Crystallization and Optical Compensation by Fluorinated Rod Liquid Crystals for Ternary Organic Solar Cells

Cyanobiphenyl type liquid crystal molecules used as additives for polymer solar cells (PSCs) have been proved as a promising strategy to increase donor crystallinity, short circuit current density (Jsc) and result in a higher power conversion efficiency (PCE) of PSCs. However, they would sacrifice light absorption in long wavelength region due to their large bandgap and weak absorbance. To overcome this shortage, a fluorinate donor–acceptor–donor rodlike liquid crystal (RLC), 5,6-difluoro-4,7-bis(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (DFBT-TT6), is designed and synthesized as additive for bulk-heterojunction organic solar cells. DFBT-TT6 shows a broad absorption from 300 to 650 nm. Quite different from the commonly used liquid crystal additives, DFBT-TT6 can effectively compensate the energy loss of P3HT in the short wavelength without sacrificing the absorbance in long wavelength region. Remarkably, grazing incident X-ray diffraction (GIXRD), electron microscopy, and X-ray photoelectron spectroscopy (XPS) prove that, driven by the cooperative effect of the RLC orientation and its surface segregation, a favorable morphology with improved crystallinity of poly(3-hexyl)thiophene (P3HT) in the active layer has been developed. Therefore, incorporation of DFBT-TT6 into P3HT: [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend achieved an improved PCE from 2.34% to 3.91% with an increased Jsc, fill factor (FF), and charge mobility.