Abstract: The invention provides a bio-photovoltaic device, in which a photoelectric center, exemplified by a biological photosynthetic reaction center (RC), is dispersed and mobile in a medium, such as an aqueous solution. The charges generated by the illuminated RC are transferred to electrodes via one or more mediators. In selected embodiments, the difference between the reaction rates of two types of mediator at the electrode surfaces, in conjunction with other charge transfer reaction equilibria, determines the direction of the photocurrent in the device. In an exemplified embodiment, the magnitude of the photocurrent is proportional to the incident light intensity, and the current increases nonlinearly with an increase in the RC concentration in the medium.