Stoichiometric Engineering of Ag2s Nanocrystals to Realize High-Performance for Organic-Inorganic Hybrid Photodiode

Heavy metal-free Ag2S nanocrystals (NCs) were synthesized with stoichiometric engineering for organic-inorganic hybrid photodiode (HPDs) in which polymeric semiconductor poly(3-hexylthiophene) (P3HT) was co-used. For the high-performance HPDs, 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 is essentials. To this end, the surface stoichiometry of Ag2S NCs was strategically tuned from Ag-rich, near stoichiometric to 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 HPDs was demonstrated by the structure of ITO/PEDOT:PSS/P3HT:Ag2S NCs/Al with the Ag2S NCs with Ag-rich surface. The Ag2S NCs with Ag-rich surface 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 HPDs. The optimized HPDs exhibited a high peak EQE of 170,000%, responsivity of 580 A/W, and specific detectivity of 3 ⨯ 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.