Regio-regular poly(3-hexylthiophene) (rr-P3HT) has been the most investigated conjugated polymer, as electron-donor, in Organic Photovoltaic Cells (OPVs). Upon combination with electron-accepting soluble fullerenes, such as PCBM, OPVs with power conversion efficiencies reaching up to 6%, depending on the processing conditions, can be fabricated. The use of soluble materials allows the improvement of OPVs performance by modification of the active layer morphology upon adequate selection of the solvent(s) or by thermal annealing [1]. Morphology may also change during operation lifetime, as OPVs are exposed to relatively high temperatures. To circumvent this instability, the use of cross-linkable materials has been proposed in order to “freeze” a specific morphology. We have also been investigating cross-linkable conjugated polymers towards that purpose. In addition, we found that nanostructured conjugated polymer films, formed by phase separation from an assisting polymer [2], can also be turned into insoluble networks, allowing further deposition of other components from solution, namely the deposition of PCBM.
In this communication we report on the synthesis and characterization of a series of new regio-regular poly(3-hexylthiophene) copolymers with cross-linking ability due to the functionalization of the side groups with oxetane moieties. Upon addition of a catalytic amount of a photo-acid generator, insoluble films of conjugated polymer networks can be obtained by UV-irradiation and temperature annealing. The composition of the copolymers was varied in terms of the content of oxetane-carrying monomers, in order to determine the minimum concentration required to obtain completely insoluble films while minimizing any detrimental effects on the copolymer photophysical properties. Additionally, aiming to obtain copolymers with broader absorption spectra, to better match solar emission, and with improved charge mobility, various co-monomers were also combined. Some of these copolymers were used to fabricate OPVs and their performance will be related to copolymers chemical structure.

References
[1] - R. Alex Marsh, Justin M. Hodgkiss, Sebastian Albert-Seifried, and Richard H. Friend, Nano Letters, 10 (2010) 923
[2] – A. Charas, Q. Ferreira, J. Farinhas, M. Matos, S. M. Fonseca, H. Burrows, L. Alcácer, J. Morgado, Macromolecules 42 (2009) 7903