Effect of Alkyl Chain Lengths of Highly Crystalline Nonfullerene Acceptors on Open-Circuit Voltage of All-Small-Molecule Organic Solar Cells

Three dicyanostyrylbenzene (DCS)-based small-molecule nonfullerene acceptors (SM-NFAs), NIDCS-MO, NIDCS-PrO, and NIDCS-PeO, bearing alkoxy chains with different lengths from methoxy to pentyloxy are synthesized to investigate the alkyl chain length effect on the performance of all-small-molecule organic solar cells (SM-OSCs). The optical properties and frontier molecular orbital (FMO) energy levels of the three SM-NFAs are virtually identical in both solution and neat films. In contrast, their photovoltaic performance in SM-OSCs with a common small-molecule donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]- dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]5-yl)benzo[c]- [1,2,5]thiadiazole) (DTS-F) is significantly varied. In particular, the open-circuit voltage (VOC) is gradually increased from 0.85 to 1.00 V with the increase in the alkyl chain length. By cyclic voltammetry (CV) and impedance spectroscopy, it is revealed that the energetic landscape of the blend films is negligibly changed depending on the alkyl chain length. However, it is found that voltage loss from the nonradiative recombination in SM-OSCs with longer alkyl chain acceptors is considerably reduced due to denser blend film formation, which is evidenced by CV studies with various conditions and the morphology analysis of the blend films using X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM).