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Semiconductor photochemistry of BiVO4 photoanodes and sensitized wide band gap p-type oxides for tandem water splitting devices

  • Author(s): Nair, Vineet Vijayakrishnan
  • Advisor(s): Law, Matt D
  • et al.
Creative Commons 'BY' version 4.0 license

Solar driven water splitting has the potential to be a true

terawatt-scale solution to the current clean energy crisis. The broad goal

of this thesis is to outline and demonstrate two novel device architectures

for high efficiency tandem water splitting.

Initially focus was dedicated towards the bottlenecks encountered in

fabricating standalone high efficiency p-type dye sensitized solar cells

(DSCs). Synthesis and characterization of 5 novel nanocrystalline potential

hole conducting oxides will be discussed.

Special emphasis will be devoted to Cr-based spinels, which have the

potential to constitute a new class of wide band gap, p-type oxides with

substantially deeper valence bands than all current existing p-type oxides

in literature.Finally, hole injection from CdSe quantum dots into the VB of

zinc chromate (ZnCr2O4) will be demonstrated with photovoltages as high as

350mV using an polysuflide-based redox couple, making it one of the largest

photovoltages of all p-type oxides using this redox couple.

The latter half will be devoted towards fabricating high efficiency

BiVO4 photoanodes for oxygen evolution. The first strategy utilizes the

enhanced reactivity of the (004) facet of BiVO4, wherein a textured

nanostructured Mo:BiVO4 photoanode shows near unity carrier separation and

record catalytic efficiency towards oxygen evolution for a standalone BiVO4

photoanode. These photoanodes increased the electron diffusion length to ~

200nm. The second involves tackling the lack of photoresponse at low biases

(~0.6V vs RHE) via phosphate treatments of undoped BiVO4. Using appropriate

phosphate precursor treatments, photocurrent densities in excess of 3 mA/cm2

were achieved at 0.6V vs RHE and greater than 4 mA/cm2 at 1.23V vs RHE

towards sulfite oxidation. It was found that the electron diffusion length

using these treatments could be further increased to ~ 300nm which is one of

the highest reported for all known metal oxide semiconductors.

This thesis can be used as a guideline for the fabrication of a broad range

of device architectures to achieve low-cost, high efficiency

solar-to-hydrogen conversion efficiencies. This work is aimed at improving

the existing library of high work function p-type oxides as well as setting

the benchmark and improving photoanode-based PEC via a rational approach

towards improving the electronic properties of BiVO4 photoanodes.

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