Thermal behaviour of dicarboxylic ester bithiophene polymers exhibiting a high open-circuit voltage. Issue 14 (10th November 2017) - Heuvel, Ruurd
Abstract : Polythiophene derivatives with ester side chains enable high open-circuit voltages in polymer–fullerene solar cells. The side chains affect the solubility, thermal properties, tendency to aggregate, and photovoltaic performance by modulating the morphology of the blends. Abstract : Nine different polythiophene derivatives based on dialkyl-(2, 2′-bithiophene-5, 5′-diyl)-4, 4′-dicarboxylate (DCB) alternating with thiophene (T), bithiophene (2T) or thienothiophene (TT) as co-monomer have been synthesized to study the effect of the polymer backbone and side chain length on the thermal properties, the tendency to aggregate, and the photovoltaic performance. Polymers incorporating DCB and 2T show increased crystallinity and a large effect of the side chain length on the morphology of the photoactive layer blends. Thermal annealing increases the crystallinity of the polymers and enhances the long-wavelength light absorption. The concomitant increase in polymer fibre width, however, deteriorates the photovoltaic performance. The best devices were made using the PDCB-2T polymer with 2-butyloctyl side chains providing a power conversion efficiency of 5.18%. The PDCB-T polymer with 2-ethylhexyl substituents shows a comparable efficiency (5.08%), but with a significantly higher open-circuit voltage due to deeper frontier orbitals levels.