Flexible Organic Single Crystals with Unconventional Negative Photoconductivity for Intelligent Security Systems.

Negative photoconductivity (NPC) is a rare phenomenon in which electrical conductivity decreases upon exposure to light. Although uncommon, NPC provides unique advantages for advanced electronic devices that are difficult to achieve with positive photoconductivity alone. Typically observed in low-dimensional inorganic materials, NPC results from mechanisms such as carrier trapping, the formation of deep acceptor-level states, enhanced recombination, or environmental factors like oxygen and moisture adsorption. Organic single-crystal-based electronic devices are garnering attention because of their high efficiency achieved from highly ordered molecular packing and efficient charge transport properties. However, NPC has yet to be observed in organic single crystals, and the fundamental mechanisms behind this behavior remain largely unexplored. This study presents NPC in long, flexible single crystals of a small organic compound, shedding light on the underlying physical mechanisms of this unconventional photoconduction response. A salicylideneimine-boron complex was designed by integrating donor and acceptor moieties to promote intramolecular charge transfer, which, in turn, facilitated the formation of deep-level trap states responsible for NPC under both UV and visible light. The single crystals demonstrated a detection sensitivity of ∼45% and a photoresponsivity of 7.18 μA/W. These properties were utilized to fabricate a human-machine interface for advanced user authentication. The development of customized single crystals exhibiting stable light-modulated switching and sensing firmly establishes organic single crystals as a versatile platform for harnessing NPC in next-generation device applications, while opening new opportunities in advanced optoelectronic technologies.