Tailoring D-π-A architectures with hybridized local and charge transfer fluorophores exhibiting high electroluminescence exciton utilization and low threshold amplified spontaneous emission.

Novel amorphous compounds which could simultaneously use 25% singlet excitons and 75% triplet excitons as the energy source for light amplification enable the reduction of the threshold current density for electrically pumped organic semiconductor laser diodes (OSLDs); however, there is always a trade-off between the high radiative decay rate of the local excited (LE) state that is required for amplified spontaneous emission (ASE) and high exciton utilization benefiting from the charge-transfer (CT) state during electroluminescence (EL). Herein, we have explored a delicate balance to achieve both low ASE threshold and high EL exciton utilization by adopting a carefully tailored hybridized local and charge-transfer (HLCT) molecular design. A series of donor-π-acceptor (D-π-A) molecules (SBz-1, SBz-2 and SBz-3) are synthesized, and the structural change mainly refers to the spatial distance between D and A which could regulate the excited-state character via adjusting the CT length. Notably, the ASE phenomenon with a low threshold (2.97 µJ cm-2) and a high exciton utilization of 57.6% are achieved at the same time for SBz-2 with an appropriate CT length. The results provide guidance for molecular design toward harvesting triplet excitons in organic laser materials.

Alternative Organic Spacers for More Efficient Perovskite Solar Cells Containing Ruddlesden-Popper Phases.

The halide perovskite Ruddlesden-Popper (RP) phases are a homologous layered subclass of solution-processable semiconductors that have aroused great attention, especially for developing long-term solar photovoltaics. They are defined as (A')2(A)n-1PbnX3n+1 (A' = spacer cation, A = cage cation, and X = halide anion). The orientation control of low-temperature self-assembled thin films is a fundamental issue associated with the ability to control the charge carrier transport perpendicular to the substrate. Here we report new chemical derivatives designed from a molecular perspective using a novel spacer cation 3-phenyl-2-propenammonium (PPA) with conjugated backbone as a low-temperature strategy to assemble more efficient solar cells. First, we solved and refined the crystal structures of single crystals with the general formula (PPA)2(FA0.5MA0.5)n-1PbnI3n+1 (n = 2 and 3, space group C2) using X-ray diffraction and then used the mixed halide (PPA)2(Cs0.05(FA0.88MA0.12)0.95)n-1Pbn(I0.88Br0.12)3n+1 analogues to achieve more efficient devices. While forming the RP phases, multiple hydrogen bonds between PPA and inorganic octahedra reinforce the layered structure. For films we observe that as the targeted layer thickness index increases from n = 2 to n = 4, a less horizontal preferred orientation of the inorganic layers is progressively realized along with an increased presence of high-n or 3D phases, with an improved flow of free charge carriers and vertical to substrate conductivity. Accordingly, we achieve an efficiency of 14.76% for planar p-i-n solar cells using PPA-RP perovskites, which retain 93.8 ± 0.25% efficiency with encapsulation after 600 h at 85 °C and 85% humidity (ISOS-D-3).

Unsymmetric Heteroleptic Ir(III) Complexes with 2-Phenylquinoline and Coumarin-Based Ligand Isomers for Tuning Character of Triplet Excited States and Achieving High Electroluminescent Efficiencies.

2-Phenylquinoline (PQ) and four coumarin-based ligand isomers with ease of synthesis have been selected to construct the unsymmetric heteroleptic [Ir(C1∧N)(C2∧N)(acac)]-type complex phosphors for organic light-emitting diodes (OLEDs). Six unsymmetric heteroleptic Ir(III) complexes have been obtained by employing four coumarin-based ligand isomers (L-C5/L-C6/L-C7/L-C8) in the [Ir(PQ)(C∧N)(acac)] structure due to two different coordinating carbon atoms in ligands L-C6 and L-C7 to form C-Ir bond. Through adopting unsymmetric heteroleptic [Ir(C1∧N)(C2∧N)(acac)] structure, these Ir(III) complexes can not only achieve impressive absolute quantum yield Φp (ca. 0.5-1.0), higher than that of complex [Ir(PQ)2(acac)] (ca. 0.4), but also realize a dual modulation of both emission color from orange (AIrC6out, λ = 578 nm) to red (AIrC5, λ = 622 nm) and the character of the lowest triplet excited states (T1), showing both 3MLCT character and 3ILCT (intraligand charge transfer) character in their T1 states. AIrC5, AIrC7out, and AIrC7in show MLCT character from Ir(III) center to ligand L-C5 or L-C7 and ILCT character in ligand L-C5 or L-C7 in their T1 states, while AIrC6out, AIrC6in, and AIrC8 show MLCT character from Ir(III) center to ligand PQ and ILCT character in ligand PQ in their T1 states. Moreover, the color-tuning mechanism and the lowest triplet state characters are investigated in detail. AIrC6in and AIrC8 were selected as emitters to evaluate the electroluminescent (EL) performance due to their high ΦP of nearly up to unity. Optimal orange-emitting device B2 based on AIrC8 can give a maximum external quantum efficiency (ηext) of 23.9%, a maximum current efficiency (ηL) of 70.9 cd A-1, and a maximum power efficiency (ηP) of 60.7 lm W-1. All these impressive results can definitely demonstrate the effectiveness of our simple approach for tuning character of the triplet excited states and achieving high-performance Ir-based phosphors in OLEDs.

Exciton and bi-exciton mechanisms in amplified spontaneous emission from CsPbBr3 perovskite thin films.

We studied temperature-dependent amplified spontaneous emission (ASE) in CsPbBr3 perovskite thin films. For temperatures 180-360 K, a narrow-band lasing is observed. However, a new accompanying ASE band appears below 180 K, indicating a more complicated behavior. The two ASE bands are strongly correlated and in competition; they are assigned as exciton and bi-exciton recombination. We estimated the exciton binding energy (EB = 27.3 meV) and that of the bi-exciton, which is lower than the EB. The reduced effective mass of the exciton is estimated as µ = 0.11 me. This discovery identifies more details of the ASE phenomenon.

Isomers of Coumarin-Based Cyclometalated Ir(III) Complexes with Easily Tuned Phosphorescent Color and Features for Highly Efficient Organic Light-Emitting Diodes.

Three Ir(C∧N)2(acac)-type and one Ir(C1∧N)(C2∧N)(acac)-type coumarin-based cyclometalated Ir(III) complex isomers (IrC5, IrC7, IrC7-A, and IrC8) have been obtained using three coumarin-based isomers of 2-phenylpyridine (ppy)-type cyclometalating ligands (L-C5, L-C7, and L-C8). Two coordination isomers emerging as principal products (IrC7 and IrC7-A) are obtained in the synthesis of corresponding coumarin-based cyclometalated Ir(III) complexes because of two different coordination sites in ligand L-C7 to form a C-Ir bond. To the best of our knowledge, there are no such isomers reported to date. Interestingly, a broad range of phosphorescent color tuning from green (IrC8, λ = 516 nm) to red (IrC5, λ = 608 nm) has been realized through variation of the pyridyl substitution positions on the fused phenyl ring of the coumarin skeleton. In addition, based on natural transition orbital (NTO) analyses, features of the lowest triplet excited states (T1) from these coumarin-based cyclometalated Ir(III) complex isomers can be tuned easily by these ligand isomers as well. IrC5, IrC7, and IrC7-A show prevailing 3MLCT character associated with their T1 states which emit the phosphorescent signals, while the T1 state of IrC8 exhibits the dominant ligand-centered π-π* transition feature. Importantly, owing to the strong rigidity of the coumarin skeleton, all the coumarin-based cyclometalated Ir(III) complex isomers can show high phosphorescent quantum yields Φp (ca. 0.4-1). Together with the improved electron-injection/electron-transport (EI/ET) ability, all the phosphorescent emitters display impressive electroluminescence (EL) performance. The device based on IrC8 gives the highest EL efficiencies of external quantum efficiency (ηext) 22.7%, current efficiency (ηL) 79.7 cd A-1, and power efficiency (ηP) 58.2 lm W-1, representing the most state-of-the-art EL ability ever achieved by coumarin-based phosphorescent emitters. All these encouraging data definitely suggest the great potential of the coumarin skeleton in both easy tuning of the photophysical properties of ppy-type Ir(III) phosphorescent complexes and developing high-performance phosphorescent emitters.

High-Quality Cs2 AgBiBr6 Double Perovskite Film for Lead-Free Inverted Planar Heterojunction Solar Cells with 2.2 % Efficiency.

All-inorganic double-metal perovskite materials have recently gained much attention due to their three dimensionality (3D) and non-toxic nature to replace lead-based perovskite materials. Among all those double perovskite materials, theoretical works have demonstrated that Cs2 AgBiBr6 shows high stability and possesses a suitable band gap for solar-cell applications. However, the film-forming ability of Cs2 AgBiBr6 is found to be the utmost challenge hindering its development in thin-film solar-cell devices. In this work, a high-quality Cs2 AgBiBr6 film with ultra-smooth morphology, micro-sized grains, and high crystallinity is realized via anti-solvent dropping technology and post-annealing at high temperature. After optimization, the first example of an inverted planar heterojunction solar-cell device based on Cs2 AgBiBr6 exhibits a power conversion efficiency of 2.23 % with VOC =1.01 V, JSC =3.19 mA/cm2 , and FF=69.2 %. Besides, the device shows no hysteresis and a high stability.

Asymmetric Heteroleptic Ir(III) Phosphorescent Complexes with Aromatic Selenide and Selenophene Groups: Synthesis and Photophysical, Electrochemical, and Electrophosphorescent Behaviors.

With the aim of evaluating the potential of selenium-containing groups in developing electroluminescent (EL) materials, a series of asymmetric heteroleptic Ir(III) phosphorescent complexes (Ir-Se0F, Ir-Se1F, Ir-Se2F, and Ir-Se3F) have been synthesized by using 2-selenophenylpyridine and one ppy-type (ppy = 2-phenylpyridine) ligand with a fluorinated selenide group. To the best of our knowledge, these complexes represent unprecedented examples of asymmetric heteroleptic Ir(III) phosphorescent emitters bearing selenium-containing groups. Natural transition orbital (NTO) analysis based on optimized geometries of the first triplet state (T1) have shown that the phosphorescent emissions of these Ir(III) complexes dominantly show 3π-π* features of the 2-selenophenylpyridine ligand with slight metal to ligand charge transfer (MLCT) contribution. In comparison with their symmetric parent complex Ir-Se with two 2-selenophenylpyridine ligands, these asymmetric heteroleptic Ir(III) phosphorescent complexes can show much higher phosphorescent quantum yields (ΦP) of ca. 0.90. Both the hole- and electron-trapping ability of these Ir(III) phosphorescent complexes can be enhanced by selenophene and fluorinated selenide groups to improve their EL efficiencies. The EL abilities of these asymmetric heteroleptic Ir(III) phosphorescent emitters fall in the order Ir-Se3F > Ir-Se2F > Ir-Se1F > Ir-Se0F. The highest EL efficiencies have been achieved by Ir-Se3F in the solution-processed OLEDs with external quantum efficiency (ηext), current efficiency (ηL), and power efficiency (ηP) of 19.9%, 65.6 cd A-1, and 57.3 lm W-1, respectively. These encouraging EL results clearly indicate the great potential of selenium-containing groups in developing high-performance Ir(III) phosphorescent emitters.

Theoretical evidence of low-threshold amplified spontaneous emission in organic emitters: transition density and intramolecular vibrational mode analysis.

Organic gain materials are highly attractive for lasing due to their chemical tunability and large stimulated emission cross sections. In previous reports, the radiative decay rate kr was considered as an important factor to determine outstanding amplified spontaneous emission (ASE) performance of organic molecules. In this study, we use quantum mechanics to reveal the influential factors on photophysical properties of organic emitters, and give insight into the effect of kr on ASE performance. Based on the theoretical analysis of three molecules with similar structure, calculated results show that large kr derives from enhanced transition density ρ between the electronic wave functions of the ground-state and the lowest excited singlet state as well as a handful of low-frequency torsional modes with small Huang-Rhys factor S, further, kr values are calculated depending on molecular vibration terms. In addition, through the analysis of non-radiative decay rate knr considering vibration terms (vibronic coupling constants), the comparison of kr and knr shows that the radiative decay process is promoted in the three molecules. The two aspects are desired for outstanding ASE performance. Our work shows that the roles of transition density and vibrational modes are crucial to clarify the photophysical properties, which govern the ASE performance in organic light emitters.

Charge Transport between Coupling Colloidal Perovskite Quantum Dots Assisted by Functional Conjugated Ligands.

Long alkyl-chain capping ligands are indispensable for preparing stable colloidal quantum dots. However, its insulating feature blocks efficient carrier transport among QDs, leading to inferior performance in light-emitting diodes (LEDs). The trade-off between conductivity and colloidal stability of QDs has now been overcome. Methylamine lead bromide (MAPbBr3 ) QDs with a conjugated alkyl-amine, 3-phenyl-2-propen-1-amine (PPA), as ligands were prepared. Owing to electron cloud overlapping and the delocalization effect of conjugated molecules, the conductivity and carrier mobility of PPA-QDs films increased almost 22 times over that of OA-QD films without compromising colloidal stability and photoluminescence. PPA-QDs LEDs exhibit a maximum current efficiency of 9.08 cd A-1 , which is 8 times of that of OA-QDs LEDs (1.14 cd A-1 ). This work provides critical solution for the poor conductivity of QDs in applications of energy-related devices.

Real-time image haze removal using an aperture-division polarimetric camera.

Polarimetric dehazing methods have been proven to be effective in enhancing the quality of images acquired in turbid media. We report a new full-Stokes polarimetric camera, which is based on the division of aperture structure. We design a kind of automatic polarimetric dehazing algorithm and load it into the field programmable gate array (FPGA) modules of our designed polarimetric camera, achieving a real-time image haze removal with an output rate of 25 fps. We demonstrate that the image quality can be significantly improved together with a good color restoration. This technique might be attractive in a range of real-time outdoor imaging applications, such as navigation, monitoring, and remote sensing.

Online fabrication scheme of helical long-period fiber grating for liquid-level sensing.

We present a novel online fabrication scheme of helical long-period fiber gratings (H-LPFGs) by directly twisting a standard single-mode fiber (SMF) in a microheater. This is done by taking advantage of the inherent core-cladding eccentricity in SMF. We adopt a fiber optic rotary joint to eliminate the accompanying twisting spiral for real-time spectral monitoring and a stepping mechanical system to accurately control the twisting length in fabrication. As a consequence, low-cost and high-quality H-LPFGs can be readily fabricated. Meanwhile, by using this kind of H-LPFG, we design a simple and low-cost wavelength-interrogated liquid-level sensor with a high sensitivity of 0.1 nm/mm.

Improvement of light extraction in organic light-emitting diodes using a corrugated microcavity.

Based on the phase separation effect in the film formation process of Polystyrene and Poly(methyl methacrylate) blend solution, bottom-emitting organic light-emitting diodes (OLEDs) with corrugated microcavity was demonstrated. This device exhibited high efficiency, broad spectra and Lambertian angular emission. Compared with the traditional bottom-emitting OLEDs with ITO anode and the planar microcavity OLEDs, about 57% and 41% enhancement for external quantum efficiency was achieved in this corrugated microcavity OLEDs respectively. This improvement can be understood by the scattering effect of the quasi-periodic characteristic of this corrugated microcavity which reduces the optical loss at surface plasmon polariton modes and wave-guided modes. This work provides a simple as well as efficienct method to recover trapped light in OLEDs, which will benefit the low cost fabrication process.

Enhanced lasing assisted by the Ag-encapsulated Au plasmonic nanorods.

Threshold reduction and emission enhancement were reported for the waveguided random lasing, assisted by the Ag-encapsulated Au nanorods (Au@Ag NRs). The blend of tris(8-hydroxyquinolinato)aluminum (Alq3) and 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4 H-pyran (DCJTB), which comprise the typical donor-acceptor lasing system, is used as the gain media. Compared with the Ag nanoparticles and Au nanorods, Au@Ag NRs exhibited the broad absorption spectra of localized surface plasmon resonance (LSPR) with multiple peaks, which sufficiently overlapped with both absorption and emission spectra of the donor-acceptor system of the gain media. This unique plasmonic characteristic of Au@Ag NRs leads to the lower lasing threshold and enhances the lasing efficiency by the effects of both enhancement of localized electromagnetic field and scattering.

Highly transparent, conductive, flexible resin films embedded with silver nanowires.

In this article, a low sheet resistance and highly transparent silver nanowire (AgNW) resin composite film was demonstrated, which was prepared by a simple and efficacious two-step spin-coating method. By burying the AgNWs below the surface of the transparent resin matrix which was cured at 150 °C in air, we achieved a uniform, highly transparent, conductive, flexible film. Compared to the reported transparent electrodes, this composite transparent and conductive film showed 10 Ω/□ sheet resistance and nearly 90% mean optical transmittance over the UV-visible range simultaneously. Undergoing hundreds of cycles of tensile and compression folding, the composite film slightly increased its sheet resistance by less than 5%, displaying good electromechanical flexibility. These characteristics of the composite AgNW-resin films were expected to be used in applications of flexible optoelectronics.

Novel red phosphorescent polymers bearing both ambipolar and functionalized Ir(III) phosphorescent moieties for highly efficient organic light-emitting diodes.

A series of novel red phosphorescent polymers is successfully developed through Suzuki cross-coupling among ambipolar units, functionalized Ir(III) phosphorescent blocks, and fluorene-based silane moieties. The photophysical and electrochemical investigations indicate not only highly efficient energy-transfer from the organic segments to the phosphorescent units in the polymer backbone but also the ambipolar character of the copolymers. Benefiting from all these merits, the phosphorescent polymers can furnish organic light-emitting diodes (OLEDs) with exceptional high electroluminescent (EL) efficiencies with a current efficiency (η L ) of 8.31 cd A(-1) , external quantum efficiency (η ext ) of 16.07%, and power efficiency (η P ) of 2.95 lm W(-1) , representing the state-of-the-art electroluminescent performances ever achieved by red phosphorescent polymers. This work here might represent a new pathway to design and synthesize highly efficient phosphorescent polymers.

Untargeted metabolomic analysis of the metabolites in roots of Pugionium cornutum seedlings under drought stress.

Pugionium cornutum is an annual or biennial xerophyte distributed in arid regions, with drought resistance properties. While previous studies have predominantly focused on the physiological changes of P. cornutum , the understanding of its metabolite variations remains limited. In this study, untargeted metabolomic technology was performed to analyse the change of metabolites in the roots of P. cornutum seedlings under drought stress. Our findings revealed that compared to the R1, the root water potential and the number of lateral roots increased, while the length of the tap root and fresh weight increased first and then decreased. In the R1-R2, a total of 45 differential metabolites (DMs) were identified, whereas in the R1-R3 82 DMs were observed. Subsequently, KEGG analysis revealed a significant enrichment of microbial metabolism in diverse environments and aminobenzoate degradation in the R1-R2, and phenylpropanoid biosynthesis, ubiquinone, and other terpenoid-quinone biosynthesis and isoquinoline alkaloid biosynthesis were significantly enriched in the R1-R3. The upregulation DMs, including L-arginosuccinate, L-tyrosine, p-coumarate, caffeate, ferulate, vanillin, coniferin, 5-aminopentanoate, 2-methylmaleate and 2-furoate in P. cornutum seedlings may play a crucial role in enhancing root growth and improving drought resistance. These findings provide a basis for future investigations into the underlying mechanisms of drought resistance in P. cornutum .

Efficient red thermally activated delayed fluorescence emitters achieved through precise control of excited state energy levels.

The variety of highly efficient red/near-infrared (NIR) materials with thermally activated delayed fluorescence (TADF) feature is extremely limited so far, and it is necessary to expand the candidate pool of excellent red/deep-red emitters. However, how to control the energy level alignment of the 1CT (singlet charge transfer) state and the 3LE (triplet local excitation) state to improve the emission efficiency of materials remains a challenge. Herein, based on our previously reported green fluorescent material 67dTPA-FQ, three new donor-acceptor type TADF materials (TQ-oMeOTPA, TsQ-oMeOTPA and SQ-oMeOTPA) were designed by introducing 4,4'-dimethoxy triphenylamine (MeOTPA) as the donor, and introduced S atoms on the acceptors to enhance the spin-orbit coupling (SOC) and CT effects. The theoretical calculations showed that the newly introduced MeOTPA and S atom successfully enhanced the CT effect of the materials, not only shifting the luminescence peak to the deep red region but also effectively adjusting the energy level alignment of the excited state, accelerating the reverse intersystem crossing process. Finally, the organic light-emitting diodes based on SQ-oMeOTPA exhibit an external quantum efficiency of 19.1%, with an emission peak at 619 nm. This work not only expands the candidate inventory of red TADF materials, but also proves the feasibility of designing emitters by adjusting the excited state energy levels, greatly broadening the diversity of TADF emitters in design, and providing a powerful means for rapidly screening efficient emitters in the future.

Partial Ligand Stripping from CsPbBr3 Nanocrystals Improves Their Performance in Light-Emitting Diodes.

Halide perovskite nanocrystals (NCs), specifically CsPbBr3, have attracted considerable interest due to their remarkable optical properties for optoelectronic devices. To achieve high-efficiency light-emitting diodes (LEDs) based on CsPbBr3 nanocrystals (NCs), it is crucial to optimize both their photoluminescence quantum yield (PLQY) and carrier transport properties when they are deposited to form films on substrates. While the exchange of native ligands with didodecyl dimethylammonium bromide (DDAB) ligand pairs has been successful in boosting their PLQY, dense DDAB coverage on the surface of NCs should impede carrier transport and limit device efficiency. Following our previous work, here, we use oleyl phosphonic acid (OLPA) as a selective stripping agent to remove a fraction of DDAB from the NC surface and demonstrate that such stripping enhances carrier transport while maintaining a high PLQY. Through systematic optimization of OLPA dosage, we significantly improve the performance of CsPbBr3 LEDs, achieving a maximum external quantum efficiency (EQE) of 15.1% at 516 nm and a maximum brightness of 5931 cd m-2. These findings underscore the potential of controlled ligand stripping to enhance the performance of CsPbBr3 NC-based optoelectronic devices.

2D Hybrid Perovskites: From Static and Dynamic Structures to Potential Applications.

2D perovskites have received great attention recently due to their structural tunability and environmental stability, making them highly promising candidates for various applications by breaking property bottlenecks that affect established materials. However, in 2D perovskites, the complicated interplay between organic spacers and inorganic slabs makes structural analysis challenging to interpret. A deeper understanding of the structure-property relationship in these systems is urgently needed to enable high-performance tunable optoelectronic devices. Herein, this study examines how structural changes, from constant lattice distortion and variable structural evolution, modeled with both static and dynamic structural descriptors, affect macroscopic properties and ultimately device performance. The effect of chemical composition, crystallographic inhomogeneity, and mechanical-stress-induced static structural changes and corresponding electronic band variations is reported. In addition, the structure dynamics are described from the viewpoint of anharmonic vibrations, which impact electron-phonon coupling and the carriers' dynamic processes. Correlated carrier-matter interactions, known as polarons and acting on fine electronic structures, are then discussed. Finally, reliable guidelines to facilitate design to exploit structural features and rationally achieve breakthroughs in 2D perovskite applications are proposed. This review provides a global structural landscape of 2D perovskites, expected to promote the prosperity of these materials in emerging device applications.

Optimizing the Buried Interface in Flexible Perovskite Solar Cells to Achieve Over 24% Efficiency and Long-Term Stability.

The buried interface of the perovskite layer has a profound influence on its film morphology, defect formation, and aging resistance from the outset, therefore, significantly affects the film quality and device performance of derived perovskite solar cells. Especially for FAPbI3 , although it has excellent optoelectronic properties, the spontaneous transition from the black perovskite phase to nonperovskite phase tends to start from the buried interface at the early stage of film formation then further propagate to degrade the whole perovskite. In this work, by introducing ─NH3 + rich proline hydrochloride (PF) with a conjugated rigid structure as a versatile medium for buried interface, it not only provides a solid α-phase FAPbI3 template, but also prevents the phase transition induced degradation. PF also acts as an effective interfacial stress reliever to enhance both efficiency and stability of flexible solar cells. Consequently, a champion efficiency of 24.61% (certified 23.51%) can be achieved, which is the highest efficiency among all reported values for flexible perovskite solar cells. Besides, devices demonstrate excellent shelf-life/light soaking stability (advanced level of ISOS stability protocols) and mechanical stability.