Stoichiometric Engineering of Ag₂S Nanocrystals to Realize High-Performance Organic-Inorganic Hybrid Photomultiplication-Type Photodiode

Heavy metal-free Ag₂S nanocrystals (NCs) were synthesized for a novel class of sensitizing center of photomultiplication (PM)-type photodiodes in which polymeric semiconductor poly(3-hexylthiophene) (P3HT) was co-used. The key role of PM sensitizing center is considered as efficient charge separation and trapping, which requires prolonged capturing of minority carriers and advantageous band-bending at the cathode interface for continuous tunneling injection of majority carriers. To this end, the surface stoichiometry of Ag₂S NCs was strategically tuned as Ag-rich, stoichiometric, or S-rich. The Ag₂S NCs are designed to have more acceptor-like states when the stoichiometry is dominated by Ag atoms. Conversely, they have more donor-like states when S atoms prevail. In addition to the elemental analyses, the surface stoichiometry of NCs was further characterized by time-resolved photoluminescence and space-charge limited current analyses. An optimal PM-type photodiode was demonstrated by the structure of ITO/PEDOT:PSS/P3HT:Ag₂S NCs/Al with the holes as the majority carriers, Ag-rich Ag₂S NCs as the sensitizing centers. The Ag-rich Ag₂S NCs showed enhanced electron trapping and minimized hole trapping rate due to the high density of acceptor-like states. We showcased that fine-tuning the surface stoichiometry of Ag₂S nanocrystal enables high external quantum efficiency (EQE) of the PM-type photodiode. The optimized hybrid PM-type photodiode exhibited a high peak EQE of 170,000%, responsivity of 580 A/W, and specific detectivity of 3 Χ 10¹³ Jones. Sophisticated control of NC stoichiometry is important for the photophysical properties of sensitizing centers which guarantees successful applications to optoelectronic devices such as photodiodes.