Abstract
Batch-to-batch variations remain a challenge for organic photovoltaics (OPVs) that must be solved for widespread commercialization. This work addresses the role of the molecular weight (MW), introducing a new donor polymer PM7-D5 with an extended thiophene bridge compared to that of PM6 or PM7. Devices comprising PM7-D5 and the nonfullerene acceptor L8-BO are relatively insensitive to a significant molecular weight variation and result in OPVs with photoconversion efficiencies (PCEs) of 11.4% and 12.4%, when using polymers with 26 and 125 kDa. We elucidate the origins of the small performance change and systematically address morphology, charge generation, recombination, and extraction, combining optical simulations and experimental techniques. The comprehensive analysis emphasizes the complexity of the photoelectric processes and confirms the high robustness to MW variations of this blend system. Lastly, we review reported blends and suggest systematic research of the structural parameters that lead to an increased robustness to MW changes.