Band Gap Engineering of Oxide Photoelectrodes: Characterization of ZnO1-xSex

No single material or materials system today is a clear choice for photoelectrochemical electrode applications. Generally, materials with narrow, well-aligned band gaps are unstable in solution and stable materials have band gaps that are too wide to efficiently absorb sunlight. Here, we demonstrate the narrowing of the ZnO band gap and combine a variety of electrical, spectroscopic, and photoelectrochemical methods to explore the opportunities for this so-called highly mismatched alloy in photoelectrochemical water splitting applications. We find that the conduction band edge of ZnO1-xSex is located at 4.95 eV below the vacuum level (0.5 V below the hydrogen evolution potential). Soft X-ray emission and absorption spectroscopies confirm that the previously observed similar to 1 eV reduction in the ZnO band gap with the addition of selenium result from the formation of a narrow Se-derived band. We observe that this narrow band contributes to photocurrent production using applied bias incident photon to current efficiency measurements at an electrochemical junction. Electrical measurements, electrochemical flat band, and photocurrent measurements as a function of x in ZnO1-xSex alloys indicate that this alloy is a good candidate for an oxide/silicon tandem photoelectrochemical device because of the natural band alignment between the silicon valence band and the ZnO1-xSex conduction band. We observe that the photocurrent onset in preliminary ZnO1-xSex/silicon diode tandem devices is shifted toward spontaneous hydrogen production compared to ZnO1-xSex films grown on sapphire. With these findings, we hope that our method of band gap engineering oxides for photoelectrodes can be extended to devise better materials systems for spontaneous solar water splitting.