Viable Mixing Protocol Based on Formulated Equations for Achieving Desired Molecular Weight and Maximal Charge Separation of Photovoltaic Polymer

Abstract

A major difficulty in the polymer solar cell (PSC) community is discovering a methodology capable of accessing polymeric photovoltaic materials with controllable/predictable molecular weights. Effective mathematical equations that enable the reproduction of polymer batches with precisely controlled molecular weight, by mixing as-synthesized polymer batches with the ones that have different molecular weights, are formulated in this study. The properties of both the as-synthesized and mixed-different-molecular weight PM6 polymer series are systematically investigated to determine the effect of molecular weight on the performance of related PSCs. The power conversion efficiencies (PCEs) improve monotonically with an increase in weight average molecular weight (M-w) up to 120 kDa, but a further increase in M-w results in a sharp decline in PCE. This trend in PCEs is correlated with the charge-separation efficacy as a function of the balance between the counteracting domain size and packing in the blends. Further, decreasing the polydispersity index in the optimal-molecular weight PM6 sample produces a small but noticeable positive effect on device performance, yielding the best PCE of 16.5%. This viable mixing approach is useful in producing desirable molecular weight polymers, facilitating the development of high-quality, high-reproducibility photovoltaic polymers.