The warming-up effects of quantum-dot light emitting diodes: A reversible stability issue related to shell traps.

The aging phenomenon is commonly observed in quantum-dot light emitting diodes (QLEDs), involving complex chemical or physical processes. Resolving the underlying mechanism of these aging issues is crucial to deliver reliable electroluminescent devices in future display applications. Here, we report a reversible positive aging phenomenon that the device brightness and efficiency significantly improve after device operation, but recover to initial states after long-time storage or mild heat treatment, which can be termed as warming-up effects. Steady and transient equivalent circuit analysis suggest that the radiative recombination current dramatically increases but electron leakage from the quantum dots (QDs) to hole transport layer becomes more accessible during the warming-up process. Further analysis discloses that the notable enhancement of device efficiency can be ascribed to the filling of shell traps in gradient alloyed QDs. This work reveals a distinct positive aging phenomenon featured with reversibility, and further guidelines would be provided to achieve stable QLED devices in real display applications.

Machine Learning Assisted Stability Analysis of Blue Quantum Dot Light-Emitting Diodes.

The operational stability of the blue quantum dot light-emitting diode (QLED) has been one of the most important obstacles to initialize its industrialization. In this work, we demonstrate a machine learning assisted methodology to illustrate the operational stability of blue QLEDs by analyzing the measurements of over 200 samples (824 QLED devices) including current density-voltage-luminance (J-V-L), impedance spectra (IS), and operational lifetime (T95@1000 cd/m2). The methodology is able to predict the operational lifetime of the QLED with a Pearson correlation coefficient of 0.70 with a convolutional neural network (CNN) model. By applying a classification decision tree analysis of 26 extracted features of J-V-L and IS curves, we illustrate the key features in determining the operational stability. Furthermore, we simulated the device operation using an equivalent circuit model to discuss the device degradation related operational mechanisms.

The fatigue effects in red emissive CdSe based QLED operated around turn-on voltage.

The operational stability is a current bottleneck facing the quantum dot light-emitting diodes (QLEDs). In particular, the device working around turn-on voltage suffers from unbalanced charge injection and heavy power loss. Here, we investigate the operational stability of red emissive CdSe QLEDs operated at different applied voltages. Compared to the rising luminance at higher voltages, the device luminance quickly decreases when loaded around the turn-on voltage, but recovers after unloading or slight heat treatment, which is termed fatigue effects of operational QLED. The electroluminescence and photoluminescence spectra before and after a period of operation at low voltages show that the abrupt decrease in device luminance derives from the reduction of quantum yield in quantum dots. Combined with transient photoluminescence and electroluminescence measurements, as well as equivalent circuit model analysis, the electron accumulation in quantum dots mainly accounts for the observed fatigue effects of a QLED during the operation around turn-on voltage. The underlying mechanisms at the low-voltage working regime will be very helpful for the industrialization of QLED.

Performance analysis of PQDCF-coated silicon image sensor using Monte-Carlo ray-trace simulation.

Perovskite quantum dots embedded composite film (PQDCF) exhibits strong photoluminescence emissions and is expected to be excellent down-shifting material for enhancing ultraviolet (UV) response of silicon devices. In this work, light conversion process is analyzed by combining the experiments with Monte-Carlo ray-trace simulation. Results show that external quantum efficiency (EQE) in the UV region was mainly determined by absorption loss and match of peak wavelength. Moreover, resolution was correlated with thickness and reabsorption. This conclusion provides a guideline for designing novel materials with enhanced UV sensitivity and an EQE of 28% is predicted. Our experimental results showed that the use of red emissive PQDCF achieved an EQE of 20%.

Revealing the vertical structure of in-situ fabricated perovskite nanocrystals films toward efficient pure red light-emitting diodes.

The development of efficient perovskite light-emitting diodes (PeLEDs) relies strongly on the fabrication of perovskite films with rationally designed structures (grain size, composition, surface, etc.). Therefore, an understanding of structure-performance relationships is of vital importance for developing high-performance perovskite devices, particularly for devices with in-situ fabricated perovskite nanocrystal films. In this study, we reveal the vertical structure of an in-situ fabricated quasi-two-dimensional perovskite film. By combining time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and low-temperature photoluminescence spectra, we illustrate that the resulting in-situ fabricated DPPA2Csn-1Pbn(Br0.3I0.7)3n+1 (DPPA+: 3,3-diphenylpropylammonium) film has a gradient structure with a very thin layer of ligands on the surface, predominantly small-n domains at the top, and predominantly large-n domains at the bottom owing to the solubility difference of the precursors. In addition, GIWAXS measurements show that the domain of n = 2 on the top layer has an ordered in-plane alignment. Based on the understanding of the film structure, we developed an in-situ fabrication process with ligand exchange to achieve efficient pure red PeLEDs at 638 nm with an average external quantum efficiency (EQE) of 7.4%. The optimized device had a maximum luminance of 623 cd/m2 with a peak EQE of 9.7%.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Highly luminescent red emissive perovskite quantum dots-embedded composite films: ligands capping and caesium doping-controlled crystallization process.

Perovskite quantum dots (PQDs) are emerging as functional luminescence down-shifting materials for light conversion applications. The incorporation of PQDs into a polymeric matrix is a key step to improving their stability, thus facilitating device integration. Compared to the conventional way of mixing the pre-synthesized PQDs into a polymer, the in situ fabrication of perovskite quantum dots-embedded composite films (PQDCFs) is an efficient and cost-effective method, which yields enhanced photoluminescence properties. This method has been successfully developed for green emissive CH3NH3PbBr3 PQDCFs, whereas the red CH3NH3PbI3 PQDCFs only show the photoluminescence quantum yields (PLQYs) less than 15%. By means of combining transmittance electron microscopy (TEM) and absorption spectrum analysis, we showed that the "perovskite red wall" in PQDCFs was mainly related to the phase separation, formation of large-sized particles and incomplete chemical conversion of precursors. These problems are caused by the solubility variance of perovskite precursors in the solvent as well as the solvation compatibility between perovskite precursors and the polymer during the crystallization process. Based on these findings, we introduced Cs+ as a dopant and 3,3-diphenylpropyamine (DPPA) as capping ligands, respectively, to decrease the solubility variance of the precursors and improve the compatibility between PQDs and the polymer. Consequently, highly luminescent red emissive PQDCFs with a PLQY of 91% were achieved with this strategy.

Enhanced piezo-response in copper halide perovskites based PVDF composite films.

In-situ fabricated perovskite nanocrystals in polymeric matrix provide new generation composite materials for plenty of cutting edge technology. In this work, we report the in-situ fabrication of copper halide perovskite (MA2CuCl4, MA:CH3NH3+) embedded poly(vinylidene fluoride) (PVDF) composite films. The optimized MA2CuCl4/PVDF composite films exhibit greatly enhanced piezo-response in comparasion with pure PVDF films. The enhancements were invesitgated and explained by applying piezo-response force microscopy (PFM) measurements and density functional theory (DFT) caculations. We proposed that the high piezoelectric properties of MA2CuCl4/PVDF composite films could be related to the large Cu off-centering displacement, the strong interactions between MA2CuCl4 and PVDF as well as large stress concentration around the MA2CuCl4 particles in the films. These piezoelectric composite films are expected to be suitable functional materials for flexible and/or wearable piezoelectrics.

Efficient Light-Emitting Diodes Based on in Situ Fabricated FAPbBr3 Nanocrystals: The Enhancing Role of the Ligand-Assisted Reprecipitation Process.

In this paper, we reported the in situ fabrication of highly luminescent formamidinium lead bromide (FAPbBr3) nanocrystal thin films by dropping toluene as an anti-solvent during the spin-coating with a perovskite precursor solution using 3,3-diphenylpropylamine bromide (DPPA-Br) as a ligand. The resulting films are uniform and composed of 5-20 nm FAPbBr3 perovskite nanocrystals. By monitoring the solvent mixing of anti-solvent and precursor solution on the substrates, we illustrated the difference between the ligand-assisted reprecipitation (LARP) process and the nanocrystal-pinning (NCP) process. This understanding provides a guideline for film optimization, and the optimized films obtained through the in situ LARP process exhibit strong photoluminescence emission at 528 nm, with quantum yields up to 78% and an average photoluminescence lifetime of 12.7 ns. In addition, an exciton binding energy of 57.5 meV was derived from the temperature-dependent photoluminescence measurement. More importantly, we achieved highly efficient pure green perovskite based light-emitting diode (PeLEDs) devices with an average external quantum efficiency (EQE) of 7.3% (maximum EQE is 16.3%) and an average current efficiency (CE) of 29.5 cd A-1 (maximum CE is 66.3 cd A-1) by adapting a conventional device structure of ITO/PEDOT:PSS/TFB/perovskite film/TPBi/LiF/Al. It is expected that the in situ LARP process provides an effective methodology for the improvement of the performance of PeLEDs.

Emulsion Synthesis of Size-Tunable CH3NH3PbBr3 Quantum Dots: An Alternative Route toward Efficient Light-Emitting Diodes.

We report a facile nonaqueous emulsion synthesis of colloidal halide perovskite quantum dots by controlled addition of a demulsifier into an emulsion of precursors. The size of resulting CH3NH3PbBr3 quantum dots can be tuned from 2 to 8 nm by varying the amount of demulsifier. Moreover, this emulsion synthesis also allows the purification of these quantum dots by precipitation from the colloidal solution and obtains solid-state powder which can be redissolved for thin film coating and device fabrication. The photoluminescence quantum yields of the quantum dots is generally in the range of 80-92%, and can be well-preserved after purification (∼80%). Green light-emitting diodes fabricated comprising a spin-cast layer of the colloidal CH3NH3PbBr3 quantum dots exhibited maximum current efficiency of 4.5 cd/A, power efficiency of 3.5 lm/W, and external quantum efficiency of 1.1%. This provides an alternative route toward high efficient solution-processed perovskite-based light-emitting diodes. In addition, the emulsion synthesis is versatile and can be extended for the fabrication of inorganic halide perovskite colloidal CsPbBr3 nanocrystals.

Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology.

Organometal halide perovskites are inexpensive materials with desirable characteristics of color-tunable and narrow-band emissions for lighting and display technology, but they suffer from low photoluminescence quantum yields at low excitation fluencies. Here we developed a ligand-assisted reprecipitation strategy to fabricate brightly luminescent and color-tunable colloidal CH3NH3PbX3 (X = Br, I, Cl) quantum dots with absolute quantum yield up to 70% at room temperature and low excitation fluencies. To illustrate the photoluminescence enhancements in these quantum dots, we conducted comprehensive composition and surface characterizations and determined the time- and temperature-dependent photoluminescence spectra. Comparisons between small-sized CH3NH3PbBr3 quantum dots (average diameter 3.3 nm) and corresponding micrometer-sized bulk particles (2-8 µm) suggest that the intense increased photoluminescence quantum yield originates from the increase of exciton binding energy due to size reduction as well as proper chemical passivations of the Br-rich surface. We further demonstrated wide-color gamut white-light-emitting diodes using green emissive CH3NH3PbBr3 quantum dots and red emissive K2SiF6:Mn(4+) as color converters, providing enhanced color quality for display technology. Moreover, colloidal CH3NH3PbX3 quantum dots are expected to exhibit interesting nanoscale excitonic properties and also have other potential applications in lasers, electroluminescence devices, and optical sensors.