Elucidating the relationship between crystallo-chemistry and optical properties of CIGS nanocrystals

The performance of solar cells fabricated using Cu(In,Ga)(S,Se)2 nanocrystal (NC) inks synthesized using the hot injection method has yielded efficiencies up to 12% recently. The efficiency of these devices is highly dependent on the chemical composition and crystallographic quality of the NCs. The former has been extensively discussed as it can be easily correlated to the optical properties of the film, but detailed crystallographic structure of these NCs has scarcely been discussed and it can influence both the optical and electrical properties. Hence both chemical composition and crystal structure should be explored for these NCs in order for this material to be further developed for application in thin film solar cells. In this work, a thorough investigation of the composition and crystal structure of CuIn x Ga1−x Se2 NCs synthesized using the hot injection method over the entire composition range (0 ≤ x ≤ 1) has been conducted. Raman spectroscopy of the NCs complements the information derived from x-ray diffraction (XRD) and electron probe microanalysis (EPMA). EPMA, which was carried out for the first time, indicates good controllability of the NC Ga/(In + Ga) ratio using this synthesis method. Raman spectroscopy reveals that CuInSe2 NCs are a mixture of chalcopyrite and sphalerite with disordered cations, whereas CuGaSe2 NCs are purely chalcopyrite. The lattice parameters determined from XRD were found to deviate from those calculated using Vegard's law for all compositions. Hence, it can be deduced that the lattice is distorted in the crystal. The optical and electrochemical band gap of CuIn x Ga1−x Se2 NCs increases as the Ga content increases. The energy band gap deviates from the theoretical values, which could be related to the contribution from cation disordering and strain. These results help to tailor the opto-electrical properties of semiconductors, which inherently depend on the crystalline quality, strain and composition

Boosting the oxygen evolution activity in non-stoichiometric praseodymium ferrite-based perovskites by A site substitution for alkaline electrolyser anodes

Sustainable fossil fuel free systems are crucial for tackling climate change in the global energy market, and the identification and understanding of catalysts needed to build these systems plays a vital role in their development. ABO3−δ perovskite oxides have been observed to be potential replacement materials for the high-performing, but low ionic conducting and economically unfavourable Pt and IrO2 water splitting catalysts. In this work increased addition of Sr2+ aliovalent dopant ions into the crystal lattice of Pr1−xSrxFeO3−δ perovskites via A site substitution was seen to drastically improve the electrocatalytic activity of the oxygen evolution reaction (OER) in alkaline environments. The undoped PrFeO3−δ catalyst was not catalytically active up to 1.70 V against the reversible hydrogen electrode (RHE), whilst an onset potential of 1.62 V was observed for x = 0.5. Increased strontium content in Pr1−xSrxFeO3−δ was found to cause a reduction in the lattice parameters and crystal volume whilst retaining the orthorhombic Pbnm space group throughout all dopant levels, analysed using the Rietveld method. However, it was noted that the orthorhombic distortion was reduced as more Sr2+ replaced Pr3+. The mechanism for the increased electrocatalytic activity with increased strontium is due to the increasing concentration of oxygen vacancy (δ), leading to increased catalyst site availability, and the increased average oxidation state of Fe cations, consistent with the iodometric titration results. This results in shifting the average d shell eg electron filling further towards unity. X-ray photoelectron spectrum of the O 1s core level also shows the presence of lattice oxide and surface hydroxide/carbonate. This work shows promise in that using the more abundant and more economically friendly material of strontium allows for improved OER catalytic activity in otherwise inactive perovskite catalyst oxides

Understanding surface structure and chemistry of single crystal lanthanum aluminate

The surface crystallography and chemistry of a LaAlO 3 single crystal, a material mainly used as a substrate to deposit technol ogically important thin films (e.g. for superconducting and magnetic devices), was analysed using surface X-ray diffraction and low energy ion scattering spectroscopy. The surfa ce was determined to be terminated by Al-O species, and was significantly different from th e idealised bulk structure. Termination reversal was not observed at higher temperature (600°C) and chamber pressure of 10 -10 Torr, but rather an increased Al-O occupancy occurred, which was accompanied by a larger outwards relaxation of Al from the bulk positions. Changing the oxygen pressure to 10 -6 Torr enriched the Al site occupancy fraction at the outermost surface from 0.245(10) to 0.325(9). In contrast the LaO, which is located at the next sub-surface atomic layer, showed no chemical enrichment and the structural relaxation was lower than for the top AlO 2 layer. Knowledge of the surface structure will aid the understanding of how and which type of interface will be formed when LaAlO 3 is used as a substrate as a function of temperature and pressure, and so lead to improved design of device structures

Temperature effect on surface structure of single crystal SrLaAlO₄(001)

Development of next generation electrochemical devices such as solid oxide cells requires control of the charge transferprocesses across key interfaces. Structural strain at electrolyte:electrode interfaces could potentially alter the devicescharge transport properties, therefore understanding the structural behaviour of electrode surfaces at operating conditionsis important. The functional oxide single crystal substrate SrLaAlO4 has been well-characterised with bulkstructure studies, however there are very few studies of SrLaAlO4 surface structures. Here we present an investigationof the surface structure of SrLaAlO4(001) substrates using surface X-ray diffraction, under UHV conditions (10\u10000010torr) with the substrate held at either room temperature or 650 C. Best-fit models using a 1:1 ratio of Sr:La showedsignificant distortions to the surface AlO6 octahedra

Stability of NdBaCo2−xMnxO5+δ (x = 0, 0.5) layered perovskites under humid conditions investigated by high-temperature in situ neutron powder diffraction

The double perovskites NdBaCo2−xMnxO5+δ (x = 0 and 0.5) were investigated using in situ high temperature neutron powder diffraction in dry argon and wet atmospheres (40% D2O/argon and 40% D2O/air) in order to assess their stability as cathodes in proton conducting fuel cells. The x = 0 oxide loses oxygen on heating in dry argon at T > 400 °C and exhibits an oxygen vacancy order–disorder transition as evidenced by the orthorhombic Pmmm to tetragonal P4/mmm transition. Refinement of site occupancy factors suggests that the oxygen vacancies mainly form in the Nd layers and to a lesser extent at the equatorial positions of the transition metal polyhedra; at 800 °C, δ ∼ 0. When the gas was changed to wet argon at 800 °C and the sample cooled to 260 °C, no structural modification or change in the oxygen content was detected and no impurity phases formed, highlighting the excellent stability of the sample in wet atmospheres. On switching the gas to wet air at 260 °C, thermal analysis and neutron powder diffraction data together reveal that the sample intercalates mainly oxygen rather than proton defects within a two-phase process involving two orthorhombic phases, reflecting the symmetry of the reduced and oxidised materials. On heating, the sample transforms at T ≥ 600 °C to a single tetragonal phase whose symmetry is retained up to 800 °C and on subsequent cooling. The x = 0.5 material prepared in argon adopted a tetragonal P4/mmm structure at RT with δ ∼ 0. Its symmetry remains tetragonal on heating/cooling in wet argon. On changing the gas to wet air at 260 °C, it takes up oxygen via a two-phase process involving two tetragonal phases. Since fast oxidation is the main process that fills the oxygen vacancies of these double perovskites in wet air, a large oxygen deficiency seems to be not the only requirement for effective proton incorporation in this family of materials with basic characteristics