Efficient Exciton Research Articles

Carbazole and Triazine-Based D-A Covalent Organic Framework for Visible Light-Mediated Photocatalytic C-H Activation of Imidazopyridine and Indole.

Two-dimensional donor-acceptor covalent organic frameworks (COFs) show considerable promise for metal-free and heterogeneous photocatalysis due to their efficient charge carrier separation and exciton transport upon photoexcitation. To date, numerous photocatalysts have been developed. However, they encounter several challenges, such as inadequate sunlight harvesting ability, poor photostability, and nonreusability. Fortunately, the emergence of COFs presents a promising solution to these problems. Herein, we report an imine-linked CzTA-TAPT COF featuring carbazole as the electron donor and triazine as the electron acceptor. Compared to the previously reported C2-linker-derived CzDA-TAPT COF, this C3-linked COF exhibits good charge separation and charge carrier transport. As a consequence, it demonstrates excellent photocatalytic applicability in the C-3 thiocyanation of imidazo[1,2-a]pyridine and indole under ambient conditions under visible light. Moreover, its broad substrate compatibility and high recyclability provide a green and sustainable approach for the thiocyanation of imidazopyridine and indole. To the best of our knowledge, this is the first heterogeneous catalyst demonstrated for the thiocyanation of imidazo[1,2-a]pyridine. These findings will inspire further research in the development of high-performance D-A COFs as photocatalysts for organic transformations.

Grain size control in quasi-two-dimensional perovskite thin film via intermediate phase engineering for efficient bound exciton generation

AbstractQuasi-two dimensional (2D) perovskites have emerged as a promising class of materials due to their remarkable photoluminescence efficiency, which stems from their exceptionally high exciton binding energies. The spatial confinement of excitons within smaller grain sizes could enhance the formation of biexcitons leading to higher radiative recombination efficiency. However, the synthesis of high-quality quasi-2D perovskite thin films with controllable grain sizes remains a challenging task. In this study, we present a facile method for achieving quasi-2D perovskite thin films with controllable grain sizes ranging from 500 to 900 nm. This is accomplished by intermediate phase engineering during the film fabrication process. Our results demonstrate that quasi-2D perovskite films with smaller grain sizes exhibit more efficient bound exciton generation and a reduced stimulated emission threshold down to 15.89 µJ cm−2. Furthermore, femtosecond transient absorption measurements reveal that the decay time of bound excitons is shorter in quasi-2D perovskites with smaller grain sizes compared to that of those with larger grains at the same pump density, which is 230.5 ps. This observation suggests a more efficient exciton recombination process in the smaller grain size regime. Our findings would offer a promising approach for the development of efficient bound exciton lasers.

Gradient Hole Injection Inducing Efficient Exciton Recombination in Blue (475 nm) Perovskite QLEDs.

Perovskite quantum dots (QDs) are emerging as excellent light sources for light-emitting diodes (LEDs). However, the performance of blue perovskite QD-based LEDs (QLEDs) still lags behind that of red and green counterparts, which is hindered by blue perovskite QDs with broad bandgaps that tend to increase nonradiative recombination. Here, we designed a gradient energy for hole injection utilizing multiple hole injection layers (HTLs) combined with carbazole-based small-molecule modification to reduce the hole injection barrier between HTLs and QD layers and improve the hole injection efficiency, realizing efficient exciton recombination in blue perovskite QLEDs. Moreover, the QD film on the designed HTLs demonstrates a lower surface roughness and improved photoluminescence properties. The optimized blue CsPbCl3-xBrx QLEDs exhibit an impressive external quantum efficiency of 20.7% with an electroluminescence peak at 475 nm and a turn-on voltage of 2.6 V, representing the state-of-the-art for blue perovskite LEDs emitting in the range of 460-480 nm.

Linker‐Mediated Delocalized Excited States in Dimeric Acceptors Enable Efficient Exciton Dissociation with Negligible Energy‐level Offsets

Efficient exciton dissociation at low energy offsets is key to overcoming voltage losses in organic solar cells. In this work, we developed two dimeric acceptors, i‐YT and o‐YT, by precisely controlling the position of an asymmetric electron‐donating linker. It induced the foldamer conformation of i‐YT with a para linkage (relative to the dicyano groups), while retaining the unfold conformation for o‐YT. This subtle structural modification influenced the molecular assembly properties, enabled near‐zero energy offset exciton dissociation and power conversion efficiencies exceeding 18% for i‐YT based organic solar cells. Detailed excitonic dynamics further revealed that the linker position critically influences three processes: the formation of delocalized singlet excited states, ultrafast charge transfer (~5 ps) in solid blends, and the suppression of exciton recombination. Additionally, devices based on i‐YT demonstrated outstanding long‐term stability, retaining over 85% of their initial efficiency after 1,400 hours of continuous illumination. These findings introduce a new class of dimeric acceptors that combine high efficiency with exceptional stability, offering a promising pathway toward low‐energy‐loss organic photovoltaics.

Linker-Mediated Delocalized Excited States in Dimeric Acceptors Enable Efficient Exciton Dissociation with Negligible Energy-Level Offsets.

Efficient exciton dissociation at low energy offsets is key to overcoming voltage losses in organic solar cells. In this work, we developed two dimeric acceptors, i-YT and o-YT, by precisely controlling the position of an asymmetric electron-donating linker. It induced the foldamer conformation of i-YT with a para linkage (relative to the dicyano groups), while retaining the unfold conformation for o-YT. This subtle structural modification influenced the molecular assembly properties, enabled near-zero energy offset exciton dissociation and power conversion efficiencies exceeding 18 % for i-YT based organic solar cells. Detailed excitonic dynamics further revealed that the linker position critically influences three processes: the formation of delocalized singlet excited states, ultrafast charge transfer (~5 ps) in solid blends, and the suppression of exciton recombination. Additionally, devices based on i-YT demonstrated outstanding long-term stability, retaining 85 % of their initial efficiency after 1,400 hours of continuous illumination. These findings introduce a new class of dimeric acceptors that combine high efficiency with exceptional stability, offering a promising pathway toward low-energy-loss organic photovoltaics.

Efficient Triplet Harvesting via Hot Exciton Utilization in Solution Processed OLEDs Employing Tetraphenyl Buta‐1,3‐Diene Based AIE Emitters

AbstractHigh photoluminescence quantum yield (PLQY) especially in solid‐state together with harvesting radiative triplet excitons are essential to achieve efficient organic light‐emitting diodes (OLEDs). Herein, the aggregation induced emission (AIE) active tetraphenyl buta‐1,3‐diene (TPB) moiety has been employed and tactfully modified to obtain orthogonally oriented donor‐acceptor (D‐A‐π‐A‐D) type derivatives (namely, TPB‐CHO‐TPA and TPB‐CN‐TPA) which possess additionally the hybridized local and charge transfer (HLCT) type excited‐state. Moreover, computational studies have revealed the possibility of triplet harvesting through hot exciton mechanism in these designed emitters. These key features along with excellent solubility in most of the organic solvents have encouraged to utilize these as light emitting materials for solution processed non‐doped and doped OLED devices. The optimal OLED device using TPB‐CHO‐TPA exhibits yellow light emission (ELmax = 572 nm and CIEx,y = 0.48, 0.51) having maximum external quantum efficiency (EQEmax) of 7.6% with power and current efficiency of 6.1 lm W−1 and 8.9 cd A−1 where the calculated exciton utilization efficiency (EUE) approaches to 97%, indicating efficient triplet harvesting in electroluminescence process. This work signifies a novel design strategy for AIE‐based HLCT type emitters having efficient hot exciton utilization which can pave the way for future development of TPB based highly efficient OLED devices.

Engineering exciton dissociation and intermolecular charge transfer to boost superoxide radical in conjugated microporous polymers for simultaneous elimination of coexisting contaminants

Simultaneous photocatalytic elimination of coexisting contaminants with polymeric semiconductors are highly urgent for environmental purification. Nevertheless, the insufficient exciton dissociation and charge transfer in polymeric photocatalysts results in unsatisfactory photocatalytic performance. Herein, we proposed a sulfur-contained linkages engineering strategy via regulating donor–acceptor interactions to boost exciton dissociation and intermolecular charge transfer for efficient generation of superoxide radical. With the low exciton binding energy (50.11 meV) and short average lifetime (τavg‑TAS) of the photoinduced exciton (478.1 ps), the resultant BDT-Tr deriving from BDT and 2,4,6-tris(4-bromophenyl)-1,3,5-triazine exhibited efficient exciton dissociation and charge transfer, enabling superior photocatalytic degradation of ofloxacin (∼94.0 % in 2 h) than that of TTP-Tr (∼87 %) and DTT-Tr (∼76 %) when coupled with the uranium (VI) reduction (∼96.0 % in 1 h). This is the first work highlights the photocatalytic removal of coexisting aqueous pollutants with CMPs-based photocatalysts, which might facilitate the exploration of polymeric semiconductors for wastewater purification.

CBD-synthesized Mg-doped CdS thin films for hybrid solar cells and self-powered photodetectors

Mg-doped CdS samples were deposited via chemical bath deposition onto fluorine-doped tin oxide slides with varying levels of Mg-doping. Structural analysis revealed improved crystal quality in CdS films upon Mg incorporation. Morphological examinations indicated a reduction in grain size alongside appearance of smooth, void-free surfaces particularly evident at 3 % Mg-doping. Mg-doping also resulted in enhanced transparency of CdS films, notably at 3 % and 5 % within the visible spectrum. Efficient exciton dissociation was observed in hybrid solar cells based on 1 % and 3 % Mg-doped CdS, as evidenced by photoluminescence. Top-performing solar cell achieved an efficiency of 0.220 %, nearly seven times that of control device. 5 % Mg-doped CdS-based photodetectors exhibited favorable photosensing characteristics: a responsivity of 0.011 A/W, detectivity of 4.4 × 108 Jones, external quantum efficiency of 3.1 %, and rise/decay times of 26/25 ms at zero bias. These findings underscore beneficial effects of Mg-doping on both hybrid solar cell and self-driven photodetector performance.

An efficient thermally activated delayed fluorophore featuring an oxygen-locked azaryl ketone acceptor

Thermally activated delayed fluorescence (TADF) dyes are promising for applications like bioimaging, sensing, and optoelectronic devices due to their efficient exciton utilization without noble metals, making them ideal for organic light-emitting diodes (OLEDs). In this contribution, we introduce a new TADF emitter, 3-(9,9-dimethylacridin-10(9H)-yl)-12H-chromeno[2,3-b]quinolin-12-one (CQLO-DMAC), which incorporates an intramolecular oxygen-locked azaryl ketone acceptor, CQLO. This design enhances molecular rigidity and electron-accepting strength. The CQLO-DMAC compound, featuring a bulky donor, shows significant intramolecular charge transfer and TADF emission. Comprehensive analyses, including spectroscopic studies and theoretical calculations, reveal high luminescence efficiency and reduced non-radiative losses. Utilizing CQLO-DMAC in OLEDs, we achieved a green-yellow emission peak at 548 nm and a maximum external quantum efficiency of 17.4%. The findings highlight the potential of CQLQ as a highly effective acceptor in designing of TADF electroluminescent dyes.

Achieving efficient self-trapped excitons emission by suppressing defect level in Sb3+ doped metal halide

The field of metal halide engineering has made considerable progress, yet additional studies are needed to completely grasp the intriguing principles that govern their luminescence. In this work, we report a novel zero-dimensional (0D) organic-inorganic metal halide hybrid (OIMH) (C3H12N2)2InCl7 (C3H12N2 = 1,3-propane diamine cation) and reveal its interesting luminescent properties through experimental research and theoretical calculations. The photoluminescence (PL) of (C3H12N2)2InCl7 is a synergistic combination of intrinsic self-trapped exciton (STE) emission and defect-bound exciton (DBE) emission. Doping with Sb3+ induces several changes in (C3H12N2)2InCl7, including defect passivation, a reduction in the band gap, and structural shrinkage. These modifications result in a transition of PL centers to external STE emission, with the photoluminescence quantum yield (PLQY) increasing from 4.44 % to 68.70 %. Our work proposes, for the first time, the synergistic emission of STE and DBE and reveals the effect of metal ion doping, providing new insights into the mechanisms of materials exhibiting multifunctional exciton emission.

Chlorinated Polythiophene‐Based Donors with Reduced Energy Loss for Organic Solar Cells

Comprehensive SummaryThe industrialization of organic solar cells (OSCs) faces challenges due to complex synthesis routes and high costs of organic photovoltaic materials. To address this, we designed and synthesized a series of polythiophene‐based donor materials, PTVT‐T‐xCl (20%Cl, 50%Cl and 100%Cl), by introducing different degrees of chlorine substitution within their conjugated skeletons. The incorporation of chlorine atoms does not change the planar conformation of the conjugated main chain of the control polymer, PTVT‐T, but effectively reduces their HOMO energy levels (≤ –5.3 eV) and alters the crystallinity of the polymers. In addition, when preparing OSC by blending with non‐fused electron acceptor A4T‐16, the non‐radiative energy loss of the three photovoltaic devices gradually decreased with the increase of chlorine content (0.343, 0.278 and 0.189 eV, respectively). Notably, PTVT‐T‐20%Cl exhibited a more moderate nanoscale phase separation with the acceptor, leading to efficient exciton dissociation, lower bimolecular recombination, and thus a favorable current in the OSCs. Consequently, the photovoltaic device based on PTVT‐T‐20%Cl:A4T‐16 achieved a remarkable photovoltaic efficiency of 11.8%. In addition, the PTVT‐T‐xCl series polymers show much lower material‐only‐cost (MOC) values than the other reported photoactive material systems. This work provides the way for the development of low‐cost photovoltaic materials and the industrial application of OSC, overcoming previous limitations posed by high energy losses in polythiophene‐based donors.

A Perspective on 2D Fused-Ring Quad-Rotor-Shaped Nonfullerene Acceptors with an Anthracene Core.

Previous studies have demonstrated the remarkable properties of quad-rotor-shaped two-dimensional nonfullerene acceptors (2D NFAs), which encompass exceptional electron affinity, robust sunlight absorption, effective exciton separation, and accelerated electron transfer capabilities. Naphthalene has been demonstrated to be a significant 2D fused core to construct high-performance 2D NFAs. However, synthesizing such materials through existing synthetic pathways poses a significant challenge. In this work, we designed four 2D NFAs (TEA-SIC, TEA-SIC-8F, TEA-SIC-OH, and TEA-SIC-OH-8F) with an anthracene core. These NFAs can theoretically be synthesized into a quad-rotor configuration through a seven-step synthetic process. Theoretical calculations have demonstrated that these 2D NFAs exhibit superior electron-accepting abilities, enhanced sunlight absorption, and more efficient exciton dissociation compared to Y6. Furthermore, TEA-SIC and TEA-SIC-8F exhibited impressive electron mobilities of 1.76 × 10-3 cm2 V-1 s-1 and 1.18 × 10-3 cm2 V-1 s-1, respectively, indicating their suitability for the development of high-performance organic solar cells (OSCs). Although TEA-SIC-OH and TEA-SIC-OH-8F have lower electron mobility, their high sunlight absorption and efficient exciton separation suggest potential as third components in ternary OSCs. These 2D NFAs also exhibit a commendable solubility in most alcohol-based solvents, indicating their potential for specialized applications in the fabrication of stacked OSCs. These findings provide valuable insights for the future design of synthesizable high-performance 2D NFAs.

Plasmonic molybdenum carbide MXene nanosheets for highly efficient and stable perovskite solar cells

Besides the excellent metallic electrical conductivity, high carrier mobility, and high optical transmittance, the two-dimensional MXenes exhibiting impressive optical and plasmonic properties have been explored to photodiodes. However, their potential use of surface plasmons oscillating in perovskite solar cells has not been studied. Herein, we incorporate, for the first time, Mo2CTx MXene nanosheets with characteristic plasmonic absorption in the whole visible region into the perovskite active layer. First, the presence of Mo2CTx nanosheets in the perovskite film increases the perovskite grain size due to the decreased growth rate and passivation of the defects around the grain boundaries through strong interaction with undercoordinated Pb2+. Furthermore, with the help of the localized surface plasmon resonance effect of Mo2CTx nanosheets, the exciton binding energy in the perovskite absorber is reduced, which suggests the efficient exciton dissociation and enhanced charge separation. Consequently, Mo2CTx nanosheets based perovskite solar cell device exhibits a champion power conversion efficiency (PCE) of 24.05 %. It is worth noting that the unencapsulated devices demonstrate considerably improved stability, maintaining about 89.25 % of the initial PCE after aging in air for 3000 h. This fascinating strategy provides more opportunities of the functional MXene materials for the perovskite optoelectronics.

Ultra-broadband Photodetectors Based on Formamidinium Lead Iodide Quantum Dots

Near-infrared photodetectors were fabricated by incorporating formamidinium lead iodide (FAPbI3) quantum dots (QDs) as the light-harvesting layer. Through systematic optimization of the device architecture, high device performance was achieved by utilizing PCBM as the electron transport material and a 50 nm-thick TAPC film as the hole transport layer. The energy level alignment between PCBM and the FAPbI3 QDs enabled efficient exciton dissociation and hole blocking, while the optimized TAPC thickness decrease current leakage pathways. The resulting photodetectors exhibited an impressive external quantum efficiency of 59.56 % at 750 nm, along with a high specific detectivity of 2.63 x 1011 Jones. A broadband photoresponse from 300–900 nm was observed, as well as a fast temporal response with 30.58/31.26 μs rise/fall times. A substantial linear dynamic range of 61.5 dB was achieved under 780 nm illumination. Furthermore, the low dark current densities facilitated by the judiciously selected materials and thicknesses contributed to the excellent overall device performance. The device performance of the PCBM/FAPbI3 QDs/TAPC system demonstrate its promising potential for near-infrared optoelectronic applications requiring high sensitivity, speed, and broad spectral response, opening up opportunities for further advances in photodetection as well as other relevant device technologies.

Efficient metal free organic radical scintillators

The development of high-performance metal-free organic X-ray scintillators (OXSTs), characterized by a synergistic combination of robust X-ray absorption, efficient exciton utilization, and short luminescence lifetimes, poses a considerable challenge. Here we present an effective strategy for achieving augmented X-ray scintillation through the utilization of halogenated open-shell organic radical scintillators. Our experimental results demonstrate that the synthesized scintillators exhibit strong X-ray absorption derived from halogen atoms, display efficacious X-ray stability, and theoretically achieve 100% exciton utilization efficiency with a short lifetime (∼18 ns) due to spin-allowed doublet transitions. The superior X-ray scintillation performance exhibited by these organic radicals is not only exploitable in X-ray radiography for contrast imaging of various objects but also applicable in a medical high-resolution micro-computer-tomography system for the clear visualization of fibrous veins within a bamboo stick. Our study substantiates the promise of organic radicals as prospective candidates for OXSTs, offering valuable insights and a roadmap for the development of advanced organic radical scintillators geared towards achieving high-quality X-ray radiography.

Efficient Deep-Blue Organic Light-Emitting Diodes Employing Doublet Sensitization.

Fast and efficient exciton utilization is a crucial solution and highly desirable for achieving high-performance blue organic light-emitting diodes (OLEDs). However, the rate and efficiency of exciton utilization in traditional OLEDs, which employ fully closed-shell materials as emitters, are inevitably limited by spin statistical limitations and transition prohibition. Herein, a new sensitization strategy, namely doublet-sensitized fluorescence (DSF), is proposed to realize high-performance deep-blue electroluminescence. In the DSF-OLED, a doublet-emitting cerium(III) complex, Ce-2, is utilized as sensitizer for multi-resonance thermally activated delayed fluorescence emitter ν-DABNA. Experimental results reveal that holes and electrons predominantly recombine on Ce-2 to form doublet excitons, which subsequently transfer energy to the singlet state of ν-DABNA via exceptionally fast (over 108s-1) and efficient (≈100%) Förster resonance energy transfer for deep-blue emission. Due to the circumvention of spin-flip in the DSF mechanism, near-unit exciton utilization efficiency and remarkably short exciton residence time of 1.36µs are achieved in the proof-of-concept deep-blue DSF-OLED, which achieves a Commission Internationale de l'Eclairage coordinate of (0.13, 0.14), a high external quantum efficiency of 30.0%, and small efficiency roll-off of 14.7% at a luminance of 1000cdm-2. The DSF device exhibits significantly improved operational stability compared with unsensitized reference device.

Room-temperature efficient and tunable interlayer exciton emissions in WS2/WSe2 heterobilayers at high generation rates.

Transition metal dichalcogenide (TMDC) heterobilayers (HBs) have been intensively investigated lately because they offer novel platforms for the exploration of interlayer excitons (IXs). However, the potentials of IXs in TMDC HBs have not been fully studied as efficient and tunable emitters for both photoluminescence (PL) and electroluminescence (EL) at room temperature (RT). Also, the efficiencies of the PL and EL of IXs have not been carefully quantified. In this work, we demonstrate that IX in WS2/WSe2 HBs could serve as promising emitters at high generation rates due to its immunity to efficiency roll-off. Furthermore, by applying gate voltages to balance the electron and hole concentrations and to reinforce the built-in electric fields, high PL quantum yield (QY) and EL external quantum efficiency (EQE) of ∼0.48% and ∼0.11% were achieved at RT, respectively, with generation rates exceeding 1021 cm-2·s-1, which confirms the capabilities of IXs as efficient NIR light emitters by surpassing most of the intralayer emissions from TMDCs.

High-performing organic/quantum dot hybrid upconversion device based on a single-component near-infrared-sensitive layer

A donor/acceptor (D/A) heterojunction with an interfacial energetic offset is demonstrated to enable efficient exciton dissociation in organic photodetectors and upconversion devices (UCDs). Unfortunately, this approach usually encounters complicated optimization procedures and interfacial instability. Herein, we present an alternative strategy for achieving high-performing UCDs by utilizing an organic single-component near-infrared (NIR)-sensitive layer instead of a D/A heterojunction. The showcased UCD is constructed by vertically stacking an organic single-component Y6 NIR-detection unit and a quantum dot light-emitting unit. Due to the high dielectric constant and low exciton binding energy of the non-fullerene acceptor Y6, free carriers are directly and spontaneously generated upon NIR light excitation. As a result, the single-component UCD achieves a low light detection capability of 2.5 μW/cm2, a fast refresh rate of >3.8 × 104, and a high resolution exceeding 1100 dpi, providing a stable optical response to high-frequency NIR signals and high-quality NIR imaging.

D/A heterojunction photocatalysts interspersed onto biochar to couple photocatalysis and adsorption for visible light-responsive efficient removal of pollutants

Phenol has various applications in industry, due to its hazardousness, solubility, and volatility, it persists in water and poses a risk to human health. People are excited by the progress of photodegradation technology that facilitates water pollution treatment. The effect of side-chain engineering is proven to enhance photodegradation performance by improving optoelectronic properties. Therefore, here is a strategy that introduces functional groups to change the electron cloud density, achieving excellent optoelectronic properties evidenced by efficient exciton dissociation, separation, and fine-tuned energy level. Subsequently, by coupling of photocatalysis and adsorption, the donor/acceptor (D/A) heterojunction photocatalysts (PBDB-T: BTTIC-TT, PBDB-T: BTTIC-Ph) are designed via loading on coconut-shell carbon to process phenol. The results indicate that the champion photocatalyst PBDB-T: BTTIC-TT possesses superior optoelectronic properties, which can be attributed to the stronger electron-withdrawing ability of thieno[3,2-b]thiophene than benzene. Consequently, the synergistic efficiency of adsorption and photocatalysis of phenol (10, 20 mg/L) are removed within 10 mins over 90 % which is still valid after twenty cycles. Surprisingly, a high concentration of phenol (100 mg/L) also exhibited excellent removal efficiency of over 80 % within 60 min.

Fluorine substitution for aggregation transformation and exciton dissociation acceleration in non-fused electron acceptors

In order to enable low-cost manufacture of organic solar cells (OSCs), it is very important to develop non-fused electron acceptors to match with one of the cheapest polymer donors, poly(3-hexylthiophene) (P3HT). However, the lower exciton dissociation and charge transfer are still the main factors that limit the improvement of P3HT-based OSCs. Herein, we designed one class of A2-A1-D-A1-A2 type non-fused electron acceptors and synthesized BTA34 and F-BTA34 by introducing benzo[d][1,2,3] triazole as the bridged unit. The dominated H-aggregation in F-BTA34 affords high degree of crystalline and proper phase separation when blended with polymer P3HT, which ensure efficient exciton dissociation and higher charge carrier mobility. Consequently, P3HT:F-BTA34 devices demonstrate a better performance with higher short-circuit current and fill factor. The blend of P3HT:BTA34 exhibits a broader distribution of the mixing domains, resulting in relative longer-lived charge species. This work provides useful strategies to design non-fused electron acceptors to match with P3HT from the perspective of simultaneously improving exciton dissociation and suppressing charge recombination.