The monolithic integration of all key photonic components (11-16) of an integrated-optical microsystem (1) based on organic semiconductors is disclosed. Examples of such components (11-16) are light sources (11), photodetectors (12), photovoltaic power generators (12), field-effect transistors (13, 14), resistors, capacitors (15), or waveguiding structures (11, 12). The components (11-16) are arranged on a common substrate (20), are compatible with each other, can be manufactured simultaneously and can be operated simultaneously. At least one of the components (11-14) comprises a layer (23) of organic semiconductor material. Each component (11-16) comprises a plurality of layers (21-26), at least one of which (21) has identical physical and chemical characteristics in at least two components (11-16). The large number and diversity of photonic devices that can be monolithically integrated allows a higher degree of integration, of functionality and of system complexity than was practical with the state of the art.