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

Abstract

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 SiO x 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 SiO x 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 V OC 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 V OC 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 V OC 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.

Energetic Tug‐of‐War between Pt and Leaky TiO2: Positive and Negative Effects on the Function of Molecularly‐Modified p‐Si(111)|TiO2|Pt Photocathodes

“…We propose this interfacial field offsets the Si and metal work functions and increases the Si flatband potential. This phenomenon is similar to the effects of interfacial dipoles on semiconductor surfaces we and others have reported previously . Hu et.…”

“…extensively detailed hole‐transport through “leaky” TiO 2 , in which they establish that Ti precursor choice and ALD growth conditions are the major determinants of defect density and thus the extent of current leakage . Recently, our group showed that use of ‘leaky’ TiO 2 overlayers on a photocathode in contact with the one‐electron outer‐sphere redox couple methyl viologen showed substantial decreases in photovoltage . Additionally, Ager and Javey et.…”

“…TiO 2 ‐protected Si samples were electrochemically characterized by voltammetry and IMHFR (Intensity‐Modulated High Frequency Resistivity) impedance voltammetry, a technique capable of measuring the thermodynamic flatband potential of the silicon . An outer‐sphere redox couple was used in place of fuel‐forming reactions (all inner sphere) to isolate the kinetic effects from catalysis . Thus, we present data comparing the flatband potential and voltammetry data that we use to deconvolute the individual contributions from thermodynamic and kinetic effects of the TiO 2 film.…”

“…[30] Recently, our group showed that use of 'leaky' TiO 2 overlayers on a photocathode in contact with the one-electron outer-sphere redox couple methyl viologen showed substantial decreases in photovoltage. [35] Additionally, Ager and Javey et. al.…”

“…[25] An outersphere redox couple was used in place of fuel-forming reactions (all inner sphere) to isolate the kinetic effects from catalysis. [35] Thus, we present data comparing the flatband potential and voltammetry data that we use to deconvolute the individual contributions from thermodynamic and kinetic effects of the TiO 2 film.…”