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Evaluation of exciton diffusion length in highly oriented fullerene films of fullerene/p-Si(100) hybrid solar cells
摘要： Highly oriented fullerene (C60) films on p-Si (100) substrates were fabricated to evaluate the crystallinity dependent exciton diffusion length of C60 (LC60). The crystal structure of the C60 films was examined using grazing incidence X-ray diffraction (GIXD). The results of an in-plane rocking scan and a pole figure suggested that a 12-fold-symmetry crystal was grown with the C60(111) surface interfaced to the Si(100) substrate. The photovoltaic characteristics of the oriented C60/p-Si(100) hybrid solar cells were evaluated. A masking effect was clearly evident in the incident photon-to-current conversion efficiency (IPCE) spectra. LC60 was evaluated using both experimental IPCE spectra and that produced by one-dimensional-optical simulation. It was concluded that LC60 for highly oriented C60 was 60 nm, which was longer than that of disordered C60 films.
关键词： crystal growth,hybrid solar cells,fullerene (C60),exciton diffusion length,grazing incidence X-ray diffraction (GIXD),masking effect
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS <sub/>2</sub>
摘要： Two-dimensional Transition Metal Dichalcogenides (TMDs) have been widely studied because of the peculiar electronic band structure and the strong excitonic effects. In these materials the large spin-orbit coupling lifts the spin degeneracy of the valence (VB) and the conduction band (CB) giving rise to the A and B interband excitonic transitions. In monolayer WS2, the spins of electrons in the lowest CB and in the highest VB at K/K’ point of the Brillouin zone are antiparallel resulting in an intravalley dark exciton state at a lower energy than the bright exciton. Here we use two-colour helicity-resolved pump-probe spectroscopy to directly resolve the intravalley spin-?ip process of the photoexcited electrons in the CB of single-layer WS2. We ?nd that at T=77 K this process occurs on a sub-ps time scale and it is signi?cantly dependent on the temperature, strongly pointing to a phonon assisted relaxation process. Our results shed light on the intravalley spin relaxation process in single-layer WS2, determining the formation of the intravalley dark exciton, which we measure to occur on a sub-ps timescale. The study of dark excitons formation dynamics is important for designing TMD-based electronic/photonic devices.
关键词： WS2,spin-?ip relaxation,exciton,pump-probe spectroscopy,Transition Metal Dichalcogenides
更新于2025-09-11 14:15:04
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Reversible PL Tuning by Defect Passivation via Laser Irradiation on Aged Monolayer MoS2
摘要： Atomically thin (1L) MoS2 emerged as a direct band gap semiconductor with potential optical applications. The photoluminescence (PL) of 1L-MoS2 degrades due to aging related defect formation. The passivation of these defects leads to substantial improvement in optical properties. Here we report the enhancement of PL on aged 1L-MoS2 by laser treatment. Using photoluminescence and Raman spectroscopy in a controlled gas environment, we show the enhancement is associated with efficient adsorption of oxygen on existing sulfur vacancies preceded by removal of adsorbates from the sample’s surface. Oxygen adsorption depletes negative charges, resulting in suppression of trions and improved neutral exciton recombination. The result is a 6-8 fold increase in PL emission. The laser treatment in this work does not cause any measurable damage to the sample as verified by Raman spectroscopy, which is important for practical applications. Surprisingly, the observed PL enhancement is reversible by both vacuum and ultrafast femtosecond excitation. While the former approach allows switching a designed micro pattern on the sample ON and OFF, the latte provides a controllable mean for accurate PL tuning, which is highly desirable for optoelectronic and gas sensing applications.
关键词： Raman,Exciton,Laser Annealing,Oxygen,Photoluminescence,Reversible Defect passivation,MoS2
更新于2025-09-11 14:15:04
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Structural modification as the pioneer strategy in competition of the porphyrin dye and perovskite solar cells: From dynamics to kinetics of the photovoltaic processes
摘要： Molecular engineering plays a key role in designing ef?cient dye-sensitized/perovskite solar cells (DSSCs/PSCs). Porphyrins have more favorable spectroscopic properties than AZn(COOH)3 perovskites due to their lower chemical hardness. Also, porphyrins are pioneering photosensitizers in exciton formation/dissociation, kinetically. The presence of Zn2t and butadiyne as the linker in the porphyrins and the existence of the electron donors of dimethylammonium and formamidinium in the perovskites improve their photovoltaic properties. Porphyrin dimers have a greater charge transfer distance in comparison to the monomers. However, they show less energy conversion ef?ciency, which may be related to the quenching of the interporphyrin charge transfer. As a consequence, DSSCs based on the porphyrin monomers have a higher ef?ciency than the studied PSCs.
关键词： perovskite solar cells,porphyrin dye,exciton formation/dissociation,molecular engineering,photovoltaic processes
更新于2025-09-11 14:15:04
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Plasmon-Enhanced Fluorescence in Gold Nanorod-Quantum Dot Coupled Systems
摘要： Plasmon-exciton coupling is of great importance to many optical devices and applications. One of the coupling manifestations is plasmon-enhanced fluorescence. Although this effect is demonstrated in numerous experimental and theoretical works, there are different particle shapes for which this effect is not fully investigated. In this work electrostatic complexes of gold nanorods and CdSe/CdZnS quantum dots were studied. Double-resonant gold nanorods have an advantage of the simultaneous enhancement of the absorption and emission when the plasmon bands match the excitation and fluorescence wavelengths of an emitter. A relationship between the concentration of quantum dots in the complexes and the enhancement factor was established. It was demonstrated that the enhancement factor is inversely proportional to the concentration of quantum dots. The maximal fluorescence enhancement by 10.8 times was observed in the complex with the smallest relative concentration of 2.5 quantum dots per rod and approximately 5 nm distance between them. Moreover, the influence of quantum dot location on the gold nanorod surface plays an important role. Theoretical study and experimental data indicate that only the position near the nanorod ends provides the enhancement. At the same time, the localization of quantum dots on the sides of the nanorods leads to the fluorescence quenching.
关键词： quantum dot fluorescence,gold nanorod-quantum dot complexes,plasmon-enhanced fluorescence,plasmon-exciton coupling,gold nanorods
更新于2025-09-11 14:15:04
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Surface Modification for Improving the Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene over Inorganic Lead Halide Perovskite Quantum Dots
摘要： Inorganic lead halide perovskite quantum dots (iLHP-QDs) have recently been used in the photocatalytic reaction. However, the factors that influence the photocatalytic performance of the iLHP-QDs has not been fully investigated. Herein, we synthesised a series of iLHP-QDs with varied halide ratio (CsPbX3, X=I, I0.67Br0.33, I0.5Br0.5, I0.33Br0.67, Br) and studied its influence on the photocatalytic performance by monitoring the polymerization of TerEDOT. The CsPbI3 QDs showed the best performance owing to its narrow bandgap and low exciton binding energy. Moreover, the photocatalytic performance of the iLHP-QDs could be simply improved by being treated with methyl acetate, which can be attributed to the replacement of the oleic acid by the short acetate acid, and the introduction of the traps on the surface of QDs in the post-treatment. These results could help design more efficient photocatalytic system and further promote the application of iLHP-QDs.
关键词： photocatalytic performance,quantum dots,exciton binding energy,charge transport efficiency,halide perovskite
更新于2025-09-11 14:15:04
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Reference Module in Materials Science and Materials Engineering || Excitonic Model versus Band Gap Model in Organic Materials
摘要： The binding energy Eb of an optical excitation is a key parameter for the understanding of the opto-electronic properties of organic solids in general and of conjugated polymers in particular. It controls the dissociation of an electronic excitation of either singlet or triplet character into a pair of free charges as well as the reverse process, i.e., the recombination of an electron–hole (eh) pair yielding an excitation that can decay radiatively or nonradiatively. If Eb is large, photogeneration of charge carriers is an endothermic, inefficient process. It is obvious that in an organic solar cell (OSC) one would like Eb to be as small as possible, while in a light emitting diode (LED) it is the opposite because charge recombination requires a driving force. In this article an outline of the problem of defining the exciton binding in a molecular solid will be presented followed by experimental and theoretical advances and a critical review of relevant conceptual frameworks with particular emphasis on conjugated polymers.
关键词： Organic Materials,Conjugated Polymers,Band Gap Model,Opto-electronic Properties,Exciton Binding Energy
更新于2025-09-11 14:15:04
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Triplet exciton fine structure in Pt-rich polymers studied by circularly polarized emission under high magnetic field
摘要： Incorporating heavy atoms into polymer chains represents an effective way to generate emissive triplets. Here we used magneto-optical emission spectroscopy up to 17.5 Tesla for studying the fine structure of the triplet exciton in a series of Pt-rich π-conjugated polymers with various intrachain Pt concentrations. We found that their phosphorescence emission band shows substantial field-induced circular polarization (FICPO) up to 50% with an unusual, nonmonotonic field dependence at cryogenic temperature. From the field-induced energy splitting between left and right circularly polarized phosphorescence we obtained the effective g factor of triplet exciton varying in the range of ?0.13-0.85, which depends on the Pt concentration in the polymer chains. The FICPO of triplet emission originates from the population difference in spin sublevels, which is determined by thermal equilibrium subjected to spin-orbit coupling (SOC), exchange, and Zeeman interactions. Surprisingly we also observed FICPO in the fluorescence emission that results from the singlet-triplet interaction caused by the strong SOC. From these results we extracted the various interaction parameters that describe the exciton fine structure in these Pt-contained compounds.
关键词： Pt-rich polymers,high magnetic field,circularly polarized emission,spin-orbit coupling,triplet exciton
更新于2025-09-11 14:15:04
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Dark exciton brightening and its engaged valley dynamics in monolayer
摘要： We theoretically investigate the valley Zeeman splitting, dark-exciton brightening, and magnetic-field-controlled dark-exciton engaged valley dynamics in monolayer WSe2 subjected to a tilted magnetic field B. In Faraday geometry (out-of-plane B⊥), only bright-exciton emissions emerge, and the valley polarization (VP) as a function of B⊥ for σ + and σ ? circularly polarized laser excitations features an “X” pattern, i.e., the VP is locked to the valley. We also find that the photoluminescence (PL) intensity exhibits a similar behavior to VP. In the Voigt geometry (in-plane B(cid:3)), however, aside from the bright-exciton emission, the dark exciton becomes brightening. The PL intensity of the dark exciton is enhanced parabolically with increasing B(cid:3), independent of the pumping laser helicity. For B along any other direction, as B increases, the dark-exciton emission intensity increases but depends on the pumping laser helicity, accompanied by an decrease of its VP. Furthermore, not only the bright-exciton but also the dark-exciton emission peak splits for laser excitation with different circular polarizations. In addition, the latter demonstrates a much larger peak splitting than the former, which facilitates individual manipulation of each of the two valleys. Our theory paves the way for a study of the properties of bright-dark hybrid states with lifetime orders of magnitude longer than that of the bright states, which is desirable for fields of either spintronics or valleytronics (or their combination).
关键词： monolayer WSe2,dark-exciton brightening,tilted magnetic field,valley dynamics,valley Zeeman splitting
更新于2025-09-11 14:15:04
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Exciton Peierls mechanism and universal many-body gaps in carbon nanotubes
摘要： “Metallic” carbon nanotubes exhibit quasiparticle gaps when isolated from a screening environment. The gap-opening mechanism is expected to be of electronic origin, but the precise nature is debated. In this work, we show that hybrid density functional theory predicts a set of excitonic instabilities capable of opening gaps of the size found in experiment. The excitonic instabilities are coupled to vibrational modes and, in particular, the modes associated with the (cid:2) ? E2g and K ? A(cid:2) 1 Kohn anomalies of graphene, inducing Peierls lattice distortions with a strong electron-phonon coupling. In the larger tubes, the longitudinal optical phonon mode becomes a purely electronic dimerization that is fully symmetry conserving in the zigzag and chiral tubes, but breaks the symmetry in the armchair tubes. The resulting gaps are universal (i.e., independent of chirality) and scale as 1/R with tube radius.
关键词： exciton Peierls mechanism,electron-phonon coupling,many-body gaps,carbon nanotubes,hybrid density functional theory
更新于2025-09-11 14:15:04