Morphology-performance Correlation in Near-Infrared Organic Solar Cells with Efficiency Exceeding 9%

Organic solar cells (OSCs) are attractive due to their tunable properties and rising efficiency, >18%. Recently synthesized electron-accepting molecules for OSCs show efficient absorption near IR and PCE~10% when paired with a polymer donor. Here, we utilized X-ray techniques to probe their morphology. Our x-ray, TEM, SEM and AFM results show that small-molecule domains undergo drastic change in packing and phase separating beyond the optimal processing condition, i.e. solvent additive as a plasticizer. We found that the additive increases domain size and packing, suggesting reduced charge recombination and enhanced charge transport, i.e. favorable for performance. However, beyond 1% additive, the performance significantly decreases. We attribute that to large domains reducing exciton-dissociation efficiency. We found that the elemental substitution of fluorine for chlorine in the small molecules have similar trends in performance and morphology. Such a thorough characterization provides sufficient knowledge to correlate morphology with performance, also gives insights into the morphological effects of chemical and processing modifications of organic materials for optoelectronic applications.