Unencapsulated Self-Hydroadaptive Organic Heterojunctions for Underwater Physiological Monitoring

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.