G. Nagasubramanian

Semiconductor Electrodes XLIX . Evidence for Fermi Level Pinning and Surface‐State Distributions from Impedance Measurements in Acetonitrile Solutions with Various Redox Couples

Capacitance‐voltage (C‐V) measurements were made for the single crystal semiconductors ; , , p‐Si,, , and in acetonitrile containing a number of redox couples whose potentials spanned a potential regime much wider than the bandgaps. The flatband potential evaluated from capacitance‐potential (C‐V) measurements (Mott‐Schottky plots) exhibited three types of behavior with varying solution redox potentials: (i) varied monotonically with for p‐Si, , and ; (ii) for and , did not shift for couples located negative of the midgap potential, but varied monotonically for couples positive of this value; (iii) for the layer‐type, compounds (, ), was almost independent of . These differences were ascribed to differences in surface‐state densities. For crystals, (001) face etched with molten and reduced, evidence for surface states at two different potentials was obtained from the in‐phase component of the total admittance. Tentative assignment of these states is to lattice defects. The states closer to the conduction band are assigned to oxygen vacancies and the deeper states to Ti (III), The densities of surface states evaluated from vs. ω plots for and p‐Si are around 1010 and 1013 cm−2, respectively. These two values represent different situations, i.e., while the former value of is not sufficient for pinning the Fermi level, the latter value is sufficiently high for the occurrence of Fermi level pinning.

Semiconductor Electrodes XXXVII . Photoelectrochemical Behavior of p‐Type in Acetonitrile Solutions

The photoelectrochem ical behavior of polycrystalline p-Cu~O in acetonitrile solutions containing a number of redox couples [e.g., phthalonitrile (0/--1), nitrobenzene (0/--1), methyl viologen (-t-2/-4-1)] was investigated. The p-Cu20, grown by oxidation of Cu metal by thermal methods or anodization, showed stable behavior under optical irradiation in these solutions. The bandgap, estimated from photoacoustic spectroscopy (PAS) and the photocurrent action spectrum Lu solution, was ~2.0 eV and the flatband potential was ~ -~0.16V vs. SCE. Scanning electron micrographs of the thermally grown samples reveal well-developed crystallites with distinct boundaries. A PEC cell of the form p-Cu20/Ph(CN)2,M eCN/Pt was shown to have an overall optical-to-elect rical energy conversion efficiency of only 0.05%. The low efficiency for such a cell is ascribed to rapid recombination processes in the bulk semiconductor and at the interface.

On the role of surface states in semiconductor electrode photoelectrochemical cells

Surface states that occur at the semiconductor–liquid interface play an important role in the behaviour of that interface and affect the efficiency of photoelectrochemical solar devices. The nature of such states and evidence for their existence will be briefly reviewed. Their role in dark electron transfer reactions for redox couples with energies within the band-gap region and in mediating surface recombinations will be discussed. The importance of Fermi-level pinning by surface states at moderate densities in GaAs and Si in controlling the open-circuit photovoltage and the observed electrochemical behaviour will be described. The effect of the surface pretreatment on the photoelectrochemical behaviour of p-GaAs and n-WSe2 will be demonstrated.

Semiconductor Electrodes XLII . Evidence for Fermi Level Pinning from Shifts in the Flatband Potential of p‐Type Silicon in Acetonitrile Solutions with Different Redox Couples

The flatband potential, , of p‐Si electrodes in acetonitrile solutions containing various redox couples was determined by measurement of the cell impedance. was found to depend strongly on the redox potential of the solution, indicating the occurrence of Fermi level pinning. The shift of did not depend upon the nature (cationic or anionic) of the redox couples; thus it cannot be attributed to specific adsorption. The shift in was also found with redox couples (such as oxazine and benzoquinone) which have energy levels located below the middle of the gap, implying that inversion does not occur in these cases.