Redox mediators are often added to experimental systems to improve the rate of electron export from the biological material and/or electron transfer to the anode, especially when whole cells are employed as the light harvesting material.  Quinones, phenazines, and viologens have all been successfully employed to increase current output from photosynthetic organisms in biological photovoltaic devices.   Adding artificial mediators is considered an unsustainable practice in scaled-up applications, so most modern research is on mediator-free systems.