ABSTRACT
Valence band dispersions of single-crystalline SnS1-xSex solid solutions were observed by angle-resolved photoemission spectroscopy (ARPES). The hole effective masses, crucial factors in determining thermoelectric properties, were directly evaluated. They decrease slightly with increasing Se content in the low Se composition range but sharply in the high Se composition range.
ABSTRACT
Iron pyrite [cubic FeS2 (cFeS2)] is considered as an earth-abundant and low-cost thin-film photovoltaic material. However, the conversion efficiency of cFeS2-based solar cells remains below 3%. To elucidate this limitation, we evaluate the optical and electrical characteristics of cFeS2 single crystals that are grown using the flux method, thus providing us an understanding of the electron transport behavior of cFeS2 single crystals. The oxide layer on the surface of cFeS2, which can possibly have an influence on the electrical characteristics of cFeS2, is removed prior to characterization via optical spectroscopy and electrical transport measurement. The optical property of cFeS2 was found to have both indirect and direct transitions. We also observed the presence of a band tail below the conduction band. The obtained electrical transport behavior indicates that cFeS2 bulk exhibits a high defect density and a disordered phase, thus leading to the hopping conduction mechanism. Our results will pave the way for the development of photovoltaic applications with iron pyrite.
ABSTRACT
Chalcopyrite CuGaSe2 single crystals were successfully synthesized by the flux method using a home-made Bridgman-type furnace. The grown crystals were nearly stoichiometric with a Se-poor composition. Although a wafer form of the thus-obtained single crystal showed poor p-type electrical properties due to such unfavorable off-stoichiometry, these properties were found to be improved by applying a post-annealing treatment under Se vapor conditions. As a result, an electrode derived from the Se-treated single crystalline wafer showed appreciable p-type photocurrents. After deposition of a CdS ultrathin layer and a nanoparticulate Pt catalyst on the surface of the electrode, appreciable photoelectrochemical H2 evolution was observed over the modified electrode under photoirradiation by simulated sunlight with application of a bias potential of 0 VRHE.
ABSTRACT
We report on the optimization of interface structure in ZnSnP2 solar cells. The effects of back electrode materials and related interface on photovoltaic performance were investigated. It was clarified that a conventional structure Mo/ZnSnP2 showed a Schottky-behavior, while an ohmic-behavior was observed in the Cu/ZnSnP2 structure annealed at 300 °C. STEM-EDX analysis suggested that Cu-Sn-P ternary compound was formed at the interface. This compound is considered to play an important role to obtain the ohmic contact between ZnSnP2 and Cu. In addition, it was clarified that the aqua regia etching of ZnSnP2 bulk crystals before chemical bath deposition process for the preparation of buffer layer was effective to remove the layer including lattice defects introduced by mechanical-polishing, which was supported by TEM observations and photoluminescence measurements. This means that the carrier transport across the interface was improved because of the reduced defect at the interface. Consequently, the conversion efficiency of approximately 2% was achieved with the structure of Al/ZnO;Al/ZnO/CdS/ZnSnP2/Cu, where the values of short circuit current density, JSC, open circuit voltage, VOC, and fill factor, FF, were 8.2 mA cm-2, 0.452 V, and 0.533, respectively. However, the value of VOC was largely low considering the bandgap value of ZnSnP2. To improve the conversion efficiency, the optimization of buffer layer material is considered to be essential in the viewpoint of band alignment.
ABSTRACT
The effects of partial replacement of Cu with Ag in a Cu(In,Ga)S2 (CIGS) thin film on its structural, optical, electrostructural, and photoelectrochemical (PEC) properties were investigated, in order to improve its performance for PEC water reduction under sunlight illumination. Results from X-ray diffraction (XRD) analyses revealed the successful partial replacement of Cu with Ag to form solid-solutions with different Ag/(Ag + Cu) ratios (A(x)CIGS, x = Ag/(Ag + Cu) = 0.1, 0.2, 0.3 and 0.4), as confirmed by a gradual change in the (112) reflections to smaller 2θ angles with increasing Ag/(Ag + Cu) ratio. Analyses of the photoabsorption properties of the materials using photoacoustic spectroscopy indicated changes in the band gap energies associated with increasing the Ag/(Ag + Cu) ratio. In addition, valence band maximum potentials of A(x)CIGS were deepened gradually with increasing Ag/(Ag + Cu) ratio. After modifying these A(x)CIGS films with a CdS ultrathin (ca. 70 nm) layer and a Pt catalyst, the PEC water reduction properties were evaluated in an electrolyte solution with the pH adjusted to 6.5, under simulated sunlight (AM 1.5G) radiation. Compared to the CdS- and Pt-modified Ag-free A(x)CIGS (A(0)CIGS) films, appreciable enhancements in the PEC properties were observed for electrodes based on A(x)CIGS (x > 0) films, and the best PEC performance was obtained for the electrode based on the A(0.2)CIGS film. However, the electrode derived from the A(x)CIGS film with Ag/(Ag + Cu) ratios higher than 0.3 showed diminished PEC properties due to the partial conversion of its semiconducting properties from p-type to n-type.
ABSTRACT
Copper indium disulfide (CuInS2) modified with an In2S3 layer and a Pt catalyst showed a more efficient photoelectrochemical (PEC) property for hydrogen evolution from a nearly neutral (pH 6) 0.2 M NaH2PO4 solution under simulated sunlight illumination (AM 1.5G) than that of a CuInS2 electrode modified with a CdS layer and a Pt catalyst. Analysis of the PEC properties of In2S3-modified CuInS2 (In2S3/CuInS2) and CdS-modified CuInS2 (CdS/CuInS2) in solutions containing an electron scavenger (Eu(3+)) showed identical enhancement of the PEC properties of In2S3/CuInS2 when compared to those of CdS/CuInS2, indicating the formation of a favorable heterointerface in In2S3/CuInS2 for efficient charge separation. Spectroscopic evaluation of conduction band offsets revealed that In2S3/CuInS2 had a notch-type conduction band offset, whereas a cliff-type offset was formed in CdS/CuInS2: these results also revealed a better interface electric structure of In2S3/CuInS2 than that of CdS/CuInS2.
ABSTRACT
The existence of tetravalent dysprosium in perovskite-type oxide barium zirconate is confirmed in this work. This discovery will stimulate many researchers in diverse fields such as catalysts, solid state ionics, sensors, and fluorescent materials.
