Over Twofold Fluorescence Lifetime Increase in PTCDA Organic Crystals through Sodium-Ion Insertion

Optimizing the photophysical properties of less defective organic crystals is crucial for advancing their applications in optoelectronic devices. In this study, we investigate the enhancement of fluorescence lifetime in an organic semiconductor by inserting Na ions into perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) crystals using a physical vapor transport (PVT) method. X-ray diffraction (XRD) confirms that the Na-ion insertion maintains the original crystal structure. X-ray photoemission spectroscopy (XPS) reveals electron transfer from molecular carbonyl groups (C═O) of the surrounding PTCDA molecule to the Na ions. Ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectroscopy were employed to investigate the influence of Na content on intramolecular interaction and photophysical properties, respectively. A twofold increase in fluorescence lifetime was achieved in NaxPTCDA crystals, attributed to enhanced self-trapped excitons. Our results demonstrate that the insertion of alkali metal ions into organic crystals can effectively modulate intramolecular interactions, thereby optimizing fluorescence lifetime. These findings offer an alternative strategy for the rational design of organic crystals for high-performance optoelectronic devices, improving both sensitivity and efficiency.