Ultra-wideband, wide angle, asymmetric transmission based chiral metasurface for C and X band applications.

A multi-layered chiral device manifesting asymmetric transmission (AT) facilitating one-way channeling of electromagnetic (EM) waves, based on the inherent polarization is presented. The designed metasurface depicts a high transmission contrast with an efficiency above 80% for an ultra-wide operational range of 6.3-12.3 GHz, constituting a fractional bandwidth of 64%. As an additional feature, the reported metasurface yields robustness against oblique incidences up to 45[Formula: see text] while maintaining high transmission efficiency. This report also introduces a unique analogy of the AT based communication system with logic-gates by formulating its truth-table and logic circuit. Furthermore, new insights of AT magnitude's dependence to oblique incidences are presented on the account of surface impedance mismatch due to TE and TM polarization with varying incidence angle. Moreover, avoidance of grating lobes and the associated transmission deterioration through utilization of electrically small periodic metasurface is presented. The results have been numerically and practically validated yielding state-of-the-art features. Operating within C and X band, the reported work is an ideal candidate for practical AT applications.

A 25 mA cm-2 dye-sensitized solar cell based on a near-infrared-absorbing organic dye and application of the device in SSM-DSCs.

A blue organic near infrared (NIR) absorbing sensitizer, AP25, is investigated for use in broadly absorbing dye-sensitized solar cells (DSCs). AP25 shows solar-to-electric conversion with an onset near 900 nm in DSC devices and a photocurrent near 25 mA cm-2 when co-sensitized. An all-organic SSM-DSC device reaches 10.3% PCE.

Precious metal-free solar-to-fuel generation: SSM-DSCs powering water splitting with NanoCOT and NiMoZn electrocatalysts.

A precious metal-free sequential series multijunction dye-sensitized solar cell (SSM-DSC)-powered water electrolysis system is demonstrated using NanoCOT and NiMoZn electrodes. A stable 3.9% solar-to-hydrogen (STH) efficiency is achieved using a recently reported black organic dye and graphene electrodes for DSCs.

Copper-based redox shuttles supported by preorganized tetradentate ligands for dye-sensitized solar cells.

Three copper redox shuttles ([Cu(1)]2+/1+, [Cu(2)]2+/1+, and [Cu(3)]2+/1+) featuring tetradentate ligands were synthesized and evaluated computationally, electrochemically, and in dye-sensitized solar cell (DSC) devices using a benchmark organic dye, Y123. Neutral polyaromatic ligands with limited flexibility were targeted as a strategy to improve solar-to-electrical energy conversion by reducing voltage losses associated with redox shuttle electron transfer events. Inner-sphere electron transfer reorganization energies (λ) were computed quantum chemically and compared to the commonly used [Co(bpy)3]3+/2+ redox shuttle which has a reported λ value of 0.61 eV. The geometrically constrained biphenyl-based Cu redox shuttles investigated here have lower reorganization energies (0.34-0.53 eV) and thus can potentially operate with lower driving forces for dye regeneration (ΔGreg) in DSC devices when compared to [Co(bpy)3]3+/2+-based devices. The rigid tetradentate ligand design promotes more efficient electron transfer reactions leading to an improved JSC (14.1 mA cm-2), higher stability due to the chelate effect, and a decrease in VlossOC for one of the copper redox shuttle-based devices.

SnO2 Transparent Printing Pastes from Powders for Photon Conversion in SnO2 -Based Dye-Sensitized Solar Cells.

Tin oxide (SnO2 ) is the most attractive alternative to titanium oxide (TiO2 ) with the aim of identifying a more positive conduction band material for dye-sensitized solar cells (DSCs). This study puts forward a protocol based on grinding, sonication, and centrifuge to generate transparent SnO2 pastes to minimize light reflectance losses from the metal oxide. Under optimized conditions, a highly transparent film with substantially enhanced light penetration depth through active layer SnO2 is realized for efficient light harvesting from two different commercially available powders (18 and 35 nm nanoparticle sizes). A ruthenium sensitizer (B11) and two organic sensitizers (NL3 and MK2) are shown to achieve higher or comparable photocurrent densities with SnO2 relative to standard TiO2 -based DSCs. SnO2 -based DSCs show minimum recombination losses, comparable charge collection efficiencies, and minimal photovoltage losses relative to TiO2 DSCs. Thus, the option of a transparent metal oxide, which can facilitate high photocurrents (>16 mA cm-2 observed) and lower recombination rates than TiO2 is an attractive material for DSC applications.

Unique hue stimulus selection using Munsell color chips under different chroma levels and illumination conditions.

To determine the role of chroma and illumination conditions on assessments of unique hue stimuli (UHs: Y, B, R, and G) 25 color normal observers selected Munsell color chips under controlled but different light sources that included simulated D65 daylight, incandescent (A), CWF and TL84, from two rotatable trays-one containing low and the other high chroma samples. Three independent evaluations were obtained from each observer with a gap of at least 24 h between assessments. A total of 2400 UH assessments were thus obtained. The mean, as well as the range and variability, in UH selections was calculated according to gender, illumination condition, and chroma. Results show statistically significant differences for uB and uR stimuli selections from the low to the high chroma sets under all lighting conditions, and for uY under light source A and CWF while differences in uG selections in different lighting conditions were statistically insignificant. No significant difference was found between male and female UH selections. Hue shift predictions based on CAT02 transform were found to agree well with observer responses under different illumination conditions. It was also found that observers' UH choices varied more for low chroma samples under different lighting conditions compared to high chroma samples. The highest variability was obtained under light source CWF, while the simulated D65 source resulted in the lowest variability in selections.

Near-Infrared-Absorbing Indolizine-Porphyrin Push-Pull Dye for Dye-Sensitized Solar Cells.

Porphyrins are attractive chromophores for application in dye-sensitized solar cells (DSCs), as judicious tuning of donor-acceptor properties can enable excellent near-infrared (NIR) absorption and exceptional device performance. Here, we report a porphyrin-based dye (SM85) conjugated to the planar strong electron donor, indolizine, designed to extend absorption further into the NIR region by inducing π-π interactions such as head-to-tail dye aggregation. The optoelectronic consequences of indolizine incorporation in SM85 include raising the ground-state oxidation potential and broadening and red-shifting ultraviolet-visible-NIR absorptions, along with increased molar absorptivity when compared to the dye SM315. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations confirm the push-pull character of SM85, which features an overlap of frontier occupied and unoccupied orbitals. Steady-state spectrophotometric analyses reveal the presence of solution aggregates via absorption and emission spectroscopies. Aggregate modes were probed by DFT and TD-DFT analyses, and plausible models are presented. SM85-based DSC devices demonstrate a 5.7% power conversion efficiency (PCE) at full sun (7.4% PCE at 10% sun) with an exceptional improvement to the incident photon-to-current conversion onset at ∼850 nm. Current dynamics measurements, time-correlated single photon counting, and computational analyses are used to better understand device performances. This study puts forward a novel intramolecular charge-transfer porphyrin system with a dramatic shift into the NIR region, as is needed for nonprecious metal-based sensitizers, and provides an example of controlled, donor-acceptor-mediated aggregation as a complementary strategy to traditional donor-acceptor modifications to single-molecule π-systems in accessing enhancements in long wavelength light harvesting in molecular-based optoelectronic devices.

Iodine binding with thiophene and furan based dyes for DSCs.

Iodine binding to thiophene rings in dyes for dye-sensitized solar cells (DSCs) has been hypothesized to be performance degrading in a number of literature cases. Binding of iodine to dyes near the semiconductor surface can promote undesirable electron transfers and lower the overall efficiency of devices. Six thiophene or furan containing dye analogs were synthesized to analyze iodine binding to the dyes via Raman spectroscopy, UV-Vis studies, device performance metrics and density functional theory (DFT) based computations. Evidence suggests I2 binds thiophene-based dyes stronger than furan-based dyes. This leads to higher DSC device currents and voltages from furan analogues, and longer electron lifetimes in DSC devices using furan based dyes. Raman spectrum of the TiO2 surface-bound dyes reveals additional and more instense peaks for thiophene dyes in the presence of I2 relative to no I2. Additionally, broader and shifted UV-Vis peaks are observed for thiophene dyes in the presence of I2 on TiO2 films suggesting significant interaction between the dye molecules and I2. These observations are also supported by DFT and TD-DFT calculations which indicate the absence of a key geometric energy minimum in the dye-I2 ground state for furan dyes which are readily observed for the thiophene based analogues.

A High-Voltage Molecular-Engineered Organic Sensitizer-Iron Redox Shuttle Pair: 1.4†V DSSC and 3.3†V SSM-DSSC Devices.

The development of high voltage solar cells is an attractive way to use sunlight for solar-to-fuel devices, multijunction solar-to-electric systems, and to power limited-area consumer electronics. By designing a low-oxidation-potential organic dye (RR9)/redox shuttle (Fe(bpy)33+/2+ ) pair for dye-sensitized solar-cell (DSSC) devices, the highest single device photovoltage (1.42 V) has been realized for a DSSC not relying on doped TiO2 . Additionally, Fe(bpy)33+/2+ offers a robust, readily tunable ligand platform for redox potential tuning. RR9 can be regenerated with a low driving force (190 mV), and by utilizing the RR9/Fe(bpy)33+/2+ redox shuttle pair in a subcell for a sequential series multijunction (SSM)-DSSC system, one of the highest known three subcell photovoltage was attained for any solar-cell technology (3.34 V, >1.0 V per subcell).

Ullazine Donor-π bridge-Acceptor Organic Dyes for Dye-Sensitized Solar Cells.

A series of four ullazine-donor based donor-π bridge-acceptor (D-π-A) dyes have been synthesized and compared to a prior ullazine donor-acceptor (D-A) dye as well as a triphenylamine donor with an identical π-bridge and acceptor. The D-π-A ullazine series demonstrates an unusually uniform-in-intensity panchromatic UV/Vis absorption spectrum throughout the visible region. This is in part due to the introduction of strong high-energy bands through incorporation of the ullazine building block as shown by computational analysis. The dyes were characterized on TiO2 films and in DSC devices. Performances of 5.6 % power conversion efficiency were obtained with IPCE onsets reaching 800 nm.

Panchromatic cross-conjugated π-bridge NIR dyes for DSCs.

Four organic sensitizers incorporating a cross-conjugated cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) π-bridge have been synthesized. As a result of molecular engineering, broad high energy bands and red shifted absorption maxima and onset is observed relative to a benchmark analogue (C218) using a non-cross-conjugated CPDT π-bridge. The use of a cross-conjugated bridge allows a new strategy for tuning dye energetics and introduction of increased absorption uniformity by adding additional high-energy absorption bands. These dyes show solar-to-electric conversion up to 800 nm with one derivative exceeding the performance of C218 under identical conditions.

Near-Infrared-Absorbing Metal-Free Organic, Porphyrin, and Phthalocyanine Sensitizers for Panchromatic Dye-Sensitized Solar Cells.

Dye-sensitized solar cells (DSCs) are a promising source of renewable energy. However, the power conversion efficiency (PCE) of devices has been limited largely by the difficulty of producing electricity using photons from the near-infrared (NIR) spectral region. Metal-free organic sensitizers frequently employ strong electron-donating or -withdrawing moieties to tune the optical band gap to allow the absorption of lower energy wavelengths in charge-transfer systems, whereas porphyrins and phthalocyanines use substituents to shift the Soret and Q bands toward lower energy absorption. Very few devices employing precious metal-free dyes have achieved panchromatic and NIR photon conversion for electricity generation at wavelengths >750 nm despite a tremendous number of sensitizers published over the last 25 years. This Minireview seeks to compile a summary of these sensitizers to encourage assimilation, analysis, and development of efficient future sensitizers with absorption extending into the NIR. Herein, we discuss common synthetic strategies, optical properties, and electronic properties of the most successful panchromatic organic sensitizers.

A wearable sensor based multi-criteria-decision-system for real-time seizure detection.

This paper presents a wireless, low power and low cost two part wearable for real-time epileptic seizure detection. Using parameters of Electro-cardiograph (ECG), Electro-dermal Activity (EDA), body motion and breathing rate (BR), a novel multi-criteria-decision-system (MCDS) is proposed that reduces false alarms and true negatives. The combination of a chest and hand worn wearable continuously senses these parameters transmitting the data to a smart phone application via BLE 4.0 where long-short-term-memory (LSTM) based anomaly detection algorithms and logistic classifiers decide on the occurrence of the seizure in real time. A 96% precision and 90% recall is achieved through testing on synthetic data.

Molecular Engineering of Near Infrared Absorbing Thienopyrazine Double Donor Double Acceptor Organic Dyes for Dye-Sensitized Solar Cells.

The thienopyrazine (TPz) building block allows for NIR photon absorption in dye-sensitized solar cells (DSCs) when used as a π-bridge. We synthesized and characterized 7 organic sensitizers employing thienopyrazine (TPz) as a π-bridge in a double donor, double acceptor organic dye design. Donor groups are varied based on electron donating strength and sterics at the donor-π bridge bond with the acceptor groups varied as either carboxylic acids or benzoic acids on the π-bridge. This dye design was found to be remarkably tunable with solution absorption onsets ranging from 750 to near 1000 nm. Interestingly, the solution absorption measurements do not accurately approximate the dye absorption on TiO2 films with up to a 250 nm blue-shift of the dye absorption onset on TiO2. This shift in absorption and the effect on electron transfer properties is investigated via computational analysis, time-correlated single photon counting studies, and transient absorption spectroscopy. Structure-performance relationships were analyzed for the dyes in DSC devices with the highest performance observed at 17.6 mA/cm2 of photocurrent and 7.5% PCE for a cosensitized device with a panchromatic IPCE onset of 800 nm.

Low-Recombination Thieno[3,4-b]thiophene-Based Photosensitizers for Dye-Sensitized Solar Cells with Panchromatic Photoresponses.

We report four NIR photosensitizers employing a low-recombination donor and a thieno[3,4-b]thiophene (3,4-TT) π bridge for use in dye-sensitized solar cells. The inclusion of electron rich π spacers red-shifts the dye absorbance with solution absorption onsets reaching 700 nm. Dyes were found to have suitable energy levels for rapid electron transfers using cyclic voltammetry and UV/Vis-NIR absorption spectroscopy. Computationally optimized ground-state geometries show an increased torsional angle between π spacer and π bridge brought about by an added alkyl chain. This results in a widened optical band gap and increased oxidation potentials owing to a weakening of the electron-accepting ability of 3,4-TT for solution-state measurements. Interestingly in terms of device parameters, the alkylated π spacer had a nearly identical incident photon-to-current conversion efficiency (IPCE) curve onset when compared to a non-alkylated analogue, suggesting more similar dye geometries on the surface of TiO2 . Elevated short-circuit current density (JSC ) values and comparable open-circuit voltage (VOC ) values were observed in the alkylated-π-spacer-dye-based devices with power conversion efficiencies up to 6.8 % observed with IPCE onsets exceeding 800 nm.

Effect of Donor Strength and Bulk on Thieno[3,4-b]-pyrazine-Based Panchromatic Dyes in Dye-Sensitized Solar Cells.

Near-infrared-absorbing organic dyes are critically needed in dye-sensitized solar cells (DSCs). Thieno[3,4-b]pyrazine (TPz) based dyes can access the NIR spectral region and show power conversion efficiencies (PCEs) of up to 8.1 % with sunlight being converted at wavelengths up to 800 nm for 17.6 mA cm-2 of photocurrent in a co-sensitized DSC device. Precisely controlling dye excited-state energies is critical for good performances in NIR DSCs. Strategies to control TPz dye energetics with stronger donor groups and TPz substituent choice are evaluated here. Additionally, donor size influence versus dye loading on TPz dyes is analyzed with respect to the TiO2 surface protection designed to prevent recombination of electrons in TiO2 with the redox shuttle. Importantly, the dyes evaluated were demonstrated to work well with low Li+ concentration electrolytes, with iodine and cobalt redox shuttle systems, and efficiently as part of co-sensitized devices.

Harnessing Photovoltage: Effects of Film Thickness, TiO2 Nanoparticle Size, MgO and Surface Capping with DSCs.

High photovoltage dye-sensitized solar cells (DSCs) offer an exceptional opportunity to power electrocatalysts for the production of hydrogen from water and the reduction of CO2 to usable fuels with a relatively cost-effective, low-toxicity solar cell. Competitive recombination pathways such as electron transfer from TiO2 films to the redox shuttle or oxidized dye must be minimized to achieve the maximum possible photovoltage (Voc) from DSC devices. A high Voc of 882 mV was achieved with the iodide/triiodide redox shuttle and a ruthenium NCS-ligated dye, HD-2-mono, by utilizing a combined approach of (1) modulating the TiO2 surface area through film thickness and nanoparticle size selection, (2) addition of a MgO insulating layer, and (3) capping available TiO2 film surface sites post film sensitization with an F-SAM (fluorinated self-assembled monolayer) treatment. The exceptional Voc of 882 mV observed is the highest achieved for the popular NCS containing ruthenium sensitizers with >5% PCE and compares favorably to the 769 mV value observed under common device preparation conditions.

Disposable, Paper-Based, Inkjet-Printed Humidity and H2S Gas Sensor for Passive Sensing Applications.

An inkjet-printed, fully passive sensor capable of either humidity or gas sensing is presented herein. The sensor is composed of an interdigitated electrode, a customized printable gas sensitive ink and a specialized dipole antenna for wireless sensing. The interdigitated electrode printed on a paper substrate provides the base conductivity that varies during the sensing process. Aided by the porous nature of the substrate, a change in relative humidity from 18% to 88% decreases the electrode resistance from a few Mega-ohms to the kilo-ohm range. For gas sensing, an additional copper acetate-based customized ink is printed on top of the electrode, which, upon reaction with hydrogen sulphide gas (H2S) changes, both the optical and the electrical properties of the electrode. A fast response time of 3 min is achieved at room temperature for a H2S concentration of 10 ppm at a relative humidity (RH) of 45%. The passive wireless sensing is enabled through an antenna in which the inner loop takes care of conductivity changes in the 4-5 GHz band, whereas the outer-dipole arm is used for chipless identification in the 2-3 GHz band.

Effect of field view size and lighting on unique-hue selection using Natural Color System object colors.

The aim of this study was twofold, first to determine the effect of field view size and second of illumination conditions on the selection of unique hue samples (UHs: R, Y, G and B) from two rotatable trays, each containing forty highly chromatic Natural Color System (NCS) samples, on one tray corresponding to 1.4° and on the other to 5.7° field of view size. UH selections were made by 25 color-normal observers who repeated assessments three times with a gap of at least 24h between trials. Observers separately assessed UHs under four illumination conditions simulating illuminants D65, A, F2 and F11. An apparent hue shift (statistically significant for UR) was noted for UH selections at 5.7° field of view compared to those at 1.4°. Observers' overall variability was found to be higher for UH stimuli selections at the larger field of view. Intra-observer variability was found to be approximately 18.7% of inter-observer variability in selection of samples for both sample sizes. The highest intra-observer variability was under simulated illuminant D65, followed by A, F11, and F2.

A femtosecond study of the anomaly in electron injection for dye-sensitized solar cells: the influence of isomerization employing Ru(II) sensitizers with anthracene and phenanthrene ancillary ligands.

In this study, an intriguing difference caused by structural isomerization based on anthracene and phenanthrene stilbazole type ancillary ligands in Ru(ii) sensitizers for dye sensitized solar cells (DSCs) has been investigated using femtosecond transient absorption spectroscopy. Both anthracene and phenanthrene based sensitizers HD-7 and HD-8, respectively, resulted in a similar extinction coefficient, photophysical and thermodynamic free energy of electron injection and dye regeneration as measured by UV-Vis, excited state lifetime and cyclic voltammetry measurements, respectively. However, TiO2 adsorbed HD-7 resulted in up to 45% less photocurrent density than HD-8 although photovoltage was similar owing to comparable thermodynamic characteristics. It was obvious from the measurement of incident photon to current conversion efficiency (IPCE) that excited electrons in HD-7 are prone to internal energy loss before injection into the TiO2 conduction band. Analysis of photo-induced spectral features measured by femtosecond transient absorption spectroscopy showed that excited electrons in HD-7 are prone to ISC (intersystem crossing) much more than HD-8 and those triplet electrons are not injected into TiO2 efficiently. Interestingly, from impedance measurements, HD-7 showed higher recombination resistance than HD-8 and N719, but a shorter lifetime for electrons injected into the TiO2 conduction band.