N-Type Molybdenum Diselenide-Based Photoelectrochemical Cells: Evidence for Fermi Level Pinning and Comparison of the Efficiency for Conversion of Light to Electricity with Various Solvent/Halogen/Halide Combinations

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Interfacial energetics for n-type MoSe2 (Eg = 1.4 eV, direct) and photoelectrochemical conversion of light to electrical energy in the presence of Xn-/X- (X = Cl, Br, I) have been characterized in CH3CN electrolyte solution. Data for MoSe2 in H20/I3-/I- are included for comparison, along with a comparison of MoSe2-based cells with MoS2- (Eg = 1.7 eV, direct) based cells. Cyclic voltammetry for a set of reversible (at Pt electrodes) redox couples whose formal potential, E. spans a range -0.8 to +1.5 V vs. SCE has been employed to establish the interface energetics of MoSe2. For the redox couples having E° more negative than ~-0.1 V vs. SCE, we find reversible electrochemistry in the dark at n-type MoSe2. When E° is somewhat positive of -0.1 V vs. SCE, we find that oxidation of the reduced form of the redox couple can be effected in an uphill sense by irradiation of the n-type MoSe2 with >Eg light; the anodic current peak is at a more negative potential than at Pt for such situations. The extent to which the photoanodic current peak is more negative than at Pt is a measure of the output photovoltage for a given couple. For E° more positive than ~+0.7 V vs. SCE it would appear that this output photovoltage is constant at ~0.4 V. For a redox couple such as biferrocene (E°(BF+/BF) = +0.3 V vs. SCE) we find a photoanodic current onset at -0.2 V vs. SCE; a redox couple with E° = 1.5 V vs. SCE shows an output photovoltage of 0.43 V under the same conditions. The ability to observe (i) photoeffects for redox reagents spanning a range of E°'s that is greater than the direct Eg and (ii) constant photovoltage for a range of E°'s evidences an important role for surface states or carrier inversion such that a constant amount of band bending (constant barrier height) is found for a couple having E° more positive than ~+0.7 V vs. SCE. Conversion of >g light to electricity can be sustained in CH3CN solutions of Xn-/X- (X = Cl, Br, I) with an efficiency that is ordered Cl > Br > I where n-type MoSe2 is used as a stable photoanode. In aqueous solution n-type MoSe2 is not a stable anode in the presence of similar concentrations of Br2/Br- or C12/C1-, showing an important role for solvent in thermodynamics for electrode decomposition. In CH3CN, efficiency for conversion of 632.8-nm light to electricity has been found to be up to 7.5% for C12/C1-, 1.4% for Br2/Br-, and 0.14% for Br2/Br-. Differences among these redox systems are output voltage and short-circuit current, accounting for the changes in efficiency. In H20, I3-/I- yields a stable n-type MoSe2-based photoelectrochemical cell with an efficiency for 632.8-nm light a little lower than that for the CH3CN/C12/C1- solvent/redox couple system. Data for MoS2-based cells in the CH3CN/Xn-/X- solvent/redox couple systems show that the efficiency again depends on X: Cl > Br > I. In H20/I3-/I- an efficiency a little lower than that for the CH3CN/C12/C1- is obtained. MoSe2-based cells are somewhat more efficient than MoS2-based cells, but significant variations in efficiency are found depending on the electrode sample used.



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