A new method for the preparation of active layers of polymeric solar cells without the need for thermal post-treatment to obtain optimal performance is presented. Poly(3-hexylthiophene) (P3HT) nanofibers are obtained in highly concentrated solutions, which enables the fabrication of nanostructured films on various substrates. Here, the preparation of these fibers along with their characterization in solution and in the solid state is detailed. By mixing these nanofibers with a molecular acceptor such as [6,6]-phenyl C61-butyric acid methyl ester (PCBM) in solution, it is possible to obtain in a simple process a highly efficient active layer for organic solar cells with a demonstrated power conversion efficiency (PCE) of up to 3.6 %. The compatibility of the room-temperature process developed herein with commonly used plastic substrates may lead to applications such as the development of large-area flexible solar cells.