The Impact of Sequential Fluorination of pi-Conjugated Polymers on Charge Generation in All-Polymer Solar Cells

Abstract

The performance of all-polymer solar cells (all-PSCs) is often limited by the poor exciton dissociation process. Here, the design of a series of polymer donors (P1-P3) with different numbers of fluorine atoms on their backbone is presented and the influence of fluorination on charge generation in all-PSCs is investigated. Sequential fluorination of the polymer backbones increases the dipole moment difference between the ground and excited states (Delta mu(ge)) from P1 (18.40 D) to P2 (25.11 D) and to P3 (28.47 D). The large Delta mu(ge) of P3 leads to efficient exciton dissociation with greatly suppressed charge recombination in P3-based all-PSCs. Additionally, the fluorination lowers the highest occupied molecular orbital energy level of P3 and P2, leading to higher open-circuit voltage (V-OC). The power conversion efficiency of the P3-based all-PSCs (6.42%) outperforms those of the P2 and P1 (5.00% and 2.65%)-based devices. The reduced charge recombination and the enhanced polymer exciton lifetime in P3-based all-PSCs are confirmed by the measurements of light-intensity dependent short-circuit current density (J(SC)) and V-OC, and time-resolved photoluminescence. The results provide reciprocal understanding of the charge generation process associated with Delta mu(ge) in all-PSCs and suggest an effective strategy for designing pi-conjugated polymers for high performance all-PSCs.