Multiple-bandgap photoelectrochemistry: Inverted semiconductor ohmic regenerative electrochemistry

Illuminated bipolar multiple semiconductor bandgaps can generate a photopotential larger than the thermodynamic electrochemical potential window of many solvents. From a fundamental and experimental perspective, it is demonstrated that via an inverted bandgap alignment containing a common node at the quasi-Fermi level shared by wide-bandgap and small-bandgap layers of similar n or p type, multiple bandgaps can enhance the energetics of photoelectrochemical interactions without inducing large photopotentials. The energy diagram of regenerative photocathodic inverted ohmic photoelectrochemistry has been evaluated and exemplified using GaAs (E-G = 1.4 eV) antipolar-aligned with Si (E-G = 1.0 eV). Processes examined include GaAs\Si-photodriven electrochemistry in which an iodide redox couple is in direct contact with the silicon interface. HF addition prevents Si surface passivation and stabilizes the photocurrent. Processes were also examined in which the solution-phase redox couple is isolated from the silicon interface via an electrocatalyst intermediate, preventing any electrolyte-induced semiconductor photocorrosion. Processes examined include polysulfide, iodide, and V2+/3+ electrochemistry through a CoS, Pt, or C interface. Under illumination, the total power generated is the sum of the simultaneous extractable power at a solar-to-electrical conversion efficiency of 19-20%. Incident light passes directly into the wide-bandgap semiconductor layer rather than through the solution, preventing incident photon loss through competitive electrolyte light absorption. In all processes, efficient energy conversion is achieved at low photopotentials of less than 0.9 V, and indicates basic charge-transfer pathways by which the photopotential may be in situ segmented into useful smaller constituents without loss of the energetic advantage of multiple bandgaps.