Efficiency of Splitting Water with Semiconducting Photoelectrodes

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 ). 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.