Multiple band gap semiconductor/electrolyte solar energy conversion

Fundamental and experimental multiple band gap semiconductor/electrolyte processes to convert optical energy and utilize renewable solar energy are explored. Alternate processes discussed include an inverted (1 photon per e(-)) or bipolar (n greater than or equal to 2 photons per e(-)) arrangement of successive band gaps. Such processes can incorporate either a Schottky or an ohmic photoelectrochemical solution interface that can drive either regenerative single or multiple different redox reactions. The multiple redox case permits solar energy storage or solar water splitting evolving H-2 and provides an energy reservoir that may compensate for the intermittent nature of solar energy. Generated H-2 is attractive as a clean, renewable fuel. Experimental configurations examined include GaAs/Si or AlGaAs/Si photodriven redox couples, each with a solar to electrical conversion efficiency of 19-20%. A related solar cell is configured with electrochemical storage, which provides a nearly constant energetic output in illuminated or dark conditions. Similarly, multiple band gap semiconductors can also be utilized to generate hydrogen fuel by solar driven water splitting. A cell containing illuminated AlGaAs/Si RuO2/Pt-black evolves H-2 and O-2 at record (18.3%) efficiency. Contemporary models underestimate the attainable efficiency of solar energy conversion to water splitting, and future multiple band gap/electrolyte systems are calculated as being capable of attaining over 30% solar efficiency.