Structurally Pure and Reproducible Polymer Materials for High-Performance Organic Solar Cells – Insights in the Chemical Structures of PM6 and D18

While (push-pull) semiconducting polymers are now displaying better device performances than ever before, their commercial uptake remains limited, in part due to challenges in terms of reproducibility. Next to consistent device fabrication, continuous material quality is of high importance to obtain optimal and reliable performance metrics. Often the assumption is made that the “drawn” chemical structure is representative of the active material used in applications such as polymer-based solar cells. It has, however, become clear that this is not always the case. In this work, we assess the structures of two top-performing organic solar cell donor polymers, PM6 and D18, both for commercial and synthesized samples.^[1] Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF MS) we analyze the chemical structures in detail. We also introduce an approach to limit structural defects and study the impact on device performance. Next to that, we translate the synthesis of these polymer materials to a droplet flow protocol, not only to limit batch variability but also as a means to achieve reproducible molar masses. In general, flow chemistry offers benefits such as easier scalability and enhanced safety at larger scale. Droplet-flow synthesis in particular has demonstrated excellent reproducibility, both in terms of molar mass distribution and optoelectronic properties of push-pull conjugated polymers. This allows a detailed investigation of the impact of chemical structure and molar mass on blend morphology and solar cell performance.