Planarity‑Enforced Hexagonal Fused Merocyanines for High‐Performance Green‑Selective Organic Photodiodes

Next‐generation image sensors aim to pack ever more pixels into a limited footprint by stacking organic photodiodes (OPDs) on top of silicon photodiodes. While OPDs provide exceptional color selectivity, their comparatively slow signal‐transfer speed remains a major barrier to further material advancement. To overcome this limitation, we designed a hexagonal D– π‐fused merocyanine, designated G2 , whose rigid, fully covalently fused planar backbone adopts a nearly perfectly planar conjugated structure that sharpens absorption and improves charge transport. This molecular planarity yields green‐selective OPDs that deliver a high external quantum efficiency of 63%, responsivity of 0.29 A W − 1 , and a signal bandwidth of 40 kHz (corresponding to a response time of ∼8.75 µs). The device operates in the green spectral range (peak at 555 nm, FWHM = 122 nm) and achieves a high specific detectivity (D ∗ ) of 4.02 × 10 1 2 cm Hz 1/2 W −1 under −3 V bias. These results demonstrate that absolute molecular planarity in hexagonal D –π‐fused merocyanines is a decisive design principle for next‐generation image sensors to deliver ultra‐high resolution, rapid read‐out, and reliable color reproduction under low‐light conditions.