Organic Photovoltaic Performance Resiliency: Role of Molecular Weight in a PM7 Derivative

Batch-to-batchvariations remain a challenge for organic photovoltaics(OPVs) that must be solved for widespread commercialization. Thiswork addresses the role of the molecular weight (MW), introducinga new donor polymer PM7-D5 with an extended thiophene bridge comparedto that of PM6 or PM7. Devices comprising PM7-D5 and the nonfullereneacceptor L8-BO are relatively insensitive to a significant molecularweight 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 systematicallyaddress morphology, charge generation, recombination, and extraction,combining optical simulations and experimental techniques. The comprehensiveanalysis emphasizes the complexity of the photoelectric processesand confirms the high robustness to MW variations of this blend system.Lastly, we review reported blends and suggest systematic researchof the structural parameters that lead to an increased robustnessto MW changes.