Site-Specific Carbazole-Bridged Isomeric Guests Enable Organic Solar Cells with 21.10% Efficiency and Reduced Non-Radiative Recombination

Non-radiative recombination losses in non-fullerene acceptors (NFAs) represent a critical bottleneck limiting the open-circuit voltage (V_oc) and power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, we report a molecular design strategy that harnesses luminescent-carbazole linkage site isomerism to suppress non-radiative recombination. Two carbazole-functionalized NFAs, QxCz‑C and QxCz‑N, were designed and synthesized. Theoretical calculations reveal that QxCz‑C adopts a coplanar conformation, whereas the carbazole unit in QxCz‑N is oriented nearly perpendicular to the main chain. Upon incorporating QxCz‑C as a minor guest into the host material BTP‑eC9, a favorable mixed phase is formed, accompanied by efficient energy transfer from the guest to the host. The photoluminescence quantum yield of the blend acceptor is significantly enhanced, effectively suppressing electron‑phonon coupling, thereby reducing non‑radiative recombination loss and improving V_oc. Simultaneously, guest incorporation optimizes molecular packing order and active layer morphology, facilitating exciton dissociation and charge transport. Consequently, the PM6:BTP‑eC9:QxCz‑C device achieves a PCE of 20.53%. The generality of this strategy is further validated in the D18:L8‑BO system, delivering an excellent PCE of 21.10%. This work establishes a quantitative “connectivity topology–molecular conformation–non-radiative loss” structure–property relationship and provides a generalizable approach to overcoming the voltage bottleneck in OSCs.