Quantum Dot Layers Research Articles

Fullerene‐Free p–i–n Perovskite Solar Cells: Direct Deposition of Tin Oxide on Perovskite Layer Using Ligand Bridges

AbstractIn p–i–n perovskite solar cells (PSCs), fullerene derivatives are predominantly used as an electron transport material (ETM) despite their disadvantages, such as parasitic absorption in the short wavelength range and high cost. State‐of‐the‐art n‐i‐p PSCs are fabricated using SnO2 as the ETM due to their high charge transfer ability, transparency, and low cost. However, in p–i–n PSCs, dispersing SnO2 nanoparticles in a solvent that does not damage the perovskite and forming a uniform layer is challenging. Herein, a strategy of directly depositing SnO2 quantum dots (QDs) on perovskite using ethylenediamine (EDA) for high‐performance applications is reported, which involves a SnO2 QD solution designed with a damage‐free cosolvent. Treating the SnO2 QD layer with the EDA strategy creates a conformal SnO2 QD layer and improves charge transport. This strategy achieves a high power conversion efficiency (PCE) of 18.9% in PSCs with a 1.77 eV bandgap, which is the highest PCE reported for wide bandgap p–i–n PSCs using an inorganic ETM. The top SnO2 layer enables ITO deposition without sputtering damage and achieves a bifacial factor of 99% due to the high transmittance of SnO2 QD. The resulting four‐terminal all‐perovskite tandem exhibited a PCE of 27.0%.

Direct Photopatterning of Colloidal Quantum Dots with Electronically Optimized Diazirine Cross-Linkers.

Colloidal quantum dots (QDs) with a wide color gamut and high luminescent efficiency are promising for next-generation electronic and photonic devices. However, precise and scalable patterning of QDs without degrading their properties and their integration into commercially relevant devices, such as digitally addressable QD light-emitting diode (QLED) displays, remain challenging. Here, we develop electronically optimized diazirine-based cross-linkers for nondestructive, direct photopatterning of QDs and, ultimately, building the active-matrix QLED displays. The key to the cross-linker design is the introduction of electron-donating substituents that permit the formation of ground-state singlet carbenes for air-stable and benign QD photopatterning. Under ambient conditions, these cross-linkers enable the patterning of heavy metal-free QDs at a resolution of over 13,000 pixels per inch using commercial i-line photolithography. The patterned QD layers fully preserved their optical and optoelectronic properties. Pixelated electroluminescent devices with patterned InP/ZnSe/ZnS QD layers show a peak external quantum efficiency of 15.3% and a maximum luminance of about 40,000 cd m-2, outperforming those made by existing QD patterning approaches. We further show the seamless integration of patterned QLEDs with thin-film transistor circuits and the fabrication of dual-color active-matrix displays. These results underscore the importance of designing photochemistry for QD patterning, and promise the implementation of direct photopatterning methods in manufacturing commercial QLED displays and other integrated QD device platforms.

Direct Photo‐Crosslinking Patterning for High‐Performance 0D–2D Hybrid Photodetectors

AbstractHigh‐performance 0D–2D hybrid photodetectors integrated with a crosslinker for direct pattering of quantum dots on the large‐scale synthesized MoS2 layer are reported. In the patterned hybrid structure, QD layers are patterned with a resolution of up to 2 µm, ensuring high precision. Enhanced charge transfer from QDs to 2D materials is confirmed using PL quenching, TR‐PL, and UPS analysis. As a result, the QD/2D hybrid photodetectors with crosslinker‐assisted direct patterning demonstrated a remarkable photoresponsivity of ≈105 A W−1 and a specific detectivity of over 1011 Jones, attributed to the difference in built‐in potential. The crosslinker patterning of QDs opens up potential applications for the photodetectors in highly integrated image sensors and can be further extended to high‐resolution display industries, eliminating unnecessary fabrication processes.

Enhanced detection of cardiac troponin-I using CdSe/CdS/ZnS core-shell quantum dot/TiO2 heterostructure photoelectrochemical sensor

This paper introduces a photoelectrochemical sensor employing red-emitting CdSe/CdS/ZnS core-shell quantum dot/TiO2 heterostructure for detecting cardiac troponin-I (cTnI) biomarkers. The sensor utilizes a photoactive layer-by-layer self-assembly of CdSe/CdS/ZnS nanocrystals within a TiO2 nanoparticle photon-sensitive layer to enhance charge separation, thereby boosting sensor photocurrent. Additionally, the CdSe/CdS/ZnS quantum dot layer enhances photocurrent stability by facilitating the separation of photo-excited electrons and holes. Functionalization of the CdSe/CdS/ZnS core-shell quantum dot/TiO2 heterostructure-based photoelectrochemical sensor with anti-troponin involves first activating -COOH functionalized CdSe/CdS/ZnS with carbodiimide cross-linkers, followed by cTnI protein capture. Photocurrent changes are measured using amperometry. The sensor format exhibits an enhanced photocurrent signal proportional to concentration, with a detection range of 10 pg/mL to 0.2 ng/mL of cTnI protein. The use of a red-emitting CdSe/CdS/ZnS quantum dot layer ensures chemical stability during and after chemical coupling on the sensor. This study showcases the successful application of CdSe/CdS/ZnS core-shell quantum dot/TiO2 heterostructure-based photoelectrochemical sensors for early detection of troponin-I protein in cardiac disease diagnostics.

ETL and HTL Engineering in MAPbI3 based stable and efficient perovskite solar cell with a PbS QD layer using SETFOS 5.3

ETL and HTL Engineering in MAPbI3 based stable and efficient perovskite solar cell with a PbS QD layer using SETFOS 5.3

Colloidal 2D Layered SiC Quantum Dots from a Liquid Precursor: Surface Passivation, Bright Photoluminescence, and Planar Self-Assembly.

We report the bottom-up synthesis of colloidal two-dimensional (2D) layered silicon carbide (SiC) quantum dots with a cubic structure, lateral size of 5-10 nm, ⟨110⟩ exfoliation to few atomic layers, and surface passivation with 1-dodecene. Samples shielded from oxygen and plasma-annealed for purity exhibit narrow blue photoluminescence (PL) with quantum yields (QYs) over 60% in exceptional cases, while unshielded nanocrystals (NCs) exhibit broad blue/green/white PL with 10-15% QY. The latter scenario is attributed to excess surface carbon and oxygen accrued during synthesis and processing, with size separation through ultracentrifugation revealing size-dependent impurity emission. In contrast, the shape of the bright narrow blue PL shows little variation with NC size, while in both scenarios, the maximum QY occurs near four atomic layers. When dried under heat, the disk-like NC suspensions are observed to aggregate into microscale domains, with further self-assembly into planar superlattice domains with common crystalline orientation. The results are compared with photophysical simulations and bring clarity to the broad emission commonly reported for top-down approaches, while inspiring bottom-up schemes directed at improved material quality.

Influence of carrier localization on photoluminescence emission from sub-monolayer quantum dot layers

We have investigated the origins of photoluminescence from quantum dot (QD) layers prepared by alternating depositions of sub-monolayers and a few monolayers of size-mismatched species, termed as sub-monolayer (SML) epitaxy, in comparison with their Stranski–Krastanov (SK) QD counterparts. Using measured nanostructure sizes and local In-compositions from local-electrode atom probe tomography as input into self-consistent Schrödinger–Poisson simulations, we compute the 3D confinement energies, probability densities, and photoluminescence (PL) spectra for both InAs/GaAs SML- and SK-QD layers. A comparison of the computed and measured PL spectra suggests one-dimensional electron confinement, with significant 3D hole localization in the SML-QD layers that contribute to their enhanced PL efficiency in comparison to their SK-QD counterparts.

Effects of Dislocation Filtering Layers on Optical Properties of Third Telecom Window Emitting InAs/InGaAlAs Quantum Dots Grown on Silicon Substrates.

Integrating light emitters based on III-V materials with silicon-based electronics is crucial for further increase in data transfer rates in communication systems since the indirect bandgap of silicon prevents its direct use as a light source. We investigate here InAs/InGaAlAs quantum dot (QD) structures grown directly on 5° off-cut Si substrate and emitting light at 1.5 μm, compatible with established telecom platform. Using different dislocation defect filtering layers, exploiting strained superlattices, and supplementary QD layers, we mitigate the effects of lattice constant and thermal expansion mismatches between III-V materials and Si during growth. Complementary optical spectroscopy techniques, i.e. photoreflectance and temperature-, time- and polarization-resolved photoluminescence, allow us to determine the optical quality and application potential of the obtained structures by comparing them to a reference sample-state-of-the-art QDs grown on InP. Experimental findings are supported by calculations of excitonic states and optical transitions by combining multiband k•p and configuration-interaction methods. We show that our design of structures prevents the generation of a considerable density of defects, as intended. The emission of Si-based structures appears to be much broader than for the reference dots, due to the creation of different QD populations which might be a disadvantage in particular laser applications, however, could be favorable for others, e.g., in broadly tunable devices, sensors, or optical amplifiers. Eventually, we identify the overall most promising combination of defect filtering layers and discuss its advantages and limitations and prospects for further improvements.

Quantum Dot/TiO2 Nanocomposite-Based Photoelectrochemical Sensor for Enhanced H2O2 Detection Applied for Cell Monitoring and Visualization.

This workexploits the possibility of using CdSe/ZnS quantum dot (QD)-electrodes to monitor the metabolism of living cells based on photoelectrochemical (PEC) measurements. To realize that, the PEC setup is improved with respect to an enhanced photocurrent signal, better stability, and an increased signal-to-noise ratio, but also for a better biocompatibility of the sensor surface on which cells have been grown. To achieve this, a QD-TiO2 heterojunction is introduced with the help of atomic layer deposition (ALD). The heterojunction reduces the charge carrier recombination inside the semiconductor nanoparticles and improves the drift behavior. The PEC performance is carefully analyzed by adjusting the TiO2 thickness and combining this strategy with multilayer immobilizations of QDs. The optimal thickness of this coating is ≈5nm; here, photocurrent generation can be enhanced significantly (e.g., for a single QD layer electrode by more than one order of magnitude at 0V vs Ag/AgCl). The resulting optimized electrode is used for hydrogen peroxide (H2O2) sensing with a good sensitivity down to µmolar concentrations, reusability, stability, response rate, and repeatability. Finally, the sensing system is applied to monitor the activity of cells directly grown on top of the electrode surface.

Origin of the Efficiency Roll-off in Quantum Dot Light-Emitting Diodes: An Electrically Excited Transient Absorption Spectroscopy Study.

In situ characterizations of charge injection dynamics, equilibrated concentration, and electric field distributions shed light on the critical mechanisms of quantum dot light-emitting diodes (QD-LEDs). In this work, we developed electrically excited transient absorption spectroscopy, which can provide the above key information, to investigate the efficiency roll-off of QD-LEDs. We found that the average electron populations per QD are low when QD-LEDs exhibit efficiency roll-off, excluding Auger recombination as the main cause. We also revealed that the weak electrical field inside the QD layer under forward biases has a negligible impact on the efficiency. Interestingly, we found that as the voltage increases the electron concentration in the QD layer saturates at very low levels. When combined with the concomitant efficiency roll-off, we propose electron leakage is the main loss at elevated driving voltages. We further demonstrate that increasing the electron confinement potential with the ZnS shell enables us to efficiently mitigate the efficiency roll-off.

Suppressing Charge Extraction Loss in Quantum Dot Infrared Photovoltaics by Optimizing the Charge Transport Layer.

Infrared solar cells (IRSCs), capable of converting low-energy infrared photons to electron-hole pairs, are promising infrared optoelectronic devices because of their extended utilization region of the solar to short-wavelength infrared region. For PbS QDs IRSCs, charge extraction loss, easily generated at the interfaces, has been one of the dominate obstacles impeding the improvement of device efficiencies due to too many trap states and mismatched energy levels between the photoactive layer and electron transport layer (ETL). Herein, an advanced ZnO ETL was developed to improve the extraction of photogenerated charges from the PbS QD photoactive layer to ETLs. The advanced ETL film exhibited effectively suppressed trap states and better-matched energy levels compared with the QD layer. As a consequence, high-performance PbS QD IRSCs with the highest infrared power conversion efficiencies of 1.26% under 1100 nm filtered solar illumination are achieved, suggesting an effective and facile route for enhancing the charge extraction in infrared photovoltaics.

Probing the Operation of Quantum-Dot Light-Emitting Diodes Using Electrically Pumped Transient Absorption Spectroscopy.

The quantum-dot light-emitting diode (QLED) is a new generation light emission source that holds great promise for display and lighting applications. Understanding the dynamics of electrons and holes in QLEDs during their operation is crucial for future QLED optimization, but a time-resolved technology capable of characterizing electrons is still lacking. To tackle this challenge, we develop a unique electrically pumped transient absorption (E-TA) spectroscopy to probe the density of electrons in the QD layer with a nanosecond time resolution. The E-TA result provides a comprehensive understanding of the electron dynamics in QLEDs by quantifying the electron injection time after external voltage on, electron release time after external voltage off, and equilibrated electron density (Ne) in the QD layer during device operation. By combining E-TA technology with time-resolved electroluminescence and transient current measurements, we present a comprehensive overview of the dynamics of both electrons and holes in a QLED during operation.

Chiral 3D structures through multi-dimensional transfer printing of multilayer quantum dot patterns

Three-dimensional optical nanostructures have garnered significant interest in photonics due to their extraordinary capabilities to manipulate the amplitude, phase, and polarization states of light. However, achieving complex three-dimensional optical nanostructures with bottom-up fabrication has remained challenging, despite its nanoscale precision and cost-effectiveness, mainly due to inherent limitations in structural controllability. Here, we report the optical characteristics of intricate two- and three-dimensional nanoarchitectures made of colloidal quantum dots fabricated with multi-dimensional transfer printing. Our customizable fabrication platform, directed by tailored interface polarity, enables flexible geometric control over a variety of one-, two-, and three-dimensional quantum dot architectures, achieving tunable and advanced optical features. For example, we demonstrate a two-dimensional quantum dot nanomesh with tuned subwavelength square perforations designed by finite-difference time-domain calculations, achieving an 8-fold enhanced photoluminescence due to the maximized optical resonance. Furthermore, a three-dimensional quantum dot chiral structure is also created via asymmetric stacking of one-dimensional quantum dot layers, realizing a pronounced circular dichroism intensity exceeding 20°.

Microfluid biosensor for detection of hpv in patient scraping samples: Determining E6 and E7 oncogenes

E6 and E7 oncogenes are pivotal in the carcinogenic transformation in HPV infections and efficient diagnostic methods can ensure the detection and differentiation of HPV genotype. This study describes the development and validation of an electrochemical, label-free genosensor coupled with a microfluidic system for detecting the E6 and E7 oncogenes in cervical scraping samples. The nanostructuring employed was based on a cysteine and graphene quantum dots layer that provides functional groups, surface area, and interesting electrochemical properties. Biorecognition tests with cervical scraping samples showed differentiation in the voltammetric response. Low-risk HPV exhibited a lower biorecognition response, reflected in ΔI% values of 82.33 % ± 0.29 for HPV06 and 80.65 % ± 0.68 for HPV11 at a dilution of 1:100. Meanwhile, high-risk, HPV16 and HPV18, demonstrated ΔI% values of 96.65 % ± 1.27 and 93 % ± 0.026, respectively, at the same dilution. Therefore, the biorecognition intensity followed the order: HPV16 >HPV18 >HPV06 >HPV11. The limit of detection and the limit of quantification of E6E7 microfluidic LOC−Genosensor was 26 fM, and 79.6 fM. Consequently, the E6E7 biosensor is a valuable alternative for clinical HPV diagnosis, capable of detecting the potential for oncogenic progression even in the early stages of infection.

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.

(Invited) Epitaxial III-V Nanomaterials on the Si Platform for Quantum and Data Communications

As Si-based integrated photonic components begin to replace older technologies for optical data communications, the need for better integration of semiconductor lasers on the Si platform is greater than ever. In addition, emerging interest in integrated-photonic-based quantum computing and quantum sensing presents a further need for quantum light emitters—entangled and single-photon—on Si. III-V quantum dots represent a promising solution, both for the active region of telecommunications-wavelength lasers and for quantum emitters. However, epitaxial quantum dots grown on conventional (100) substrates typically lack the high symmetry required for entangled-photon emission.This presentation will discuss our recent work to develop a novel approach for III-V direct growth on Si via an ultrathin (<10 nm) single crystal strain relieving buffer layer, realized by the selective oxidation and condensation of an amorphous SiGe surface layer. The fabrication and properties of this novel Ge-rich buffer layer, as well as its use for subsequent III-V organometallic vapor-phase epitaxy will be presented.The second part of the talk will present our recent work on the epitaxial growth and characterization of (In,Ga)As quantum dots for the above-mentioned applications. Specifically, I will show that surface-energy-modifying surfactants (Sb, Bi) present the possibility to externally direct quantum dot formation “on-demand” in strained InAs layers on GaAs(111) substrates, where quantum dot formation by the Stranski–Krastanov process does not naturally occur. These nanostructures are prospective for high-symmetry entangled-photon emitters. The three-dimensional (3D) composition profile of InAs quantum dot layers is characterized by atom-probe microscopy (APT), elucidating the real-world quantum dot matrix. This 3D composition data is input into a finite element solver to determine the quantum dot energies and wavefunctions for electrons and holes. The modeling reveals that in high-density quantum dot layers, the electron and hole states are hybridized between multiple quantum dots in the matrix. This has important consequences for design of quantum dot light emitters. The predicted transition energies are in agreement with low-temperature photoluminescence. This work constitutes a promising approach for integrating telecommunications-wavelength quantum dot lasers and entangled photon emitters on the Si integrated photonics platform.

(Invited) Electronic Trap Characterization in QD Devices Using Impedance Analysis Techniques

Colloidal quantum dots (QDs) are semiconductor nanocrystals with dimensions in the range of tens of nanometers. They exhibit unique optical and electronic properties, such as a size-dependent band gap and high absorption cross-section. These properties make them attractive for optoelectronic applications such as photovoltaics, light-emitting diodes (LEDs), and photodetectors. However, QD optoelectronics pose significant challenges due to trap states, which mainly originate from the surface states of nanocrystals. Therefore, it is necessary to analyze the trap states of QDs to understand their electronic properties and optimize their performance in optoelectronic applications. With this understanding, we want to correlate the cause of the trap states with the chemical defect to design better passivation strategies.In this discussion, we will explore the principles of electronic trap analysis in colloidal quantum dots based on impedance analysis techniques as a complementary method to spectroscopy. We will provide examples of electronic trap analysis in PbS QD photovoltaics and InP QD LEDs to demonstrate the effectiveness of this approach. We can electrically probe the QD layers in the device using an impedance analyzer to analyze the energy level and density of trap states. By combining chemical analysis and a theoretical approach with this technique, we attempted to correlate the trap states and chemical defects.

Quantum Dot-Based Three-Stack Tandem Near-Infrared-to-Visible Optoelectric Upconversion Devices.

Quantum dots (QDs) exhibit size-tunable optical properties, making them suitable for efficient light-sensing and light-emitting devices. Tandem devices that can convert near-infrared (NIR) to visible (Vis) signals can be fabricated by integrating an NIR-sensing QD device with a Vis electroluminescence (EL) QD device. However, these devices require delicate control of the QD layer during processing to prevent damage to the predeposited QD layers in tandem devices during the subsequent deposition of other functional layers. This has restricted attainable device structures for QD-based upconversion devices. Herein, we present a modular approach for fabricating QD-based optoelectric upconversion devices. This approach involves using NIR QD-absorbing (Abs) and Vis QD-EL units as building modules, both of which feature cross-linked functional layers that exhibit structural tolerance to dissolution during subsequent solution-based processes. Tandem devices are fabricated in both normal (EL unit on Abs unit) and inverted (Abs unit on EL unit) structures using the same set of NIR QD-Abs and Vis QD-EL units stacked in opposite sequences. The tandem device in the normal structure exhibits a high NIR photon-to-Vis-photon conversion efficiency of up to 1.9% in a practical transmissive mode. By extending our modular approach, we also demonstrate a three-stack tandem device that incorporates a single NIR-absorbing unit coupled with two EL units, achieving an even higher conversion efficiency of up to 3.2%.

The Effect of Quantum Dots on the Performance of the Solar Cell

Quantum dot solar cells are currently the subject of research in the fields of renewable energy, photovoltaics and optoelectronics, due to their advantages which enables them to overcome the limitations of traditional solar cells. The inability of ordinary solar cells to generate charge carriers, which is prevents them from contributing to generate the current in solar cells. This work focuses on modeling and simulating of Quantum Dot Solar Cells based on InAs/GaAs as well as regular type of GaAs p-i-n solar cells and to study the effect of increasing quantum dots layers at the performance of the solar cell. The low energy of the fell photons considers as one of the most difficult problems that must deal with. According to simulation data, the power conversion efficiency increases from (12.515% to 30.94%), current density rises from 16.4047 mA/cm2 for standard solar cell to 39.4775 mA/cm2) using quantum dot techniques (20-layers) compared to traditional type of GaAs solar cell. Additionally, low energy photons' absorption range edge expanded from (400 to 900 nm) for quantum technique. The results have been modeled and simulated using (SILVACO Software), which proved the power conversion efficiency of InAs/GaAs quantum dot solar cells is significantly higher than traditional (p-i-n) type about (247%)

Photoluminescence of dense arrays of InGaPAs/InGaAs quantum dots formed by substitution of group V elements

One drawback of self-organized quantum dots is their low optical gain. The development of new shaping methods that would allow higher gain, for example, due to a higher surface density of the QD array, remains an important task to date. In the present work, arrays of quantum dots have been formed by substituting phosphorous atoms with arsenic ones in a thin InGaP epilayer. Based on transmission electron microscopy data, the surface density was estimated to be about 1.3 × 1012 cm−2, which is one of the highest values for quantum dots on GaAs. The influence of an additional InGaAs epilayer (quantum well) on the luminescence properties of quantum dots was investigated in a wide temperature range and different optical pumping levels. It was found that placing the quantum well under the layer of quantum dots has little effect on the central emission wavelength, while quantum dots covered with the InGaAs demonstrate a strong red shift of the luminescence spectrum. A transition from a nonequilibrium to an equilibrium distribution of charge carriers over quantum-dot states, related to the lateral transport of charge carriers over quantum dot array, similar to that previously observed in In(Ga)As Stranski-Krastanow quantum dots, was revealed. This is manifested in a change in temperature dependencies of the linewidth and peak position of the photoluminescence band. For the structures under study, this transition occurs at a temperature of about 125 K. Rapid thermal annealing of the structures allows to increase the integrated photoluminescence intensity of quantum dots up to 4 times, while the maximum of the spectrum remains almost unchanged, provided that the annealing temperature does not exceed 620 °C.