Organic electronic devices are promising alternatives to conventional inorganic technologies. Ink-based processes can easily be used to obtain active films, e.g. by spray-drying or roll-to-roll techniques. But their industrial development might be limited by the use of chlorinated solvents. An alternative consists in using solvent-dispersed NPs of organic semi-conductor. Previous work have demonstrated that it was possible to obtain organic semi-conducting dispersions of P3HT and/or [6,6]-Phenyl C61-Butyric acid Methyl ester (PCBM) NPs using an antisolvent approach. For organic photovoltaic devices, the nano-morphology has to be controlled in order to have donor and acceptor domains size in the range of the exciton diffusion length (10 nm). However, conventional synthesis approaches are based on slow solvent / antisolvent mixing process, whereas strong mixing is required to access small sizes and narrow size distributions. Admittedly, micromixing has a significant effect over particles size and size distribution since homogeneous concentration distribution and high degree of supersaturation can only be reached by intense micromixing. This can be achieved by performing antisolvent processes at microscale using microfluidic devices and/or by using supercritical fluids as antisolvents. By coupling both, we will present in here the first demonstration of a supercritical antisolvent process performed within a microsystem, so called µSAS. The µSAS process was applied to the processing of P3HT NPs as small as 36 ± 8 nm. The designed set-up includes a spraying nozzle at the outlet of the back pressure regulator allowing depositing P3HT NPs films onto a substrate. The µSAS process benefits from both the advantages of supercritical antisolvent approaches and microscale processes (fast mixing, high supersaturation, etc.) and can be successfully used to synthesize semi-conducting polymeric NPs, which size are compatible with the diffusion-length of excitons in active layer of photovoltaic devices.