Tuning the Morphology of Solution-Sheared P3HT:PCBM Films

Organic bulk heterojunction (BHJ) solar cells are a promising alternative for future clean-energy applications. However, to become attractive for consumer applications, such as wearable, flexible, or semitransparent power-generating electronics, they need to be manufactured by high-throughput, low-cost, large-area-capable printing techniques. However, most research reported on BHJ solar cells is conducted using spin coating, a single batch fabrication method, thus limiting the reported results to the research lab. In this work, we investigate the morphology of solution-sheared films for BHJ solar cell applications, using the widely studied model blend P3HT:PCBM. Solution shearing is a coating technique that is upscalable to industrial manufacturing processes and has demonstrated to yield record performance organic field-effect transistors. Using grazing incident small-angle X-ray scattering, grazing incident wide-angle X-ray scattering, and UV-vis spectroscopy, we investigate the influence of solvent, film drying time, and substrate temperature on P3HT aggregation, conjugation length, crystallite orientation, and PCBM domain size. One important finding of this study is that, in contrast to spin-coated films, the P3HT molecular orientation can be controlled by the substrate chemistry, with PEDOT: PSS substrates yielding face-on orientation at the substrate-film interface, an orientation highly favorable for organic solar cells.