Shin Tajima

Selective CO2 conversion to formate in water using a CZTS photocathode modified with a ruthenium complex polymer

Highly selective photoelectrochemical CO(2) reduction (>80% selectivity) in water was successfully achieved by combining Cu(2)ZnSnS(4) (CZTS) with a metal-complex electrocatalyst. CZTS, a sulfide semiconductor that possesses a narrow band gap and consists of earth-abundant elements, is demonstrated to be a candidate photoabsorber for a CO(2) reduction hybrid photocatalyst.

6% Efficiency Cu2ZnSnS4-based thin film solar cells using oxide precursors by open atmosphere type CVD

An open atmosphere type chemical vapor deposition (OA-CVD) method is one of the most effective methods for producing functional thin films. Especially, the OA-CVD method is a unique technique which is able to deposit metal oxide thin films by decomposition of vaporized raw materials released through a nozzle onto substrates in the air. Cu2ZnSnS4 (CZTS)-based thin films as absorber layers of thin film solar cells were fabricated by sulfurizing oxide precursor thin films synthesized by the OA-CVD method. Cu(C5H7O2)2, Zn(C5H7O2)2 and Sn(C5H7O2)2 were used as raw materials. The oxide precursor thin films were sulfurized at 520–560 °C in 5 vol% H2S balanced with N2. The formed CZTS-based thin films included oxygen with the composition ratio of O/(S + O) = 0.17–0.27 according to energy dispersive X-ray spectroscopy. The thin film solar cells using the CZTS-based thin films including oxygen [CZT(S,O) films] as the absorber layers were fabricated. The CZT(S,O) thin film solar cell had a stack structure of Al/Al-doped-ZnO/CdS/CZT(S,O)/Mo/soda-lime glass substrate. The efficiency of the CZT(S,O) thin film solar cells was 6.03%, which was the high efficiency in the reported value for CZTS-based thin film solar cells using oxide thin film precursors. It was found that the OA-CVD method is suited to fabricate the absorber layers of thin film solar cells.

Thermoelectric properties of highly textured NaCo2O4 ceramics processed by the reactive templated grain growth (RTGG) method

Highly-textured NaCo2O4 ceramics were prepared and the thermoelectric properties were examined in the preferred {001} plane. Plate-like Co3O4 particles were synthesized and used as a reactive template for c-axis oriented NaCo2O4 ceramics. Tape casting of the template particles with Na2CO3 and the following in-situ reaction produced grain-oriented ceramics topotaxially as designed. The textured ceramic specimen had in-plane electrical conductivity: 600–300 S cm−1, Seebeck coefficient, 60–120 μV K−1 and thus power factor: 2–5×10−4 W mK−2 in the temperature range from room temperature to 700°C. A large excess amount of Na and inhomogeneity in its distribution could be attributed to the lower transport properties than expected from those of a single crystal.

Improvement of the open-circuit voltage of Cu2ZnSnS4 solar cells using a two-layer structure

In Cu2ZnSnS4 (CZTS) photovoltaic cells, a low open-circuit voltage (VOC) principally causes low conversion efficiency. We investigated the deposition of a CZTS layer by a two-layer process to improve the VOC of the CZTS cells. In this process, the first CZTS layers near a Mo electrode have a high Cu content and the second layer near the surface has a low Cu content. The two-layer process improved the VOC of the CZTS cells from 0.66 to 0.78 V. Finally, the best CZTS cell showed a conversion efficiency of 8.8%.

Atom-probe tomographic study of interfaces of Cu2ZnSnS4 photovoltaic cells

The heterophase interfaces between the CdS buffer layer and the Cu2ZnSnS4 (CZTS) absorption layers are one of the main factors affecting photovoltaic performance of CZTS cells. We have studied the compositional distributions at heterophase interfaces in CZTS cells using three-dimensional atom-probe tomography. The results demonstrate: (a) diffusion of Cd into the CZTS layer; (b) segregation of Zn at the CdS/CZTS interface; and (c) a change of oxygen and hydrogen concentrations in the CdS layer depending on the heat treatment. Annealing at 573 K after deposition of CdS improves the photovoltaic properties of CZTS cells probably because of the formation of a heterophase epitaxial junction at the CdS/CZTS interface. Conversely, segregation of Zn at the CdS/CZTS interface after annealing at a higher temperature deteriorates the photovoltaic properties.

Enhancement of Conversion Efficiency of Cu2ZnSnS4 Thin Film Solar Cells by Improvement of Sulfurization Conditions

To enhance the conversion efficiency of Cu2ZnSnS4 (CZTS) thin film solar cells prepared by the sulfurization method, we investigated the formation process of the CZTS thin film. The holding temperature of the sulfurization was 580 °C. This study showed that the spreading resistance (SR) of the CZTS layer strongly depends on the holding time of the sulfurization. At the intermediate holding time (~30 min), the SR of the CZTS layer came to a minimum, and the efficiency of the CZTS solar cell came to a maximum. A 7.6% efficiency CZTS solar cell without a high-resistance buffer layer and an antireflection coating was fabricated.

Direct measurement of band offset at the interface between CdS and Cu2ZnSnS4 using hard X-ray photoelectron spectroscopy

We directly and non-destructively measured the valence band offset at the interface between CdS and Cu2ZnSnS4 (CZTS) using hard X-ray photoelectron spectroscopy (HAXPES), which can measure the electron state of the buried interface because of its large analysis depth. These measurements were made using the following real devices; CZTS(t = 700 nm), CdS(t = 100 nm)/CZTS(t = 700 nm), and CdS(t = 5 nm)/CZTS(t = 700 nm) films formed on Mo coated glass. The valence band spectra were measured by HAXPES using an X-ray photon energy of 8 keV. The value of the valence band offset at the interface between CdS and CZTS was estimated as 1.0 eV by fitting the spectra. The conduction band offset could be deduced as 0.0 eV from the obtained valence band offset and the band gap energies of CdS and CZTS.

Synthesis and Magnetic Properties of Fe7C3 Particles with High Saturation Magnetization

The formation behavior and magnetic properties of Fe7C3 particles were investigated under atmospheric pressure. The starting material of barium-containing iron oxide was carbureted with CO by heat treatment. Fe7C3 particles were formed in the reaction temperature range of 300–375°C along with a small amount of Fe5C2 particles. Fe7C3 content in the product increased with the increase in the partial pressure of CO and the decrease in the reaction temperature. DTA-TG analysis revealed that the product contained about 5–10 wt% free carbon. The saturation magnetization of the product was about 110 emu/g regardless of the reaction conditions. The coercivity and the ratio of the residual magnetization to the saturation magnetization increased with the increase of the Fe7C3 content in the product.

Improving Powder Magnetic Core Properties via Application of Thin, Insulating Silica-Nanosheet Layers on Iron Powder Particles

A thin, insulating layer with high electrical resistivity is vital to achieving high performance of powder magnetic cores. Using layer-by-layer deposition of silica nanosheets or colloidal silica over insulating layers composed of strontium phosphate and boron oxide, we succeeded in fabricating insulating layers with high electrical resistivity on iron powder particles, which were subsequently used to prepare toroidal cores. The compact density of these cores decreased after coating with colloidal silica due to the substantial increase in the volume, causing the magnetic flux density to deteriorate. Coating with silica nanosheets, on the other hand, resulted in a higher electrical resistivity and a good balance between high magnetic flux density and low iron loss due to the thinner silica layers. Transmission electron microscopy images showed that the thickness of the colloidal silica coating was about 700 nm, while that of the silica nanosheet coating was 30 nm. There was one drawback to using silica nanosheets, namely a deterioration in the core mechanical strength. Nevertheless, the silica nanosheet coating resulted in nanoscale-thick silica layers that are favorable for enhancing the electrical resistivity.

Cu2Sn1− x Ge x S3 solar cells fabricated with a graded bandgap structure

We fabricated Cu2Sn1− x Ge x S3 (CTGS) solar cells with a graded bandgap structure in order to improve their photovoltaic performance. Bandgap gradation was formed by changing the Ge/Sn ratio in the depth direction of the CTGS layers. The composition profile of each sample was measured by secondary ion mass spectrometry, and we confirmed that the Ge/Sn ratio near the buffer layer was lower than that near the back electrode. This means that the bandgap increases with depth from the surface. The performance of the cells was improved to over 6.7% in conversion efficiency.