Quantum Dot Films Research Articles

(Invited) In Situ and Operando Scattering Studies on Quantum Dot-Based Devices

Colloidal quantum dots (QDs) made e.g. of PbS are attractive for various optoelectronic applications due to their tunable band gap with a large absorption wavelength range from the visible to the near-infrared region. Moreover, due to the high intrinsic permittivity, the exciton binding energy of PbS is smaller than in cadmium chalcogenide QDs and polymer materials, which is beneficial for devices driven by charge carrier generation and transport, e.g. photovoltaics and photodetectors.In this work, we introduce wet chemical deposition methods beyond spin coating such as printing and spray deposition of QDs as a facile and potentially large-scale deposition method for high-performance photodetectors. The inner structures including the inter-dot distance of neighboring QDs and the structure, in the deposited QD solids, are studied by grazing-incidence small-angle X-ray scattering (GISAXS). In addition, the facet orientation of the QDs in the QD solids is characterized by grazing-incidence wide-angle X-ray scattering (GIWAXS). In particular, in situ GISAXS/GIWAXS studies during film formation are able to provide detailed information about the complex kinetic processes [1,2]. Operando GISAXS/GIWAXS studies on QD-based devices such as solar cells bring an in-depth understanding of the degradation mechanisms, which is key for improving the device stability and thereby bringing these novel materials into real-world applications [3]. Superlattice deformation in QD films on flexible substrates via uniaxial strain is followed in situ with GISAXS, providing a fundamental understanding of the impact of strain on the performance of flexible devices based on QD films, such as wearable electronics and next-generation solar cells on flexible substrates [4]. Nanoscale Horiz. 5, 880-885 (2020)Nano Energy 78, 105254 (2020)Energy Environ. Sci. 14, 3420-3429 (2021)Mater. Horizon 8, 383-395 (2023)

Chiral Perovskite Heterostructure Films of CsPbBr3 Quantum Dots and 2D Chiral Perovskite with Circularly Polarized Luminescence Performance and Energy Transfer.

This work reports the synthesis of chiral perovskite heterostructure films by combining a two-dimensional (2D) chiral (R-/S-MBA)2PbI4 perovskite with CsPbBr3 quantum dots (QDs). The as-synthesized chiral heterostructure films exhibit obvious circularly polarized luminescence (CPL) properties, even though pure 2D chiral perovskite cannot present photoluminescence. It indicates that the chirality of the excited state of the QDs originates from the 2D chiral perovskite. The circular polarization-resolved transient absorption (TA) spectra further demonstrate that the CPL response of heterostructure films originates from the energy transfer between the chiral perovskite layer and QDs layer and the suppression of spin relaxation, which induces the imbalance of the spin population of excited states in QDs layer. In addition, the photoluminescence (PL), circular dichroism (CD), and CPL spectra of these heterostructure films can be controlled by varying the thickness and component of the chiral perovskite layer, which demonstrates that the anion exchange between chiral perovskite and CsPbBr3 QDs can tune the chemical composition and optoelectronic properties due to the low bonding energy difference between them and decrease the strain within the QDs layer to reduce the radiative recombination lifetime. This work provides guidance for the synthesis of chiral perovskites with a strong CPL response and further provides insight into the origination of CPL.

Stability in Photoluminescence and Photovoltaic Properties of Formamidinium Lead Iodide Quantum Dots

Formamidinium lead iodide (FAPI) thin films in the perovskite crystalline phase have piqued interest for single-junction solar cells due to their optimal bandgap, long photocarrier lifetime, and intrinsic structural stability. However, trap states on the surface and within the lattice impede the performance and stability of FAPI bulk crystalline film cells. Strategies such as film crystallization, deposition optimization, precursor engineering, and interface property tailoring aim to overcome these limitations. One approach involves forming FAPI quantum dots (QDs). By reducing the size, we achieve better quality and stable alpha-phase FAPI on a nanometer scale. In this contribution, we report synthesizing of alpha-phase FAPI QDs with a mean size distribution of 20 nm. These QDs exhibit a narrow photoluminescence emission peak at 1.61 eV and higher absolute quantum yields ranging from 70% to 86%. Compared to bulk FAPI with a 1.54 eV gap, a blue shift is observed due to the quantum confinement effect.However, achieving degradation-free perovskite solar cells under prolonged light soaking remains a challenge. Furthermore, we investigate the stability of FAPI bulk and QDS films in a humid chamber and after exposure to AM1.5G light in a humid setting (RH ~ 30%, T ~ 28°C). Our approach allows us to form homogeneously dispersed films of quantum dots with thicknesses of several hundred nanometers. These films are studied for photoluminescence and photovoltaic properties. Prototype solar cells incorporating synthesized FAPI QDs showed a power conversion efficiency of around 8%. These findings indicate considerably more stability in photoluminescence and photovoltaic properties of FAPI QDs for enhanced light-soaking stability compared to bulk FAPI films.

(Invited) Surface State Modulation of Colloidal Quantum Dots and Construction of Quantum Dot Based Infrared Detectors

Near-infrared photodetection and imaging devices play an important role in fields such as biological detection, information communication, military meteorology, etc. Traditional imaging devices require integration with readout circuits, and the complex integration process limits the development of infrared imaging systems. Lead sulfide colloidal quantum dots have excellent photoelectric properties in the infrared region, with a tunable bandgap width ranging from 0.4 to 1.5 eV, and spectral response covering below 2400 nm. Lead sulfide quantum dot films exhibit high absorption coefficients in the infrared region and excellent carrier mobility. In recent years, the performance of infrared detectors based on quantum dot systems has rapidly improved, and they are expected to develop into a new type of infrared detection and imaging technology. The speaker's team has combined infrared quantum dots with visible light quantum dots to prepare a simple, low-cost, and efficient infrared upconversion imaging device. By using the method of inducing band bending through interface electron storage, the efficiency of carrier tunneling and photon conversion has been improved, and the application of this technology in biological imaging has been preliminarily demonstrated. This report will introduce the development of quantum dot infrared detection materials and devices in recent years, as well as the research progress of the speaker's team in this area.

Achieving Image Encryption Quantum Dot-Functionalized Encryption Camera with Designed Films.

The risk of information leaks increases as images become a crucial medium for information sharing. There is a great need to further develop the versatility of image encryption technology to protect confidential and sensitive information. Herein, using high spatial redundancy (strong correlation of neighboring pixels) of the image and the in situ encryption function of a quantum dot functionalized encryption camera, in situ image encryption is achieved by designing quantum dot films (size, color, and full width at half maximum) to modify the correlation and reduce spatial redundancy of the captured image during encryption processing. The correlation coefficients of simulated encrypted image closely apporach to 0. High-quality decrypted images are achieved with a PSNR of more than 35dB by a convolutional neural network-based algorithm that meets the resolution requirements of human visual perception. Compared with the traditional image encryption algorithms, chaotic image encryption algorithms and neural network-based encryption algorithms described previously, it provides a universal, efficient and effective in situ image encryption method.

Dynamic fluorescent patterns inspired by cuttlefish for anti-counterfeiting applications

Dynamic fluorescent patterns that convert external stimulation into intuitive color changes have drawn significant interest in various fields. However, the complex synthesis of materials with multiple emissions and technical barriers in the flexible fabrication of full-color fluorescent patterns hinder their applicability in diverse scenarios. Here, we present a bioinspired dynamic fluorescent pattern fabricated by femtosecond laser-induced forward transfer (FsLIFT), inspired by cuttlefish. The elastomeric substrate can contract and expand as muscle fibers of cuttlefish, and the patterned red, green, and blue quantum dot (QD) films can be regarded as pigment cells, achieving bionic preparation of function and morphology similar to cuttlefish. Notably, the fluorescent patterns can be modulated by manipulating strains and excitations. Using this bioinspired strategy, we successfully produced an advanced fluorescent anti-counterfeiting label via FsLIFT, which can only be recognized under specific strain and excitation light conditions. These dynamic fluorescent patterns show broad application potential in anti-counterfeiting, wearable devices, and information encryption.

A strategy for the preparation of low dielectric FGQD/PSPI composite films in wafer-level packaging

Advancement of wafer-level packaging (WLP) has heightened the requirements for photosensitive polyimide (PSPI) materials, where the low dielectric property is a key element to realize high frequency communication. Herein, a poly(amidate) (PAE) was prepared using Pyromellitic Dianhydride, 4,4’-diaminodiphenyl ether, and hydroxyethyl methacrylate as the photosensitive group. Specifically, the fluorinated graphene quantum dots (FGQD) was introduced into the PAE resin by in situ polymerization, and the prepared composite films of photosensitive polyimide-fluorinated graphene quantum dots (PSPI-FGQD) showed excellent dielectric constants and other comprehensive properties. Compared with pure PSPI, the dielectric constant of PSPI-FGQD decreases from 3.8 to 3.0 at 1 MHz; the tensile strength increases from 84.5 to 110.1 MPa, the elongation at break raises from 5.24 % to 17.3 %, and the Young's modulus increases from 2.68 GPa to 2.88 GPa. The glass transition temperature (Tg) is also increased to 420 °C with the introduction of FGQD. Apart of these, the fine patterning ability was observed for the FGQD/PSPI composite film, achieving a high photolithography resolution of 10 μm (L/S). To conclude, the preparation method of low dielectric FGQD/PSPI materials has promising potential applications in advanced packaging.

Synthesis and Processing Strategy for High-Bandgap PbS Quantum Dots: A Promising Candidate for Harvesting High-Energy Photons in Solar Cells.

The wide tunability of the energy bandgap of colloidal lead sulfide (PbS) quantum dots (QDs) has uniquely positioned them for the development of single junction and tandem solar cells. While there have been substantial advancements in moderate and narrow bandgap PbS QDs-ideal for single junction solar cells and the bottom cell in tandem solar cells, respectively; progress has been limited in high-bandgap PbS QDs that are ideally suited for the formation of the top cell in tandem solar cells. The development of appropriate high bandgap PbS QDs would be a major advancement toward realizing efficient all-QD tandem solar cells utilizing different sizes of PbS QDs. Here, we report a comprehensive approach encompassing synthetic strategy, ligand engineering, and hole transport layer (HTL) modification to implement high-bandgap PbS QDs into solar cell devices. We achieved a greater degree of size homogeneity in high-bandgap PbS QDs through the use of a growth retarding agent and a partial passivation strategy. By adjusting the ligand polarity, we successfully grow HTL over the QD film to fabricate solar cells. With the aid of an interface modifying layer, we incorporated an organic HTL for the realization of high-performance solar cells. These solar cells exhibited an impressive open-circuit voltage of 0.824 V and a power conversion efficiency of 10.7%, marking a 360% improvement over previous results.

Pre-sensor computing with compact multilayer optical neural network.

Moving computation units closer to sensors is becoming a promising approach to addressing bottlenecks in computing speed, power consumption, and data storage. Pre-sensor computing with optical neural networks (ONNs) allows extensive processing. However, the lack of nonlinear activation and dependence on laser input limits the computational capacity, practicality, and scalability. A compact and passive multilayer ONN (MONN) is proposed, which has two convolution layers and an inserted nonlinear layer, performing pre-sensor computations with designed passive masks and a quantum dot film for incoherent light. MONN has an optical length as short as 5 millimeters, two orders of magnitude smaller than state-of-the-art lens-based ONNs. MONN outperforms linear single-layer ONN across various vision tasks, off-loading up to 95% of computationally expensive operations into optics from electronics. Motivated by MONN, a paradigm is emerging for mobile vision, fulfilling the demands for practicality, miniaturization, and low power consumption.

Efficient and Stable Pure-Red Perovskite LED Based on Uniform Arrangement Strongly Confined Quantum-Dot Film

Efficient and Stable Pure-Red Perovskite LED Based on Uniform Arrangement Strongly Confined Quantum-Dot Film

Ultraviolet encryption information storage using hydrophobic poly(ε-caprolactone)/carbon quantum dot composite film

Luminescent poly(ε-caprolactone)/nitrogen-doped carbon quantum dots (PCL/N-CQDs) film with the function of information protection and encryption was realized based on the fluorescent N-CQDs. The N-CQDs were synthesized from citric acid with urea as a dopant for the enhancement of fluorescent properties. The facilely prepared composite film retained the hydrophobic nature of PCL and achieved the enhanced mechanical property. The appropriate crystallization temperature and degree of crystallinity of PCL provide the film with opaque characteristics at room temperature, rendering it an ideal carrier for carrying information. The information encryption and decryption are achieved through ionoprinting technique, exploiting the ability of Fe3+ ions to quench the fluorescence of N-CQDs, allowing for the encryption of designed information on the film’s surface. With its opaque, hydrophobic, and photoluminescent properties, the PCL/N-CQDs composite film holds significant promise for applications in information encryption.

Monolithic full-color micro-LED displays featuring three-dimensional chip bonding and quantum dot-based color conversion layer

What we believe to be a novel fabrication process for monolithic full-color (RGB) micro-LED (µLED) display technology, featuring three-dimensional (3D) and quantum dot (QD)-based color conversion layer, has been proposed. This method offers advantages such as a wide color gamut, high pixel density, high yield, and low cost. A 16 × 16 passive matrix (PM) RGB µLED array, with a pitch size of 80 µm and a pixel density of 328 pixels per inch (PPI), has been successfully realized using flip-chip bonding technology. When measuring the electroluminescence (EL) spectra of the green and red pixels with the addition of color filters, the color gamut can achieve a maximum of 124% of the National Television System Committee (NTSC) standard. Additionally, this process significantly reduces the risk of damage to the QD film during photolithography compared to using two different colored QDs for RGB µLED arrays. The proposed manufacturing process shows considerable promise for commercialization.

Luminescent Quantum Dot Films Increase the Radiation Capture and Yield of Lettuce and Sweet Basil Compared to a Traditional/Neutral-density Greenhouse Glazing

Utilizing quantum dot (QD) luminescent films as a greenhouse covering material is an innovative method of modifying the greenhouse light spectrum. The QD films convert a portion of high-energy ultraviolet and blue photons to lower-energy photons. Previous research has shown that the application of QD films in greenhouses led to improved crop yields of red lettuce and tomatoes. However, the underlying mechanism of the yield increases has not been fully explored. We quantified the effects of solar spectral shifts attributable to QD films on plant morphology, radiation capture, and, subsequently, crop yield. Green and red leaf lettuces and basil were grown in a greenhouse under four treatments: regular-concentration QD film (reg QD film); high-concentration QD film (high QD film); color-neutral polyethylene (PE) film; and control treatment without any films. Compared to the reg QD film, the high QD film converted a higher fraction of blue photons into longer-wavelength photons, resulting in enhanced leaf expansion, stem elongation, and shoot fresh weight of red lettuce and basil compared with those grown under the PE film without spectral modifications. No significant growth differences were observed between the control and high QD film treatments of red lettuce and basil despite a 23% reduction in the average daily light integral (DLI) under the high QD film treatment. Compared to that grown under the control treatment, green lettuce grown under the high QD film treatment had a similar total leaf area but reduced shoot biomass; this was likely associated with reductions in leaf thickness and chlorophyll content. In contrast, the red lettuce showed more pronounced leaf expansion and reduced leaf anthocyanin content under the high QD film, which likely helped to offset the reduction in DLI. Overall, our results indicated that modifying the solar spectrum with QD films as greenhouse covering material could result in improved crop radiation capture and yield in greenhouse production of lettuce and basil. However, the spectral shifts caused by the QD films may affect crop quality attributes, such as anthocyanin levels and the production of other beneficial secondary metabolites. This effect on crop quality should be carefully considered and requires further study.

Slow Hot-Exciton Cooling and Enhanced Interparticle Excitonic Coupling in HgTe Quantum Dots.

Rapid hot-carrier/exciton cooling constitutes a major loss channel for photovoltaic efficiency. How to decelerate the hot-carrier/exciton relaxation remains a crux for achieving high-performance photovoltaic devices. Here, we demonstrate slow hot-exciton cooling that can be extended to hundreds of picoseconds in colloidal HgTe quantum dots (QDs). The energy loss rate is 1 order of magnitude smaller than bulk inorganic semiconductors, mediated by phonon bottleneck and interband biexciton Auger recombination (BAR) effects, which are both augmented at reduced QD sizes. The two effects are competitive with the emergence of multiple exciton generation. Intriguingly, BAR dominates even under low excitation fluences with a decrease in interparticle distance. Both experimental evidence and numerical evidence reveal that such efficient BAR derives from the tunneling-mediated interparticle excitonic coupling induced by wave function overlap between neighboring HgTe QDs in films. Thus, our study unveils the potential for realizing efficient hot-carrier/exciton solar cells based on HgTe QDs. Fundamentally, we reveal that the delocalized nature of quantum-confined wave function intensifies BAR. The interparticle excitonic coupling may cast light on the development of next-generation photoelectronic materials, which can retain the size-tunable confinement of colloidal semiconductor QDs while simultaneously maintaining high mobilities and conductivities typical for bulk semiconductor materials.

Electron-related properties in a GaAs/GaAlAs Ultra-thin Core/Shell Film through external field direction for energy and photonic devices

In this paper, we investigate electron-related properties in GaAs/GaAlAs Ultra-thin Core/Shell Film Quantum dot (UTFQD), examining the effects of external electric fields, impurity position, and aluminum fraction. Using the finite element method (FEM) within the effective mass approximation, we numerically solve the Schrödinger equation to study electron probability density, polarizability, and diamagnetic susceptibility. Our results reveal that electric fields can shift electron probability density due to the Stark effect. Without an electric field, the density centers near the shell's core. However, with electric fields, the density shifts opposite to the field's direction. Moreover, polarizability and diamagnetic susceptibility are influenced by impurity position and aluminum fraction. Polarizability increases with electric field intensity, indicating a transition from geometrical confinement to the Stark effect. Diamagnetic susceptibility rises with more aluminum, indicating stronger electron confinement. These findings suggest that adjusting electric fields, impurity position, and aluminum fraction in GaAs/GaAlAs core/shell QDs can control electron behavior, providing flexibility for photonic devices. This study offers a foundation for optimizing quantum dot-based components in photonics.

Enable strong electrical coupling between the InZnP@PbS collodial quantum dots in films via the two-step ligand exchange method

Colloidal quantum dots (CQDs) are promising materials for building next-generation solution-processed, high-performance, and low-cost optoelectronic devices. The photoelectric properties of the resulting films are not only determined by the intrinsic characteristics of the CQDs but are also significantly affected by the electrical coupling between them. However, enhancing the electrical coupling in CQD-based films is still a big challenge. In this work, a two-step ligand exchange strategy has been conducted to solve the above issue, where the InZnP@PbS CQDs have been used as the case. First, the Meerwein salt (Et3OBF4) solution in N, N-dimethylformamide (DMF) has been employed to peel off the native ligands that are strongly capped on the surface of oil-soluble CQDs, resulting in a neat surface. Second, iodide ions are linked to the neat surface of the CQDs to narrow the distance between them and passivate their defects, establishing strong electrical coupling. Transient surface photovoltage (TSPV) and photo-conductance tests confirm that the resulting InZnP@PbS–I CQDs films possess good photoelectric properties and stability. In the fabricated solid-state solar cells, 3.1 % of the photoelectric conversion efficiency (PCE) has been achieved under 1 sun. The current results are outstanding for III-V group CQD-based all solid-state solar cells.

Improving the Device Stability by Controlling the Morphology of Quantum Dot Emissive Layer Via a Coating Process in Blue QLEDs

AbstractBlue light‐emitting CdSe@ZnS/ZnS quantum dot (QD) nanoparticles (NPs) were synthesized and their photophysical properties in both solution and film phases were investigated. The morphological properties of films prepared by different coating methods i. e. single layer coating from low to high concentrations of QD solutions and layer‐by‐layer (multilayer) coating within constant low QD solution concentration, were also examined in detail. Varying the concentration (1–10 mg/mL) and the number of layers (from 1–16) did not essentially affect the photophysical properties of QD films, although it resulted in a direct increment in QD film thickness. The concentration and layer‐dependent films were used as an emissive layer (EML) in QD light‐emitting diodes (QLEDs). Although the “6 mg/ml−1 Layer” QD EML‐based device exhibited relatively high device efficiency compared to the “1 mg/ml−10 Layers” based one at working voltage region, it had ~2‐fold higher efficiency roll‐off at high voltage region. The performance differences for both devices with the same QD EML thickness were attributed to the morphological variations for the QD layer in terms of surface roughness, void density, aggregates/clusters, and trap sites that were directly related to the charge injection balance and Auger recombination.

Negative corona discharge strategy for efficient quantum dot light-emitting diodes.

In colloid quantum dot light-emitting diodes (QLEDs), the control of interface states between ZnO and quantum dots (QDs) plays a vital role. We present a straightforward and efficient method using a negative corona discharge to modify the QD film, creating a dipole moment at the interface of QDs and magnesium-doped ZnO (ZnMgO) for balanced charge carrier distribution within the QDs. This process boosts external quantum efficiencies in red, green, and blue QLEDs to 17.71%, 14.53%, and 9.04% respectively. Notably, optimized devices exhibit significant enhancements, especially at lower brightness levels (1000 to 10,000 cd·m-2), vital for applications in mobile displays, TV screens, and indoor lighting.

P‐244: Late‐News Poster: Flexible mini‐LED Backplane with Laminated QDF for Optimized Illuminating

To fulfill the criteria for a lightweight portable device, this report introduces a skin‐friendly flexible backplane featuring laminated Quantum Dot film. Report details the mechanical and optical performance of this backplane and discusses its applicability on curved surfaces, making it suitable for XR and wearable displays.

The "energy gap law" for mid-infrared nanocrystals.

Colloidal quantum dots are of increasing interest for mid-infrared detection and emission, but device performances will vastly benefit from reducing the non-radiative recombination. Empirically, the photoluminescence quantum yield decreases exponentially toward the mid-infrared, which appears similar to the energy gap law known for molecular fluorescence in the near-infrared. For molecules, the mechanism is electron-vibration coupling and fast internal vibrational relaxation. Here, we explore the possible mechanisms for inorganic quantum dots. The primary mechanism is assigned to an electric dipole near-field energy transfer from the quantum dot electronic transitions to the infrared absorption of surface organic ligands and then to the multiphonon absorption of the quantum dot inorganic core or the surrounding inorganic matrix. In order to obtain luminescent quantum dots in the 3-10μm range, we motivate the importance of using inorganic matrices, which have a higher infrared transparency compared to organic materials. At longer wavelengths, inter-quantum dot energy transfer is noted to be much faster than radiative relaxation, indicating that bright mid-infrared colloidal quantum dot films might then benefit from dilution.