Exciton Lifetime Research Articles

Achieving High External Quantum Efficiency for ITIC‐Based Organic Solar Cells with Negligible Homo Energy Offsets

AbstractMinimizing energy loss in organic solar cells (OSCS) is critical for attaining high photovoltaic performance. Among the parameters that correlated to photovoltaic performance, the energy offsets between donor–acceptor pairs play a vital role in photoelectric conversion processes. For so far reported a large number of non‐fullerene acceptors (NFAs), only Y6 and its derivatives can achieve external quantum efficiencies (EQEs) over 80% with negligible energy offsets when combined with polymeric donors. Thus, understanding the relationship between energy offsets and energy losses in representative NFAs is the key to further enhancing the efficiency of OSCs. In this study, a series of wide‐bandgap polymer donors based on pyrrolo[3,4‐f]benzotriazole‐5,7(6H)‐dione (TzBI) and benzo[1,2‐c:4,5‐c′] dithiophene‐4,8‐dione building blocks are combined with representative NFAs, including ITIC and Y6, to gain deep insights into their photovoltaic performances and related energy losses. Outstanding EQEs (≈70%) and suppressed non‐radiative recombination are achieved at negligible energy offsets. Moreover, it is noted that a prolonged exciton lifetime of acceptor is not essential to obtain high EQEs in OSCs with negligible energy offsets. Eventually, ITIC derivatives with high electroluminescence efficiencies and near‐infrared absorptions have the potential to be assembled to obtain high‐efficiency OSCs.

Synergetic Enhancement of Quantum Yield and Exciton Lifetime of Monolayer WS2 by Proximal Metal Plate and Negative Electric Bias.

The efficiency of light emission is a critical performance factor for monolayer transition metal dichalcogenides (1L-TMDs) for photonic applications. While various methods have been studied to compensate for lattice defects to improve the quantum yield (QY) of 1L-TMDs, exciton-exciton annihilation (EEA) is still a major nonradiative decay channel for excitons at high exciton densities. Here, we demonstrate that the combined use of a proximal Au plate and a negative electric gate bias (NEGB) for 1L-WS2 provides a dramatic enhancement of the exciton lifetime at high exciton densities with the corresponding QY enhanced by 30 times and the EEA rate constant decreased by 80 times. The suppression of EEA by NEGB is attributed to the reduction of the defect-assisted EEA process, which we also explain with our theoretical model. Our results provide a synergetic solution to cope with EEA to realize high-intensity 2D light emitters using TMDs.

Correlation between Dynamics of Polaronic Photocarriers and Photoelectrochemical Performance in Mo-Doped Bismuth Vanadate.

Bismuth vanadate (BiVO4) has received intense research interest due to its outstanding performance for solar water splitting, and doping it with molybdenum (Mo) ions can effectively boost photoelectrochemical performance. In this material, highly localized polarons play a key role in the photoconversion process. Herein, we uncovered the influence of Mo dopants on the dynamics of polaronic transient species using transient absorption spectroscopy. We find that the preexisting electron small polarons stemming from the thermal ionization of dopants provide additional centers to capture itinerant holes, which significantly decrease the hole lifetime. However, the introduction of dopants increases the lifetime of self-trapped excitons that arise from the binding of electron polarons and holes. The dependence of the photoelectrochemical performance of BiVO4 photoelectrodes on doping levels can be well explained by combining the dopant effects on the lifetimes of delocalized and self-trapped transient species. Our findings provide guidance for rational optimization of dopant concentration to maximize the PEC efficiency.

Recombination dynamics and pronounced re-absorption effect in ZnO single crystals under two-photon excitation

Time-resolved photoluminescence measurements were conducted on ZnO single crystals using two-photon (2P) excitation at low temperatures ranging from 25 to 260 K. The decay constants of various emission peaks are determined and compared between one-photon and two-photon excitations using a recently modified localized-state ensemble model. The analysis is focused on the redshift of the phonon-assisted free exciton transition energy with increasing temperature. A significantly longer radiative recombination lifetime of approximately 840 ps is observed under 2P excitation. The extended lifetime of excitons during 2P excitation is achieved through the interplay of multiple mechanisms. Photon-recycling, enabled by high absorption coefficients and total internal reflection, facilitates re-absorption and new carrier generation. Phonon scattering lowers photon energy, allowing their escape from the crystal, while exciton–polariton interactions delay photon travel, collectively contributing to the extension of exciton lifetimes. The re-absorption effect and elongated carrier lifetime observed from our work could be beneficial for applications of ZnO materials in 2P imaging, particularly of biological samples.

Probing the Optoelectronic Properties and Energy Conversion of Boron Nitride Quantum Dots: Impact of Oxygen Doping and Chemical Functionalization

Boron nitride quantum dots (BNQDs) are emerging nanomaterials with promising applications in photocatalysis and optoelectronics. Chemical functionalization effectively tunes the optoelectronic properties and photocatalytic performance of BNQDs. This study investigates the impact of oxygen doping and functionalization with urea, thiourea, and p‐phenylenediamine (PPD) on BNQD properties. Density functional theory calculations reveal that these functional groups introduce new electronic states within the bandgap, shifting absorption spectra and reducing the bandgap due to ligand molecular orbital contributions. Functionalization also adjusts the energy‐level alignment between BNQDs and cocatalysts, enhancing interfacial charge transfer. Oxygen‐doped, chemically functionalized BNQDs exhibit significantly higher first‐order hyperpolarizability, up to 17 times greater than pristine BNQDs, enabling superior optical nonlinearities. Additionally, PPD functionalization leads to a remarkable 18.2% photocatalytic energy conversion efficiency under simulated solar irradiation. These achieved quantum yields result from bandgap engineering, expanded light harvesting, increased exciton densities, prolonged exciton lifetimes, and improved charge separation and transfer dynamics. This study provides crucial insights into property tuning mechanisms, establishing the promising potential of chemically functionalized BNQDs for energy conversion and optoelectronic technologies.

Circularly Polarized Light Emission from Nonchiral Perovskites Incorporated into Nanoporous Cholesteric Polymer Templates.

Chiral perovskites have garnered significant attention, owing to their chiroptical properties and emerging applications. Current fabrication methods often involve complex chemical synthesis routes. Herein, an alternative approach for introducing chirality into nonchiral hybrid organic-inorganic perovskites (HOIPs) using nanotemplates composed of cholesteric polymeric networks is proposed. This method eliminates the need for additional molecular design. In this process, HOIP precursors are incorporated into a porous cholesteric polymer film, and two-dimensional (2D) HOIPs grow inside the nanopores. Circularly polarized light emission (CPLE) was observed even though the selective reflection band of the cholesteric polymer films containing a representative HOIP deviated from the emission wavelength of the 2D HOIP. This effect was confirmed by the induced circular dichroism (CD) observed in the absorbance band of the HOIP. The observed CPLE and CD are attributed to the chirality induced by the template in the originally nonchiral 2D HOIP. Additionally, the developed 2D HOIP exhibited a long exciton lifetime and good stability under harsh conditions. These findings provide valuable insights into the development and design of innovative optoelectronic materials.

Large-scale synthesis and exciton dynamics of monolayer MoS2 on differently doped GaN substrates

Abstract Mixed dimensional van der Waals heterostructure based on layered two-dimensional molybdenum disulfide (MoS2) interfaced to gallium nitride (GaN) has attracted tremendous attention due to its unique properties and application in novel electronic, optoelectronic, and quantum devices. However, developing facile synthesis methods and insights into the exciton dynamics for this system still remains a major challenge. Here, a simple and cost-effective method is demonstrated for large-scale synthesis of monolayer MoS2 on differently doped GaN substrates. A mixed aqueous solution of Na2MoO4 and NaOH is spin-coated on GaN and sulfurated in one step by chemical vapor deposition (CVD). High quality monolayer MoS2 nanosheets with side length over 400 μm and surface coverage ratio of more than 90 % are achieved on GaN. Furthermore, the PL intensity, excitonic transition ratios and ultrafast exciton dynamics of MoS2 are observed to be largely modulated by the doping type of GaN, owing to substrate-induced doping, which is proved by Raman, PL and transient absorption spectroscopy. Notably, p-GaN can attract electrons from monolayer MoS2 and weaken its intrinsic n-doping, thereby facilitating higher PL intensity as well as longer exciton lifetime, while n-GaN provides strong n-doping and generates opposite effect. This work hereby presents a pathway for large-scale synthesis of MoS2/GaN heterostructures and further understanding of their charge transfer properties and exciton dynamics, which should inspire their applications for optoelectronic devices.

Efficient Exciton Dissociation on Ceria Chelated Cerium-Based MOF Isogenous S-Scheme Photocatalyst for Acetaldehyde Purification.

Long-term exposure to low concentration indoor VOCs of acetaldehyde (CH3CHO) is harmful to human health. Thus, a novel isogenous heterojunction CeO2/Ce-MOF photocatalyst is synthesized via a one-step hydrothermal method for the effective elimination of CH3CHO in this work. This CeO2/Ce-MOF photocatalyst performs well in CH3CHO removal and achieves an apparent quantum efficiency of 7.15% at 420nm, which presents ≈6.7 and 3.4times superior to those generated by CeO2 and Ce-MOF, respectively. The enhanced efficiency is due to two main aspects including i) an effective photocarrier separation ability and the prolonged reaction lifetime of excitons play crucial roles and ii) the formation of an internal electric field (IEF) is sufficient to overcome the considerable exciton binding energy, and increases the exciton dissociation efficiency by up to 50.4%. Moreover, the reasonable pathways and mechanisms of CH3CHO degradation are determined by in situ DRIFTS analysis and simulated DFT calculations. Those results demonstrated that S-scheme heterojunction successfully increases the efficiency of harmful volatile organic compounds elimination, and it offers essential guidance for designing rare earth-based MOF photocatalysts.

Frequency-domain time-resolved luminescence for In-operando study of efficiency roll-off in organic light-emitting diodes

Time-resolved luminescence measurements are invaluable for understanding light-emitting materials but are difficult to perform on operating organic light-emitting diodes (OLEDs) because the light generated by the OLED can disturb common time-resolved techniques. We show that frequency-domain luminescence can be used to overcome this problem, and apply it to study efficiency roll-off in OLEDs. We measure the exciton lifetime of a phosphorescent OLED based on the emitter bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate) iridium(III) (Ir(MDQ)2(acac)) whilst it is operating. By driving the OLED continuously at current densities of up to 128 mA/cm2, we observe that the exciton lifetime decreases with increasing current density as a result of triplet-polaron annihilation (TPA). The measurement presented can guide the development of new materials and device structures with improved efficiency at high brightness.

Effects of the Exciton Fine Structure Splitting on the Entanglement-Based Quantum Key Distribution

The reliable transmission of secure keys is one of the essential tasks to be efficiently accomplished by quantum information processing, and the use of entangled particles is a very important tool toward that goal. However, efficient production of maximally entangled states is still a challenge for further progress in quantum computing and quantum communication. In the search for optimal sources of entanglement, quantum dots have emerged as promising candidates, but the presence of dephasing in the generated entangled states raises questions about their real usefulness in large-scale quantum networks. In this work, we evaluate the effects of the exciton fine structure splitting, present in most quantum dot samples, on the fidelity of the BBM92 protocol for quantum key distribution. We find that the protocol’s performance is heavily impacted by such splitting and establish an upper limit for the product between the energy splitting and the exciton lifetime to have a dependable distributed key.

Modulating the Photoresponsivity of Perovskite Photodetectors through Interfacial Engineering of Self‐Assembled Monolayers

AbstractPhotodetectors are one of the most essential optoelectronics nowadays. Perovskites have emerged as one of the potential materials for photodetectors due to their high absorption efficiency over a wide range of wavelengths, high charge mobility, strong mechanical deformability, and solution processability. In this study, interfacial engineering using self‐assembled monolayers (SAMs) is explored to improve the performance of perovskite photodetectors. SAMs with different terminal groups exhibit distinct surface energy, and they all result in a reduced exciton lifetime of the perovskite due to charge transfer between polycrystalline perovskite and the SAMs. In particular, the photodetector modified with (4‐hydroxyphenyl)phosphonic acid (OH) exhibits the highest electron mobility of 0.8 cm2 V−1 s−1 under 450‐nm light illumination. In addition, it shows excellent detector performance with an ultrafast photoresponse of < 14 ms, a high photoresponsivity of 5600 AW−1, and a high specific detectivity of 5.9 × 1010 Jones at a low light intensity of 18 µW cm−2. The excellent photoresponse and device switchability are attributed to the high surface energy of OH, the resulting homogeneous perovskite film morphology, the high charge transfer efficiency, and the favorable energy level alignment. The studies provide new prospects for the development of emerging perovskite photodetectors.

Easily hydroxylated PtxPd1-x-TiO2 nanotube arrays: An energetic Schottky photocatalyst for synergistically boosted degradation of Microcystin-LR

The metal co-catalyst-based photocatalytic oxidation technology provides a promising approach to eliminate contaminants from water. However, the applicability of these catalysts often suffer from severe carrier recombination and high activation energy barriers of reactive oxygen species (ROS). Herein, self-supporting TiO2 nanotube arrays were engineered with PtPd atom clusters having atomically controlled molar ratio, assembling a dynamic Mott-Schottky catalyst for photocatalytic removal of Microcystin-LR, a toxin produced by cyanobacteria. The separation of photogenerated carriers and the creation of reactive oxygen species (ROS) are optimized by regulating electronic construction via tailoring the Pd ratio in PtPd atom clusters (ca. 4.6 nm in size). The exciton lifetime of the optimum Pt0.9Pd0.1-TiO2 is shortened from 2.51 ns on pure TiO2 to 1.16 ns, and the Gibbs free energy for forming hydroxyl radicals (•OH) varies from −0.826 eV (pure TiO2) to −2.576 eV, powering plentiful •OH when Pt0.9Pd0.1-TiO2 illuminated under solar lights. The kinetic constant of Pt0.9Pd0.1-TiO2 for MC-LR removal is 12.7 times higher than that of the pristine TiO2, presenting exceptional photocatalytic capacity in turning Microcystin-LR into harmless chemicals.

Emergent properties resulting from type-II band alignment in a new lateral ([formula omitted]-PbO/[formula omitted]-SnO) heterostructure

α-PbO and α-SnO monolayers are stable nanostructures that are separately synthesized by epitaxial growth on the same oxidized silicon substrate (SiO2/Si). In this study, we theoretically designed new (α-PbO)m/(α-SnO)m lateral heterostructures formed by periodically repeating narrow PbO and SnO strips joined along their adjacent one-dimensional interface. By means of first-principles calculations, structural, mechanical and electronic properties are investigated for different widths (from m = 1 to m = 10). Interface and mechanical stability of these heterostructures are demonstrated from m = 1 to m = 8. The advantage of heterostructures of this kind is that their band gaps can be adjusted by variation of their width, which may provide new opportunities for ultimate band gap engineering at the nanoscale and for new applications. Electronic calculations for the (α-PbO)7/(α-SnO)7 heterostructure show that it has an Anderson type-II band alignment, where the conduction band minimum and valence band maximum are located separately at PbO and SnO strips, respectively. Consequently, the photon-generated electron–hole pairs spatially separated, resulting in much a longer exciton lifetime than in the two monolayers separately, which needs to be confirmed by further experimental measurements.

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.

Correlation of Functional Coumarin Dye Structure with Molecular Packing and Organic Solar Cells Performance

Nowadays, ternary organic solar cells (OSCs) based on all‐small molecules have the convenient strategy toward high‐performance and stable devices. Herein, two coumarin‐based dyes C4 and C4–CN are designed and synthesized with a simple synthetic route. The molecules display different intra‐ and intermolecular interactions in the solid state originating from acceptor substitution, as revealed by the single‐crystal structure analysis. The introduction of dicyanomethylene acceptor group in C4–CN plays a synergistic role in molecular packing and orientation in the solid state, thus fostering the elongation of exciton lifetime and improved charge transport in photoactive layer. The optimized C4–CN:AQx–3‐based binary OSCs show a power conversion efficiency (PCE) of 15.07%, which is appreciably higher than the C4:AQx–3‐based OSC (11.39%) due to the appropriate energy‐level alignment, balanced charge transport, favorable phase separation, and lower charge recombination. Furthermore, by introduction of C4–CN as the third component into C4:AQx–3 blend, the ternary OSC shows an improved PCE of 15.34%, which is linked with the enhanced crystallization, optimized morphology, balanced charge carrier mobility, and suppressed recombination with low energy loss.

Full‐Landscape Condensation Phases for Long‐Lived Excitons in 2D Tellurium: Crystal‐Field Splitting and Finite‐Momentum Excitons

AbstractThe realization of high‐temperature exciton macroscopic quantum phases in materials still remains challenging due to strict constraints in thermal stabilities and excitonic lifetimes. In this work, by using first‐principles calculations, the exciton dispersions and macroscopic quantum phase transitions of 2D α‐ and β‐Te are investigated. The excitons with lowest eigen energy for both α‐ and β‐Te are dark restricted by the crystal‐field symmetries. The Bose–Einstein condensation (BEC) transition for α‐ and β‐Te can be realized at 165.4 and 32.8 K with low excitation power densities of 0.42 × 1012 and 1.0 × 1012 cm−2, respectively. Furthermore, the phase transitions from insulating free exciton (FE) gas to conducting electron–hole plasma (EHP) and electron–hole liquid (EHL), as well as that from BEC to superfluidity phases are also predicted. Finally, the authors investigate the microscopic dynamics for bright excitons of 2D tellurium and find that they reach thermal equilibrium at 1–50 fs and excitonic lifetimes can reach 1–40 ns, beneficial for experimental observation of quantum condensate states. The findings in this work not only demonstrate the excellent optoelectronic properties of 2D tellurium allotropes, but also provide a promising platform for experimental realization of high‐temperature excitonic macroscopic quantum condensates.

Strongly Localized Moiré Exciton in Twisted Homobilayers.

Artificially molding exciton flux is the cornerstone for developing promising excitonic devices. In the emerging hetero/homobilayers, the spatial separated charges prolong exciton lifetimes and create out-plane dipoles, facilitating electrically control exciton flux on a large scale, and the nanoscale periodic moiré potentials arising from twist-angle or/and lattice mismatch can substantially alter exciton dynamics, which are mainly proved in the heterostructures. However, the spatially indirect excitons dynamics in homobilayers without lattice mismatch remain elusive. Here the nonequilibrium dynamics of indirect exciton in homobilayers are systematically investigated. The homobilayers with slightly twist-angle can induce a deep moiré potential (>50 meV) in the energy landscape of indirect excitons, resulting in a strongly localized moiré excitons insulating the transport dynamics from phonons and disorder. These findings provide insights into the exciton dynamics and many-body physics in moiré superlattices modulated energy landscape, with implications for designing excitonic devices operating at room temperature.

Development of a novel flexible thin PWO(Er)/ZnO(Ag) nanocomposite for ionizing radiation sensing

Radiation dosimetry is an essential aspect of modern radiotherapy. Optically stimulated luminescence (OSL) dosimetry is a promising technique that uses materials to measure radiation doses. However, current OSL dosimeters are limited in their ability to provide comprehensive validation of spatial dose distributions, particularly in flexibility radiation sensitivity. In this regard, a novel flexible PWO(Er)/ZnO(Ag) nanocomposite sensor has been developed by a solvothermal procedure and its ionization radiation responses were investigated under UV, proton, laser 980 nm, and 241Am sources. The calculated alpha detection efficiency showed that the prepared PWO(Er)/ZnO(Ag) sensor is highly sensitive to alpha radiation (99.6 %) with good stability/repeatability and linear response. According to the Photo/Ionoluminescence study, the doped dual nanocomposite sample showed the strongest blue, green, and yellow emissions at room temperature compared with pure and single doped samples with an exciton lifetime of τ = 143.30 ±0.07 μs. The XRD, FTIR, Raman, and EDX results indicated that the prepared pure and doped nanocomposites have two-phase compositions of PWO and ZnO and related chemical compounds/elements in the nanocomposite structure. The morphologies of the PWO and PWO(Er) nanoparticles (NPs) were mostly cubic and heterogeneous with average particle sizes of 125 and 82 nm. ZnO and ZnO(Ag) had hexagonal, spherical, and rod-shaped particles with typical particle diameters of 130 and 90 nm, respectively. Our results indicate the prepared luminescent nanocomposites are transparent, flexible, highly stable, highly sensitive to ionizing radiation, and have the potential for practical use in optoelectronics and medical imaging in the future.

Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals.

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA0.9Cs0.1PbI2.8Br0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 μeV at a temperature of 1.5 K. The corresponding exciton coherence time T2 = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Stable electroluminescence in ambipolar dopant-free lateral p–n junctions

Dopant-free lateral p–n junctions in the GaAs/AlGaAs material system have attracted interest due to their potential use in quantum optoelectronics (e.g., optical quantum computers or quantum repeaters) and ease of integration with other components, such as single electron pumps and spin qubits. A major obstacle to integration has been the unwanted charge accumulation at the p–n junction gap that suppresses light emission, either due to enhanced non-radiative recombination or due to inhibition of p–n current. Typically, samples must frequently be warmed to room temperature to dissipate this built-up charge and restore light emission in a subsequent cooldown. Here, we introduce a practical gate voltage protocol that clears this parasitic charge accumulation, in situ at low temperature, enabling the indefinite cryogenic operation of devices. This reset protocol enabled the optical characterization of stable, bright, dopant-free lateral p–n junctions with electroluminescence linewidths among the narrowest (<1 meV; <0.5 nm) reported in this type of device. It also enabled the unambiguous identification of the ground state of neutral free excitons (heavy and light holes) as well as charged excitons (trions). The free exciton emission energies for both photoluminescence and electroluminescence are found to be nearly identical (within 0.2 meV or 0.1 nm). The binding and dissociation energies for free and charged excitons are reported. A free exciton lifetime of 237 ps was measured by time-resolved electroluminescence, compared to 419 ps with time-resolved photoluminescence.