Over the last decade, great advances have been made in the field of organic electronics. A first generation of visible organic light emitting devices has been commercialized (MP3 players, TV displays, ...). Solar cells (Organic PhotoVoltaic ; OPV) with high power conversion efficiencies have been reported. Printed organic thin film transistor circuits containing hundreds of active gates have been operated at frequencies exceeding 10 MHz. For solar-cells, semiconducting polymers (and more generally organic materials) provide an alternative route to usual Silicon technology. Organic PV represents a low-cost, lightweight, printable solution, suitable for large areas that would be particularly applicable in the mid-term to roof-top panels, but, in the short-term, to novel applications, such as smart fabrics, nomad devices, and textiles. Most currently-available organic solar cells consist of a mixture of a conjugated polymer, such as poly(3-hexylthiophene), P3HT, as a donor, and C60 fullerene derivatives as acceptors [1]. The best published power-conversion efficiency (world-record) is about 6% [2], but with small cells. Efficiency will certainly be improved, particularly by using large-scale devices. In this paper, the nano-structure of an organic bulk heterojunction solar cells based on a polythiophene/fullerene blend was investigated. The device was coated with a continuous roll-to-roll impression technique. The nano-structure was observed by AFM microscopy, and different nano-morphologies are shown versus device performances. We show that annealing the blend improved nano-segregation, leading to better efficiencies. [1] M. Urien, L. Bailly, L. Vignau, E. Cloutet, A. de Ciendas, G. Wantz, H. Cramail, L. Hirsch and J.P. Parneix, Polym. Int., 57, 764 (2008) [2] Y.K. Jin, K. Lee, N.E. Coates, D. Moses, T.Q. Nguyen, M. Dante, A.J. Heeger, Science, 317, 5835, 222 (2007) Work supported by Conseil Régional d'Aquitaine, ANRT and CNRS