Combined systems of semiconducting polymers and aqueous electrolytes are emerging as the new frontier of organic electronics, with many promising applications in biology, neuroscience and medicine. A detailed characterization of polymer/water interfaces is thus urgently needed. In particular, the combined effect of contact with electrolytes and visible illumination should be taken into account, since many applications rely on exposure to light, or are meant to work in ambient room light conditions. In this work, we first extensively characterize the chemical-physical processes occurring in thin films of poly(3-hexylthiophene) exposed to water saline solutions and visible light. Through combination of different spectroscopic techniques, we demonstrate that prolonged contact with saline solutions does not add further degree to photo-activated doping processes of the polymer; instead, it turns out that the reduced number of oxygen molecules present in water, compared to open air, acts as a limiting factor, thus fully validating the use of semiconducting polymers in contact with electrolytes. In addition, we demonstrate that the recently demonstrated technique of cell stimulation by polymer photo-excitation (CSP) represent a versatile platform for full-optical control of cell excitation/inhibition. We report examples of functional interfaces between several combinations of conjugated polymers and different cell cultures (HeK cells, astrocytes, neuronal networks). A detailed model of the mechanisms occurring at the polymer/electrolyte interface and leading to living cell photoexcitation, based on electrical and optical measurements, will be presented and critically discussed. The most obvious application of the polymer-based cell photostimulation protocol is in the artificial visual prosthetics: recent results of in-vivo implantation in rats of organic-based artificial prosthesis will be finally presented and critically evaluated.