Short Exciton Lifetimes Research Articles

Efficient Light-Driven Ion Pumping for Deep Desalination via the Vertical Gradient Protonation of Covalent Organic Framework Membranes.

Traditional desalination methods face criticism due to high energy requirements and inadequate trace ion removal, whereas natural light-driven ion pumps offer superior efficiency. Current synthetic systems are constrained by short exciton lifetimes, which limit their ability to generate sufficient electric fields for effective ion pumping. We introduce an innovative approach utilizing covalent-organic framework membranes that enhance light absorption and reduce charge recombination through vertical gradient protonation of imine linkages during acid-catalyzed liquid-liquid interfacial polymerization. This technique creates intralayer and interlayer heterojunctions, facilitating interlayer hybridization and establishing a robust built-in electric field under illumination. These improvements enable the membranes to achieve remarkable ion transport across extreme concentration gradients (2000:1), with a transport rate of approximately 3.2 × 1012 ions per second per square centimeter and reduce ion concentrations to parts per billion. This performance significantly surpasses that of conventional reverse osmosis systems, representing a major advancement in solar-powered desalination technology by substantially reducing energy consumption and secondary waste.

Leveraging Compatible Iridium(III) Complexes to Boost Performance of Green Solvent‐Processed Non‐Fullerene Organic Solar Cells

AbstractIn organic solar cells (OSCs), the short exciton lifetime poses a significant limitation to exciton diffusion and dissociation. Extending exciton lifetime and suppressing recombination are crucial strategies for improving the OSC performance. Herein, an effective approach is proposed by introducing the phosphorescent emitter, tris(2‐(4‐(tert‐butyl)phenyl)‐5‐fluoropyridine)Iridium(III), with long‐lived triplet exciton lifetime in OSCs. This research reveals that the steric structure of fac‐Ir(tBufppy)3 exhibits excellent compatibility with both the donor PM6 and acceptor BTP‐eC9, maintaining efficiencies of over 90% even with a 30% third component loading. Moreover, a 10% addition of fac‐Ir(tBufppy)3 mitigates excessive aggregation in the acceptor BTP‐eC9, optimizing the active layer morphology and improving the fill factor. Transient absorption spectroscopy and transient photoluminescence measurements demonstrate that the introduction of fac‐Ir(tBufppy)3 significantly extends exciton lifetimes and suppresses recombination, which increases the short‐circuit current (JSC). Ultimately, employing the non‐halogenated solvent o‐xylene for processing, an impressive power conversion efficiency (PCE) of 18.54% is achieved in devices based on PM6:10%fac‐Ir(tBufppy)3:BTP‐eC9, surpassing the efficiency of binary PM6:BTP‐eC9 devices (17.41%). This work provides a promising approach to further improve the PCEs in binary OSCs by introducing a phosphorescent iridium(III) complex as the third component.

Surface-Engineered Cesium Lead Bromide Perovskite Nanocrystals for Enabling Photoreduction Activity.

In recent years, colloidal lead halide perovskite (LHP) nanocrystals (NCs) have exhibited such intriguing light absorption properties to be contemplated as promising candidates for photocatalytic conversions. However, for effective photocatalysis, the light harvesting system needs to be stable under the reaction conditions propaedeutic to a specific transformation. Unlike photoinduced oxidative reaction pathways, photoreductions with LHP NCs are challenging due to their scarce compatibility with common hole scavengers like amines and alcohols. In this contribution, it is investigated the potential of CsPbBr3 NCs protected by a suitably engineered bidentate ligand for the photoreduction of quinone species. Using an in situ approach for the construction of the passivating agent and a halide excess environment, quantum-confined nanocubes (average edge length = 6.0 ± 0.8 nm) are obtained with a low ligand density (1.73 ligand/nm2) at the NC surface. The bifunctional adhesion of the engineered ligand boosts the colloidal stability of the corresponding NCs, preserving their optical properties also in the presence of an amine excess. Despite their relatively short exciton lifetime (τAV = 3.7 ± 0.2 ns), these NCs show an efficient fluorescence quenching in the presence of the selected electron accepting quinones (1,4-naphthoquinone, 9,10-phenanthrenequinone, and 9,10-anthraquinone). All of these aspects demonstrate the suitability of the NCs for an efficient photoreduction of 1,4-naphthoquinone to 1,4-dihydroxynaphthalene in the presence of triethylamine as a hole scavenger. This chemical transformation is impracticable with conventionally passivated LHP NCs, thereby highlighting the potential of the surface functionalization in this class of nanomaterials for exploring new photoinduced reactivities.

Highly efficient ionic thermally activated delayed fluorescence emitters with short exciton lifetimes towards High-Performance Solution-Processed OLEDs

Ionic thermally activated delayed fluorescence (iTADF) materials are newly emerging as promising candidates for constructing solution-processed electroluminescence devices. Developing iTADF materials with high emission efficiency, short exciton lifetime, and fast reverse intersystem crossing (RISC) is crucial for achieving stable and efficient organic light-emitting diodes (OLEDs), but it remains challenging. Here we report two high-performance iTADF emitters, namely DMAC-TPPO[BF4] and DMAC-TPPS[BF4], which consist of a conventional acridine as the electron-donor and newly developed oxygen/sulfur-fused arylphosphonium derivatives as the electron-acceptor. DMAC-TPPO[BF4] and DMAC-TPPS[BF4] exhibit high photoluminescence quantum yields of 77 % and 83 %, coupled with short exciton lifetimes of 1.2 and 1.1 μs, and RISC rate constants reaching up to 2.60 × 106 and 2.73 × 106 s−1, respectively. Theoretical and experimental investigations demonstrate that the imbedding of bridging heteroatoms into the phosphonium cation is pivotal for enhancing emission efficiency through molecular skeleton rigidification, as well as for accelerating the RISC process by capitalizing on the non-metal heavy-atom effect. Eventually, solution-processed OLEDs utilizing DMAC-TPPS[BF4] realize a peak external quantum efficiency (EQE) of 17.8 % and show a minor efficiency roll-off of 7.3 % (EQE = 16.5 %) even at a practical high luminance of 1000 cd/m2.

A Rising Star: Luminescent Carbene-Metal-Amide Complexes.

Coinage metal (gold, silver, and copper) complexes are attractive candidates to substitute the widely studied noble metal complexes, such as, iridium(III) and platinum(II), as luminescent materials in organic light-emitting diodes (OLEDs). However, the development of coinage metal complexes exhibiting high emission quantum yields and short exciton lifetimes is still a formidable challenge. In the past few years, coinage metal complexes featuring a carbene-metal-amide (CMA) motif have emerged as a new class of luminescent materials in OLEDs. Thanks to the coinage metal-bridged linear geometry, coplanar conformation, and the formation of excited states with dominant ligand-to-ligand charge transfer character and reduced metal d-orbital participation, most CMA complexes have high radiative rates via thermally activated delayed fluorescence. Currently, the family of CMA complexes have rapidly evolved and remarkable progresses in CMA-based OLEDs have been made. Here, a Concept article on CMA complexes is presented, with a focus on molecular design principles, the correlation between molecular structure/conformation and optoelectronic properties, as well as OLED performance. The future prospects of CMA complexes are also discussed.

Hybridized Local and Charge-Transfer Excited-State Emitter for a Blue Organic Solid-State Laser.

The use of triplet excitons harvesting and short exciton lifetime organic emitters is important to improve the exciton utilization in organic semiconductor laser diodes. In this study, a hybridized local and charge-transfer (HLCT)-type molecule, 11-(3-(10-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)anthracen-9-yl)phenyl)-11H-benzofuro[3,2-b]carbazole (PhAnMBf), is used as an emitter for blue-emitting organic solid-state lasers (OSSLs). The short exciton lifetime and high photoluminescence quantum yield of the PhAnMBf emitter allowed the fabrication of an organic laser with an emission wavelength of 453 nm, a small full width at half-maximum of 1.2 nm, and a threshold of 105 nJ/pulse, corresponding to 44 μJ/cm2, on the distributed feedback substrate. The anthracene-based PhAnMBf material showed the potential of the HLCT emitter as an OSSL.

Sulfone-incorporated thermally activated delayed fluorescence emitters enable organic light-emitting diodes with low efficiency roll-off

Purely organic thermally activated delayed fluorescence (TADF) emitters have shown enormous potential yet still suffer from the serious efficiency roll-off in organic light-emitting diodes (OLEDs) due to the long exciton lifetimes, thus impeding the wide-range commercialization. In this work, phenothiazine dioxide is incorporated into the diphenyl sulfone derivatives to construct two novel emitters, namely 10-(4-((4-(9,9-dimethylacridin-10(9H)-yl)phenyl)sulfonyl)phenyl)-10H-phenothiazine 5,5-dioxide (DMAC-DPS-DOPTZ) and 10-(4-((4-(10H-phenoxazin-10-yl)phenyl)sulfonyl)phenyl)-10H-phenothiazine 5,5-dioxide (PXZ-DPS-DOPTZ). Heavy atom effect of sulfur (S) atom can efficiently improve the spin-orbit coupling (SOC) and accelerate the reverse intersystem crossing (RISC) process of the emitters. In addition, the S atom could promote intersystem crossing (ISC) channel. Expectedly, both emitters exhibit the tiny energy splitting (ΔEST = 0.03 eV) between the lowest excited single state (S1) and triplet state (T1). The short exciton lifetimes (τd = 3.2 μs) and rapid RISC rates (kRISC = 2.7 × 106 s−1) are realized without introducing noble metals, which demonstrate the efficient TADF. Moreover, the emitters display obvious aggregation-induced emission (AIE) property. The OLEDs fabricated with the emitters achieve a maximum quantum efficiency (EQE) of 11.7%. Significantly, even at a luminance of 1000 cd m−2, the device EQE is retained to be 10.1% for the device based on PXZ-DPS-DOPTZ emitter, which represents the extremely low efficiency roll-off among reported TADF OLEDs.

Realizing high-performance bluish-green ionic thermally activated delayed fluorescence materials through counterion regulation

Ionic thermally activated delayed fluorescence (iTADF) materials have rarely been reported and applied in organic light emitting diodes (OLEDs) owing to their poor sublimability, photoluminescence (PL) and electroluminescence (EL) performance. It is appealing and challenging to develop iTADF emitters for high-performance OLEDs based on a deep understanding of their structure–property relationship. In this work, we report three new iTADF materials, DMAC-TPP[Br], DMAC-TPP[BF4] and DMAC-TPP[BArF24], which have the same phosphonium-cation-based chromophore and different counter anions. These materials show excellent PL properties, e.g., high photoluminescence quantum yields (PLQYs), short exciton lifetimes and fast reverse intersystem crossing (RISC). The comparative studies reveal that the photophysical properties of these iTADF materials are slightly affected by the anions via molecular configuration, intra-/inter-molecular interactions and molecular stacking. More importantly, we find that the anions play a key role in determining the EL performance of the iTADF emitters. The change of anions can lead to 3-fold increased external quantum efficiency (EQE), over 21-fold increased luminance and significantly suppressed efficiency roll-offs for the iTADF-OLEDs. The solution-processed OLED based on DMAC-TPP[BF4] achieved a EQE of 15.1 % with small efficiency roll-offs of only 0.7 % and 10.6 % at luminance of 100 cd/m2 and 1000 cd/m2, and a maximum luminance of 10400 cd/m2. The significant difference in EL performance is ascribed to the different charge recombination governed by the migration and hole-trapping ability of anions under an electric field. These results motivate the further development of high-performance iTADF materials for EL applications by anion engineering.

Exciton and vibrational dynamics of MAu24(SR)18 (M=Pd, Pt) nanoclusters

The optical properties of doped metal nanoclusters (NCs) have stimulated great research interests because of their applications in biosensing and photocatalysis. The photoluminescence and excited state dynamics of MAu24(SR)18 are complicated and the detailed mechanism has not been fully understood. Here, we investigate the exciton and vibrational dynamics of two doped NCs MAu24(SR)18 (M=Pd, Pt; SR stands for phenylethanethiolate) by ultrafast spectroscopy. In contrast to the parent Au25(SR)18 NCs, Pd and Pt doping significantly reduce the exciton lifetime by several orders of magnitude. We find that the ultrashort exciton lifetimes of PtAu24 (5 ps) and PdAu24 (30 ps) are ascribed to the ultrasmall energy gap (Eg=0.3 eV). In both two doped NCs, we observe significant coherent vibrations (2.4 THz) that arise from the metal core, which indicates these oscillations can survive regardless of the short exciton lifetime. Unravelling the effect of foreign atom doping on the exciton and vibrational dynamics of metal NCs will provide new insight into their optical properties and help designing these molecular-like nanostructures for specific applications.

Spectra stable deep-blue light-emitting diodes based on cryolite-like cerium(III) halides with nanosecond d-f emission.

Next-generation wide color gamut displays require the development of efficient and toxic-free light-emitting materials meeting the crucial Rec. 2020 standard. With the rapid progress of green and red perovskite light-emitting diodes (PeLEDs), blue PeLEDs remain a central challenge because of the undesirable color coordinates and poor spectra stability. Here, we report Cs3CeBrxI6-x (x=0to 6) with the cryolite-like structure and stable and tunable color coordinates from (0.17, 0.02) to (0.15, 0.04). Further encouraged by the short exciton lifetime (26.1ns) and high photoluminescence quantum yield (~76%), we construct Cs3CeBrxI6-x-based rare-earth LEDs via thermal evaporation. A seed layer strategy is conducted to improve the device's performance. The optimal Cs3CeI6 device achieves a maximum external quantum efficiency of 3.5% and a luminance of 470 cd m-2 with stable deep-blue color coordinates of (0.15, 0.04). Our work opens another avenue to achieving efficient and spectrally stable deep-blue LEDs.

Highly Emissive Luminogens in Both Solution and Aggregate States

Highly Emissive Luminogens in Both Solution and Aggregate States

Metal and halogen-free purely organic room temperature phosphorescence material using heavy atom effect of phenoselenazine

Development of purely organic room temperature phosphorescent (RTP) emitters is challenging and promising to various applications. However, it is very difficult to observe phosphorescence from common organic emitters. Herein, we introduced phenoselenazine (PSeZ) as a moiety to induce phosphorescence and evaluated the emission properties of (10 H -phenoselenazin-10-yl)(phenyl)methanone (BzylPSeZ) and demonstrated effects of PSeZ on room temperature phosphorescent properties in comparison with (10 H -phenothiazin-10-yl)(phenyl)methanone (BzylPTZ) which contains a phenothiazine. As a result, selenium in the PSeZ enhanced spin-orbit coupling by heavy atom effect and BzylPSeZ showed phosphorescence in crystalline state. The BzylPSeZ exhibited phosphorescence quantum yield of 7.7% and phosphorescence lifetime of 670 μs. This work proved the possibility of the PSeZ as a unit of pure organic RTP emitters. • Room temperature phosphorescence using selenium based organic units. • Strong spin-orbit coupling by heavy atom effect of selenium. • Short triplet exciton lifetime in pure organic phosphorescent emitter.

Long-range transport and ultrafast interfacial charge transfer in perovskite/monolayer semiconductor heterostructure for enhanced light absorption and photocarrier lifetime.

Atomically thin two-dimensional transition metal dichalcogenides (TMDs) have shown great potential for optoelectronic applications, including photodetectors, phototransistors, and spintronic devices. However, the applications of TMD-based optoelectronic devices are severely restricted by their weak light absorption and short exciton lifetime due to their atomically thin nature and strong excitonic effect. To simultaneously enhance the light absorption and photocarrier lifetime of monolayer semiconductors, here, we report 3D/2D perovskite/TMD type II heterostructures by coupling solution processed highly smooth and ligand free CsPbBr3 film with MoS2 and WS2 monolayers. By time-resolved spectroscopy, we show interfacial hole transfer from MoS2 (WS2) to the perovskite layer occurs in an ultrafast time scale (100 and 350fs) and interfacial electron transfer from ultrathin CsPbBr3 to MoS2 (WS2) in ∼3 (9) ps, forming a long-lived charge separation with a lifetime of >20ns. With increasing CsPbBr3 thickness, the electron transfer rate from CsPbBr3 to TMD is slower, but the efficiency remains to be near-unity due to coupled long-range diffusion and ultrafast interfacial electron transfer. This study indicates that coupling solution processed lead halide perovskites with strong light absorption and long carrier diffusion length to monolayer semiconductors to form a type II heterostructure is a promising strategy to simultaneously enhance the light harvesting capability and photocarrier lifetime of monolayer semiconductors.

73‐2: Invited Paper: High‐Efficiency Organic Light‐Emitting Diodes Based on Purely Organic Emitters

Horizontal dipole orientation of light‐emitting materials has a significant effect on the optical out‐coupling efficiency of organic light‐emitting diodes (OLEDs). This paper will present the molecular design of planar or linear‐shaped donor‐acceptor type purely organic light‐emitting materials towards high horizontal dipole orientation of the light emitters. In addition, host‐guest engineering is also investigated to induce further improved horizontal dipole ratio. Besides, owing to the tuned exciton dynamics like fast radiative rate, suppressed non‐radiation and short exciton lifetime, highly efficient monochromatic and white OLEDs are achieved.

Blue Thermally Activated Delayed Fluorescence with Sub‐Microsecond Short Exciton Lifetimes: Acceleration of Triplet–Singlet Spin Interconversion via Quadrupolar Charge‐Transfer States

AbstractExciton lifetime is a critical factor in determining the performance of optoelectronic functional systems and devices. Thermally activated delayed fluorescence (TADF) emitters that can concurrently achieve a high fluorescence quantum yield and short exciton lifetime are desirable for application in organic light‐emitting diodes (OLEDs) with suppressed efficiency roll‐off. Herein, phenoxaborin and xanthone‐cored TADF emitters with quadrupolar electronic structures are reported to exhibit sub‐microsecond TADF lifetimes as short as 650 and 970 ns, respectively, while preserving high fluorescence quantum yields. By extending the El‐Sayed rule to the quadrupolar π‐systems, the contribution of doubly degenerate charge‐transfer excited states induced by dual donor units can enhance the spin–orbit coupling between them, leading to a spin‐flip acceleration between the excited triplet and singlet states. This electronic feature is advantageous for mitigating exciton annihilation processes in the emission layer, thereby reducing the efficiency roll‐offs in OLEDs. Consequently, a high external electroluminescence quantum efficiency over 20% can be retained, even under operating the device at a high luminance of 1000 cd m−2.

Boosting quantum yields in two-dimensional semiconductors via proximal metal plates

Monolayer transition metal dichalcogenides (1L-TMDs) have tremendous potential as atomically thin, direct bandgap semiconductors that can be used as convenient building blocks for quantum photonic devices. However, the short exciton lifetime due to the defect traps and the strong exciton-exciton interaction in TMDs has significantly limited the efficiency of exciton emission from this class of materials. Here, we show that exciton-exciton interaction in 1L-WS2 can be effectively screened using an ultra-flat Au film substrate separated by multilayers of hexagonal boron nitride. Under this geometry, induced dipolar exciton-exciton interaction becomes quadrupole-quadrupole interaction because of effective image dipoles formed within the metal. The suppressed exciton-exciton interaction leads to a significantly improved quantum yield by an order of magnitude, which is also accompanied by a reduction in the exciton-exciton annihilation (EEA) rate, as confirmed by time-resolved optical measurements. A theoretical model accounting for the screening of the dipole-dipole interaction is in a good agreement with the dependence of EEA on exciton densities. Our results suggest that fundamental EEA processes in the TMD can be engineered through proximal metallic screening, which represents a practical approach towards high-efficiency 2D light emitters.

Managing local triplet excited states of boron-based TADF emitters for fast spin-flip process: Toward highly efficient TADF-OLEDs with low efficiency roll-off

Improving the rate of a spin-flip process from a triplet to a singlet excited state, which is known as reverse intersystem crossing (RISC), is a challenging issue in realizing efficient and stable organic light-emitting diodes based on thermally activated delayed fluorescence (TADF-OLEDs). Herein, we report two donor−acceptor-type TADF emitters (TMCzBCO and DMACBCO) employing a boron-carbonyl (BCO) hybrid acceptor unit. The emitters possess a 3nπ* state of the BCO unit as an intermediate local triplet excited state (3LE, T2), leading to the large spin–orbit coupling between the T2 and excited singlet (S1) states with significantly low activation barriers <10 meV. Thus, a fast RISC with rate constants (kRISC) exceeding 106 s−1, as well as short exciton lifetimes (~1 μs), are realized. Using the TADF emitters, high-performance TADF-OLEDs with maximum external quantum efficiencies of 24.7% for the green (TMCzBCO) and 28.4% for the yellow (DMACBCO) are presented. Remarkably, the devices sufficiently maintain high efficiency at high luminance, exceeding 20% at 5,000 cd m−2 and 18% at 10,000 cd m−2. These results indicate the crucial role of the 3nπ* state of BCO in the improvement of the RISC rate and shortening of the exciton lifetime, enhancing the optoelectrical performance of TADF-OLEDs.

Developing solid-state photon upconverters based on sensitized triplet–triplet annihilation

Photon upconversion assisted by sensitized triplet–triplet annihilation (sTTA-UC) is a wavelength-shifting technique where high-energy photons are emitted from the radiative recombination of high-energy singlets populated through the annihilation of the metastable triplets of two annihilator/emitter molecules. The emitter triplets are previously populated via energy transfer from a light-harvester/sensitizer moiety that absorbs the incident low-energy photons. In solutions, this process is efficient even at low excitation powers, whereas the limited molecular mobility and short exciton lifetimes typically observed in solid matrices hinder the bi-molecular interactions making the sTTA-UC process rather ineffective. We show here that controlling the confinement of the upconverting dye pairs in nanostructured or nanosized materials results in an increased effective local density of the excitation energy. This also activates a specific sTTA-UC kinetics independent of the triplet excitons’ mobility that improves the material performance at low powers. We provide a complete modeling of the sTTA-UC process in confined systems. The results obtained afford useful guidelines for the future development of upconverting photonic devices operating at subsolar irradiances suitable for technological implementation.

Assessing the effects of increasing conjugation length on exciton diffusion: from small molecules to the polymeric limit.

Organic solar cells (OSC) generally contain long-chain π-conjugated polymers as donor materials, but, more recently, small-molecule donors have also attracted considerable attention. The nature of these compounds is of crucial importance concerning the various processes that determine device performance, among which singlet exciton diffusion is one of the most relevant. The efficiency of the diffusion mechanism depends on several aspects, from system morphology to electronic structure properties, which vary importantly with molecular size. In this work, we investigated the effects of conjugation length on the exciton diffusion length through electronic structure calculations and an exciton diffusion model. By applying extrapolation procedures to thiophene and phenylene vinylene oligomer series, we investigate their electronic and optical properties from the small-molecule point of view to the polymeric limit. Several properties are calculated as a function of oligomer size, including transition energies, absorption and emission spectra, reorganization energies, exciton coupling and Förster radii. Finally, an exciton diffusion model is used to estimate diffusion lengths as a function of oligomer size and for the polymeric limit showing agreement with experimental data. Results also show that longer conjugation lengths correlate with longer exciton diffusion lengths in spite of also being associated with shorter exciton lifetimes.

InGaN quantum dots with short exciton lifetimes grown on polar c-plane by metal-organic chemical vapor deposition

An investigation of self-assembled polar InGaN quantum dots (QDs) on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD) is reported. The radiative exciton lifetime is measured by time-resolved photoluminescence at a low temperature of 18 K, where the non-radiative recombination can be negligible. A mono-exponential exciton decay with a radiative exciton lifetime of 480 ps for uncapped QDs is revealed. With an optimized GaN capping layer grown by a two-step method, a radiative exciton lifetime of 707 ps for the capped QDs is preserved. The short radiative exciton lifetime is much shorter than that for previously studied polar QDs and is even comparable with those grown along non-polar QDs, which is strong evidence of the reduction of built-in fields in these polar InGaN QDs.