This study presents a new non-fullerene acceptor, (5Z,5′Z)-5, 5′-[(9,9-diocetyl-9H-fluorene-2,7-diyI)bis[2,3-benzothiadiazole-7, 4-diyI(Z)methylidene)]bis(3-ethyl-2-thioxo-1,3-thiazolidin-4-one) (IDTBR), designed to improve the performance of poly(3-hexylthiophene) (P3HT) in organic photovoltaics (OPV). IDTBR is synthesized with an indacenodithiophene core and benzothiadiazole and rhodanine flanking groups, offering a more planar molecular backbone, delocalized electronic structure, and push-pull molecular orbital hybridization compared to previous non-fullerene acceptors. This design results in a UV–vis absorption profile that complements P3HT, allowing broader photon harvesting across the incident solar spectrum. The absorption onset of IDTBR can be tuned by the choice of solubilizing alkyl chains, with linear chains promoting stronger intermolecular packing and a further red-shifted absorption compared to branched chains. IDTBR devices exhibit higher short-circuit currents and power conversion efficiencies (PCE) of up to 6.4%, the highest reported for fullerene-free P3HT devices. Additionally, IDTBR devices demonstrate superior oxidative stability compared to benchmark PC60BM:P3HT devices and several high-performance polymers, showing minimal degradation over 1,200 hours under ambient conditions. The improved stability and performance of IDTBR:P3HT devices make them promising candidates for large-area, scalable OPV with practical operating lifetimes.This study presents a new non-fullerene acceptor, (5Z,5′Z)-5, 5′-[(9,9-diocetyl-9H-fluorene-2,7-diyI)bis[2,3-benzothiadiazole-7, 4-diyI(Z)methylidene)]bis(3-ethyl-2-thioxo-1,3-thiazolidin-4-one) (IDTBR), designed to improve the performance of poly(3-hexylthiophene) (P3HT) in organic photovoltaics (OPV). IDTBR is synthesized with an indacenodithiophene core and benzothiadiazole and rhodanine flanking groups, offering a more planar molecular backbone, delocalized electronic structure, and push-pull molecular orbital hybridization compared to previous non-fullerene acceptors. This design results in a UV–vis absorption profile that complements P3HT, allowing broader photon harvesting across the incident solar spectrum. The absorption onset of IDTBR can be tuned by the choice of solubilizing alkyl chains, with linear chains promoting stronger intermolecular packing and a further red-shifted absorption compared to branched chains. IDTBR devices exhibit higher short-circuit currents and power conversion efficiencies (PCE) of up to 6.4%, the highest reported for fullerene-free P3HT devices. Additionally, IDTBR devices demonstrate superior oxidative stability compared to benchmark PC60BM:P3HT devices and several high-performance polymers, showing minimal degradation over 1,200 hours under ambient conditions. The improved stability and performance of IDTBR:P3HT devices make them promising candidates for large-area, scalable OPV with practical operating lifetimes.