Ultrafast Kinetics Investigation of a Fluorinated-Benzothiadiazole Polymer with an Increased Excited State Transition Dipole Moment Applied in Organic Solar Cells

The fluorination of polymer donors as an encouraging strategy has been widely applied in organic solar cells (OSCs). In this contribution, the two polymers PDTBDT-SBTEH and PDTBDT-SFBTEH with hydrogen and fluorine substitution were synthesized and used as donors in OSCs. In comparison to the PDTBDT-SBTEH:ITIC-based device, the PDTBDT-SFBTEH:ITIC-based device showed a remarkable enhancement in power conversion efficiency (PCE) of 8.61%, achieving a 20% improvement. The multiple effects of the fluorination on the morphology and device performance were systematically investigated. The density functional theory (DFT) calculations indicated that this fluorinated PDTBDT-SFBTEH with a greater molecular electrostatic potential (ESP) could enhance the intramolecular charge transfer (ICT) effect. In addition, its larger ground to excited state dipole moment (Δμge) induces a lower Coulomb binding energy of the excitons, which is beneficial for charge separation. The DFT-calculated results show that PDTBDT-SFBTEH has a lower bonding energy (Eb) of 0.30 eV and the PDTBDT-SFBTEH exhibits a higher Eb value of 0.38 eV. Ultrafast transient absorption (TA) spectroscopy measurements suggested that the pristine PDTBDT-SFBTEH exhibited a faster electron injection after excitation and that the generated excitons can more easily and quickly separate at the donor/acceptor (D/A) interface. Our results provide deeper insights into fluorinated polymer donors with better performance achieved in OSCs.