Transparent Electrical Contacts Research Articles

Validation of transparent and flexible neural implants for simultaneous electrophysiology, functional imaging, and optogenetics.

The combination of electrophysiology and neuroimaging methods allows the simultaneous measurement of electrical activity signals with calcium dynamics from single neurons to neuronal networks across distinct brain regions in vivo. While traditional electrophysiological techniques are limited by photo-induced artefacts and optical occlusion for neuroimaging, different types of transparent neural implants have been proposed to resolve these issues. However, reproducing proposed solutions is often challenging and it remains unclear which approach offers the best properties for long-term chronic multimodal recordings. We therefore created a streamlined fabrication process to produce, and directly compare, two types of transparent surface micro-electrocorticography (μECoG) implants: nano-mesh gold structures (m-μECoGs) versus a combination of solid gold interconnects and PEDOT:PSS-based electrodes (pp-μECoGs). Both implants allowed simultaneous multimodal recordings but pp-μECoGs offered the best overall electrical, electrochemical, and optical properties with negligible photo-induced artefacts to light wavelengths of interest. Showing functional chronic stability for up to four months, pp-μECoGs also allowed the simultaneous functional mapping of electrical and calcium neural signals upon visual and tactile stimuli during widefield imaging. Moreover, recordings during two-photon imaging showed no visible signal attenuation and enabled the correlation of network dynamics across brain regions to individual neurons located directly below the transparent electrical contacts.

Ultrathin Wide‐Bandgap a‐Si:H‐Based Solar Cells for Transparent Photovoltaic Applications

Herein, the fabrication of UV‐blue selective transparent solar cells based on ultrathin (<30 nm) intrinsic hydrogenated amorphous silicon films (a‐Si:H) as absorber and using a fully inorganic architecture is reported, using metal‐oxide thin films as carrier selective contacts and as transparent electrical contacts. These transparent ultrathin devices present a photovoltaic effect and high average visible transmittance (AVT), showing their potential as candidates for implementation as a transparent energy harvester. Potential applications range from Building‐Integrated PV to agrophotovoltaics, and can also be of interest as a ubiquitous and inexpensive power source integrated in functional devices such as low‐power devices, Internet of Things devices, and other sensors. Glass/FTO/ZnO/a‐Si:H/MoO3/ITO device prototypes are produced. These devices present an AVT ranging from 50% to 69%, present a photovoltaic effect with a power conversion efficiency up to 0.5% calculated for an AM1.5G spectrum and have light utilization efficiency (LUE) values of 0.25%, confirming the potential of the proposed device architectures for the development of highly transparent devices with improved LUE.

InP nanowires on Si(111) for piezotronic applications

III-V nanowires (NWs) are a promising technology for piezotronic and nanooptoelctronic applications. In this work, we investigate the processes of fabricating a structure with InP NW arrays on a silicon substrate for piezotronic applications. The coating of the NW array with a polymer and the fabrication of a transparent electrical contact to NWs is studied. The piezoelectric effect for the structures with a nonzero piezoelectric modulus d 33 is demonstrated in an oil environment at voltage amplitudes of about 100 mV. The experimental electromechanical coupling is about 0.049, which is close to the theoretical estimate of 0.053.

Effectively transparent electrical contacts for thermally sensitive solar cells

High conductivity silver pastes are usually incompatible for use with thermally sensitive devices due to their higher sintering temperatures while nanoparticle inks are costly and undergo high particle agglomeration. Effectively transparent contacts (ETCs) have been designed to increase the transmittance, reduce reflectance and overcome shadowing losses in solar cells while enhancing the overall conductivity and charge collection through the grid. In this study, reactive silver ink (RSI) is mixed with a low temperature sintering conductive silver paste (150 °C) for deposition via a microchannel approach. ETCs deposited using the resulting composite were cured at a lower temperature (100 °C) with a conductivity of the order of 103 1/Ω-cm and a transparency >90% in the visible region, promising a reduction in optical and electrical losses when compared to bare transparent conductive oxides. The use of low temperature curing conductive material makes ETCs suitable for use in thermally sensitive optoelectronic and photovoltaic devices, hence they were ultimately deposited on perovskite solar cells and the solar cell parameters were measured.

Monolithically-integrated BiVO4/p+-n GaAs1-xPx tandem photoanodes capable of unassisted solar water splitting

Monolithically-integrated tandem photoanodes were fabricated on substrates consisting of epitaxial n-GaAs1-xPx (x ≅ 0.32) grown on n+-GaAs wafers. A p+-n junction photovoltaic (PV) cell was first formed by zinc diffusion into the n-GaAs0.68P0.32 from a deposited ZnO coating. After diffusion the ZnO serves as a transparent electrical contact to the resulting p+-GaAs0.68P0.32 surface layer. Transparent, conducting SnO2:F provides chemical and mechanical protection for the ZnO and the underlying PV cell, and it electrically connects this cell to a top BiVO4 photocatalyst layer. In some photoanodes, a WO3 thin film was interposed between the SnO2:F and BiVO4. All oxide coatings were produced by ultrasonic spray pyrolysis except WO3, which was spin coated. Unassisted (unbiased) solar water splitting was achieved, with a solar-to-hydrogen efficiency approaching 2%, without addition of any co-catalyst to the BiVO4 surface. This work can provide insights to other researchers regarding scalable, low cost approaches for the planar monolithic integration of oxide photoanode materials with PV cells to create new tandem devices.

Fabrication and Characterization of Superconducting Films for Superconductor-Topological Insulator Hybrid Devices

Proximation of s-wave superconductivity to topological materials is an effective way to realize Majorana modes in condensed matter physics. In order to fabricate a successful device, the superconducting material plays a crucial role since selection of material, growth method, applicable critical temperature, adequate critical magnetic field, transparent electrical contact, and thus proper work function, needs to be deliberately considered. Here, we report the fabrications and transport measurements for two types of superconducting thin films, tantalum (Ta) and niobium nitride (NbN), at various growth conditions. Both Ta and NbN films show applicable potentials as the superconducting electrodes. We also demonstrate the proximity effect induced supercurrent state in an InAs/GaSb bilayer device with superconducting Ta electrodes.

Nanoscale Back Contact Perovskite Solar Cell Design for Improved Tandem Efficiency.

Tandem photovoltaics, combining absorber layers with two distinct band gap energies into a single device, provide a practical solution to reduce thermalization losses in solar energy conversion. Traditionally, tandem devices have been assembled using two-terminal (2-T) or four-terminal (4-T) configurations; the 2-T limits the tandem performance due to the series connection requiring current matching, while the standard 4-T configuration requires at least three transparent electrical contacts, which reduce the total collected power due to unavoidable parasitic absorption. Here, we introduce a novel architecture based on a nanoscale back-contact for a thin-film top cell in a three terminal (3-T) configuration. Using coupled optical–electrical modeling, we optimize this architecture for a planar perovskite-silicon tandem, highlighting the roles of nanoscale contacts to reduce the required perovskite electronic quality. For example, with an 18% planar silicon base cell, the 3-T back contact design can reach a 32.9% tandem efficiency with a 10 μm diffusion length perovskite material. Using the same perovskite quality, the 4-T and 2-T configurations only reach 30.2% and 24.8%, respectively. We also confirm that the same 3-T efficiency advantage applies when using 25% efficient textured silicon base cells, where the tandems reach 35.2% and 32.8% efficiency for the 3-T, and 4-T configurations, respectively. Furthermore, because our design is based on the individual subcells being back-contacted, further improvements can be readily made by optimizing the front surface, which is left free for additional antireflective coating, light trapping, surface passivation, and photoluminescence outcoupling enhancements.

A New Look at the Electronic Structure of Transparent Conductive Oxides—A Case Study of the Interface between Zinc Magnesium Oxide and Cadmium Telluride

By combining X‐ray and ultraviolet photoelectron spectroscopy, X‐ray excited Auger electron spectroscopy, and inverse photoemission spectroscopy, a detailed characterization of the chemical and electronic structure at the CdTe/(Zn,Mg)O interface is presented. In doing so, a novel approach to characterize the electronic structure of transparent conductive oxides, namely, to distinguish between optically and transport‐relevant states is presented. Consequently, “optical” and “electronic” band offsets at the CdTe/(Zn,Mg)O interface are derived and it is demonstrated that the “electronic” band offset is suitable for electron transport across the interface (a flat conduction band alignment of 0.02 ± 0.20 eV), while the “optical band offset” is not (a significant 0.48 ± 0.20 eV spike). The new approach can easily be applied to many other transparent conductive oxides and is thus of relevance for a large variety of optoelectronic applications involving transparent electrical contacts.

Structural, optical and electrical properties of SnO2:F thin films deposited by spray pyrolysis for application in thin film solar cells

Fluorine doped tin oxide (FTO) thin films with adequate properties to be used as transparent electrical contact for PV solar cells were synthesised using the spray pyrolysis technique, which provides a low cost operation. The deposition temperature and the fluorine doping have been optimized for achieving a minimum resistivity and maximum optical transmittance. No post-deposition annealing treatments were carried out. The X-ray diffraction study showed that all the FTO films were polycrystalline with a tetragonal crystal structure and preferentially oriented along the (200) direction. The grain size ameliorates with the increase in substrate temperature. The samples deposited with the substrate temperature at 440 °C and fluorine content of 20 wt % exhibited the lowest electrical resistivity (1.8 × 10−4 Ω cm), as measured by four-point probe. Room-temperature Hall measurements revealed that the 20 wt% films are degenerate and exhibit n-type electrical conductivity with carrier concentration of ~4.6 × 1020 cm−3, sheet resistance of 6.6 Ω/□ and a mobility of ~25 cm2 V−1 s−1. In addition, the optimized growth conditions resulted in thin films (~500 nm thickness) with average visible transmittance of 89 % and optical band-gap of 3.90 eV. The electrical and optical characteristics of the deposited films revealed their excellent quality as a TCO material.

Characterization of plasmonic hole arrays as transparent electrical contacts for organic photovoltaics using high-brightness Fourier transform methods

We present a methodology for probing light-matter interactions in prototype photovoltaic devices consisting of an organic semiconductor active layer with a semitransparent metal electrical contact exhibiting surface plasmon-based enhanced optical transmission. We achieve high-spectral irradiance in a spot size of less than 100 μm using a high-brightness laser-driven light source and appropriate coupling optics. Spatially resolved Fourier transform photocurrent spectroscopy in the visible and near-infrared spectral regions allows us to measure external quantum efficiency with high sensitivity in small-area devices (<1 mm2). This allows for rapid fabrication of variable-pitch sub-wavelength hole arrays in metal films for use as transparent electrical contacts, and evaluation of the evanescent and propagating mode coupling to resonances in the active layer.

Lithium intercalation in sputter deposited antimony-doped tin oxide thin films: Evidence from electrochemical and optical measurements

Transparent conducting oxides are used as transparent electrical contacts in a variety of applications, including in electrochromic smart windows. In the present work, we performed a study of transparent conducting antimony-doped tin oxide (ATO) thin films by chronopotentiometry in a Li+-containing electrolyte. The open circuit potential vs. Li was used to investigate ATO band lineups, such as those of the Fermi level and the ionization potential, as well as the dependence of these lineups on the preparation conditions for ATO. Evidence was found for Li+ intercalation when a current pulse was set in a way so as to drive ions from the electrolyte into the ATO lattice. Galvanostatic intermittent titration was then applied to determine the lithium diffusion coefficient within the ATO lattice. The electrochemical density of states of the conducting oxide was studied by means of the transient voltage recorded during the chronopotentiometry experiments. These measurements were possible because, as Li+ intercalation took place, charge compensating electrons filled the lowest part of the conduction band in ATO. Furthermore, the charge insertion modified the optical properties of ATO according to the Drude model.

Ultra-thin gold films on transparent polymers

AbstractFabrication of continuous ultra-thin gold films (&lt;10 nm) on the surface of optical polymers (CYCLOTENE and ORMOCLEAR) is reported. Using a range of electrical, optical and structural characterization techniques, we show that polymers can be superior to more conventional (inorganic) materials as optical substrates for realizing ultra-thin gold films. Using these transparent polymer substrates, smooth, patternable gold films can be fabricated with conventional deposition techniques at room temperature, without adhesion or seeding layers, facilitating new photonic and plasmonic nanostructures, including transparent electrical contacts, thin film waveguides, metamaterials, biosensors and high-contrast superlenses.

Comparing resonant photon tunneling via cavity modes and Tamm plasmon polariton modes in metal-coated Bragg mirrors

Resonant photon tunneling was investigated experimentally in multilayer structures containing a high-contrast (TiO(2)/SiO(2)) Bragg mirror capped with a semitransparent gold film. Transmission via a fundamental cavity resonance was compared with transmission via the Tamm plasmon polariton resonance that appears at the interface between a metal film and a one-dimensional photonic bandgap structure. The Tamm-plasmon-mediated transmission exhibits a smaller dependence on the angle and polarization of the incident light for similar values of peak transmission, resonance wavelength, and finesse. Implications for transparent electrical contacts based on resonant tunneling structures are discussed.

Spray-deposited Al-doped ZnO transparent contacts for CdTe solar cells

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited by ultrasonic spray pyrolysis of alcoholic solutions in ambient atmosphere without any post-growth treatment. Spraying parameters, composition and acidity of the sprayed solution, and doping level were systematically investigated in order to obtain AZO films suitable for applications in solar cells. It was confirmed that the amount of aluminum chloride as the dopant and acetic acid as the stabilizing agent are decisive for film morphology and opto-electrical properties. Films of 1.1μm in thickness deposited under optimized conditions on borosilicate substrates exhibited resistivity of 5×10−3Ωcm and an average transmittance of 82% in the wavelength range 400–900nm. A resistivity of 3×10−3Ωcm, one of the lowest values reported in literature for sprayed AZO, was obtained for a film thickness of 3μm maintaining a high optical transmittance of >74%. The latter film was applied as a transparent electrical contact in a CdTe solar cell and an efficiency of 12.1% without antireflection coating was achieved. The average efficiency value of >11% measured for 22 fabricated solar cells demonstrates the feasibility of using spray-pyrolysis technique for the deposition of large area AZO contacts for solar cell applications.

Highly Transparent AZO/Ag/AZO Multilayer Front Contact for n-i-p Silicon Thin-Film Solar Cells

ABSTRACTOxide-metal-oxide structures are an alternative to single material transparent electrical contacts. Among other advantages, these multilayer systems provide good conductivity and transmittance, even when fabricated at room temperature. Low temperature processing is a requirement for silicon thin-film solar cells on various flexible substrates. The design and fabrication of oxide-metal-oxide structures based on ZnO:Al and Ag are investigated in this work. Further the integration of an optimized multilayer electrode into an amorphous silicon solar cell in substrate configuration was performed. Measurement results and possible loss mechanisms are discussed.

Photo-EMF Sensitivity of Porous Silicon Thin Layer–Crystalline Silicon Heterojunction to Ammonia Adsorption

A new method of using photo-electromotive force in detecting gas and controlling sensitivity is proposed. Photo-electromotive force on the heterojunction between porous silicon thin layer and crystalline silicon wafer depends on the concentration of ammonia in the measurement chamber. A porous silicon thin layer was formed by electrochemical etching on p-type silicon wafer. A gas and light transparent electrical contact was manufactured to this porous layer. Photo-EMF sensitivity corresponding to ammonia concentration in the range from 10 ppm to 1,000 ppm can be maximized by controlling the intensity of illumination light.

Development of new transparent conductors and device applications utilizing a multidisciplinary approach

Transparent and conductive films are key components for optoelectronic devices. They are applied as n-type transparent electrical contacts for inorganic and organic light emitting diodes, solar cells and flat panel displays as well as p- and n-type active semiconductive oxides to setup wide band gap p–n junctions and devices for the emerging field of transparent and radiation hard electronics. The demand for these films is strongly increasing due to the extensive market growth in these areas but the solutions available today only partially fulfill the requirements on low resistivity, high transmittance, large area deposition, low cost manufacturing, and ability for fine patterning, light scattering and precise alignment of the electronic structure to surrounding semiconductors. The cooperation of five Fraunhofer Institutes within the “Fraunhofer Project MAVO METCO” aims towards establishing fundamental knowledge and control about the defect chemistry, structure and morphology of the transparent semiconductive oxides. The goal is to achieve materials with outstanding properties such as n-type transparent conductive oxides with tailored work function and excellent durability, novel delafossite based p-type materials allowing cost effective large area deposition, oxide based p–n heterojunctions and Ag based electrodes to be used for thin film photovoltaics and organic light emitting diodes. Starting from first-principle modelling of the electronic structure, we address the development of new transparent conductive layers by PVD and Sol–Gel ending up with device implementation for OLEDs and organic as well as Si based a-Si:H/µc-Si:H and HIT solar cells.

Oxide Contacts in Organic Photovoltaics: Characterization and Control of Near-Surface Composition in Indium−Tin Oxide (ITO) Electrodes

The recent improvements in the power conversion efficiencies of organic photovoltaic devices (OPVs) promise to make these technologies increasingly attractive alternatives to more established photovoltaic technologies. OPVs typically consist of photoactive layers 20-100 nm thick sandwiched between both transparent oxide and metallic electrical contacts. Ideal OPVs rely on ohmic top and bottom contacts to harvest photogenerated charges without compromising the power conversion efficiency of the OPV. Unfortunately, the electrical contact materials (metals and metal oxides) and the active organic layers in OPVs are often incompatible and may be poorly optimized for harvesting photogenerated charges. Therefore, further optimization of the chemical and physical stabilities of these metal oxide materials with organic materials will be an essential component of the development of OPV technologies. The energetic and kinetic barriers to charge injection/collection must be minimized to maximize OPV power conversion efficiencies. In this Account, we review recent studies of one of the most common transparent conducting oxides (TCOs), indium-tin oxide (ITO), which is the transparent bottom contact in many OPV technologies. These studies of the surface chemistry and surface modification of ITO are also applicable to other TCO materials. Clean, freshly deposited ITO is intrinsically reactive toward H(2)O, CO, CO(2), etc. and is often chemically and electrically heterogeneous in the near-surface region. Conductive-tip atomic force microscopy (C-AFM) studies reveal significant spatial variability in electrical properties. We describe the use of acid activation, small-molecule chemisorption, and electrodeposition of conducting polymer films to tune the surface free energy, the effective work function, and electrochemical reactivity of ITO surfaces. Certain electrodeposited poly(thiophenes) show their own photovoltaic activity or can be used as electronically tunable substrates for other photoactive layers. For certain photoactive donor layers (phthalocyanines), we have used the polarity of the oxide surface to accelerate dewetting and "nanotexturing" of the donor layer to enhance OPV performance. These complex surface chemistries will make oxide/organic interfaces one of the key focal points for research in new OPV technologies.

Preparation of indium tin oxide thin films without external heating for application in solar cells

Indium tin oxide (ITO) films were grown without external heating in an ambient of pure argon by RF-magnetron sputtering method. The influence of argon ambient pressure on the electro-optical properties of as-deposited ITO films was investigated. The morphology, structural and optical properties of ITO films were examined and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and UV–VIS transmission spectroscopy. The deposited ITO films with a thickness of 300 nm show a high transparency between 80% and 90% in the visible spectrum and 14–120 Ω/□ sheet resistance under different conditions. The ITO films deposited in the optimum argon ambient pressure were used as transparent electrical contacts for thin film Cu(In,Ga)Se2 (CIGS) solar cells. CIGS solar cells with efficiencies of the order of 7.0% were produced without antireflective films. The results have demonstrated that the developed ITO deposition technology has potential applications in thin film solar cells.

Replacement of Transparent Conductive Oxides by Single-Wall Carbon Nanotubes in Cu(In,Ga)Se2-Based Solar Cells

Thin films of single-wall carbon nanotubes were used as the transparent top electrical contact in Cu(In,Ga)Se2- based solar cells. Specifically, we demonstrate that thin layers of carbon nanotubes in combination with insulating polymer layers can effectively replace the metal oxide layers typically used in polycrystalline thin-film solar cells. Replacing the standard n-type ZnO layer with a thin film of carbon nanotubes yielded energy conversion efficiencies up to 13%. The optical and electrical transport properties of the single-wall carbon nanotubes suggest that suitable applications for these materials include multiple-junction solar cells, thermophotovoltaics, and other applications benefiting from a p-type transparent conductor with high near-infrared transmission.