Conjugated organic polymers as photocathode materials in organic photoelectrochemical cells

The work presented in this thesis aims at studying the photoelectrochemical properties of conjugated organic polymers, with a particular interest in their application as photocathode materials to perform the hydrogen evolution reaction. Whereas inorganic semiconductors have been the primary focus in the area of solar water splitting, organic semiconductors have only recently emerged as a new class of photoelectrode material. Furthermore, the research emphasis on organic semiconductor systems used for water splitting applications has largely been focused on improving performance through device engineering. Reports concerning the interfacial thermodynamic and electronic processes are less frequent. Given the difference in photophysical properties compared to inorganic semiconductors and the change in chemical environment compared to solid-state organic electronics, the characterization and elucidation of interfacial processes at the organic semiconductor-electrolyte remains an area of need. The ability of uncatalyzed P3HT photocathodes to perform the hydrogen evolution reaction in acidic aqueous media is first investigated. Whereas previous reports in the literature have attributed the aqueous photoactivity of P3HT to the hydrogen evolution reaction, this study reveals that residual dissolved oxygen is largely responsible for the observed photocurrents and that, despite favorable thermodynamics, hydrogen is not evolved at P3HT photocathodes in the absence of a catalyst. Following the initial investigation of P3HT photocathode performance, strategies to improve photocurrent densities at organic photocathodes are explored. In these studies, nanostructured photocathodes are prepared from P3HT:PCBM nanoparticles. The optoelectronic and morphological properties, as well as the photoelectrochemical performance of the nanostructured photocathodes are compared to planar P3HT:PCBM photocathodes. To achieve hydrogen production, platinum nanoparticle catalysts are deposited onto the organic layer photoelectrochemically, where the increased surface area of the nanostructured electrodes leads to enhanced catalyst loadings and increased photocurrent densities compared to the planar photocathodes. Finally, the influence of PCBM on interfacial energy alignment of the redox couple at the semiconductor-electrolyte interface is investigated in a non-aqueous electrolyte using a benzoquinone redox couple. Photoelectrochemical measurements show that the presence of PCBM at the semiconductor-electrolyte interface leads to the formation of an interfacial dipole layer and decreases the built-in potential developed at the semiconductor-electrolyte interface.