Luminescence Quantum Efficiency Research Articles

Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells.

Organic solar cells benefit from non-fullerene acceptors (NFA) due to their high absorption coefficients, tunable frontier energy levels, and optical gaps, as well as their relatively high luminescence quantum efficiencies as compared to fullerenes. Those merits result in high yields of charge generation at a low or negligible energetic offset at the donor/NFA heterojunction, with efficiencies over 19% achieved for single-junction devices. Pushing this value significantly over 20% requires an increase in open-circuit voltage, which is currently still well below the thermodynamic limit. This can only be achieved by reducing non-radiative recombination, and hereby increasing the electroluminescence quantum efficiency of the photo-active layer. Here, current understanding of the origin of non-radiative decay, as well as an accurate quantification of the associated voltage losses are summarized. Promising strategies for suppressing these losses are highlighted, with focus on new material design, optimization of donor-acceptor combination, and blend morphology. This review aims at guiding researchers in their quest to find future solar harvesting donor-acceptor blends, which combine a high yield of exciton dissociation with a high yield of radiative free carrier recombination and low voltage losses, hereby closing the efficiency gap with inorganic and perovskite photovoltaics.

Designing Nonconventional Luminescent Materials with Efficient Emission in Dilute Solutions via Modulation of Dynamic Hydrogen Bonds.

Nonconventional luminescent materials (NLMs) which do not contain traditional aromatic chromophores are of great interest due to their unique chemical structures, optical properties, and their potential applications in various areas, such as cellular imaging and chemical sensing. However, most reported NLMs show weak or no emission in dilute solutions, which severely limits their applications. In this work, dynamic hydrogen bonds were utilized to design NLMs with efficient emission in dilute solutions. To further validate the results, polymers P1 and P2 were successfully prepared and investigated. It was found that the luminescence quantum efficiency of P1 and P2 at a concentration of 0.1 mg/mL in water solution was 8.9 and 0.6%, respectively. The high efficiency can be attributed to the fact that polymer P1 has more intra- or intermolecular dynamic hydrogen bonds and other short interactions than P2 in dilute solutions, allowing P1 to achieve the through-space conjugation effect to increase the degree of system conjugation, restrict molecular motion, and decrease nonradiative transitions, which can effectively improve luminescence. In addition, polymer P2 exhibits the characteristics of clustering-triggered emission, excitation wavelength-dependent and concentration-dependent fluorescence properties, excellent photobleaching resistance, low cytotoxicity, and selective recognition of Fe3+. The present study investigates the manipulation of luminescence properties of NLMs in dilute solutions through the modulation of dynamic hydrogen bonds. This approach can serve as a semi-empirical technique for designing and building innovative NLMs in the times ahead.

An efficient strategy of charge compensation for Ni2+-activated SrTiO3 perovskite near-infrared phosphor toward optical imaging illumination

Near-infrared (NIR) light sources have great potential in applications such as optical imaging, non-destructive medical diagnosis, and non-visual lighting. Ni2+ activated phosphors as a class of newly discovered NIR emitting material shows broadband luminescent properties that covers NIR-II (1000–1600 nm) region which is relatively transparent and less scattering in tissues. However, it has shortcomings, such as low luminous efficiency and poor thermal stability that need to be improved. Here, we report the synthesis of new Ni2+-activated SrTiO3 perovskite-based broadband NIR phosphor by high-temperature solid-state reaction method, with M5+ (M = Ta, Sb, Nb, P) as the co-doping agent to compensate charges. By charge compensation, the new composition significantly improved the NIR emission intensity by 12 folds, increased the luminescence quantum efficiency from 7.9% to 19.7%, and improved the resistance to thermal quenching. We studied the synergistic effect of charge compensation on the luminescence performance of NIR phosphor by experimental and simulation approaches. We showed that co-doping of charge compensator improved NIR phosphors by eliminating electronic defects and introducing defect or distortion to lattice octahedra. We further demonstrate that NIR pc-LED can be made by packaging the optimized NIR phosphor with a UV LED chip. Its potential application in the field of optical imaging was discussed.

Chiral Luminescence Quantum Efficiency Engineered by Plasmonic Antennas

Chiral detection is critical for investigating many biological processes. We use numerical calculations to study the quantum efficiency (QE) of a chiral emitter modulated by plasmonic antennas. The chiral difference in QE is discovered when the chiral emitter is placed in the vicinity of the antenna. The chiral QE is position-dependent for symmetric and asymmetric antennas and can be modulated by designing chiral configurations. The local optical chirality and the coupled harmonic oscillator model can qualitatively explain the QE difference revealed by the numerical methods. That originates from the interference between resonant modes with different phases induced by chiral interactions, resulting in the different loss of coupled modes. Our results provide extended design ideas for enantioselective fluorescent recognition of chiral molecules.

Enhanced Luminescence of Long-Wavelength Broadband Near-Infrared Germanate Phosphors.

Long-wavelength broadband near-infrared (NIR) phosphors have attracted considerable interest in the fields of medical cosmetology and organic detection because of their special emission band. Herein, Ca2GeO4(CGO): Cr4+ NIR phosphor, presenting a broadband emission with longer wavelength ranging from 1100 to 1600 nm, has been synthesized. Further, the luminescence intensity and quantum efficiency of Cr4+ could be obviously improved via the energy transfer from Eu3+ to Cr4+. The energy transfer is dominated by the dipole-dipole mechanism, which can be inferred from the spectra and the decay curves. Furthermore, in order to evaluate the potential application, an NIR phosphor-converted light-emitting diode (pc-LED) based on blue chip has been prepared. Consequently, CGO: Eu3+, Cr4+ exhibits proper output power and wider half-width than the NIR LED chip, indicating its great prospect for long-wavelength NIR pc-LED applications.

Crystal growth and spectral properties of Eu3+-doped La2CaB10O19 single crystals

The invention and commercial application of InGaN/GaN-laser diodes have made it feasible to directly obtain the visible lasing radiation by pumping rare earth (RE) doped laser crystals. Hence searching for new laser crystals with efficient luminescence and applicable optical and physical properties in the visible range is an essential work. Here, the crystal growth and spectral properties of a series of large-size Eu3+ doped La2CaB10O19 (Eu3+-LCB) single crystals with the nominal atomic ratios of 3%, 7%, and 10% have been well studied. High-quality Eu3+-LCB single crystals with the size of 45 × 30 × 17 mm3, was grown by top-seeded solution growth (TSSG) method. The luminescence probe was employed as the tool to identify occupation sites, demonstrating that Eu3+ ions enter into La3+ sites first, then occupy Ca2+ sites after the nominal atomic ratio of Eu3+ ions exceeds 7%. Based on the absorption and emission spectra, the intensity parameters, spontaneous transition probability, radiation lifetime, and fluorescence branch ratio for Eu3+-LCB single crystals are studied with the Judd–Ofelt theory. From the transient spectra, the emission cross-section, fluorescence decay lifetime and luminescence quantum efficiency are deduced, which exhibit the maximum performance at the nominal atomic ratio of 7% Eu3+. Combining the luminescence probe and spectral properties, it can be concluded that Eu3+ doped crystals will have the best spectral properties when Eu3+ occupation at the single La3+ sites reaches the maximum in LCB single crystals, which may be universal for RE-doped crystal with two possible occupation sites. Besides, our work demonstrates that the 7% Eu3+-LCB single crystal may be a promising medium for laser radiation in the visible region in comparison with other Eu3+ doped single crystals.

Efficient red-emitting SrZrO3:Eu3+ phosphor superstructures for display device applications

In the present report, a series of intense red light emitting SrZrO3:Eu3+ nanopowders were fabricated via low-cost eco-friendly hydrothermal route using Aloe vera gel as surfactant. Upon utilizing the powder X-ray diffraction (PXRD) method the phase of the prepared nanopowders were evaluated. Pure single-phase tetragonal structure was observed from the PXRD results with no additional impurity peaks. The energy band gap of the fabricated powders was calculated by diffuse reflectance spectra (DRS) and found to be around 3.98 – 4.27 eV. High resolution scanning electron microscope (SEM) was utilized to study the morphological behavior of the prepared powders. Honeycomb-like structures were observed from the SEM micrographs. The particle size was evaluated by transmission electron microscopy (TEM) and observed to be ∼ 42 nm. The interplanar distance was found to be around 0.32 nm. Photoluminescence properties were systematically studied in detail. The phosphors were successfully excited at 395 nm NUV light, producing intense red characteristics emission. The emission peaks were centered at 585 nm (5D0→7F1), 613 nm (5D0→7F2) and 648 nm (5D0→7F3). The photoluminescent concentration quenching was noticed beyond 5 mol% Eu3+ ion doping content. The dipole-dipole interaction resulting in cross relaxation was found to be the principal cause of concentration quenching mechanism. The color features such as Commission International de I'Eclairage (CIE) and correlated color temperature (CCT) were studied in detail. The optimized chromaticity coordinates were estimated to be (0.67, 0.32), which falls in the intense red region. The average CCT value obtained was 4328 K. The color purity was observed around ∼ 88%. In addition, the average internal quantum efficiency was calculated, and it was found to be 48%. The prepared powders exhibit excellent luminescence properties, quantum efficiency and high color purity. The obtained results indicate the possibility of the present powders in the fabrication of the red-light emitting diode applications.

Making Eu2+- and Sm2+-Doped Borates Fit for Solar Energy Applications

Despite the development of many luminescent materials for various applications, only a few of these phosphors are applicable for solar energy generation applications. This study used the conventional solid-state reaction method to synthesize different strontium borate compounds codoped with divalent europium and samarium ions. The material was optimized by varying the experimental procedure, the molar ratio of the boron, and the molar ratios of both codopants. Strontium hexaborate doped with a relatively high europium concentration and a low samarium concentration (Sr0.89B6O10:Eu0.1, Sm0.01) gave the optimum optical properties. These properties included a broad excitation range from 220 to 600 nm containing contributions from divalent europium and samarium ions. The material exhibited strong and narrow emission in the region from 650 to 850 nm following radiative transitions within the divalent samarium ions. The internal luminescence quantum efficiency of the optimized material was 79% (λex = 508.5 nm). There was no spectral overlap between the excitation and emission spectra, thereby reducing the reabsorption probability. These marked characteristics make this phosphor material appropriate for use as a solar radiation converter or for use in luminescent solar concentrators.

Fourier-transform infrared photoreflectance spectroscopy of the InSb/InAs/In(Ga,Al)As/GaAs metamorphic heterostructures with a superlattice waveguide

Fourier-transform infrared photoreflectance (PR) spectroscopy was used to study the energy spectrum of InSb/InAs/In(Ga,Al)As/GaAs metamorphic heterostructures with a superlattice waveguide at room temperature (RT). Theoretical calculations in the framework of the eight-band Kane model were performed to obtain a reliable knowledge of the actual energies of the most probable optical transitions. The experimental results were analyzed to determine the influence of the design features and stress balance on the energy spectra of the structures. Photoluminescence studies performed at 11 K and RT, as well as the determination of the internal quantum efficiency of luminescence, enabled us to characterize the emission characteristics of the structures, regardless of their waveguide efficiency. The structure with a 5-nm-thick GaAs insertion within the metamorphic buffer layer exhibited the highest probability of the main optical transition observed in the PR spectra as well as the highest luminescence intensity and quantum efficiency.

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone-Triarylboron Acceptor.

10H-Dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B)-a high triplet (T1 =3.05 eV) strongly electron-accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B. We demonstrate the near-complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S1 →S0 CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ-Dipp-SO2B and PXZ-Dipp-SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ-Dipp-SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m-2 with low efficiency roll off at high brightness. For CZ-Dipp-SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (3 LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light-emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor‐Acceptor Emitter Featuring a Hybrid Sulfone‐Triarylboron Acceptor**

Abstract10H‐Dibenzo[b,e][1,4]thiaborinine 5,5‐dioxide (SO2B)—a high triplet (T1=3.05 eV) strongly electron‐accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ‐Dipp‐SO2B and CZ‐Dipp‐SO2B. We demonstrate the near‐complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S1→S0 CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ‐Dipp‐SO2B and PXZ‐Dipp‐SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ‐Dipp‐SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m−2 with low efficiency roll off at high brightness. For CZ‐Dipp‐SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (3LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light‐emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

A highly efficient Mn4+ activated Nb-based oxyfluoride red fluorescent material with excellent water stability: preparation and performance analysis.

In this study, an efficient non-rare earth Mn4+-doped K3(NbOF5)(HF2) red fluorescent material was synthesized by using the coprecipitation method. Replacing KF with K2CO3 effectively solved the problem that KF was difficult to stir due to its strong water absorption. The sample was composed of rods. The excitation spectra consisted of two strong excitation peaks at 366 nm and 468 nm. The emission spectra consisted of a series of narrow-band emissions between 580 nm and 680 nm. Besides, the luminescence quantum efficiency (QE) reached 84.3% under the excitation of 468 nm. The fluorescent lifetime of K3(NbOF5)(HF2):Mn4+ was less than 4 ms, which can achieve fast response display in backlight display applications. The WLED was fabricated with K3(NbOF5)(HF2):Mn4+ and commercial YAG:Ce3+ and the commercial InGaN blue chip. At a 30 mA drive current, the WLED device exhibited excellent luminescence properties. The correlated color temperature (CCT) was 3853 K, the Ra was 90.1 and the luminous efficiency was 310.432 lm W-1. Therefore, K3(NbOF5)(HF2):Mn4+ has very broad prospects in WLED lighting and backlight display applications.

Controllable structural and optical properties of NaYF4:Tm, Yb microparticles by Yb3+ doping for anti-counterfeiting.

Hexagonal NaYF4:Tm, Yb upconversion (UC) phosphors with excellent UC luminescence quantum efficiency and chemical stability meet demands for applications in bioimaging and anti-counterfeiting printing. In this work, a series of NaYF4:Tm, Yb upconversion microparticles (UCMPs) with different concentrations of Yb were synthesized by a hydrothermal method. Then, the UCMPs become hydrophilic through surface oxidation of the oleic acid (C-18) ligand to azelaic acid (C-9) using the Lemieux-von Rodloff reagent. The structure and morphology of UCMPs were investigated by X-ray diffraction and scanning electron microscopy. The optical properties were studied using diffusion reflectance spectroscopy and photoluminescent spectroscopy under 980 nm laser irradiation. The emission peaks of the Tm3+ ions are 450, 474, 650, 690, and 800 nm, attributed to the transitions from the excited state to ground state 3H6. These emissions are the results of two or three photon absorption through multi-step resonance energy transfer from excited Yb3+, confirmed via a power-dependent luminescence study. The results show that the crystal phases and luminescence properties of the NaYF4:Tm, Yb UCMPs are controlled by changing the Yb doping concentration. The printed patterns are readable under the excitation of a 980 nm LED. Moreover, the zeta potential analysis shows that the UCMPs after surface oxidation are water dispersible. In particular, the naked eye can observe the enormous upconversion emissions in UCMPs. These findings indicated that this fluorescent material is an ideal candidate for anti-counterfeiting and biological applications.

Investigation of quantum dot luminescent solar concentrator single, double and triple structures: A ray tracing simulation study

Quantum dot based luminescent solar concentrators (QDLSCs) are a special class of transparent photovoltaics (TPV), especially suited for building integrated photovoltaics (BIPV). Photons are absorbed by luminescent species in a waveguide and emitted at a red-shifted wavelength. Due to total internal reflection, these photons are absorbed by the solar cells attached to the sides. Successful deployment requires high conversion efficiency and high transparency, which are contradictory requirements. We have performed Monte-Carlo ray tracing simulations to investigate single, double, and triple QDLSCs and have assessed their optical and electrical performance. To this end, eight different semiconductor quantum dot materials have been used with various absorption and emission properties, and Stokes’ shift. Device efficiency is analyzed for different average visible transmission (AVT) values, thus considering the human photopic response. The range of luminescent quantum efficiencies (30%–70%) leads to maximum efficiency of 2% for a single QDLSC, 2.4% for a double, and 2.7% for a triple structure, at high transparency and good color rendering index. Further improvements are possible towards >5% at high transparency with near-unity quantum efficiencies.

Low turn-on voltage CsPbBr3 perovskite light-emitting diodes with regrowth crystal MAPbBr3 hole transport layer

This work reports the effect of a MAPbBr3 layer in CsPbBr3 perovskite light-emitting diodes (PeLEDs). The MAPbBr3 layer employed MAPbBr3 powder as the evaporation source, which was obtained from using MAPbBr3 bulk crystals prepared by the inverse temperature crystallization method. The devices exhibited the best quality and performance when a 20-nm-thick MAPbBr3 layer was used as the nucleation and hole transport layer. The rc-MAPbBr3/CsPbBr3 structure apparently boosted the carrier lifetime such that the injection holes and electrons quickly recombine and then transfer into photons and emissions. The champion luminescence and external quantum efficiency were 21,850 cd/m2 at a bias of 7.0 V and 1.18% at a bias of 6.5 V, respectively.

Scintillation property and highly efficient photoluminescence of cerium-doped BaF2–Al2O3–B2O3 glass for thermal neutron detection

Glass has several advantages over single crystals and transparent ceramics, including easier formability, increased productivity, and higher versatility in composition. However, the scintillation properties should be further enhanced for practical applications. Herein, we focused on fluoroborate glass, which has a high quantum yield. The xCeF3–(50-x)BaF2–25Al2O3–25B2O3 glass showed luminescence quantum efficiency up to 83% with a broad peak at 380 nm, and its concentration in the quantum yield was remarkably low. The lifetime of the X-ray-induced luminescence was also insignificant to the concentration. The light yields were obtained from the pulse height spectrum: for gamma rays from 241Am, up to 411 photons MeV−1; and for neutrons with 252Cf as the source, up to 1717 photons n−1 were observed. The performance of glass for thermal neutron detection using inexpensive natural abundant B2O3 was notably good. Thus, the possibility of developing an inexpensive glass scintillator has been presented here.

Novel Sm3+-activated YCa4O(BO3)3 phosphors with high quantum efficiency and thermal stability for red-backlight display and indoor illumination

Novel Sm3+-activated YCa4O(BO3)3 (YCOB) phosphors with high color purity, excellent thermal stability and high luminescence quantum efficiency were developed. Upon 405 nm excitation, all samples displayed strong orange-red emissions. The crystal structure and luminescence characteristics of YCOB in relation to Sm3+ concentrations were explored. The optimal doping concentration was found to be 5% mol with remarkable color purity of 97.73%. The concentration quenching mechanism in YCOB host lattices was dominated by dipole-dipole interaction. Meanwhile, temperature-dependent luminescence spectra revealed that the investigated phosphors have excellent thermal stability. Impressively, it was calculated that the internal quantum efficiency of synthesized phosphors was as high as 76.54%. To further verify the practicability of synthesized phosphors, both red light-emitting diodes and white light-emitting diodes were fabricated using YCOB: 5%Sm3+ phosphors, and they exhibited bright red and white light, respectively. These results suggest that the novel YCOB: Sm3+ phosphors with high quantum efficiency and luminescent stability were promising for red-backlight display and indoor illumination.

Organic Phosphorescence Lasing Based on a Thermally Activated Delayed Fluorescence Emitter.

Organic phosphorescence materials provide an opportunity to use triplets for lasing. However, population inversion based on phosphorescence is hard to establish, owing to low luminescent quantum efficiency and intensive optical loss. By comparison, thermally activated delayed fluorescence emitters exhibit excellent optical gain with the aid of the reverse intersystem crossing (RISC) process. In this work, we designed a multifunctional gain material, not only serving as a thermally activated delayed fluorescence (TADF) emitter with excellent optical gain but also working as a phosphorescence source with high utilization of triplets. The lone pair of electrons in oxygen substitutions promotes a fast spin-flip and high delayed fluorescence quantum yield (ΦDF = 55%), enabling TADF amplified spontaneous emissions (ASE) of CH2Cl2 solution. Single-crystalline nanowires of H-aggregates effectively lower triplet energy levels with high phosphorescence quantum yield (ΦP = 27%), demonstrating Fabry-Perot mode phosphorescence lasing at 630 nm.

Color-tunable donor-acceptor-type B-embedded dioxygen-bridged π-conjugated molecules: Synthesis, structures, and optical properties

Three novel donor-acceptor-type B-embedded dioxygen-bridged π-conjugated molecules (BO-CZ, BO-N and BO-NO) featuring different electron donors have been synthesized by Suzuki-Miyaura C–C coupling. Single crystal structures show that both BO-CZ and BO-N exhibit obvious π-π interactions in solid state due to the good planarity of the B-embedded dioxygen-bridged π-conjugated unit. More importantly, by controlling different electron donor units, the luminescence color of these compounds can be changed from deep blue to yellow with high luminescence quantum efficiency, up to close to 100%. Notably, the deep-blue-emitting molecule BO-CZ in film exhibits an ultrahigh color purity with a very narrow full width at half maximum (FWHM = 36 nm) and CIE coordinates (0.164, 0.019). Interestingly, these donor-acceptor-type boron-containing molecules with Lewis acidity can interact with Lewis bases (pyridine and DMAP) and induce interesting optical changes.

Predicting and Designing Thermally Activated Delayed Fluorescence Molecules with Balanced ΔEST and Transition Dipole Moment

AbstractThe investigations on the relationship between geometrical structures, excited state properties, and photophysical performance can accelerate the design of thermally activated delayed fluorescence (TADF) molecules. Herein, the photophysical properties of newly proposed organic TADF molecule: [4‐(9,9‐dimethyl‐9,10‐dihydroacridine) phenyl] (pyridin‐4‐yl)‐methanone (PyB‐DMAC), are theoretically investigated by using the quantum mechanics and molecular mechanics method. The photophysical performance dependency of PyB‐DMAC molecule on its geometrical and electronic structures, and aggregation effects are systematically analyzed. The calculated fluorescence quantum efficiencies are in good agreement with the experimental results, for instance, the calculated fluorescence efficiency of 61.2% (exp. 49%) for PyB‐DMAC molecule in the crystal phase. Based on these investigations, novel organic TADF molecules can be designed by balancing the singlet–triplet energy gap (ΔEST) and transition dipole moment. Sixteen unexplored candidates are screened with similar structures to PyB‐DMAC molecule, while two new TADF organic molecules (PyBF‐DMAC and OSOF‐DMAC) are selected with enhanced luminescence performance. The newly designed TADF molecules reveal lower reorganization energies and reduced nonradiative decay rates. The simulation results exhibit that their luminescence quantum efficiencies in toluene solvent and amorphous state are as high as 63.7%, 53.2% and 90%, 97.4%, respectively.