Light-induced Voltage Research Articles

On the photovoltaic effect asymmetry in ferroelectrics

Despite symmetrical polarization, the magnitude of a light-induced voltage is known to be asymmetric with respect to poling sign in many photovoltaic (PV) ferroelectrics (FEs). This asymmetry remains unclear and is often attributed to extrinsic effects. We show here for the first time that such an asymmetry can be intrinsic, steaming from the superposition of asymmetries of internal FE bias and electro-piezo-strictive deformation. This hypothesis is confirmed by the observed decrease of PV asymmetry for smaller FE bias. Moreover, the both PV effect and remanent polarization are found to increase under vacuum-induced expansion and to decrease for gas-induced compression, with tens percents tunability. The change in cations positions under pressure is analysed through the first-principle density functional theory calculations. The reported properties provide key insight for FE-based solar elements optimization.

Organic Semiconductors for Optically Triggered Neural Interfacing: The Impact of Device Architecture in Determining Response Magnitude and Polarity

The use of organic semiconductor devices as photocapacitors is an innovation with promising applications in neural interface technologies, particularly for retinal prosthetics. Here we report the characterization of four distinct photocapacitor device architectures that were fabricated by depositing ultra-thin layers of poly-3-hexylthiophene and C60 fullerene in various combinations on tin-doped indium oxide (ITO) electrodes. We used electrophysiological recordings to measure the intrinsic photoresponse at 470 nm, and also to determine light-induced voltage perturbations in electrolyte solutions interfaced with these semiconductors. We also determined the light-induced intracellular voltage changes in co-cultured sensory neurons. Electrochemical impedance spectroscopy was used to establish the photocapacitive response mechanism of the intracellular changes upon illumination. The largest amplitude photocapacitive response was elicited from a neuron/acceptor-donor bilayer/ITO device architecture, whilst reversing the donor and acceptor layer order enabled a neuron response of the opposite polarity. The photoresponse in the neuron/acceptor/donor/ITO bilayer device configuration was significantly enhanced in both amplitude and time duration by electrical grounding of the indium tin oxide layer. We describe the advantages and limitations of each configuration of these device and discuss pathways towards the creation of optically triggered neural interfaces that do not require an external electrical power supply.

Photovoltage generation by photosystem II core complexes immobilized onto a Millipore filter on an indium tin oxide electrode.

The light-induced functioning of photosynthetic pigment-protein complex of photosystem II (PSII) is linked to the vectorial translocation of charges across the membrane, which results in the formation of voltage. Direct measurement of the light-induced voltage (∆V) generated by spinach oxygen-evolving PSII core complexes adsorbed onto a Millipore membrane filter (MF) on an indium tin oxide (ITO) electrode under continuous illumination has been performed. PSII was shown to participate in electron transfer from water to the ITO electrode, resulting in ∆V generation. No photovoltage was detected in PSII deprived of the water-oxidizing complex. The maximal and stable photoelectric signal was observed in the presence of disaccharide trehalose and 2,6-dichloro-1,4-benzoquinone, acting as a redox mediator between the primary quinone acceptor QA of PSII and electrode surface. Long time preservation of the steady-state photoactivity at room temperature in a simple in design ITO|PSII-MF|ITO system may be related to the retention of water molecules attached to the PSII surface in the presence of trehalose.

Purely Protonic Solar Cells Fabricated from Photoacid-Modified Ion-Exchange Membranes

Analogies have been drawn between electronic semiconductors and water since 1956 when Fuller explained the similarity between silicon ionization, which generates an electron and a hole as mobile charge carriers, and water ionization, which yields a hydroxide and a proton as mobile charge carriers.1 Since then our group has taken steps to produce a water-based pn-junction solar cell that uses water as a protonic semiconductor. Toward this, we have demonstrated photovoltaic action using photoacid-modified Nafion membranes alone2 and when laminated to an anion-exchange membrane,3 yet each demonstration required the membrane to be wetted by an aqueous acid–base gradient. In this report, a liquid-electrolyte-free photoacid-modified ion-exchange membrane–electrode–assembly (MEA) driving reversible H2 redox chemistry is described. The cell architecture consists of a three-layer membrane (cation-selective Nafion membrane, aminopyrene-functionalized polyphenylene oxide, anion-selective X37 membrane) that was characterized using direct membrane contacts consisting of platinized platinum mesh. The MEA achieved V oc = 10 mV and J sc = 7.5 μA/cm2 under low-intensity 405 nm illumination, resulting in an internal quantum efficiency of ~0.2%. The reported composite MEA overcomes the problem of co-ion crossover by eliminating co-ions altogether while still exhibiting photovoltaic action due to the charge-separating potential across the membrane interfaces. Because photovoltaic performance persists in the absence of electrolyte gradients and because the potentiometric measurements conducted here are only sensitive to the electrochemical potential of protons, we can conclude that the light-induced voltage and current originate from protons and/or hydroxides, and no other ionic species. This MEA thus serves as a robust proof-of-concept of water’s role as a protonic semiconductor. It also offers a promising format for fundamental studies on the origin of photovoltaic action in photoacid–polymer-electrolyte membranes in the absence of the convoluting effects of liquid electrolyte. Acknowledgments: This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Early Career Research Award under award number DE-SC0019162 from the Solar Photochemistry Program. Fuller, C. S. Chem. Prog. 1956, 17, 75-93.White, W.; Sanborn, C. D.; Reiter, R. S.; Fabian, D. M.; Ardo, S. J. Am. Chem. Soc. 2017, 139, 11726–11733.White, W.; Sanborn, C. D.; Fabian, D. M.; Ardo, S. Joule 2018, 2, 94-109.

Transparent Cu4O3/ZnO heterojunction photoelectric devices

The present article reports the development of flexible, self-biased, broadband, high speed and transparent heterojunction photodiode, which is essentially important for the next generation electronic devices. We grow semitransparent p-type Cu4O3 using the reactive sputtering method at room temperature. The structural and optical properties of the Cu4O3 film were investigated by using the X-ray diffraction and UV–visible spectroscopy, respectively. The p-Cu4O3/n-ZnO heterojunction diode under dark condition yields rectification behavior with an extremely low saturation current value of 1.8 × 10−10 A and a zero bias photocurrent under illumination condition. The transparent p-Cu4O3/n-ZnO heterojunction photodetector can be operated without an external bias, due to the light-induced voltage production. The metal oxide heterojunction based on Cu4O3/ZnO would provide a route for the transparent and flexible photoelectric devices, including photodetectors and photovoltaics.

Characterization of electrically-active defects in ultraviolet light-emitting diodes with laser-based failure analysis techniques

Laser-based failure analysis techniques demonstrate the ability to quickly and non-intrusively screen deep ultraviolet light-emitting diodes (LEDs) for electrically-active defects. In particular, two laser-based techniques, light-induced voltage alteration and thermally-induced voltage alteration, generate applied voltage maps (AVMs) that provide information on electrically-active defect behavior including turn-on bias, density, and spatial location. Here, multiple commercial LEDs were examined and found to have dark defect signals in the AVM indicating a site of reduced resistance or leakage through the diode. The existence of the dark defect signals in the AVM correlates strongly with an increased forward-bias leakage current. This increased leakage is not present in devices without AVM signals. Transmission electron microscopy analysis of a dark defect signal site revealed a dislocation cluster through the pn junction. The cluster included an open core dislocation. Even though LEDs with few dark AVM defect signals did not correlate strongly with power loss, direct association between increased open core dislocation densities and reduced LED device performance has been presented elsewhere [M. W. Moseley et al., J. Appl. Phys. 117, 095301 (2015)].

Non-Invasive Recognition of Poorly Resolved Integrated Circuit Elements

We present a non-invasive method for recognition of components in a digital CMOS integrated circuit (IC). We use a confocal infrared laser scanning optical microscope to collect multimodal images through the backside of the IC. Individual modes correspond to passive reflectivity measurements or active measurements, such as light-induced voltage alteration. The modes are registered and stored in a multidimensional data cube. We apply a machine learning algorithm using a binary representation to identify a variety of data structures from transistors to entire logic cells. Because of the compact representation, objects can be detected rapidly. We show that by increasing the number of imaging modes used to develop the descriptor, we can significantly increase recognition accuracy. The approach allows recognition of poorly resolved components, whose primary distinguishing features are below traditional optical resolution limits, and is general enough to be applied to multiple design processes. We believe this represents a significant step toward a fully non-invasive IC reverse engineering system.

Light-induced Open-circuit Voltage Increase in Amorphous Silicon/Microcrystalline Silicon Tandem Solar Cells

ABSTRACTLight-induced metastability of amorphous/microcrystalline (micromorph) silicon tandem solar cell, in which the microcrystalline bottom cell was deposited in a single-chamber system, has been studied under a white light for more than 1000 hours. Two different light-induced metastable behaviors were observed. The first type was the conventional light-induced degradation, where the open-circuit voltage (Voc), fill factor (FF), and short-circuit current density (Jsc) were degraded, hence the efficiency was degraded as well. This phenomenon was observed mainly in the tandem cells with a bottom cell limited current mismatch. The second type was with a light-induced increase in Voc, which sometimes resulted in an increase in efficiency. The second type of light-induced metastability was observed in the tandem cells with a top cell limited current mismatch. The possible mechanisms for these phenomena are discussed.

Photosysem II: where does the light-induced voltage come from?

Photosystem II (PS II) is a biological energy transducer. The enzyme catalyses the light-driven oxidation of water and reduction of plastoquinone. The aim of this work was to review the mechanisms of electrical events in PS II. The major contribution to the total photoelectric response is due to the charge-separation between the primary chlorophyll donor P680 and quinone acceptor QA accompanied by re-reduction of P680+ by tyrosine residue YZ. The remaining part of the membrane potential is believed to be associated mainly with electron and proton transfer events due to the S-state transitions of the oxygen-evolving complex and proton uptake associated with protonation of the doubly reduced secondary quinone acceptor QB. Under certain non-physiological conditions, some other electrogenic reactions are observed, namely: proton-coupled electron transfer between QA and non-heme Fe3+ and electron transfer from the protein-water interface to the YZ radical in the presence of artificial electron donors. These data may provide a good platform for further development of artificial photosynthetic constructs and bio-inspired catalysts.

A Novel Fluorescence Assay for Voltage-Gated Ion Channels Based upon Light Induced Voltage Clamp

Ion channels are a key target class with a high therapeutic potential in virtually all possible disease indications. In addition, a potential side effect of pharmaceutical compounds is the blocking of hERG channels in heart cells making easy and cost effective hERG safety screening necessary for drug development today. Conventional screening techniques yield insufficient data quality particularly when assessing voltage-gated ion channels. Thus, the development of new reliable technologies is desirable to integrate ion channel screening into early lead generation stages of drug discovery.Here we demonstrate a method that allows light-induced activation of voltage-gated ion channels and the concurrent imaging of membrane potential changes using voltage-sensitive dyes. This light-induced voltage clamp (LIVC) method uses photostimulation through channelrhodopsin-2 (ChR2) to activate voltage-gated ion channels. ChR2 allows light to be immediately transduced into a depolarizing ionic current, which in turn causes voltage-gated ion channels to open. In our system we coexpressed ChR2 either with the voltage-gated potassium channels hERG or hKv1.5 in cell lines and in Xenopus oocytes. In electrophysiological experiments we show that light-induced depolarization through ChR2 sufficed to activate hERG as well as hKv1.5 channels. We were further able to optically monitor the light-induced membrane de- and hyperpolarizations on a millisecond timescale with the voltage-sensitive RH421 and Annine6. The fluorescence readout reflected the dose-response relationships of the hERG blocker Terfenadine and the hKv1.5 inhibitor DPO-1 obtained from patch-clamp measurements.LIVC represents a solely optical technology with remote activation of the target voltage-gated ion channels by the delivery of a flash of blue light and simultaneous detection of their activity employing voltage-sensitive dyes. It combines the high-throughput of optical methods with the high-content of patch clamp concerning high temporal resolution, membrane potential control and repetitive stimulation.

A Novel Screening Tool for Voltage-Gated Ion Channels: Light Induced Voltage Clamp

Ion channels are a key target class with a high therapeutic potential. Conventional screening techniques yield insufficient data quality particularly when assessing voltage-gated ion channels. Thus, the development of new reliable technologies is desirable to integrate ion channel screening into early lead generation of drug discovery.Here we demonstrate a method that allows non-invasive, millisecond light-induced activation of voltage-gated ion channels and the concurrent imaging of membrane potential changes using fast voltage-sensitive dyes. This light-induced voltage clamp method (LIVC) uses photostimulation through channelrhodopsin-2 (ChR2), to activate voltage-gated ion channels. ChR2 allows blue light (∼ 470 nm) to be immediately transduced into a depolarizing ionic current, which causes voltage-gated ion channels to open. We coexpressed ChR2 with the voltage-gated potassium channel hKv1.5 in HEK293 cells and in Xenopus oocytes. In electrophysiological experiments we show that the light-induced cell depolarization through ChR2 sufficed to open hKv1.5 channels; the light-induced membrane depolarization was greatly reduced with active hKv1.5 channels compared to the full ChR2 response during hKv1.5 inhibition. We were further able to optically monitor the light-induced membrane depolarizations on a millisecond timescale with the voltage-sensitive RH421. The fluorescence readout reflected the concentration-inhibition relationship of the hKv1.5 inhibitor DPO-1.LIVC represents a solely optical technology with remote activation of the target voltage-gated ion channels simply by the delivery of a flash of blue light and simultaneous detection of their activity employing voltage-sensitive dyes. It combines the high-throughput of optical methods with the high-content of patch clamp concerning and possible repetitive stimulation. Proof of concept and results from assay development for voltage-gated sodium and calcium channels as well as for the hERG channel underline the potential for LIVC to evolve into a high-throughput, high reliability assay for voltage-gated ion channels in general.

Technique to detect two-photon absorption in Si

A technique to detect two-photon absorption in Si is presented. The light-induced voltage rather than the transmitted light is measured. The sample acts as the detector and the measurement is easy to be carried out. The nonlinear coefficients can be calculated conveniently from experimental data. From the experimental data measured at different light powers and different temperatures, the simultaneous two-photon absorption coefficients at 800nm in Si are calculated to be about 230cm∕MW at 300K and 360cm∕MW at 77K.

Light Soaking and Thermal Annealing Effects on the Micro-Electrical Properties of Amorphous and Nanocrystalline Mixed-phase Silicon Solar Cells

AbstractWe report on the measurement of local current flow in hydrogenated amorphous and nanocrystalline mixed-phase n-i-p silicon solar cells in the initial, light-soaked, and annealed states using conductive atomic force microscopy (C-AFM). The C-AFM measurement shows that the nanometer-size grains aggregate, and the local current densities in the nanocrystalline aggregation areas decreased significantly after light soaking and recovered to values similar to the initial state after annealing at a high temperature in a vacuum. This result supports the model of two parallel-connected diodes for explaining the light-induced open-circuit voltage increase in the mixed-phase solar cells.

Local Current Flow in Mixed-Phase Silicon Solar Cells and Correlation to Light-Induced Open-Circuit Voltage Enhancement

AbstractWe use conductive atomic force microscopy (C-AFM) to measure the local current flow in the mixed-phase hydrogenated silicon n-i-p solar cell structure without the top ITO contact. The forward biased C-AFM images reveal that for the fully amorphous region the current is very low on the entire surface. However, high current spikes appear in the mixed-phase region, where the current spikes are correlated to the formation of nanocrystallite aggregations with a diameter of ~500 nm. Furthermore, the density of the current spikes increases from the mixed-phase to the substantially nanocrystalline regions. The nanocrystallite aggregation supports our previously proposed parallel-connected two-diode model for Voc drops with crystalline volume fraction and light-induced Voc increase in the mixed-phase solar cells. Adding a 50-nm thick a-Si:H buffer layer between the p and i layers significantly reduces the magnitude of the high current spikes, even the top morphology appears unaffected. This result is also consistent with the previously proposed two-diode model for explaining the carrier transport in the mixed-phase solar cells.

Optical rectification effect in nano-carbon CVD films

We report on the study of the optical rectification effect in the nano-carbon (nC) films. The effective second-order susceptibility of the film is found to be 2 × 10 − 5 CGSE, which is two orders higher than that of the conventional nonlinear crystals. The nC films were obtained by CVD method and consist of nano-sized graphite crystallites preferably oriented along the substrate normal. In the experiment, we measure the potential difference between two electrodes attached to surface of the nC film, which is irradiated by a nanosecond laser pulse. The temporal profile of measured electric signal well reproduces the envelope of incident light pulse. We found that the observed in the experiment dependence of the light-induced voltage on intensity, polarization, wavelength and angle of incidence are well described in terms of the optical rectification in nonlinear optical media. The experimental results indicate that the effect may originate from the quadrupole and magnetic dipole mechanism of optical nonlinearity. In the nC films this mechanism may be pronounced because of the strong delocalization of sp 2 orbitals within the graphitic layers. The observed nonlinear optical effect indicates that nC films is a prospective material to be employed in fast wide band optical detectors and position sensitive devices.

The Proton-Driven Rotor of ATP Synthase: Ohmic Conductance (10 fS), and Absence of Voltage Gating

The membrane portion of F 0F 1-ATP synthase, F 0, translocates protons by a rotary mechanism. Proton conduction by F 0 was studied in chromatophores of the photosynthetic bacterium Rhodobacter capsulatus. The discharge of a light-induced voltage jump was monitored by electrochromic absorption transients to yield the unitary conductance of F 0. The current-voltage relationship of F 0 was linear from 7 to 70 mV. The current was extremely proton-specific (>10 7) and varied only slightly (≈threefold) from pH 6 to 10. The maximum conductance was ≈10 fS at pH 8, equivalent to 6240 H + s −1 at 100-mV driving force, which is an order-of-magnitude greater than of coupled F 0F 1. There was no voltage-gating of F 0 even at low voltage, and proton translocation could be driven by ΔpH alone, without voltage. The reported voltage gating in F 0F 1 is thus attributable to the interaction of F 0 with F 1 but not to F 0 proper. We simulated proton conduction by a minimal rotary model including the rotating c-ring and two relay groups mediating proton exchange between the ring and the respective membrane surface. The data fit attributed pK values of ≈6 and ≈10 to these relays, and placed them close to the membrane/electrolyte interface.

Failure Analysis Turned Upside Down: A Review of Backside Analysis Techniques

Abstract This article provides a high-level review of the tools and techniques used for backside analysis. It discusses the use of laser scanning and conventional microscopy, liquid and solid immersion lenses, photon emission microscopy (PEM), and laser-based fault isolation methods with emphasis on light-induced voltage alteration (LIVA). It explains how laser voltage probing is used for backside waveform acquisition and describes backside sample preparation and deprocessing techniques including parallel polishing and milling, laser chemical etching, and FIB circuit edit and modification.

ABCs of IVAs

Abstract This article provides a qualitative overview of several new defect localization techniques, including charge-induced voltage alteration (CIVA), light-induced voltage alteration (LIVA), thermally-induced voltage alteration (TIVA), and Seebeck effect imaging (SEI). It explains how each method works in terms of the physics of signal generation and the types of images they produce. It also includes a summary highlighting the similarities and differences of each technique.

Techniques for Localization of IC Interconnection Defects

AbstractThe advances in integrated circuit technology has made failure site localization extremely challenging. Charge-Induced Voltage Alteration (CIVA), Low Energy CIVA (LECIVA), Light-Induced Voltage Alteration (LIVA), Seebeck Effect Imaging (SEI) and Thermally-Induced Voltage Alteration (TIVA) are five recently developed failure analysis techniques which meet the challenge by rapidly and non-destructively localizing interconnection defects on ICs. The techniques take advantage of voltage fluctuations in a constant current power supply as an electron or photon beam is scanned across an IC. CIVA and LECIVA are scanning electron microscopy (SEM) techniques that yield rapid localization of open interconnections. LIVA is a scanning optical microscopy (SOM) method that yields quick identification of damaged semiconductor junctions and determines transistor logic states. SEI and TIVA are SOM techniques that rapidly localize open interconnections and shorts respectively. LIVA, SEI, and TIVA can be performed from the backside of ICs by using the proper photon wavelength. CIVA, LECIVA, LIVA, TIVA, and SEI techniques in terms of the physics of signal generation, data acquisition system required, and imaging results displaying the utility of each technique for localizing interconnection defects. In addition to the techniques listed above, the Resistive Contrast Imaging (RCI) for localizing opens on metal test patterns will be described as a starting point for the “IVA” technologies.

Global fault localization using induced voltage alteration

Advances in integrated circuit technology have made failure site localization extremely challenging. Charge-induced voltage alteration (CIVA), low energy CIVA (LECIVA), light-induced voltage alteration (LIVA), Seebeck effect imaging (SEI) and thermally-induced voltage alteration (TIVA) are five recently developed failure analysis techniques which meet the challenge by rapidly and non-destructively localizing interconnection defects on ICs. The techniques take advantage of voltage fluctuations in a constant current power supply as an electron or photon beam is scanned across an IC. CIVA and LECIVA are scanning electron microscopy techniques that yield rapid localization of open interconnections. LIVA is a scanning optical microscopy (SOM) method that yields quick identification of damaged semiconductor junctions and determines transistor logic states. SEI and TIVA are SOM techniques that rapidly localize open interconnections and shorts respectively. LIVA, SEI, and TIVA can be performed from the backside of ICs by using the proper photon wavelength. This paper describes the CIVA, LECIVA, LIVA, TIVA, and SEI techniques in terms of the physics of signal generation, general data acquisition system required, and imaging results displaying the utility of each technique for localizing interconnection defects.