(Invited) Element-Specific Measurement of Hole Transport in a Photoanode By Transient Extreme Ultraviolet Spectroscopy

Abstract

A passivating oxide layer is critical for the performance and stability of solar-fuel photoelectrodes. While the semiconductor surface can be passivated by a few nanometer oxide film, the best performance often correlates with a thicker and defect-rich amorphous TiO2 layer. The defect states are suggested to facilitate hole transport between the semiconductor and metal catalyst. In this presentation, transient extreme ultraviolet (XUV) absorption spectroscopy is used to measure the electron and hole transport between each element of a photoexcited Ni-TiO2-Si junction. The transient changes in the Ni M2,3 edge, Si L2,3­ edge, and Ti M2,3 edge are quantified using a broad-band, high harmonic XUV continuum that spans 30 to 120 eV. Hole tunneling from the p-type Si to the Ni metal is measured immediately following 800 nm optical excitation. The measured transport time and electric field indicate a ballistic tunneling through the TiO2 that originates from a Si hole with a mobility of ~400 cm2/Vs. An appreciable transfer of photoexcited electrons is not measured. The quantum yield of the hole tunneling is calculated to be ~50% percent by using the photoexcited carrier density in Si and the measured shift in the Ni M2,3 edge Fermi energy. On a picoseconds time scale, the holes are measured to back-transfer from the Ni to the Si through the mid-gap states in the TiO2. The extracted kinetics suggest a hole diffusion constant of ~1 cm2/s. The surface recombination velocity of electrons and holes at the Si-TiO2 interface is also quantified as ~200 cm/s. These results suggest that transient XUV spectroscopy can be used to quantitatively measure electron and hole transport in solar-fuel relevant nanoscale junctions.