Water splitting cells with direct semiconductor/liquid contacts are attractive because they avoid significant fabrication and systems costs involved with the use of separate electrolyzers wired to p-n junction solar cells. Another attractive advantage of photoelectrochemical water splitting directly at the semiconductor surface is the ease with which an electric field can be created at a semiconductor/liquid junction. Water splitting cells require semiconductor materials that are able to support rapid charge transfer at a semiconductor/aqueous interface, that exhibit long-term stability, and that can efficiently harvest a large portion of the solar spectrum. In contrast to the use of a single band gap configuration (S2) to split water, the use of a dual band gap (D4) water splitting cell configuration, where the electric field is generated at a semiconductor liquid junction or through a buried junction, appears to be the most efficient and robust use of complementary light absorbing materials.
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