Wet Chemical Modification of Crystalline Silicon Interfaces for Heterogeneous Charge Transfer and Quantitative Analysis

Abstract

This thesis describes the covalent bonding of organic monolayers on crystalline silicon (Si) interfaces for the advancement of photovoltaic and photoelectrochemical applications. The first section of this thesis explores the premise that surface bonding will influence the resultant electronic properties of the interface. Wet chemical Grafting via Grignard reagents form Si-C bonds that are known to highly passivate Si(111) eliminating surface state traps. Other methods, such as silane dehydrogenative couplings, that form a Si-Si bond are relatively unexplored. Furthermore, bis(trifluoromethanesulfonyl)-based solutions are known to mask surface defects temporarily on etched crystalline silicon. The combination of wet-chemical grafting methods in tandem with these bis(trifluoromethanesulfonyl)-based solutions has not been explored previously. Specifically, n-Si(111) surfaces were decorated with an octadecyl functional group using either a two-step Grignard reaction or silane dehydrogenative coupling compare the putative surface bond. Both surface types were treated with a solution of trifluoromethanesulfonic anhydride to detail the observable chemical properties on the interfacial bond. This work demonstrated that all surface types exhibited an initial beneficial lowering of surface recombination from this treatment. However, the efficacy using trifluoromethanesulfonic anhydride of in conjunction with Si-C bond at the interface was shown to improve suppression of charge recombination for at least one week. The second focus of my work is a detailed framework for modeling and interpreting the entire voltammetric response recorded for adsorbed redox monolayers on n-Si(111) electrodes. The quantitative model uses a set of equations that encapsulate the explicit forward and back charge-transfer rate constants of an adsorbed species as a function of potential. To validate accuracy, the analytical model was used in conjunction with an experimental system using covalently bound viologens on n-Si(111). Irreversibly bound surface molecules are one of the most researched directions in semiconductor photoelectrochemistry, namely dye-sensitized solar cells and water-splitting photoelectrochemical systems with molecular electrocatalysts. This work not only fills a long-standing knowledge gap in semiconductor physics but also will aid in advancing photoelectrochemical energy conversion/storage strategies. The final chapter of this thesis details unresolved work to help future students using surface chemistry to improve (photo)electrochemical systems. First, a project that details the synthesis and attempted absorption of triarylmethane dyes for GaP(111a) is discussed. Second, many additional parameters were explored for adsorbed viologens for quantitative modeling to understand the experimental system reported in Chapter 4, select parameters include changing viologen distance from the electrode, dopant density, electrolyte concentration, and solvent. Third, redox active molecules, including viologens, were adsorbed in a Nafion film to generate electrochemical data for quantitative modeling. n-Si(111) electrodes were etched or passivated with -CH3 groups to prevent surface oxidation. Finally, the last section details preliminary results of bipyridinium dimers that were used for electrochemical carbon dioxide capture in aqueous solutions. These works serve to understand heterogeneous charge transfer processes.