Nickel Oxide Hole Injection Layers for Balanced Charge Injection in Quantum Dot Light-Emitting Diodes

Quantum dot (QD) light-emitting diodes (QLEDs) are promising for next-generation displays, but suffer from carrier imbalance arising from lower hole injection compared to electron injection. A defect engineering strategy is reported to tackle transport limitations in nickel oxide-based inorganic hole-injection layers (HILs) and find that hole injection is able to enhance in high-performance InP QLEDs using the newly designed material. Through optoelectronic simulations, how the electronic properties of NiOx affect hole injection efficiency into an InP QD layer, finding that efficient hole injection depends on lowering the hole injection barrier and enhancing the acceptor density of NiOx is explored. Li doping and oxygen enriching are identified as effective strategies to control intrinsic and extrinsic defects in NiOx, thereby increasing acceptor density, as evidenced by density functional theory calculations and experimental validation. With fine-tuned inorganic HIL, InP QLEDs exhibit a luminance of 45 200 cd m-2 and an external quantum efficiency of 19.9%, surpassing previous inorganic HIL-based QLEDs. This study provides a path to designing inorganic materials for more efficient and sustainable lighting and display technologies.

Dicarboxylic Acid-Assisted Surface Oxide Removal and Passivation of Indium Antimonide Colloidal Quantum Dots for Short-Wave Infrared Photodetectors

Heavy-metal-free III-V colloidal quantum dots (CQDs) are promising materials for solution-processed short-wave infrared (SWIR) photodetectors. Recent progress in the synthesis of indium antimonide (InSb) CQDs with sizes smaller than the Bohr exciton radius enables quantum-size effect tuning of the band gap. However, it has been challenging to achieve uniform InSb CQDs with band gaps below 0.9 eV, as well as to control the surface chemistry of these large-diameter CQDs. This has, to date, limited the development of InSb CQD photodetectors that are sensitive to ≥ ${\ge }$ 1400 nm light. Here we adopt solvent engineering to facilitate a diffusion-limited growth regime, leading to uniform CQDs with a band gap of 0.89 eV. We then develop a CQD surface reconstruction strategy that employs a dicarboxylic acid to selectively remove the native In/Sb oxides, and enables a carboxylate-halide co-passivation with the subsequent halide ligand exchange. We find that this strategy reduces trap density by half compared to controls, and enables electronic coupling among CQDs. Photodetectors made using the tailored CQDs achieve an external quantum efficiency of 25 % at 1400 nm, the highest among III-V CQD photodetectors in this spectral region.

Arresting Ion Migration from the ETL Increases Stability in Infrared Light Detectors Based on III-V Colloidal Quantum Dots

III-V colloidal quantum dots (CQDs) are of interest in infrared photodetection, and recent developments in CQDs synthesis and surface engineering have improved performance. Here this work investigates photodetector stability, finding that the diffusion of zinc ions from charge transport layers (CTLs) into the CQDs active layer increases trap density therein, leading to rapid and irreversible performance loss during operation. In an effort to prevent this, this work introduces organic blocking layers between the CQDs and ZnO layers; but these negatively impact device performance. The device is then, allowing to use a C60:BCP as top electron-transport layer (ETL) for good morphology and process compatibility, and selecting NiOX as the bottom hole-transport layer (HTL). The first round of NiOX -based devices show efficient light response but suffer from high leakage current and a low open-circuit voltage (Voc) due to pinholes. This work introduces poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) with NiOX NC to form a hybrid HTL, an addition that reduces pinhole formation, interfacial trap density, and bimolecular recombination, enhancing carrier harvesting. The photodetectors achieve 53% external quantum efficiency (EQE) at 970 nm at 1 V applied bias, and they maintain 95% of initial performance after 19 h of continuous illuminated operation. The photodetectors retain over 80% of performance after 80 days of shelf storage.

Transient Measurements and Simulations Correlate Exchange Ligand Concentration and Trap States in Colloidal Quantum Dot Photodetectors

Colloidal quantum dot (CQD) photodetectors (PDs) can detect wavelengths longer than the 1100 nm limit of silicon because of their highly tunable bandgaps. CQD PDs are acutely affected by the ligands that separate adjacent dots in a CQD-solid. Optimizing the exchange solution ligand concentration in the processing steps is crucial to achieving high photodetector performance. However, the complex mix of chemistry and optoelectronics involved in CQD PDs means that the effects of the exchange solution ligand concentration on device physics are poorly understood. Here we report direct correspondence between simulated and experimental transient photocurrent responses in CQD PDs. For both deficient and excess conditions, our model demonstrated the experimental changes to the transient photocurrent aligned with changes in trap state density. Combining transient photoluminescence, absorption, and photocurrent with this simulation model, we revealed that different mechanisms are responsible for the increased trap density induced by excess and deficient active layer ligand concentrations.

Halide-Driven Synthetic Control of InSb Colloidal Quantum Dots Enables Short-Wave Infrared Photodetectors

In the III-V family of colloidal quantum dot (CQD) semiconductors, InSb promises access to a wider range of infrared wavelengths compared to many light-sensing material candidates. However, achieving the necessary size, size-dispersity, and optical properties has been challenging. Here the synthetic challenges associated with InSb CQDs are investigated and it is found that uncontrolled reduction of the antimony precursor hampers the controlled growth of CQDs. To overcome this, a synthetic strategy that combines nonpyrophoric precursors with zinc halide additives is developed. The experimental and computational studies show that zinc halide additives decelerate the reduction of the antimony precursor, facilitating the growth of more uniformly sized CQDs. It is also found that the halide choice provides additional control over the strength of this effect. The resultant CQDs exhibit well-defined excitonic transitions in spectral range of 1.26-0.98 eV, along with strong photoluminescence. By implementing a postsynthesis ligand exchange, colloidally stable inks enabling the fabrication of high-quality CQD films are achieved. The first demonstration of InSb CQD photodetectors is presented reaching 75% external quantum efficiency (QE) at 1200 nm, to the knowledge the highest short-wave infrared (SWIR) QE reported among heavy-metal-free infrared CQD-based devices.

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

III-V colloidal quantum dots (CQDs) are promising materials for optoelectronic applications, for they avoid heavy metals while achieving absorption spanning the visible to the infrared. However, the covalent nature of III-V CQDs requires the development of new passivation strategies to fabricate conductive CQD solids for optoelectronics: we show herein that ligand exchanges, previously developed in II-VI and IV-VI quantum dots and employing a single ligand, do not fully passivate CQDs, and that this curtails device efficiency. Guided by density functional theory (DFT) simulations, we develop a co-passivation strategy to fabricate indium arsenide CQD photodetectors, an approach that employs the combination of X-type methyl ammonium acetate (MaAc) and Z-type ligands InBr₃ . This approach maintains charge carrier mobility and improves passivation, seen in a 25% decrease in Stokes shift, a four-fold reduction in the rate of first-exciton absorption linewidth broadening over time-under-stress, and leads to a doubling in photoluminescence lifetime. The resulting devices show 37% external quantum efficiency at 950 nm, the highest value reported for InAs CQD photodetectors. This article is protected by copyright. All rights reserved.

Control over Charge Carrier Mobility in the Hole Transport Layer Enables Fast Colloidal Quantum Dot Infrared Photodetectors

Solution-processed colloidal quantum dots (CQDs) are promising materials for photodetectors operating in the short-wavelength infrared region (SWIR). Devices typically rely on CQD-based hole transport layers (HTL), such as CQDs treated using 1,2-ethanedithiol. Herein, we find that these HTL materials exhibit low carrier mobility, limiting the photodiode response speed. We develop instead inverted (p-i-n) SWIR photodetectors operating at 1370 nm, employing NiOx as the HTL, ultimately enabling 4× shorter fall times in photodiodes (∼800 ns for EDT and ∼200 ns for NiOx). Optoelectronic simulations reveal that the high carrier mobility of NiOx enhances the electric field in the active layer, decreasing the overall transport time and increasing photodetector response time.

Electron-Transport Layers Employing Strongly Bound Ligands Enhance Stability in Colloidal Quantum Dot Infrared Photodetectors

Solution-processed photodetectors based on colloidal quantum dots (CQDs) are promising candidates for short-wavelength infrared light sensing applications. Present-day CQD photodetectors employ a CQD active layer sandwiched between carrier-transport layers in which the electron-transport layer (ETL) is composed of metal oxides. Herein, a new class of ETLs is developed using n-type CQDs, finding that these benefit from quantum-size effect tuning of the band energies, as well as from surface ligand engineering. Photodetectors operating at 1450 nm are demonstrated using CQDs with tailored functionalities for each of the transport layers and the active layer. By optimizing the band alignment between the ETL and the active layer, CQD photodetectors that combine a low dark current of ≈1 × 10^-3 mA cm^-2 with a high external quantum efficiency of ≈66% at 1 V are reported, outperforming prior reports of CQD photodetectors operating at >1400 nm that rely on metal oxides as ETLs. It is shown that stable CQD photodetectors rely on well-passivated CQDs: for ETL CQDs, a strongly bound organic ligand trans-4-(trifluoromethyl)cinnamic acid (TFCA) provides improved passivation compared to the weakly bound inorganic ligand tetrabutylammonium iodide (TBAI). TFCA suppresses bias-induced ion migration inside the ETL and improves the operating stability of photodetectors by 50× compared to TBAI.

Quantum-Size-Effect Tuning Enables Narrowband IR Photodetection with Low Sunlight Interference

Infrared photodetection enables depth imaging techniques such as structured light and time-of-flight. Traditional photodetectors rely on silicon (Si); however, the bandgap of Si limits photodetection to wavelengths shorter than 1100 nm. Photodetector operation centered at 1370 nm benefits from lower sunlight interference due to atmospheric absorption. Here, we report 1370 nm-operating colloidal quantum dot (CQD) photodetectors and evaluate their outdoor performance. We develop a surface-ligand engineering strategy to tune the electronic properties of each CQD layer and fabricate photodetectors in an inverted (PIN) architecture. The strategy enables photodetectors with an external quantum efficiency of 75% and a low dark current (1 μA/cm²). Outdoor testing demonstrates that CQD-based photodetectors combined with a 10 nm-line width bandpass filter centered at 1370 nm achieve over 2 orders of magnitude (140× at incident intensity 1 μW/cm²) higher signal-to-background ratio than do Si-based photodetectors that use an analogous bandpass filter centered at 905 nm.

Controlled Crystal Plane Orientations in the ZnO Transport Layer Enable High-Responsivity, Low-Dark-Current Infrared Photodetectors

Colloidal quantum dots (CQD) have emerged as attractive materials for infrared (IR) photodetector (PD) applications because of their tunable bandgaps and facile processing. Presently, zinc oxide is the electron-transport layer (ETL) of choice in CQD PDs; however, ZnO relies on continuous ultraviolet (UV) illumination to remove adsorbed oxygen and maintain high external quantum efficiency (EQE), speed, and photocurrent. Here, it is shown that ZnO is dominated by electropositive crystal planes which favor excessive oxygen adsorption, and that this leads to a high density of trap states, an undesired shift in band alignment, and consequent poor performance. Over prolonged operation without UV exposure, oxygen accumulates at the electropositive planes, trapping holes and degrading performance. This problem is addressed by developing an electroneutral plane composition at the ZnO surface, aided by atomic layer deposition (ALD) as the means of materials processing. It is found that ALD ZnO has 10× lower binding energy for oxygen than does conventionally deposited ZnO. IR CQD PDs made with this ETL do not require UV activation to maintain low dark current and high EQE.