Organic Excitons Research Articles

Crystalline and Amorphous Interface Simulations of Donor-Acceptor Blends.

In an organic solar cell, exciton dissociation and charge transport to generate current depend on interface and bulk morphology, respectively, and their rates dictate device performance. Blend miscibility and processing determine the final morphology. We investigate the blend miscibility of P3HT and O-IDTBR, employing our recently developed "push-pull" computational technique, and explore its effect on nanoscale morphology. The resulting Flory-Huggins χ parameter is primarily enthalpic, and the blend exhibits UCST behavior. To directly simulate an equilibrated morphology in such a blend requires large (30 nm) systems and long (10 μs) simulations, which we attain with our virtual site coarse graining technique. In the resulting amorphous phase-separated state, O-IDTBR swells P3HT by about 15%, but does not percolate within the P3HT-rich phase. The interfacial profile depends on crystallinity: the amorphous interface is several nanometers wide, whereas an acceptor crystal induces order in the adjacent donor polymer, forming a sharp interface quite similar to a crystal-crystal interface. The wide amorphous interface promotes more donor-acceptor contacts with a wide range of relative orientations; however, the acceptors do not form a percolating network, which will likely lead to charge recombination. A crystal interface exhibits fewer donor-acceptor contacts with primarily face-on orientation that are more likely to charge-separate because of instant access to the preferred domain.

Embrace the darkness: An experimental perspective on organic exciton–polaritons

Organic polaritonics has emerged as a captivating interdisciplinary field that marries the complexities of organic photophysics with the fundamental principles of quantum optics. By harnessing strong light–matter coupling in organic materials, exciton–polaritons offer unique opportunities for advanced device performance, including enhanced energy transport and low-threshold lasing, as well as new functionalities like polariton chemistry. In this review, we delve into the foundational principles of exciton–polaritons from an experimental perspective, highlighting the key states, processes, and timescales that govern polariton phenomena. Our review centers on the spectroscopy of exciton–polaritons. We overview the primary spectroscopic approaches that reveal polariton phenomena, and we discuss the challenges in disentangling polaritonic signatures from spectral artifacts. We discuss how organic materials, due to their complex photophysics and disordered nature, not only present challenges to the conventional polariton models but also provide opportunities for new physics, like manipulating dark electronic states. As the research field continues to grow, with increasingly complex materials and devices, this review serves as a valuable introductory guide for researchers navigating the intricate landscape of organic polaritonics.

Regulating Exciton Diffusion in Antimony‐Based Perovskite‐Like Single Crystals toward Highly Efficient and Stable Luminescence

AbstractOrganic‐inorganic hybrid metal halide perovskite has played an important role in optoelectronics and photonics due to their extremely high photoluminescence quantum yield (PLQY). However, considering the strong nonmetallic property of antimony (Sb), the excitons in antimony‐based single crystals are usually dissipated in the form of non‐radiative recombination under organic‐inductive effect, which leads to the low PLQY. Herein, two kinds of perovskite‐like single crystals, i.e., (8‐HQ)2SbCl3 and (8‐HQH)SbCl4, are innovatively grown. The density functional theory (DFT) calculations reveal that the unique luminescence characteristics are associated with the inductive effect of organic coordination and exciton effect in the metal octahedral skeleton. When Cl− replaces the organic ligand in (8‐HQ)2SbCl3, the inductive effect is reduced while the self‐trapped exciton effect in the metal octahedron is enhanced. Besides, the theoretical research on electron transmission in a modeling device reveals that the hydrogen bond of organic A‐site in the single crystal promotes the transition of electrons. Based on the luminescent properties of (8‐HQH)SbCl4, the white LED devices are obtained by coating (8‐HQH)SbCl4 and red phosphor on 450 nm LED chips.

Vibrational polariton transport in disordered media.

Chemical reactions and energy transport phenomena have been experimentally reported to be significantly affected by strong light-matter interactions and vibrational polariton formation. These quasiparticles exhibit nontrivial transport phenomena due to the long-range correlations induced by the photonic system and elastic and inelastic scattering processes driven by matter disorder. In this article, we employ the Ioffe-Regel criterion to obtain vibrational polariton mobility edges and to identify distinct regimes of delocalization and transport under variable experimental conditions of light-matter detuning, disorder, and interaction strength. Correlations between the obtained trends and recent observations of polariton effects on reactivity are discussed, and essential differences between transport phenomena in organic electronic exciton and vibrational polaritons are highlighted. Our transport diagrams show the rich diversity of transport phenomena under vibrational strong coupling and indicate that macroscopic delocalization is favored at negative detuning and large light-matter interaction strength. We also find the surprising feature that, despite the presence of dephasing-induced inelastic scattering processes, macroscopic lower polariton delocalization and wave transport are expected to persist experimentally, even in modes with small photonic weight.

Multiexciton Generation from a 2D Organic-Inorganic Hybrid Perovskite with Nearly 200% Quantum Yield of Red Phosphorescence.

2D organic-inorganic hybrid perovskites (OIHPs) show obvious advantages in the field of optoelectronics due to their high luminescent stability and good solution processability. However, the thermal quenching and self-absorption of excitons caused by the strong interaction between the inorganic metal ions lead to a low luminescence efficiency of 2D perovskites. Herein, a 2D Cd-based OIHP phenylammonium cadmium chloride(PACC) with a weak red phosphorescence (ΦP < 6%) at 620nm and a blue afterglow is reported. Interestingly, the Mn-doped PACC exhibits very strong red emission with nearly 200% quantum yield and 15ms lifetime, thus resulting in a red afterglow. The experimental data prove that the doping of Mn2+ not only induces the multiexciton generation (MEG) process of the perovskite, avoiding the energy loss of inorganic excitons, but also promotes the Dexter energy transfer from organic triplet excitons to inorganic excitons, thus realizing the superefficient red-light emission of Cd2+ . This work suggests that guest metal ions can induce host metal ions to realize MEG in 2D bulk OIHPs, which provides a new idea for the development of optoelectronic materials and devices with ultrahigh energy utilization.

Energy-Transfer-Induced Enhanced Valley Splitting of Excitonic Emission of Inorganic CdTe@ZnS QDs in the Presence of Organic J-Aggregates: A Spectroscopic Insight into the Efficient Exciton (Inorganic)–Exciton (Organic) Coupling

An inorganic–organic nanohybrid material comprising an inorganic exciton (CdTe@ZnS) and an organic exciton (J-aggregates) has been synthesized, and subsequently investigations on exciton–exciton coupling between two components of the materials are carried out by employing various spectroscopic techniques. Analysis of the data has revealed very high Förster-type resonance energy transfer (FRET) from the donor quantum dots (QDs) to the acceptor J-aggregates. Interestingly, a clear splitting of the QD emission band during the FRET event is observed, indicating efficient coupling between inorganic and organic excitons. The observation of valley splitting of QD emission by organic J-aggregates, even in the absence of an external field, is new and exciting. More interestingly, the investigations have also shown that the valley splitting of the QD emission can be controlled by regulating the FRET process. In fact, a linear relationship between the energy separation value of the valley splitting and the FRET efficiency between QDs and J-aggregates is obtained. Moreover, in the present hybrid system, the presence of an organic moiety becomes advantageous in the sense that the valley splitting can also be controlled easily through chemical modification of the organic moiety. The outcome of the present investigation essentially demonstrates that the present nanohybrid system can be considered as a potential candidate for the fabrication of valleytronic materials.

SPIN-SELECTIVE INTERACTION OF TRIPLET-EXCITED MOLECULES ON THE SURFACE OF A FERROMAGNETIC NANOPARTICLE

Influence of a magnetic field generated by a ferromagnetic nanoparticle on the annihilation of triplet-excited organic molecules or triplet excitons in a near-surface particle layer is studied. A detailed mathematical model is presented that accounts for electron excitation diffusive mobility and geometry of the system. The kinetic operator is given in the complete 9x9 basis of triplet- triplet pair spin states. Time dependencies of the singlet spin state population of the triplet-triplet pair and the dependence of the triplet-triplet annihilation magnetic response profile (magnetic reaction effect) from the magnetic field induction are obtained. It is found that the influence of a magnetic field gradient on the reaction yield dominates over the other known mechanisms of spin-dynamics in triplet- triplet pairs.

Surface energetics mediated charge transfer and exciton transfer in cation-adsorbed rubrene/PbS nanocrystal hybrids

Organic molecule and inorganic nanocrystal (NC) hybrids have become a promising platform for photon energy conversion. Although surface energetics modification has proven effective in promoting triplet energy transfer, singlet energy transfer and charge transfer have been barely investigated. Here, we systematically clarify the photophysical dynamics of charge, singlet exciton, and triplet exciton within the energy conversion process based on hybrids of rubrene and Cd2+-adsorbed PbS NCs. It is found that a considerable number of charges in rubrene molecules can be transferred to cation-induced surface states in the ∼2 ps time scale with high efficiency to trigger a delayed biexciton effect, which provides a novel approach to uncover the intermediate role of NC surface states. For the triplet exciton, strong interaction with surface states is investigated with a recycling energy transfer of around 14% efficiency, which is found to be insensitive to changes in NC surface energetics. As a result, the maximum photoluminescence lifetime of PbS NCs was enhanced by about 38%. This work reveals the neglected photo-physical dynamics in the transfer process between organic molecules/inorganic NCs and validates the capability of the surface state in sensitization of organic charges and excitons.

Low Threshold, High Q-Factor Optically Pumped Organic Lasers and Exciton Dynamics in OLEDs under High Current Density: Singlet–Triplet Annihilation Effect and toward Electrical Injection Lasing

Low Threshold, High <i>Q</i>-Factor Optically Pumped Organic Lasers and Exciton Dynamics in OLEDs under High Current Density: Singlet–Triplet Annihilation Effect and toward Electrical Injection Lasing

Photocurrent generation following long-range propagation of organic exciton–polaritons

Natural photosynthesis exploited by both plants and bacteria provides essential energy and chemicals for sustaining life. A photosynthetic system often comprises antenna complexes (ACs) that absorb incident sunlight and deliver the molecular excited state energy to a central reaction center (RC) that converts the energy to charge. Numerous efforts have been made to mimic natural photosynthesis using organic materials. However, short-range energy transport due to material disorder has hindered their success. Here, we demonstrate a photodetector that mimics this natural architecture. It employs an AC that exploits exceptionally long-range ( ∼ 100 µ m ) Bloch surface wave polariton propagation that directs the excited states to an organic heterojunction detector serving as the RC. Exciton–polaritons are largely immune to localization due to the partial photonic character of the polariton, which consequently transports energy over extraordinary distances from its point of origin. Exciton–polaritons are edge-coupled into an RC that harvests excitation energy by dissociation into an electron and hole. This device relies on polariton-to-charge conversion combined with long-range energy transport for the direct detection of excited states in organic semiconductors. Our work paves the way for the realization of large-scale artificial photosynthetic systems comprising integrated organic photonic and optoelectronic devices with efficient energy transport and harvesting under ambient conditions.

High‐Performance Blue Electroluminescence Devices Based on Linear Gold(I) Complexes as Ultrafast Triplet Exciton Harvesters

AbstractAchieving large external quantum efficiencies, narrow bandwidths, and a long operational lifetime at high brightness remains the largest hurdle to developing organic blue‐emitting devices. Here, a material strategy is demonstrated that can meet these conditions. The strategy is based on linear heteroleptic Au(I) complex exciton harvesters and multiresonance thermally activated delayed fluorescence (MR‐TADF) emitters. The organic electroluminescence devices produce blue emission with Commission Internationale de l'Eclairage chromaticity coordinates of (0.108, 0.160), a narrow full‐width at half‐maximum value of 20 nm, and a maximum external quantum efficiency (EQE) as high as 30.2%. Notably, the EQE value remains 22.2% at 2000 cd m−2, whereas conventional control devices with an organic exciton harvester suffer from huge roll‐offs in quantum efficiency. An additional benefit of the device is a one‐order‐of‐magnitude improvement in its operational lifetime compared with that of the control device. Finally, the investigations reveal that the improvements are attributable to the unique ability of the Au(I) complexes for ultrafast triplet exciton harvest. In addition, the Au(I) complexes can facilitate Förster energy transfer to the MR‐TADF emitter, with effective suppression of hazardous triplet–triplet Dexter energy transfer. It is believed that the research is helpful in commercializing high‐efficiency and stable blue electroluminescence devices.

Understanding the Interaction between Inorganic and Organic Excitonic Components of an Inorganic‐Organic Nanohybrid Associate

AbstractThe present study deals with the design, formation, and photophysical studies on light‐harvesting inorganic‐organic nanohybrid associates comprising a less‐toxic, inorganic core‐shell QD (Cu−In−S/ZnS) and an organic S0041 dye aggregate. One of the main objectives of this study is to understand the interaction between the inorganic and organic excitonic components of the hybrid system and its implication on the photophysical response of the system. The nanohybrid associate have been obtained through the self‐assembly of S0041 dye molecules (organic exciton) onto Cu−In−S/ZnS QD (inorganic exciton). The characterization of as‐synthesized QD have been done by conventional microscopic and spectroscopic measurements. The exciton domain length for the organic dye aggregates is estimated to be 24 nm. Steady‐state absorption, emission, and time‐resolved photoluminescence (PL) intensity decay measurements have been carried out to understand the interaction mechanism between inorganic and organic excitonic components and the overall photophysical behaviour of the hybrid system. The studies have revealed that the formation of hybrid associate is electrostatically driven and thermodynamically feasible. Interestingly, steady‐state and time‐resolved PL decay measurements and their analysis have revealed that the interaction between inorganic QD and organic aggregate is mediated through dipole‐dipole Förster energy transfer (ET) mechanism and the ET efficiency is found to be as high as 95%. The present system is expected to be used in fabrication of less‐toxic nanoscale light‐harvesting system.

Dispersion of organic exciton polaritons—a novel undergraduate experiment

We report on an innovative and simple way to perform an experiment which utilizes the properties of a quantized electromagnetic field coupled to Frenkel excitons in organic materials, forming exciton–polaritons (EP). We present an optical setup, which allowed to perform precise measurements of transmission of organic optical cavities at different angles of incidence and to study the dispersion relation of EP. We provided the full theoretical background of EP and demonstrated that the experimental results are in good agreement with theoretical predictions. Furthermore, we extracted the strong coupling strength and the excitonic–photonic weights of organic cavity samples.

Realization of Polaritonic Topological Charge at Room Temperature Using Polariton Bound States in the Continuum from Perovskite Metasurface

AbstractExciton‐polaritons are mixed light–matter excitations resulting from the strong coupling regime between an active excitonic material and photonic resonances. Harnessing these hybrid excitations provides a rich playground to explore fascinating fundamental features, as out‐of‐equilibrium Bose–Einstein condensation and quantum fluids of light, plus novel mechanisms to be exploited in optoelectronic devices. The formation of exciton‐polaritons arising from the mixing between hybrid inorganic–organic perovskite excitons and an optical bound state in a continuum (BIC) of a subwavelength‐scale metasurface, are experimentally investigated at room temperature. These polaritonic eigenmodes, hereby called polariton BICs (pol‐BICs) are revealed in reflectivity, resonant scattering, and photoluminescence measurements. Although pol‐BICs only exhibit a finite quality factor bounded by the nonradiative losses of the excitonic component, they fully inherit BIC peculiar features: a full uncoupling from the radiative continuum in the vertical direction, which is associated to a locally vanishing farfield radiation in momentum space. Most importantly, the experimental results confirm that the topological nature of the photonic BIC is perfectly transferred to the pol‐BIC. This is evidenced by the observation of a polarization vortex in the farfield of polaritonic emission. The results pave the way to engineer BIC physics of interacting bosons and novel room temperature polaritonic devices.

Analysis of luminescence decay dynamics in metal-dielectric photonic structures with organic layers

Metal-dielectric photonic structures with organic materials 4,4-bis(N-carbazolyl)-1,1-biphenyl and 4,4'-bis[4-(di-ptolylamino)styryl]biphenyl) as light emitting layers were investigated. The formation of the polaritonic modes in the investigated structures was experimentally demonstrated and a correlation between theoretical and experimental dispersion was shown. Was shown, that increase in interaction between organic exciton and optical mode leads to a significant decrease in the lower polariton emission bandwidth. Keywords: light-emitting layers, exciton, luminescence.

Анализ динамики затухания люминесценции в металл-диэлектрических фотонных структурах с органическими слоями

Metal-dielectric photonic structures with organic materials 4,4-bis(N-carbazolyl)-1,1-biphenyl and 4,4′-bis[4-(di-ptolylamino)styryl] biphenyl) as light emitting layers were investigated. The formation of the polaritonic modes in the investigated structures was experimentally demonstrated and a correlation between theoretical and experimental dispersion was shown. Was shown, that increase in interaction between organic exciton and optical mode leads to a significant decrease in the lower polariton emission bandwidth.

Single-layer Thermally Activated Delayed Fluorescent Organic Light-emitting Devices and Exciton Distribution Profiles

Single-layer Thermally Activated Delayed Fluorescent Organic Light-emitting Devices and Exciton Distribution Profiles

Towards Understanding Excited-State Properties of Organic Molecules Using Time-Resolved Soft X-ray Absorption Spectroscopy.

The extension of the pump-probe approach known from UV/VIS spectroscopy to very short wavelengths together with advanced simulation techniques allows a detailed analysis of excited-state dynamics in organic molecules or biomolecular structures on a nanosecond to femtosecond time level. Optical pump soft X-ray probe spectroscopy is a relatively new approach to detect and characterize optically dark states in organic molecules, exciton dynamics or transient ligand-to-metal charge transfer states. In this paper, we describe two experimental setups for transient soft X-ray absorption spectroscopy based on an LPP emitting picosecond and sub-nanosecond soft X-ray pulses in the photon energy range between 50 and 1500 eV. We apply these setups for near-edge X-ray absorption fine structure (NEXAFS) investigations of thin films of a metal-free porphyrin, an aggregate forming carbocyanine and a nickel oxide molecule. NEXAFS investigations have been carried out at the carbon, nitrogen and oxygen K-edge as well as on the Ni L-edge. From time-resolved NEXAFS carbon, K-edge measurements of the metal-free porphyrin first insights into a long-lived trap state are gained. Our findings are discussed and compared with density functional theory calculations.

Untargeted effects in organic exciton-polariton transient spectroscopy: A cautionary tale.

Strong light-matter coupling to form exciton- and vibropolaritons is increasingly touted as a powerful tool to alter the fundamental properties of organic materials. It is proposed that these states and their facile tunability can be used to rewrite molecular potential energy landscapes and redirect photophysical pathways, with applications from catalysis to electronic devices. Crucial to their photophysical properties is the exchange of energy between coherent, bright polaritons and incoherent dark states. One of the most potent tools to explore this interplay is transient absorption/reflectance spectroscopy. Previous studies have revealed unexpectedly long lifetimes of the coherent polariton states, for which there is no theoretical explanation. Applying these transient methods to a series of strong-coupled organic microcavities, we recover similar long-lived spectral effects. Based on transfer-matrix modeling of the transient experiment, we find that virtually the entire photoresponse results from photoexcitation effects other than the generation of polariton states. Our results suggest that the complex optical properties of polaritonic systems make them especially prone to misleading optical signatures and that more challenging high-time-resolution measurements on high-quality microcavities are necessary to uniquely distinguish the coherent polariton dynamics.

Understanding radiative transitions and relaxation pathways in plexcitons

Summary Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of polaritonic light-matter states, called plexcitons. Such systems are tunable, scalable, easy to synthesize, and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime, promising a plethora of applications. However, the complexity of these materials prevented the understanding of their excitation and relaxation phenomena. Here, we follow the relaxation pathways in plexcitons and conclude that while the metal destroys the optical coherence, the molecular aggregate coupled to surface processes significantly contributes to the energy dissipation. We use two-dimensional electronic spectroscopy with theoretical modeling to assign the different relaxation processes to either molecules or metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be considered in the language of hot electron distributions instead of the accepted lower and upper polariton branches and establish the framework for further understanding.