Halogen-substituted phenazine cores reduce energy losses and optimize carrier dynamics in tethered acceptors for 19.8% efficient and stable polymer solar cells

Abstract

The thermodynamic relaxation and rigidity of small-molecule acceptors (SMAs) drive an oligomeric design approach to enhance both operational stability and mechanical flexibility in polymer solar cells (PSCs). While tethered SMAs with multiple subunits connected via flexible linkers to an aromatic core address these challenges, their device efficiencies often remain limited to 16-18%, lagging behind their SMA counterparts due to significant energy losses (similar to 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 donors. 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 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 similar to 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 of high-performance tethered acceptors.