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Suggestions on Efficiency Droop of GaN-based LEDs
摘要： InGaN/GaN-based light-emitting diodes (LEDs) are widely used in modern society and industry among different areas. However, InGaN/GaN LEDs suffer from an efficiency droop issue: The internal efficiency decreases during high current injection. The efficiency droop significantly affects the development of GaN-based LEDs devices in efficiency and light-output areas. Therefore, the improvement of the droop phenomenon has become a significant topic. This paper introduces several possible mechanisms of droop phenomenon based on different hypotheses including Auger Recombination, Carrier Delocalization and Electron Leakage. Furthermore, some proposals to mitigate efficiency droop, including semipolar LEDs, electron blocking layer(EBL), quaternary alloy and chip design will be discussed and analyzed. Also, it will provide some suggestions for the further optimization of droop phenomenon in each proposal.
关键词： electron blocking layer,semipolar LEDs,GaN-based LEDs,Auger Recombination,chip design,quaternary alloy,Carrier Delocalization,Electron Leakage,efficiency droop
更新于2025-09-23 15:21:01
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Robust and Fragile Quantum Effects in the Transfer Kinetics of Delocalized Excitons between B850 Units of LH2 Complexes
摘要： Aggregates of light harvesting 2 (LH2) complexes form the major exciton-relaying domain in the photosynthetic unit of purple bacteria. Application of a generalized master equation to pairs of the B850 units of LH2 complexes, where excitons predominantly reside, provides quantitative information on how the inter-LH2 exciton transfer depends on the distance, relative rotational angle, and the relative energies of the two LH2s. The distance dependence demonstrates significant enhancement of the rate due to quantum delocalization of excitons, the qualitative nature of which remains robust against the disorder. The angle dependence reflects isotropic nature of exciton transfer, which remains similar for the ensemble of disorder. The variation of the rate on relative excitation energies of LH2 exhibits resonance peaks, which, however, is fragile as the disorder becomes significant. Overall, the average transfer times between two LH2s are estimated to be in the range of 4?25 ps for physically plausible inter-LH2 distances.
关键词： light harvesting 2 (LH2) complexes,quantum delocalization,exciton transfer,photosynthetic unit,purple bacteria,disorder
更新于2025-09-23 15:21:01
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The Role of Delocalization and Excess Energy in the Quantum Efficiency of Organic Solar Cells and the Validity of Optical Reciprocity Relations
摘要： The photon energy dependence of long-range charge separation is studied for two prototypical polymer:fullerene systems. The internal quantum efficiency (IQE) of PCDTBT:PC61BM is experimentally shown to be independent on the excitation energy. In contrast, for TQ1:PC71BM the IQE is strongly energy-dependent for excitation energies close to CT electroluminescence peak maximum while it becomes energy-independent at higher excitation energies. Kinetic Monte Carlo simulations reproduce the experimental IQE and reveal that the photon energy-dependence of the IQE is governed by charge delocalization. Efficient long-range separation at excitation energies corresponding to the CT electroluminescence peak maximum or lower requires an initial separation of the hole-electron pair by ～4-5nm, whereas delocalization is less important for charge separation at higher photon energies. Our modeling results suggest that a phenomenological reciprocity between CT electroluminescence and external quantum efficiency does not necessarily prove that commonly employed reciprocity relations between these spectra are valid from a fundamental perspective.
关键词： delocalization,optical reciprocity relations,quantum efficiency,organic solar cells,charge separation
更新于2025-09-23 15:19:57
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Density Functional Theory Investigation of Nonlinear Optical Properties of T-Graphene Quantum Dots
摘要： Using density functional theory calculations, we have analyzed nonlinear optical properties of a series of T-graphene quantum dots differing in their shape and size. Electronic polarizability, first-order and second-order hyperpolarizability of these systems are investigated and shed light on their stability and electronic properties. Negative cohesive energy shows that they are energetically stable. The effect of size and incident frequency on their nonlinear responses are comprehensively discussed. Most of the systems exhibit strong NLO response and it enhanced in the presence of external field. All these systems show absorption maximum ranging from UV to visible window. Overall, this theoretical framework highlighted the nonlinear optical properties of T-graphene quantum dots that may provide valuable information in designing potential NLO materials.
关键词： T-graphene quantum dot,electron delocalization,nonlinear optical property,UV-visible absorption spectra,Density functional theory
更新于2025-09-16 10:30:52
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Disorder vs Delocalization: Which Is More Advantageous for High-Efficiency Organic Solar Cells?
摘要： We investigate the combined influence of energetic disorder and delocalization on electron-hole charge-transfer state separation efficiency in donor-acceptor organic photovoltaic systems using an analytical hopping model and Monte Carlo calculations, coupled with an effective mass model. Whereas energetic disorder increases the separation yield at intermediate and low electric fields for low efficiency blends with strongly localized carriers, we find that it reduces dramatically the fill factors and power conversion efficiencies in high efficiency solar cells that require high carrier delocalization within the conjugated segment and high mobility-lifetime product. We further demonstrate that the initial electron-hole distance and thermalization processes play only a minor role in the separation dynamics.
关键词： Charge separation,delocalization,organic photovoltaics,disorder,Monte Carlo simulations
更新于2025-09-12 10:27:22
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Coupling of deterministically activated quantum emitters in hexagonal boron nitride to plasmonic surface lattice resonances
摘要： The cooperative phenomena stemming from the radiation field-mediated coupling between individual quantum emitters are presently attracting broad interest for applications related to on-chip photonic quantum memories and long-range entanglement. Common to these applications is the generation of electro-magnetic modes over macroscopic distances. Much research, however, is still needed before such systems can be deployed in the form of practical devices, starting with the investigation of alternate physical platforms. Quantum emitters in two-dimensional (2D) systems provide an intriguing route because these materials can be adapted to arbitrarily shaped substrates to form hybrid systems wherein emitters are near-field-coupled to suitable optical modes. Here, we report a scalable coupling method allowing color center ensembles in a van der Waals material (hexagonal boron nitride) to couple to a delocalized high-quality plasmonic surface lattice resonance. This type of architecture is promising for photonic applications, especially given the ability of the hexagonal boron nitride emitters to operate as single-photon sources at room temperature.
关键词： coupling,strain,delocalization,defect,quantum emission,photoluminescence,surface plasmons,hexagonal boron nitride,2D materials,surface lattice resonance
更新于2025-09-11 14:15:04
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Investigation of parameters governing damage resistance of nematic liquid crystals for high-power or peak-intensity laser applications
摘要： We investigate the damage resistance of saturated and unsaturated liquid crystals (Lc’s) under a wide range of laser excitation conditions, including 1053-nm pulse durations between 600 fs and 1.5 ns and nanosecond pulse excitation at 351 nm and 532 nm. This study explores the relationship between the LC’s resistance to laser-induced breakdown (damage) and the electronic structure (π-electron delocalization) of the constituent molecules. the laser-induced damage threshold at all wavelengths and pulse durations was consistently higher in saturated materials than in their unsaturated counterparts. the wavelength’s dependence in the results suggests that the energy coupling process that leads to laser-induced breakdown is governed by the energy separation between the ground state and the first and second excited states, while the pulse duration’s dependence in the results reveals the important role of electron relaxation between the excited states. A qualitative description was developed to interpret the experimental observations.
关键词： laser-induced damage threshold,liquid crystals,saturated materials,unsaturated materials,π-electron delocalization
更新于2025-09-11 14:15:04
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Engineering Photon Delocalization in a Rabi Dimer with a Dissipative Bath
摘要： A Rabi dimer is used to model a recently reported circuit quantum electrodynamics system composed of two coupled transmission-line resonators with each being coupled to one qubit. In this study, a phonon bath is adopted to mimic the multimode micromechanical resonators and is coupled to the qubits in the Rabi dimer. The dynamical behavior of the composite system is studied by the Dirac–Frenkel time-dependent variational principle combined with the multiple Davydov D2 ans¨atze. Initially, all the photons are pumped into the left resonator, and the two qubits are in the down state coupled with the phonon vacuum. In the strong qubit–photon coupling regime, the photon dynamics can be engineered by tuning the qubit–bath coupling strength α and photon delocalization is achieved by increasing α. In the absence of dissipation, photons are localized in the initial resonator. Nevertheless, with moderate qubit–bath coupling, photons are delocalized with quasiequilibration of the photon population in two resonators at long times. In this case, high-frequency bath modes are activated by interacting with depolarized qubits. For strong dissipation, photon delocalization is achieved via frequent photon-hopping within two resonators and the qubits are suppressed in their initial down state.
关键词： Rabi dimer,photon delocalization,qubit–bath coupling,Davydov ans¨atze,quantum electrodynamics
更新于2025-09-10 09:29:36
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Coupling to Charge Transfer States Is the Key to Modulate the Optical Bands for Efficient Light-Harvesting in Purple Bacteria.
摘要： The photosynthetic apparatus of purple bacteria uses exciton delocalization and static disorder to modulate the position and broadening of its absorption bands, leading to efficient light harvesting. Its main antenna complex, LH2, contains two rings of identical bacteriochlorophyll pigments, B800 and B850, absorbing at 800 nm and at 850 nm, respectively. It has been an unsolved problem why static disorder of the strongly coupled B850 ring is several times larger than that of the B800 ring. Here we show that mixing between excitons and charge transfer states in the B850 ring is responsible for the effect. The linear absorption spectrum of the LH2 system is simulated by using a multi-scale approach with an exciton Hamiltonian generalized to include the charge transfer states that involve adjacent pigment pairs, with static disorder modelled microscopically by molecular dynamics simulations. Our results show that a sufficient inhomogeneous broadening of the B850 band, needed for efficient light-harvesting, is only obtained by utilizing static disorder in the coupling between local excited and inter-pigment charge transfer states.
关键词： exciton delocalization,B850,B800,charge transfer states,purple bacteria,LH2,molecular dynamics simulations,static disorder,bacteriochlorophyll pigments,photosynthetic apparatus,light harvesting
更新于2025-09-10 09:29:36
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Designing Long-Range Charge Delocalization from First Principles
摘要： Efficient electronic communication over long distances is a desirable property of molecular wires. Charge delocalization in mixed-valence (MV) compounds where two redox centers are linked by a molecular bridge is a particularly well-controlled instance of such electronic communication, thus lending itself to comparisons between theory and experiment. We study how to achieve and control long-range charge delocalization in cationic organic MV systems by means of Kohn–Sham density functional theory (DFT), and show that a captodative substitution approach recently suggested for molecular conductance (T. Stuyver et al., J. Phys. Chem. C 122, 3194 (2018)) greatly enhances charge delocalization in para-phenylene-based wires. To ensure the adequacy of our DFT methods, we validate different protocols for organic MV systems of different lengths. The BLYP35 hybrid functional combined with a polarizable continuum model, established by Renz and Kaupp, is indeed capable of correctly describing experimentally observed length-dependent charge delocalization, in contrast to the long-range corrected functionals ω-B97X-D and ω-PBE. We also discuss the implications of these results for a first-principles description of the transition between coherent tunneling and incoherent hopping regimes in molecular conductance.
关键词： Charge Delocalization,Molecular Wires,Mixed-Valence Compounds,Quantum Electronic Structure,Density Functional Theory
更新于2025-09-09 09:28:46