Enhanced Surface Passivation of Lead Sulfide Quantum Dots for Short-Wavelength Photodetectors

Abstract

Lead sulfide (PbS) quantum dots are promising solution-processed materials for short-wave infrared (SWIR) photodetectors due to their tunable band gap and solution processability. Phase-transfer ligand exchange is a common method to prepare quantum dot (QD) inks used in device fabrication. For large-sized PbS QDs used for SWIR detection, the conventional phase-transfer ligand exchange has been problematic due to the densely packed organic ligands and charge-neutral (100) facets.Here, we report a new strategy to carry out the efficient phasetransfer ligand exchange in large-sized QDs. Specifically, using lead acetate trihydrate (PbAc 2 •3H 2 O) as a precursor and methylammonium acetate (MAAc) as an additive in the ligand solution, we can facilitate the efficient phase-transfer ligand exchange and epitaxial growth of perovskite intermediate (MAPbI 3−x Ac x ) on the (100) facets, resulting in a significant improvement in film quality suitable for device fabrication. The resulting photodiodes show a 2.5× enhancement in external quantum efficiency (EQE) compared to devices using QD inks obtained using the conventional method. Considering the low transmittance of the ITO electrode in the SWIR regime, our devices exhibit an internal quantum efficiency of over 90%.

Understanding Colloidal Quantum Dot Device Characteristics with a Physical Model

“…Under a given irradiance, the V oc decreases when the barrier height increases. The results in Figures a,b agree with studies that have shown that increasing ligand passivation by employing two-step instead of one-step ligand-exchange or by using a different ligand (methylammonium acetate vs ammonium acetate) leads to an increase in forward bias current.…”

Sequential Co‐Passivation in InAs Colloidal Quantum Dot Solids Enables Efficient Near‐Infrared Photodetectors

“…[23] MaAc can bind to the vacant sites of the III-V CQDs, given that a small positive ammonium ion and negative acid moiety would make an efficient co-passivation of the CQDs. [18,24] Hence, to maximize the effect of co-passivation, we introduce MaAc in the final-step solution ligand exchange to treat the unpassivated surface sites leftover after the standard InBr 3 /NH 4 Br exchange step, see schematics summarizing the ligand exchange reaction in Figure 2a. We compare the results of InAs CQDs capped with MaAc (MaAc-doped) to those of InAs CQDs samples (standard)-without adding MaAc, to study the effect of the co-passivation.…”

Lead Sulfide Quantum Dots for Synaptic Transistors: Modulating the Learning Timescale with Ligands

“…[8,[34][35][36][37] One notable advantage of CQDs is their design flexibility, not limited to chemical composition [38][39][40][41] but, more importantly, to size [42,43] and surface chemistry. [44][45][46][47] One of the workhorses of the field is lead-sulfide-based nanocrystals, PbS CQDs, because of their highly reproducible chemistry and their broad tunability (from the visible to the short wavelength infrared) of the absorption. [48][49][50] Colloidal quantum dots have been applied in various neuromorphic devices: in transistor architecture [8,51,52] as an additional charge-trapping layer within the dielectric and in two-terminal devices.…”

“…[ 8,34–37 ] One notable advantage of CQDs is their design flexibility, not limited to chemical composition [ 38–41 ] but, more importantly, to size [ 42,43 ] and surface chemistry. [ 44–47 ] One of the workhorses of the field is lead‐sulfide‐based nanocrystals, PbS CQDs, because of their highly reproducible chemistry and their broad tunability (from the visible to the short wavelength infrared) of the absorption. [ 48–50 ]…”