Synergism in Binary Nanocrystals Enables Top-Illuminated HgTe Colloidal Quantum Dot Short-Wave Infrared Imager.

Thanks to their tunable infrared absorption, solution processability, and low fabrication costs, HgTe colloidal quantum dots (CQDs) are promising for optoelectronic devices. Despite advancements in device design, their potential for imaging applications remains underexplored. For integration with Si-based readout integrated circuits (ROICs), top illumination is necessary for simultaneous light absorption and signal acquisition. However, most high-performing traditional HgTe CQD photodiodes are p-on-n stack and bottom-illuminated. Herein, we report top-illuminated inverted n-on-p HgTe CQD photodiodes using a robust p-type CQD layer and a thermally evaporated Bi2S3 electron transport layer. The p-type CQD solid is achieved by exploring the synergism in binary HgTe and Ag2Te CQDs. These photodetectors show a room-temperature detectivity of 3.4 × 1011 jones and an EQE of ∼44% at ∼1.7 µm wavelength, comparable to the p-on-n HgTe CQD photodiodes. A top-illuminated HgTe CQD short-wave infrared imager (640 × 512 pixels) was fabricated, demonstrating successful infrared imaging.

In-Synthesis Se-Stabilization Enables Defect and Doping Engineering of HgTe Colloidal Quantum Dots.

Colloidal Quantum Dots (CQDs) of mercury telluride (HgTe) hold particular appeal for infrared photodetection due to their widely tunable infrared absorption and good compatibility with silicon electronics. While advances in surface chemistry have led to improved CQD solids, the chemical stability of HgTe material is not fully emphasized. In this study, it is aimed to address this issue and identifies a Se-stabilization strategy based on the surface coating of Se on HgTe CQDs via engineering in the precursor reactivity. The presence of Se-coating enables HgTe CQDs with improved colloidal stability, passivation, and enhanced degree of freedom in doping tuning. This enables the construction of optimized p-i-n HgTe CQD infrared photodetectors with an ultra-low dark current 3.26 × 10-6 A cm⁻2 at -0.4 V and room-temperature specific detectivity of 5.17 × 1011 Jones at wavelength ≈2 um, approximately one order of magnitude improvement compared to that of the control device. The stabilizing effect of Se is well preserved in the thin film state, contributing to much improved device stability. The in-synthesis Se-stabilization strategy highlights the importance of the chemical stability of materials for the construction of semiconductor-grade CQD solids and may have important implications for other high-performance CQD optoelectronic devices.

Short-Wave Infrared Detection and Imaging Employing Size-Customized HgTe Nanocrystals.

HgTe nanocrystals (NCs) possess advantages including tunable infrared absorption spectra, solution processability, and low fabrication costs, offering new avenues for the advancement of next-generation infrared detectors. In spite of great synthetic advances, it remains essential to achieve customized synthesis of HgTe NCs in terms of industrial applications. Herein, by taking advantage of a high critical nucleation concentration of HgTe NCs, a continuous-dropwise (CD) synthetic approach that features the addition of the anion precursors in a feasible drop-by-drop fashion is demonstrated. The slow reaction dynamics enable size-customized synthesis of HgTe NCs with sharp band tails and wide absorption range fully covering the short- and mid-infrared regions. More importantly, the intrinsic advantages of CD process ensure high-uniformity and scale-up synthesis from batch to batch without compromising the excitonic features. The resultant HgTe nanocrystal photodetectors show a high room-temperature detectivity of 8.1 × 1011 Jones at 1.7 µm cutoff absorption edge. This CD approach verifies a robust method for controlled synthesis of HgTe NCs and might have important implications for scale-up synthesis of other nanocrystal materials.

Colloidal PbS Quantum Dot Photodiode Imager with Suppressed Dark Current.

Lead sulfide (PbS) colloidal quantum dots (CQDs) for photodetectors (PDs) have garnered great attention due to their potential use as low-cost, high-performance, and large-area infrared focal plane arrays. The prevailing device architecture employed for PbS CQD PDs is the p-i-n structure, where PbS CQD films treated with thiol molecules, such as 1,2-ethanedithiol (EDT), are widely used as p-type layers due to their favorable band alignment. However, PbS-EDT films face a critical challenge associated with low film quality, resulting in many defects that curtail the device performance. Herein, a controlled oxidization process is developed for better surface passivation of the PbS-EDT transport layer. The dark current density (Jd) of PbS CQD PDs based on optimized PbS-EDT layer shows a dramatic decrease by nearly 2 orders of magnitude. The increase of carrier lifetime and suppression of carrier recombination via controlled oxidation in PbS-EDT CQDs were confirmed by transient absorption spectra and electrochemical impedance spectra. The device based on the optimized PbS-EDT hole transport layer (HTL) exhibits a specific detectivity (D*) that is 3.4 times higher compared to the control device. Finally, the CQD PD employing oxidization PbS-EDT CQDs is integrated with a thin film transistor (TFT) readout circuit, which successfully accomplishes material discrimination imaging, material occlusion imaging, and smoke penetration imaging. The controlled oxidization strategy verifies the significance of surface management of CQD solids and is expected to help advance infrared optoelectronic applications based on CQDs.

Fluoride passivation of ZnO electron transport layers for efficient PbSe colloidal quantum dot photovoltaics.

Lead selenide (PbSe) colloidal quantum dots (CQDs) are suitable for the development of the next-generation of photovoltaics (PVs) because of efficient multiple-exciton generation and strong charge coupling ability. To date, the reported high-efficient PbSe CQD PVs use spin-coated zinc oxide (ZnO) as the electron transport layer (ETL). However, it is found that the surface defects of ZnO present a difficulty in completion of passivation, and this impedes the continuous progress of devices. To address this disadvantage, fluoride (F) anions are employed for the surface passivation of ZnO through a chemical bath deposition method (CBD). The F-passivated ZnO ETL possesses decreased densities of oxygen vacancy and a favorable band alignment. Benefiting from these improvements, PbSe CQD PVs report an efficiency of 10.04%, comparatively 9.4% higher than that of devices using sol-gel (SG) ZnO as ETL. We are optimistic that this interface passivation strategy has great potential in the development of solution-processed CQD optoelectronic devices.

Cation-Exchange Enables In Situ Preparation of PbSe Quantum Dot Ink for High Performance Solar Cells.

Lead selenide (PbSe) colloidal quantum dots (CQDs) are promising candidates for optoelectronic applications. To date, PbSe CQDs capped by halide ligands exhibit improved stability and solar cells using these CQDs as active layers have reported a remarkable power conversion efficiency (PCE) up to 10%. However, PbSe CQDs are more prone to oxidation, requiring delicate control over their processability and compromising their applications. Herein, an efficient strategy that addresses this issue by an in situ cation-exchange process is reported. This is achieved by a two-phase ligand exchange process where PbI2 serves as both a passivating ligand and cation-source inducing transformation of CdSe to PbSe. The defect density and carrier lifetime of PbSe CQD films are improved to 1.05 × 1016  cm-3 and 12.2 ns, whereas the traditional PbSe CQD films possess 1.9 × 1016  cm-3 defect density and 10.2 ns carrier lifetime. These improvements are translated into an enhancement of photovoltaic performance of PbSe solar cells, with a PCE of up to 11.6%, ≈10% higher than the previous record. Notably, the approach enables greatly improved stability and a two-month stability is successfully demonstrated. This strategy is expected to promote the fast development of PbSe CQD applications in low-cost and high-performance optoelectronic devices.

Ligand-Engineered HgTe Colloidal Quantum Dot Solids for Infrared Photodetectors.

HgTe colloidal quantum dots (CQDs) are promising absorber systems for infrared detection due to their widely tunable photoresponse in all infrared regions. Up to now, the best-performing HgTe CQD photodetectors have relied on using aggregated CQDs, limiting the device design, uniformity and performance. Herein, we report a ligand-engineered approach that produces well-separated HgTe CQDs. The present strategy first employs strong-binding alkyl thioalcohol ligands to enable the synthesis of well-dispersed HgTe cores, followed by a second growth process and a final postligand modification step enhancing their colloidal stability. We demonstrate highly monodisperse HgTe CQDs in a wide size range, from 4.2 to 15.0 nm with sharp excitonic absorption fully covering short- and midwave infrared regions, together with a record electron mobility of up to 18.4 cm2 V-1 s-1. The photodetectors show a room-temperature detectivity of 3.9 × 1011 jones at a 1.7 µm cutoff absorption edge.

Phase-Transfer Exchange Lead Chalcogenide Colloidal Quantum Dots: Ink Preparation, Film Assembly, and Solar Cell Construction.

Solution-processed colloidal quantum dots (CQDs) are promising candidates for the third-generation photovoltaics due to their low cost and spectral tunability. The development of CQD solar cells mainly relies on high-quality CQD ink, smooth and dense film, and charge-extraction-favored device architectures. In particular, advances in the processing of CQDs are essential for high-quality QD solids. The phase transfer exchange (PTE), in contrast with traditional solid-state ligand exchange, has demonstrated to be the most promising approach for high-quality QD solids in terms of charge transport and defect passivation. As a result, the efficiencies of Pb chalcogenide CQD solar cells have been rapidly improved to 14.0%. In this review, the development of the PTE method is briefly reviewed for lead chalcogenide CQD ink preparation, film assembly, and device construction. Particularly, the key roles of lead halides and additional additives are emphasized for defect passivation and charge transport improvement. In the end, several potential directions for future research are proposed.