Colloidal Quantum Dot Layer Research Articles

Advancing the Coupling of III–V Quantum Dots to Photonic Structures to Shape Their Emission Diagram

AbstractThe development of optoelectronic devices based on III–V semiconductor colloidal quantum dots (CQDs) is highly sought after due to their reduced toxicity. While devices based on conventional CQDs (II–VI semiconductors, halide perovskites) have achieved impressive technological leaps since their discovery, the most mature of these compounds contain toxic heavy metal elements (Cd, Hg, or Pb), which are highly undesirable for safe industrial scale applications. The strong covalent bonds of III–V compounds like InP, InAs, or InSb prevent the release of their toxic atoms, making them safer. However, these same bonds create severe material constraints. Namely, their harsher reaction conditions and increased sensitivity to oxidation have kept most of the research focused on material development. Meanwhile, their integration into devices and their coupling to photonic structures lag behind. Here, the integration of InAs/ZnSe core‐shell CQDs is advanced. First, the material parameters necessary to design plasmonic gratings coupled to the CQDs are elucidated and those gratings are fabricated. Angle‐resolved spectroscopy shows that the plasmon modes successfully couple to the CQD layer's emission leading to a tunable directivity with a 15° linewidth. A 3‐fold increase of the PL signal is achieved at normal incidence, thus advancing toward the goal of efficient outcoupling in LEDs.

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.

A Colloidal-Quantum-Dot Integrated U-Shape Micro-Light-Emitting-Diode and Its Photonic Characteristics.

A special micro LED whose light emitting area is laid out in a U-like shape is fabricated and integrated with colloidal quantum dots (CQDs). An inkjet-type machine directly dispenses the CQD layer to the central courtyard-like area of this U-shape micro LED. The blue photons emitted by the U-shape mesa with InGaN/GaN quantum wells can excite the CQDs at the central courtyard area and be converted into green or red ones. The U-shape micro LEDs are coated with Al2O3 by an atomic layer deposition system and exhibit moderate external quantum efficiency (6.51% max.) and high surface recombination because of their long peripheries. Low-temperature measurement also confirms the recovery of the external quantum efficiency due to lower non-radiative recombination from the exposed surfaces. The color conversion efficiency brought by the CQD layer can be as high as 33.90%. A further continuous CQD aging test, which was evaluated by the strength of the CQD emission, under current densities of 100 A/cm2 and 200 A/cm2 injected into the micro LED, showed a lifetime extension of the unprotected CQD emission up to 1321 min in the U-shape device compared to a 39 min lifetime in the traditional case, where the same CQD layer was placed on the top surface of a squared LED.

Thiol‐Free p‐Type Colloidal Quantum Dot for Efficient Broadband Optoelectronics

Colloidal quantum dots (CQDs) are promising semiconductors for optoelectronic applications owing to their bandgap tunability and solution processability. Historically, ethanedithiol has been widely employed as a surface ligand to form a p‐type CQD layer via a layer‐by‐layer process. However, the limited control of p‐type characteristics reduces the device performance, and the high reactivity of ligand and processing solvents degrade the underlying layer. In this study, a thiol‐free p‐type CQD is demonstrated by creating native p‐type CQDs during the synthesis process. Sulfurization of PbS CQD results in p‐type properties that enable the avoidance of any further thiol‐ligand treatment process. Further, alternative surface passivation of the sulfurized CQD using halide ligand yields a robust, p‐type, morphologically uniform, and trap‐suppressed film. The developed CQD film is then employed as a hole‐transporting layer (HTL) for both broadband solar cells and photodetectors. The resulting CQD devices exhibit improved performance with a 65.7% increase in photovoltaic efficiency and a 1.7‐fold improvement in responsivity for infrared photodetection. The devices also demonstrate higher ambient stability, retaining 85% of the initial performance after 1,000 hr owing to the uniform top HTL morphology, indicating the potential of the new p‐type layer to be utilized in various emerging optoelectronics.

Computer Simulation of Charge Carrier Transport Processes in the Conducting Layers of Colloidal Quantum Dots

Computer Simulation of Charge Carrier Transport Processes in the Conducting Layers of Colloidal Quantum Dots

Molecular surface programming of rectifying junctions between InAs colloidal quantum dot solids

Heavy-metal-free III-V colloidal quantum dots (CQDs) show promise in optoelectronics: Recent advancements in the synthesis of large-diameter indium arsenide (InAs) CQDs provide access to short-wave infrared (IR) wavelengths for three-dimensional ranging and imaging. In early studies, however, we were unable to achieve a rectifying photodiode using CQDs and molybdenum oxide/polymer hole transport layers, as the shallow valence bandedge (5.0 eV) was misaligned with the ionization potentials of the widely used transport layers. This occurred when increasing CQD diameter to decrease the bandgap below 1.1 eV. Here, we develop a rectifying junction among InAs CQD layers, where we use molecular surface modifiers to tune the energy levels of InAs CQDs electrostatically. Previously developed bifunctional dithiol ligands, established for II-VI and IV-VI CQDs, exhibit slow reaction kinetics with III-V surfaces, causing the exchange to fail. We study carboxylate and thiolate binding groups, united with electron-donating free end groups, that shift upward the valence bandedge of InAs CQDs, producing valence band energies as shallow as 4.8 eV. Photophysical studies combined with density functional theory show that carboxylate-based passivants participate in strong bidentate bridging with both In and As on the CQD surface. The tuned CQD layer incorporated into a photodiode structure achieves improved performance with EQE (external quantum efficiency) of 35% (>1 μm) and dark current density < 400 nA cm-2, a >25% increase in EQE and >90% reduced dark current density compared to the reference device. This work represents an advance over previous III-V CQD short-wavelength IR photodetectors (EQE < 5%, dark current > 10,000 nA cm-2).

Unlocking the Potential of Colloidal Quantum Dot/Organic Hybrid Solar Cells: Band Tunable Interfacial Layer Approach.

Hybrid colloidal quantum dot (CQD)/organic architectures are promising candidates for emerging optoelectronic devices having high performance and inexpensive fabrication. For unlocking the potential of CQD/organic hybrid devices, enhancing charge extraction properties at electron transport layer (ETL)/CQD interfaces is crucial. Hence, we carefully adjust the interface properties between the ETL and CQD layer by incorporating an interfacial layer for the ETL (EIL) using several types of cinnamic acid ligands. The EIL having a cascading band offset (ΔEC) between the ETL and CQD layer suppresses the potential barrier and the local charge accumulation at ETL/CQD interfaces, thereby reducing the bimolecular recombination. An optimal EIL effectively expands the depletion region that facilitates charge extraction between the ETL and CQD layer while preventing the formation of shallow traps. Representative devices with an EIL exhibit a maximum power conversion efficiency of 14.01% and retain over 80% of initial performances after 300 h under continuous maximum power point operation.

Organic Solvent Dispersible MXene Integrated Colloidal Quantum Dot Photovoltaics

AbstractDespite recent advances in colloidal quantum dot (CQD) photovoltaics, several challenges persist and hinder further improvements. In particular, the Fermi level mismatch between the iodide‐treated photoactive and thiol‐treated hole‐transporting CQD layers creates an unfavorable energy band for hole collection. Furthermore, the numerous surface cracks in the thiol‐treated CQD layer facilitate direct contact between the photoactive CQD layer and the metal electrode, consequently leading to reduced device performance. To address these issues, a polycatechol functionalized MXene (PCA‐MXene) that can serve both as a dopant and an interlayer for CQD photovoltaics is developed. By achieving a uniformly dispersed mixture in a butylamine solvent, PCA‐MXene enables the effective combination of MXene and CQDs. This results in the modification of the work function of CQDs and the modulation of the energy band alignment, ultimately promoting enhanced hole extraction. Moreover, the PCA‐MXene employed as an interlayer effectively covers the surface cracks present in the thiol‐treated CQD layer. This coverage inhibits both metal electrode penetration and moisture intrusion into the device. Owing to these advantages, the CQD photovoltaics incorporating PCA‐MXene achieve a power conversion efficiency (PCE) of 13.6%, accompanied by enhanced thermal stability, in comparison to the reference device with a PCE of 12.8%.

To improve device performance of self-driven heterojunction photodetectors by inserting a thin layer of silver nanoparticles into the electron-transporting layer

Lead selenide colloidal quantum dots (CQDs) are widely used in infrared photodetectors due to their size-dependent bandgap tunability, facile solution-processing techniques and low cost. Heterojunctions (HJs) are usually used to construct device to facilitate the separation of excitons and the transportation of photogenerated carriers. Based on the solution-processed HJ photodetector ITO/ZnO/PbSe/Ag, in which PbSe CQDs layer acts as the active layer and ZnO nanoparticles (NPs) layer as the electron-transporting layer, a greatly enhanced-performance was obtained after inserting 10 nm Ag NPs layer within the ZnO NPs layer close to the HJ ZnO/PbSe interface. By optimizing the concentration of Ag NPs solutions and the location of Ag NPs thin layer in ZnO film, the maximum responsivity of the self-driven HJ photodetector ITO/ZnO(50 nm):Ag-NPs(10 nm):ZnO(50 nm)/PbSe(300 nm)/Ag reaches to 6.25 mA/W with a specific detectivity D* of 5.17 ×1011 Jones under 8.5 μW/cm² 1550 nm illumination at zero bias. Further, the underlain physical mechanisms for the enhanced-performance were discussed in details. In this way, it provides a very efficient method for enhanced-performance self-driven HJ photodetectors by inserting a thin layer of metal NPs into the electron-transporting layer close to the heterojunction interface.

Impact of Photo-induced Physical and Chemical Processes on Lateral Transport of Excitons in Quantum Dot Thin Films

We study how lateral transport of excitons (exciton diffusion) in quantum dot thin films is influenced by the photo-physical and photo-chemical properties of the quantum dots. For this, thin layers of colloidal quantum dots on glass substrates are exposed to laser pulses with different total average powers. Spatial mappings of the lifetimes of the quantum dots within the diffraction-limited spot sizes of the laser beam show that their photo-induced properties can be influenced by the excitation intensity mode profiles significantly, leading to three distinct regimes of exciton diffusion across the films. These include the cases wherein the lateral energy transport (i) is linearly influenced by the laser intensity mode profiles, (ii) is convoluted by variations of laser intensity and photo-physical and photo-chemical properties of the quantum dots, and (iii) is significantly influenced by the formation of oxidation hole burning. The third regime, oxidation hole burning effect, occurs at the center of the excited regions of the quantum dot thin films, suppressing their emission and transport of energy considerably. We will discuss variations of the diffusivity as a function of the laser intensity mode profiles and time. The results shed light on the impact of photo-chemical and photo-physical processes on transport of energy in quantum dot thin films, highlight the regimes of efficient exciton diffusion, and demonstrate the prominent impact of the intensity mode profiles of the light used to excite quantum dot thin films.

Photo-sensitive Structures with Cascade Radiation Concentrators Based on Colloidal Quantum Dots of Metal Chalcogenides

Introduction . The problem of increasing the efficiency of existing photodetectors and creating their new types attracts much research attention. Among new photodetector types are photosensitive structures based on cascade concentrators, whose operational principle involves the absorption of optical radiation followed by its reemission at a longer wavelength and radiation concentration onto a highly efficient small-area photodetector. The absorption and re-emission spectra of each cascade layer depend on the characteristics of the material used. Сolloidal quantum dots are among the most promising materials for cascade layers due to their manufacturing technology, which provides for accurate control over the photoluminescence maximum position. It seems highly relevant to develop and to study photosensitive structures with cascade concentrators of various shapes based on CdS, CdSe/ZnS, and PbS colloidal quantum dots. Aim . To develop photosensitive structures with a wide-range sensitivity spectrum based on concentrators containing arrays of metal chalcogenide CQDs and to study their characteristics. Materials and methods . Cascade photosensitive structures were manufactured based on layers made of polymethyl methacrylate and layers of colloidal quantum dots embedded in a polystyrene matrix. Results . Three-layer concentrators were manufactured with different colloidal quantum dots in each concentrator layers. A 22 % increase in the output power was observed for a three-layer cascade structure based on different cascade layer materials compared to a similar structure using a single layer concentrator. Conclusion . The conducted studies showed an increase in the efficiency of photosensitive structures with a cascade concentrator based on colloidal quantum dots of various types (CdS, CdSe/ZnS, and PbS) in the cascade layers.

Tailored Band Edge Positions by Fractional Ligand Replacement of Nonconductive Colloidal Quantum Dot Films

The tunable band edge position of colloidal quantum dot (CQD) films is a key part of efficient optoelectronic device design of various forms such as photovoltaics, light-emitting diodes, and photodetectors. An accurate estimation of shifts in the band edge position of CQD layers is still considered challenging, especially when the CQD films are nonconductive. Here, we investigate the effect of nonconductive CQD films on photoelectron spectroscopy (PES) and photoelectron yield spectroscopy (PYS). We demonstrate control of systematic band edge positions by fractional ligand replacement of nonconductive CQD film characterized with photoelectron yield spectroscopy and density functional theory calculations. As-synthesized CQDs with insulating oleate ligands were fractionally replaced with trans-3,5-difluorocinnamic acid molecules in a nonpolar solution. The fractionally replaced surface-bound ligands are quantitatively analyzed using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. We found that the energy levels of nonconducting CQD films shift linearly as a function of the number of bound trans-3,5-difluorocinnamates with specific dipole moments while retaining hydrophobic wettability.

Improved Operation Stability of Colloidal Quantum Dots Photodiodes via Suppressing Ion Migration

Solution-processed colloidal quantum dots (CQDs) photodiodes are compatible for monolithic integration with silicon-based readout circuit, enabling high-performance and low-cost infrared imaging. However, operation stability of CQDs photodiodes has rarely been explored, which is one of the main obstacles for CQDs imaging commercialization. Here, we reveal that the performance deterioration of the CQDs photodiodes under strong electric field at high temperature is mainly attributed to ion migration in CQDs film, clearly clarified by aging photoconductors based on each functional layer of CQDs photodiodes. The halide ion migration through vacancy increases the defect density of the CQDs layer and blocks device interfaces, resulting in increased dark current density and postponed saturation voltage. We introduce polyimide (PI) into CQDs film to efficiently block the pathway for halide ion migration. As a result, the CQDs photodiodes with PI achieve greatly improved operation stability over 27 h under 104 V/cm electric field at 85 °C.

Improvement of Dynamic Performance and Detectivity in Near-Infrared Colloidal Quantum Dot Photodetectors by Incorporating Conjugated Polymers.

Colloidal quantum dots (CQDs) have a unique advantage in realizing near-infrared (NIR) photodetection since their optical properties are readily tuned by the particle size, but CQD-based photodetectors (QPDs) presently show a high dark current density (Jd) and insufficient dynamic characteristics. To overcome these two problems, we synthesized and introduced two types of conjugated polymers (CPs) by replacing the p-type CQD layer in the QPDs. The low dielectric constant and insulating properties of CPs under dark conditions effectively suppressed the Jd in the QPDs. In addition, the energy-level alignment and high-hole mobility of the CPs facilitated hole transport. Therefore, both the responsivity and specific detectivity were highly enhanced in the CP-based QPDs. Notably, the dynamic characteristics of the QPDs, such as the -3 dB cut-off frequency and rising/falling response times, were significantly improved in the CP-based QPDs owing to the sizable molecular ordering and fast hole transport of the CP in the film state as well as the low trap density, well-aligned energy levels, and good interfacial contact in the CP-based devices.

Efficient Short-Wave Infrared Light-Emitting Diodes Based on Heavy-Metal-Free Quantum Dots.

Short-wave infrared (SWIR) light emission is important for a diverse range of modern applications, such as eye-safe depth sensing, light detection and ranging (LiDAR), facial recognition, eye tracking, optical communication, and health-monitoring technologies. However, there are a very limited number of known semiconductors that can emit efficiently in the SWIR spectral range. Presently, SWIR light-emitting diodes (LEDs) based on colloidal quantum dots (CQD) are dominated by lead chalcogenide systems, despite the presence of heavy metal and modest efficiencies. Here, a highly efficient SWIR LED based on heavy-metal-free indium arsenide (InAs) core-shell CQDs is presented. In the LED design, the implementation of an otherwise hole-transporting poly(vinylcarbazole) (PVK) layer on the electron-injecting side of the device stack leads to a surprising enhancement in device performance, giving remarkably high external quantum efficiencies (EQEs) of 13.3% at 1006 nm. Single-carrier device and optical investigations reveal the origins of enhancement to be the electronic decoupling of the CQD layer with the electron-injecting zinc oxide (ZnO) layer, which mitigates luminescence quenching and improves charge balance. This work marks one of the highest efficiencies reported for heavy-metal-free solution-processed LEDs in the SWIR spectral region, and can find significant applications in emerging consumer electronic technologies.

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.

Self-Aligned Emission of Distributed Feedback Lasers on Optical Fiber Sidewall

This article assembles a distributed feedback (DFB) cavity on the sidewalls of the optical fiber by using very simple fabrication techniques including two-beam interference lithography and dip-coating. The DFB laser structure comprises graduated gratings on the optical fiber sidewalls which are covered with a layer of colloidal quantum dots. Directional DFB lasing is observed from the fiber facet due to the coupling effect between the grating and the optical fiber. The directional lasing from the optical fiber facet exhibits a small solid divergence angle as compared to the conventional laser. It can be attributed to the two-dimensional light confinement in the fiber waveguide. An analytical approach based on the Bragg condition and the coupled-wave theory was developed to explain the characteristics of the laser device. The intensity of the output coupled laser is tuned by the coupling coefficient, which is determined by the angle between the grating vector and the fiber axis. These results afford opportunities to integrate different DFB lasers on the same optical fiber sidewall, achieving multi-wavelength self-aligned DFB lasers for a directional emission. The proposed technique may provide an alternative to integrating DFB lasers for applications in networking, optical sensing, and power delivery.

Graphene-Quantum Dot Hybrid Photodetectors with Low Dark-Current Readout.

Graphene-based photodetectors have shown responsivities up to 108 A/W and photoconductive gains up to 108 electrons per photon. These photodetectors rely on a highly absorbing layer in close proximity to graphene, which induces a shift of the graphene chemical potential upon absorption, hence modifying its channel resistance. However, due to the semimetallic nature of graphene, the readout requires dark currents of hundreds of microamperes up to milliamperes, leading to high power consumption needed for the device operation. Here, we propose a different approach for highly responsive graphene-based photodetectors with orders of magnitude lower dark-current levels. A shift of the graphene chemical potential caused by light absorption in a layer of colloidal quantum dots induces a variation of the current flowing across a metal-insulator-graphene diode structure. Owing to the low density of states of graphene near the neutrality point, the light-induced shift in chemical potential can be relatively large, dramatically changing the amount of current flowing across the insulating barrier and giving rise to an alternative gain mechanism. This readout requires dark currents of hundreds of nanoamperes up to a few microamperes, orders of magnitude lower than that of other graphene-based photodetectors, while keeping responsivities of ∼70 A/W in the infrared, almost 2 orders of magnitude higher than that of established germanium on silicon and indium gallium arsenide infrared photodetectors. This makes the device appealing for applications where high responsivity and low power consumption are required.

Design of a Unipolar Barrier for a Nanocrystal-Based Short-Wave Infrared Photodiode

Nanocrystals are promising materials for the design of low-cost, infrared (IR) detectors. Here we focus on HgTe colloidal quantum dots (CQDs) as an active material for detection in the extended short-wave infrared (2.5 μm as cutoff wavelength). In this paper, we propose a strategy to enhance the performances of previously reported photodiodes. In particular, we integrate in this diode a unipolar barrier, whose role is to reduce the dark current injection and subsequently enhance the signal-to-noise ratio. We demonstrate that such unipolar barrier can be designed from another layer of HgTe CQDs with a wider band gap. Using a combination of IR spectroscopy and photoemission, we show that the barrier is resonant with the absorbing layer valence band, while presenting a clear offset with the conduction band. The combination of contacts with improved design and use of a unipolar barrier allows us to reach a detectivity as high as 3 × 108 Jones at room temperature with 3 dB cut off frequency above 10 kHz.

Near infrared-induced optical gating at the lead-sulfide (PbS)/pentacene interface

We report optical gating at the pentacene/lead sulfide (PbS) functional interface through which mobile carriers are confined in the pentacene layer close to the PbS colloidal quantum dot (CQD) layer. Using a bottom-contact pentacene/PbS field effect transistor (FET) structure, hole doping in a pentacene layer is demonstrated and the mechanism by which mobile carriers are created is elucidated by probing threshold voltage shift in the FET and the pentacene/PbS interfacial trap density. A large threshold voltage shift under selective illumination (780 nm) of the PbS CQD layer is interpreted as signature of hole transfer from the PbS to the pentacene. Electron trapping at the pentacene/PbS interface is suggested to be involved in the optical gating process.