Electrical properties of WSe2, WS2, MoSe2, MoS2, and their use as photoanodes in a semiconductor liquid junction solar cell
The electrical resistivity and Hall effect perpendicular to the c-axis of single crystals of n-type WSe(,2), MoSe(,2), WS(,2), and MoS(,2) were studied in the extrinsic conduction temperature range (77 K to 300 K). It was found that a single donor energy level can be assigned to the crystals of WSe(,2) and MoSe(,2), even for crystals from different growth ampoules. E(,D) = (108 (+OR-) 4) meV for WSe(,2) and (64 (+OR-) 2) meV for MoSe(,2). The electron Hall mobility of these two compounds depends strongly on temperature. (mu)(,H)(TURN)T('-2.4) for WSe(,2) and (TURN)T('-2.6) for MoSe(,2). For WS(,2) and MoS(,2), the results were not as consistent as those for WSe(,2) and MoSe(,2) because of poorer crystal quality. The resistivity parallel to the c-axis was also studied on single crystals of WSe(,2) and MoSe(,2). The resistivity anisotropy of these two compounds is small, compared to that reported on natural single crystals of MoS(,2);Single crystals of n-type WSe(,2), MoSe(,2), WS(,2), and MoS(,2) were employed as photoanodes in a photoelectrochemical solar cell with a variety of redox couples as the charge-transfer agents. It was found that WSe(,2) is the best photoelectrode material of all, and the redox couple, I('-)/I(,3)('-), provides a fast-kinetics path for electron transfer. An efficiency as high as 10.2% was achieved for an n-WSe(,2)/I('-)-I(,3)('-)/Pt cell, and 9.4% when MoSe(,2) was used. The cell (n-WSe(,2)/I('-)-I(,3)('-)/C) is stable for at least one year. Polycrystalline samples of n-WSe(,2) were also studied as photoanodes in such a cell, but their performance is poor, compared with that of single crystals, because defects, such as steps on the crystal surface and grain boundaries, act as recombination centers;Transmission spectra of single crystals of WSe(,2) were studiednear the fundamental absorption edge. Based on the experimental results, it was concluded that WSe(,2) is an indirect band gap material;*DOE Report IS-T-1045. This work was performed under ContractNo. W-7405-Eng-82 with the U.S. Department of Energy.