Bob L. Wheeler

Semiconductor Electrodes XXIX . High Efficiency Photoelectrochemical Solar Cells with Electrodes in an Aqueous Iodide Medium

which maximizes the output voltage, and cond i t ions which minimize recombination processes in the semiconductor and so lu t ion and at the in ter face. The solut ion redox couple and solvent are selected to s tab i l i ze the semiconductor from photoinduced corrosion processes (8,9) . The highest published power e f f i c iency for such ce l l s is the 12% reported for a s ingle crysta l n-GaAs electrode in a selenide medium ( I0 ) . We describe here a PEC cel l based on s ingle crysta l nWSe 2 which shows comparable e f f i c i enc ies .

Semiconductor Electrodes XXXIX . Techniques for Stabilization of n‐Silicon Electrodes in Aqueous Solution Photoelectrochemical Cells

A number of different approaches have been used in the stabilization of small band gap n-type semiconductors against photocorrosion in photoelectrochemical (PEC) cells. Such stabilization is necessary in the design of practical cells for conversion of solar energy to electricity, and is of critical importance in photoelectrosynthetic systems where the photogenerated holes produce species (e.g. 02, C12) at quite positive potentials at the semiconductor surface, i) One approach involves the utilization of thin films of metals (I) or semiconductors (2) to protect the surface. For example, n-GaAs electrodes can be stabilized o in a solution of Fe(II) EDTA by a thin (~60A) film of gold (3). 2) Use of nonaqueous solvents (4) in PEC cells has been shown to aid in the stabilization of the semiconductor electrode by decreasing the extent of solvation of the oxidation products. A similar approach involves the use of concentrated electrolytes in aqueous solutions. Wrighton and co-workers (5) have recently demonstrated that n-MoSe 2 and n-MoS 2 electrodes are stable in concentrated LiCI solutions even in the presence of chlorine evolution. 3) The deposition of polymer layers on the semiconductor surface can also decrease photocorrosion. Noufi and co-workers (6) have described the stabilization of n-GaAs and n-Si by the electrodeposition of polypyrrole films on the surface. We demonstrate here that by combining these approaches even better stabilization can be accomplished and describe the photo-oxidation of Fe 2+ at n-Si electrodes covered by thin gold and thicker polypyrrole film and immersed in a concentrated electrolyte aqueous solution. Electrodes were fabricated from n-Si single crystals (0.4 to 0.6 ~ cm) donated by Texas Instruments. Ohmic contacts were made with In-Ga alloy. They were connected to a copper wire with silver epoxy cement. The crystals were mounted in a glass tube with all sides insulated with 5 min epoxy and covered by silicone rubber sealant leaving an area of 0.2-0.5 *Electrochemical Society Active Member.

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 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.