Interface by design : manipulating interfacial energetics and carrier dynamics of Si(111) via surface functionalization

Boucher, Dylan Gregory
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Semiconductor|liquid junctions are a critical component of photoelectrochemical (PEC) devices for the conversion of sunlight to chemical fuels. PEC device efficiency depends heavily on the energetics and band-alignment of the junction. Exerting energetic control over the junction via chemical functionalization is an extremely attractive strategy for tuning device efficiency. Here we explore the molecular and materials functionalization of Si(111) (photo)electrodes and the resulting impact on junction energetics and device behavior. Utilizing the principles of interfacial dipoles, we illustrated how covalently tethered polar organic moieties (e.g. PhNO₂) can modulate the energetic position of the semiconductor band-edges. We demonstrated how these changes to the interfacial energetics impact PEC performance in contact with an outer-sphere redox couple. Expanding the suite of surface modifiers to include polyaromatic moieties revealed an even wider range of accessible band-edge energies and PEC performances. In addition we showed that these surface modifications can be applied to tune the efficiency of PEC hydrogen evolution on TiO₂ protected photocathodes. Interestingly, functionalization of Si(111) with extended aromatics resulted in electronic coupling between the semiconductor and attached molecule. These interfacial states were charterized via density functional theory calculations and their impact on PEC performance is discussed. Using the same suite of molecules we explored tuning the rate constant of electron transfer at n-Si(111)|liquid junctions via interfacial band-edge modulation. Modulation of the energetic position of the band-edges resulted in over an order of magnitude variation in the electron transfer rate constant. These results are discussed in the context of Marcus theory of electron transfer. Finally, we explored the impact of material overlayer (protective metal-oxide and catalyst) on junction energetics. With a combination of electrochemistry and XPS and UPS studies, we highlighted charging of the metal-oxide overlayer as a major contributor to the junction energetics and resulting solar fuels performance. Overall, these studies show how molecularly precise semiconductor junctions can be exploited to tune interfacial energetics, charge carrier dynamics, and ultimately control PEC device efficiency.