Improved Photoluminescence Intensity Research Articles

Amplified Photoluminescence of CsPbX3 Perovskites Confined in Silica Film with a Chiral Nematic Structure

AbstractMetal halide perovskites (MHPs, CsPbX3: X = Cl, Br, I) have advanced the field of optoelectronic devices due to their remarkable light‐emitting capabilities, stemming from the large overlap between their emission and absorption spectra, offering the possibility to reabsorb their own emitted photons. Herein, a straightforward method is reported to confine CsPbBr3 into mesoporous silica films with a chiral nematic structure, allowing the amplification of the photoluminescence (PL). The simple room temperature ligand‐free synthesis allows facile growth of CsPbBr3 in silica photonic films, in which the Bragg peak position can be tuned from the UV to the visible range. The perovskite/silica films demonstrate a remarkable improvement in PL intensity and lifetime compared to the as‐synthesized non‐confined perovskite nanocrystals (NCs) due to the overlap of the Bragg peak position of the chiral nematic photonic films and CsPbBr3 absorption band. Such a PL enhancement stems from the slow photon effect induced at blue and red Bragg peak edges that facilitates the photon recycling of the emitted photons. This innovative approach offers a new way to fabricate highly emissive and long‐lived photoluminescent films at ambient conditions, potentially advancing perovskite utilization in light‐emitting devices.

Photolithographic patterning on multi-wavelength quantum dot film of the improved conversion efficiency for digital holography.

A triple-wavelength patterned quantum dot film was fabricated for the light source of digital holography to improve both the axial measurement range and noise reduction. The patterned quantum dot film was fabricated after optimizing the photolithography process condition based on the UV-curable quantum dot solution, which was capable of multiple patterning processes. In addition, an optimized pattern structure was developed by adding TiO2 nanoparticles to both the quantum dot and bank layers to increase the scattering effect for the improved photoluminescence intensity. Finally, the newly developed light source with the balanced spectral distribution was applied to the digital holography, rendering it applicable as an improved light source.

Lead Telluride Nanowires for Surface Passivation in Cadmium Telluride Photovoltaics

Cadmium telluride (CdTe) is one of the leading technologies in thin-film solar cells; however, its open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ) is limited by defects associated with polycrystalline nature, carrier concentration, and interfaces. To overcome the deep work function of the CdTe absorber and create a passivating back contact interface, a lattice-matched and highly doped p-type material is needed to function as a buffer layer between the CdTe and a highly conductive electrode. Here, we report that lead telluride (PbTe) nanowires (NWs) serve to passivate the CdTe back contact, enhancing the optoelectronic properties and device performance. The steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements showed improved PL intensity and minority carrier lifetime for devices utilizing a PbTe NW back buffer, consistent with surface passivation. Additionally, the devices employing PbTe NWs showed improved V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> and fill factor (FF). For instance, implementation of PbTe NWs as a passivating layer to CdS/CdTe solar cells yielded a photoconversion efficiency of 14.8% (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> = 855 mV and FF = 77.3%), whereas the control device reached 14.0% (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> = 840 mV and FF = 75.9%). This improvement in V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> and FF is consistent with the reduction in both back-barrier height and recombination losses observed for devices with the PbTe NW back buffer.

Pressure‐Induced Indirect‐Direct Bandgap Transition of CsPbBr 3 Single Crystal and Its Effect on Photoluminescence Quantum Yield (Adv. Sci. 29/2022)

CsPbBr3 Single Crystal High pressure exerted on CsPbBr3 single crystal using diamond anvil cell leads to improved photoluminescence intensity due to the decreased dynamic Rashba splitting. Application of higher pressure transforms it into a pure indirect bandgap phase, which can be maintained at near-ambient pressure. These phenomena provide a novel framework to understand the optoelectronic properties of perovskite materials. More details can be found in article number 2201554 by Junbo Gong, Zengming Zhang, Xudong Xiao, and co-workers.

Multifunctional Additive of Potassium Cinnamate Improve Crystallization and Passivate Defect for Perovskite Solar Cell with Efficiency Exceeding 22%

Perovskites with high film quality and low defect density are considered the essential condition for obtaining high‐performance perovskite devices. Herein, a simple and effective multifunctional additive of potassium cinnamate (PC) is doped into a perovskite precursor solution to improve the film quality and passivate the defect. It is revealed that the PC additives can react with the perovskite compositions, which affect the crystallization of perovskite and eventually form a high‐quality perovskite film. In addition, the PC additive still exists in the perovskite film after annealing, which effectively passivates the defects in perovskites. It is found that the PC‐doped devices exhibit improved photoluminescence intensity, increased carrier lifetimes, and reduced charge recombination. As a result, due to the improvement of film quality and efficient defect passivation, the doped device delivers a champion photoelectric conversion efficiency of 22.00%, which is significantly higher than 18.96% of the pristine device. The doped device maintains 86% of its maximum efficiency after aging for 5 months, which significantly improves long‐term stability compared to 62% of the original device. Herein, a new idea for designing efficient additives is provided to further enhance the performance and stability of perovskite solar cells.

Direct-Bandgap Bilayer WSe2 /Microsphere Monolithic Cavity for Low-Threshold Lasing.

Monolayer transition metal dichalcogenides (TMDs) have emerged as widely accepted 2D gain materials in the field of light sources owing to their direct bandgap and high photoluminescence quantum yield. However, the monolayer medium suffers from weak emission because only a single layer of molecules can absorb the pump energy. Moreover, the material degradation when transferring these fragile materials hinders their cooperation with the optical cavity further. In this study, for the first time, a high-quality monolithic structure is developed by directly growing single-domain tungsten diselenide (WSe2 ) bilayers on single silica microsphere (MS) cavities. Such a completely wrapped structure guides the indirect-to-direct bandgap transition of WSe2 bilayers, leading to a significantly improved photoluminescence intensity by about 60-fold. Moreover, the high-quality monolithic structure enhances the confinement factor of the cavity by more than 20-fold. Based on the above advantages, a bilayer WSe2 /MS microlaser is realized with an ultralow threshold of 0.72 Wcm-2 , nearly an order of magnitude lower than the existing records. The results demonstrate the possibility of using multilayer TMD materials as 2D gain media and provide insights into a new ultracompact monolithic platform of TMD material/cavity for lasing devices.

Improvement of photoluminescence intensity and film morphology of perovskite by Ionic liquids additive

Metal halide perovskites have received much attention for their application in light-emitting diodes (LEDs) and solar cells in the past several years. Among them, 2D and quasi-2D perovskite with organic long-chain cations introduced have drawn significant attention. However, while improving wet and thermal stability, as the grain size becomes smaller, more defects introduced at the grain boundary and surface, resulting in the increase of non-radiative recombination is becoming the main problem which should be faced by 2D/quasi-2D perovskite materials. Here, we report a new strategy employing ionic liquid named 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide(EMB).By adding a small amount of ionic liquid to the precursor, the defect was effectively passivated and the photoluminescence intensity was increased by 11 times and the fluorescent lifetime was increased by about 1.5 times. The flatness of the prepared perovskite thin films has also been effectively improved.

Structural and photoluminescent investigations of SrAl2O4:Eu[formula omitted],RE[formula omitted] improved nanophosphors for solar cells

The long afterglow green emitting materials were prepared by fast gel-combustion procedure (600 °C) using urea as fuel. This paper reports the improvement in photoluminescence intensity and persistency of Eu2+ incorporated long-persistent SrAl2O4 nanophosphors by the addition of rare earth ions (Dy3+, Pr3+, Y3+) as codopants. The materials were investigated using X-ray diffraction (XRD), Fourier transformation infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. X-ray diffraction analysis exhibited the formation of pure SrAl2O4 phase with monoclinic crystal system and space group P21. FTIR spectra were used to determine the stretching and bending modes of vibrations to confirm the formation of metal–oxygen bonds. The microstructures were observed using SEM and TEM analysis. For the synthesized strontium aluminate phosphors, the photoluminescence characteristics including excitation, emission and decay kinetics were also examined. PL emission spectra, recorded at the excitation wavelength of 362 nm, exhibited the characteristics peak at 510 nm corresponding to the parity allowed 5d–4f electronic transition of Eu2+ ions. The phosphorescence intensity of the materials were greatly enhanced on addition of codopants (Dy3+, Pr3+, Y3+), without any change in the position of the peak in spectra. The outcomes of these investigations made the focused materials worthy for the applications in the solar cells and lighting technology.

Improved efficiency of methylammonium-free perovskite thin film solar cells by fluorinated ammonium iodide treatment

Abstract Volatile, CH3NH3+-free, [CH(NH2)2]0.9Cs0.1PbI3 organic-inorganic perovskite thin films were treated with fluorinated ammonium iodide {4-fluoro-phenethylammonium iodide (4F-PEAI)} solution and thin two-dimensional (4F-PEA)2PbI4 films were formed on the surface, resulting in a significant improvement in photoluminescence intensity. When applied to low-temperature-processed PSCs with planar structure, untreated PSCs exhibited PCEs of 18.6% and 17.0% during reverse and forward scanning, respectively. The PSCs treated with 4F-PEAI showed improved open-circuit voltage, fill factor, and PCEs of 20.6% and 18.5% during reverse and forward scanning, respectively.

Preparation and characterization of sol-gel derived highly transparent aluminosilicate glass thin films with ultrahigh doping level of Yb3+ ions

A series of highly transparent aluminosilicate glass films with ultrahigh doping level of Yb3+ ions have been prepared by Sol-Gel method and deposited on transparent silica glass substrates with thickness ranging from 0.901 μm to 1.314 μm and doping level of Yb3+ ions up to 30 mol%. Each film exhibits a characteristic photoluminsecence spectrum of Yb3+ ions with a maximum at 975 nm and high transmission in VIS-NIR region. The refractive index for the film has been measured by spectroscopic ellipsometry. AFM images and EPMA cross-sectional images have presented the formation of film surface and cross section respectively and a uniform film formation has been achieved with the composition of 10YbO1.5-15AlO1.5-75SiO2 and 20YbO1.5-12AlO1.5-58SiO2. It has been verified that multilayer films get a major improvement in photoluminescence intensity still with high transmission in VIS-NIR region.

Reduced power consumption in nickel zinc ferrite nanoparticles doped blue phase chiral nematic liquid crystal devices

The effect of nickel zinc ferrite (NZFO (Ni0.5Zn0.5Fe2O4)) nanoparticles in reducing the threshold voltage of blue phase (BP) in chiral nematic liquid crystal (CNLC) was studied at room temperature. Due to low concentration selected for doping, the structure of CNLC and the temperature window of BP is maintained in entire LC range. The significant reduction in threshold voltage (5.5V) for Freedericksz transition is recorded for doped CNLC. Strong interaction between guest NZFO nanoparticles and host, aligns the CNLC molecule in an ordered structure to considerably enhance dielectric permittivity and anisotropy of CNLC doped with 0.05wt% NZFO nanoparticles to reduce threshold voltage. The presence of NZFO nanoparticles reduces the impurity ions and associated free energy in CNLC, which lead to the observed decrease in dielectric loss and elastic constant, respectively, to contribute towards reduction in driving power (by ~50%) of BP based displays. The improved photoluminescence intensity of CNLC, in the presence of NZFO, promises as a potential material for optoelectronic devices.

Fabrication and Refractive Index Control of Transparent and Luminescent HfO2:Ln3+ (Ln3+ = Eu3+, Tb3+) Thin Films for Enhanced Surface Emissions

Dense and porous thin films of luminescent Ln3+-doped HfO2 materials (Ln3+ = Eu3+ and Tb3+) were fabricated on silica glass substrates by a sol–gel dip-coating method using different kinds of coating solutions. The refractive index of the HfO2:Ln3+ thin films was controlled by changing their porosity based on the Lorentz–Lorenz relationship. Pores could be introduced systematically into the thin films with the addition of different amounts of a poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer to the coating solutions and the subsequent heat-treatment at 700°C. While photoluminescence (PL) of the dense HfO2:Ln3+ thin films with higher refractive index was inefficient mainly due to the reflection problem, the porous HfO2:Ln3+ thin films with lower refractive index exhibited much improved PL intensity by suppressing total internal reflection of the emitted light. The existence of the pores was also beneficial to an increase in the absorption of the excitation light due to its multiple scattering inside the thin films. Our results then demonstrate that the porous luminescent thin films are suitable for applications where the transparency and the strong surface emission are both necessary.

Optical and luminescence characteristics for Cd1-xErxTe quantum dots prepared via furan-2-ylmethanethiol assisted hydrothermal approach

Cd1-xErxTe nanocrystals are synthesized via furan-2-ylmethanethiol assisted hydrothermal approach. The CdTe nanocrystals preserved their cubic structure at all concentrations of Er ions. The Er ions caused an increase of the lattice constant and crystallite size of CdTe QDs. The prepared nanocrystals showed a spherical shape with high monodispersion. The d-spacing was found to be increase due to the increase of Er ions into the CdTe QDs. The EDAX spectra suggested the stoichiometry of the synthesized nanocrystals. The partial replacement of the Er ions in Cd sites gives rise to an improvement of the photoluminescence by more than 4 times and a red shift of their emission peaks was observed. At optimum Er ions concentration, an improvement of their photoluminescence intensity by more than four orders of magnitude and also for quantum yield by 76% are obtained via the increase of the hydrothermal temperature from 50 °C to 250 °C. The increase of the reaction time caused further improvement of photoluminescence intensity by more than 20 times and the quantum yield increased to 91% at reaction time of 3 h. The narrow emission band, high luminescent and the high quantum yield for the Cd0.95Er0.05Te nanocrystals may render them to be employed as efficient material for the manufacturing of light emitting devices.

Optimization of graphene quantum dots by chemical exfoliation from graphite powders and carbon nanotubes

This work adopts one- and two-step modified Hummers' method to chemically exfoliate graphene nanosheets (GNs) from graphite flake powders and carbon nanotubes (CNTs). The protocol for making the GN products involves one- and two-step exfoliation using KMnO4 oxidation agent. Experimental results reveal that the two-step exfoliation induce high efficiency of chemical oxidation, especially for the synthesis of GNs from CNT precursor. The thermal reduction enables the removal of surface functionalities, thus narrowing the band gap of GN-based electrodes, e.g., from ∼2.12 eV to ∼1.60 eV. The two series of graphene samples, prepared from graphite flake and CNTs, are chemically cut into nanoscale and serve as graphene quantum dots (GQDs). The photoluminescence (PL) excitation behavior is strongly affected by oxidation level and band gap of GQD products. After the thermal reduction, the PL peak shifts to lower wavelength region (i.e., 427–451 nm), offering a blue-shift behavior. The blue-shift behavior primarily originated from surface functionalization, particle size and shape of GQDs. Without any thermal reduction, the GQD suspensions exhibit an improved PL intensity, attributed to fast carrier‒carrier scattering dominates over the electron‒phonon scattering onto the basal plane of graphene sheets due to their high oxidation level and large band gap. As a result, this study offers an efficient way to tune the band gap of GQDs, favoring the development of optoelectronic and biological applications using graphene-based materials in the future.

Achieving the high phase purity of CH3NH3PbI3 film by two-step solution processable crystal engineering

To date, it is still a great challenge for highly efficient perovskite devices to realize the high quality perovskite films with high purity, high coverage ratio and good crystallization by two-step scalable solution method. In this study, a series PbI2 films with tunable micro-architecture of PbI2 crystals are prepared via solution processable crystal engineering. The perovskite film, prepared by optimized pit spacing in gas pumped PbI2 film at 1000 Pa, shows the highest film quality, including no residual PbI2 phase, compact morphology, and improved photoluminescence intensity. A transformation kinetics shows that the pit spacing strongly influences both the mass transfer and the sequential intercalation reaction between CH3NH3I and PbI2 crystals, which ultimately determines the full reaction state of the perovskite film. The perovskite solar cells assembled by the perovskite film show both high power-conversion efficiency and good reproducibility of photovoltaic performance due to the restrained charge recombination arising from the high quality perovskite film.

Effect of high temperature rapid thermal annealing on optical properties of InGaAsP grown by molecular beam epitaxy

The effect of rapid thermal annealing (RTA) on the optical properties of InGaAsP with band-gap energy of around 1.05 eV for quadruple-junction solar cells grown by molecular beam epitaxy (MBE) has been investigated. The photoluminescence (PL) spectrum of InGaAsP film annealed at 800 °C has strong integrated intensity and low activation energy of band-tail states. The time-resolved PL measurement shows that the decay time of the InGaAsP annealed at 800 °C and as-grown one are 11.6 ns and 3.0 ns at 10 K, respectively. An S-shape PL decay time as a function of temperature for the InGaAsP annealed at 800 °C is observed and is explained by the carrier relaxation dynamics. The RTA process induces reorganization of In and Ga inside the alloy due to the existence of miscibility gap in InGaAsP grown by MBE owing to the Be diffusion at high temperature and results in an increased composition uniformity and an improved PL intensity.

Annihilation of arsenic-nitrogen bonding defects in annealed InAs1−xNx quantum dots grown through nitrogen background pressure–controlled SS-MBE

We report the self-assembled growth of InAs1−xNx/GaAs quantum dots (QDs) through solid-source molecular beam epitaxy on GaAs (001) substrates. High-resolution X-ray diffraction confirmed that a low nitrogen background pressure of 4 × 10−6 Torr facilitates 2.2% incorporation of nitrogen into the InAs1−xNx QDs, whereas a relatively high pressure of 5 × 10−6 Torr facilitates a low nitrogen incorporation of 0.6%. 19 K photoluminescence (PL) measurements exhibited wide and multi-peaked spreading in nitride QDs, which can be conflicted with nucleation at dislocations caused by nitrogen. As-grown InAs0.978N0.022 QDs exhibited maximum red-shift upto 1358 nm but with a considerably reduced PL intensity, attributed to nitrogen incorporation and As–N defects generated in nitride QD, respectively. The Raman spectra of the InAs0.978N0.022 QDs exhibited a high intensity and evidently reduced full width at half maximum of the In-N like local vibrational mode as compared to the InAs0.994N0.006 QDs, attributed to less number of defects formed during growth. Rapid thermal annealing was conducted in nitrogen atmosphere for 30 s, and a continuous blue-shift was observed upto 800 °C because of the In/Ga interchange and As/N diffusion. For InAs0.978N0.022 QDs annealed at 750 °C, 440-fold higher improvement in PL intensity was realized because of the annihilation of the arsenic-nitrogen (As–N) bonding defects, whereas the InAs/GaAs QDs used as the reference sample exhibited negligible enhancement. The proposed method eases high nitrogen incorporation into InAs QDs, and the annealed nitride QDs have highly favorable optical properties. These experimental results offer insights into the performance of InAs1−xNx-based lasers.

High nitrogen composition–induced low interfacial roughness of GaAs0.978N0.022/GaAs multiple quantum wells grown through solid-source molecular beam epitaxy

GaAs1−xNx/GaAs multiple quantum wells (MQWs) were grown on GaAs(001) substrates through solid-source molecular beam epitaxy under various nitrogen background pressures (NBPs), and the crystal quality at the interface of GaAs1−xNx and GaAs was investigated. X-ray diffraction and electron microscopy confirmed the low interface roughness of MQWs grown at a NBP of 5×10−6Torr. Surface morphology measurements revealed a smooth surface without whisker-like defect structures. The fabricated MQWs exhibited high photoluminescence intensity because of the reduction in surface recombination with high nitrogen incorporation. Raman spectroscopy confirmed the presence of N-like local vibrational mode, and this was attributed to the presence of phase separation in GaAsN alloys. Rapid thermal annealing improved photoluminescence intensity by 100-fold and substantially reduced full width at half maximum because of MQW homogenization. These results evidence the favorable crystal interface of GaAs0.978N0.022 alloys. Hence, GaAs0.978N0.022/GaAs MQWs grown under high pressure might be useful in fabricating optoelectronic devices.

Improved photoluminescence intensity and thermal stability brought by increasing Eu3+ content in KBaY1−xEuxSi2O7 solid-solution phosphors

Developing high brightness, high color purity and thermal stable red phosphors has gained an increasing attention for enhancing the performance of white light emitting diodes (w-LEDs). Herein, a series of red-emitting KBaY1−xEuxSi2O7 (0 < x < 1) solid-solution phosphors were designed and fabricated. The phosphors can be excited by 394 nm near-UV light and emit red light peaking at 612 nm due to the 5D0 → 7F2 transition of Eu3+. The increasing proportion of Eu3+ in the solid-solution system can improve the photoluminescence intensity and thermal stability of the phosphors continuously until the Eu3+ concentration comes to 100%, while no deformation of the crystal structure has been brought. The as-prepared KBaEuSi2O7 phosphor shows excellent thermal photoluminescence stability, its emission integrated intensity can still maintain ∼90% of the initial value at room temperature when heating to 180 °C. The CIE coordinates (0.652, 0.348) of KBaEuSi2O7 phosphor are very close to the NTSC (National television system committee) standard red chromaticity (0.67, 0.33), and the color shift at high temperature is very slight. The KBaEuSi2O7 phosphor can be regarded as a promising red luminescence candidate for w-LEDs.

Cu–In–S/ZnS Quantum Dots Embedded in Polyvinylpyrrolidone (PVP) Solids for White Light-Emitting Diodes (LEDs)

Here, we report a facile and green route to prepare Cu–In–S (CIS)/ZnS core/shell quantum dots (QDs) embedded in polyvinylpyrrolidone (PVP) solids and their application as phosphors for white light-emitting diodes (WLEDs). CIS/ZnS core–shell QDs were easily synthesized in the aqueous phase, assisted by microwave irradiation within 20 min at 95 °C. The emission wavelength of CIS/ZnS QDs could be tunable between a large range of 543–700 nm by adjusting the [Cu]/[In] ratios, and the photoluminescence (PL) quantum yield could be up to 43% by coating a ZnS layer. QD-embedded PVP solids then were facilely fabricated by mixing CIS/ZnS QDs and PVP in water, followed by drying, to show improved PL intensity, higher photostability, and higher thermostability. Finally, we demonstrated their potential application as a white light source by using green- and red-emitting CIS/ZnS/PVP nanocomposite solids as color converters, in combination with a blue light-emitting diode (LED) chip.