Methods and compositions modulate the activity of electrically excitable cells. Photovoltaic compounds which, upon exposure to light energy, increase or decrease the electrical activity of cells. These supplement and/or replace of vision based on the conversion of light energy to electrical energy within certain cells of the visual system. A “patch” or bridge to circumvent one or more defective, damaged, or diseased cells in the visual system. Additionally, in several embodiments, subjects with normal vision can benefit from the methods, compositions, systems, and/or devices disclosed herein as normal visual acuity can be heightened. The exposure induces an energy (e.g., a receipt of light energy, conversion to electrical energy, and passage of that electrical energy) from the photovoltaic compound to the cell, thereby altering the transmembrane potential of the cell and/or the opening of one or more ion channels, thereby modulating the activity of the electrically excitable cell.