This study reports for the first time the elaboration of core-shell organic nanoparticles through successive solvent displacements. The concept has been applied to generate donor-acceptor nanoparticles with the commonly used organic semiconductors P3HT and PCBM. The strategy is based on the sequential nanoprecipitation of P3HT and PCBM. Starting from a P3HT:PCBM solution in THF, the first solvent displacement with DMSO triggers the formation of the P3HT core while the second displacement with water generates the PCBM shell. The core-shell morphology is evidenced by both DLS and TEM. Efficient quenching of the P3HT photoluminescence is observed after the second solvent displacement procedure, confirming the formation of a well-defined PCBM shell around the P3HT core. Studying the emission of such core-shell nanoparticles leads to an estimation of the diffusion length of photogenerated excitons in P3HT of around ~14 nm.