Charge-Transfer through Ultrathin Film TiO2 on n-Si(111) Photoelectrodes: Experimental and Theoretical Investigation of Electric Field-Enhanced Transport with a Nonaqueous Redox Couple

Ultrathin film amorphous (a-TiO₂) and anatase crystalline (c-TiO₂) titanium dioxide were investigated as corrosion passivation layers on n-type Si(111). Varying thicknesses of TiO₂ (5–60 Å) were deposited on n-Si(111)–CH₃ substrates by atomic layer deposition (ALD) at 150 and 240 °C, thus yielding a-TiO₂ and c-TiO₂, respectively. The phase and morphology of the TiO₂ films were determined using a combination of XRD, Raman spectroscopy, XPS, AFM and SEM. The electronic properties of a-TiO₂ and c-TiO₂ films were compared using 4-point sheet resistance and electrochemical impedance spectroscopy. Substrates functionalized with c-TiO₂ exhibited higher conductivity, lower charge transfer resistance, and comparable anticorrosion behavior to a-TiO₂. The photoelectrochemical response of n-Si(111)–CH₃|TiO₂ electrodes as a function of TiO₂ thickness was characterized using cyclic voltammetry with ferrocene in acetonitrile. A thickness of 20 or 40 Å was required to block charge transport through a-TiO₂ and c-TiO₂, respectively. Lastly, the charge transport behaviors of both the amorphous and crystalline n-Si(111)–CH₃|TiO₂ constructs were enhanced via the deposition of platinum nanoparticles (ALD) on the TiO₂ layer. Using a solid-state drift diffusion simulation package (wxAMPS), a theoretical basis for the charge-transport behavior was developed. The experimental thickness-dependence results were used as a basis of comparison to determine the charge transfer mechanism across the n-Si(111)–CH₃|TiO₂ electrodes. The simulations suggest that the charges conduct via field-assisted thermionic emission across the Si(111)–CH₃|TiO₂ interface, utilizing a defect band that is consistent with the “leaky dielectric” attribute of TiO₂ films. In addition, the simulations suggest that the defects present in a-TiO₂ behave as trap states, while the defects present in c-TiO₂ behave as recombination centers; this is derived from the observed difference in photoelectrochemical behavior between the two films.