The Influence of Surface Chemistry on the Photoelectrochemical Properties of FeS(2) Photoanodes

Abstract

The recurring theme of this dissertation is the correlation between FeS 2 surface chemistry and key electrical and electronic properties of FeS 2. Efforts have been made to identify and characterize the FeS 2 surface, investigate the photoelectrochemistry of FeS 2 photoanodes under anhydrous and anoxic conditions, and investigate the influence of deliberate surface chemistry on FeS 2 photoelectrochemistry. Infrared reflection-absorption spectroscopy (IRRAS) was used to investigate a thin adsorbate layer on pyrite. The results showed that the combination of angledependent studies and computational efforts are a powerful tool for characterizing the pyrite surface. The photoelectrochemistry of FeS 2 photoanodes was investigated in an I-/I 3 acetonitrile electrolyte. The results revealed that the non-aqueous system was suitable for strictly anhydrous and anoxic photoelectrochemical studies. A model was proposed to explain the observed influence of concentration of dissolved I 2 on the photovoltage. A central component of the proposed model was that shunting was assumed to take place at physically distinct regions of the electrode and that mass-transport to and from these regions could be treated separately from mass-transport to the regions responsible for the rectifying behavior of the FeS 2 /liquid junction. The implication of the agreement between experimental and calculated J-E curves is that macroscopic photoelectrochemical investigations may underestimate the quality of FeS 2 photoanodes due to the presence of defects. ii The influence of surface treatments on FeS 2 photoelectrochemistry was further studied using non-coordinating redox species. A statistically significant increase of photovoltage was observed after treating FeS 2 surfaces with KCN. X-ray photoelectron spectroscopy was used to study chemical bond formation between the electron donating ligands and iron(II) centers on the pyrite surface. The results were discussed in terms of charge recombination models and surface coordination chemistry. Unfinished work is also presented. Cathodic polarization in acidic media is a prerequisite for any detectable photoresponse. The exact function of the electrochemical activations was further investigated by electropolishing pyrite electrode under different experimental conditions including etchant identity and applied bias. The results suggested that the electrochemical treatment removes the damaged surface layer caused by mechanical polishing, and might also stabilize the surface states. Further experiments can be focus on anhydrous etching of pyrite photoanode. The research presented in this dissertation guides future studies of thin film FeS 2 photovoltaics. iii Dedication This dissertation is dedicated to my family. iv Acknowledgments I wish to express my sincere gratitude to my advisor Prof. Erik Johansson. His broad knowledge and logical way of thinking have been of great value for me. The present dissertation would not have been possible without his encouragement and supervision. I would like to thank Pro...