Photocurrent generation at organic heterojunction-electrolyte interface for optoelectronic biosensor implementation

This thesis project investigates an organic heterojunction made by common and largely available inert dyes which can operate in cellular fluid without being toxic for their environment. The heterojunction/electrolyte interface converts light into an ionic current pulse strong enough to depolarize adjacent cells. Indeed, such a photoelectrode is envisioned to act as a building block in an artificial retina. In order to achieve this goal, detailed characterization and understanding of the charge generation mechanism is necessary. To this end I realized two experimental configurations that characterize a photoelectrode made of a p/n junction cast over ITO. In the first configuration, the photoelectrode was studied in a three-electrode electrochemical cell under pulsed LED light stimulation. Using different analytical techniques, I characterized the photoelectrodes impedance and photocurrent transients as a function of a well-defined externally applied potential. In the second configuration I extended the experiment to assess the impact of a more realistic floating photoelectrode operation. In this situation the photoelectrode circuit is closed by a second, passive electrode interface that converts the ionic current back into an electronic one. As a consequence, the potential is no longer defined and I demonstrate that the properties of the passive electrode can be optimized to maximize the capacitive currents and minimize the faradic ones. The following thesis is organized in three main chapters: first an introduction, where I provide an essential background; then methods and materials, where are described the investigated system, the models used for its interpretation and the methods used to take the measurements; then result and discussion, where the measurements made in the non-floating and floating arrangements and their explicative model are presented and discussed, and simulation are performed and compared to the experimental results.