Photoelectrochemical characterization of III-V semiconductors utilizing impedance spectroscopy methods.

摘要

The thermodynamic reversible potential required to decompose water into hydrogen and oxygen is about 1.23 eV. An overpotential of 100-400 mV may have to be overcome at the semiconductor/electrolyte interface to successfully photo-electrolyze water. An optimal bandgap of a semiconductor utilized in photoelectrolysis of water would be in the range 1.6-2.0 eV. Single crystal GaInP{dollar}sb2{dollar}, (Eg = 1.83 eV) and Ga{dollar}sb{lcub}rm 1-x{rcub}{dollar}Al{dollar}sb{lcub}rm x{rcub}{dollar}As (Eg = 1.5-1.8 eV), Ga{dollar}sb{lcub}rm 1-x{rcub}{dollar}Al{dollar}sb{lcub}rm x{rcub}{dollar}As with embeded single quantum wells of GaAs, Ga{dollar}sb{lcub}rm x{rcub}{dollar}As{dollar}sb{lcub}rm 1-x{rcub}{dollar}P, Ga{dollar}sb{lcub}rm x{rcub}{dollar}Al{dollar}sb{lcub}rm y{rcub}{dollar}In1-x-yP, GaAs and InP grown epitaxially in a MOCVD reactor (at NREL) were the subject of this study since, by virtue of their bandgaps, they show promise of being capable of photodecomposition of water in the absence of an external bias. These materials were studied when immersed in various aqueous electrolyte solutions of different pHs. Electrochemical impedance spectroscopy (EIS) was used to investigate the semiconductor-electrolyte interface, which was modeled using an equivalent electrical circuit analog that represented the physical phenomena. Based on the space-charge layer capacitance, the flat-band potential and hence, the corresponding position of band edges relative to the hydrogen and oxygen redox levels were determined. Capacitance measurements, current-voltage measurements and photocurrent spectroscopy were also carried out on these interfaces. Based on favorable results obtained for GaInP{dollar}sb2{dollar}, a tandem cell structure consisting of GaInP{dollar}sb2{dollar} and GaAs was utilized in photoelectrolysis of water in acidic solutions and efficiencies of 7-14% reported.