Up to date, the most efficient organic photovoltaic cells (OPVs) are based on blends of an electron-donor conjugated polymer and a soluble fullerene, as the electron-acceptor, assembled as interpenetrating networks where donor and acceptor phases are separated at the nanoscale. However, achieving such efficient morphologies for the active layer is often difficult, due to the wide variety of processing conditions affecting the blend morphology. An alternative approach consists on firstly developing nanostructured layers of the donor polymer and depositing the acceptor on top, in order to facilitate exciton dissociation at the donor-acceptor interface and to provide continuous transfer pathways for the charge carriers towards the electrodes, thereby contributing to photocurrent improvement.
Here, we report on OPV architectures consisting on a columnar-grain layer of a cross-linked donor polymer, an oxetane-functionalized polyfluorene-alt-bithiophene (F8T2Ox1) coated with an electron accepting layer of 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM). The active layers preparation involved only spin coating [1]. We show that a significant improvement in cells performance upon decreasing the column diameter of the donor phase occurs, which is attributed to an improvement of the exciton dissociation. Furthermore, the high robustness of the cross-linked polymer phase provides higher thermal stability for the cell performance when compared to reference devices made with blends of non-cross-linkable analogue polymers.

References
[1] A. Charas, Q. Ferreira, J. Farinhas, S. M. Fonseca, H. D. Burrows, M. Matos, L. Alcácer, J. Morgado, Macromolecules, 2010, 42(20), 7903.