(The) impact of dipole moment of conjugated polymers on charge generation in all-polymer solar cells

Abstract

The performance of all-polymer solar cells (all-PSCs) is often limited by poor exciton dissociation process. Herein, we present the design of a series of polymer donors (P1-P3) with different numbers of fluorine atoms on their backbone and investigate the influence of fluorination on charge generation in all-PSCs. 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. Also, the fluorination lowered the highest occupied molecular orbital energy level of P3 and P2, leading to higher open-circuit voltage ($V_{OC}$). The power conversion efficiency (PCE) 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 dependence of short-circuit current density ($J_{SC}$) and $V_{OC}$, and time-resolved photoluminescence. Therefore, our 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.