Photoelectrochemical behavior of n-type Si(100) electrodes coated with thin films of manganese oxide grown by atomic layer deposition

Thin (10 nm) films of manganese oxide have been deposited by atomic layer deposition (ALD) onto n-type silicon and onto degenerately doped p-type silicon. The photoelectrochemical properties of the resulting semiconductor/metal-oxide structures were evaluated in contact with aqueous 0.35 M K₄Fe(CN) ₆-0.05 M K₃Fe(CN)₆, 1.0 M KOH(aq), as well as in contact with a series of nonaqueous one-electron, reversible, outer-sphere redox systems. Under simulated air mass (AM) 1.5 illumination in contact with 0.35 M K₄Fe(CN)₆-0.05 M K₃Fe(CN) ₆(aq), MnO-coated n-Si photoanodes displayed open-circuit voltages of up to 550 mV and stable anodic currents for periods of hours at 0.0 V versus the solution potential. In contact with 1.0 M KOH(aq), at current densities of ∼25 mA cm^-2, MnO|Si photoanodes under 100 mW cm^-2 of simulated AM 1.5 illumination yielded stable oxygen evolution for 10-30 min. Variation in the thickness of the MnO films from 4 to 20 nm indicated the presence of a series resistance in the MnO film that limited the fill factor and thus the solar energy-conversion efficiency of the photoelectrodes. Open-circuit photovoltages of 30 and 450 mV, respectively, were observed in contact with cobaltocene^+/0 or ferrocene^+/0 in CH ₃CN, indicating that the energetics of the MnO-coated Si surfaces were a function of the electrochemical potential of the contacting electrolyte solution.