Fu Ren F. Fan

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. 24. Behavior and photoelectrochemical cells based on p-type GaAs in aqueous solutions

The electrochemical behavior of single-crystal p-type GaAs in aqueous solutions containing several redox couples (I/sub 3//sup -//I/sup -/,Fe(III)/Fe(II),SN(IV)/Sn(II),Eu(III)/Eu(II)) in the dark and under irradiation is described. The observation that the difference in potential between that for the onset of photocurrent and the standard potential for the redox couple was 0.4-0.5V, independent of the couple, leads to a revised model for semiconductor/electrolyte solution interface with semiconductors having a high density of surface states with energies within the band-gap region. In such a surface controlled system the Fermi level of the semiconductor is pinned at the surface state level. Several solar cells in which p-GaAs shows stable behavior are described. The cell p-GaAs/I/sub 3//sup -/(0.25 M),I/sup -/(0.75 M)/Pt showed an open-circuit voltage of 0.20 V and a short-circuit current density of 30 mA/cm/sup 2/ under irradiation with 1.7-mW He-Ne laser. The quantum efficiency at the maximum photocurrent in this cell was about 95%.

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.

Electrochemical Vapor Deposition of Semiconductors from Gas Phase with a Solid Membrane Cell

We demonstrate the feasibility of semiconductor deposition via the electrochemical reduction of gaseous precursors by the use of an anhydrous proton-conducting membrane, the solid acid CsHSO4, at 165 °C. This membrane electrode assembly was operated within the oxidation of hydrogen on a porous Pt anode and the deposition of Si or Ge under bias at the cathode from chloride-based gaseous precursors; SiCl4 and GeCl4 in an Ar flow with a reduction potential over -1.0 V (vs RHE).