Lowest Exciton Research Articles

Achieving Low Exciton–Exciton Annihilation Rates and High Exciton Diffusion Coefficients in Monolayer WS2 on Solid-State Electrolytes

Achieving Low Exciton–Exciton Annihilation Rates and High Exciton Diffusion Coefficients in Monolayer WS₂ on Solid-State Electrolytes

Low temperature exciton lifetime enhancement of MAPbBr3 studied by broadband terahertz spectroscopy

Over the past few years, lead halide perovskites have demonstrated significant potential in the field of solar cells, light-emitting diodes, and field-effect transistors. We investigated the transient photoconductivity and temperature-induced phase transitions in MAPbBr3 thin films, using ultrabroadband optical pump-terahertz probe spectroscopy with varying excitation fluences and temperature conditions. As the temperature drops from 293 to 123 K, MAPbBr3 undergoes two phase transitions, resulting in a significant extension of exciton lifetime in the tetragonal phase near 150 K, where MAPbBr3 exhibits a longer exciton lifetime. Our research contributes to a better understanding of the temperature-induced phase transition mechanism of MAPbBr3, enhancing its potential applications in optoelectronic devices.

Single-layer GaInTe3: Water-splitting photocatalyst low exciton binding and ultrahigh solar conversion efficiency

Herein, we analyze the water decomposition possibility of single-layer (SL) GaInTe3 under the −2% to +2 % biaxial strain via first-principles stimulations. Our results find that SL GaInTe3 exhibits effective separation of photogenerated electrons and holes, which is strongly affirmed by the electronic property and the quite loose exciton binding. Furthermore, SL GaInTe3 has a suitable band edge and excellent visible-light capture ability, making it an ideal candidate for solar-assisted water decomposition. In addition, SL GaInTe3 shows a solar-to-hydrogen (STH) conversion of more than 33.0 %, further emphasizing its potential as a highly efficient photocatalyst. In conclusion, SL GaInTe3 has been supported by conclusive evidence as a photocatalytic material with excellent performance, providing new support and encouragement for the development of hydrogen production technology. Moreover, the effective thickness of 2D materials is defined as the largest spreading height of the electron cloud in covalent bonds perpendicular to the 2D atomic plane.

Quantum-Confined Perovskite Nanocrystals Enabled by Negative Catalyst Strategy for Efficient Light-Emitting Diodes.

The perovskite nanocrystals (PeNCs) are emerging as a promising emitter for light-emitting diodes (LEDs) due to their excellent optical and electrical properties. However, the ultrafast growth of PeNCs often results in large sizes exceeding the Bohr diameter, leading to low exciton binding energy and susceptibility to nonradiative recombination, while small-sized PeNCs exhibit a large specific surface area, contributing to an increased defect density. Herein, Zn2+ ions as a negative catalyst to realize quantum-confined FAPbBr3 PeNCs with high photoluminescence quantum yields (PL QY) over 90%. Zn2+ ions exhibit robust coordination with Br- ions is introduced, effectively retarding the participation of Br- ions in the perovskite crystallization process and thus facilitating PeNCs size control. Notably, Zn2+ ions neither incorporate into the perovskite lattice nor are absorbed on the surface of PeNCs. And the reduced growth rate also promotes sufficient octahedral coordination of PeNC that reduces defect density. The LEDs based on these optimized PeNCs exhibits an external quantum efficiency (EQE) of 21.7%, significantly surpassing that of the pristine PeNCs (15.2%). Furthermore, the device lifetime is also extended by twofold. This research presents a novel approach to achieving high-performance optoelectronic devices.

Solution‐Processed Tin‐Antimony Quaternary Chalcohalides for Self‐Powered Broadband Photodetectors

The mixed‐metal quaternary chalcohalides of group IV and V elements are a promising class of low‐toxicity perovskite‐inspired materials with tunable bandgaps and desirable defect tolerance. Sn2SbS2I3 is known for its broadband absorption, low exciton binding energy, ambient stability, and solution processability in thin films. However, its use in optoelectronic devices has so far been only limited to solar cells. In this work, the first self‐powered photodetectors based on Sn2SbS2I3 thin films, sandwiched in an n–i–p device configuration are reported. The insertion of an interlayer at the hole‐transport layer/gold top‐electrode interface reduces the dark current and improves the device performance. The high external quantum efficiency of the devices in the range of 350−900 nm hints to a broadband spectral photoresponsivity. The devices indeed exhibit promising photodetection properties, namely a photoresponsivity of 0.33 A W−1, a specific detectivity of 1.55 × 1012 Jones, and photoresponse/decay times of 0.52 and 0.45 s at zero bias voltage. These results, combined with the excellent operational stability of the photodetectors, encourage the exploration of a wide range of practical light‐sensing applications for quaternary chalcohalides and stimulate device and material engineering to further enhance photodetection across the UV‐Visible‐NIR spectrum.

Dipole polarization modulating of vinylene-linked covalent organic frameworks for efficient photocatalytic hydrogen evolution

Photocatalytic hydrogen (H2) evolution using covalent organic frameworks (COFs) is an attractive and promising avenue for exploration, but one of its big challenges is low photo-induced charge separation. In this study, we present a straightforward and facile dipole polarization engineering strategy to enhance charge separation efficiency, achieved through atomic modulation (O, S, and Se) of the COF monomer. Our findings demonstrate that incorporating atoms with varying electronegativities into the COF matrix significantly influences the local dipole moment, thereby affecting charge separation efficiency and photostability, which in turn affects the rates of photocatalytic H2 evolution. As a result, the newly developed TMT-BO-COF, which contains highly electronegative O atoms, exhibits the lowest exciton binding energy, the highest efficiency in charge separation and transportation, and the longest lifetime of the active charges. This leads to an impressive average H2 production rate of 23.7 mmol g−1 h−1, which is 2.5 and 24.5 times higher than that of TMT-BS-COF (containing S atoms) and TMT-BSe-COF (containing Se atoms), respectively. A novel photocatalytic hydrogen evolution mechanism based on proton-coupled electron transfer on N in the structure of triazine rings in vinylene-linked COFs is proposed by theoretical calculations. Our findings provide new insights into the design of highly photoactive organic framework materials for H2 evolution and beyond.

Facilitating exciton dissociation in conjugated microporous polymers for photocatalytic degradation of tetracycline via a gradient N doping strategy

As a leading green wastewater treatment technology, organic semiconductor photocatalysis for tetracycline (TC) degradation is widely recognized but currently constrained by excessive exciton binding energies and rapid charge recombination. In this work, conjugated microporous polymers (CMPs) with five-membered heterocyclic ring have been fabricated by gradient doping of nitrogen at the monomer level. It demonstrates satisfactory photocatalytic performance, with thiadiazole introduced into the unit, achieving a TC removal rate of 95.6% within 30 min under visible light. The underlying origin of the preferable property lies in the lower exciton binding energy (Eb) with the assistance of the donor–acceptor (D-A) structure constructed by thiadiazole monomer and triazine skeleton. Density Functional Theory (DFT) calculations reveal that this five-membered heterocyclic structure can appropriately regulate the local charge distribution and increase the dipole moment, thus accelerating the migration dynamics of charge carriers in the degradation process. This study provides a novel insight into the modulation of polymer exciton Eb and enlightens the designation of metal-free photocatalysts based on antibiotic degradation.

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.

Boosting exciton dissociation in conjugated microporous polymers via molecular isomerization for photocatalytic degradation of antibiotics

Photocatalytic degradation of antibiotics from wastewater by polymeric photocatalysts is a powerful strategy to alleviate ecological damage and environmental pollution. Conjugated microporous polymers (CMPs) are promising in photocatalysis due to their tunable electronic structure and excellent light absorption. Nonetheless, CMPs still suffer from low exciton separation efficiency. Here, we propose a molecular isomerism strategy to promote exciton dissociation in CMPs by modulating the position of the carbonyl moiety. As a result, the introduction of carbonyl moiety with different substitution positions into the backbone of CMPs could minimize the exciton binding energy (Eb) and thus promote charge separation. PyCMPs-3 with phenanthrenequinone moiety as the acceptor featured a lower Eb of 37.19 meV, which are much smaller than that of carbonyl-free PyCMPs-1 (80.49 meV) and PyCMPs-2 (59.21 meV) with anthraquinone as the acceptor. Among them, PyCMPs-3 performed the best in photocatalytic degradation of ofloxacin (99 %) at a concentration of 144 ppm, which was significantly higher than that of its analogues PyCMPs-1 (26 %) and PyCMPs-2 (57 %) within 100 min under visible light exposure. In short, this work delivered a novel molecular isomerism strategy to minimize Eb by regulating the position of the carbonyl moiety for enhancing the photocatalytic degradation efficiency, promoting the development of CMPs for practical environmental remediation applications.

Ultrahigh carrier mobility and anisotropy of the layered semiconductor ATiN2 (A = Ca, Sr and Ba)

Semiconductors with a suitable band gap and high carrier mobility are highly desirable in the electronics, optoelectronic and photovoltaic applications. The mechanical, electronic, optical and transport properties of the layered nitrides ATiN2 (A = Ca, Sr, and Ba) have been systematically studied by theoretical calculations. These nitrides show good ductility with moderate moduli, larger Poisson's ratio than 0.26 and Pugh's modulus ratio exceeding 1.75. CaTiN2 and SrTiN2 are direct bandgap semiconductors, while BaTiN2 is an indirect bandgap semiconductor, showing suitable band gaps (1.54–1.78 eV) and band edge positions for optoelectronic and photocatalytic water-splitting applications. More intriguingly, they possess superhigh carrier mobility with remarkable anisotropy. Particularly, the in-plane electron mobility reaches an ultrahigh order of 104 cm2V−1s−1, whereas those along the out-of-plane direction are almost zero. In addition, the hole mobilities are also very large along the in-plane direction and the anisotropic ratios are as high as about 30 for all these nitrides. Furthermore, these ATiN2 nitrides exhibit high optical absorption coefficients (∼105 cm−1) and lower exciton binding energies. Due to the suitable band gaps and band alignments, ultrahigh carrier mobility and huge anisotropy, excellent visible light absorption performance, the ATiN2 nitrides will be promising candidate semiconductors in electronics, optoelectronic, photovoltaic and photocatalytic applications.

Front Cover: Steric hindrance induced low exciton binding energy enables low‐driving‐force organic solar cells

Front Cover: Steric hindrance induced low exciton binding energy enables low‐driving‐force organic solar cells

Long-Lived Topological Flatband Excitons in Semiconductor Moiré Heterostructures: A Bosonic Kane-Mele Model Platform.

Moiré superlattices based on two-dimensional transition metal dichalcogenides (TMDs) have emerged as a highly versatile and fruitful platform for exploring correlated topological electronic phases. One of the most remarkable examples is the recently discovered fractional quantum anomalous Hall effect (FQAHE) under zero magnetic field. Here, we propose a minimal structure that hosts long-lived excitons-a ubiquitous bosonic excitation in TMD semiconductors-with narrow topological bosonic bands. The nontrivial exciton topology originates from hybridization of moiré interlayer excitons and is tunable by controlling twist angle and electric field. At small twist angle, the lowest exciton bands are isolated from higher energy bands and provide a solid-state realization of the bosonic Kane-Mele model with topological flatbands, which could potentially support the bosonic version of FQAHE.

Deep blue high-efficiency solution-processed triplet-triplet annihilation organic light-emitting diodes using bis(8-carbazol-N-yl)fluorene- and benzonitrile-modified anthracene/chrysene fluorescent emitters

Triplet-triplet annihilation (TTA) emitters can effectively utilize non-radiative triplet excitons through the interaction of low triplet energy excitons to produce high energy singlet excitons, but they are mostly restricted by their multilayered device structure fabricated using layer-by-layer thermal vacuum evaporation. It is a great challenge to develop, for the first time, efficient solution-processed non-doped TTA organic light-emitting diodes (OLEDs). In this study, two solution-processable blue emissive TTA molecules (FAnCN and FCsCN) bearing (anthracen-9-yl)benzonitrile (AnCN) and (chrysen-6-yl)benzonitrile (CsCN) as TTA emissive cores modified with 9,9′-bis(8-(carbazole-N-yl)octyl)fluorene (F) are designed and synthesized, respectively. The experimental and theoretical studies reveal that both molecules exhibit deep blue emissions, amorphous morphology with good thermal stability, high-quality solution-cast thin films, decent hole mobility, high-lying HOMO levels (∼-5.45 eV), and suitable lowest singlet (S1)/triplet (T1) excited states (2T1 > S1) for TTA process. FAnCN and FCsCN are successfully employed as solution-processed non-doped emissive layers (EML) in simple structured TTA OLEDs. These devices show intense blue emissions, low turn-on voltages (∼3.6 V), excellent electroluminescent (EL) performances (EQEmax = 5.47–6.84 % and LEmax = 5.66–5.83 cd/A), and TTA characteristics. Especially, FCsCN-based TTA OLED emits deep blue EL emission peaked at 435 nm with a high EQEmax of 6.84 %. This work not only presents a new strategic design for the preparation of solution-processable TTA emitter, but also further ratifies that the TTA mechanism can also be applicable in solution-processed OLEDs.

Study of electrical properties of fullerene based all-small-molecule low donor organic devices with and without cathode buffer layers for photovoltaic application

This article examines electrical properties of fullerene (C70) based all-small-molecule organic photovoltaics with low donor concentration. Enhancements in the properties of devices are observed with the incorporation of lithium fluoride (LiF) and molybdenum trioxide (MoO3) as cathode buffer layers (CBLs). Samples are characterized using field emission-scanning electron microscopy (FESEM), optical absorption spectra, current-voltage (I-V) characteristics in dark and under illumination, and capacitance-frequency (C-ω) measurements. Device with single CBL (LiF) exhibits reduced photoabsorption leading to less saturation photocurrent density (Jsat) and lower maximum exciton generation rate (Gmax), due to non-uniform deposition of LiF in device. In contrast, device having two CBLs (LiF & MoO3) has enhanced the photoabsorption exhibiting high Jsat and Gmax attributed to uniform deposition of MoO3. Moreover, dielectric properties and AC conductivity of devices are also measured confirming the results and these are found to be dependent on the frequency and voltage.

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.

Phosphonium Iodide Featuring Blue Thermally Activated Delayed Fluorescence for Highly Efficient X‐Ray Scintillator

AbstractOrganic scintillators are praised for their abundant element reserves, facile preparation procedures, and rich structures. However, the weak X‐ray attenuation ability and low exciton utilization efficiency result in unsatisfactory scintillation performance. Herein, a new family of highly efficient organic phosphonium halide salts with thermally activated delayed fluorescence (TADF) are designed by innovatively adopting quaternary phosphonium as the electron acceptor, while dimethylamine group and halide anions (I−) serve as the electron donor. The prepared butyl(2‐[2‐(dimethylamino)phenyl]phenyl)diphenylphosphonium iodide (C4‐I) exhibits bright blue emission and an ultra‐high photoluminescence quantum yield (PLQY) of 100 %. Efficient charge transfer is realized through the unique n‐π and anion‐π stacking in solid‐state C4‐I. Photophysical studies of C4‐I suggest that the incorporation of I accounts for high intersystem crossing rate (kISC) and reverse intersystem crossing rate (kRISC), suppressing the intrinsic prompt fluorescence and enabling near‐pure TADF emission at room temperature. Benefitting from the large Stokes shift, high PLQY, efficient exciton utilization, and remarkable X‐ray attenuation ability endowed by I, C4‐I delivers an outstanding light yield of 80721 photons/MeV and a low limit of detection (LoD) of 22.79 nGy ⋅ s−1. This work would provide a rational design concept and open up an appealing road for developing efficient organic scintillators with tunable emission, strong X‐ray attenuation ability, and excellent scintillator performance.

Phosphonium Iodide Featuring Blue Thermally Activated Delayed Fluorescence for Highly Efficient X-Ray Scintillator.

Organic scintillators are praised for their abundant element reserves, facile preparation procedures, and rich structures. However, the weak X-ray attenuation ability and low exciton utilization efficiency result in unsatisfactory scintillation performance. Herein, a new family of highly efficient organic phosphonium halide salts with thermally activated delayed fluorescence (TADF) are designed by innovatively adopting quaternary phosphonium as the electron acceptor, while dimethylamine group and halide anions (I-) serve as the electron donor. The prepared butyl(2-[2-(dimethylamino)phenyl]phenyl)diphenylphosphonium iodide (C4-I) exhibits bright blue emission and an ultra-high photoluminescence quantum yield (PLQY) of 100 %. Efficient charge transfer is realized through the unique n-π and anion-π stacking in solid-state C4-I. Photophysical studies of C4-I suggest that the incorporation of I accounts for high intersystem crossing rate (kISC) and reverse intersystem crossing rate (kRISC), suppressing the intrinsic prompt fluorescence and enabling near-pure TADF emission at room temperature. Benefitting from the large Stokes shift, high PLQY, efficient exciton utilization, and remarkable X-ray attenuation ability endowed by I, C4-I delivers an outstanding light yield of 80721 photons/MeV and a low limit of detection (LoD) of 22.79 nGy ⋅ s-1. This work would provide a rational design concept and open up an appealing road for developing efficient organic scintillators with tunable emission, strong X-ray attenuation ability, and excellent scintillator performance.

Comprehensive study on structural, electronic, optical, elastic, and transport properties of natural mercury sulphohalides via DFT computation

The Mercury Sulphohalides have attracted significant attention in the fields of solar cells and thermoelectric applications. This study delves into the fundamental characteristics, including structural, elasticity, electronic behavior, phonon stability, optical properties, and transport features of AgHgSZ (Z = Br, I) through computational simulations based on Density Functional Theory (DFT) using WIEN2k software. Meticulous calculations of the phonon band structure ensure dynamic stability. The semiconductor nature with indirect band gaps (1.833 eV and 1.832 eV) for Mercury Sulphohalides (Br, I), as revealed by their band structures, suggests diverse photovoltaic and transport applications. Mechanical assessments show stable ductility for AgHgSBr and brittleness for AgHgSI, along with anisotropy and resistance to scratching. Optical properties exhibit anisotropy and significant UV absorption. Analysis of effective masses, exciton binding energy, and exciton Bohr radius suggests low exciton binding energy and classification under Mott-Wannier excitons. Positive thermopower results indicate holes as the predominant charge carriers in AgHgSBr and AgHgSI materials. Moreover, essential thermoelectric factors are examined, revealing the compounds’ potential for thermoelectric applications. Notably, the figure of merit (ZT) at 300 K for AgHgSBr and AgHgSI are calculated to be 0.41 and 0.13, respectively. While these values are low at 300 K, they indicate promising potential for thermoelectric applications at higher temperatures. In summary, this investigation provides valuable understanding into the photovoltaic and thermoelectric properties of AgHgSZ (Z = Br, I) materials, potentially paving the way for further exploration in this domain.

(Invited) Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System

3-dimensional hybrid halide perovskites have been intensely researched because of their extraordinary optoelectronic properties with spectacular quantum efficiencies arising primarily from their suitable bandgaps to harvest the solar spectrum, low excitonic binding energies, and low-cost synthetic processes. However, stabilizing these materials under real-life operating conditions has proven to be a challenge, leading to extensive explorations of lower dimensional analogous hybrid halide materials that are often more stable, though generally having considerably higher bandgaps and excitonic binding energies. I shall present some of our investigations leading to the synthesis of a highly moisture-stable 2-dimensional hybrid lead halide material with the lowest bandgap and exciton binding energy within this class of compounds.1 I shall detail the origin of such a low bandgap and exciton binding energy, identifying the most critical control parameters to determine these properties. In addition, I shall also present preliminary results to suggest the beneficial impacts of incorporating the same organic ligand while making a solar photovoltaic device based on three-dimensional hybrid lead halide perovskites.Reference: Debasmita Pariari, Sakshi Mehta, Sayak Mandal, Arup Mahata, Titas Pramanik, Sujit Kamilya, Arya Vidhan, Tayur N. Guru Row, Pralay K. Santra, Shaibal K. Sarkar, Filippo De Angelis, Abhishake Mondal, and D. D. Sarma, Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System, J. Am. Chem. Soc. 145, 15896 (2023).

Molecular engineering push-pull structural versatility in the triphenylamine-thienodioxine-based hole transporting materials for high-performance organic and perovskite solar cells

This study proposes a push-pull molecular engineering approach to fabricate versatile hole-transporting materials (HTMs) for photovoltaic cells. Herein, we introduce a D-π-A strategy for fabricating four highly functional small molecules of thienodioxine core-based HTMs (CJ06C1 to CJ06C4), via core functionalization with diversified electron-withdrawing acceptor moieties. The engineered HTMs are evaluated by employing quantum calculations to analyze the structure-property relationships from optoelectronic, electrochemical, and solvability attributes. The findings reveal that acceptor-integrated HTMs unveil excellent band alignment with active perovskite layer with deeper HOMO energy levels (-4.96 eV to 5.02 eV), transparency in the visible portion λmaxabs<521, which entail appropriate photophysical attributes. In comparison to the reference HTM (CJ06), the acceptor-integrated HTMs have exhibited potentially higher hole mobility rate, accredited to smaller hole reorganization energies (0.15 eV to 0.21 eV), greater transfer integral values (0.26 eV to 0.31 eV) and greater hole hopping rates (1.08×1028 S−1 to 3.70×1028 S−1). The electron-excitation investigation exhibited stronger electronic coupling, smaller hole-electron spatial overlapping, and higher charge transference length (19.42 Å to 21.14 Å) in the fabricated HTMs. Thus ensued a significant upsurge in intrinsic charge transference (74.22–99.74 %) and lower exciton binding energies, leading to easy charge dissociation and low recombination chances compared to the parent HTM (CJ06C1). Moreover, the findings from dipole moments (12.56 D to 13.05 D) and solvation-free energy (-91.08 kJ/mol) to −108.1 kJ/mol),imply easier processability and aptness for film formation. Overall, this study ensures the efficiency of thienodioxine core functionalization with acceptor moieties for fabricating state-of-the-art HTMs. The promising attributes of fabricated HTMs pave the way for forthcoming perovskite solar cells.