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

Solar driven water splitting has the potential to be a trueterawatt-scale solution to the current clean energy crisis. The broad goalof this thesis is to outline and demonstrate two novel device architecturesfor high efficiency tandem water splitting.Initially focus was dedicated towards the bottlenecks encountered infabricating standalone high efficiency p-type dye sensitized solar cells(DSCs). Synthesis and characterization of 5 novel nanocrystalline potentialhole conducting oxides will be discussed.Special emphasis will be devoted to Cr-based spinels, which have thepotential to constitute a new class of wide band gap, p-type oxides withsubstantially deeper valence bands than all current existing p-type oxidesin literature.Finally, hole injection from CdSe quantum dots into the VB ofzinc chromate (ZnCr2O4) will be demonstrated with photovoltages as high as350mV using an polysuflide-based redox couple, making it one of the largestphotovoltages of all p-type oxides using this redox couple.The latter half will be devoted towards fabricating high efficiencyBiVO4 photoanodes for oxygen evolution. The first strategy utilizes theenhanced reactivity of the (004) facet of BiVO4, wherein a texturednanostructured Mo:BiVO4 photoanode shows near unity carrier separation andrecord catalytic efficiency towards oxygen evolution for a standalone BiVO4photoanode. 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 appropriatephosphate precursor treatments, photocurrent densities in excess of 3 mA/cm2were achieved at 0.6V vs RHE and greater than 4 mA/cm2 at 1.23V vs RHEtowards sulfite oxidation. It was found that the electron diffusion lengthusing these treatments could be further increased to ~ 300nm which is one ofthe highest reported for all known metal oxide semiconductors.This thesis can be used as a guideline for the fabrication o