Decoupling Effects of Surface Recombination and Barrier Height on p-Si(111) Photovoltage in Semiconductor|Liquid Junctions via Molecular Dipoles and Metal Oxides

This work provides insight into carrier dynamics in a model photoelectrochemical system comprised of a
semiconductor, metal oxide, and metal. To isolate carrier dynamics from catalysis, a common catalytic metal (Pt) is used in
concert with an outer-sphere redox couple. Silicon (111) substrates were surface-functionalized with electronegative aryl
moieties (p-nitrophenyl and m-dinitrophenyl). A mixed monolayer using p-nitrophenyl/methyl exhibited high surface quality as
determined by X-ray photoelectron spectroscopy (low surface SiOx content) and low surface recombination velocity. This
substrate also exhibited the expected positive surface dipole, as evidenced by rectifying J−V behavior on p-type substrates, and
by positive photovoltage measured by surface photovoltage spectroscopy. Its close molecular relative m-dinitrophenyl exhibited
poor electronic surface quality as indicated by high SiOx coverage and high surface recombination velocities (S > 3000 cm s−1
).
Photoelectrochemical J−V measurements of p-type Si-functionalized surfaces in contact with a high concentration (50 mM) of
methyl viologen (MV2+) in aqueous media revealed VOC values that are correlated with the measured barrier heights. In
contrast, low-concentration (1.5 mM) MV2+ experiments revealed significant contributions from surface recombination. Next,
the electronic and (photo)electrochemical properties were studied as a function of ALD metal oxide deposition (TiO2, Al2O3)
and Pt deposition. For the m-dinitrophenyl substrate, ALD deposition of both TiO2 and Al2O3 (150 °C, amorphous) decreased
the surface recombination velocity. Surprisingly, this TiO2 deposition resulted in negative shifts in VOC for all surfaces (possibly
ALD-TiO2 defect band effects). However, Pt deposition recovered the efficiencies beyond those lost in TiO2 deposition,
affording the most positive VOC values for each substrate. Overall, this work demonstrates that (1) when carrier collection is
kinetically fast, p-Si(111)−R devices are limited by thermal emission of carriers over the barrier, rather than by surface
recombination. And (2) although TiO2|Pt improves the PEC performance of all substrates, the beneficial effects of the
underlying (positive) surface dipole are still realized. Lastly (3) Pt deposition is demonstrated to provide beneficial charge
separation effects beyond enhancing catalytic rates.