Reduced-dimensional perovskites are attractive light-emitting materials due to their efficient luminescence, color purity, tunable bandgap, and structural diversity. A major limitation in perovskite ...light-emitting diodes is their limited operational stability. Here we demonstrate that rapid photodegradation arises from edge-initiated photooxidation, wherein oxidative attack is powered by photogenerated and electrically-injected carriers that diffuse to the nanoplatelet edges and produce superoxide. We report an edge-stabilization strategy wherein phosphine oxides passivate unsaturated lead sites during perovskite crystallization. With this approach, we synthesize reduced-dimensional perovskites that exhibit 97 ± 3% photoluminescence quantum yields and stabilities that exceed 300 h upon continuous illumination in an air ambient. We achieve green-emitting devices with a peak external quantum efficiency (EQE) of 14% at 1000 cd m
; their maximum luminance is 4.5 × 10
cd m
(corresponding to an EQE of 5%); and, at 4000 cd m
, they achieve an operational half-lifetime of 3.5 h.
Quantum dot (QD) solids are an emerging platform for developing a range of optoelectronic devices. Thus, understanding exciton dynamics is essential towards developing and optimizing QD devices. ...Here, using transient absorption microscopy, we reveal the initial exciton dynamics in QDs with femtosecond timescales. We observe high exciton diffusivity (~10
cm
s
) in lead chalcogenide QDs within the first few hundred femtoseconds after photoexcitation followed by a transition to a slower regime (~10
-1 cm
s
). QD solids with larger interdot distances exhibit higher initial diffusivity and a delayed transition to the slower regime, while higher QD packing density and heterogeneity accelerate this transition. The fast transport regime occurs only in materials with exciton Bohr radii much larger than the QD sizes, suggesting the transport of delocalized excitons in this regime and a transition to slower transport governed by exciton localization. These findings suggest routes to control the optoelectronic properties of QD solids.
Colloidal quantum dot (CQD) solar cells are solution-processed photovoltaics with broad spectral absorption tunability. Major advances in their efficiency have been made via improved CQD surface ...passivation and device architectures with enhanced charge carrier collection. Herein, we demonstrate a new strategy to improve further the passivation of CQDs starting from the solution phase. A cosolvent system is employed to tune the solvent polarity in order to achieve the solvation of methylammonium iodide (MAI) and the dispersion of hydrophobic PbS CQDs simultaneously in a homogeneous phase, otherwise not achieved in a single solvent. This process enables MAI to access the CQDs to confer improved passivation. This, in turn, allows for efficient charge extraction from a thicker photoactive layer device, leading to a certified solar cell power conversion efficiency of 10.6%, a new certified record in CQD photovoltaics.
Graphdiyne, a novel large π‐conjugated carbon hole transporting material, is employed as anode buffer layer in colloidal quantum dots solar cells. Power conversion efficiency is notably enhanced to ...10.64% from 9.49% compared to relevant reference devices. Hole transfer from the quantum dot solid active layer to the anode can be appreciably enhanced only by using graphdiyne to lower the work function of the colloidal quantum dot solid. It is found that the all‐carbon buffer layer prolongs the carrier lifetime, reducing surface recombination on the previously neglected back side of the photovoltaic device. Remarkably, the device also shows high long‐term stability in ambient air. The results demonstrate that graphdiyne may have diverse applications in enhancing optoelectronic devices.
The novel large π‐conjugated carbon hole transporting material, graphdiyne (GD) is used as anode buffer layer in PbS CQD solar cells. GD increases carrier lifetime and reduces carrier recombination inside the PbS CQD solar cells. Notably, a high power conversion efficiency of 10.64% is achieved, and the device also shows high long‐term stability in ambient air.
Bandtail states in disordered semiconductor materials result in losses in open-circuit voltage (V
) and inhibit carrier transport in photovoltaics. For colloidal quantum dot (CQD) films that promise ...low-cost, large-area, air-stable photovoltaics, bandtails are determined by CQD synthetic polydispersity and inhomogeneous aggregation during the ligand-exchange process. Here we introduce a new method for the synthesis of solution-phase ligand-exchanged CQD inks that enable a flat energy landscape and an advantageously high packing density. In the solid state, these materials exhibit a sharper bandtail and reduced energy funnelling compared with the previous best CQD thin films for photovoltaics. Consequently, we demonstrate solar cells with higher V
and more efficient charge injection into the electron acceptor, allowing the use of a closer-to-optimum bandgap to absorb more light. These enable the fabrication of CQD solar cells made via a solution-phase ligand exchange, with a certified power conversion efficiency of 11.28%. The devices are stable when stored in air, unencapsulated, for over 1,000 h.
Colloidal quantum dots (CQDs) are promising photovoltaic (PV) materials because of their widely tunable absorption spectrum controlled by nanocrystal size
. Their bandgap tunability allows not only ...the optimization of single-junction cells, but also the fabrication of multijunction cells that complement perovskites and silicon
. Advances in surface passivation
, combined with advances in device structures
, have contributed to certified power conversion efficiencies (PCEs) that rose to 11% in 2016
. Further gains in performance are available if the thickness of the devices can be increased to maximize the light harvesting at a high fill factor (FF). However, at present the active layer thickness is limited to ~300 nm by the concomitant photocarrier diffusion length. To date, CQD devices thicker than this typically exhibit decreases in short-circuit current (J
) and open-circuit voltage (V
), as seen in previous reports
. Here, we report a matrix engineering strategy for CQD solids that significantly enhances the photocarrier diffusion length. We find that a hybrid inorganic-amine coordinating complex enables us to generate a high-quality two-dimensionally (2D) confined inorganic matrix that programmes internanoparticle spacing at the atomic scale. This strategy enables the reduction of structural and energetic disorder in the solid and concurrent improvements in the CQD packing density and uniformity. Consequently, planar devices with a nearly doubled active layer thicknesses (~600 nm) and record values of J
(32 mA cm
) are fabricated. The V
improved as the current was increased. We demonstrate CQD solar cells with a certified record efficiency of 12%.
The stability of solution-processed semiconductors remains an important area for improvement on their path to wider deployment. Inorganic caesium lead halide perovskites have a bandgap well suited to ...tandem solar cells
but suffer from an undesired phase transition near room temperature
. Colloidal quantum dots (CQDs) are structurally robust materials prized for their size-tunable bandgap
; however, they also require further advances in stability because they are prone to aggregation and surface oxidization at high temperatures as a consequence of incomplete surface passivation
. Here we report 'lattice-anchored' hybrid materials that combine caesium lead halide perovskites with lead chalcogenide CQDs, in which lattice matching between the two materials contributes to a stability exceeding that of the constituents. We find that CQDs keep the perovskite in its desired cubic phase, suppressing the transition to the undesired lattice-mismatched phases. The stability of the CQD-anchored perovskite in air is enhanced by an order of magnitude compared with pristine perovskite, and the material remains stable for more than six months at ambient conditions (25 degrees Celsius and about 30 per cent humidity) and more than five hours at 200 degrees Celsius. The perovskite prevents oxidation of the CQD surfaces and reduces the agglomeration of the nanoparticles at 100 degrees Celsius by a factor of five compared with CQD controls. The matrix-protected CQDs show a photoluminescence quantum efficiency of 30 per cent for a CQD solid emitting at infrared wavelengths. The lattice-anchored CQD:perovskite solid exhibits a doubling in charge carrier mobility as a result of a reduced energy barrier for carrier hopping compared with the pure CQD solid. These benefits have potential uses in solution-processed optoelectronic devices.
Control over carrier type and doping levels in semiconductor materials is key for optoelectronic applications. In colloidal quantum dots (CQDs), these properties can be tuned by surface chemistry ...modification, but this has so far been accomplished at the expense of reduced surface passivation and compromised colloidal solubility; this has precluded the realization of advanced architectures such as CQD bulk homojunction solids. Here we introduce a cascade surface modification scheme that overcomes these limitations. This strategy provides control over doping and solubility and enables n-type and p-type CQD inks that are fully miscible in the same solvent with complete surface passivation. This enables the realization of homogeneous CQD bulk homojunction films that exhibit a 1.5 times increase in carrier diffusion length compared with the previous best CQD films. As a result, we demonstrate the highest power conversion efficiency (13.3%) reported among CQD solar cells.
A solution‐based passivation scheme is developed featuring the use of molecular iodine and PbS colloidal quantum dots (CQDs). The improved passivation translates into a longer carrier diffusion ...length in the solid film. This allows thicker solar‐cell devices to be built while preserving efficient charge collection, leading to a certified power conversion efficiency of 9.9%, which is a new record in CQD solar cells.
Abstract
While total internal reflection (TIR) lays the foundation for many important applications, foremost fibre optics that revolutionised information technologies, it is undesirable in some other ...applications such as light-emitting diodes (LEDs), which are a backbone for energy-efficient light sources. In the case of LEDs, TIR prevents photons from escaping the constituent high-index materials. Advances in material science have led to good efficiencies in generating photons from electron–hole pairs, making light extraction the bottleneck of the overall efficiency of LEDs. In recent years, the extraction efficiency has been improved, using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum. However, the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and time-consuming fabrication, which causes a gap between lab demonstration and industrial-level applications. Inspired by lightning bugs, we propose and realise a disordered metasurface for light extraction throughout the visible spectrum, achieved with single-step fabrication. By applying such a cost-effective light extraction layer, we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs, demonstrating a substantial potential for global energy-saving and sustainability.
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