Solar conversion efficiencies for splitting water with semiconducting photoelectrodes are calculated from basic thermodynamic principles combined with transport properties matching those of the best materials presently available. Assuming no further constraints, we derive in this way “upper limit” estimates of efficiencies achievable via semiconductor photoelectrochemical cells (PEC's), operating with no external electrical bias. Both one‐ and two‐photon configurations are considered. A one‐photon PEC is found to have an “upper limit” efficiency of ∼7% (AM 1.2 solar energy to chemical potential energy stored as H2 ). For two‐photon configurations, the “upper limit” for a p‐n PEC is ∼10%, while for a tandem PEC it is ∼18%. The tandem cell configuration is the least sensitive to the choice of materials parameters and transport losses and yields the highest efficiencies. Significant increases in conversion efficiencies result from assuming lower oxygen overpotentials and higher photoelectrode fill factors than have been achieved so far, with the latter being the more important, however.

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A vacuum microbalance and furnace assembly was constructed and employed to determine the growth kinetics of oxide films on aluminum. Using electropolished foil samples of geometric area 80 cm2, and making corrections for spurious weight changes arising from thermal diffusion effects, film thickness changes as small as 0.3Aå could be reliably detected. In addition to kinetic data, the structure of representative oxide films was examined by electron microscopy. It was found that only “amorphous” oxide grew for about the first 10 hr at 454°C and for shorter periods at 478° and 505°C, the weight gain data being in good accord with an equation similar to that proposed by Mott and Cabrera for the growth of very thin oxide films, an approximate integrated form of which is sometimes referred to as the inverse logarithmic equation. The data could not be represented satisfactorily by either a parabolic oxidation law or a direct logarithmic law.

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