Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO2-Si photoanode

Scott K Cushing; Ilana J Porter; Bethany R de Roulet; Angela Lee; Brett M Marsh; Szilard Szoke; Mihai E Vaida; Stephen R Leone

doi:10.1126/sciadv.aay6650

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO₂-Si photoanode

Sci Adv. 2020 Apr 3;6(14):eaay6650. doi: 10.1126/sciadv.aay6650. eCollection 2020 Apr.

Authors

Scott K Cushing¹, Ilana J Porter^{2

3}, Bethany R de Roulet², Angela Lee², Brett M Marsh², Szilard Szoke¹, Mihai E Vaida^{2

4}, Stephen R Leone^{2

3

5}

Affiliations

¹ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
² Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720, USA.
³ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
⁴ Department of Physics, University of Central Florida, Orlando, FL 32816, USA.
⁵ Department of Physics, University of California, Berkeley, Berkeley, CA 94720, USA.

Abstract

Metal oxide semiconductor junctions are central to most electronic and optoelectronic devices, but ultrafast measurements of carrier transport have been limited to device-average measurements. Here, charge transport and recombination kinetics in each layer of a Ni-TiO₂-Si junction is measured using the element specificity of broadband extreme ultraviolet (XUV) ultrafast pulses. After silicon photoexcitation, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in characteristic spectral shifts in the XUV edges. Meanwhile, the electrons remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO₂, shifting the Ti spectrum to a higher oxidation state, followed by electron-hole recombination at the Si-TiO₂ interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO₂ and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.

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