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.
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.