Designing multiple charge carrier separation pathways in core-type near infrared colloidal nanocrystal for broadband photodetector†

Abstract

Near-infrared colloidal nanocrystals (NIR-CNCs) have been widely utilized in optoelectronic applications due to their exceptional optical properties and suitability for mass production. However, their practical application is often hindered by poor chemical stability and suboptimal electronic properties. In this work, four different surface ligand systems—insulating ligands, organic molecular linkers, inorganic molecular linkers, and matrix-type ligands—were systematically investigated to evaluate their effects on the transport and recombination behavior of NIR-CNCs via photoinduced carriers. While molecular linkers enhance transport behavior by improving electronic coupling, they tend to induce photoinduced charge carrier accumulation under AM1.5 illumination due to a high degree of Fermi-level pinning caused by unfavorable electronic structures. In contrast, the matrix-type band-like transport ligand significantly reduced dark current and hysteresis characteristics in CNCs, demonstrating superior performance. Impedance and capacitance analyses revealed that the matrix-type ligand, with its multiple carrier separation pathways, enhanced carrier transport through sub-states facilitated by amorphous MoS_x and effectively passivated CNC trap states, thereby reducing the Fermi-level pinning effect. This approach dramatically suppressed hot carrier-induced trap state generation, minimized photoinduced recombination, and improved operational stability. Overall, this study presents a significant advancement in developing cost-efficient, chemically stable NIR optoelectronic devices with outstanding electronic properties.