Competition between singlet fission and charge separation in solution-processed blend films of 6,13-bis(triisopropylsilylethynyl)pentacene with sterically-encumbered perylene-3,4:9,10-bis(dicarboximide)s

The photophysics and morphology of thin films of N,N-bis(2,6- diisopropylphenyl)perylene-3,4:9,10-bis(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10- bis(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ∼100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast singlet fission ( ^1*TIPS-Pn + TIPS-Pn → 2 ^3*TIPS-Pn) and charge transfer from ^1*TIPS-Pn to PDIs 1-3. As the blend films become more homogeneous across the series TIPS-Pn:PDI 1 → 2 → 3, charge separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state ¹(TIPS-Pn ^+•-PDI ^-•) that undergoes radical pair intersystem crossing to form ³(TIPS-Pn ^+•-PDI ^-•), which then undergoes charge recombination to yield either ^3*PDI or ^3*TIPS-Pn. Energy transfer from ^3*PDI to TIPS-Pn also yields ^3*TIPS-Pn. These results show that multiple pathways produce the ^3*TIPS-Pn state, so that OPV design strategies based on this system must utilize this triplet state for charge separation.