Naphtho[1,2-c:5,6-c′]bis([1,2,3]thiadiazole) enables the design of efficient halogen-free polymer donors and fabrication of organic solar cells with >19% efficiency

Polymer donors with aromatic fused rings are considered significant in achieving efficient non-fullerene organic solar cells (OSCs). In this study, a novel fused electron-withdrawing motif naphtho[1,2-c:5,6-c′]bis([1,2,3]thiadiazole) (iNT) was developed to design a cost-effective polymer donor PiNT with high luminous efficiency and large ionization potential. PiNT demonstrated a superior efficiency of 19.1% in OSCs, while the control 1,2,3-benzothiadiazole (iBT)-based polymer (PiBT) and naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) (NT)-based polymer (PNT) reached inferior efficiencies of 9.2% and 14.2%, respectively. Systematic studies revealed that fusion and isomerization strategies endowed PiNT with a higher extinction coefficient, larger ionization potential, and stronger crystallinity. Moreover, the photoluminescence quantum yield increased from 9.3% for PNT to 30.7% for PiNT. Efficient Förster resonance energy transfer from the donor PiNT exciton to acceptor BTP-eC9, followed by reverse hole transfer, improved exciton dissociation and suppressed non-radiative loss. Consequently, PiNT-based OSCs reached the highest short-circuit current density (J_SC) of 27.8 mA cm⁻², open-circuit voltage (V_OC) of 0.88 V and fill factor (FF) of 77.6%. Notably, PiNT combines simple synthesis, high efficiency, excellent device stability, and scalability, making it a cost-effective donor alternative for future commercial production. Overall, this study highlights that iNT is an effective candidate for designing efficient polymer donors.