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

The dual-working-electrode technique enables the deconvolution of the intrinsic properties of the buried p–n junction and the electrocatalyst on the surface for water splitting photocathodes

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 MAþ 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

Tuning the selectivity of biomass oxidation over oxygen evolution on NiO-OH electrodes

Electrochemical reactions powered by renewable electricity are an important means of reducing the carbon footprint of large-scale chemical processes. Here, we investigate the efficient conversion of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), an important building block in the polymer and pharmaceutical industries, using a cheap and abundant nickel-based electrocatalyst. We elucidate the key factors for tuning the chemical selectivity for HMF oxidation over the competing oxygen evolution reaction (OER) at the catalyst surface. We show that the selectivity for HMF oxidation is enhanced by removing trace impurities of iron species as well as adjusting the composition of the alkali hydroxide electrolyte solution. LiOH solution without iron impurities is more favorable for HMF oxidation, whereas CsOH solution with iron species present is more active for the OER and unfavorable for HMF oxidation. Under optimized conditions, HMF oxidation in 1 M LiOH electrolyte solution without iron (pH 14) achieved 98% faradaic efficiency for the production of FDCA. The principles used in this work can be applied to other electrosynthetic reactions, in particular where the OER is the main competing side reaction

Revealing and Accelerating Slow Electron Transport in Amorphous Molybdenum Sulphide Particles for Hydrogen Evolution Reaction

Electrochemical impedance spectroscopy is used to identify a slow electron transport process in hydrogen evolution catalysed by amorphous molybdenum sulphides on glassy carbon. A new chemical synthesis leads to an amorphous molybdenum sulfide catalyst with a higher electronic conductivity

Preparation and Characterization of High-Entropy Alloy (TaNb)1−x_{1-x}(ZrHfTi)x_x Superconducting Films

We report on the preparation and the physical properties of superconducting (TaNb)_1-x(ZrHfTi)_x high-entropy alloy films. The films were prepared by means of magnetron sputtering at room temperature, with x ranging from 0 to 1 with an average thickness of 600 - 950 nm. All films crystallize in a pseudo body-centered cubic (BCC) structure. For samples with x < 0.65, the normal-state properties are metallic, while for x > 0.65 the films are weakly insulating. The transition from metallic to weakly insulating occurs right at the near-equimolar stoichiometry. We find all films, except for x = 0 or 1, to become superconducting at low temperatures, and we interpret their superconducting properties within the Bardeen-Cooper-Schrieffer (BCS) framework. The highest transition temperature T_c = 6.9 K of the solid solution is observed for x = 0.43. The highest upper-critical field B_c2(0) = 11.05 T is found for the near-equimolar ratio x = 0.65, where the mixing entropy is the largest. The superconducting parameters derived for all the films from transport measurements are found to be close to those that are reported for amorphous superconductors. Our results indicate that these films of high-entropy alloys are promising candidates for superconducting device fabrication

Solution phase treatments of Sb2_2Se3_3 heterojunction photocathodes for improved water splitting performance

Antimony selenide (Sb$_2$Se$_3$) 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 Sb$_2$Se$_3$/TiO$_2$-based photocathodes for photoelectrochemical water splitting were explored to address this challenge. The FTO/Ti/Au/Sb$_2$Se$_3$ (substrate configuration) stack was treated with (NH$_4$)$_2$S as an etching solution, followed by CuCl$_2$ treatment prior to deposition of the TiO$_2$ by atomic layer deposition. The different treatments show different mechanisms of action compared to similar reported treatments of the back Au/Sb$_2$Se$_3$ 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 Sb$_2$Se$_3$ films. From SEM and XPS studies, it is clear that the etching treatment induces a morphological change and removes the surface Sb$_2$Se$_3$ layer, which eliminates the Fermi-level pinning that the oxide layer generates. CuCl$_2$ 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 Sb$_2$Se$_3$ for large-scale water splitting

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 favorable

Impedance spectroscopy of Sb₂Se₃ photovoltaics consisting of (Sb₄Se₆)_<i>n</i> 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

Long-Range π-Conjugation in Phenothiazine-containing Donor-Acceptor Dyes for Application in Dye-Sensitized Solar Cells

Four organic donor–π-bridge–acceptor dyes containing phenothiazine as a spacer and cyanoacrylic acid as an acceptor were synthesized and tested as sensitizers in dye-sensitized solar cells (DSCs). The influence of iodide- and cobalt-based redox electrolytes on the photovoltaic device performance was investigated. In these new dyes, systematic π-conjugation was extended by inserting one or two phenothiazine moieties and investigated within the context of the resulting photoinduced charge-transfer properties. A detailed investigation, including transient absorption spectroscopy and quantum chemical methods, provided important information on the role of extended π-conjugation on the photophysical properties and photovoltaic device performance. Overall, the results showed that the extension of π-conjugation by one phenothiazine unit resulted in the best device performance owing to reduced recombination rates, whereas extension by two phenothiazine units reduced dye adsorption on TiO2 probably owing to the increase in molecular size. The performance of the dyes in DSCs was found to be a complex interaction between dye structure and size