Increase In Photoluminescence Efficiency Research Articles

Harvesting Triplet Excitons in High Mobility Emissive Organic Semiconductor for Efficiency Enhancement of Light-Emitting Transistors.

Organic light-emitting transistors (OLETs), a kind of highly integrated and minimized optoelectronic device, demonstrate great potential applications in various fields. The construction of high-performance OLETs requires the integration of high charge carrier mobility, strong emission, and high triplet exciton utilization efficiency in the active layer. However, it remains a significant long-term challenge, especially for single component active layer OLETs. Herein, the successful harvesting of triplet excitons in a high mobility emissive molecule, 2,6-diphenylanthracene (DPA), through the triplet-triplet annihilation process is demonstrated. By incorporating a highly emissive guest into the DPA host system, an obvious increase in photoluminescence efficiency along with exciton utilization efficiency results in an obvious enhancement of external quantum efficiency of 7.2times for OLETs compared to the non-doped devices. Moreover, well-tunable multi-color electroluminescence, especially white emission with Commission Internationale del'Eclairage of (0.31, 0.35), from OLETs is also achieved by modulating the doping concentration with a controlled energy transfer process. This work opens a new avenue for integrating strong emission and efficient exciton utilization in high-mobility organic semiconductors for high-performance OLETs and advancing their related functional device applications.

Direct Electron Transfer Enables Highly Efficient Dual Emission Modes of Mn2+-Doped Cs2Na1-xAgxBiCl6 Double Perovskites.

Double perovskites with bright emission, low toxicity, and excellent stability have drawn considerable attention. Herein, we report the hydrothermal synthesis of Mn2+-doped Cs2Na1-xAgxBiCl6 double perovskites that exhibit dual emission modes. Introducing Ag+ ions to Cs2NaBiCl6 samples enables a bright self-trapped exciton (STE) emission in orange-red color, whereas Mn2+ dopants induce a yellow-orange emission. Importantly, Mn2+ doping into Cs2Na1-xAgxBiCl6 double perovskites with an indirect bandgap enables a high photoluminescence quantum yield of 49.52 ± 2%. Density functional theory calculations reveal that bringing Ag+ ions into Cs2NaBiCl6 can localize wave function to the [AgCl6]5- octahedron and convert dark transitions to bright STE transitions. Moreover, the 3d orbitals of Mn2+ dopants hybridize with Bi-6p and Cl-3p orbitals at the conduction band minimum, resulting in direct electron transfer from the host to Mn2+ and a significant increase in photoluminescence efficiency. These results shed light on the optical physical process of Mn2+-doped systems, providing useful information for further improvement of the photoluminescence efficiency of double perovskites.

Efficient and stable emission of warm-white light from lead-free halide double perovskites.

Lighting accounts for one-fifth of global electricity consumption1. Single materials with efficient and stable white-light emission are ideal for lighting applications, but photon emission covering the entire visible spectrum is difficult to achieve using a single material. Metal halide perovskites have outstanding emission properties2,3; however, the best-performing materials of this type contain lead and have unsatisfactory stability. Here we report a lead-free double perovskite that exhibits efficient and stable white-light emission via self-trapped excitons that originate from the Jahn-Teller distortion of the AgCl6 octahedron in the excited state. By alloying sodium cations into Cs2AgInCl6, we break the dark transition (the inversion-symmetry-induced parity-forbidden transition) by manipulating the parity of the wavefunction of the self-trapped exciton and reduce the electronic dimensionality of the semiconductor4. This leads to an increase in photoluminescence efficiency by three orders of magnitude compared to pure Cs2AgInCl6. The optimally alloyed Cs2(Ag0.60Na0.40)InCl6 with 0.04 per cent bismuth doping emits warm-white light with 86 ± 5 per cent quantum efficiency and works for over 1,000 hours. We anticipate that these results will stimulate research on single-emitter-based white-light-emitting phosphors and diodes for next-generation lighting and display technologies.

Enhanced Luminescence of InGaN Quantum Well Structures with Localized Surface Plasmon by Using Sputter‐Fabricated Ag Nanoparticles in an Ionic Liquid

Small, uniform Ag nanoparticles (NPs) in ionic liquids (ILs) are synthesized via simple sputtering route. The NPs embedded in polymer film are coated on top of InGaN/GaN active multiple quantum‐well light‐emitting structures and their optical characteristics are compared. The InGaN structures coated with Ag NPs show a significant increase in photoluminescence efficiency, mainly caused by localized surface plasmon (LSPs) coupling. Time‐resolved (TR) photoluminescence analysis demonstrates that the coupling efficiency is 47.2%. The increased luminescence is caused by good energy matching between the surface plasmon and the light emitter. The authors believe that present work takes a first step to utilize the advantages of metal NPs synthesized in ILs and has a great potential for use in various optoelectronic devices.

Role of Surface Recombination in Halide Perovskite Nanoplatelets.

Halide perovskites are an extremely promising material platform for solar cells and photonic devices. The role of surface carrier recombination-well known to detrimentally affect the performance of devices-is still not well understood for thin samples where the thickness is comparable to or less than the carrier diffusion length. Here, using time-resolved microspectroscopy along with modeling, we investigate charge-carrier recombination dynamics in halide perovskite CH3NH3PbI3 nanoplatelets with thicknesses from ∼20 to 200 nm, ranging from much lesser than to comparable to the carrier diffusion length. We show that surface recombination plays a stronger role in thin perovskite nanoplatelets, significantly decreasing photoluminescence (PL) efficiency, PL decay lifetime, and photostability. Interestingly, we find that both thick and thin nanoplatelets exhibit a similar increase in PL efficiency with increasing excitation fluence, well described by our excitation saturation model. We also find that the excited carrier distribution along the depth impacts the surface recombination. Using the diffusion-surface recombination model, we determine the surface recombination velocity. This work provides a comprehensive understanding of the role of surface recombination and charge-carrier dynamics in thin perovskite platelets and reveals valuable insights useful for applications in photovoltaics and photonics.

Influence of the halogen atom on the solid-state fluorescence properties of 2-phenyl-benzoxazole derivatives

Four halogenated derivatives of 2-phenyl-benzoxazole and the unsubstituted analog were studied in solution and as powders by absorption and steady-state fluorescence spectroscopy. In solution, the fluorescence quantum yield was progressively decreased with increasing the size of the halogen atom, as explained by heavy atom effect. In the solid state, an increase in photoluminescence efficiency was observed when passing from the fluorinated to chlorinated derivative, and then the fluorescence efficiency was decreased for the brominated and iodinated derivatives, the latter compound being virtually not emissive. To explain the solid-state behavior, the influence of molecular arrangement on the emission properties was discussed on the basis of X-ray data. Finally, microcrystals were generated using a solvent-exchange method. An evolution from elongated platelets to cubic particles was observed with increasing the size of the halogen atom, providing preliminary information for the use of these compounds as micro- and nano-materials.

(Invited) Photoluminescence Efficiency of Germanium Dots Self-Assembled on Oxides

Self-assembled Ge quantum dots were formed by in-situ thermal annealing of a thin amorphous Ge layer deposited by molecular beam epitaxy either on a thin porous TiO2 layer grown on SiO2 on Si(001) or directly on the SiO2 layer itself. For samples with dot diameters ranging from 10 to 35 nm, the dot photoluminescence (PL) appeared primarily as a wide near-infrared band peaked near 800 meV. The peak energy of the PL band reflects the average dot size and its shape depends on the dot size distribution. Using tight binding and effective mass theoretical models, we have analyzed the PL spectrum in terms of the dot size distribution. The observed size distribution determined from transmission electron and atomic force microscopy allowed the determination of the nonlinear increase in the PL efficiency with decreasing dot diameter. Although the absolute intensities of the PL from the samples vary, the calculated efficiency curves are all well fitted by straight lines on a log-log plot, with essentially the same slope for all samples, thereby demonstrating that under the weak confinement regime investigated here there is a universal power-law increase in PL efficiency with decreasing dot size. Knowing this generic PL efficiency, we show that it is possible to evaluate the size distribution of Ge dots from their PL energy dependence.

Silica-coated CdTe Quantum Dots of Unchanged Size with Intense Photoluminescence at Various Wavelengths

A new method for controlling the photoluminescence (PL) wavelength of silica-coated quantum dots (QDs) without changing the sizes of the core QDs is reported. When silica-coated green-emitting CdTe QDs were refluxed in aqueous solution containing Cd2+ and a sulfur-containing surfactant (thioglycolic acid (TGA)), the PL peak wavelength shifted to red with a significant increase in PL efficiency. This spectral shift was attributed to the reduction of the quantum confinement effect by the formation of small CdS-like clusters very close to the QDs in the silica shells. When the silica-coated QDs were refluxed in a solution of higher pH, the extent of this red shift increased. The larger red shift of the PL with pH is attributed to the hydrolysis speed of TGA increasing with pH. There is faster generation of S2− and as a consequence the growth rate of CdS-like clusters is enhanced. When a different sulfur-containing surfactant (thioglycerol (TG)) was used for reflux, the PL spectral shift and enhancement of PL efficiency became very small. This was because TG did not hydrolyze significantly enough to release S2−. These results demonstrate that the reported reflux procedure is a promising method for tuning the PL color of QDs through selection of the solution pH and the surfactant during reflux.

Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends

AbstractHere, studies on the evolution of photophysics and device performance with annealing of blends of poly(3‐hexylthiophene) with the two polyfluorene copolymers poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(3‐hexylthien‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2′′‐diyl) (F8TBT) and poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) are reported. In blends with F8TBT, P3HT is found to reorganize at low annealing temperatures (100 °C or below), evidenced by a redshift of both absorption and photoluminescence (PL), and by a decrease in PL lifetime. Annealing to 140 °C, however, is found to optimize device performance, accompanied by an increase in PL efficiency and lifetime. Grazing‐incidence small‐angle X‐ray scattering is also performed to study the evolution in film nanomorphology with annealing, with the 140 °C‐annealed film showing enhanced phase separation. It is concluded that reorganization of P3HT alone is not sufficient to optimize device performance but must also be accompanied by a coarsening of the morphology to promote charge separation. The shape of the photocurrent action spectra of P3HT:F8TBT devices is also studied, aided by optical modeling of the absorption spectrum of the blend in a device structure. Changes in the shape of the photocurrent action spectra with annealing are observed, and these are attributed to changes in the relative contribution of each polymer to photocurrent as morphology and polymer conformation evolve. In particular, in as‐spun films from xylene, photocurrent is preferentially generated from ordered P3HT segments attributed to the increased charge separation efficiency in ordered P3HT compared to disordered P3HT. For optimized devices, photocurrent is efficiently generated from both P3HT and F8TBT. In contrast to blends with F8TBT, P3HT is only found to reorganize in blends with F8BT at annealing temperatures of over 200 °C. The low efficiency of the P3HT:F8BT system can then be attributed to poor charge generation and separation efficiencies that result from the failure of P3HT to reorganize.

Effect of laser annealing on the distribution of defect levels in CdSe films

Effects of laser treatment on the modification and/or distribution of defect levels in chemically-deposited CdSe thin films have been discussed. By annealing the CdSe films with laser in air, we could observe a drastic reduction of defect density and a redistribution of defect states in the low quality chemically deposited films. Increase of photoluminescence efficiency by laser treatment demonstrates the effect of recombination enhanced defect reactions at the annealed region. By manipulating the laser power and the time of annealing, we could obtain the films of better optoelectronic quality. The process might be useful for the control of defect levels in chalcogenide thin films for their applications in optoelectronic devices.

Visible light emission from thin films containing Si, O, N, and H

We report the fabrication, chemical, optical, and photoluminescence characterization of amorphous silicon-rich oxynitride (SiOxNy:H) thin films by plasma-enhanced chemical-vapor deposition. The film compositions were followed by changes in the refractive index. X-ray photoelectron and Fourier transform infrared spectroscopy indicate that the chemical composition is dominated by silicon suboxide bonding with N present as a significant impurity. A broad tunable photoluminescence (PL) emission is visible at room temperature with a quantum efficiency of 0.011% at peak energies to 3.15 eV. The radiative lifetimes are less than 10 ns, and there is nearly no temperature dependence of the PL intensity down to 80 K. Ex situ annealing at temperatures above 850 °C results in an increase in PL efficiency by nearly three orders of magnitude, and the PL intensity is independent of the annealing ambient. The PL results are remarkably similar to literature results in oxidized porous silicon and oxidized nanocrystalline Si thin films, and suggest that the radiative center is due to the defect structure in the silicon suboxide moiety.