Design Rules for Efficient Photogeneration in Metal Oxides

Introduction

Photoelectrochemical (PEC) water splitting is an attractive route for green hydrogen production. Despite nearly half a century of research efforts, no material has successfully met the stringent requirements for a photoelectrode material, the light harvesting semiconductor within the PEC cell.

Challenges with Current Materials

Metal-oxides are widely viewed as the most promising photoelectrode materials for their exceptional stability in aqueous electrolytes. However, those with suitable band gaps for visible light absorption typically have open d shell configurations and suffer from low photoconversion efficiencies.

I hypothesize that the underperformance of such materials is related to their electronic configuration, which reduces the photogeneration yield of mobile charge carriers. This is an overlooked yet critical loss mechanism in metal-oxides.

Photogeneration Mechanism

Unlike conventional semiconductors where all absorbed photons generate electrons and holes, in metal-oxides with open d shell configuration, many of the photons give rise to localized electronic transitions that do not contribute to the photocurrent.

In addition, polaronic transport and charge carrier recombination reduce the charge carrier collection efficiency.

Project Goals

DREAM will address these challenges and provide a leap forward in understanding the photogeneration processes in metal-oxide photoelectrodes and their effect on photoconversion efficiency.

To achieve these goals, we will:

1. Couple systematic control of crystallographic structure, d orbital occupancy, and local cation environment using heteroepitaxial thin film growth.
2. Utilize wavelength and temperature-resolved characterization of the photogeneration yield spectrum.

Expected Outcomes

The knowledge gained by these fundamental investigations will lead to new design rules. We will employ these rules to engineer new metal-oxides with near unity photogeneration yield and integrate them into novel device architectures. This will enable highly efficient PEC-PV tandem cells for unassisted solar water splitting.