Electronic and Excitonic Processes in Quantum Dot Light-Emitting Diodes

A modified Langevin model has been proposed to study the electronic and excitonic dynamic processes in quantum dot light-emitting diodes (QLEDs), and the electroluminescence onset processes of the QLEDs under different charge-injection conditions have been explored. The simulation results are in good agreement with experimental curves, confirming the feasibility of this model. It is demonstrated that the formation of an exciton on the quantum dots (QDs) with one electron injected first followed by one hole is much more effective than that with the reverse sequence. That is, charging a QD with one electron first is more favorable for device performance enhancement, which is attributed to the low (high) Auger recombination rate of negative (positive) trions of commonly used type I QDs. Additionally, we demonstrate that enough electron injection is one of the prerequisites for high-performance QLEDs based on these type I QDs

Color-Tunable Highly Bright Photoluminescence of Cadmium-Free Cu-Doped Zn–In–S Nanocrystals and Electroluminescence

A series of Cu doped Zn–In–S quantum dots (Cu:Zn–In–S d-dots) were synthesized via a one-pot noninjection synthetic approach by heating up a mixture of corresponding metal acetate salts and sulfur powder together with dodecanethiol in oleylamine media. After overcoating the ZnS shell around the Cu:Zn–In–S d-dot cores directly in the crude reaction solution, the resulting Cu:Zn–In–S/ZnS d-dots show composition-tunable photoluminescence (PL) emission over the entire visible spectral window and extending to the near-infrared spectral window (from 450 to 810 nm), with the highest PL quantum yield (QY) up to 85%. Importantly, the initial high PL QY of the obtained Cu:Zn–In–S/ZnS d-dots in organic media can be preserved when transferred into aqueous media via ligand exchange. Furthermore, electroluminescent devices with good performance (with a maximum luminance of 220 cd m^–2, low turn-on voltages of 3.6 V) have been fabricated with the use of these Cd-free low toxicity yellow-emission Cu:Zn–In–S/ZnS d-dots as an active layer in these QD-based light-emitting diodes

Efficient and Stable Red Emissive Carbon Nanoparticles with a Hollow Sphere Structure for White Light-Emitting Diodes

Red-emissive solid-state carbon nanoparticles (CNPs) with a hollow sphere structure for white light-emitting diodes (WLEDs) were designed and synthesized by molecular self-assembly and microwave pyrolysis. Highly ordered graphite-like structures for CNPs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible (UV–vis) spectroscopy. The emission mechanism of the red-emissive solid-state CNPs was investigated in detail by steady-state and time-resolved photoluminescence (PL) spectroscopy. The as-prepared CNPs showed a red emission band centered at 620 nm with excitation wavelength independence, indicating uniform size of sp² carbon domains in the CNPs. The CNPs also had a PL quantum yield (QY) of 17% under 380 nm excitation. Significantly, the PL QY of the organosilane-functionalized CNPs was 47%, which is the highest value recorded for red-emissive solid-state carbon-based materials under UV-light excitation. More importantly, the red-emissive CNPs exhibited a PL QY of 25% after storage in air for 12 months, indicating their excellent stability. The red-emissive CNP powders were used as environmentally friendly and low-cost phosphors on a commercial 460 nm blue GaN-based chip, and a pure white light with CIE coordinates of (0.35, 0.36) was achieved. The experimental results indicated that the red-emissive CNP phosphors have potential applications in WLEDs

Controlled Photoluminescence Lifetimes and Quantum Efficiencies in Mn-Doped Two-Dimensional Perovskite via A‑Site Cation Engineering

Low-dimensional metal halide perovskites are widely studied due to their excellent optoelectronic properties and environmental stability. Recently, transition metal Mn2+ ions were introduced into two-dimensional (2D) perovskite to modify their photophysical properties. However, the Mn luminescence mechanism still needs further exploration, which includes the role played by the A-site cation engineering. Herein, we prepared 2D perovskite microcrystals with four A-site cations: butylamine (BA), phenethylamine (PEA), octylamine (OCA) and oleylamine (OAM) by hot-injection method, namely Mn2+:BA2PbBr4, Mn2+:PEA2PbBr4, Mn2+:OCA2PbBr4, and Mn2+:OAM2PbBr4. The Mn PL lifetime is tuned from 0.12 to 0.75 ms, achieving a maximum PL QY of 83% in BA-based 2D perovskite. The obtained perovskite samples exhibited micrometer-sized morphology, and the periodic arrangement of X-ray diffraction demonstrated the formation of a single-layer 2D structure. The temperature-dependent PL spectra revealed that the enhancement mechanism of Mn emission was related to short-chain ligand surface passivation (BA and PEA) and improvement of the exciton-to-Mn2+ energy transfer. The experimental results indicate that A-site cation engineering enriches the diversity of Mn-doped low-dimensional perovskites, which provides a new approach to regulate Mn luminescence kinetics

Efficient Self-Trapped Exciton Emission in Ruddlesden–Popper Sb-Doped Cs₃Cd₂Cl₇ Perovskites

Metal halide perovskites (MHPs) have attracted extensive attention due to their excellent optoelectronic properties. Among them, layered two-dimensional (2D) metal halide materials with special structures have attracted extensive attention due to their superior stability and optoelectronic properties. Here, we report the 2D Ruddlesden–Popper (RP) phase Cs3Cd2Cl7 synthesized by a solvothermal method, and the photoluminescence quantum yields (PLQYs) of the pristine Cs3Cd2Cl7 sample (PLQY ∼ 10%) can be increased to 68 ± 5% through appropriate Sb3+ doping. This should be the highest PLQY of all reported all-inorganic RP-phase Cd-based perovskites so far. Our results indicate that the highly efficient cyan emission can be attributed to the common self-trapped exciton (STE) emission of the triplet states of Cd2+ and Sb3+ induced by strong electron–phonon coupling, and Sb3+:Cs3Cd2Cl7 has excellent structural and spectral stability. This new material should be a promising candidate for optoelectronic applications in the future

Controlled Synthesis, Formation Mechanism, and Great Enhancement of Red Upconversion Luminescence of NaYF₄:Yb³⁺, Er³⁺ Nanocrystals/Submicroplates at Low Doping Level

Strong red upconversion luminescence of rare-earth ions doped in nanocrystals is desirable for the biological/biomedical applications. In this paper, we describe the great enhancement of red upconversion emission (4F9/2 → 4I15/2 transition of Er3+ ion) in NaYF4:Yb3+, Er3+ nanocrystals at low doping level, which is ascribed to the effectiveness of the multiphonon relaxation process due to the existence of citrate as a chelator and cross relaxation between Er3+ ions. The dissolution−recrystallization transformation, governing both the intrinsic crystalline phase (cubic and/or hexagonal phase) and the growth regime (thermodynamic vs kinetic), is responsible for the phase control of the NaYF4 crystals. The possible formation mechanism of the NaYF4 crystals and the role of trisodium citrate which acts as a chelating agent and shape modifier are discussed in detail. It is also found that the α → β phase transition is favored by the high molar ratio of fluoride to lanthanide and high hydrothermal temperature as well as long hydrothermal time

Improving Performance of InP-Based Quantum Dot Light-Emitting Diodes by Controlling Defect States of the ZnO Electron Transport Layer

ZnO nanoparticles (NPs) are currently the benchmark of electron transport materials for preparing indium phosphide (InP)-based environmentally friendly quantum dot light-emitting diodes (QLEDs). However, the defect-dependent exciton quenching and charge injection limiting behavior at the ZnO/quantum dot (QD) interface seriously restrict the improvement in device performance. Herein, we report a strategy based on Li doping and MgO shell coating to regulate the defect state of ZnO to improve the performance of InP-based QLEDs. It is found that Li doping passivates the intrinsic defect states of ZnO NPs and improves the electron mobility and reduces the spontaneous charge transfer at the ZnO/QD interface and the current leakage of QLEDs. The MgO shell passivates the surface oxygen defects of ZnO NPs, thus reducing the exciton quenching and non-radiative recombination centers at the ZnO/QD interface, resulting in enhanced QLED performance. As a result, the optimized QLED prepared by Li-doped and MgO shell-coated ZnO NPs shows an external quantum efficiency of 9.7% and a brightness of 22,200 cd m–2 at 4.2 V, which are, respectively, 2.6 and 7 times higher than those of a QLED based on pure ZnO. This work shows that controlling the defect states of the ZnO electron transport layer by ion doping and shell coating provides an effective way to obtain high-performance environment-friendly QLEDs

Exploring the Effect of Band Alignment and Surface States on Photoinduced Electron Transfer from CuInS₂/CdS Core/Shell Quantum Dots to TiO₂ Electrodes

Photoinduced electron transfer (ET) processes from CuInS₂/CdS core/shell quantum dots (QDs) with different core sizes and shell thicknesses to TiO₂ electrodes were investigated by time-resolved photoluminescence (PL) spectroscopy. The ET rates and efficiencies from CuInS₂/CdS QDs to TiO₂ were superior to those of CuInS₂/ZnS QDs. An enhanced ET efficiency was surprisingly observed for 2.0 nm CuInS₂ core QDs after growth of the CdS shell. On the basis of the experimental and theoretical analysis, the improved performances of CuInS₂/CdS QDs were attributed to the passivation of nonradiative traps by overcoating shell and enhanced delocalization of electron wave function from core to CdS shell due to lower conduction band offset. These results indicated that the electron distribution regulated by the band alignment between core and shell of QDs and the passivation of surface defect states could improve ET performance between donor and acceptor

Thickness-Dependent Photoluminescence Properties of Mn-Doped CsPbBr₃ Perovskite Nanoplatelets Synthesized at Room Temperature

Mn-doped CsPbBr3 perovskite nanoplatelets (NPLs) with multicolor emission have very promising applications in light-emitting devices. However, the effects of NPL thickness on the Mn2+ luminescence properties remain to be investigated. Herein, a series of Mn-doped CsPbBr3 NPLs and nanocubes with different Mn/Pb molar ratios were synthesized by a supersaturated crystallization method at room temperature. The incorporation of Mn2+ ions into CsPbBr3 perovskites is attributed to the formation of the L2[Pb1–xMnx]Br4 intermediate structure in the precursor. The excitonic peak is tuned from 437 to 488 nm and the morphology evolves from NPLs to nanocubes with an increasing Mn2+ ion doping concentration due to the excess Br– from MnBr2. The photoluminescence quantum yields (PL QYs) of NPLs/nanocubes were greatly enhanced, achieving the maximum PL QYs of 88.7% at the Mn/Pb molar ratio of 3/1. The PL lifetime of Mn2+ emission is tuned from 0.19 to 0.44 ms due to the passivation of defect states and morphology transformation. Temperature-dependent steady-state and time-resolved PL spectra revealed that deep defect states in the NPLs/nanocubes were significantly reduced as the thickness increased. The Mn-doped CsPbBr3 NPLs/nanocubes show great potential for application in white light-emitting diodes