Quantifying unit ratios in semiconducting copolymers and effect of structural deviations on photovoltaic performance

Large batch-to-batch differences in the performance of organic photovoltaic (OPV) devices are a serious problem, often attributed to structural inconsistencies in semiconducting copolymers. This work uses X-ray photoelectron spectroscopy (XPS) to quantify the monomer unit ratio in widely used electron donor copolymers for OPVs, achieving high accuracy through optimized measurement conditions and calibration. The results show that the monomer unit ratios in different polymer batches deviate from the ideal 1:1 ratio and vary depending on the supplier. Additionally, we synthesize copolymers with intentionally altered unit ratios, which are accurately quantified by XPS. For both commercial and synthesized polymers, larger deviations from a 1:1 ratio are associated with reduced fill factor and open-circuit voltage in bulk-heterojunction OPVs. Quantitative XPS analysis of copolymer structures provides valuable insight into the relationship between variations in OPV performance and the structural integrity of the polymer across batches. Minor structural inconsistencies in semiconducting copolymers lead to serious performance differences in devices. Here, X-ray photoelectron spectroscopy is used to quantify the monomer unit ratio in electron donor copolymers, and their structural deviations are linked to characteristics in organic photovoltaic devices.