An above-room-temperature ferroelectric two-dimensional halide double perovskite with direction dependent properties.

Herein, we introduce a new non-centrosymmetric 2D halide double perovskite Cl0.26Br3.74PA4AgSbBr8 (CPAS) with A-site linker, 3-chloropropylamine. CPAS exhibits above room-temperature ferroelectricity, with spontaneous polarization of 3.4 µC cm-2. The polarization is direction dependent, where ferroelectricity is observed perpendicular to octahedral sheets but mere capacitive phenomena between the sheets. The material shows defect- or phonon-mediated charge conduction. The optical properties reveal a bandgap of 2.48 eV with blue emission.

Direction Dependent Ferroelectricity and Conductivity in a Single Crystal 2D Halide Double Perovskite.

Halide ferroelectric materials have garnered a lot of interest because of their distinctive electrical and structural characteristics. In this study, the design and development of a new non-centrosymmetric 2D layered halide double perovskite material, Cl1.14Br2.86PA4AgInBr8 (CPAIn) is reported. This material shows ferroelectric properties above room temperature, with a Curie temperature of 190 °C. This behavior is achieved through the substitution of the halogenated A-site organic linker, 3-chloropropylammonium. CPAIn exhibits anisotropic ferroelectric behavior with higher spontaneous polarization of 6.25 µC cm-2 along the perpendicular direction to the octahedral layers, whereas the value decreases to 0.174 µC cm-2 between sheets. While using bottom contact to study the nature of polarity within a sheet, the P-E loop displays capacitive loop. The nature and value of polarization is highly direction dependent, and to further understand the mechanism of conduction, a combination of temperature-dependent impedance studies and poling dependent conductivity techniques are employed. These directional dependent properties hold immense potential in memory devices, sensors and photovoltaics, piezoelectric devices and energy storage.

Bilayer Porous Electrocatalysts for Stable and Selective Electrochemical Reduction of CO2 to Formate in the Presence of Flue Gas Containing NO and SO2.

The development of stable and selective electrocatalysts for converting CO2 to value-added chemicals or fuels has gained much interest in terms of their potential to mitigate anthropogenic carbon emissions. Most of the electrocatalysts are tested under pure CO2; however, industrial outlet flue gas contains numerous impurities, such as NO and SO2, which poison the electrocatalysts and alter the product selectivity. Developing electrocatalysts that are resistant to such impurities is essential for commercial implementation. Herein, we prepared bilayer porous electrocatalysts, namely, Sn, Bi, and In, on porous Cu foam mesh (Sn/Cu-f, Bi/Cu-f, and In/Cu-f) by a two-step electrodeposition process and employed these electrodes for the electrochemical reduction of CO2 to formate. It was observed that the bilayer porous electrocatalysts exhibited high CO2 reduction activity compared to catalysts coated on a Cu mesh. Among bilayer porous electrocatalysts, Sn/Cu-f and Bi/Cu-f electrocatalysts showed more than 80% faradaic efficiency (FE) toward formate production, with a formate partial current density of around -16 and -10.4 mA cm-2, respectively, at -1.02 V vs RHE. In/Cu-f electrocatalyst showed nearly 40% formate FE with formate partial current density of -15 mA cm-2 at -1.22 V vs RHE. We investigated the effect of NO and SO2 impurities (500 ppm of NO, 800 ppm of SO2, and 500 ppm of NO + 800 ppm of SO2) on these electrocatalysts' selectivity and stability toward formate. It was observed that the Bi/Cu-f electrocatalyst showed 50 h stability with 80 ± 5% formate FE, and Sn/Cu-f showed 18 h stability with above 80 ± 5% efficiency in the presence of NO and SO2 mixed with CO2. Furthermore, we studied the effect of CO2 concentration with Sn/Cu-f and Bi/Cu-f catalysts in the range of 15-100% CO2, for which formate FEs of 45-80% were observed.

Conducting Gold Nanoparticle Films via Sessile Drop Evaporation.

The deposit patterns obtained from the evaporation of drops containing insoluble solute particles are vital for several technologies, including inkjet printing and optical and electronic device manufacturing. In this work, we consider the evaporation of an aqueous reaction mixture typically used for gold nanoparticle (AuNP) synthesis. The patterns obtained from the evaporation-driven assembly of in situ generated AuNPs are studied using optical microscopy and SEM analyses. The evaporation of drops withdrawn at different reaction times is found to significantly influence the distribution of AuNPs in the dried patterns. The evolution of the deposit patterns is also explored by drying multiple drops on the solid substrate, wherein a drop of a fresh reaction mixture is introduced over the deposit pattern left by the evaporation of the drop dispensed at an earlier time. Using quantitative image analysis, we show that the interparticle separation between the AuNPs in the dried patterns left on the solid substrate decreases when the number of drops is increased. We find optimal conditions to achieve solid-supported AuNP films, wherein the particles are in close physical contact, leading to a conducting deposit. The current through the AuNP deposit is found to increase with increase in the number of drops due to evaporation-driven self-assembly of AuNPs into branch-like structures with reduced interparticle separation. In addition, we also show that it is possible to produce conducting AuNP deposits by drying multiple drops withdrawn from the same reaction mixture. The evaporation-driven assembly of the in situ grown nanoparticles from a reaction mixture presented in this work can be further exploited in optical and electronic device fabrication.

Dimensional Reduction of Cs2AgBiBr6 Using Alkyl Ammonium Cations CH3(CH2)nNH3+ (n = 1, 2, 3, and 6) of Varying Chain Lengths and Their Role in Structural and Optoelectronic Properties.

Tuning the dimensionality in halide perovskites provides an opportunity to obtain the properties desired for optoelectronic devices. In this work, we demonstrate the dimensional reduction of 3D halide double-perovskite Cs2AgBiBr6 by systematically introducing alkylammonium organic spacer CH3(CH2)nNH3+ (n = 1, 2, 3, and 6) of varying chain lengths. The single crystals of these materials were grown, and their structures were studied at 23 and -93 °C. The ethylammonium cation led to a formation of a 0D material, whereas all the other three higher alkyl ammonium spacers resulted in two-dimensional materials. The parent material possessed symmetric octahedra, whereas the modified samples led to both inter- and intra-octahedral distortion, thereby reducing the symmetry of constituent octahedra. The reduction in dimensionality led to a blue shift in the optical absorption spectrum. All these low-dimensional materials show excellent stability, and they are employed as absorbers for solar photovoltaics.

The effect of halogenated spacer cations on structural symmetry-breaking in 2D halide double perovskites.

Herein, we present a strategy to introduce above-room temperature non-centrosymmetry into two-dimensional halide double perovskites (A'4M'M''X8) using a halogenated A'-site organic linker, 3-chloro/bromo propyl amine. These crystals exhibit anisotropic polarization with three orders of magnitude variation between different crystallographic axes. The non-centrosymmetry is further confirmed by piezo-force microscopy studies and its role in the thermal and optical properties was investigated.

Structural distortion induced broad emission in vacancy-ordered halide triple perovskites.

Structural distortion in halide perovskites is important to tune the optical properties of the materials. The octahedra formed by metal cations and halide anions in these classes of materials remain symmetric; however, the introduction of asymmetry provides enormous opportunities to improve the photoluminescence emission and excited-state lifetimes for their application in white light emitters. In this work, we have systematically introduced asymmetry in vacancy-ordered halide triple perovskite materials Cs3M2X9 (M = Bi3+, Sb3+; X = Cl-, Br-, I-) by mixing trivalent sites in three different halide compounds. The Raman and FT-far-IR measurements were used to investigate the distortion introduced in these materials. The distortion is shown to (i) enhance self-trapped excitonic emission, which is broad and intense leading to emission in the complete visible region and (ii) improve excited-state lifetimes. This strategy to create distortion and its proven ability to improve light emission will find application in light-emitting diodes.

BiVO4/Cs2PtI6 Vacancy-Ordered Halide Perovskite Heterojunction for Panchromatic Light Harvesting and Enhanced Charge Separation in Photoelectrochemical Water Oxidation.

Photoelectrochemical water oxidation is a challenging reaction in solar water splitting due to the parasitic recombination process, sluggish catalytic activity, and electrode stability. Oxide semiconductors are stable in an aqueous medium but show huge charge carrier recombination. Creation of a heterojunction is found to be effective for extracting the photogenerated electrons/holes before they recombine to the ground state. In this work, we created a heterojunction of BiVO4 with vacancy-ordered halide perovskite Cs2PtI6 and used it as a photoanode in PEC water oxidation. Cs2PtI6 is the only halide perovskite that is found to be extremely stable even in strong acids and bases. We utilized the stability of this material and its panchromatic visible light absorption property and made the first unprotected heterojunction dual-absorber photoanode for PEC water oxidation. At 1.23 V (vs RHE), bare BiVO4 gave 0.6 mA cm-2 photocurrent density, whereas the BiVO4/Cs2PtI6 heterojunction shows 0.92 mA cm-2. With the addition of IrOx cocatalyst, at 1.23 V (vs RHE), the heterojunction gave ∼2 mA cm-2. To obtain 2 mA cm-2 photocurrent, pure BiVO4 requires 560 mV overpotential, whereas the heterojunction requires 250 mV. The increase in the photocurrent arises from the increase in the efficiency of charge separation from BiVO4 to Cs2PtI6 and the complementary absorption offered by the latter.

Solar energy storage in a Cs2AgBiBr6 halide double perovskite photoelectrochemical cell.

Storing solar energy using a stable visible light absorbing Cs2AgBiBr6 double perovskite is achieved using a photoelectrochemical (PEC) device with cobalt complexes and methyl viologen redox mediators. Under illumination, a potential gain of nearly 500 mV is achieved for charging. The charge-discharge cycling was carried out, and using in situ emission and FTIR studies, the self-discharge and solvent crossover were investigated.

Cs2 PtI6 Halide Perovskite is Stable to Air, Moisture, and Extreme pH: Application to Photoelectrochemical Solar Water Oxidation.

Halide perovskites show incredible photovoltaic power conversion efficiency coupled with several hundreds of hours of device stability. However, their stability is poor in aqueous electrolyte media. Reported here is a vacancy ordered halide perovskite, Cs2 PtI6 , which shows extraordinary stability under ambient conditions (1 year), in aqueous media of extreme acidic (pH 1), basic (pH 13), and under electrochemical reduction conditions. It was employed as an electrocatalyst and photoanode for hydrogen production and water oxidation, respectively. The catalyst remains intact for at least 100 cycles of electrochemical cycling and six hours of hydrogen production at pH 1. Cs2 PtI6 was employed as a photoanode for PEC water oxidation, and the material displayed a photocurrent of 0.8 mA cm-2 at 1.23 V (vs. RHE) under simulated AM1.5G sunlight. Using constant voltage measurement, Cs2 PtI6 exhibited over 12 hours of PEC stability without loss of performance.

Investigation on the Interface Modification of TiO2 Surfaces by Functional Co-Adsorbents for High-Efficiency Dye-Sensitized Solar Cells.

The influence of interface modification of sensitized TiO2 surfaces by co-adsorbents on photovoltaic performance is detailed. We investigated different functional groups of co-adsorbents, such as carboxylic (4-guanidino butyric acid, chenodeoxycholic acid), phosphinic (dineohexyl phosphinic acid), and phosphonic (dodecyl phosphonic acid), to better highlight their influence on the device performance and accurately classify them into de-aggregating agents or agents with both de-aggregating and co-adsorbing properties. By optimizing the type of co-adsorbent and its concentration in the dye solution, we reached an efficiency of 11.0 % using 4-guanidino butyric acid or dineohexyl phosphinic acid, compared to 10.6 % when the benchmark chenodeoxycholic acid was used. The presence of co-adsorbents on the TiO2 surface was studied using ATR-FTIR spectroscopy. The role of these co-adsorbents on the band edge shift versus the recombination resistance is discussed.

Pyridyl- and Picolinic Acid Substituted Zinc(II) Phthalocyanines for Dye-Sensitized Solar Cells.

A series of tri-tert-butyl zinc(II) phthalocyanines (Pcs) substituted with pyridyl, carboxyl, or picolinic acid anchoring groups on the periphery were prepared. Photovoltaic (PV) studies on these dyes were carried out revealing some interesting features. In the case of the pyridyl-substituted Pcs, the PV properties were found to depend strongly on the the pyridyl substitution pattern (meta or para) and the number of pyridyl units at the macrocycle's periphery (one or two). For these four pyridyl-substituted Pcs, higher photovoltaic efficiencies were obtained for 1) the para- versus the meta-substituted Pcs, and 2) the mono- versus the bis-functionalized dyes. In order to improve the poor adsorption of the pyridyl-substituted Pcs onto TiO2 , a new dye was tested bearing a picolinic acid unit. This moiety combines a carboxylic acid function, as a strong anchoring group for binding to TiO2 , with an electron-withdrawing nitrogen atom for better electron injection into the semiconductor's conduction band. For this latter system, an improvement in the PV efficiency up to 2.1 % was obtained.

Double D-π-A dye linked by 2,2'-bipyridine dicarboxylic acid: influence of para- and meta-substituted carboxyl anchoring group.

Starting from 2,2'-bipyridine dicarboxylic acid, two new (D-π-A)2 sensitizers, including m-DA with the carboxyl anchoring group substituted meta to the donor-bridge moiety and p-DA with a para-substituted anchoring group, were synthesized in order to evaluate the impact of the position of the anchoring group on the optical, electrochemical, and photovoltaic properties of dye-sensitized solar cells. p-DA exhibits red-shifted absorption behavior compared to m-DA, owing to the more efficiently extended π-conjugation with para substitution. Both m-DA and p-DA are adsorbed on the mesoporous TiO2 surface by using both of their carboxylic acid groups in a bianchoring mode, which is confirmed through attenuated total reflectance FTIR analysis. Red-shifted absorption of p-DA assists the achievement of a red-shifted incident photon-to-electron conversion efficiency and a higher short-circuit current density than m-DA. The photogenerated electron lifetime in TiO2 is also found to be higher for para substituted p-DA than the meta-substituted m-DA, which results in a higher open-circuit voltage. All of the results suggest that dicarboxyl-2,2'-bipyridine can be used as an acceptor for metal-free organic sensitizers. However, the anchoring segments should be adjusted to the favorable position of the corresponding donor-bridge moieties for better conjugation.

Sterically hindered phthalocyanines for dye-sensitized solar cells: influence of the distance between the aromatic core and the anchoring group.

A new phthalocyanine (Pc) bearing bulky peripheral substituents and a carboxylic anchoring group directly attached to the macrocycle has been prepared and used as a sensitizer in DSSCs, reaching 5.57% power conversion efficiency. In addition, an enhanced performance for the TT40 dye, previously reported by us, was achieved in optimized devices, obtaining a new record efficiency with Pc-sensitized cells.

Quantum-confined ZnO nanoshell photoanodes for mesoscopic solar cells.

We present a photoanode for dye-sensitized solar cell (DSC) based on ZnO nanoshell deposited by atomic layer deposition at 150 °C on a mesoporous insulating template. An ultrathin layer of ZnO between 3 and 6 nm, which exhibits quantum confinement effect, is found to be sufficient to transport the photogenerated electrons to the external contacts and exhibits near-unity collection efficiency. A 6 nm ZnO nanoshell on a 2.5 µm mesoporous nanoparticle Al2O3 template yields photovoltaic power conversion efficiency (PCE) of 4.2% in liquid DSC. Perovskite absorber (CH3NH3PbI3) based solid state solar cells made with similar ZnO nanostructures lead to a high PCE of 7%.

Analysis of electron transfer properties of ZnO and TiO2 photoanodes for dye-sensitized solar cells.

Mesoporous TiO2 nanoparticle films are used as photoanodes for high-efficiency dye-sensitized solar cells (DSCs). In spite of excellent photovoltaic power conversion efficiencies (PCEs) displayed by titanium dioxide nanoparticle structures, the transport rate of electrons is known to be low due to low electron mobility. So the alternate oxides, including ZnO, that possesses high electron mobility are being investigated as potential candidates for photoanodes. However, the PCE with ZnO is still lower than with TiO2, and this is typically attributed to the low internal surface area. In this work, we attempt to make a one-to-one comparison of the photovoltaic performance and the electron transfer dynamics involved in DSCs, with ZnO and TiO2 as photoanodes. Previously such comparative investigations were hampered due to the morphological differences (internal surface area, pore diameter, porosity) that exist between zinc oxide and titanium dioxide films. We circumvent this issue by depositing different thicknesses of these oxides, by atomic layer deposition (ALD), on an arbitrary mesoporous insulating template and subsequently using them as photoanodes. Our results reveal that at an optimal thickness ZnO exhibits photovoltaic performances similar to TiO2, but the internal electron transfer properties differ. The higher photogenerated electron transport rate contributed to the performances of ZnO, but in the case of TiO2, it is the low recombination rate, higher dye loading, and fast electron injection.

Adapting Ruthenium Sensitizers to Cobalt Electrolyte Systems.

In this work, we report the use of bulky substitutions in a new heteroleptic ruthenium(II) bipyridine complex, Ru(NCS)2LL', coded TT-230 to obtain high open-circuit potential in a dye-sensitized solar cell (where L is a bipyridine ligand appended with two cyclopenta(2,1-b;3,4-bA)dithiophene moieties, and L' = 4,4,'-dicarboxylic acid 2,2'-bipyridine). The electrolytes based on cobalt complexes have shown significant advantages in terms of attainable open-circuit potential compared to the standard iodide/tri-iodide redox mediators. These merits of the cobalt complexes were previously realized with a porphyrin sensitizer, achieving a VOC greater than 1 V in DSC. However, with conventional Ru(II)-polypyridyl complexes such as the C101 dye, similar increase in the VOC could not be attained due to the enhanced recombination. In this work, we have shown that the use of bulky substituents can prevent the back reaction of photogenerated electron and subsequently increase the open-circuit potential of the device. The recombination processes were investigated by transient photovoltage decay measurements.

Yttrium-substituted nanocrystalline TiO2 photoanodes for perovskite based heterojunction solar cells.

We report the use of Y(3+)-substituted TiO2 (0.5%Y-TiO2) in solid-state mesoscopic solar cells, consisting of CH3NH3PbI3 as the light harvester and spiro-OMeTAD as the hole transport material. A power conversion efficiency of 11.2% under simulated AM 1.5 full sun illumination was measured. A 15% improvement in the short-circuit current density was obtained compared with pure TiO2, due to the effect of Y(3+) on the dimensions of perovskite nanoparticles formed on the semiconductor surface, showing that the surface modification of the semiconductor is an effective way to improve the light harvesters' morphology and electron transfer properties in the solid-state mesoscopic solar cells.

The application of electrospun titania nanofibers in dye-sensitized solar cells.

Titania nanofibers were fabricated using the industrial Nanospider(TM) technology. The preparative protocol was optimized by screening various precursor materials to get pure anatase nanofibers. Composite films were prepared by mixing a commercial paste of nanocrystalline anatase particles with the electrospun nanofibers, which were shortened by milling. The composite films were sensitized by Ru-bipyridine dye (coded C106) and the solar conversion efficiency was tested in a dye-sensitized solar cell filled with iodide-based electrolyte solution (coded Z960). The solar conversion efficiency of a solar cell with the optimized composite electrode (η = 7.53% at AM 1.5 irradiation) outperforms that of a solar cell with pure nanoparticle film (η = 5.44%). Still larger improvement was found for lower light intensities. At 10% sun illumination, the best composite electrode showed η = 7.04%, referenced to that of pure nanoparticle film (η = 4.69%). There are non-monotonic relations between the film's surface area, dye sorption capacity and solar performance of nanofiber-containing composite films, but the beneficial effect of the nanofiber morphology for enhancement of the solar efficiency has been demonstrated.