On the homocoupling of trialkylstannyl monomers in the synthesis of diketopyrrolopyrrole polymers and its effect on the performance of polymer-fullerene photovoltaic cells
Homocoupling of monomers in a palladium-catalyzed copolymerization of donor-acceptor polymers affects the perfect alternating structure and may deteriorate the performance of such materials in solar cells. Here we investigate the effect of homocoupling bis(trialkylstannyl)-thiophene and -bithiophene monomers in two low band gap poly(diketopyrrolopyrrole-alt-oligothiophene) polymers by deliberately introducing extended oligothiophene defects in a controlled fashion. We find that extension of the oligothiophene by one or two thiophenes and creating defects up to at least 10% does not significantly affect the opto-electronic properties of the polymers or their photovoltaic performance as electron donor in solar cells in combination with [6,6]-phenyl C-71 butytic acid methyl ester as acceptor. By using model reactions, we further demonstrate that for the optimized synthetic protocol and palladium-catalyst system the naturally occurring defect concentration in the polymers is expected to be less than 0.5%. ; We thank Dr Koen Hendriks for valuable discussions. G. H. L. Heintges acknowledges the Agency for Innovation by Science and Technology in Flanders (IWT). This project has further received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 339031 and from the Ministry of Education, Culture and Science (Gravity program 024.001.035).