Unencapsulated Self‐Hydroadaptive Organic Heterojunctions for Underwater Physiological Monitoring

ABSTRACT Water exposure is generally considered detrimental to organic semiconductor films, causing irreversible defects, dielectric perturbation, and device failure. Here, n‐type giant‐molecule‐type organic semiconductors GSY‐C4 and GSY‐C10 with tailored alkyl chains were developed for waterproof organic photodetectors (OPDs). D18:GSY‐C4 and D18:GSY‐C10 bulk heterojunctions enabled stable underwater photoplethysmography sensing, with peak responsivities of ∼1.31 and ∼0.7 A W −1 , respectively. Short‐term water exposure did not cause obvious irreversible degradation, but induced water‐triggered adaptive structural reorganization, supported by reduced lamellar d‐spacing, increased lamellar coherence length, and preserved π–π stacking. After 4 h water immersion, unencapsulated D18:GSY‐C4 arrays ( n = 49) showed a ∼9% reduction in dark‐current density and nearly unchanged responsivity ( R λ ) and specific detectivity ( D *). Moreover, key current parameters could be recovered through ethanol rinsing and mild annealing at 100°C for 20 min. D18:GSY‐C10 OPDs maintained underwater functionality for 9 days with <10% variation in dark‐ and photocurrent densities, highlighting alkyl‐chain engineering as an effective strategy for aqueous‐stable organic optoelectronics.