Oligoethylene Glycol Side‐Chain Engineering of Y6‐Derived Nonfullerene Acceptors for Thermally Induced Spontaneous Pseudo‐Planar Heterojunction Organic Photodetectors

ABSTRACT Dark current density ( J D ) fundamentally limits the noise characteristics and detectivity of organic photodetectors (OPDs), arising primarily from reverse‐bias carrier injection and trap‐mediated leakage pathways in donor‐acceptor (D‐A) bulk‐heterojunction (BHJ) active layers. Although pseudo‐planar heterojunction ( p ‐PHJ) architectures effectively suppress these losses, their implementation often requires complex layer engineering or controlled processing conditions. Here, we report two Y6‐derived nonfullerene acceptors (NFAs), YTEG‐4F and YTEG‐4Cl, bearing hydrophilic oligoethylene glycol (OEG) side chains that induce thermally driven spontaneous vertical phase separation within the D‐A BHJ layer. This molecular modification enables p‐PHJ formation, suppressing reverse‐bias carrier injection and leakage pathways, thereby reducing J D . In contrast to the thermally annealed PM6:Y6 device, which exhibits an increased J D of 6.64 × 10 −7 A cm −2 , the PM6:YTEG‐4F and PM6:YTEG‐4Cl devices achieve low J D s of 2.76 × 10 −9 and 3.24 × 10 −9 A cm −2 , corresponding to approximately 260‐fold and 210‐fold reductions after thermal annealing, respectively. Consequently, shot‐noise‐limited detectivities ( D sh *) of 1.17 × 10 13 and 9.74 × 10 12 Jones are achieved at −0.5 V for PM6:YTEG‐4F and PM6:YTEG‐4Cl p ‐PHJ OPDs, respectively. This work demonstrates that OEG side‐chain engineering of NFAs provides a broadly applicable molecular strategy for inducing p ‐PHJ architectures and regulating J D toward high‐performance OPDs.