Colloidal Quantum Dot Films Research Articles

Single-Mode Lasing from a Monolithic Microcavity with Few-Monolayer-Thick Quantum Dot Films

We monolithically fabricated vertical cavity lasers with densely packed colloidal quantum dot films and demonstrated single-mode lasing operation using nanosecond optical excitation. Due to the accurate spectral and spatial alignment of the cavity optical modes and the quantum dot gains, in addition to the high quality factors of our devices, we observed lasing from a colloidal quantum dot gain medium only 35 nm thick (6 or 7 monolayers thick) with a threshold of 20 mJ/cm2. We fabricated a laser with more numerous quantum dot layers as well, which exhibited a lower lasing threshold of 9 mJ/cm2 due to the enhanced modal gain. This work represents an important step toward fabricating monolithic colloidal quantum dot lasers for practical photonic devices.

Advancing colloidal quantum dot photovoltaic technology

Abstract Colloidal quantum dots (CQDs) are attractive materials for solar cells due to their low cost, ease of fabrication and spectral tunability. Progress in CQD photovoltaic technology over the past decade has resulted in power conversion efficiencies approaching 10%. In this review, we give an overview of this progress, and discuss limiting mechanisms and paths for future improvement in CQD solar cell technology.We briefly summarize nanoparticle synthesis and film processing methods and evaluate the optoelectronic properties of CQD films, including the crucial role that surface ligands play in materials performance. We give an overview of device architecture engineering in CQD solar cells. The compromise between carrier extraction and photon absorption in CQD photovoltaics is analyzed along with different strategies for overcoming this trade-off. We then focus on recent advances in absorption enhancement through innovative device design and the use of nanophotonics. Several light-trapping schemes, which have resulted in large increases in cell photocurrent, are described in detail. In particular, integrating plasmonic elements into CQD devices has emerged as a promising approach to enhance photon absorption through both near-field coupling and far-field scattering effects. We also discuss strategies for overcoming the single junction efficiency limits in CQD solar cells, including tandem architectures, multiple exciton generation and hybrid materials schemes. Finally, we offer a perspective on future directions for the field and the most promising paths for achieving higher device efficiencies.

Highly efficient quantum dot near-infrared light-emitting diodes

Embedding quantum dots in a perovskite matrix yields efficient light emitters. Colloidal quantum dots (CQDs) are emerging as promising materials for constructing infrared sources in view of their tunable luminescence, high quantum efficiency and compatibility with solution processing1. However, CQD films available today suffer from a compromise between luminescence efficiency and charge transport, and this leads to unacceptably high power consumption. Here, we overcome this issue by embedding CQDs in a high-mobility hybrid perovskite matrix. The new composite enhances radiative recombination in the dots by preventing transport-assisted trapping losses; yet does so without increasing the turn-on voltage. Through compositional engineering of the mixed halide matrix, we achieve a record electroluminescence power conversion efficiency of 4.9%. This surpasses the performance of previously reported CQD near-infrared devices two-fold, indicating great potential for this hybrid QD-in-perovskite approach.

Graphene Doping Improved Device Performance of ZnMgO/PbS Colloidal Quantum Dot Photovoltaics

Lead sulfide (PbS) colloidal quantum dots (CQDs) solar cells possess the advantages of absorption into the infrared, solution processing, and multiple exciton generation, making them very competitive as a low‐cost photovoltaic alternative. Employing an n‐i‐p ZnO/tetrabutylammonium (TBAI)–PbS/ethanedithiol (EDT)–PbS device configuration, the present study reports a 9.0% photovoltaic device through ZnMgO electrode engineering and graphene doping. Sol–gel‐derived Zn0.9Mg0.1O buffer layer shows better transparency and higher conduction band maximum than ZnO, and incorporation of graphene and chlorinated graphene oxide into the TBAI–PbS and EDT–PbS layer respectively boosts carrier collection, leading to device with significantly enhanced open circuit voltage and short‐circuit current density. It is believed that incorporation of graphene into PbS CQD film as proposed here, and more generally nanosheets of other materials, would potentially open a simple and powerful avenue to overcome the carrier transport bottleneck of CQD optoelectronic device, thus pushing device performance to a new level.

Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz.

Infrared thermal imaging devices rely on narrow band gap semiconductors grown by physical methods such as molecular beam epitaxy and chemical vapor deposition. These technologies are expensive, and infrared detectors remain limited to defense and scientific applications. Colloidal quantum dots (QDs) offer a low cost alternative to infrared detector by combining inexpensive synthesis and an ease of processing, but their performances are so far limited, in terms of both wavelength and sensitivity. Herein we propose a new generation of colloidal QD-based photodetectors, which demonstrate detectivity improved by 2 orders of magnitude, and optical absorption that can be continuously tuned between 3 and 20 μm. These photodetectors are based on the novel synthesis of n-doped HgSe colloidal QDs whose size can be tuned continuously between 5 and 40 nm, and on their assembly into solid nanocrystal films with mobilities that can reach up to 100 cm(2) V(-1) s(-1). These devices can be operated at room temperature with the same level of performance as the previous generation of devices when operated at liquid nitrogen temperature. HgSe QDs can be synthesized in large scale (>10 g per batch), and we show that HgSe films can be processed to form a large scale array of pixels. Taken together, these results pave the way for the development of the next generation mid- and far-infrared low-cost detectors and camera.

Background limited mid-infrared photodetection with photovoltaic HgTe colloidal quantum dots

The photovoltaic response of thin films of HgTe colloidal quantum dots in the 3–5 μm range is observed. With no applied bias, internal quantum efficiency exceeding 40%, specific detectivity above 1010 Jones and microseconds response times are obtained at 140 K. The cooled devices detect the ambient thermal radiation. A detector with 5.25 μm cut-off achieves Background Limited Infrared Photodetection at 90 K.

Plasmon-mediated emergence of collective emission and enhanced quantum efficiency in quantum dot films

We present experimental and theoretical results on monolayer colloidal cadmium selenide quantum dot films embedded with tiny gold nanoparticles. By varying the density of the embedded gold nanoparticles, we were able to engineer a plasmon-mediated crossover from emission quenching to enhancement regime at interparticle distances for which only quenching of emission is expected. This crossover and a nonmonotonic variation of photoluminescence intensity and decay rate, in experiments, is explained in terms of a model for plasmon-mediated collective emission of quantum emitters which points to the emergence of a new regime in plasmon-exciton interactions. The presented methodology to achieve enhancement in optical quantum efficiency for optimal doping of gold nanoparticles in such ultrathin high-density quantum dot films can be beneficial for new-generation displays and photodetectors.

Microsecond-sustained lasing from colloidal quantum dot solids

Colloidal quantum dots have grown in interest as materials for light amplification and lasing in view of their bright photoluminescence, convenient solution processing and size-controlled spectral tunability. To date, lasing in colloidal quantum dot solids has been limited to the nanosecond temporal regime, curtailing their application in systems that require more sustained emission. Here we find that the chief cause of nanosecond-only operation has been thermal runaway: the combination of rapid heat injection from the pump source, poor heat removal and a highly temperature-dependent threshold. We show microsecond-sustained lasing, achieved by placing ultra-compact colloidal quantum dot films on a thermally conductive substrate, the combination of which minimizes heat accumulation. Specifically, we employ inorganic-halide-capped quantum dots that exhibit high modal gain (1,200 cm−1) and an ultralow amplified spontaneous emission threshold (average peak power of ∼50 kW cm−2) and rely on an optical structure that dissipates heat while offering minimal modal loss.

Nanoscale and Single-Dot Patterning of Colloidal Quantum Dots.

Using an optimized lift-off process we develop a technique for both nanoscale and single-dot patterning of colloidal quantum dot films, demonstrating feature sizes down to ~30 nm for uniform films and a yield of 40% for single-dot positioning, which is in good agreement with a newly developed theoretical model. While first of all presenting a unique tool for studying physics of single quantum dots, the process also provides a pathway toward practical quantum dot-based optoelectronic devices.

The Silicon:Colloidal Quantum Dot Heterojunction.

A heterojunction between crystalline silicon and colloidal quantum dots (CQDs) is realized. A special interface modification is developed to overcome an inherent energetic band mismatch between the two semiconductors, and realize the efficient collection of infrared photocarriers generated in the CQD film. This junction is used to produce a sensitive near infrared photodetector.

Identifying and Eliminating Emissive Sub-bandgap States in Thin Films of PbS Nanocrystals.

Chemical oxidation of under-charged Pb atoms reduces the density of trap states by a factor of 40 in films of colloidal PbS quantum dots for devices. These emissive sub-bandgap states are a byproduct of several standard ligand-exchange procedures. X-ray photoelectron spectro-scopy measurements and density function theory simulations demonstrate that they are associated with under-charged Pb.

Magnetoresistance of Manganese-Doped Colloidal Quantum Dot Films

The magnetoresistance of films of manganese-doped colloidal quantum dots of CdSe, ZnO, HgS, and ZnTe is investigated. At low concentration of manganese ions (1% or less), the hyperfine splitting of the Mn2+ electron spin resonance is resolved and similar to that of the bulk doped materials, indicating successful doping into the nanocrystals. At high Mn concentration (∼10%), the hyperfine splitting disappears because of interaction between the Mn2+ ions. Thin films of Mn:CdSe, Mn:ZnO, and Mn:HgS quantum dots are charged negative by applying an electrochemical potential, and the magnetoresistance is measured down to 2 K and up to 9 T. At low charging level, the magnetoresistance of thin films is positive, exhibits little effect of the manganese dopant, and is instead consistent with predictions from the variable range hopping model and the squeezing of the wave function of the quantum dots. At high charging level, the magnetoresistance becomes linear both for Mn:CdSe and Mn:ZnO, and this is not explained. A...

Stability and aging studies of lead sulfide quantum dot films: Photoabsorption, morphology, and chemical state characteristics

The stability of colloidal PbS quantum dot (QD) films deposited on glass substrates was studied over a short term and a long term periods, when left unprotected in air under ambient conditions. Two deposition approaches were used, traditional solvent deposition method (SDM) and supercritical fluid CO2 deposition (SFD). To track possible changes to the QDs over time, near infrared (NIR) spectrometry, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and optical microscopy, were employed. Evidence points towards initial oxidation-induced shrinking of the active QD volume causing a blue shift of the absorption over the first four months, and then experiencing competition from oxidation and sintering/ripening of the QDs, resulting in a red shift. The presented studies have important consequences for the long term reliability of light emitters based on PbS QDs.

Photojunction field-effect transistor based on a colloidal quantum dot absorber channel layer.

The performance of photodetectors is judged via high responsivity, fast speed of response, and low background current. Many previously reported photodetectors based on size-tuned colloidal quantum dots (CQDs) have relied either on photodiodes, which, since they are primary photocarrier devices, lack gain; or photoconductors, which provide gain but at the expense of slow response (due to delayed charge carrier escape from sensitizing centers) and an inherent dark current vs responsivity trade-off. Here we report a photojunction field-effect transistor (photoJFET), which provides gain while breaking prior photoconductors' response/speed/dark current trade-off. This is achieved by ensuring that, in the dark, the channel is fully depleted due to a rectifying junction between a deep-work-function transparent conductive top contact (MoO3) and a moderately n-type CQD film (iodine treated PbS CQDs). We characterize the rectifying behavior of the junction and the linearity of the channel characteristics under illumination, and we observe a 10 μs rise time, a record for a gain-providing, low-dark-current CQD photodetector. We prove, using an analytical model validated using experimental measurements, that for a given response time the device provides a two-orders-of-magnitude improvement in photocurrent-to-dark-current ratio compared to photoconductors. The photoJFET, which relies on a junction gate-effect, enriches the growing family of CQD photosensitive transistors.

Magneto-optical controlled transmittance alteration of PbS quantum dots by moderately applied magnetic fields at room temperature

We observed changes of the transmitted monochromatic light passing through a colloidal PbS quantum dot film on glass owing to an applied moderate (smaller than 1 T) magnetic field under ambient conditions. The observed alterations show a square dependence on the magnetic field increase that cannot be achieved with bulk semiconductors. The findings point to so far not recognized application potentials of quantum dots.

Electronically active impurities in colloidal quantum dot solids.

Colloidal quantum dot films have seen rapid progress as active materials in photodetection, light emission, and photovoltaics. Their processing from the solution phase makes them an attractive option for these applications due to the expected cost reductions associated with liquid-phase material deposition. Colloidally stable nanoparticles capped using long, insulating aliphatic ligands are used to form semiconducting, insoluble films via a solid-state ligand exchange in which the original ligands are replaced with short bifunctional ligands. Here we show that this ligand exchange can have unintended and undesired side effects: a high molecular weight complex can form, containing both lead oleate and the shorter conductive ligand, and this poorly soluble complex can end up embedded within the colloidal quantum dot (CQD) active layer. We further show that, by adding an acidic treatment during film processing, we can break up and wash away these complexes, producing a higher quality CQD solid. The improved material leads to photovoltaic devices with reduced series resistance and enhanced fill factor relative to controls employing previously reported CQD solids.

Epitaxially connected PbSe quantum-dot films: controlled neck formation and optoelectronic properties.

Ligand exchange is a much-used method to increase the conductivity of colloidal quantum-dot films by replacing long insulating ligands on quantum-dot surfaces with shorter ones. Here we show that while some ligands indeed replace the original ones as expected, others may be used to controllably remove the native ligands and induce epitaxial necking of specific crystal facets. In particular, we demonstrate that amines strip lead oleate from the (100) surfaces of PbSe quantum dots. This leads to necking of QDs and results in cubic superlattices of epitaxially connected QDs. The number of amine head-groups as well as the carbon chain length of linear diamines is shown to control the extent of necking. DFT calculations show that removal of Pb(oleate)2 from (100) surfaces is exothermic for all amines, but the driving force increases as monoamines < long diamines < short diamines < tetramines. The neck formation and cubic ordering results in a higher optical absorption cross section and higher charge carrier mobilities, thereby showing that the use of the proper multidentate amine molecules is a powerful tool to create supercrystals of epitaxially connected PbSe QDs with controlled electronic coupling.

Colloidal quantum dot solar cells on curved and flexible substrates

Colloidal quantum dots (CQDs) are semiconductor nanocrystals synthesized with, processed in, and deposited from the solution phase, potentially enabling low-cost, facile manufacture of solar cells. Unfortunately, CQD solar cell reports, until now, have only explored batch-processing methods—such as spin-coating—that offer limited capacity for scaling. Spray-coating could offer a means of producing uniform colloidal quantum dot films that yield high-quality devices. Here, we explore the versatility of the spray-coating method by producing CQD solar cells in a variety of previously unexplored substrate arrangements. The potential transferability of the spray-coating method to a roll-to-roll manufacturing process was tested by spray-coating the CQD active layer onto six substrates mounted on a rapidly rotating drum, yielding devices with an average power conversion efficiency of 6.7%. We further tested the manufacturability of the process by endeavoring to spray onto flexible substrates, only to find that spraying while the substrate was flexed was crucial to achieving champion performance of 7.2% without compromise to open-circuit voltage. Having deposited onto a substrate with one axis of curvature, we then built our CQD solar cells onto a spherical lens substrate having two axes of curvature resulting in a 5% efficient device. These results show that CQDs deposited using our spraying method can be integrated to large-area manufacturing processes and can be used to make solar cells on unconventional shapes.

Synergetic Effect of Silver Nanocrystals Applied in PbS Colloidal Quantum Dots for High-Performance Infrared Photodetectors

PbS colloidal quantum dot (CQD) photodetectors hold great potential for near-infrared detection due to their extremely high sensitivity and low-cost solution processing. In this paper we report that incorporation of 0.5% to 1% (by weight) Ag nanocrystals (NCs) into the PbS CQDs film could simultaneously enhance the photocurrent and suppress dark current and hence significantly boost device detectivity. A set of control experiments suggested that Ag NCs, once added to the PbS CQD film, could trap photogenerated electrons from neighboring PbS CQDs, extend carrier lifetime, and increase photocurrent. We further built a sensitive flexible photodetector using the optimized composite on stone paper, achieving an estimated detectivity as high as 1.5 × 1010 Jones. The synergetic effect found in our PbS CQD/Ag NC composite photodetectors is expected to be extendable to other binary NC systems for various applications.

Surface-emitting red, green, and blue colloidal quantum dot distributed feedback lasers.

We demonstrate surface emitting distributed feedback (DFB) lasers across the red, green, and blue from densely packed colloidal quantum dot (CQD) films. The solid CQD films were deposited on periodic grating patterns to enable 2nd-order DFB lasing action at mere 120, 280, and 330 μJ/cm2 of optical pumping energy densities for red, green, and blue DFB lasers, respectively. The lasers operated in single mode operation with less than 1 nm of full-width-half-maximum. We measured far-field patterns showing high degree of spatial beam coherence. Specifically, by taking advantage of single exciton optical gain regime from our engineered CQDs, we can significantly suppress the Auger recombination to reduce lasing threshold and achieve quasi-steady state, optically pumped operation.