Multiple Exciton Research Articles

Efficient Multiple Exciton Generation in Monolayer MoS2.

Utilization of the excess energy of photoexcitation that is otherwise lost as thermal effects can improve the efficiency of next-generation light-harvesting devices. Multiple exciton generation (MEG) in semiconducting materials yields two or more excitons by absorbing a single high-energy photon, which can break the Shockley-Queisser limit for the conversion efficiency of photovoltaic devices. Recently, monolayer transition metal dichalcogenides (TMDs) have emerged as promising light-harvesting materials because of their high absorption coefficient. Here, we report efficient MEGs with low threshold energy and high (86%) efficiency in a van der Waals (vdW) layered material, MoS2. Through different experimental approaches, we demonstrate the signature of exciton multiplication and discuss the possible origin of decisive MEG in monolayer MoS2. Our results reveal that vdW-layered materials could be a potential candidate for developing mechanically flexible and highly efficient next-generation solar cells and photodetectors.

Moiré Enhanced Potentials in Twisted Transition Metal Dichalcogenide Trilayers Homostructures.

The exploration of moiré superlatticesholds promising potential to uncovernovel quantum phenomena emerging from the interplay of atomic structure and electronic correlation . However, the impact of the moiré potential modulation on the number of twisted layers has yet to be experimentally explored. Here, this work synthesizes a twisted WSe2 homotrilayer using a dry-transfer method and investigates the enhancement of the moiré potential with increasing number of twisted layers. The results of the study reveal the presence of multiple exciton resonances with positive or negative circularly polarized emission in the WSe2 homostructure with small twist angles, which are attributed to the excitonic ground and excited states confined to the moiré potential. The distinct g-factor observed in the magneto-optical spectroscopy is also shown to be a result of the confinement of the exciton in the moiré potential. The moiré potential depths of the twisted bilayer and trilayer homostructures are found to be 111 and 212 meV, respectively, an increase of 91% from the bilayer structure. These findings demonstrate that the depth of the moiré potential can be manipulated by adjusting the number of stacked layers, providing a promising avenue for exploration into highly correlated quantum phenomena.

Improving Ultraviolet Stability of Perovskite Solar Cells via Singlet Fission Down‐Conversion†

Comprehensive SummaryThe instability of perovskite materials under continuous ultraviolet (UV) light irradiation and high sensitivity in humid environments remain obstacles to future commercialization. Especially, the photovoltaic performance of perovskite solar cells (PVSCs) is prone to decline under UV light exposure for sustained periods of time. However, in conventional methods, preventing UV light from entering PVSCs usually comes at the expense of reducing short circuit photocurrent (Jsc). Herein, the UV stability of PVSCs is modified by introducing a singlet fission down‐conversion layer 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐PEN) via one‐step anti‐solvent method without sacrificing device efficiency. The introduction of down conversion layer can not only improve the Jsc by converting UV light into multiple excitons, but also enhance the open‐circuit voltage (Voc) owing to a better matched energy level alignment at the perovskite/spiro‐OMeTAD interface. Consequently, the TIPS‐PEN incorporated PVSCs attain the champion power conversion efficiency (PCE) up to 22.92% accompanied with dramatically increased UV photostability which can retain 80% of its primitive PCE under continuous UV light soaking for 150 h. Moreover, the unencapsulated PVSCs with TIPS‐PEN exhibit remarkable moisture stability which can sustain over 80% of the initial value under air conditions (50% relative humidity, 25 °C) after 1000 h.

Strong coupling between a plasmon mode and multiple different exciton states

Strong coupling between a plasmon mode and multiple different exciton states

Synthesis of single‐crystalline InP tetrapod nanocrystals via addition of ZnCl2

AbstractRecently, single‐crystalline indium phosphide (InP) tetrapods, which allow an exciton to act in a true tetrapodal geometry, were successfully synthesized and proved as a useful platform to study multiple exciton behaviors. We used ZnCl2 precursor, a commonly used additive in the synthesis of aminophosphine‐based InP nanocrystals to enhance the optical performance, and successfully synthesized InP tetrapods (InP‐ZnCl2) with the narrower arm length distribution. Interestingly, no distinct enhancement in photoluminescence was observed while the arm length of the InP tetrapod was suppressed with the addition of ZnCl2. Further shell growth on InP‐ZnCl2 successfully preserved tetrapodal geometry in InP‐ZnCl2/ZnSe and showed higher photoluminescence than that of tetrapodal InP/ZnSe. The crystal structure of InP‐ZnCl2 was also retained even along with various Zn feed ratios as observed in x‐ray diffraction (XRD) patterns and showed no peak shift as Zn only passivates the surface of InP tetrapods. These results provide a platform to study the role of Zn precursor.

Semiconducting Quantum Dots for Energy Conversion and Storage

AbstractSemiconducting quantum dots (QDs) have received huge attention for energy conversion and storage due to their unique characteristics, such as quantum size effect, multiple exciton generation effect, large surface‐to‐volume ratio, high density of active sites, and so on. However, the holistic and systematic understanding of the energy conversion and storage mechanism centering on QDs in specific application is still lacking. Herein, a comprehensive introduction of these extraordinary 0D materials, e.g., metal oxide, metal dichalcogenide, metal halides, multinary oxides, and nonmetal QDs, is presented. It starts with the synthetic strategies and unique properties of QDs. Highlights are focused on the rational design and development of advanced QDs‐based materials for the various applications in energy‐related fields, including photocatalytic H2 production, photocatalytic CO2 reduction, photocatalytic N2 reduction, electrocatalytic H2 evolution, electrocatalytic CO2 reduction, electrocatalytic N2 fixation, electrocatalytic O2 evolution, electrocatalytic O2 reduction, solar cells, metal‐ion batteries, lithium–sulfur batteries, metal–air batteries, and supercapacitors. At last, challenges and perspectives of semiconducting QDs for energy conversion and storage are detailedly proposed.

Strong coupling of multiple plasmon modes and excitons with excitation light controlled active tuning

Abstract While the strong coupling between cavity modes and quantum emitters has been investigated in various systems, multiple surface plasmon modes in single nanostructures strongly coupling with excitons are rarely explored. Here, we investigate the strong coupling between three surface plasmon modes in silver nanowires and excitons in monolayer WSe2 at room temperature. Four plasmon-exciton polariton (plexciton) states are observed in the scattering spectra. The photoluminescence (PL) spectra of the hybrid system show clear splitting due to strong coupling, and the energies of the emission corresponding to the two lower plexciton states agree with that of the scattering very well. In addition, we show that the plasmon-exciton interaction in this system can be efficiently tuned by controlling the excitation power. These results reveal the fundamental properties of strong coupling between multiple plasmon modes and excitons, deepen the understanding of the correlation between scattering and PL spectra of plasmon-exciton strong coupling systems, and open up a new way to actively control the coupling between plasmonic nanostructures and two-dimensional semiconductors.

Multiple Exciton Generation Solar Cells: Numerical Approaches of Quantum Yield Extraction and Its Limiting Efficiencies

Multiple exciton generation solar cells exhibit low power conversion efficiency owing to non-radiative recombination, even after the generation of numerous electron and hole pairs per incident photon. This paper elucidates the non-idealities of multiple exciton generation solar cells. Accordingly, we present mathematical approaches for determining the quantum yield to discuss the non-idealities of multiple exciton generation solar cells by adjusting the delta function. We present the use of the Gaussian distribution function to present the occupancy status of carriers at each energy state using the Dirac delta function. Further, we obtained ideal and non-ideal quantum yields by modifying the Gaussian distribution function for each energy state. On the basis of this approach, we discuss the material imperfections of multiple exciton generations by analyzing the mathematically obtained quantum yields. Then, we discuss the status of radiative recombination calculated from the ratio of radiative to non-radiative recombination. Finally, we present the application of this approach to the detailed balance limit of the multiple exciton generation solar cell to evaluate the practical limit of multiple exciton generation solar cells.

Coherent excitation energy transfer processes in two-dimensional para-sexiphenyl molecular clusters

Excitation energy transfer is one of the most important factors affecting the applications of para-sexiphenyl devices. The study of exciton dynamics and exciton coherence effect of para-sexiphenyl clusters under external field excitation is important in order to improve the performance of molecular devices composed of para-sexiphenyl and its related derivatives. In this work, the two-dimensional disc-like para-sexiphenyl molecular cluster is used as the object of study. The molecular system is simplified into a two-level model based on its structural features and energy level distribution. Within the framework of density matrix theory, the exciton dynamics and exciton coherence behavior of disk-like para-hexaphene molecular clusters excited by different pulse fields are analyzed through using the mathematical mean value approximation of the operator. The results show that when long pulses are used to excite para-sexiphenyl clusters, the single exciton state characteristic appears and is insensitive to the change of excited external field strength. When the clusters are subjected to strong pulsed fields with short pulse widths, multiple excitons are excited simultaneously in the cluster, forming multiple exciton states, with the exciton energy levels shifting toward lower energy and new hybrid states appearing. In the optical response spectrum, there appear multiple resonance peaks. And as the pulse field is enhanced, the multi-exciton effect becomes apparent and the hybridization energy level increases. Under short pulse excitation, the excited states are distributed differently in different energy regions, but all of them show obvious symmetry. As the highest-energy exciton states of H-type clusters are preferentially excited, we analyze the exciton state population and the exciton coherence evolution with time in the high-energy exciton state. With the pulse field increases, Rabi oscillations appear and the exciton coherence effect increases. When the pulsed field reaches a certain field strength, the exciton oscillation cooperativity disappears in the first 100 fs, showing the non-local characteristic. The position of the wave trough of the exciton state population corresponds to the peak in the exciton coherence size. It indicates that when the pulse field is intense enough, a large number of molecules are in the exciton coherent state during the pulsed excitation, and transient out-of-domain phenomena occur.

Room temperature exciton formation and robust optical properties of CVD-grown ultrathin Bi2O2Se crystals on arbitrary substrates.

The appealing success of non-van der Waals (non-VdW) two-dimensional (2D) bismuth oxyselenide (Bi2O2Se) crystals in optoelectronics provides an exciting avenue to investigate their fundamental physical properties. To date, the majority of efforts have focused on understanding the properties of 2D Bi2O2Se, usually grown on a mica substrate. However, a gap exists in realizing the origin of photoluminescence (PL) of new age non-VdW Bi2O2Se at visible and near-infrared (NIR) wavelengths and the effect of growth substrates on the structure and optical properties. Herein, we report that the formation of multiple excitons in momentum valleys is responsible for broadband absorption and visible PL from a few layer thick 2D Bi2O2Se. The effect of growth substrates on the structure and optical properties is investigated in detail. Our studies unfold that the growth substrates (mica, sapphire, quartz, SiO2, glass) introduce strain/doping in chemical vapor deposition (CVD)-grown Bi2O2Se crystals, and consequently, the morphology, lattice constant, absorption coefficient, optical bandgap, refractive index, and PL properties are modulated. In addition, the possible direct/indirect multiple exciton formation at the valence band to the conduction band at different symmetry points of Bi2O2Se is analyzed from experimental data on different growth substrates and corroborated with the density functional theory (DFT) calculation of the electronic band structure. Furthermore, temperature-dependent photo-carrier dynamics discloses an A/Γ-exciton activation energy of 209.6 meV in Bi2O2Se. These findings are significant for the futuristic optoelectronic applications of Bi2O2Se and the choice of growth substrates on directly fabricated nanodevices.

Regulating multiple self-trapped exciton emissions in zero-dimensional antimony halides with pyramidal units

The [SbCl5]2− distortions modulate triplet STE emissions. The enhancement of high-energy triplet emission is ascribed to enhanced electron–phonon coupling, promoted intersystem crossing process, as well as restrained nonradiative transitions.

Highly efficient deep-blue organic light-emitting diodes (OLEDs) based on hot-exciton materials with multiple triplet exciton conversion channels

High-energy triplet exciton conversion, high exciton utilization efficiency and high device efficiency were achieved through multiple hot-exciton conversion channels.

Quantum shells versus quantum dots: suppressing Auger recombination in colloidal semiconductors.

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of attention in recent decades. The quantum efficiency of many optoelectronic processes based on these nanomaterials, however, declines with increasing optical or electrical excitation intensity. This issue is caused by Auger recombination of multiple excitons, which converts the NC energy into excess heat, whereby reducing the efficiency and lifespan of NC-based devices, including lasers, photodetectors, X-ray scintillators, and high-brightness LEDs. Recently, semiconductor quantum shells (QSs) have emerged as a viable nanoscale architecture for the suppression of Auger decay. The spherical-shell geometry of these nanostructures leads to a significant reduction of Auger decay rates, while exhibiting a near unity photoluminescence quantum yield. Here, we compare the optoelectronic properties of quantum shells against other low-dimensional semiconductors and discuss their emerging opportunities in solid-state lighting and energy-harvesting applications.

Signature of Cascade Transitions between Interlayer Excitons in a Moiré Superlattice.

A moiré superlattice in transition metal dichalcogenides heterostructure provides an exciting platform for studying strongly correlated electronics and excitonic physics, such as multiple interlayer exciton (IX) energy bands. However, the correlations between these IXs remain elusive. Here, we demonstrate the cascade transitions between IXs in a moiré superlattice by performing energy- and time-resolved photoluminescence measurements in the MoS_{2}/WSe_{2} heterostructure. Furthermore, we show that the lower-energy IX can be excited to higher-energy ones, facilitating IX population inversion. Our finding of cascade transitions between IXs contributes to the fundamental understanding of the IX dynamics in moiré superlattices and may have important applications, such as in exciton condensate, quantum information protocols, and quantum cascade lasers.

Impact of Morphological Inhomogeneity on Excitonic States in Highly Mismatched Alloy ZnSe1-XTeX Nanocrystals.

ZnSe1-XTeX nanocrystals (NCs) are promising photon emitters with tunable emission across the violet to orange range and near-unity quantum yields. However, these NCs suffer from broad emission line widths and multiple exciton decay dynamics, which discourage their practicable use. Here, we explore the excitonic states in ZnSe1-XTeX NCs and their photophysical characteristics in relation to the morphological inhomogeneity of highly mismatched alloys. Ensemble and single-dot spectroscopic analysis of a series of ZnSe1-XTeX NC samples with varying Te ratios coupled with computational calculations shows that, due to the distinct electronegativity between Se and Te, nearest-neighbor Te pairs in ZnSe1-XTeX alloys create localized hole states spectrally distributed approximately 130 meV above the 1Sh level of homogeneous ZnSe1-XTeX NCs. This forms spatially separated excitons (delocalized electron and localized hole in trap), accounting for both inhomogeneous and homogeneous line width broadening with delayed recombination dynamics. Our results identify photophysical characteristics of excitonic states in NCs made of highly mismatched alloys and provide future research directions with potential implications for photonic applications.

(Keynote) Colloidal Quantum Shells: An Emerging Class of 2D Semiconductors

Low-dimensional colloidal semiconductors are inexpensive to process and offer a broad spectrum of attractive quantum-mechanical properties. The notorious problem of these nanostructures lies in their limited performance under high energetic loads, when more than one exciton per particle is created. Multiple excitons undergo fast Auger recombination, causing efficiently roll-off in energy-intensive applications, including high-brightness LEDs, X-ray scintillators, and solar cells. This presentation will discuss an emerging type of low-dimensional semiconductors, known as colloidal quantum shells that allow avoiding such multi-exciton (MX) energy losses. The geometry of quantum shells benefits from the spatial separation of multiple excitons, which leads to extraordinary improvements to MX lifetimes and MX quantum yield. We will compare quantum shells with other nanoscale geometries in three categories: Auger suppression, multi-exciton level structure, and solution-processed film conductivity, highlighting the potential benefits quantum shells can contribute to various applications. Theoretical analysis discussing the role of the quantum shell geometry in suppressing Auger recombination will also be presented.

(Invited) Quantum Well Architecture to Realize High Optical Gain Lasing Media

Photonic applications of semiconductor quantum dots (QDs) had significant drawbacks due to their ultrafast Auger decay of multiple excitons. These nonradiative processes have hindered the performance of electroluminescent and lasing devices. The emergence of the colloidal quantum well (QW) architecture where semiconductor quantum shell with an inverted QD geometry has led to suppressed Auger recombination. Ultra long biexciton lifetime (>10 ns) and large biexciton quantum yields has been achieved through spatial separation promoted by the QW geometry. Moreover, architecture induced exciton-exciton repulsions have led to the splitting of exciton and biexciton optical transitions which resulted in the single exciton gain mode along with the biexciton gain mode. High biexciton lifetime and the longest reported single exciton gain lifetime (>6 ns) has rendered this geometry a worthy candidate for the development of optically and electrically pumped gain media

Ternary atoms alloy quantum dot assisted hole transport in thin film polymer solar cells

Quantum confinement effects and multiple exciton generation (MEG) were explored in this investigation to assist the hole transport and improved trapping of light in thin film solar cells (TFOSCs). The popular organic transport material Poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT:PSS) is employed here for effective electron blocking and hole transport processes through doping with quantum dots (QD). Alloyed CdTeSe QD is used, at different concentrations, in PEDOT:PSS layer to fabricate TFOSCs in the convectional device structure consisting of molecules blended as poly (3-hexylthiophene): (6, 6)-phenyl- C61-butyric acid methyl ester (P3HT: PCBM) solar absorber. A remarkable increase in performance of the devices was achieved from TFOSC whose HTL doped with QDs that resulted in an optimal boost in power conversion efficiency (PCE) as high as 111.7% relative to the reference cell. The results suggest that the doping of HTL with the alloy of CdTeSe quantum dots assisted in electron blocking, and improved hole collection as well as improved light trapping at the interface between the solar absorber layer and HTL.

Dye-Sensitized Multiple Exciton Generation in Lead Sulfide Quantum Dots.

Multiple exciton generation (MEG), the generation of multiple excitons from the absorption of a single high-energy photon, is a strategy to go beyond the limiting efficiencies that define current-day solar cells by harvesting some of the thermalization energy losses that occur when photons with an energy greater than the semiconductor bandgap are absorbed. In this work, we show that organic dyes can sensitize MEG in semiconductor quantum dots (QDs). In particular, we found that surface-anchored pyrene ligands enhanced the photon-to-charge carrier quantum yield of PbS QDs from 113 ± 3% to 183 ± 7% when the photon energy was 3.9 times the band gap. A wavelength dependence study shows that the enhancement is positively correlated with the pyrene absorptivity. Transient absorption and steady-state photoluminescence measurements suggest that the MEG sensitization is based on an initial fast electron transfer from the pyrene ligands to the PbS QDs producing hot-electrons in the QDs that subsequently undergo MEG. This work demonstrates that hybrid and synergistic organic/inorganic interactions can be a successful strategy to enhance MEG.

Understanding inclusive quantum dots hollow CN@CIZS heterojunction for enhanced photocatalytic CO2 reduction

Photocatalytic conversion of carbon dioxide (CO2) to fuel or useful chemicals is of great significance for energy and environmental issues. Herein, we successfully prepare a hollow g-C3N4@Cu-In-Zn-S quantum dots (CN@CIZS QDs) photocatalytic heterojunction employing the method of electrostatic self-assembly. The hollow structure, multiple exciton effect of quantum dots and the discrepancy of band structure generate excellent photocatalytic performance. The yield of CO2 photocatalytic reduction to CO and CH4 respectively is 23.04 μmol g−1 and 13.92 μmol g−1 after simulated visible light irradiation for 6 h for hollow CN@CIZS QDs 20 wt% sample. Moreover, the results of in-situ characterization and other photochemical tests strongly demonstrate the significant optimization of the photocatalytic performance. The research offers a feasible green thought for the resource utilization of CO2.