How Oxygen Exposure Improves the Back Contact and Performance of Antimony Selenide Solar Cells.

The improvement of antimony selenide solar cells by short-term air exposure is explained using complementary cell and material studies. We demonstrate that exposure to air yields a relative efficiency improvement of n-type Sb2Se3 solar cells of ca. 10% by oxidation of the back surface and a reduction in the back contact barrier height (measured by J-V-T) from 320 to 280 meV. X-ray photoelectron spectroscopy (XPS) measurements of the back surface reveal that during 5 days in air, Sb2O3 content at the sample surface increased by 27%, leaving a more Se-rich Sb2Se3 film along with a 4% increase in elemental Se. Conversely, exposure to 5 days of vacuum resulted in a loss of Se from the Sb2Se3 film, which increased the back contact barrier height to 370 meV. Inclusion of a thermally evaporated thin film of Sb2O3 and Se at the back of the Sb2Se3 absorber achieved a peak solar cell efficiency of 5.87%. These results demonstrate the importance of a Se-rich back surface for high-efficiency devices and the positive effects of an ultrathin antimony oxide layer. This study reveals a possible role of back contact etching in exposing a beneficial back surface and provides a route to increasing device efficiency.

Modular Design via Multiple Anion Chemistry of the High Mobility van der Waals Semiconductor Bi4O4SeCl2.

Making new van der Waals materials with electronic or magnetic functionality is a chemical design challenge for the development of two-dimensional nanoelectronic and energy conversion devices. We present the synthesis and properties of the van der Waals material Bi4O4SeCl2, which is a 1:1 superlattice of the structural units present in the van der Waals insulator BiOCl and the three-dimensionally connected semiconductor Bi2O2Se. The presence of three anions gives the new structure both the bridging selenide anion sites that connect pairs of Bi2O2 layers in Bi2O2Se and the terminal chloride sites that produce the van der Waals gap in BiOCl. This retains the electronic properties of Bi2O2Se while reducing the dimensionality of the bonding network connecting the Bi2O2Se units to allow exfoliation of Bi4O4SeCl2 to 1.4 nm height. The superlattice structure is stabilized by the configurational entropy of anion disorder across the terminal and bridging sites. The reduction in connective dimensionality with retention of electronic functionality stems from the expanded anion compositional diversity.

A CO2 -Tolerant Perovskite Oxide with High Oxide Ion and Electronic Conductivity.

Mixed ionic-electronic conductors (MIECs) that display high oxide ion conductivity (σo ) and electronic conductivity (σe ) constitute an important family of electrocatalysts for a variety of applications including fuel cells and oxygen separation membranes. Often MIECs exhibit sufficient σe but inadequate σo . It has been a long-standing challenge to develop MIECs with both high σo and stability under device operation conditions. For example, the well-known perovskite oxide Ba0.5 Sr0.5 Co0.8 Fe0.2 O3- δ (BSCF) exhibits exceptional σo and electrocatalytic activity. The reactivity of BSCF with CO2 , however, limits its use in practical applications. Here, the perovskite oxide Bi0.15 Sr0.85 Co0.8 Fe0.2 O3- δ (BiSCF) is shown to exhibit not only exceptional bulk transport properties, with a σo among the highest for known MIECs, but also high CO2 tolerance. When used as an oxygen separation membrane, BiSCF displays high oxygen permeability comparable to that of BSCF and much higher stability under CO2 . The combination of high oxide transport properties and CO2 tolerance in a single-phase MIEC gives BiSCF a significant advantage over existing MIECs for practical applications.

Low-Temperature Wafer-Scale Deposition of Continuous 2D SnS2 Films.

Semiconducting 2D materials, such as SnS2 , hold immense potential for many applications ranging from electronics to catalysis. However, deposition of few-layer SnS2 films has remained a great challenge. Herein, continuous wafer-scale 2D SnS2 films with accurately controlled thickness (2 to 10 monolayers) are realized by combining a new atomic layer deposition process with low-temperature (250 °C) postdeposition annealing. Uniform coating of large-area and 3D substrates is demonstrated owing to the unique self-limiting growth mechanism of atomic layer deposition. Detailed characterization confirms the 1T-type crystal structure and composition, smoothness, and continuity of the SnS2 films. A two-stage deposition process is also introduced to improve the texture of the films. Successful deposition of continuous, high-quality SnS2 films at low temperatures constitutes a crucial step toward various applications of 2D semiconductors.

Bi4O4Cu1.7Se2.7Cl0.3: Intergrowth of BiOCuSe and Bi2O2Se Stabilized by the Addition of a Third Anion.

Layered two-anion compounds are of interest for their diverse electronic properties. The modular nature of their layered structures offers opportunities for the construction of complex stackings used to introduce or tune functionality, but the accessible layer combinations are limited by the crystal chemistries of the available anions. We present a layered three-anion material, Bi4O4Cu1.7Se2.7Cl0.3, which adopts a new structure type composed of alternately stacked BiOCuSe and Bi2O2Se-like units. This structure is accessed by inclusion of three chemically distinct anions, which are accommodated by aliovalently substituted Bi2O2Se0.7Cl0.3 blocks coupled to Cu-deficient Bi2O2Cu1.7Se2 blocks, producing a formal charge modulation along the stacking direction. The hypothetical parent phase Bi4O4Cu2Se3 is unstable with respect to its charge-neutral stoichiometric building blocks. The complex layer stacking confers excellent thermal properties upon Bi4O4Cu1.7Se2.7Cl0.3: a room-temperature thermal conductivity (κ) of 0.4(1) W/mK was measured on a pellet with preferred crystallite orientation along the stacking axis, with perpendicular measurement indicating it is also highly anisotropic. This κ value lies in the ultralow regime and is smaller than those of both BiOCuSe and Bi2O2Se. Bi4O4Cu1.7Se2.7Cl0.3 behaves like a charge-balanced semiconductor with a narrow band gap. The chemical diversity offered by the additional anion allows the integration of two common structural units in a single phase by the simultaneous and coupled creation of charge-balancing defects in each of the units.

Reversible electron-transfer reactions within a nanoscale metal oxide cage mediated by metallic substrates.

Transition metal oxides exhibit a rich collection of electronic properties and have many practical applications in areas such as catalysis and ultra-high-density magnetic data storage. Therefore the development of switchable molecular transition metal oxides has potential for the engineering of single-molecule devices and nanoscale electronics. At present, the electronic properties of transition metal oxides can only be tailored through the irreversible introduction of dopant ions, modifying the electronic structure by either injecting electrons or core holes. Here we show that a molybdenum(VI) oxide 'polyoxometalate' molecular nanocluster containing two embedded redox agents is activated by a metallic surface and can reversibly interconvert between two electronic states. Upon thermal activation two electrons are ejected from the active sulphite anions and delocalized over the metal oxide cluster cage, switching it from a fully oxidized state to a two-electron reduced state along with the concomitant formation of an S-S bonding interaction between the two sulphur centres inside the cluster shell.

A high-resolution photoemission study of nanoscale aluminum oxide films on NiAl(110).

Using high-resolution soft X-ray photoemission, Al 2p, we have been able to quantify the relative populations of tetrahedrally (Al(tet)) and octahedrally (Al(oct)) coordinated Al(3+) in three distinct phases of nanoscale aluminum oxide films on NiAl(110). We have hence determined the bulk alumina phases that the nanoscale films most resemble. Adsorption of oxygen at room temperature produces a layer which predominately (90%) contains Al(tet) and is analogous to the amorphous bulk phase of alumina. Annealing this layer results in an Al enrichment of the oxide layer, through the diffusion of metal from the substrate, and an increase in the relative amount of Al(oct), producing a gamma-alumina-like layer with a relative Al(oct)/Al(tet) occupancy of 28 +/- 3%/72 +/- 3%. Oxygen adsorption at 823 K also produces a gamma-like phase, with a relative Al(oct)/Al(tet) occupancy of 27 +/- 3%/73 +/- 3%, although this layer is thicker than that formed at room temperature. Both oxidation methods produce gamma-alumina layers that display poor translational order. However, these poorly ordered layers have a relative Al(oct)/Al(tet) occupation similar to that of well-ordered oxide films produced using different oxidation conditions in previous studies. Both gamma layers undergo partial decomposition upon annealing to 1273 K, producing an alpha-alumina-like oxide, which contains only Al(oct), and is highly deficient in Al. There are significant oxide-free areas within the alpha-alumina oxide layer, which is characteristic of crystallite formation. Repeated cycles of oxidation and annealing to 1273 K do not produce a homogeneous film, but they do make the alpha-like oxide more Al rich.

Molecular ordering in isonicotinic acid on rutile TiO2(110) investigated with valence band photoemission.

The adsorption of isonicotinic acid on rutile TiO(2)(110) has been investigated using synchrotron-based valence band photoemission. Structural ordering in multilayer films of the molecules is found to give rise to a strong angular dependence in the valence band intensities when measured using linearly polarized radiation. Molecular ordering in this case is proposed to be induced by intermolecular hydrogen bonding which is found to be highly dependent upon the deposition rate of the isonicotinic acid. Through comparison of the experimental data with density functional calculated valence band spectra of hydrogen-bonded isonicotinic acid molecules, we can account for the angular dependence in terms of the spatial distribution of the molecular orbitals.

The Mobile Luminescence End-Station, MoLES: a new public facility at Daresbury Synchrotron.

A new mobile end-station is described for use on multiple beamlines at the Daresbury synchrotron radiation source (overall excitation range 5 eV to 70 keV) that allows for the detection and dispersion of photoluminescence from solid-state samples in the emission range 190-1000 nm (1.2-6.5 eV). The system is fully self-contained and includes sample-cooling facilities for the temperature range 8-330 K using a closed-cycle refrigerator, thus eliminating the need for liquid cryogens. The system also includes solid-state laser sources for use with a variety of pump-probe-type experiments, and an Ar(+) surface-cleaning facility. In order to demonstrate the various capabilities of the system, the results of a variety of experiments are summarized, carried out over the excitation range 5-5000 eV on beamlines 3.2, MPW6.1 and 4.2. These include the optical detection of XAS of L-edge structure in natural minerals and archaeological ceramics, band-gap determinations of wide-band-gap silicates, and pump-probe studies of quartz.