Exciton Annihilation Research Articles

Exciton lifetimes play a critical role in the performance of organic optoelectronic devices. In this work, we investigate how the presence of multiple rotational domains and therefore grain boundaries impacts exciton dynamics in thin films of C60/Au(111) using time- and angle-resolved photoemission spectroscopy (TR-ARPES). We find that films with multiple rotational domains exhibit shorter exciton lifetimes and higher susceptibility to exciton-exciton annihilation, even when one domain dominates. Scanning tunneling microscopy (STM) measurements reveal electronic structure changes resulting from a locally reduced dielectric constant at grain boundaries, suggesting a mechanism for lifetime reduction through exciton funneling and other additional decay channels. These findings highlight the critical role of film quality in determining intrinsic exciton lifetimes and show that minuscule amounts of disorder that are nearly undetectable by ensemble measurements can significantly impact dynamics. These results imply that precise structural control is essential for optimizing the performance of organic optoelectronic devices.

Achieving high-efficiency white organic light-emitting diodes (WOLEDs) based on simplified architecture with low efficiency roll-off at high luminance remains a challenge. Herein, we manufacture high-efficiency phosphorescent WOLEDs with extremely low roll-off by strategically combining an ultrathin emitting layer (UEML) with a novel deep-blue exciplex host. This architecture significantly suppresses exciton annihilation owing to the expansion of the exciton diffusion region. Meanwhile, the bipolar transport properties of the exciplex host and the UEML design help reduce triplet exciton accumulation by expanding the recombination zone and facilitating free exciton diffusion. The resulting WOLED exhibits the maximum current efficiency (CE) and power efficiency (PE) of 59.5 cd/A and 61.1 lm/W with the Commission Internationale de L’Eclairage (CIE) coordinates variation of (0.002, 0.002) over a wide luminance range. Notably, the CE still retains 57.6 cd/A at 1000 cd/m2, corresponding to a roll-off of only 3.2%, which is one of the best performances in phosphorescent WOLEDs based on ultrathin-layer architecture. These results provide an effective approach to construct high-performance WOLEDs.

Bandgap engineering in lead halide perovskites through the lead-site doping is a promising strategy to achieve blue-shifted emission in nanocrystals (NCs) without relying on quantum confinement or halide mixing. Here, the structure and photophysical properties of CsPb1-xCdxBr3 NCs with a varied amount (3, 8, and 15%) of Cd(II) doping are explored. The incorporation of the increasing amount of Cd2+ ions results in an up to 5nm decrease of the average NC size, while the emission is blue-shifted from 515 to 485nm. Applying the ultrafast transient absorption spectroscopy, a significant enhancement is observed in the absorption oscillator strength of CsPb1-xCdxBr3 NCs along with an almost threefold increase in the hot carrier temperature, which indicates more efficient population of the band edge compared to pristine CsPbBr3. Furthermore, it is demonstrated that CsPb1-xCdxBr3 NCs exhibit their own volume scaling law for the exciton-exciton annihilation threshold and rate. Specifically, Cd(II)-doped CsPbBr3 NCs with a smaller size exhibit a higher Auger threshold than the larger pristine CsPbBr3 NCs, which makes them potentially useful for light-emitting and lasing applications. The insights gained into the excited carrier dynamics in CsPb1-xCdxBr3 NCs open new pathways for the development of efficient nanoscale emitters in the blue spectral range.

Non-photochemicalquenching (NPQ) protects plants fromexcess lightby dissipating excitation energy as heat. Limited exciton migrationis a key feature of NPQ, reducing the energy flux to reaction centers;however, quantitative evidence for this mechanism in native thylakoidmembranes has been lacking. Here, we investigate the correlation betweenNPQ activity and exciton diffusion length (LD) using Nicotiana benthamiana mutants withdistinct NPQ capacities. NPQ under both light- and dark-acclimatedconditions was quantified for each genotype via fluorescence lifetimesnapshots, and exciton mobility was probed using transient absorptionspectroscopy with exciton–exciton annihilation analysis. Bycomparing the relationship between chlorophyll fluorescence lifetimeand LD across mutants, we observed thatNPQ activation quantitatively limits the spatial range of excitonmigration, thereby reducing the access to reaction centers. Our findingsprovide direct experimental evidence that NPQ modulates the dynamicsof energy transport, advancing our understanding of photoprotectiveregulation in photosynthetic systems.

Cubic GaN (c-GaN) has attracted increasing interest because has several intrinsic advantages over its wurtzite-GaN polymorph. Namely, no polarization in the <100> growth direction, smaller bandgap, lighter electron and heavy hole effective masses, larger optical gain, shorter radiative recombination lifetime, smaller p-doping activation energy (Mg acceptor), higher hole mobility, and larger conduction band offset. Consequently, c-GaN could enable the next-generation of high efficiency visible light-emitting diodes, normally-off AlGaN/GaN high speed power transistors, and resonant tunnel diodes. However, the synthesis of c-GaN has been a great challenge due to its metastability. To date, c- GaN films have been deposited on cubic substrates such as GaAs, c-SiC, Si, and more recently on patterned Si(100) wafers [1-5]. This work demonstrates that c-GaN can be deposited on diamond substrates, which could considerably improve thermal management, enabling its potential application for high-speed and high-power devices.c-GaN epitaxial film was grown directly on a boron nitride nucleation layer, previously deposited on <100> type IIA CVD-diamond, by plasma-assisted molecular-beam epitaxy with no additional surface preparation and under conditions (e.g. substrate temperature, nitrogen and gallium fluxes) that result in wurtzite-GaN when grown on hexagonal substrates [6]. The thickness of the GaN top layer is estimated to be ~120 nm. XRD measurements verified that <100> zincblende is the dominant crystalline phase, with a small contribution from the wurtzite phase at some sample regions. However, no wurtzite-GaN inclusion were observed in the film region dominated by crystal with cubic symmetry, as highlighted in the AFM imaging. Polarized Raman scattering measurements of these regions confirmed the XRD findings. Low temperature (6K) photoluminescence spectra show a dominant line near 3.12 eV and a weaker line near 3.21 eV. The former is assigned to a recombination process involving electrons bound to shallow donors with holes bound to shallow acceptors (DAP recombination). The latter, which becomes dominant at temperatures above 50K, is assigned to recombination processes involving the annihilation of excitons bound to shallow impurities. Room temperature transmission measurements yielded a direct near bandgap of 3.21 eV, which is close to previously reported values. These results verified that c-GaN can be deposited on high thermal conductivity diamond substrates [7]. If the time allows higher lateral resolution measurements will be presented. Our results provide a starting point of a new approach for realizing the potential of c-GaN devices for high-power applications, considering that large area diamond substrates are becoming commercially available. J.N. Kuznia, et al, Appl. Phys. Lett. 65 (1994) 2407 D.J. As, et al., Appl. Phys. Lett. 70 (1997) 1311 J. Wu, et al., Jpn. J. Appl. Phys. 37 (1998) 1440 M. Feneberg, et al., Phys. Rev. B 85 (2012) 155207 R. Liu, et al., ACS Photonics, 5 (2018) 9551 D.F. Storm, et al., Phys. Status Solid RRL 2022, 220003 J.A. Freitas, et al., accepted for publication in JAP This work supported by the Office of Naval Research

Abstract Solution‐processed organic light‐emitting diodes (SOLEDs) are promising candidates for cost‐efficient and scalable displays, if their operational stability can be increased to match that of thermal‐evaporation‐processed OLEDs. This study provides a comprehensive analysis of the fundamental degradation mechanisms in SOLEDs. This work reveals a self‐reinforcing degradation cycle in which charge accumulation in molecular aggregates and hetero‐interfaces strengthens exciton‐polaron interactions, thereby promoting further molecular aggregation and nonradiative exciton annihilation. This work applies material engineering strategies, including the use of host‐guest combination with minimal intermolecular interactions, modification of the hole injection interface, and suppression of self‐reinforcing degradation pathway to obtain SOLEDs that have luminous efficiency exceeding 100 cd A−1 and a half lifetime of over 700 h. These findings establish a strategic approach to increase SOLED efficiency and longevity, while offering insights into the design principles required for next‐generation organic optoelectronic devices.

The conversion of gamma particles into optical photons in state-of-the-art scintillator materials is limited to maximum 10 emitted photons per MeV of energy deposited per picosecond, when the material is excited at room-temperature. Breaking this limit has both fundamental and applied importance, and motivates the search for fast and efficient optical emitters at excitation densities relevant for particle detection, up to 1020 electron–hole pairs (eh) per cm3. In this work, we address this challenge by probing the optical response of a promising nanomaterial, CdSe/CdS core/crown nanoplatelets (NPLs), in the shape of drop cast films using intense femtosecond laser pulses. The study finds that the NPL films exhibit a bright optical response at low to medium excitation densities but suffer from high levels of nonlinear quenching, dominated by exciton-exciton annihilation (EEA), at densities exceeding 1017 eh/cm3. The experimental data and theoretical calculations suggest that EEA is enhanced in the drop cast film by the close packing of NPLs which allows excitons to migrate between NPLs in the film. Despite this, light yield estimations based on a simulated distribution of excitation densities predict values upwards of 2000 ph/MeV, while showing ample room for improvement and the future potential of surpassing the 10 ph/MeV/ps benchmark.

Understanding charge separation dynamics in organic semiconductor blends is crucial for optimizing the performance of organic photovoltaic solar cells. In this study, the optoelectronic properties and charge separation dynamics of a PCE10:FOIC blend, by combining steady-state and time-resolved spectroscopies with high-level DFT calculations. Femtosecond transient absorption spectroscopy revealed a significant reduction of the exciton-exciton annihilation recombination rate in the acceptor when incorporated into the blend, compared to its pristine form. This reduction is attributed to a decrease in exciton density within the acceptor, driven by an efficient hole-separation process that is characterized by following the temporal evolution of the transient signals associated with the excited states of the donor when the acceptor is selectively excited within the blend. The analysis of these dynamics enabled the estimation of the hole separation time constant from the acceptor to the donor, yielding a time constant of (1.3±0.3)ps. Additionally, this study allowed the quantification of exciton diffusion and revealed a charge separation efficiency of ≈60%, providing valuable insights for the design of next-generation organic photovoltaic materials with enhanced charge separation and improved device efficiency.

Excitons in monolayer transition metal dichalcogenides (TMDCs) are highly sensitive to their environment, allowing for screening to modulate exciton radiative recombination and photoluminescence (PL) intensity. While previous studies have explored modifying the dielectric environment and introducing metals separated from TMDCs by a dielectric spacer to influence screening effects and PL, the case where a metal is in contact with the TMDC monolayer by a van der Waals gap—has not been demonstrated. Here, we demonstrate that a metal layer in contact with the TMDC monolayer through a van der Waals gap achieves significant PL enhancement. By stacking metals with appropriate workfunctions above or below a TMDC monolayer, we achieved up to two-orders-of-magnitude increase in PL intensity. This enhancement is due to reduced exciton-exciton interactions from strong screening by the metal layer. Our measurements show reduced exciton-exciton annihilation, even at high generation rates, facilitated by the metal’s strong screening effect.

We directly characterize nanoscale spatiotemporal inhomogeneities of multi-layered molybdenum diselenide (MoSe2) in real space and time – the nanometre–femtosecond scale, attributing to local mechanical structures such as strain and surface/subsurface defects, which are critical in semiconductor and optoelectronic applications. This remarkable precision is achieved through the development of a hyper-temporal transient nanoscopy incorporating a sideband-coupled generalized lock-in amplification technique, allowing for characterization of local spatiotemporal defects at each pixel within a subwavelength mapping region. By utilizing this technique, we characterize the nanoscale strain-induced spatiotemporal defects of multi-layered MoSe2, including nano-bubbles that exhibit a noticeable reduction in exciton-exciton annihilation rates, which may attribute to the suppressed probability of bimolecular interaction of excitons due to the strain-induced band distortion. Moreover, we visualize topographically hidden spatiotemporal defects such as lattice mismatches, which induce mid-gap states that traps charge carriers and thereby slow down recombination process. We propose that this hyper-temporal approach to resolving intricate spatiotemporal inhomogeneities in van der Waals materials provides significant insights into their optoelectronic properties and opens new avenues for innovative material design and characterization.

The stability of efficient, deep blue organic light‐emitting diodes (OLEDs) remains a major challenge in the field of organic electronics. Poor device stability originates from the high probability for destructive non‐radiative triplet exciton annihilation events. Phosphor‐sensitized fluorescence (PSF) is proposed to achieve an efficient, deep blue color by energy transfer from phosphors to fluorophores. Recently, the polariton‐enhanced Purcell (PEP) effect is introduced to decrease the triplet radiative lifetime and density, resulting in an increase in blue phosphorescent OLED lifetime. Here, the PEP effect is introduced to enhance the stability of the PSF‐OLEDs. It is shown that the PEP effect increases all radiative decay rates of the phosphors and fluorophores, leading to a reduction in the triplet annihilation events. Using a Pt‐complex phosphor sensitizer and a so‐called multi‐resonance fluorescent emitter in a PEP cavity, a 3.1‐fold lifetime increase is observed at a current density of J = 10 mA cm−2, reduced EQE roll‐off and deep blue color with Commission Internationale de l'Eclairage coordinates of (0.13, 0.09). The PEP effect maximally extends PSF‐OLED lifetimes in devices with the highest triplet‐to‐singlet energy transfer rates. Moreover, this work suggests that the benefits of PEPs apply to all triplet‐based OLEDs.

Excitation energy transfer between photosynthetic light-harvesting complexes is vital for highly efficient primary photosynthesis. Controlling this process is the key for advancing the emerging artificial photosynthetic systems. Here, we experimentally demonstrate the enhanced excitation energy transfer between photosynthetic light-harvesting 2 complexes (LH2) mediated through the Fabry-Pérot optical microcavity. Using intensity-dependent pump-probe spectroscopy, we analyse the exciton-exciton annihilation (EEA) due to inter-LH2 energy transfer. Comparing EEA in LH2 within cavity samples and the bare LH2 films, we observe enhanced EEA in cavities indicating improved excitation energy transfer via coupling to a common cavity mode. Surprisingly, the effect remains even in the weak coupling regime. The enhancement is attributed to the additional connectivity between LH2s introduced by the resonant optical microcavity. Our results suggest that optical microcavities can be a strategic tool for modifying excitation energy transfer between molecular complexes, offering a promising approach towards efficient artificial light harvesting.

Exciton Annihilation by Lanthanide Dopants: An Atomic Probe of Sub-Diffraction Exciton Diffusion in Ferromagnetic CrI₃

The stability of efficient, deep blue organic light‐emitting diodes (OLEDs) remains a major challenge in organic electronics and industry. The poor device stability originates from the high probability of non‐radiative triplet exciton annihilation. Phosphor‐sensitized fluorescence (PSF) has been proposed to reduce triplet exciton density and enable deep blue emission by efficient energy transfer to fluorophores. Recently, the polariton‐enhanced Purcell (PEP) effect has been introduced to decrease the triplet radiative lifetime, thereby reducing reduce triplet density resulting an increase in blue phosphorescent OLED lifetime. Here, we introduce the PEP effect to enhance the stability of the PSF‐OLEDs. Applying a full PEP cavity to the device using Pt‐complex phosphor and a so‐called multi‐resonance (MR) fluorescent emitter, we observe a 3.1‐fold lifetime increase at a current density of J = 10 mA/cm 2 . reduced EQE roll‐off and a deep blue color with Commission Internationale de l'Eclairage coordinates of (0.13, 0.09).

The research on multiresonance thermally activated delayed fluorescence (MR-TADF) emitters has garnered increasing attention due to the exceptional photophysical properties of their corresponding organic light-emitting diodes (OLEDs), such as high efficiency and narrow emission features. However, they still face intractable issues like concentration-induced emission quenching, exciton annihilation, and spectral broadening. This review focuses on a sophisticated molecular design strategy named "sterically wrapping of MR fluorophores" to tackle the aforementioned problems. Bulky substituents isolate the MR emission core, thereby significantly reducing intermolecular interactions. Therefore, with these MR-TADF emitters, the OLEDs are capable of maintaining narrow emission bands while achieving high external quantum efficiencies across a wide concentration range from 1 to 20 wt% and even at higher concentrations. This article reviews the latest advancements in MR-TADF emitters with suppressed concentration quenching and spectral broadening, emphasizing their chemical structures, optoelectronic properties, and device performances. Finally, the potential challenges and future perspectives of MR-TADF materials are analyzed to better comprehend the potential of efficient narrowband OLEDs.

Photosynthesis converts solar energy into chemical energy through coordinated energy transfer between light-harvesting complexes and reaction centers (RCs). Understanding exciton motion, particularly the exciton diffusion length, is essential for optimizing energy efficiency in photosystems. In this work, we combine intensity-cycling transient absorption spectroscopy with kinetic Monte Carlo (kMC) simulation to investigate exciton motion in the C2S2 photosystem II supercomplex of spinach. Using exciton-exciton annihilation, revealed in the fifth-order response, we experimentally estimate an exciton diffusion length of 10.9nm based on a 3D normal diffusion model, suggesting the ability of excitons to traverse the supercomplex. However, kMC simulations reveal that exciton motion is sub-diffusive because of spatial constraints and the strong RC traps. An anomalous diffusion model analysis of the experimental data yields a diffusion length of 9.7nm, while the simulated diffusion length is 7.4nm. The variable exciton residence time across subunits, partly influenced by their connectivity to the trap, indicates inhomogeneous annihilation probability and suggests how plants balance efficient light harvesting with photoprotection. We also explore the influence of specific assumptions in the annihilation simulation, which are challenging to access in more complex environments, such as the thylakoid membrane. Our study provides a framework for studying exciton dynamics using exciton-exciton annihilation, which can be extended to understand the light-harvesting efficiencies of larger, more complex photosynthetic assemblies.

Enhanced photoluminescence (PL) in transition metal dichalcogenides (TMDs) is critical for their application in optoelectronics. In this study, we report a 99 × PL enhancement in a superacid bis(trifluoromethane) sulfonimide (TFSI)-treated MoS2(1-x)Se2x alloy. The alloy's optical bandgap is tunable by changing its stoichiometry, allowing for PL enhancement at tailored positions. The PL enhancement is robust even at high excitation power, overcoming the limitation of exciton-exciton annihilation observed in MoS2. Through molecular dynamics simulations and spectroscopic analysis, we demonstrate that the MoS2(1-x)Se2x monolayer inherently exhibits moderate strain, which shifts the van Hove singularity in the S-rich domain. Furthermore, density functional theory calculations reveal the absence of a pronounced van Hove feature in the alloy configuration. Our findings extend the range of materials amenable to superacid treatment and open new avenues for optoelectronic applications, particularly those requiring high excitation powers.

A combination of ultrafast, long-range, and low-loss excitation energy transfer from the photoreceptor location to a functionally active site is essential for cost-effective polymeric semiconductors. Delocalized electronic wavefunctions along π-conjugated polymer (CP) backbone can enable efficient intrachain transport, while interchain transport is generally thought slow and lossy due to weak chain-chain interactions. In contrast to the conventional strategy of mitigating structural disorder, amorphous layers of rigid CPs, exemplified by highly planar poly(indacenodithiophene-co-benzothiadiazole) (IDT-BT) donor-accepter copolymer, exhibit trap-free transistor performance and charge-carrier mobilities similar to amorphous silicon. Here, we report long-range exciton transport in HJ-aggregated IDTBT thin-film, in which the competing exciton transport and exciton-exciton annihilation (EEA) dynamics are spectroscopically separated using a phase-cycling-based scheme and shown to depart from the classical diffusion-limited and strong-coupling regime. In the thin film, we find an annihilation-limited mechanism with ≪100% per-encounter annihilation probability, facilitating the minimization of EEA-induced excitation losses. In contrast, excitons on isolated IDTBT chains diffuse over 350 nm with 0.56 cm2 s-1 diffusivity, before eventually annihilating with unit probability on first contact. We complement the pump-probe studies with temperature-dependent photocurrent and EEA measurements from 295 K to 77 K and find a remarkable correspondence of annihilation rate and photocurrent activation energies in the 140 K to 295 K temperature range.

Drift-diffusion modeling of blue OLED degradation

Multidimensional optical spectroscopy observes transient excitation dynamics through time evolution of spectral correlations. Its action-detected variants offer several advantages over the coherent detection and are thus becoming increasingly widespread. Nevertheless, a drawback of action-detected spectra is the presence of a stationary background of the so-called incoherent mixing of excitations from independent states that resembles a product of ground-state absorption spectra and obscures the excited-state signal. This issue is especially problematic in fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) and fluorescence-detected pump-probe spectroscopy (F-PP) of extended systems, where large incoherent mixing arises from efficient exciton-exciton annihilation. In this work, we demonstrate on the example of F-2DES and F-PP an inherent spectro-temporal symmetry of action-detected spectra, which allows general, system-independent subtraction of any stationary signals including incoherent mixing. We derive the expressions for spectra with normal and reversed time ordering of the pulses, relating these to the symmetry of the system response. As we show both analytically and numerically, the difference signal constructed from spectra with normal and reversed pulse ordering is free of incoherent mixing and highlights the excited-state dynamics. We further verify the approach on the experimental F-PP spectra of a molecular squaraine heterodimer and the F-2DES spectra of the photosynthetic antenna light-harvesting complex 2 of purple bacteria. The approach is generally applicable to action-detected 2DES and pump-probe spectroscopy without experimental modifications and is independent of the studied system, enabling their application to large systems such as molecular complexes.