Halogen-Substituted Phenazine Cores Reduce Energy Losses and Optimize Carrier Dynamics in Tethered Acceptors for 19.8% Efficient and Stable Polymer Solar Cells

Abstract

Tethered small-molecule acceptors (SMAs), featuring multiple SMA subunits connected to an aromatic core via flexible chains, effectively suppressing thermodynamic relaxation and enhancing stretchability in polymer solar cells (PSCs). However, these devices typically exhibit power conversion efficiencies (PCEs) below 19%, lagging behind their SMA counterparts due to significant energy losses (~0.6 eV) and suboptimal charge transport. To address this, we incorporated phenazine moieties into the SMA subunits and employed a halogenation strategy to tune aggregation behavior and compatibility with polymer donor. The phenazine-modified acceptors reduced energy losses to 0.525 eV by suppressing non-radiative recombination. Specifically, the fluorine-modified acceptor (DPz-F) exhibited a homogeneous fibrous morphology and optimal phase separation, achieving a record PCE of 19.80% along with an unprecedented high fill factor of 82.42% for tethered acceptors. In contrast, DPz-Cl and DPz-Br blends showed looser aggregation and larger phase separation, yielding moderate PCEs of 17.95% and 18.50%, respectively. Notably, DPz-F-based devices demonstrated exceptional long-term stability, with a T80 lifetime of ~1000 h, outperforming their Br- and Cl-based counterparts. This work underscores the vital significance of reducing energy losses and enhancing carrier dynamics in the design high-performance tethered acceptors.