Balancing intermolecular interactions by variation of pendent alkyl chains for high performance organic photovoltaics

Understanding the role of appended alkyl side chains is a key issue to enhance photovoltaic performance because the side chains control molecular solubility, film crystallinity, and morphology. For this purpose, we synthesized three new planar aromatic backbone molecules (Eh-Hex, Eh-Eh and Oct-Eh), where the appended alkyl side chains on the aromatic core unit were either 2-ethylhexyl (Eh) or n-octyl (Oct) and those on the side ends were either Eh or hexyl (Hex). Characterization of the molecular films revealed that all of the molecules were crystalline with mainly edge-on orientation, while the Eh-Hex and Eh-Eh molecules having Eh groups on the core units manifested higher crystallinity than the molecule (Oct-Eh) having Oct groups on the core units. Of the small molecule:PC71BM blend films, only the Eh-Hex case exhibited both good intermolecular packing and uniformly distributed nanoscopic networks of molecular channels, maintaining extensive interfacial contacts between the Eh-Hex and PC71BM domains. On the other hand, the Eh-Eh molecules showed poor miscibility with PC71BM, and the Oct-Eh molecules displayed excessive miscibility with PC71BM. These morphological differences resulted in significant differences in the photovoltaic performance of the devices using small molecule:PC71BM blend films. Inverted-type photovoltaic devices fabricated using the Eh-Hex:PC71BM blend had the highest power conversion efficiency among the three molecule-based devices. Overall, this work demonstrated that both the type of alkyl side chain and its position are crucial factors controlling the balance of interactions between small molecules and PC71BM molecules, which can significantly alter blend morphology and photovoltaic performance.