Optical and Electrical Transport Evaluations of n-Type Iron Pyrite Single Crystals.

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.

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.

Reactive Epitaxial Formation of a Mg-P-Zn Ternary Semiconductor in Mg/Zn3P2 Solar Cells.

Zinc phosphide (Zn3P2) has attracted considerable attention as an environmentally benign and earth-abundant photoabsorber for thin-film photovoltaics. It is known that interdiffusion occurs at the Mg/Zn3P2 interface, which is a component of the record device, but the micro- and nanoscopic structures of the interface after interdiffusion have been controversial for over three decades. Here, we report on the formation of a Mg-P-Zn ternary semiconductor, Mg(Mg xZn1- x)2P2, at the Mg/Zn3P2 interface. Interestingly, Mg(Mg xZn1- x)2P2 is epitaxially grown on Zn3P2 with the orientation relationship of [21̅1̅0](0001)Mg(Mg xZn1- x)||[100](011)Zn3P2 due to interdiffusion. The lattice mismatch of the Mg(Mg xZn1- x)2P2 layer on the Zn3P2 substrate is less than 0.5%, and this is favorable for carrier transport across the interface. Mg(Mg xZn1- x)2P2 is the material suggested as "n-type Mg-doped Zn3P2" or "a Mg-P-Zn alloy" in the previous studies. Thus, only the optimization of Mg treatment as conducted in the previous studies is insufficient for the improvement of the cell performance. This work clarified that a suitable microstructure and band structure around Mg(Mg xZn1- x)2P2/Zn3P2 heterointerface should be established.