V^?1 s^?1 and an on‐to‐off current ratio of ≈10⁸. Interestingly, through prolonging the annealing time to 4 h, the electrical parameters of IZO TFTs annealed at 230 °C are comparable with the TFTs annealed at 300 °C. Finally, fully WI IZO TFT based on YO_x dielectric is integrated and investigated. This TFT device can be regarded as “green electronics” in a true sense, because no organic‐related additives are used during the whole device fabrication process. The as‐fabricated IZO/YO_x TFT exhibits excellent electron transport characteristics with low operating voltage (≈1.5 V), small subthreshold swing voltage of 65 mV dec^?1 and the mobility in excess of 25 cm² V^?1 s^?1.