Regio-regular poly(3-hexylthiophene) (rr-P3HT) has been the most investigated conjugated polymer, as electron-donor, in bulk heterojunction Organic Photovoltaic Cells (OPVs). However, although the resulting cells typically exhibit relatively high efficiency values (3-5 %) when soluble fullerenes are used as electron-acceptors, their performance is strongly dependent on the processing conditions (solvents and/or annealing processes, etc.) that modulate the morphology of the active layer. As a result, the best performing blend morphology is often difficult to reproduce. In addition, it may even be unstable at high temperatures such as those attained when OPVs are used in outdoor environment. In this regard, the use of cross-linkable polymers could circumvent this drawback, as the blend morphology may be “frozen” when the polymer is cross-linked. Furthermore, cross-linkable conjugated polymers have also been employed to prepare solvent resistant nanostructured polymer films through solution methods [1], which can be used to fabricate OPVs with nanostructured donor/acceptor interfaces.
In this communication we present a series of new regio-regular poly(3-hexylthiophene) derivatives possessing the cross-linking ability, introduced through oxetane moieties anchored as terminal groups into 3-alkylsubstituted monomers. Upon addition of a photo-catalyst followed by UV-irradiation plus temperature annealing, insoluble polymer films can be generated whereas the photophysical properties of the polymers are not significantly affected. OPVs are fabricated with these new polythiophenes and their performance will be discussed.

References:
[1] – Charas A. et al, “Insoluble Patterns of Cross-linkable Conjugated Polymers from Blends Demixing in Spin Cast Films”, Macromolecules 42 (2009) 20, 7903-7912

Acknowledgements:
This work was funded by Fundação para a Ciência e Tecnologia under contracts SFRH/BD/62130/2009 and PTDC/CTM-NAN/111263/2009.