Multiple-bandgap photoelectrochemistry: Bipolar semiconductor ohmic regenerative electrochemistry

Multiple semiconductor bandgaps can enhance the energetics of photoelectrochemical interactions. However, illuminated bipolar multiple bandgaps can generate a photopotential larger than the thermodynamic electrochemical potential window of many solvents and induce parasitic chemical reactions including solvent decomposition and electrode deactivation, which could inhibit regenerative photoelectrochemistry. From a fundamental and experimental perspective, it is demonstrated that these parasitic reactions may be avoided and that several efficient regenerative bipolar bandgap photoelectrochemical processes can occur. A bipolar ohmic regenerative photoelectrochemical cell contains wide bandgap pn junctions in tandem monolithic contact with smaller bandgap pn junctions. The small-bandgap semiconductor is either in direct ohmic contact with a regenerative solution-phase redox couple or in indirect contact through an electrocatalyst intermediate. The photocathodic bipolar ohmic regenerative chemistry is given by hv --> pn (wide gap)\pn (small gap)\(redox couple) I(electrocatalyst anode). Energy diagrams of photocathodic bipolar ohmic regenerative photoelectrochemistry are evaluated and studied with several AlGaAs\Si\electrolyte photoelectrochemical cells. The individual multiple-bandgap components include a graded band emitter, varying from Al0.3-0.15Ga0.7-0.85As, and a Si layer either in direct contact with a V2+/3+ electrolyte or in indirect contact with a V2+/3+, polysulfide, or iodide aqueous electrolyte through a carbon, CoS, or Pt electrocatalyst, respectively. The electrocatalyst prevents any electrolyte-induced semiconductor photocorrosion. Under solar illumination, photopotentials larger than the 1.2-V solvent-decomposition potential are generated (open circuit potentials from 1.4 to 1.5 V), and efficient regenerative photoelectrochemistry occurs at 19%-20% solar energy conversion efficiency.