Controlling the Performance of Si(111) Photoelectrodes Via Molecular Modification

Abstract

The small bandgap of silicon (1.1 eV) and large photocurrent density (~25 mA/cm2 under 100 mW/cm2 illumination) renders silicon a nearly ideal photocathode material for photoelectrochemical hydrogen evolution (PEC-HER). However, freshly etched silicon surfaces are highly unstable and readily react with both air and solution to form chemically amorphous junctions that limit device efficacy. Stabilization of Si(111) by alkylation has resulted in well-passivated and chemically well-defined surfaces. However, the resulting interfacial dipole of the alkylated surface shifts the onset of HER to negative efficiencies, even on Pt decorated surfaces. To this end, our group has explored low-temperature, solution-based processing methods to significantly alter the interfacial properties of Si(111). Chemically derivatized surfaces (R: phenyl, para-nitrophenyl, 9-anthracene, and 9-nitroanthracene) have been achieved using organolithium precursors. Mott-Schottky analysis revealed energetic shifts in the band-edge positions of the surfaces with unity correlation to potential shifts in PEC-HER onset of pSi-R|TiO2|Pt devices. This indicates quantitative control of the photovoltage through molecular design, even in the presence of MOx and catalyst overlayers. In addition, we find that the molecular identity of the modifier plays a significant role in determining rates of surface recombination; we explore this relationship with two model systems: para-nitrophenyl (low surface recombination) and meta-dinitrophenyl (high surface recombination) and develop design principles for retaining high-quality surfaces with low rates of recombination. Finally, we utilized kinetically simple systems (one-electron, outer sphere redox couple) to demonstrate the effects of surface recombination, TiO2 deposition, and Pt deposition on the current-voltage character of these surfaces in the absence of a catalytic overpotential. Without the convoluted kinetics of catalysis, we develop a detailed view of the interfacial energetics of the chemically complex pSi-R|TiO2|Pt junctions.