Newly synthetized perylene diimide (PDI) dimers were investigated as non-fullerene electron acceptors for organic solar cells.  In particular, two analogous positional isomers exhibiting twisted vs. planar geometries were prepared to see through a direct comparison if twisting in the molecular structure (hence consequent reduction in aggregation) is really important for enhanced photovoltaic performances of the PDI derivatives, as repeatedly reported in the literature.  The two isomers exhibited strikingly different optical and photophysical properties in solution as well as in film. Our investigation indicates that the photobehavior exhibited by the two molecules in solution, severely influenced by the conformation, can give valuable information about the solid state device. The ultrafast spectroscopic investigation in solution revealed the occurrence of charge separation upon photoexcitation, with charge transfer taking place more efficiently and through a different mechanism (excited state symmetry breaking) in the case of the planar isomer as confirmed by the theoretical calculations. Both isomers exhibit remarkable Two Photon Absorption cross sections, pointing indeed to their important charge transfer character. A significant photovoltaic efficiency (ca. 5 %) was measured for the two isomers employed as electron acceptors. Blends of the two isomers with the donor polymer surprisingly exhibited similar morphology (smooth films with no aggregate domains). According to our study, twisting in the backbone of the PDI dimer structure inhibits aggregation in the neat film of the acceptor but does not significantly affect the active layer morphology. Our results suggest instead that the extent of intra-acceptor charge separation in twisted vs. planar PDI derivatives is a crucial factor determining improved power conversion efficiency in organic solar cells.
 B. Carlotti, Z. Cai, H. Kim, V. Shaparov, I. K. Madu, D. Zhao, W. Chen, P. Zimmerman, L. Yu, T. Goodson Chem. Mater., 2017, submitted.
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