Research Highlight: High-Efficiency Photoelectrochemical Water Splitting through Graded Doping

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Top left: Graded doping in an n-STO photoanode is achieved by controlled reoxidation of the n-STO electrode doped by oxygen vacancies (VO). Top right: The graded doping widens the depletion region for more effective photon collection. Bottom: The widened depletion region significantly improves the water-splitting incident photon-to-current efficiency (IPCE), particularly for the weak indirect band gap absorption, achieving an IPCE higher than 70%.

What is the scientific achievement? 

In semiconductor-based photoelectrochemical (PEC) water splitting, charge carrier separation and delivery largely rely on the depletion region formed at the semiconductor-water interface. In a Schottky junction device, the trade-off between photon collection and minority charge carrier delivery remains a persistent obstacle for maximizing the performance of a water-splitting photoelectrode. 

This investigation demonstrated that the PEC water-splitting efficiency for an n-SrTiO3 (n-STO) photoanode improves very significantly—despite the photoanode’s weak, indirect band gap optical absorption (a < 104 cm-1)—by widening the depletion region through engineering the region’s doping density and profile. A simple low-temperature reoxidation technique was used to fabricate graded, doped n-STO photoanodes, with their bulk heavily doped with oxygen vacancies and their surface lightly doped over a tunable depth of a few 100 nm. The graded doping profile widened the depletion region to more than 500 nm, leading to a very efficient charge carrier separation and high quantum efficiency (> 70%) for the weak indirect transition. This simultaneous optimization of light absorption, minority carrier (hole) delivery, and majority carrier (electron) transport through a graded doping architecture may inform the design of other indirect band gap photocatalysts that suffer from similar problems with weak optical absorption.

Why does this achievement matter?

This research pushes the depletion width of a water-splitting photoelectrode to an unprecedented limit (> 500 nm). The depletion width in previous studies barely exceeds 100 nm for metal oxides.  In addition to offering a new path for optimizing visible light photocatalysts that have indirect band gaps (e.g., WO3, Fe2O3), this approach fostered a more comprehensive understanding of the carrier-relaxation process in the depletion region. For example, the effect of point defects can be considered at densities that are traditionally considered negligible.

What are the details?

CFN Capabilities: CFN’s Materials Synthesis and Nanofabrication Facilities were used to fabricate the photoelectrochemical water-splitting devices. Characterization of the devices was performed using CFN’s electrical probe station and photoelectrochemistry test station resources in the Materials Synthesis and Characterization Facility.

Publication Reference

“Semiconductor-based Photoelectrochemical Water Splitting at the Limit of Very Wide Depletion Region”

M.Z. Liu, J.L. Lyons, D.H. Yan, and M.S. Hybertsen

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973

Advanced Functional Materials 26 (2), 219–225 (2016)

Acknowledgement of Support 

This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02–05CH11231.

2016-6340  |  INT/EXT  |  Newsroom