Solution-processed Quantum Dot Research Articles

Solution-processed quantum dot short-wavelength infrared photodetectors via integrating 2D MXenes into electron transport layer

Solution-processed quantum dots (QDs) have attracted intensive attention in short-wavelength infrared (SWIR) photodetectors due to their excellent optical properties and tunable size-dependent bandgap. However, the intrinsic defects of the widely used ZnO electron-transport layer (ETL) and the inappropriate band alignment at the interface between the ETL and photoactive QDs inevitably impact the optoelectronic performance of the photodetector. Herein, a high-performance self-powered SWIR photodetector is developed by using 2D Ti3C2 MXenes to passivate the defects of ZnO ETL and to tailor the band alignment at the interface of the ETL/photoactive layer through interface engineering. As a result, the SWIR photodetectors demonstrate the ultra-high light-to-dark current (Ilight/Idark) ratio of ∼ 105 and the low dark current density of 8.33 × 10-4 mA/cm2, and the high specific detectivity (D*) of 1.99 × 1011 Jones at 0 V. By the systematic characterizations, it is proofed that 2D MXene can efficiently passivate defects in ZnO and optimize the alignment of energy levels between the ETL and photoactive layer, which will suppress the recombination of electrons and holes and eventually promote photogenerated carrier transfer at the interface and thus improve device performance. This work revealed that the high-performance solution-processed QD based SWIR photodetector enabled by interface engineering will promote the development of optoelectronic devices in the future.

Direct Optical Patterning of MoO3 Nanoparticles and Their Application as a Hole Injection Layer for Solution-Processed Quantum Dot Light-Emitting Diodes

Direct Optical Patterning of MoO₃ Nanoparticles and Their Application as a Hole Injection Layer for Solution-Processed Quantum Dot Light-Emitting Diodes

Record high external quantum efficiency of 20% achieved in fully solution-processed quantum dot light-emitting diodes based on hole-conductive metal oxides

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been widely used as a hole injection material in quantum dot (QD) light-emitting diodes (QLEDs). However, it degrades the organic materials and electrodes in QLEDs due to its strong hydroscopicity and acidity. Although hole-conductive metal oxides have a great potential to solve this disadvantage, it is still a challenge to achieve efficient and stable QLEDs by using these solution-processed metal oxides. Herein, the state-of-the-art QLEDs fabricated by using hole-conductive MoOx QDs are achieved. The α-phase MoOx QDs exhibit a monodispersed size distribution with clear and regular crystal lattices, corresponding to high-quality nanocrystals. Meanwhile, the MoOx film owns an excellent transmittance, suitable valence band, good morphology and impressive hole-conductivity, demonstrating that the MoOx film could be used as a hole injection layer in QLEDs. Moreover, the rigid and flexible red QLEDs made by MoOx exhibit peak external quantum efficiencies of over 20%, representing a new record for the hole-conductive metal oxide based QLEDs. Most importantly, the MoOx QDs afford their QLEDs with a longer T95 lifetime than these devices made by PEDOT:PSS. As a result, we believe that the MoOx QDs could be used as efficient and stable hole injection materials used in QLEDs.

Gallium Sulfide Quantum Dots with Zinc Sulfide and Alumina Shells Showing Efficient Deep Blue Emission.

Solution-processed quantum dot (QD) based blue emitters are of paramount importance in the field of optoelectronics. Despite large research efforts, examples of efficient deep blue/near UV-emitting QDs remain rare due to lack of luminescent wide band gap materials and high defect densities in the existing ones. Here, we introduce a novel type of QDs based on heavy metal free gallium sulfide (Ga2 S3 ) and their core/shell heterostructures Ga2 S3 /ZnS as well as Ga2 S3 /ZnS/Al2 O3 . The photoluminescence (PL) properties of core Ga2 S3 QDs exhibit various decay pathways due to intrinsic defects, resulting in a broad overall PL spectrum. We show that the overgrowth of the Ga2 S3 core QDs with a ZnS shell results in the suppression of the intrinsic defect-mediated states leading to efficient deep-blue emission at 400 nm. Passivation of the core/shell structure with amorphous alumina yields a further enhancement of the PL quantum yield approaching 50 % and leads to an excellent optical and colloidal stability. Finally, we develop a strategy for the aqueous phase transfer of the obtained QDs retaining 80 % of the initial fluorescence intensity.

Low-Temperature Cross-Linkable Hole Transport Materials for Solution-Processed Quantum Dot and Organic Light-Emitting Diodes with High Efficiency and Color Purity.

Cross-linkable hole transport materials (HTMs) are ideal for improving the performance of solution-processed quantum dot light-emitting diodes (QLEDs) and phosphorescent light-emitting diodes (OLEDs). However, previously developed cross-linkable HTMs possessed poor hole transport properties, high cross-linking temperatures, and long curing times. To achieve efficient cross-linkable HTMs with high mobility, low cross-linking temperature, and short curing time, we designed and synthesized a series of low-temperature cross-linkable HTMs comprising dibenzofuran (DBF) and 4-divinyltriphenylamine (TPA) segments for highly efficient solution-processed QLEDs and OLEDs. The introduction of divinyl-functionalized TPA in various positions of the DBF core remarkably affected their chemical, physical, and electrochemical properties. In particular, cross-linked 4-(dibenzo[b,d]furan-3-yl)-N,N-bis(4-vinylphenyl)aniline (3-CDTPA) exhibited a deep highest occupied molecular orbital energy level (5.50 eV), high hole mobility (2.44 × 10-4 cm2 V-1 s-1), low cross-linking temperature (150 °C), and short curing time (30 min). Furthermore, a green QLED with 3-CDTPA as the hole transport layer (HTL) exhibited a notable maximum external quantum efficiency (EQEmax) of 18.59% with a remarkable maximum current efficiency (CEmax) of 78.48 cd A-1. In addition, solution-processed green OLEDs with 3-CDTPA showed excellent device performance with an EQEmax of 15.61%, a CEmax of 52.51 cd A-1, and outstanding CIE(x, y) color coordinates of (0.29, 0.61). This is one of the highest reported EQEs and CEs with high color purity for green solution-processed QLEDs and OLEDs using a divinyl-functionalized cross-linked HTM as the HTL. We believe that this study provides a new strategy for designing and synthesizing practical cross-linakable HTMs with enhanced performance for highly efficient solution-processed QLEDs and OLEDs.

Inverted Solution-Processed Quantum Dot Light-Emitting Devices with Wide Band Gap Quantum Dot Interlayers.

Despite its benefits for facilitating device fabrication, utilization of a polymeric hole transport layer (HTL) in inverted quantum dots (QDs) light-emitting devices (IQLEDs) often leads to poor device performance. In this work, we find that the poor performance arises primarily from electron leakage, inefficient charge injection, and significant exciton quenching at the HTL interface in the inverted architecture and not due to solvent damage effects as is widely believed. We also find that using a layer of wider band gap QDs as an interlayer (IL) in between the HTL and the main QDs' emission material layer (EML) can facilitate hole injection, suppress electron leakage, and reduce exciton quenching, effectively mitigating the poor interface effects and resulting in high electroluminescence performance. Using an IL in IQLEDs with a solution-processed poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB), HTL improves the efficiency by 2.85× (from 3 to 8.56%) and prolongs the lifetime by 9.4× (from 1266 to 11,950 h at 100 cd/m2), which, to the best of our knowledge, is the longest lifetime for an R-IQLED with a solution-coated HTL. Measurements on single-carrier devices reveal that while electron injection becomes easier as the band gap of the QDs decreases, hole injection surprisingly becomes more difficult, indicating that EMLs of QLEDs are more electron-rich in the case of red devices and more hole-rich in the case of blue devices. Ultraviolet photoelectron spectroscopy measurements verify that blue QDs have a shallower valence band energy than their red counterparts, corroborating these conclusions. The findings in this work, therefore, provide not only a simple approach for achieving high performance in IQLEDs with solution-coated HTLs but also novel insights into charge injection and its dependence on QDs' band gap as well as into different HTL interface properties of the inverted versus upright architecture.

Vertical Architecture Solution-Processed Quantum Dot Photodetectors with Amorphous Selenium Hole Transport Layer

Colloidal quantum dots (CQDs) provide wide spectral tunability and high absorption coefficients owing to quantum confinement and large oscillator strengths, which along with solution processability, allow a facile, low-cost, and room-temperature deposition technique for the fabrication of photonic devices. However, many solution-processed CQD photodetector devices demonstrate low specific-detectivity and slow temporal response. To achieve improved photodetector characteristics, limiting carrier recombination and enhancing photogenerated carrier separation are crucial. In this study, we develop and present an alternate vertical-stack photodetector wherein we use a solution-processed quantum dot photoconversion layer coupled to an amorphous selenium (a-Se) wide-bandgap charge transport layer that is capable of exhibiting single-carrier hole impact ionization and is compatible with active-matrix readout circuitry. This a-Se chalcogenide transport layer enables the fabrication of high-performance and reliable solution-processed quantum dot photodetectors, with enhanced charge extraction capabilities, high specific detectivity (D* ∼ 0.5–5 × 1012 Jones), fast 3 dB electrical bandwidth (3 dB BW ∼ 22 MHz), low dark current density (JD ∼ 5–10 pA/cm2), low noise current (in ∼ 20–25 fW/Hz1/2), and high linear dynamic range (LDR ∼ 130–150 dB) across the measured visible electromagnetic spectrum (∼405–656 nm).

High-speed and high-contrast two-channel all-optical modulator based on solution-processed CdSe/ZnS quantum dots

Recently, all-optical modulators are potentially the most promising candidate to achieve high-bit rate modulation in high-speed all-optical communication technologies and signal processing. In this study, a two-channel all-optical modulator based on a solution-processed quantum dot structure is introduced for two sizes of quantum dots to operate at two wavelengths of MIR spectra (3 µm and 5 µm). To perform numerical and theoretical analysis and evaluate the optical behavior of the proposed all-optical modulator, the coupled rate and propagation equations have been solved by considering homogeneous and inhomogeneous broadening effects. The modulation depth at the 50 GHz frequency and 3 mW probe power is attained, about 94% for channel-1 with the wavelength of 559 nm at 300 Wcm−2 pump power density as well as approximately 83.5% for channel-2 with the wavelength of 619 nm at 500 Wcm−2 pump power density. The introduced two-channel all-optical modulator can operate simultaneously at two wavelengths during the modulation process in which information could be transmitted through both signals from the control light. This approach can present the practical device as a high-contrast and high-speed two-channel all-optical modulator with a high modulation depth in numerous applications such as thermal imaging in night vision cameras, wavelength de-multiplexing, signal processing, free-space communication.

Solution-processed quantum dot SnO2 as an interfacial electron transporter for stable fully-air-fabricated metal-free perovskite solar cells

Perovskite solar cells could strongly compete with the silicon solar cells in the market soon as illustrated in recent studies. In this work, promising and stable metal-free perovskite solar cells (PSCs) has been successfully fabricated using an inorganic SnO2/Quantum dot SnO2 (QD-SnO2) double layer as an efficient electron transport layer via a low-temperature solution process. The fully-air fabricated PSCs in the form of FTO/SnO2/QD-SnO2/CH3NH3PbI3/Carbon were tested at different annealed QD-SnO2 between 300 and 500 °C. The as-prepared QD-SnO2 and the fabricated devices are characterized by various techniques, including XRD, XPS, HR-TEM, FE-SEM, UV–vis–NIR spectroscopy, PL, and solar simulator. The prepared QD-SnO2 at 300 °C has shown well-ordered nanoparticles of 5.6 nm in diameter with superior carrier density (1.5 × 1015 cm−3) and highest carrier mobility (64.1 cm2·V-1·s−1), accelerating the carriers separation process within the cell. The best devices demonstrated a maximum power conversion efficiency (PCE) of 11.7%, VOC 0.81 V, JSC 19.5 mA·cm−2, and FF 74%. The presence of an interfacial layer of QD-SnO2 over the blocking SnO2 upsurges the band gaps alignment and accelerates the carriers extraction rate affecting the performance of the fabricated perovskite devices. Moreover, the optimized fabricated devices revealed a shelf stability-life of four months in humid air (40%–50%) with >83% of its initial PCE. This simple synthetic approach can develop the opportunities to transfer the cell from the lab to the market, which will be compatible with large-scale production.

Improving hole injection ability using a newly proposed WO3/NiOx bilayer in solution processed quantum dot light-emitting diodes

We propose a novel device structure with a WO3/NiOx bilayer to improve the hole injection ability in QLEDs fabricated mainly by a solution-based process. First, we employed a spin-coated NiOx thin film as a hole injection layer (HIL) to replace Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) PEDOT:PSS which corrodes indium tin oxide (ITO) used as an anode in QLEDs. We showed a simply optimized annealing process, instead of a rather complicated process like doping, can improve the electrical conductivity of the NiOx thin film. The reason for the dependency of conductivity on the post-metallization annealing is because of the change of the total amount of Ni vacancy in the NiOx thin film as a function of annealing temperature: The electrical conductivity of the NiOx thin film annealed at 275 °C was the best in this work. Second, we inserted the WO3 thin film in between ITO electrode and NiOx HIL to form an ITO/WO3/NiOx structure which reduces the hole injection barrier to 0.35 eV, resulting in the excellent characteristic in view of charge balance. Finally, we measured the properties of QLEDs with the WO3/NiOx bilayer to check the effects of the proposed device structure and showed the substantial improvement of the electrical conductivity of NiOx, the luminance, and the current efficiency of the QLEDs.

Simple strategies deployed for developing efficient and stable solution processed quantum dot solar cells

Photoanode passivation, electrolyte additives and electrocatalytic and high surface area counter electrodes control the liquid junction quantum dot solar cell (QDSC) performance.

Fast-response, high-stability, and high-efficiency full-color quantum dot light-emitting diodes with charge storage layer

Recently, solution-processed quantum dot light-emitting diodes (QLEDs) have emerged as a promising candidate for next-generation lighting and display devices. However, when given a constant voltage or current, the QLEDs need a certain working time to reach their maximum brightness. Such positive aging challenge, dramatically reducing the response speed of the device and causing a luminescence delay, is urgent to be investigated and resolved. In the current work, we introduce a charge-storage layer architecture by inserting copper(I) thiocyanate (CuSCN) between the organic hole-injection layer and hole-transport layer. The extracted holes will be released during the next electrical signal stimulation to increase the efficiency of charge transport. As a result, the response speed of the QLEDs is improved by an order of magnitude. In addition, by inserting an inorganic CuSCN layer, the efficiency, lifetime, and environmental stability of red/green/blue full-color QLEDs are enhanced simultaneously. Moreover, this work provides a generic strategy for the fabrication of fast-response and high-efficiency full-color QLEDs without luminescence delay, which plays a critical role in the practical industrialization of QLEDs.

Selective Energy Contacts and Multi-Wavelength Solution-Processed Quantum Dot Infrared Photodetector

In this work, a three-channel infrared photodetector with three independent related electric signals as a single optoelectronic chip has been introduced. The proposed structure has been implemented by exploiting an array of PbSe/ZnSe quantum dots and the PbSe/ZnS selective energy contacts considering solution-processed nanotechnology. The electron transport system has been designed to allow the transmission of electrons stimulated by three wavelengths simultaneously from different paths to the corresponding output. The high efficiency, simplicity and fabrication ease, cost-effectiveness, and high tunability of the photodetector have been accomplished as a consequence of solution-process technology. Moreover, the dark current has been significantly declined as a consequence of resonance tunneling in the proposed selective energy contacts. Last but not least, our proposed multi-wavelength ( λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> =2.9 μm, λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> =2.25 μm, λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> =1.75 μm) infrared photodetector has been promising candidates for a wideband spectrum analyzer or multi-wavelength infrared camera.

Comprehensive Performance Calibration Guidance for Perovskites and Other Emerging Solar Cells

AbstractEmerging photovoltaic (PV) technologies (e.g., organic, perovskite, and solution processed quantum dot) have attracted remarkable attention with the rapid growth of their efficiencies, and their transition toward commercialization. Accurate and reliable efficiency measurements of these PV technologies are crucial, yet much more complicated than for conventional PV technologies due to the former's pronounced dynamic responses to changes in measurement conditions (e.g., current–voltage (I–V) scan rate and preconditioning) and their susceptibility to degradation. Adjustments to the measurement procedures are therefore necessary so that a reproducible “steady state” is reached during measurement. Furthermore, given the small size of many emerging cells, inappropriate device area definition and solar simulator setup can lead to measurement errors. Here, comprehensive efficiency calibration guidance is offered for emerging solar cells, including area measurement; spectral irradiance translation to standard test conditions; and steady‐state electrical performance. The necessity of reporting steady‐state efficiency is justified with a statistical performance comparison between conventional and steady‐state I–V scans over hundreds of cells the group has received globally for efficiency certifications. The procedures described here do not require specialized measurement instrumentation; what matters most are changes to the measurement protocols. These described changes aim to enable better comparisons between reported efficiencies.

Quantitative Electrochemical Control over Optical Gain in Quantum-Dot Solids.

Solution-processed quantum dot (QD) lasers are one of the holy grails of nanoscience. They are not yet commercialized because the lasing threshold is too high: one needs >1 exciton per QD, which is difficult to achieve because of fast nonradiative Auger recombination. The threshold can, however, be reduced by electronic doping of the QDs, which decreases the absorption near the band-edge, such that the stimulated emission (SE) can easily outcompete absorption. Here, we show that by electrochemically doping films of CdSe/CdS/ZnS QDs, we achieve quantitative control over the gain threshold. We obtain stable and reversible doping of more than two electrons per QD. We quantify the gain threshold and the charge carrier dynamics using ultrafast spectroelectrochemistry and achieve quantitative agreement between experiments and theory, including a vanishingly low gain threshold for doubly doped QDs. Over a range of wavelengths with appreciable gain coefficients, the gain thresholds reach record-low values of ∼1 × 10–5 excitons per QD. These results demonstrate a high level of control over the gain threshold in doped QD solids, opening a new route for the creation of cheap, solution-processable, low-threshold QD lasers.

High-performance full-solution-processed quantum dot light-emitting diodes with a doped hole injection layer

<p indent="0mm">Full solution-processed quantum dot light-emitting diodes are the potential ones for future flat panel displays and solid state lighting. The imbalance of holes and electrons due to the larger hole injection barrier is still the critical obstacle for full solution-processed quantum dot light-emitting diodes with high performance. Therefore, improving the hole injection level is the first consideration for us. In this paper, we doped graphene oxide into PEDOT:PSS (volume ratio: 1:2) to form hole injection layer PEDOT:PSS-GO, and then fabricated the high-efficiency full solution-processed quantum dot light-emitting diodes with inverted architecture: ITO/ZnMgO/QDs/PVK/PEDOT:PSS-GO/Al. The maximum brightness of <sc>51392 cd/m²,</sc> and peak current efficiency of <sc>7.60 cd/A</sc> were achieved, showing about 1.3 fold improvement compared to the reference ones. Meanwhile, the conventional ones: ITO/PEDOT:PSS-GO/PVK/QDs/ZnMgO/Al, also fabricated by the same mean, exhibited 0.29 fold increase in maximum brightness. The above results demonstrate that the introduction of GO is beneficial for both normal and inverted full solution-processed quantum dot light-emitting diodes, and is a more effective method for high-performance full solution-processed quantum dot light-emitting diode with inverted structure.

Horizontally Oriented Exciton Dipoles in Solution-Processed Quantum Dot Solids

Horizontally Oriented Exciton Dipoles in Solution-Processed Quantum Dot Solids

Horizontally Oriented Exciton Dipoles in Solution-Processed Quantum Dot Solids

Horizontally Oriented Exciton Dipoles in Solution-Processed Quantum Dot Solids

Multi-wavelength solution-processed quantum dot laser

In this paper, the multi-wavelength solution-processed quantum dot laser has been proposed by the superimposed quantum dots. To this end, the rate equation framework has been developed to model the simultaneous lasing in desired wavelengths by considering inhomogeneous broadening of energy level as a result of the size distribution of quantum dots and the homogeneous broadening due to the carries scattering inside quantum dots. Moreover, the solution process technology has been utilized in order to synthesize the quantum dots in various sizes according to the desired wavelengths. The simulation results show that simultaneous lasing has been realized by superimposition of two sizes of InGaAs quantum dots in a single cavity at two wavelengths of 1.31μm and 1.55μm which have special importance in telecommunication applications. Besides, the output power intensity at the desired wavelengths can be managed by controlling the density of quantum dots.

Enhancing PbS Colloidal Quantum Dot Tandem Solar Cell Performance by Graded Band Alignment.

Colloidal quantum dot solids are attractive candidates for tandem solar cells because of their widely tunable bandgaps. However, the development of the quantum dot tandem solar cell has lagged far behind that of its single-junction counterpart. One of the fundamental problems with colloidal quantum dot solar cells is the relatively small diffusion length, which limits the quantum dot absorbing layer thickness and hence the power conversion efficiency. In this research, guided by optical modeling and utilizing a graded band alignment strategy, a two-terminal monolithic solution-processed quantum dot tandem solar cell has been successfully fabricated and a power conversion efficiency of 6.8% has been achieved. The band grading approach utilized the complementary tuning of work functions and band alignment through judicious choices of the nanoparticle surface chemistry and quantum dot confined size. This work demonstrates a general approach to improving the efficiency for tandem thin-film solar cells.