Synthesis of alkaline-earth Zintl phosphides MZn₂P₂ (M = Ca, Sr, Ba) from Sn solutions

Exploration of suitable partner materials (so-called buffer layer or n-type emitter) for each light-absorbing material is essential to practicalize various emerging photovoltaic devices. Motivated by our recent discovery of a partner material, Mg(Mg x Zn₁−x)₂P₂, in Mg/Zn₃P₂ solar cells, the related series of materials MZn₂P₂ (M = Ca, Sr, Ba) is of interest to the application in pnictide-based solar cells. In this study, we synthesize these materials to evaluate the optoelectronic properties concerning photovoltaic applications. To deal with the difficulties of the high vapor pressure and reactivity of the constituent elements, we utilized Sn as a solvent to reduce their activities during heat treatments. Powders that are mainly composed of MZn₂P₂ were obtained by crushing the samples after solution growth, although single-phase crystals of MZn₂P₂ could not be obtained in this study. The optical bandgap and the ionization potential of each MZn₂P₂ were evaluated through the diffuse reflectance and the photoelectron yield spectroscopy measurements of the powder. As a result, we found that CaZn₂P₂ would be a promising partner material in photovoltaics based on Zn₃P₂ and ZnSnP₂

Fabrication process and device application of chalcopyrite phosphides based on thermodynamics

In this article, we introduce our studies on the application of chalcopyrite phosphides to solar cells, including bulk crystal growth based on the phase diagram, and thin film deposition through chemical potential diagram. A phase diagram is a useful tool for the fabrication of multi-component materials, while the chemical potential diagram, which is a kind of phase diagram with chemical potentials as axes, is well suited for discussing vapor growth. As another example of using the chemical potential diagram, the stability of a hetero-interface is presented. In addition, bandgap control through order-disorder phenomena is described

Band offset at the heterojunction interfaces of CdS/ZnSnP₂, ZnS/ZnSnP₂, and In₂S₃/ZnSnP₂

Heterojunctions were formed between ZnSnP₂ and buffer materials, CdS, ZnS, and In₂S₃, using chemical bath deposition. The band offset was investigated by X-ray photoelectron spectroscopy based on Kraut method. The conduction band offset, ΔEC, between ZnSnP₂ and CdS was estimated to be -1.2 eV, which significantly limits the open circuit voltage, VOC. Conversely, ΔEC at the heterojunction between ZnSnP₂ and ZnS was +0.3 eV, which is within the optimal offset range. In the case of In₂S₃, ΔEC was a relatively small value, -0.2 eV, and In₂S₃ is potentially useful as a buffer layer in ZnSnP₂ solar cells. The J-V characteristics of heterojunction diodes with an Al/sulfides/ZnSnP₂ bulk/Mo structure also suggested that ZnS and In₂S₃ are promising candidates for buffer layers in ZnSnP₂ thin film solar cells, and the band alignment is a key factor for the higher efficiency of solar cells with heterojunctions

High Performance Protonic Ceramic Fuel Cells with Acid-Etched Surfaces

We focus on the proton-conducting doped - barium zirconate, which has large proton conductivity at intermediate temperature. We found here a phenomenon that acid - etching of the doped barium zirconate in a solution having a specific pH leaves a porous structure on the surface, and demonstrated power generation of hydrogen fuel cell using electroless-plated Pd and Pt on the porously acid-etched electrolyte surface. The short circuit current density of the hydrogen fuel cell was about 430 mA/cm2 at 600°C using thick electrolyte of 500 μm

Preparation of a CuGaSe2 single crystal and its photocathodic properties

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

Revaluation of equilibrium quotient between titanium ions and metallic titanium in NaCl–KCl equimolar molten salt

In this study, the effect of oxide ion on the equilibrium between titanium ions and metallic titanium was investigated in NaCl–KCl equimolar molten salt. The soluble species containing Ti^[3+] and O^[2−] was not confirmed in the molten salt by absorption spectroscopy. The result of XRD for the solidified sample containing Ti^[3+] and O^[2−] implies that O2− reacts with Ti^[3+] and Cl^− to form TiOCl(s). The concentration quotient of the equilibrium between titanium ions (Ti^[2+], Ti^[3+]) and metallic Ti, Kc, was revaluated at 740℃ as follows:3Ti^[2+]⇄2Ti^[3+]+Ti, Kc=8.0×10^[−2] mol L−1=1.2×10^[−1] mol kg^[−1]. In addition, the solubility product of TiOCl(s) in NaCl–KCl equimolar molten salt, Ksp, at 700℃ was determined using the results of absorption spectra, Ksp=1.2×10^[−2] mol^2 L−2=5.1×10^[−3] mol^2 kg^[−2]

Order-Disorder Phenomena and Their Effects on Bandgap in ZnSnP₂

A ternary compound semiconductor ZnSnP₂ has the order-disorder transition, where the ordered and disordered phases are chalcopyrite (I42d, a = 5.651 Å, c = 2a) and sphalerite (F43m, a = 5.651 Å), respectively. In this study, the quantitative relationship between bandgap and long-range order parameter η was investigated by analyzing ZnSnP₂ bulk crystals obtained by various cooling rates in crystal growth. The Chipman and Warren method was used to evaluate the long-range order parameters from X-ray difffaction profiles. The results showed that the long-range order parameter η decreases from 0.94 to 0.54 with the increase in cooling rates, and the bandgap gradually reduced from 1.60 to 1.37 eV, corresponding to the η value. It was also demonstrated that bandgap tuning of ZnSnP₂ was possible by controlling the long-range order parameter through annealing process. This study clarified the effects of the order-disorder phenomena on bandgap in a model material, ZnSnP₂, based on the evaluation of long-range order parameter, which is also a promising technique to tune the bandgap without composition control

A pn-junction between chalcopyrite phosphide semiconductors for photovoltaic application

We report on the fabrication of a pn-junction between II-IV-V₂ type compounds with a chalcopyrite crystal structure such as CdSnP₂ and ZnSnP₂ for photovoltaic application. In the fabrication process, Cd–Sn precursor thin films were prepared on ZnSnP₂ bulk crystals grown by the flux method and the precursor thin films reacted with phosphorus gas to form CdSnP₂/ZnSnP₂ junction. STEM-EDX analysis and SAED patterns revealed that CdSnP₂ was epitaxially grown on ZnSnP₂ bulk crystals, indicating that the favourable junction was obtained in the view point of carrier transport. In addition, Zn was also detected in the region of the CdSnP₂ thin film due to the diffusion of Zn during phosphidation. This suggests the formation of solid solution (Cd, Zn)SnP₂ between ZnSnP₂ and CdSnP₂, leading to realization of a homojunction. In the J−V measurements of the n-(Cd, Zn)SnP₂/p-ZnSnP₂ junction, a rectifying behavior was observed. The results in this work are cornerstones for photovoltaic application using II-IV-V₂ type compound semiconductors including phosphides

Fabrication of CdSnP₂ Thin Films by Phosphidation for Photovoltaic Application

We report on the fabrication of CdSnP₂ thin films for photovoltaic applications. The phosphidation method, where a cosputtered Cd–Sn precursor thin film reacts with phosphorus gas, was utilized for the preparation of CdSnP₂ thin films. In order to establish the fabrication process, the temperature dependence on product phases was investigated, and CdSnP₂ thin films were obtained by the phosphidation at 350 °C for 30 min under the phosphorus vapor pressure of 10⁻² atm. CdSnP₂ thin films showed an n-type conduction. The resistivity, the carrier concentration and the mobility were evaluated to be 1.7–1.9 × 10² Ω cm, 2–7 × 10¹⁵ cm⁻³, and 4.7–17 cm² V⁻¹ s⁻¹, respectively. CdSnP₂ thin films with relatively flat and smooth surfaces were obtained, although it was reported that ZnSnP₂ with the same crystal structure grew as the protrusion shape by the VLS growth mode. In order to investigate these difference in growth mechanism between CdSnP₂ and ZnSnP₂, the reaction process in the Cd–Sn–P system was investigated and discussed on the basis of the chemical potential diagrams. As the results, it was understood that Cd, Sn, and P₄ directly reacted to form CdSnP₂, while ZnSnP₂ was formed via the reaction among Zn₃P₂, Sn, and P₄ after Zn reacts with P₄ to produce Zn₃P₂. Therefore, it is speculated that this simple reaction route results in the high growth speed, and a smooth flat morphology was obtained in the fabrication of CdSnP₂ thin films