Studies of Metal Dichalcogenide Formation Using Electrochemical ALD

Group VI transition metal dichalcogenides (TMDCs) are graphene-like 2D layered semiconductors that exhibit anisotropic conductivity, which favors conductivity along the basal planes.¹ In particular, MoS₂, MoSe₂ and WSe₂ have received special interests for their potential toward photoelectrochemical and photoelectronic applications. These materials have been shown to be viable hydrogen evolution reaction (HER) catalysts in photoelectrochemical water-splitting devices.^1-2 Applications in solar energy conversion are feasible due to the direct band gaps these materials possess, which have energies range from near-IR to visible. They are also highly stable against photocorrosion due to the d-d transition that occur upon photoexcitation.³ Among the TMDCs, MoSe₂ synthesis and characterization is currently the main focus in this group. In photovoltaic (PV) devices that commonly use Mo as the back contact, having MoSe₂ as the intermediate layer between the PV absorber and the Mo back contact may create a more favorable ohmic-type contact that reduces resistive losses.⁴ A lower resistivity ohmic contact helps to reduce carrier recombination and improves conversion efficiency of a PV device.

Formation of MoSe₂ nanofilms by electrochemical atomic layer deposition (E-ALD) is currently being investigated by this group. E-ALD can be used to grow high quality nanofilms by alternating precursor solutions through a flow cell to deposit the desired elements up to one atomic layer at a time. Such fine control is possible by utilizing the underpotential deposition (UPD) of elements on each other. Pure Mo electrodeposition is hampered by the presence of HER, but “induced co-deposition” is possible by electrodepositing Mo in the presence of other metal ions.⁵ In E-ALD of MoSe₂, UPD Se plays the role of this other metal ion. Electrochemical studies for growing MoSe₂ by E-ALD as well as characterization of preliminary MoSe₂ films will be presented.

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3.            Pathak, V. M.; Patel, K. D.; Pathak, R. J.; Srivastava, R., Improved photoconversion from MoSe2 based PEC solar cells. Sol Energ Mat Sol C 2002, 73 (2), 117-123.

4.            Wada, T.; Kohara, N.; Nishiwaki, S.; Negami, T., Characterization of the Cu(In,Ga)Se-2/Mo interface in CIGS solar cells. Thin Solid Films 2001, 387 (1-2), 118-122.

5.            (a) Chandra, S.; Sahu, S. N., Electrodeposited semiconducting molybdenum selenide films. I. Preparatory technique and structural characterisation. Journal of Physics D: Applied Physics 1984, 17 (10), 2115; (b) Delphine, S. M.; Jayachandran, M.; Sanjeeviraja, C., Pulsed electrodeposition and characterization of molybdenum diselenide thin film. Mater Res Bull 2005, 40 (1), 135-147.