Electrocatalytic Ammonia Oxidation with a Tailored Molecular Catalyst Heterogenized via Surface Host-Guest Complexation.

Macrocyclic host molecules bound to electrode surfaces enable the complexation of catalytically active guests for molecular heterogeneous catalysis. We present a surface-anchored host-guest complex with the ability to electrochemically oxidize ammonia in both organic and aqueous solutions. With an adamantyl motif as the binding group on the backbone of the molecular catalyst [Ru(bpy-NMe2)(tpada)(Cl)](PF6) (1) (where bpy-NMe2 is 4,4'-bis(dimethylamino)-2,2'-bipyridyl and tpada is 4'-(adamantan-1-yl)-2,2':6',2″-terpyridine), high binding constants with ß-cyclodextrin were observed in solution (in DMSO-d6:D2O (7:3), K11 = 492 ± 21 M-1). The strong binding affinities were also transferred to a mesoporous ITO (mITO) surface functionalized with a phosphonated derivative of ß-cyclodextrin. The newly designed catalyst (1) was compared to the previously reported naphthyl-substituted catalyst [Ru(bpy-NMe2)(tpnp)(Cl)](PF6) (2) (where tpnp is 4'-(naphthalene-2-yl)-2,2':6',2″-terpyridine) for its stability during catalysis. Despite the insulating nature of the adamantyl substituent serving as the binding group, the stronger binding of this unit to the host-functionalized electrode and the resulting shorter distance between the catalytic active center and the surface led to better performance and higher stability. Both guests are able to oxidize ammonia in both organic and aqueous solutions, and the host-anchored electrode can be refunctionalized multiple times (>3) following the loss of the catalytic activity, without a reduction in performance. Guest 1 exhibits significantly higher stability in comparison to guest 2 toward basic conditions, which often constitutes a challenge for anchored molecular systems. Ammonia oxidation in water led to the selective formation of NO3- with Faradaic efficiencies of up to 100%.

Triarylamine-based hydrido-carboxylate rhenium(i) complexes as photosensitizers for dye-sensitized solar cells.

Two new dyes based on a dinuclear rhenium complex and (E)-3-(5-(4-(bis(2',4'-dibutoxy-[1,1'-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (namely D35) have been investigated as sensitizers for dye sensitized solar cells (DSSCs). Two different pyridazine ligands have been used, namely 4-pyridazine-carboxylic acid for dye 2 ([Re2(µ-H)(-D35)(CO)6(µ-pyridazine-4-COOH)]) and 4-pyridazinyl-butanoic acid for dye 3 ([Re2(µ-H)(-D35)(CO)6(µ-pyridazine-4-C3H6-COOH)]). The performances of these new dyes have been compared with those of the dye containing the bare 4-diphenylaminobenzoic acid, namely TPA, as the ancillary ligand (dye 1). Compared to dye 1, dyes 2 and 3 show an impressive tenfold increase in the absorption intensity in the range of 487-493 nm on TiO2 films, with great improvement of the light harvesting. Cyclic voltammetry experiments, performed on derivatives containing the methyl ester of the pyridazine ligands, show narrow electrochemical band gaps in the range of 1.36-1.84 eV. Solar cells with each dye have been prepared, using both iodide/triiodide and cobalt redox couples as the electrolytes, platinum or carbon as the counter electrodes, and TiO2 or SnO2 as the metal oxide photoelectrodes, respectively. The best DSSC results have been obtained using dye 3, with an overall solar-to-electric conversion efficiency of 3.5%, which greatly overcomes the previous result of 1.0% obtained for dye 1 in a not-optimized setup of the device. The performances of dye 3 are due to the presence of D35 ligand, which further suppresses the recombination of the injected electron with the electrolyte and with the oxidized state of the dye.

Covalent Immobilization of a Molecular Catalyst on Cu2O Photocathodes for CO2 Reduction.

Sunlight-driven CO2 reduction is a promising way to close the anthropogenic carbon cycle. Integrating light harvester and electrocatalyst functions into a single photoelectrode, which converts solar energy and CO2 directly into reduced carbon species, is under extensive investigation. The immobilization of rhenium-containing CO2 reduction catalysts on the surface of a protected Cu2O-based photocathode allows for the design of a photofunctional unit combining the advantages of molecular catalysts with inorganic photoabsorbers. To achieve large current densities, a nanostructured TiO2 scaffold, processed at low temperature, was deposited on the surface of protected Cu2O photocathodes. This led to a 40-fold enhancement of the catalytic photocurrent as compared to planar devices, resulting in the sunlight-driven evolution of CO at large current densities and with high selectivity. Potentiodynamic and spectroelectrochemical measurements point toward a similar mechanism for the catalyst in the bound and unbound form, whereas no significant production of CO was observed from the scaffold in the absence of a molecular catalyst.

Ligand Engineering for the Efficient Dye-Sensitized Solar Cells with Ruthenium Sensitizers and Cobalt Electrolytes.

Over the past 20 years, ruthenium(II)-based dyes have played a pivotal role in turning dye-sensitized solar cells (DSCs) into a mature technology for the third generation of photovoltaics. However, the classic I3(-)/I(-) redox couple limits the performance and application of this technique. Simply replacing the iodine-based redox couple by new types like cobalt(3+/2+) complexes was not successful because of the poor compatibility between the ruthenium(II) sensitizer and the cobalt redox species. To address this problem and achieve higher power conversion efficiencies (PCEs), we introduce here six new cyclometalated ruthenium(II)-based dyes developed through ligand engineering. We tested DSCs employing these ruthenium(II) complexes and achieved PCEs of up to 9.4% using cobalt(3+/2+)-based electrolytes, which is the record efficiency to date featuring a ruthenium-based dye. In view of the complicated liquid DSC system, the disagreement found between different characterizations enlightens us about the importance of the sensitizer loading on TiO2, which is a subtle but equally important factor in the electronic properties of the sensitizers.

Nanowire perovskite solar cell.

Organolead iodide perovskite, CH3NH3PbI3, was prepared in the form of nanowire by means of a small quantity of aprotic solvent in two-step spin-coating procedure. One-dimensional nanowire perovskite with the mean diameter of 100 nm showed faster carrier separation in the presence of hole transporting layer and higher lateral conductivity than the three-dimensional nanocuboid crystal. Reduction in dimensionality resulted in the hypsochromic shift of both absorption and fluorescence spectra, indicative of more localized exciton states in nanowires. The best performing device employing nanowire CH3NH3PbI3 delivered photocurrent density of 19.12 mA/cm(2), voltage of 1.052 V, and fill factor of 0.721, leading to a power conversion efficiency (PCE) of 14.71% at standard AM 1.5G solar illumination. A small I-V hysteresis was observed, where a PCE at forward scan was measured to be 85% of the PCE at reverse scan.

Stable and Efficient Perovskite Solar Cells Based on Titania Nanotube Arrays.

Highly ordered 1D TiO2 nanotube arrays are fabricated and applied as nanocontainers and electron transporting material in CH3 NH3 PbI3 perovskite solar cells. The optimized device shows a power conversion efficiency of 14.8%, and improved stability under an illumination of 100 mW cm(-2). This is the best result based on 1D TiO2 nanostructures so far.

Influence of the donor size in D-π-A organic dyes for dye-sensitized solar cells.

We report two new molecularly engineered push-pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, especially in redox electrolyte employing Co(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of IQ4, YA421, and YA422 devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor-π-acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of YA422 is slower than that of the other two dyes, the slower back electron transfer of YA422 contributes to the higher device performance.

One-Step Hydrothermal Synthesis of Sn-Doped Sb2Se3 for Solar Hydrogen Production.

Antimony selenide (Sb2Se3) has recently been intensively investigated and has achieved significant advancement in photoelectrochemical (PEC) water splitting. In this work, a facile one-step hydrothermal method for the preparation of Sn-doped Sb2Se3 photocathodes with improved PEC performance was investigated. We present an in-depth study of the performance enhancement in Sn-doped Sb2Se3 photocathodes using capacitance-voltage (CV), drive-level capacitance profiling (DLCP), and electrochemical impedance spectroscopy (EIS) techniques. The incorporation of Sn2+ into the Sb2Se3 results in increased carrier density, reduced surface defects, and improved charge separation, thereby leading to improved PEC performance. With a thin Sb2Se3 absorber layer (270 nm thickness), the Sn-doped Sb2Se3 photocathode exhibits an improved photocurrent density of 17.1 mA cm-2 at 0 V versus RHE (V RHE) compared to that of the undoped Sb2Se3 photocathode (14.4 mA cm-2). This work not only highlights the positive influence of Sn doping on Sb2Se3 photocathodes but also showcases a one-step method to synthesize doped Sb2Se3 with improved optoelectronic properties.

Solution phase treatments of Sb2Se3 heterojunction photocathodes for improved water splitting performance.

Antimony selenide (Sb2Se3) is an auspicious material for solar energy conversion that has seen rapid improvement over the past ten years, but the photovoltage deficit remains a challenge. Here, simple and low-temperature treatments of the p-n heterojunction interface of Sb2Se3/TiO2-based photocathodes for photoelectrochemical water splitting were explored to address this challenge. The FTO/Ti/Au/Sb2Se3 (substrate configuration) stack was treated with (NH4)2S as an etching solution, followed by CuCl2 treatment prior to deposition of the TiO2 by atomic layer deposition. The different treatments show different mechanisms of action compared to similar reported treatments of the back Au/Sb2Se3 interface in superstrate configuration solar cells. These treatments collectively increased the onset potential from 0.14 V to 0.28 V vs. reversible hydrogen electrode (RHE) and the photocurrent from 13 mA cm-2 to 18 mA cm-2 at 0 V vs. RHE as compared to the untreated Sb2Se3 films. From SEM and XPS studies, it is clear that the etching treatment induces a morphological change and removes the surface Sb2O3 layer, which eliminates the Fermi-level pinning that the oxide layer generates. CuCl2 further enhances the performance due to the passivation of the surface defects, as supported by density functional theory molecular dynamics (DFT-MD) calculations, improving charge separation at the interface. The simple and low-cost semiconductor synthesis method combined with these facile, low-temperature treatments further increases the practical potential of Sb2Se3 for large-scale water splitting.

Impedance spectroscopy of Sb2Se3 photovoltaics consisting of (Sb4Se6)n nanoribbons under light illumination.

Sb2Se3, consisting of one-dimensional (Sb4Se6)n nanoribbons has drawn attention as an intriguing light absorber from the photovoltaics (PVs) research community. However, further research is required on the performance-limiting factors in Sb2Se3 PVs. In this study, we investigated the charge carrier behavior in Sb2Se3 PVs by impedance spectroscopy (IS) under light illumination. (Sb4Se6)n nanoribbons with two different orientations were used to investigate the effect of crystal orientation on the device performance. Regardless of the (Sb4Se6)n orientation, negative capacitance was observed at forward bias, representing a recombination pathway at the TiO2/Sb2Se3 interface. A comparison of the recombination resistances and lifetimes of two different Sb2Se3 PVs showed that a better interface could be formed by placing the (Sb4Se6)n ribbons parallel to the TiO2 layer. Based on these observations, an ideal structure of the Sb2Se3/TiO2 interface is proposed, which will enhance the performance of Sb2Se3 PVs toward its theoretical limit.

An organic D-π-A dye for record efficiency solid-state sensitized heterojunction solar cells.

The high molar absorption coefficient organic D-π-A dye C220 exhibits more than 6% certified electric power conversion efficiency at AM 1.5G solar irradiation (100 mW cm(-2)) in a solid-state dye-sensitized solar cell using 2,2',7,7'-tetrakis(N,N-dimethoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) as the organic hole-transporting material. This contributes to a new record (6.08% by NREL) for this type of sensitized heterojunction photovoltaic device. Efficient charge generation is proved by incident photon-to-current conversion efficiency spectra. Transient photovoltage and photocurrent decay measurements showed that the enhanced performance achieved with C220 partially stems from the high charge collection efficiency over a wide potential range.

High-efficiency dye-sensitized solar cell with three-dimensional photoanode.

Herein, we present a straightforward bottom-up synthesis of a high electron mobility and highly light scattering macroporous photoanode for dye-sensitized solar cells. The dense three-dimensional Al/ZnO, SnO(2), or TiO(2) host integrates a conformal passivation thin film to reduce recombination and a large surface-area mesoporous anatase guest for high dye loading. This novel photoanode is designed to improve the charge extraction resulting in higher fill factor and photovoltage for DSCs. An increase in photovoltage of up to 110 mV over state-of-the-art DSC is demonstrated.

Crystal orientation-dependent etching and trapping in thermally-oxidised Cu2O photocathodes for water splitting.

Ammonia solution etching was carried out on thermally-oxidised cuprous oxide (TO-Cu2O) in photocathode devices for water splitting. The etched devices showed increased photoelectrochemical (PEC) performance compared to the unetched ones as well as improved reproducibility. -8.6 mA cm-2 and -7 mA cm-2 photocurrent density were achieved at 0 V and 0.5 V versus the reversible hydrogen electrode (VRHE), respectively, in the champion sample with an onset potential of 0.92 VRHE and a fill factor of 44%. An applied bias photon-to-current efficiency of 3.6% at 0.56 VRHE was obtained, which represents a new record for Cu2O-based photocathode systems. Capacitance-based profiling studies showed a strong pinning effect from interfacial traps in the as-grown device, and these traps were removed by ammonia solution etching. Moreover, the etching procedure gave rise to a diverse morphology of Cu2O crystals based on the different crystallographic orientations. The distribution of crystallographic orientations and the relationship between the crystal orientation and the morphology after etching were examined by electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM). The high-index crystal group showed a statistically higher PEC performance than the low-index group. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) revealed metallic copper at the Cu2O/Ga2O3 interface, which we attribute as the dominant trap that limits the PEC performance. It is concluded that the metallic copper originates from the reduction of the CuO impurity layer on the as-grown Cu2O sample during the ALD process, while the reduction from Cu2O to Cu is not favourable.

Energy and hole transfer between dyes attached to titania in cosensitized dye-sensitized solar cells.

Cosensitization of broadly absorbing ruthenium metal complex dyes with highly absorptive near-infrared (NIR) organic dyes is a clear pathway to increase near-infrared light harvesting in liquid-based dye-sensitized solar cells (DSCs). In cosensitized DSCs, dyes are intimately mixed, and intermolecular charge and energy transfer processes play an important role in device performance. Here, we demonstrate that an organic NIR dye incapable of hole regeneration is able to produce photocurrent via intermolecular energy transfer with an average excitation transfer efficiency of over 25% when cosensitized with a metal complex sensitizing dye (SD). We also show that intermolecular hole transfer from the SD to NIR dye is a competitive process with dye regeneration, reducing the internal quantum efficiency and the electron lifetime of the DSC. This work demonstrates the general feasibility of using energy transfer to boost light harvesting from 700 to 800 nm and also highlights a key challenge for developing highly efficient cosensitized dye-sensitized solar cells.

Probing the solid-liquid interface with tender x rays: A new ambient-pressure x-ray photoelectron spectroscopy endstation at the Swiss Light Source.

A new endstation to perform operando chemical analysis at solid-liquid interfaces by means of ambient pressure x-ray photoelectron spectroscopy (APXPS) is presented. The endstation is located at the Swiss Light Source and can be attached to the soft x-ray in situ spectroscopy beamline (X07DB) for solid-gas type experiments and to a tender x-ray beamline (PHOENIX I) for solid-liquid interface experiments. The setup consists of three interconnected ultrahigh vacuum chambers: one for sample preparation using surface science techniques, the analysis chamber for APXPS experiments, and an entry-lock chamber for sample transfer across the two pressure regimes. The APXPS chamber is designed to study solid-liquid interfaces stabilized by the dip and pull method. Using a three-electrode setup, the potential difference across the solid-electrolyte interface can be controlled, as is demonstrated here using an Ir(001) electrode dipped and pulled from a 0.1M KOH electrolyte. The new endstation is successfully commissioned and will offer unique opportunities for fundamental studies of phenomena that take place at solid-liquid interfaces and that are relevant for fields such as electrochemistry, photochemistry, or biochemistry, to name a few.

Operando deconvolution of photovoltaic and electrocatalytic performance in ALD TiO2 protected water splitting photocathodes.

In this work, we demonstrate that buried junction photocathodes featuring an ALD TiO2 protective overlayer can be readily characterized using a variation of the dual working electrode (DWE) technique, where the second working electrode (WE2) is spatially isolated from the hydrogen-evolving active area. The measurement of the surface potential during operation enables the operando deconvolution of the photovoltaic and electrocatalytic performance of these photocathodes, by reconstructing J-ΔV curves (reminiscent of photovoltaic J-V curves) from the 3-electrode water splitting data. Our method provides a clearer understanding of the photocathode degradation mechanism during stability tests, including loss of the catalyst from the surface, which is only possible in our isolated WE2 configuration. A pn+Si/TiO2 photocathode was first investigated as a well behaved model system, and then the technique was applied to an emerging material system based on Cu2O/Ga2O3, where we uncovered an intrinsic instability of the Cu2O/Ga2O3 junction (loss of photovoltage) during long term stability measurements.

Investigation of (Leaky) ALD TiO2 Protection Layers for Water-Splitting Photoelectrodes.

Protective overlayers for light absorbers in photoelectrochemical water-splitting devices have gained considerable attention in recent years. They stabilize light absorbers which would normally be prone to chemical side reactions leading to degradation of the absorber. Atomic layer deposition (ALD) enables conformal and reproducible ultrathin protective layer growth even on highly structured substrates. One of the most widely investigated protective layers is amorphous TiO2, deposited by ALD at a relatively low temperature (120-150 °C). We have deposited protective layers from tetrakis(dimethylamido)titanium(IV) at two different temperatures and investigated their chemical composition as well as optical and electrochemical properties. Our main findings reveal a change in the flat band potential with thickness, reaching a stable value of about -50 to -100 mV versus reversible hydrogen electrode for films >30 nm, with doping densities of ∼1020 cm3. Practical thicknesses to achieve pinhole-free films are evaluated and discussed.

Ionic polarization-induced current-voltage hysteresis in CH3NH3PbX3 perovskite solar cells.

CH3NH3PbX3 (MAPbX3) perovskites have attracted considerable attention as absorber materials for solar light harvesting, reaching solar to power conversion efficiencies above 20%. In spite of the rapid evolution of the efficiencies, the understanding of basic properties of these semiconductors is still ongoing. One phenomenon with so far unclear origin is the so-called hysteresis in the current-voltage characteristics of these solar cells. Here we investigate the origin of this phenomenon with a combined experimental and computational approach. Experimentally the activation energy for the hysteretic process is determined and compared with the computational results. First-principles simulations show that the timescale for MA(+) rotation excludes a MA-related ferroelectric effect as possible origin for the observed hysteresis. On the other hand, the computationally determined activation energies for halide ion (vacancy) migration are in excellent agreement with the experimentally determined values, suggesting that the migration of this species causes the observed hysteretic behaviour of these solar cells.

Unraveling the Dual Character of Sulfur Atoms on Sensitizers in Dye-Sensitized Solar Cells.

Cyclometalated ruthenium sensitizers have been synthesized that differ with number of thiophene units on the auxiliary ligands. Sensitizers possessing four (SA25, SA246, and SA285) or none (SA282) sulfur atoms in their structures, were tested in solar cell devices employing I3-/I- redox mediator, enabling an estimation of the influence of sulfur-iodine/iodide interactions on dye-sensitized solar cell (DSC) performance. Power conversion efficiencies over 6% under simulated AM 1.5 illumination (1 Sun) were achieved with all the sensitizers. Consistently higher open-circuit voltage (VOC) and fill factor (FF) values were measured using SA282. Scrutinizing the DSCs with these dyes by transient absorption spectroscopy (TAS) and electrochemical impedance spectroscopy (EIS) indicate that sulfur atom induced recombination cancels favorable increased regeneration resulting in decreased power conversion efficiencies (PCEs). The data indicate that, to reduce charge recombination channels, the use of sulfur-containing aromatic rings should be avoided if possible in the dye structure when I3-/I- redox mediator is used.

Understanding the Impact of Bromide on the Photovoltaic Performance of CH3 NH3 PbI3 Solar Cells.

An optimum amount of lead bromide (1%) can enhance the power conversion efficiency of CH3 NH3 PbI3-x Brx (where x ≈ 0) devices from 14.7% to 16.9% without altering the bandgap of the perovskite material.