Although organometal halide perovskites have garnered enormous interest in solar cells, scarce attention has been paid to light-emitting devices using formamidinium lead halide perovskite as the emitting layer. Highly luminescent and air-stable formamidinium lead halide perovskite quantum dots using high-melting-point ligands have been synthesized. Through compositional engineering, the emission spectra are readily tunable over the entire visible spectral region of 409–817 nm. The photoluminescence of FAPbX3 nanocrystals has narrow emission line widths of 21–34 nm, high quantum yields of up to 88%, and a photoluminescence lifetime of 54.6–68.6 ns for single halide FAPbBr3, which could be stable for several months. We have demonstrated the fabrication of highly efficient formamidinium lead halide perovskite quantum dot-based green-light-emitting diodes with a moderately high luminance of 33993 cd m–2, current efficiency of 20.3 cd A–1, and moderately high maximum external quantum efficiency of 4.07%.Keywords: light-emitting diodes; perovskite quantum dot; quantum yields; solid ligands; solution processing;

•A novel all-quantum-dot multilayer photoluminescent film (PLF) was fabricated.•The color and PL intensities of these hybrid films can be precisely controlled.•The prepared PLFs were uniform and smooth with high visible light transmittance.•They can be potentially used in the lighting and display fields.In this report, all-quantum-dot multilayer photoluminescent films (PLFs) based on two types of modified quantum dots (MA-C8-QDs and PEI-QDs) were fabricated through the layer-by-layer (LBL) self-assembly method, providing a new kind of luminescent material with emission color covering blue to red spectral region. Aqueous QDs with high stability and photoluminescence properties were obtained by an efficient phase transfer, followed by fabrication of (PEI-QDs/MA-C8-QDs)n PLFs with alternate adsorbing the layer of the MA-C8-QDs endowed with negative (COO−) charges and PEI-QDs endowed with positive (NH3+) charges using electrostatic interactions between each layer. The resulting single color films preserved good color purity and strong luminescence of original QDs, PL intensities increased linearly with the number of bilayers n, which indicated that growth of the film is regular and uniform. In addition, the uniform and smooth PLFs as prepared have high visible light transmittance. Furthermore, by emission tuning, multicolor and white light-emitting PLFs with the color coordinates at (0.3292, 0.3418) have been easily obtained by assembly of two types of modified red, green, blue QDs. Therefore, these PLFs (especially white-light PLFs) show promise for the development of novel multiplexed biological sensors, full-color displays, intelligent response, photonic, and optoelectronic devices.

CuInS2 based quantum dots are emerging as low toxic materials for new generation white lighting technology due to their broad and color-tunable emissions as well as large Stokes shifts. Here, we developed a simple and in situ ligand exchange strategy for the fabrication of hydroxyl-terminated CuInS2 based quantum dots capped with 6-mercaptohexanol. During the ligand exchange, long-chain methyl-terminated oleylamine on the quantum dots’ surface can be effectively and efficiently replaced by the short-chain hydroxyl-terminated 6-mercaptohexanol, enabling their solubility in polar organic solvents such as methanol, ethanol, and dimethylformamide. Moreover, the resulting hydroxyl-terminated quantum dots exhibit well-preserved photoluminescence properties with quantum yields of ∼70%. Using these hydroxyl-terminated CuInS2 based quantum dots as an emitting layer, we fabricated efficient and bright light emitting diodes by adopting an inverted device structure. The optimized devices show a maximum luminance of 8,735 cd/m2 and an external quantum efficiency of 3.22%. Furthermore, the performance enhancement can be explained by considering the decreased energy barriers between the electron transport layer and the emitting layer. The combination of high efficiency and enhanced brightness as well as the potential all-solution processability using green solvents makes hydroxyl-terminated quantum dots capped with 6-mercaptohexanol a new generation of materials for light emitting applications.

We report a type of highly efficient deep-red light-emitting diode based on type-II CdTe/CdSe core/shell quantum dots (QDs). High-quality CdTe/CdSe core/shell QDs with three different photoluminescence emissions peaked at 642, 664 and 689 nm are, respectively, synthesized through a green process. Correspondingly, three devices employing QDs with different PL peaks show the maximum external quantum efficiency (EQE) of 5.24%, 5.74% and 6.19% with a low turn-on voltage of about 1.8 V, respectively. Interestingly, EQE is kept above 5% in a certain range of luminance (from 101 to 103 cd m−2). To our knowledge, this is the first report that uses type-II core/shell QDs as deep-red emitters and these results may offer a practicable reference for applications that require a high efficiency deep-red illuminant.

Colloidal nanoplatelets (NPLs) have recently been introduced as semiconductor emissive materials for the fabrication of quantum dot light-emitting diodes (QLED) on account of their ultra-narrow photoluminescence (PL) linewidth. In this paper, we report a multilayer all solution-processed green QLED based on colloidal CdSe/CdS core/shell NPLs with a narrow PL full-width-at-half-maximum (FWHM) of 12 nm. Our characterization results reveal that this kind of NPL containing QLED exhibit a low operating voltage of 2.25 V and a maximum luminance up to 33000 cd m−2, and peak external quantum efficiency (EQE) of 5%, corresponding to 12.5 cd A−1 in luminance efficiency. Particularly, these devices show ultra-high color purity for electroluminescence (EL) with FWHM of 14 nm. As extremely narrow EL and ultra-pure color is highly attractive in the applications of LED industries, this work signifies the unique potential application of one new class of colloidal core/shell NPLs in achieving bright and efficient LEDs with superior color saturation.

Here, we report the influence of the ambient gas on the performance of quantum dot-based light-emitting diodes (QD-LEDs). The blue QD-LED devices with the maximum external quantum efficiency of 8.1% and the turn-on voltage of 2.7 V could be obtained in air. The efficiency decreases by 12% and turn-on voltage increases by 0.3 V relative to the control devices fabricated in a N2-filled glovebox. The histogram of maximum external quantum efficiency (EQE) shows average peak EQE of 8.08% and a low standard deviation of 3.63%, suggesting high reproducibility. Correspondingly, the operational lifetime of 376 h is obtained, which is on par with 408 h of devices fabricated in N2. For the devices fabricated in air, relatively high efficiency could be maintained only at low voltages, because of the near balanced injection of carriers under low bias. The measurements of contact potential difference, chemical composition, and surface roughness are used to verify the variation of energy level and surface morphology of films influenced by different ambient gas. These results would offer reasonable guidance for the application of QD-LEDs in actual large-scale production.Keywords: ambient gas; blue emission; electroluminescence; light-emitting diodes; quantum dot

•Highly bright and efficient azure blue ZnCdSe core/multishell QDs-based LEDs have been demonstrated.•The performance of QD-LEDs was improved by using QDs capped with short-chain 1-octanethiol and PVK dopants.•Such azure blue quantum-dot LEDs show a 140% increase in external quantum efficiency compared with QD-LEDs without PVK.•The high efficiency (EQE > 8%) can be maintained in the range of 200–2400 cd/m2.Highly bright and efficient azure blue quantum dot-based light-emitting diodes (QD-LEDs) have been demonstrated by employing ZnCdSe core/multishell QDs as emitters and the crucial development we report here is the ability to dramatically enhance the efficiency and brightness through doping poly vinyl(N-carbazole) (PVK) in the emissive layer to balance the charge injection. The best device displays remarkable features like maximum luminance of 13,800 cd/m2, luminous efficiency of 6.41 cd/A, and external quantum efficiency (EQE) of 8.76%, without detectable red-shift and broadening in electroluminescence (EL) spectra with increasing voltage as well as good spectral matching between photoluminescence (PL) and EL. Such azure blue quantum-dot LEDs show a 140% increase in external quantum efficiency compared with QD-LEDs without PVK. More important, the peak efficiency of the QD-LEDs with PVK dopant is achieved at luminance of about 1000 cd/m2, and high efficiency (EQE > 8%) can be maintained with brightness ranging from 200 to 2400 cd/m2. There are two main aspects of the role of PVK in the proposed system. Firstly, the lower HOMO of PVK than (poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) can reduce the potential barrier for 0.4 eV at the interface of QDs and hole transport layer which could result in higher hole injection efficiency along with good EQE as compared to TFB-only HTLs. Secondly, with PVK acting as buffer layer of TFB and QDs, the exciton energy transfer from the organic host to the QDs can be effectively improved.

In this paper, highly stable violet-blue emitting ZnSe/ZnS core/shell QDs have been synthesized by a novel “low temperature injection and high temperature growth” method. The resulting nearly monodisperse ZnSe/ZnS core/shell QDs exhibit excellent characteristics such as a high color saturation (typical spectral full width at half-maximum between 12 and 20 nm), good emission tunability in the violet-blue range of wavelengths from 400 to 455 nm, a high absolute PL quantum yield (up to 83%), and superior chemical and photochemical stability. By employing ZnSe/ZnS core/shell quantum dots (QDs) as emitters with a fully solution processable method, bright, efficient, and color-stable violet Cd-free quantum dot-based light-emitting diodes (QD-LEDs) with maximum luminance up to 2632 cd m−2 and a peak EQE of 7.83% have been demonstrated successfully. Considering the factors of the photopic luminosity function, the brightness and efficiency results of such violet QD-LEDs not only represent a 12-fold increase in device efficiency and an extraordinary 100 times increase in luminance compared with previous Cd-free QD-LEDs but also can be much superior to the best performance (1.7%) of their Cd-based violet counterparts. These results demonstrate significant progress in short-wavelength QD-LEDs and shed light on the acceleration of commercial application of environmentally-friendly violet QD-based displays and lighting.

•Violet cadmium-free quantum dot-based light-emitting diodes (QD-LEDs) were demonstrated.•The performance of QD-LEDs was optimized by selecting the hole transport materials.•Violet QD-LEDs with high efficiency and low turn-on voltage could be obtained via controlling the amount of TFB mixed PVK.•No obvious broad emission could be recorded in all the regions of the EL spectrum of TFB–PVK mixture HTL devices.Bright and efficient violet quantum dot (QD) based light-emitting diodes (QD-LEDs) with heavy-metal-free ZnSe/ZnS have been demonstrated by choosing different hole transport layers, including poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), and poly-N-vinylcarbazole (PVK). Violet QD-LEDs with maximum luminance of about 930 cd/m2, the maximum current efficiency of 0.18 cd/A, and the peak EQE of 1.02% when poly-TPD was used as HTL. Higher brightness and low turn-on voltage (3.8 V) violet QD-LEDs could be fabricated when TFB was used as hole transport material. Although the maximum luminance could reach up to 2691 cd/m2, the devices exhibited only low current efficiency (∼0.51 cd/A) and EQE (∼2.88%). If PVK is used as hole transport material, highly efficient violet QD-LEDs can be fabricated with lower maximum luminance and higher turn-on voltages compared with counterpart using TFB. Therefore, TFB and PVK mixture in a certain proportion has been used as HTL, turn-on voltage, brightness, and efficiency all have been improved greatly. The QD-LEDs is fabricated with 7.39% of EQE and 2856 cd/m2 of maximum brightness with narrow FWHM less than 21 nm. These results represent significant improvements in the performance of heavy-metal-free violet QD-LEDs in terms of efficiency, brightness, and color purity.

We report a facile method for the synthesis of size-controlled triangular CuInS2 (CIS) semiconductor nanocrystals (NCs) in the organic phase, and then, molecular metal chalcogenide complexes capped CIS NCs can be synthesized by exchanging original organic compounds with (NH4)4Sn2S6 inorganic ligands in environmentally benign solvent. The properties of CIS NCs (coated by both organic and inorganic ligands) were characterized by UV–Vis spectroscopy, fourier transform infrared, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, and dynamic light scattering. CuInS2 NCs (before and after ligand exchange) films were spin coated on cleaned ITO glass substrates, and the charge transport properties were detected by current-voltage characteristic. We observed that the ligands on the surface of CIS NCs have been exchanged successfully, and the electrical transparency of (NH4)4Sn2S6-CIS NCs films was obviously increased than CIS NCs with organic capping ligands.

Size- and shape-controllable Ag2Se-ZnS nanorods (NRs) and nanowires (NWs) have been synthesized successfully by the solution–liquid–solid (SLS) method. By using Ag2Se nanocrystals (NCs) as seeds and catalyst, colloidal Ag2Se-ZnS NRs and NWs with controllable diameters and lengths in ranges of 5–12 nm and 15–600 nm were successfully synthesized by altering the experimental variables, such as diameter of Ag2Se NCs, amount of precursor, reaction time, and reaction temperature. The Ag2Se NCs not only played a key role in the control of the shape of ZnS NCs but also influenced the crystal structure of ZnS NCs. The related surface photovoltage of heterostructured Ag2Se-ZnS NWs have also been studied and the formation of Ag2Se-ZnS heterostructure was confirmed. Moreover, this SLS method was successfully exploited to synthesize Ag2S-ZnS heterostructures.Keywords: Ag2Se-ZnS; heterostructures; nanorod/wire; solution−liquid−solid; surface photovoltage;

A facile seed-growth method was developed to synthesize AgAu alloy and core/shell nanocrystals (NCs) using different-sized Ag NCs (6.1, 7.4, and 9.6 nm) as seeds and octadecylamine as the reducing agent, surface ligand, and solvent. Pre-synthesized Ag NCs acted as catalysts for the reduction of Au precursors at 130 °C. Transmission electron microscopy, energy-dispersive spectroscopy, and optical absorption spectroscopy were used to characterize as-synthesized NCs. Spherical AgAu alloy NCs were obtained when pre-synthesized 6.1 and 7.4 nm Ag NCs were used as seeds. While, if 9.6 nm Ag NCs were used as seeds, cubic Ag/Au core/shell NCs were finally obtained. The shapes and structures of AgAu NCs are related to the Ag seed sizes and the growth mechanism of alloy and core/shell NCs is discussed in detail. Different reaction temperatures were tested to optimize the synthesis of AgAu alloy NCs, and it was found that the optimum reaction temperature for the growth of AgAu alloy NCs is 130 °C.

High quality PbSe and PbTe nanocrystals were synthesized with non-injection one-pot method which could be scale up easily. The use of traditional dangerous pyrophoric trioctylphosphine (TOP) and tributylphosphine (TBP) reagents was avoided. The crystal size was controlled by the reaction time and temperature, and the shape (sphere and cube) was controlled by the use of different combinations of surfactants. X-ray diffraction (XRD) and selective area electron diffraction (SAED) characterizations demonstrated the rock salt cubic structures; the transition electron microscopy (TEM) images show narrow size distributions of as-synthesized PbSe and PbTe nanocrystals. The uniform size of as-synthesized nanocrystals promoted the self-assemble of PbSe and PbTe nanocrystals into ordered superstructures.Graphical abstractHighlights► High quality PbSe and PbTe nanocrystals were synthesized by one-pot method. ► The use of traditional dangerous pyrophoric trioctylphosphine (TOP) and tributylphosphine (TBP) reagents was avoided. ► Crystal shape was controlled by the selective ligand adsorbing on different facets. ► As-synthesized nanocrystals self assembled into ordered superstructures.

We report a type of highly efficient deep-red light-emitting diode based on type-II CdTe/CdSe core/shell quantum dots (QDs). High-quality CdTe/CdSe core/shell QDs with three different photoluminescence emissions peaked at 642, 664 and 689 nm are, respectively, synthesized through a green process. Correspondingly, three devices employing QDs with different PL peaks show the maximum external quantum efficiency (EQE) of 5.24%, 5.74% and 6.19% with a low turn-on voltage of about 1.8 V, respectively. Interestingly, EQE is kept above 5% in a certain range of luminance (from 101 to 103 cd m−2). To our knowledge, this is the first report that uses type-II core/shell QDs as deep-red emitters and these results may offer a practicable reference for applications that require a high efficiency deep-red illuminant.