Lead Sulfide Quantum Dot Photodetector with Enhanced Responsivity through a Two-Step Ligand-Exchange Method

Abstract

Recently, lead sulfide (PbS) quantum dots (QDs) have demonstrated great potential in becoming one of the most promising next-generation photoelectrical materials for photodetectors. PbS QDs provide fascinating properties including size-controllable spectral sensitivity, a wide and tunable absorption range, cost-efficient solution processability, and flexible substrate compatibility. One of the key problems that limit the performance of PbS QDs-based photodetectors is inefficient carrier transfer. Long ligands decorating the outside surface of PbS QDs to protect them against degeneration inhibit the transfer of electrical charge carriers and thereby limit the device performance. To overcome this problem, the long ligands need to be effectively exchanged. Here, a two-step ligand-exchange method is demonstrated. The QDs are pretreated using methylammonium iodide in solution as the first step ligand exchange before the layer-by-layer deposition process and solid-state ligand exchange. The grazing-incidence small-angle X-ray scattering and X-ray photoelectron spectroscopy analyses prove a smaller spacing among the QDs and an increased ligand-exchange ratio by adopting the two-step method. This strongly indicates a better capability of charge transfer than the traditional one-step solid-state ligand-exchange technology. Devices fabricated using the two-step method present an enhancement of the charge-transfer capability with a larger current. The efficient charge transfer is further demonstrated by a significant 94% increase of the responsivity and a 57% enhancement of the detectivity of the PbS QDs-based photodetector, reaching 3302 mA/W and 5.06 × 1012 J, respectively.

Facet Control for Trap‐State Suppression in Colloidal Quantum Dot Solids

“…The o ‐QDs produce a denser QD solid film, which is seen from the Yoneda peak position shifting from 0.27° to 0.30° (inset of Figure 6b). [ 41–42 ] In order to get more details about the inner morphologies of the QD solids, horizontal line cuts are also analyzed (Figure 6c). Cylinders and spheres are used to model large structures (QD aggregations) and small structures (individual QDs), respectively, and to fit the scattering intensity distribution in horizontal direction.…”

“…The solution‐phase ligand‐exchange process was carried out in air, and followed the literature with a minor modification. [ 42 ] Briefly, 460 mg (1 mmol) of PbI 2 , 74 mg (0.22 mmol) of PbBr 2 , and 40 mg (0.04 mol) of AA were completely dissolved in 10 mL DMF. Subsequently, 80 mg of QDs in octane were added to the 10 mL of the precursor solution.…”

Air-stable and ultrasensitive solution-cast SWIR photodetectors utilizing modified core/shell colloidal quantum dots

“…Moreover, the type-I structure is not favorable for solar cell and photodetector applications with respect to charge separation and transport. Improved charge transfer properties of the core/shell structure, and increased photodetector efficiency have been achieved by replacing the long-carbon-chain ligands of QDs with a shorter ligand [ 29 – 32 ]. Nevertheless, because the ligand exchange process is performed several times, long processing time can induce binding to the substrate, which acts as a trap and negatively affects photodetector performance [ 33 ].…”

Colloidal PbS Quantum Dots for Visible-to-Near-Infrared Optical Internet of Things

“…Taking advantage of the stronger quantum confinement present in QDs, as compared to other low-dimensional structures, the development in this regard has enabled numerous applications in the field of optoelectronics, e.g. LEDs [13], [17] and photodetectors [13], [15], [18]. The inherent advantages of high quantum efficiency and short lifetime present in colloidal QDs could potentially open up a plethora of opportunities in the field of high-speed OWC systems, such as the phosphor-based transmitter [19] and receiver modules [20] as demonstrated in our prior works.…”