Synthesis of Crown Ether End-Capped PM6 Copolymers with Tunable Molecular Weight for Organic Solar Cells

Poly-[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PM6) is one of the star molecules in organic solar cells (OSCs). However, PM6 has been suffering from molecular weight control and unreacted terminal groups. Thus, synthesizing high-quality polymers without batch-to-batch variations has been challenging for many years. Herein, a one-step protocol of both end-capping and polymerization at the same time was developed to prepare end-capped PM6 with molecular weight controlled by changing the monomer ratio. Specifically, non-end-capped PM67 and a series of PM6En with different molecular weights were synthesized and characterized. Our research findings indicated that (1) these copolymers' number-average molecular weights (Mns) and weight-average molecular weights (Mws) were measured to be almost linearly increased. Although textbooks have mentioned the nonequivalent ratio monomer strategy to regulate the molecular weight of polymers, the end-capping yield has not been reported in the research of OSCs due to a lack of efficient experiments and instruments to measure the low percentage of end groups in the polymer. (2) Since the selected end-caping reagent 4′-bromobenzo-18-crown-6 (EBr) can be easily distinguished from the PM6 main backbone, we first measured the end-capping yield by both nuclear magnetic resonance (NMR) and matrix-assisted laser desorption/ionization time-of-flight mass spectra (MALDI-TOF-MS). We applied this approach for the first time to synthesize polymers with reasonable end-capping yields (40–97%) for OSCs. (3) Surprisingly, 18-crown-6 can reduce end-group defects, lower the polydispersity index (PDI) of the polymer, and achieve suitable morphology and mobility. As a result, the end-capped PM6E7-based device exhibited the best stability and power conversion efficiency (PCE) compared to those of the other end-capped or non-end-capped polymers. Both the end groups and molecular weight significantly affect the performance of the OSCs. PM6 and other copolymers could be finely tuned by subtly changing the molecular weights or end-cap groups with functional building blocks. Our research will stimulate the development of OSCs and other applications of copolymers.