Organic Semiconductors For Photoelectrochemical Applications

The capture and conversion of solar energy into other useful forms has been the topic of discussion for the past few decades, accentuated by the need to combat climate change and face the challenge of non-renewability of conventional sources of fuel for our rising energy demands. Photoelectrochemical (PEC) fuel generation from sources like water is a promising method to achieve a carbon free path towards cleaner energy. As this method combines multiple physico-chemical processes: light harvesting, charge generation and fuel generation in one component, it is easier and inexpensive to make and use. Usable solar energy is mostly concentrated in the near visible to near infrared region of the spectrum, and due to bandgap and other material considerations, a single photoabsorbing semiconductor system has made way for dual absorbing PEC semiconductors, which work in tandem to carry out the reduction (photocathode) and oxidation (photoanode) halves of a redox splitting reaction. While the use of inorganic semiconductors in photoelectrodes is well established, to overcome their inherent drawbacks organic semiconductors (OSCs) are being touted as possible replacements. Indeed, the field of organic photovoltaics has demonstrated how OSCs are quite superior in many aspects, especially due to our ability to tune their properties based on their chemical structures. Therefore, many researchers around the globe are focusing towards OSCs for PEC applications. While photocathodic OSCs have indeed been successfully demonstrated to have competing performance and stability, OSCs in photoanodes is at its nascent stage. This arises due to the inherent difficulties in photooxidation itself: the use of harsh electrolytic conditions, high oxidative potentials placing strain on the components of photoanode etc. In this thesis, a complete overview of how an OSC based photoanode can be designed: right from the molecular level to the final device fabrication and optimization is elaborated. In the first chapter, an introduction to this area of research, its motivation and context is provided, with details from previous peer-reviewed research along with a bit of theoretical insight. The second chapter briefly discusses the materials, methods and techniques involved in this thesis. The third chapter is based on the Quaterrylene di-imide (QDI) molecule, its chosen reason, synthesis, and practical difficulties in implementing a workable photoanode using it. After realizing that neat OSCs themselves probably would not make good photoanodes, a transition to using bulk heterojunction (BHJ) type photoanodes is looked at. The fourth chapter discusses the donors: their synthesis, analytical and initial PEC characterization, while the fifth chapter deals with acceptors for similar aspects. Moving to BHJ, the sixth chapter elaborates optimizations and characterization of thin film BHJs and delves deep into techniques to obtain high performance and stability. The seventh chapter involves possible applications for these optimized photoanodes, and how these can be used for unassisted solar fuel generation.