Understanding\nOxygen-Induced Reactions and Their Impact\non n‑Type Polymeric Mixed Conductor-Based Devices

Electron transporting\n(n-type) polymeric mixed conductors\nare an\nexciting class of materials for devices with aqueous electrolyte interfaces,\nsuch as bioelectronic sensors, actuators, and soft charge storage\nsystems. However, their charge transport performance falls short of\ntheir p-type counterparts, primarily due to electrochemical side reactions\nsuch as the oxygen reduction reaction (ORR). To mitigate ORR, a common\nstrategy in n-type organic semiconductor design focuses on lowering\nthe lowest unoccupied molecular orbital (LUMO) level. Despite empirical\nobservations suggesting a correlation between deep LUMO levels, low\nORR, and enhanced electrochemical cycling stability in water, this\nrelationship lacks robust evidence. In this work, we delve into the\nelectrochemical reactions of n-type polymeric mixed conductors with\nvarying LUMO levels and assess the impact of ORR on charge storage\nperformance and organic electrochemical transistor (OECT) operation.\nOur results reveal a limited correlation between LUMO levels and ORR\ncurrents, as well as the electrochemical operational stability of\nthe films. While ORR currents minimally contribute to OECT channel\ncurrents under fixed biasing conditions, n-type films self-discharge\nrapidly at floating potentials in a capacitor-like configuration.\nThe density functional theory analysis, complemented by X-ray photoelectron\nspectroscopy, underscores the critical role of backbone chemistry\nin controlling O<sub>2</sub>-related degradation pathways and device\nperformance losses. These findings highlight the persistent challenge\nposed by ORR in n-type semiconductor design and advocate for shifting\nthe focus toward exploring chemical moieties with limited O<sub>2</sub> interactions to enhance operational stability and performance at\nn-type film/water interfaces.