Effects of Crystalline Perylenediimide Acceptor Morphology on Optoelectronic Properties and Device Performance

The perylenediimide (PDI)-based molecules N,N-bis(1-ethylpropyl)-2,5,8,11-tetraphenyl-PDI (3-pentyl), N,N-bis(3,7-dimethyloctyl)-2,5,8,11-tetraphenyl-PDI (3,7-DMO), N,N-bis(2-ethylhexyl)-2,5,8,11-tetraphenyl-PDI (2-EH), and N,N-bis(n-octyl)-2,5,8,11-tetraphenyl-PDI (n-octyl) are synthesized and investigated for photovoltaic response. Single-crystal X-ray structures reveal that these molecules crystallize in either herringbone or slip-stacked geometries and that the crystal packing morphology can be manipulated by changing the solubilizing alkyl substituents or the crystallization conditions. The herringbone structure is shown to result in limited electronic coupling between adjacent chromophores, while the slip-stacked geometry promotes strong coupling. In bulk-heterojunction blend films with the donor polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl) (PBDTT-FTTE), the herringbone acceptors undergo more rapid charge separation than the slip-stacked acceptors but also suffer from increased geminate recombination, as measured by femtosecond transient absorption. This tendency toward recombination decreases short-circuit currents and therefore decreases power conversion efficiency from 3.9% in the purely slip-stacked system to 2.5% in the purely herringboned system. The ratio between the slip-stacked geometry and the herringbone geometry can be reliably controlled in PBDTT-FTTE:3,7-DMO blends using the solvent additive diiodooctane (DIO),and is monitored using grazing incidence wide-angle X-ray scattering (GIWAXS). At low DIO concentrations, diffraction peaks corresponding to the slip-stacked geometry predominate while at high concentrations those corresponding to the herringbone geometry predominate. This microstructural change correlates with changes in charge carrier generation efficiency and thus device power conversion efficiency. This work also provides insights on crystalline acceptor materials which are rare in comparison to amorphous materials, and these results argue that strong coupling between neighboring acceptor molecules is important for efficient charge separation in such systems.