Photoelectrochemical (PEC) cells offer a promising method of hydrogen production driven directly by solar energy, however materials limitations have significantly hindered their efficiency. The objective of our research is to improve the efficiencies of PEC cells by identifying and engineering corrosion-resistant semiconductors that exhibit the optimal conduction and valence band edge alignment for PEC applications.
PEC cells utilize light energy (photons) to perform a chemical reaction, in this case the splitting of water into hydrogen (H2) and oxygen (O2) gases. They consist of an anode and a cathode immersed in an electrolyte and connected in an external circuit. Typically, the anode or the cathode consists of a semiconductor that absorbs sunlight, and the other electrode is typically a metal. Photons with energies greater than the semiconductor band gap can be absorbed by the semiconductor, creating electron-hole pairs which are split by the electric field in the space-charge region between the semiconductor and the electrolyte. The electric field reflects the band bending of the conduction and valence band edges at the semiconductor surface and is necessary to supply the free carriers to the appropriate electrode. Water is oxidized at the anode according to the reaction:
2h+ + H2O (l) -> ½O2 (g) + 2H+, (1)
provided that the hole potential at the anode is lower than the O2/H2O potential, which lies approximately 5.7 eV below the vacuum level. At the cathode, H+ ions are reduced to form hydrogen gas via the reaction:
2H+ + 2e- -> 2H2 (g), (2)
as long as the electron potential in the cathode is greater than the H+/H2O potential, which is located 1.2 eV above the O2/H2O half-reaction. H+ ions migrate through the electrolyte from the anode to the cathode, and electrons or holes flow through an external circuit to reach the proper electrode.
2hν + H2O (l) -> ½O2 (g) + H2 (g). (3)
For this reaction to take place, several material requirements must be met. First, the semiconductor must provide a sufficient overpotential to drive the reaction in (3) at a reasonable rate. Considering that the potential difference between the two half-reactions is 1.23 eV, the bandgap of the semiconductor should be at least 1.8 eV. However, the semiconductor must also be able to absorb a significant portion of the solar spectrum; thus the bandgap should be minimized. Second, the conduction and valence band edges must straddle the hydrogen and oxygen redox potentials unless an external bias is applied. Finally, it is essential that the semiconductor be resilient to corrosion by the electrolyte.
These requirements impose considerable constraints and challenges to the choice of the semiconductor electrode. Oxides, such as TiO2, are the most common materials used as photoelectrodes in PEC cells due to their corrosion resistance and generally acceptable band edge alignments relative to the redox potentials . However, their large bandgaps (~ 3 eV) restrict the usable portion of the solar spectrum, limiting the efficiency of these cells to less than 10% . Alternatively, nitride semiconductors exhibit substantial corrosion resistance yet have bandgaps ranging from 0.7 eV (InN) to 6.0 eV (AlN), allowing for bandgap engineering through alloying. GaN-based alloys in particular are prime candidates for PEC cell applications. The addition of In to GaN (Eg = 3.4 eV) lowers the bandgap, making InxGa1-xN an excellent match to the solar spectrum , while shifting both the conduction and valence band edges closer to the redox potentials . Furthermore, the incorporation of an InxGa1-xN photovoltaic cell of a much lower bandgap (Eg < 1.8 eV) into the PEC cell design will provide the bias needed to move the band edges of an InxGa1-xN photoelectrode with a bandgap less that 2.8 eV into favorable alignment. Within this tandem arrangement, the photovoltaic cell would absorb photons with energies below the threshold for water splitting, and should not decrease the absorption of the photoelectrode. Such a design should be simpler and less costly than the semiconductor structures currently used in tandem PEC cells since it would require only one alloy system.
Recent discoveries involving the incorporation of “highly mismatched” isoelectronic impurities onto the anion sublattice of compound semiconductors also show promise for the band edge engineering of semiconductor photoelectrodes. The addition of a few percent of As to GaN produces a narrow As-derived band approximately 0.6 eV above the valence band of GaN, pushing the band edge upward toward the O2/H2O potential. The conduction band edge also moves slightly downward in energy due to a linear shift between the positions of the two endpoint materials (GaN and GaAs). These two effects drastically reduce the bandgap of GaAsxN1-x to coincide with a usable portion of the solar spectrum. Together, these GaN-based semiconductor alloys present significant promise for the realization of high efficiency PEC cells.
 A. K. Fujishima and K. Honda. Nature 238, 37738 (1972)