Quantifying the role of poly(3-hexylthiophene) and fullerene crystallinity on performance of polymer solar cells

Since the device performance depends on the structure of the organic semiconductor blend that makes up the active layer, understanding structure-function relationships is crucial for the development of next-generation polymer solar cells. Through a combination of grazing incidence X-ray diffraction and X-ray rocking scans, we have quantified the crystallinity of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) in the active layer of bulk heterojunction polymer solar cells. Contrary to common belief that the device J_SC is only dictated by P3HT crystallinity, our study - varied over a wide range of processing conditions - shows that the device J_JC is limited, to first order, by the PCBM crystallinity. However, when the PCBM crystallinity is greater than 50% of the maximum crystallinity achievable in our samples through thermal annealing, the extent of P3HT out-of plane p-stacking is directly correlated with the device J_SC. By quantifying the relationship between the crystallinity of both organic semiconductors of the active layer and device performance, we have developed a description of the complex interplay between structure and performance in polymer solar cells.