Thin Film Architectures Research Articles

Design freedom in multilayer thin-film devices.

In fabricating inorganic thin-film devices, the relative etch rates of materials in a given etch chemistry often limit the obtainable multilayer structures. Alternatively, in fabricating multilayer organic devices by solution processing, the ability to formulate the active organic materials in orthogonal solvent systems is limiting. The pattered-by-printing method uses the combination of selective area deposition (SAD) and atomic layer deposition (ALD) to form high-quality metal oxide thin-film devices. We print an inhibiting polymer ink that patterns the functional inorganic materials that are deposited via spatial ALD (SALD). The process is inherently orthogonal, as the polymer ink does not etch or swell the inorganic functional layers. Each functional layer is additively patterned as deposited, with device isolation and vias defined by the printed inhibitor. The combination of process orthogonality and additive patterning removes processing-related constraints on device design, and readily allows for any combination of bottom- and top-gate thin-film transistor architectures to be formed on the same substrate. The freedom of this approach is further demonstrated by both all-enhancement-mode circuits and enhancement-depletion-mode circuits. In addition, we present a new tool to tune circuit performance by local control of dielectric thickness.

Efficient white polymer light-emitting electrochemical cells

An efficient white polymer light-emitting electrochemical cell has been fabricated with a thin-film sandwich architecture of glass/indium tin oxide/electroluminescent layer/aluminum.

Organic–Inorganic Halide Perovskites: Perspectives for Silicon-Based Tandem Solar Cells

We investigate the efficiency potential of organic-inorganic halide perovskite/crystalline silicon tandem solar cells, a new class of photovoltaic devices targeting long-term cost reductions by ultrahigh conversion efficiencies. Methyl ammonium lead triiodide perovskite solar cells are particularly interesting as the top cell in Si-based tandem devices due to their suitable band gap, high photovoltage, and low sub-bandgap absorption. We derive optical models for a perovskite/Si tandem cell with Lambertian light trapping in the perovskite top cell, as well as for a top cell in the single pass limit. We find that unlike for other thin-film device architectures, light trapping is not required for the triiodide perovskite/Si tandem to reach matched top and bottom cell currents. While a Lambertian top cell could be employed in a four-terminal tandem, a top cell in the single pass limit enables a current-matched monolithic device with realistic top cell thicknesses. We calculate a limiting efficiency of 35.67% for an ideal (no parasitic absorption, ideal contacts) monolithic tandem, assuming a top cell open-circuit voltage of 1100 mV.

Analysis of Architecture Effect on Hysteretic Behavior of 3-D Spin Crossover Nanostructures

We have analyzed the hysteretic properties of thermal and pressure induced low spin (LS)↔high spin (HS) phase transition of a 3-D spin crossover (SCO) nanostructure when the cluster architecture is progressively changed from a cubic to a thin film. For solving the 3-D Ising-like Hamiltonian, the Monte Carlo metropolis technique is used, with the edge molecules trapped in HS state. The role of short- and long-range interactions on the SCO cooperativity has been investigated as well. The obtained results show that the cooperativity of a 3-D SCO system, characterized by the same number of molecules and the same interactions strength are strongly affected when going from a cubic 3-D to thin-film architecture. These results are discussed in terms of surface effects.

Nanostructured functional Ti–Ag electrodes for large deformation sensor applications

This paper reports on the development of thin film-based stretchable electrodes, suitable for different types of sensors. Columnar Ti–Ag thin films with a Ag content of 8at.% were prepared by D.C. magnetron sputtering on carbon nanotube/poly(vinylidene fluoride) CNT/PVDF piezoresistive composites. GLancing Angle Deposition, GLAD, technique was used to change the typical normal columnar growth microstructure obtained by conventional sputtering, into different growing architectures, such as inclined columns and zigzag profiles, in order to tune mechanical and electrical responses of the materials. Three different incident angles of the particle flux, α=40°, 60° and 80°, were used to deposit Ti–Ag thin films with the different architectures. Upon uniaxial stretching of the prepared zigzag thin films, the resistance of the thin film starts increasing smoothly for strains up to 3%. Above 10% strain, a sharp increase of the electrical resistance is observed due to film mechanical failure and therefore interruption of the electrical conductivity pathways. Furthermore, the influence of the thin film architecture was also studied with respect to the performance of piezoresistive sensors based on carbon nanotube/poly(vinylidene fluoride), CNT/PVDF, composites in which Ti–Ag coated films served as electrodes for signal acquisition. Electromechanical tests were thus performed in composites with CNT contents close to the percolation threshold, where the electromechanical response, characterized by the Gauge Factor, is the largest. The stability of the piezoresistive response was analyzed for the various architectures of the GLAD sputtered Ti–Ag electrodes, including incident angle and number of zigzag periods. It is shown that thin film architecture has a pronounced influence in the overall sensor response, where the best results are obtained for piezoresistive polymer composites coated with Ti–Ag films, produced with intermediate (α=60°) incident angles.

Solution-phase epitaxial growth of quasi-monocrystalline cuprous oxide on metal nanowires.

The epitaxial growth of monocrystalline semiconductors on metal nanostructures is interesting from both fundamental and applied perspectives. The realization of nanostructures with excellent interfaces and material properties that also have controlled optical resonances can be very challenging. Here we report the synthesis and characterization of metal–semiconductor core–shell nanowires. We demonstrate a solution-phase route to obtain stable core–shell metal–Cu2O nanowires with outstanding control over the resulting structure, in which the noble metal nanowire is used as the nucleation site for epitaxial growth of quasi-monocrystalline Cu2O shells at room temperature in aqueous solution. We use X-ray and electron diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, photoluminescence spectroscopy, and absorption spectroscopy, as well as density functional theory calculations, to characterize the core–shell nanowires and verify their structure. Metal–semiconductor core–shell nanowires offer several potential advantages over thin film and traditional nanowire architectures as building blocks for photovoltaics, including efficient carrier collection in radial nanowire junctions and strong optical resonances that can be tuned to maximize absorption.

Unipolar CMOS Logic for Thin-Film Transistor Technology

For most thin-film transistors, it is very difficult, if not impossible, to fabricate CMOS circuits because of the difficulty in realizing both p-type and n-type channels in the same thin-film semiconductor, despite the well-known advantage of CMOS circuits’ lower stand-by power.The operating principle of the proposed unipolar logic circuits will be described in the context of both the double-gate transistor architecture and the single-gate transistor architecture. Figure 1 shows a circuit diagram for a two-input unipolar NOR and OR logic gates based on all n-channel MOSFETs, and Fig. 2 shows a circuit diagram for a two-input unipolar NAND and AND logic gates, also based on all n-channel MOSFETs. Figure 3 shows a schematic diagram of the possible implementation of a unipolar NAND and AND circuits based on the IGZO thin-film transistor double-gate architecture, and Fig.4 shows its counterpart based on single-gate architecture. The unipolar logic circuits typically suffers from the “threshold voltage loss” problem at the “HIGH” output, and we have developed a novel bootstrapping block to restore the voltage for the “HIGH” output. Examples will be presented to show the effectiveness of the bootstrapping circuits. The power savings resulting from the unipolar approach as compared to its conventional counterparts will be shown by the use of simulation. This work has been partially supported by the Sandia National Laboratory PO#1383897.

Electronic properties of meso-superstructured and planar organometal halide perovskite films: charge trapping, photodoping, and carrier mobility.

Solution-processed organometal trihalide perovskite solar cells are attracting increasing interest, leading to high performances over 15% in thin film architectures. Here, we probe the presence of sub gap states in both solid and mesosuperstructured perovskite films and determine that they strongly influence the photoconductivity response and splitting of the quasi-Fermi levels in films and solar cells. We find that while the planar perovskite films are superior to the mesosuperstructured films in terms of charge carrier mobility (in excess of 20 cm(2) V(-1) s(-1)) and emissivity, the planar heterojunction solar cells are limited in photovoltage by the presence of sub gap states and low intrinsic doping densities.

Hierarchical thin film architectures for enhanced sensor performance: liquid crystal-mediated electrochemical synthesis of nanostructured imprinted polymer films for the selective recognition of bupivacaine.

Nanostructured bupivacaine-selective molecularly imprinted 3-aminophenylboronic acid-p-phenylenediamine co-polymer (MIP) films have been prepared on gold-coated quartz (Au/quartz) resonators by electrochemical synthesis under cyclic voltammetric conditions in a liquid crystalline (LC) medium (triton X-100/water). Films prepared in water and in the absence of template were used for control studies. Infrared spectroscopic studies demonstrated comparable chemical compositions for LC and control polymer films. SEM studies revealed that the topologies of the molecularly imprinted polymer films prepared in the LC medium (LC-MIP) exhibit discernible 40 nm thick nano-fiber structures, quite unlike the polymers prepared in the absence of the LC-phase. The sensitivity of the LC-MIP in a quartz crystal microbalance (QCM) sensor platform was 67.6 ± 4.9 Hz/mM under flow injection analysis (FIA) conditions, which was ≈250% higher than for the sensor prepared using the aqueous medium. Detection was possible at 100 nM (30 ng/mL), and discrimination of bupivacaine from closely related structural analogs was readily achieved as reflected in the corresponding stability constants of the MIP-analyte complexes. The facile fabrication and significant enhancement in sensor sensitivity together highlight the potential of this LC-based imprinting strategy for fabrication of polymeric materials with hierarchical architectures, in particular for use in surface-dependent application areas, e.g., biomaterials or sensing.

Polymer‐Waveguide‐Based Flexible Tactile Sensor Array for Dynamic Response

A polymer-waveguide-based transparent and flexible force sensor array is proposed, which satisfies the principal requirements for a tactile sensor working on curvilinear surfaces, such as thinfilm architecture (thickness < 150 μm), localized force sensing (ca. 0-3 N), multiple-point re cognition (27 points), bending robustness (10.8% degradation at R = 1.5 mm), and fast response (bandwidth > 16 Hz).

Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells

The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitized and organic photovoltaics. High-power conversion efficiency can be realized in both mesoporous and thin-film device architectures. We address the origin of this success in the context of the materials chemistry and physics of the bulk perovskite as described by electronic structure calculations. In addition to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses), the materials are structurally and compositionally flexible. As we show, hybrid perovskites exhibit spontaneous electric polarization; we also suggest ways in which this can be tuned through judicious choice of the organic cation. The presence of ferroelectric domains will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and reduction of recombination through segregation of charge carriers. The combination of high dielectric constant and low effective mass promotes both Wannier-Mott exciton separation and effective ionization of donor and acceptor defects. The photoferroic effect could be exploited in nanostructured films to generate a higher open circuit voltage and may contribute to the current–voltage hysteresis observed in perovskite solar cells.

Development of highly flexible and ultra-low permeation rate thin-film barrier structure for organic electronics

A flexible thin-film encapsulation architecture for organic electronics was built and consisted of a silicon oxide/alumina and parylene layer deposited over Ca sensors on a barrier-coated polyethylene terephthalate substrate. The film's effective water vapor transmission rate was 2.4±1.5×10−5g/m2/day at 20°C and 50% relative humidity. Flexural tests revealed that for films deposited on the polyethylene terephthalate substrate, the barrier layer failed due to cracking at a curvature radius of 6.4mm, corresponding to a strain of 0.8%. Adding an epoxy top coat of suitable thickness shifted the neutral axis toward the encapsulation layer, reducing the induced strain. Barrier performance was maintained under the 6.4mm radius of curvature in this encapsulation structure. Thus, shifting the neutral axis via device structural design is an effective method of extending the flexibility of thin-film encapsulation structure without compromising the performance loss as a barrier layer.

Enhancing Seebeck Effects by Using Excited States in Organic Semiconducting Polymer MEH-PPV Based on Multilayer Electrode/Polymer/Electrode Thin-Film Structure

This article reports the experimental studies on using photoexcitation-generated excited states as a new approach to increase Seebeck coefficient as well as electrical conductivity in an organic semiconducting polymer MEH-PPV (poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) with multilayer electrode/polymer/electrode thin-film architecture. Our experimental results show that a photoexcitation can lead to a large Seebeck coefficient of 305 μV/K in the ITO/MEH-PPV/Au device under the light intensity of 16 mW/cm2. Simultaneously, the electrical conductivity is increased to 8.2 × 10–5 S/cm from the dark conductivity of 3.6 × 10–6 S/cm. Clearly, using excited states can lead to a simultaneous increase on Seebeck coefficient and electrical conductivity, as compared to opposite phenomenon induced by doping: increasing electrical conductivity but decreasing Seebeck coefficient. Our experimental studies suggest that excited states can increase both the entropy difference between high and low-temperature ...

Scanning Probe Microscopy of Nanocomposite Membranes and Dynamic Organization

AbstractNanocomposite membrane assemblies are a class of materials that incorporate inorganic/organic nanoscale materials, such as fullerenes and gold nanoparticles or nanostructured materials with bio‐inspired amphiphilic structures composed of molecules such as lipids or block copolymers. One of the intrigues of such materials is the potential to develop programmable membrane assemblies that mimic biological membrane complexity, dynamics and function. Due to the nanoscale nature of the assemblies, it becomes necessary to understand interactions between these materials with nanoscale resolution. Although many techniques are able to provide information as to the overall organization of membrane‐based assemblies, only scanning probe microscopy (SPM) methods allow for a direct visualization of stochastic processes under environmentally relevant conditions. Here, an overview of nanocomposite membrane and thin film architecture investigations is presented with an emphasis on using in situ atomic force microscopy (AFM) in combination with fluorescence microscopy/spectroscopy techniques to understand organization and dynamics, in relation to activities and capabilities at the Center for Integrated Nanotechnologies.

High-performance flexible asymmetric supercapacitors based on 3D porous graphene/MnO2nanorod and graphene/Ag hybrid thin-film electrodes

We demonstrate a simple method for preparing flexible, free-standing, three-dimensional porous graphene/MnO2 nanorod and graphene/Ag hybrid thin-film electrodes using a filtration assembly process. These graphene hybrid films, which accelerate ion and electron transport by providing lower ion-transport resistances and shorter diffusion-distances, exhibit high specific capacitances and power performances, and excellent mechanical flexibility. A novel asymmetric supercapacitor (SC) has been fabricated by using a graphene/MnO2 nanorod thin film as the positive electrode and a graphene/Ag thin film as the negative electrode. These devices exhibit a maximum energy density of 50.8 W h kg−1 and present a high power density of 90.3 kW kg−1, even at an energy density of 7.53 W h kg−1. The bent hybrid nanostructured asymmetric SC is connected to spin a fan, which also proved the high power density of the fabricated asymmetric SCs. These results suggest that such asymmetric graphene/MnO2 nanorod and graphene/Ag hybrid thin-film architectures are promising for next-generation high-performance flexible supercapacitors.

Low-temperature processed meso-superstructured to thin-film perovskite solar cells

We have reduced the processing temperature of the bulk absorber layer in CH3NH3PbI3−xClx perovskite solar cells from 500 to <150 °C and achieved power conversion efficiencies up to 12.3%. Remarkably, we find that devices with planar thin-film architecture, where the ambipolar perovskite transports both holes and electrons, convert the absorbed photons into collected charge with close to 100% efficiency.

Plasmon–Exciton Hybridization in ZnO Quantum-Well Al Nanodisc Heterostructures

We demonstrate the formation of a hybridized plasmon-exciton state exhibiting strong exciton-plasmon coupling in ZnO/Zn(0.85)Mg(0.15)O single quantum wells capped with arrays of Al nanodiscs. Tuning the quantum-well width and the diameter and pitch of the Al nanodisc arrays facilitates a transition from the weak-coupling regime into the strong coupling regime. Finite-difference time-domain simulations substantiate the localization of the plasmonic quadrupole moment within the ZnO quantum-well layer, resulting in a hybridized plasmonexciton state demonstrating a Rabi splitting of roughly 15 meV in heterostructures that exhibit a prominent plasmon quadrupole mode. The significant tunability offered by quantum-well heterostructures like those discussed here provides a flexible system for controlling exciton plasmon coupling in a device-compatible thin-film architecture.

Nanophotonic light trapping in 3-dimensional thin-film silicon architectures

Emerging low cost and large area periodic texturing methods promote the fabrication of complex absorber structures for thin film silicon solar cells. We present a comprehensive numerical analysis of a 2 μm square periodic polycrystalline silicon absorber architecture designed in our laboratories. Simulations are performed on the basis of a precise finite element reconstruction of the experimentally realized silicon structure. In contrast to many other publications, superstrate light trapping effects are included in our model. Excellent agreement to measured absorptance spectra is obtained. For the inclusion of the absorber into a standard single junction cell layout, we show that light trapping close to the Yablonovitch limit can be realized, but is usually strongly damped by parasitic absorption.

Nanostructured columnar heterostructures of TiO2 and Cu2O enabled by a thin-film self-assembly approach: Potential for photovoltaics

Significant efforts are being devoted to the development of multifunctional thin-film heterostructures and nanostructured material architectures for components with novel applications of superconductivity, multiferroicity, solar photocatalysis and energy conversion. In particular, nanostructured assemblies with well-defined geometrical shapes have emerged as possible high efficiency and economically viable alternatives to planar photovoltaic thin-film architectures. By exploiting phase-separated self-assembly, here we present advances in a vertically oriented two-component system that offers potential for future development of nanostructured thin film solar cells. Through a single-step deposition by magnetron sputtering, we demonstrate growth of an epitaxial, composite film matrix formed as self-assembled, well ordered, phase segregated, and oriented nanopillars of n-type TiO2 and p-type Cu2O. The composite films were structurally characterized to atomic resolution by a variety of analytical tools, and evaluated for preliminary optical properties using absorption measurements. We find nearly atomically distinct TiO2–Cu2O interfaces (i.e., needed for possible active p–n junctions), and an absorption profile that captures a wide range of the solar spectrum extending from ultraviolet to visible wavelengths. This high-quality materials system could lead to photovoltaic devices that can be optimized for both incident light absorption and carrier collection.

Flux Engineering To Control In-Plane Crystal and Morphological Orientation

We tailored nanostructured morphology and crystal texture of iron nanocolumns by engineering the inclination and azimuthal directions of the collimated flux characteristic of glancing angle deposition (GLAD). Under continuous substrate rotation, the flux is azimuthally isotropic within one rotation. With large substrate rotation speeds, we can deposit vertical nanocolumns with a faceted, tetrahedral apex, BCC crystal structure and ⟨111⟩ fiber texture. Designing the flux to have an azimuthal 3-fold symmetry, which reflects the symmetry of the tetrahedral apex, allows us to induce both an in-plane and out-of-plane texture (biaxial texture) by evolutionary selection. In-plane crystal orientation is accompanied by a preferential azimuthal nanocolumn orientation, where the sides of tetrahedral apex are directed toward the flux direction. This work demonstrates the flux engineering technique, which can orient in-plane crystal texture and morphology of crystalline nanocolumns on amorphous substrates. This control is a useful addition to vapor–solid, physical self-assembly with the potential to improve the performance of porous thin film architectures as biaxial buffer layers, and in a variety of device applications such as photovoltaics and energy storage.