Fluorine Tuning of Morphology, Energy Loss, and Carrier Dynamics in Perylenediimide Polymer Solar Cells

We investigate backbone fluorination effects in bulk-heterojunction (BHJ) polymer solar cells (PSCs) with the fluorine-poor PBDTT-FTTE and fluorine-rich PBDTTF-FTTE donor polymers, paired with the perylenediimide (PDI) 3D "propeller acceptor" Ph(PDI)(3). The PBDTTF-FTTE:Ph(PDI)(3) devices exhibit a >50% power conversion efficiency (PCE, up to 9.1%) increase versus PBDTT-FTTE:Ph(PDI)(3). This enhancement reflects structurally optimized phase separation due to templating effects, affording reduced energy loss, higher electron mobility, greater free charge lifetimes and yields, and lower bimolecular recombination, as quantified by UPS, AFM, TEM, GIWAXS, SCLC, light intensity dependence measurements, and fs/ns transient absorption (TA) spectroscopy. In PBDTTF-FTTE, the DFT-computed dipole orientations of the ground and excitonic states are nearly antiparallel, explaining the longer free charge lifetimes, minimized recombination, and lowered exciton binding energy. The PBDTTF-FTTE:Ph(PDI)(3) performance enhancement vs that of the fluorine-poor PBDTT-FTTE:Ph(PDI)(3) analogue as well as the overall PSC performance exceeds that of the corresponding PC71BM- and ITIC-Th-based cells.