Molecular Exciton Research Articles

The Kuramoto–Lohe model and collective absorption of a photon

Light absorption by molecular exciton states in disordered networks is studied. The main purpose of this paper is to look at how phases of the intermediate ground–excited state superposition interfere during the absorption process. How does this phase average enable, or suppress, absorption to a delocalized state? To address this question, a theory for phase oscillators is used to predict the purity of the collective excited state of the network. The results of the study suggest that collective absorption by molecular exciton states requires a sufficiently large electronic coupling between molecules in the network compared to the random distribution of transition energies at the sites, even when the molecular network is completely isolated from the environment degrees of freedom. The ‘dividing line’ between absorption to a mixture of, essentially, localized excited states and coherent excitation of a pure delocalized exciton state is suggested to be predicted by the threshold of phase synchronization.

Nonconventional 1,8-Diazafluoren-9-One Aggregates for Green Light Enhancement in Hybrid Biocompatible Media.

Organic aggregates currently play a prominent role, mainly for their unique optoelectronic properties in the aggregated state. Such properties can be related to the aggregates’ structure and the molecular packing mode. In the literature, we have well-established models of H and J aggregates defined based on the molecular exciton model. However, unconventional aggregates, the most unrecognized forms, have been generating interest among researchers recently. Within unconventional aggregation, aggregation-induced emission systems (AIE) are considered. In the present work, we discuss the effect of the forming of unconventional aggregation together with the change in dye concentration on the surface energy characteristics of the materials. All materials were prepared as hybrid biocompatible thin films where the matrix is TiO2 or TiO2/carbon nanowalls (CNWs) with the incorporated dye in the form of 1,8-diazafluoren-9-one (DFO). Using the time-resolved emission spectra and the determination of surface parameters from contact angle measurements, we indicated the correlation between the changes in such parameters and the concentration of DFO dye in two types of TiO2 and TiO2/CNW structures. To examine the propensity of DFO for aggregation, the internal energy of the dye was assessed in several aggregate structures using Quantum chemistry calculations. The results emphasize that DFO is an attractive structure in the design of new fluorophores due to its low molecular weight, the presence of a nitrogen atom that provides good coordination properties, and the ability to form hydrogen bonds. Our studies show that when using suitable matrices, i.e., rigid media, it forms the preferred forms of aggregates in the excited state, characterized by high emission efficiency in the band maximum of around 550 nm.

(Digital Presentation) Angle-Resolved Photoluminescence Measurements of Rubrene Single Crystals to Study the Exciton Energy Dispersion

Angle-resolved spectroscopic measurements provide information on the electronic structure of single crystals in reciprocal space. So far the single crystal of rubrene (RubSC), a representative of p-type organic semiconductor, has been investigated by using photoelectron spectroscopies and the highest occupied molecular orbital band energy dispersion of ~ 0.4 eV was characterized [1]. The degree of the dispersion was depended on the crystal axes [2]. The formation of energy band dispersion means that the hole carrier can be delocalized in certain crystal axes. An anisotropic hole carrier mobility is anticipated for RubSC and is indeed demonstrated in organic filed-effect transistors [3]. Considering the anisotropy of electronic structure and hole delocalization, it is questioned whether excitons can be also delocalized in RubSC. Delocalization of molecular excitons is relevant for organic devices, such as organic light-emitting diodes and organic photovoltaic cells, and is of fundamental interest. To investigate the possibility of such an exciton delocalization, we investigated angle-resolved photoluminescence (AR-PL) measurement RubSC using a newly built AR-PL apparatus.RubSC samples were prepared by using a physical vapor transport technique in a purified nitrogen stream, where “sublimed grade” (99.99% purity) source materials purchased from Sigma-Aldrich were used as received. AR-PL spectra were measured by a home-made apparatus consisting of a picosecond pulsed laser light source and some optical elements bought by Thorlabs, Inc. The excitation wavelength was 532 nm (the second harmonics of Nd:YAG laser; Ekspla, PL-2211) and the laser power was 4.9 mW with a (Gaussian) beam diameter of ~ 2 mm and a repetition rate of 1 kHz. The p-polarized light was prepared by using a half-wave plate and a beam splitter. The p-polarized light was focused on the rubrene single crystals using a convex lens (f=600 mm) at an incident angle of 34 degree (°) relative to the surface [(100) plane] normal of the RubSC. The emitted PL spectra were recorded by using a photonic multichannel spectrometer (PMA-12, Hamamatsu) connecting an optical bundled fiber (light-receiving area Φ 1mm), which was placed at a PL emission angle of 70° with a distance of 5.9 cm between RubSC and the fiber edge. To measure AR-PL spectra, the fiber mount was rotated step-by-step at a rotation center, which is a surface of rubrene single crystal. The resolution of the detected angle was estimated to be ±1degree. The alignment of the apparatus, namely, setting the in-plane and out-of-plane rotation center of the system, was done by using a line laser maker and digital cameras, which were used to check the light irradiation position on the crystals. All the measurements were done at 295 K in air.Figure 1(a) shows the results of AR-PL spectra of RubSC measured at an azimuthal angle (α, see in Fig.1(a)) of 200 °and 290°. For two spectra, the strongest peak was found at around 655 nm and the second strongest one is at around 705 nm. These peaks are consistent with those reported ones of RubSc; the largest one is the main PL band, and the second one is its vibronic progression. Note that photo-oxidization of the RubSC can possibly occur at the surface of the SC and, if in that case, its PL band would be seen at around 643 nm. However, our spectra do not show such a band and there was almost no irradiation time dependence, indicating the detected PL originates mostly from the bulk region of the RubSC. At the different α conditions, the main band shifted apparently, while the second one stayed unchanged. To more clarify the angle dependency, the second derivatives of all the data set of the AR-PL spectra were calculated and the results are shown in Fig. 1(b) as an intensity color map. It is clear that the first band shifts zigzag by every 90±10 °, while the second largest one is unchanged. The crystal structure of our RubSC grown by our way is orthorhombic and the angle between a and b axes of the RubSC is 90°. The periodicity of the turning point of the first band, i.e. 90°, is coincidently matched with the ab angle of the RubSC. This implies that the zigzag shift of the first PL band is related to the crystal structure of RubSC, whispering the detection of an energy dispersion of excitons of RubSC.Ref.[1] S. Machida, et al., Phys. Rev. Lett. 104, 156401 (2010). [2] Y. Nakayama, et al., Appl. Phys. Express 5,111601 (2012). [3] C. Reese and Z. Bao, Adv. Mater. 19, 4535 (2007). Figure 1

Polaron lifetime enhancement of P3HT thin film under femtosecond laser irradiation and its ultrafast mechanism

The morphology and properties of poly (3-hexylthiophene) (P3HT) thin films after femtosecond laser irradiation with various fluences were studied. Two types of final ablation morphology were characterized through optical microscope (OM), atomic force microscope (AFM) and scanning electron microscope (SEM). At lower fluences, nano-bumps were formed in the central area. At higher fluences, nano-bumps were formed in the entire irradiated area and the material in the center disintegrated. Time-resolved pump–probe system was used to investigate laser − P3HT ultrafast interaction and the principle of morphological differences, which revealed that the Coulomb explosion was the reason for the morphology change with fluence increasing. Photoluminescence (PL) spectra indicated that the transition of the PL signal from enhancement to quenching with various laser fluences was related to the effect of laser on morphology and molecular exciton. A maximal 2 fold lifetime enhancement of bound polaron pair dissociation and 1.3 fold lifetime improvement of defect trapping were achieved at higher fluences, which contributed to obtaining more effective free carriers and subsequent photocurrent. These results promote the potential applications of femtosecond laser on P3HT and other organic semiconductors, especially organic photovoltaics (OPV) devices.

Tuning between Quenching and Energy Transfer in DNA-Templated Heterodimer Aggregates.

Molecular excitons, which propagate spatially via electronic energy transfer, are central to numerous applications including light harvesting, organic optoelectronics, and nanoscale computing; they may also benefit applications such as photothermal therapy and photoacoustic imaging through the local generation of heat via rapid excited-state quenching. Here we show how to tune between energy transfer and quenching for heterodimers of the same pair of cyanine dyes by altering their spatial configuration on a DNA template. We assemble “transverse” and “adjacent” heterodimers of Cy5 and Cy5.5 using DNA Holliday junctions. We find that the transverse heterodimers exhibit optical properties consistent with excitonically interacting dyes and fluorescence quenching, while the adjacent heterodimers exhibit optical properties consistent with nonexcitonically interacting dyes and disproportionately large Cy5.5 emission, suggestive of energy transfer between dyes. We use transient absorption spectroscopy to show that quenching in the transverse heterodimer occurs via rapid nonradiative decay to the ground state (∼31 ps) and that in the adjacent heterodimer rapid energy transfer from Cy5 to Cy5.5 (∼420 fs) is followed by Cy5.5 excited-state relaxation (∼700 ps). Accessing such drastically different photophysics, which may be tuned on demand for different target applications, highlights the utility of DNA as a template for dye aggregation.

Searching for a Unique Exciton Model of Photosynthetic Pigment–Protein Complexes: Photosystem II Reaction Center Study by Differential Evolution

Studying the optical properties of photosynthetic pigment–protein complexes (PPCs) in the visible light range, both experimentally and theoretically, is one of the ways of gaining knowledge about the function of the photosynthetic machinery of living species. To simulate the PPC optical response, it is necessary to use semiclassical theories describing the effect of external fields–matter interaction, energy migration in molecular crystals, and electron–phonon coupling. In this paper, we report the results of photosystem II reaction center (PSIIRC) linear optical response simulations. Applying the multimode Brownian oscillator model and the theory of molecular excitons, we have demonstrated that the absorption, circular and linear dichroism, and steady-state fluorescence of PSIIRC can be accurately fitted with the help of differential evolution (DE), the multiparametric evolutionary optimization algorithm. To explore the effectiveness of DE, we used the simulated experimental data as the target functions instead of those actually measured. Only 2 of 10 DE strategies have shown the best performance of the optimization algorithm. With the best tuning parameters of DE/rand-to-best/1/exp strategy determined from the strategy tests, we found the exact solution for the PSIIRC exciton model and fitted the spectra with a reasonable convergence rate.

Liquid-crystalline ordering of davydov-split aggregates of cyanine dyes

3,3′-ditetradecyloxacarbocyanine (DiOC14(3)) and 3,3′-Dioctadecyloxacarbocyanine (DiOC18(3)) are lipophilic carbocyanine dyes with a conjugated polymethine backbone and two long hydrocarbon tails. In this paper, we report the formation of Davydov-split aggregates of DiOC14(3) and DiOC18(3) in methanol/water mixtures with and without ammonia, which show a blue-shifted H-band and a red-shifted J-band with respect to the monomer band in their UV–vis absorption spectra. The Davydov-split aggregates of DiOC14(3) show a fiber-like morphology in the absence of ammonia and a vesicle-like morphology in the presence of ammonia. While the Davydov-split aggregates of DiOC18(3) show a vesicle-like morphology with the different lamellarities, depending on the presence of ammonia in the methanol/water mixtures. These Davydov-split aggregates with different morphologies are birefringence when being viewed through crossed polarizers, suggesting that they have liquid-crystalline phases. The Davydov-split aggregate fibers and vesicles with liquid-crystalline phases and unique excitations are great potential for photonic applications because the liquid-crystalline order and molecular exciton are synergized.

Influence of Hydrophobicity on Excitonic Coupling in DNA-Templated Indolenine Squaraine Dye Aggregates.

Control over the strength of excitonic coupling in molecular dye aggregates is a substantial factor for the development of technologies such as light harvesting, optoelectronics, and quantum computing. According to the molecular exciton model, the strength of excitonic coupling is inversely proportional to the distance between dyes. Covalent DNA templating was proved to be a versatile tool to control dye spacing on a subnanometer scale. To further expand our ability to control photophysical properties of excitons, here, we investigated the influence of dye hydrophobicity on the strength of excitonic coupling in squaraine aggregates covalently templated by DNA Holliday Junction (DNA HJ). Indolenine squaraines were chosen for their excellent spectral properties, stability, and diversity of chemical modifications. Six squaraines of varying hydrophobicity from highly hydrophobic to highly hydrophilic were assembled in two dimer configurations and a tetramer. In general, the examined squaraines demonstrated a propensity toward face-to-face aggregation behavior observed via steady-state absorption, fluorescence, and circular dichroism spectroscopies. Modeling based on the Kühn–Renger–May approach quantified the strength of excitonic coupling in the squaraine aggregates. The strength of excitonic coupling strongly correlated with squaraine hydrophobic region. Dimer aggregates of dichloroindolenine squaraine were found to exhibit the strongest coupling strength of 132 meV (1065 cm–1). In addition, we identified the sites for dye attachment in the DNA HJ that promote the closest spacing between the dyes in their dimers. The extracted aggregate geometries, and the role of electrostatic and steric effects in squaraine aggregation are also discussed. Taken together, these findings provide a deeper insight into how dye structures influence excitonic coupling in dye aggregates covalently templated via DNA, and guidance in design rules for exciton-based materials and devices.

Self‐assembly into Polymorphic 2D Nanosheets with Crystallization‐Induced Emission Enhancement

Comprehensive SummaryOrganic micro/nanocrystals with polymorphic tailorability and perfect platform to modulate decode the complex relationship among molecular structures, supramolecular/noncovalent interactions, aggregates and exciton behaviors. Herein, we precisely tune the polymorphic two‐dimensional organic nanosheets (2DONs) of cyano‐spirocyclic aromatic hydrocarbon (2'‐CN‐SFX), where the crystal growth processes are dominated by precursor concentration. The low concentration (ca. 2 mmol/L) affords the rhombus‐like nanosheets with monoclinic P21/c space group, in which molecules adopt antiparallel and slipped stacking mode. In contrast, the high concentration (ca. 4 mmol/L) favors herringbone‐like molecular packing pattern and then generates shuttle‐like nanosheets with monoclinic C2/c space group. Both above polymorphic nanosheets exhibit unprecedented crystallization‐induced emission enhancement (CIEE) feature that increases the photoluminescence quantum yields (PLQYs) from 16% in solution, 21% in amorphous film to 37%—39% in crystalline states. This result will offer promising opportunities for nanophotonics and organic intelligent semiconductors.

Mapping Lamb, Stark, and Purcell Effects at a Chromophore-Picocavity Junction with Hyper-Resolved Fluorescence Microscopy

The interactions of the excited states of a single chromophore with static and dynamic electric fields spatially varying at the atomic scale are investigated in a joint experimental and theoretical effort. In this configuration, the spatial extension of the fields confined at the apex of a scanning tunneling microscope tip is smaller than that of the molecular exciton, a property used to generate fluorescence maps of the chromophore with intramolecular resolution. Theoretical simulations of the electrostatic and electrodynamic interactions occurring at the picocavity junction formed by the chromophore, the tip, and the substrate reveal the key role played by subtle variations of Purcell, Lamb, and Stark effects. They also demonstrate that hyper-resolved fluorescence maps of the line shift and linewidth of the excitonic emission can be understood as images of the static charge redistribution upon electronic excitation of the molecule and as the distribution of the dynamical charge oscillation associated with the molecular exciton, respectively.6 MoreReceived 30 April 2021Revised 29 September 2021Accepted 3 November 2021DOI:https://doi.org/10.1103/PhysRevX.12.011012Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectroluminescencePlasmonicsTechniquesFirst-principles calculationsScanning tunneling microscopyCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Superradiance and Directional Exciton Migration in Metal-Organic Frameworks.

Crystalline metal-organic frameworks (MOFs) are promising synthetic analogues of photosynthetic light-harvesting complexes (LHCs). The precise assembly of linkers (organic chromophores) around the topology-defined pores offers the evolution of unique photophysical behaviors that are reminiscence of LHCs. These include MOF excited states with photoabsorbed energy that is spatially dispersed over multiple linkers defining the molecular excitons. The multilinker molecular excitons display superradiance─a hallmark of coupled oscillators seen in LHCs─with radiative rate constant (krad) exceeding that of a single linker. Our theoretical model and experimental results on three zirconium MOFs, namely, PCN-222(Zn), NU-1000, and SIU-100, with similar topology but varying linkers suggest that the size of such molecular excitons depends on the electronic symmetry of the linker. This multilinker exciton model effectively predicts the energy transfer rate constant; corresponding single-step exciton hopping time, ranging from a few picoseconds in SIU-100 and NU-1000 to a few hundreds of picoseconds in PCN-222(Zn), matches well with the experimental data. The model also predicts the anisotropy of exciton displacement with preferential migration along the crystallographic c-axis. Overall, these findings establish various missing links defining the exciton size and dynamics in MOF-assembled linkers. The understandings will provide design principles, especially, positioning the catalysts or electrode relative to the linker orientation for low-density solar energy conversion systems.

(De)localization dynamics of molecular excitons: comparison of mixed quantum-classical and fully quantum treatments

Molecular excitons play a central role in processes of solar energy conversion, both natural and artificial. It is therefore no wonder that numerous experimental and theoretical investigations in the last decade, employing state-of-the-art spectroscopic techniques and computational methods, have been driven by the common aim to unravel exciton dynamics in multichromophoric systems. Theoretically, exciton (de)localization and transfer dynamics are most often modelled using either mixed quantum-classical approaches (e.g., trajectory surface hopping) or fully quantum mechanical treatments (either using model diabatic Hamiltonians or direct dynamics). Yet, the terms such as "exciton localization" or "exciton transfer" may bear different meanings in different works depending on the method in use (quantum-classical vs. fully quantum). Here, we relate different views on exciton (de)localization. For this purpose, we perform molecular surface hopping simulations on several tetracene dimers differing by a magnitude of exciton coupling and carry out quantum dynamical as well as surface hopping calculations on a relevant model system. The molecular surface hopping simulations are done using efficient long-range corrected time-dependent density functional tight binding electronic structure method, allowing us to gain insight into different regimes of exciton dynamics in the studied systems.

Exciton Delocalization and Scaffold Stability in Bridged Nucleotide-Substituted, DNA Duplex-Templated Cyanine Aggregates.

Molecular excitons play a foundational role in chromophore aggregates found in light-harvesting systems and offer potential applications in engineered excitonic systems. Controlled aggregation of chromophores to promote exciton delocalization has been achieved by covalently tethering chromophores to deoxyribonucleic acid (DNA) scaffolds. Although many studies have documented changes in the optical properties of chromophores upon aggregation using DNA scaffolds, more limited work has investigated how structural modifications of DNA via bridged nucleotides and chromophore covalent attachment impact scaffold stability as well as the configuration and optical behavior of attached aggregates. Here we investigated the impact of two types of bridged nucleotides, LNA and BNA, as a structural modification of duplex DNA-templated cyanine (Cy5) aggregates. The bridged nucleotides were incorporated in the domain of one to four Cy5 chromophores attached between adjacent bases of a DNA duplex. We found that bridged nucleotides increase the stability of DNA scaffolds carrying Cy5 aggregates in comparison with natural nucleotides in analogous constructs. Exciton coupling strength and delocalization in Cy5 aggregates were evaluated via steady-state absorption, circular dichroism, and theoretical modeling. Replacing natural nucleotides with bridged nucleotides resulted in a noticeable increase in the coupling strength (≥10 meV) between chromophores and increased H-like stacking behavior (i.e., more face-to-face stacking). Our results suggest that bridged nucleotides may be useful for increasing scaffold stability and coupling between DNA templated chromophores.

Effect of a Dielectric Spacer on Electronic and Electromagnetic Interactions at Play in Molecular Exciton Decay at Surfaces and in Plasmonic Gaps

The deposition of individual molecules, molecular networks, and molecular layers at surfaces is at the core of surface reactivity, energy harvesting, molecular electronics, and (single) photon sour...

Photosynthetic pigment-protein complexes optical response modeling optimized by Differential evolution: algorithm convergence study

Photosynthetic pigment-protein complexes are the essential parts of thylakoid membranes of higher plants and cyanobacteria. Besides many organic and inorganic molecules they contain pigments like chlorophyll, bacteriochlorophyll, and carotenoids, which absorb the incident light and transform it into the energy of the excited electronic states. The semiclassical theories such as molecular exciton theory and the multimode Brownian oscillator model allows us to simulate the linear and nonlinear optical response of any pigment-protein complex, however, the main disadvantage of those approaches is a significant amount of effective parameters needed to be found in order to reproduce the experimental data. To overcome these difficulties we used the Differential evolution method (DE) that belongs to the family of evolutionary optimization algorithms. Based on our preliminary studies of the linear optical properties of monomeric photosynthetic pigments using DE, we proceed to more complex systems like the reaction center of photosystem II isolated from higher plants (PSIIRC). PSIIRC contains only eight chlorophyll pigments, and therefore it is potentially a very promising subject to test DE as a powerful optimization procedure for simulation of the optical response of a system of interacting pigments. Using the theoretically simulated linear spectra of PSIIRC (absorption, circular dichroism, linear dichroism, and fluorescence), we investigated the dependence of the algorithm convergence on DE settings: strategies, crossover, weighting factor; eventually finding the optimal mode of operation of the optimization procedure.

Giant Enhancement of Excitonic Electro-optic Response in Trap-Reduced Organic Transistors

Organic field-effect transistors (FETs) exhibit excellent switching characteristics when they involve trap-eliminated semiconductor interfaces with highly hydrophobic gate dielectric layers. In this study, we investigate the excitonic electro-optic response by delocalized carrier accumulation at the trap-eliminated interfaces of pentacene single-crystal FETs. We find that gate-modulation (GM) imaging, which sensitively visualizes the variation in optical microscope images between the gate-on and gate-off states, exclusively reveals the unique enhancement of electro-optic response under the application of drain voltages (${V}_{d}$). The ${V}_{d}$-unbiased GM image exhibits a uniform spatial distribution, which is consistent with the accumulated carrier density in the channel. In contrast, the ${V}_{d}$-biased GM image presents a peculiar spatial distribution with fairly sharp increases around the edges of the source and drain electrodes. Furthermore, the following intriguing features are observed: (1) The sharp increase in the GM signal distribution around the electrode edge is similar to the lateral electric field distribution as measured by Kelvin probe force microscopy, and (2) the GM spectra, extracted from the respective GM images measured at different wavelengths, present a second-derivative-like shape that implies the broadening of exciton absorption. Based on these observations, we investigate the origin of this unique effect in terms of the enhanced violation of exciton coherence by delocalized carrier accumulations under drain bias. The gate-induced holes that are weakly bound to shallow traps should be detrapped by lateral electric fields, which eventually generate valence band holes and thus enhance the electro-optic response. These findings should elucidate the spatial coherence of the molecular excitons that are responsible for the various unique photoelectric characteristics of organic electronic devices.

Toward witnessing molecular exciton entanglement from spectroscopy

Entanglement is a defining feature of quantum mechanics that can be a resource in engineered and natural systems, but measuring entanglement in experiment remains elusive especially for large chemical systems. Most practical approaches require determining and measuring a suitable entanglement witness which provides some level of information about the entanglement structure of the probed state. A fundamental quantity of quantum metrology is the quantum Fisher information (QFI) which is a rigorous witness of multipartite entanglement that can be evaluated from linear response functions for certain states. In this work, we explore measuring the QFI of molecular exciton states of the first-excitation subspace from spectroscopy. In particular, we utilize the fact that the linear response of a pure state subject to a weak electric field over all possible driving frequencies encodes the variance of the collective dipole moment in the probed state, which is a valid measure for QFI. The systems that are investigated include the molecular dimer, $N$-site linear aggregate with nearest-neighbor coupling, and $N$-site circular aggregate, all modeled as a collection of interacting qubits. Our theoretical analysis shows that the variance of the collective dipole moment in the brightest dipole-allowed eigenstate is the maximum QFI. The optical response of a thermally equilibrated state in the first-excitation subspace is also a valid QFI. Theoretical predictions of the measured QFI for realistic linear dye aggregates as a function of temperature and energetic disorder due to static variations of the host matrix show that 2- to 3-partite entanglement is realizable. This work lays some groundwork and inspires measurement of multipartite entanglement of molecular excitons with ultrafast pump-probe experiments.

The role of charge recombination to triplet excitons in organic solar cells.

The use of non-fullerene acceptors (NFAs)in organic solar cells has led to power conversion efficiencies as high as 18%1. However, organic solar cells are still less efficient than inorganic solar cells, which typically have power conversion efficiencies of more than 20%2. A key reason for this difference is that organic solar cells have low open-circuit voltages relative to their optical bandgaps3, owing to non-radiative recombination4. For organic solar cells to compete with inorganic solar cells in terms of efficiency, non-radiative loss pathways must be identified and suppressed. Here we show that in most organic solar cells that use NFAs, the majority of charge recombination under open-circuit conditions proceeds via the formation of non-emissive NFA triplet excitons; in the benchmark PM6:Y6 blend5, this fraction reaches 90%, reducing the open-circuit voltage by 60 mV. We prevent recombination via this non-radiative channel by engineering substantial hybridization between the NFA triplet excitons and the spin-triplet charge-transfer excitons. Modelling suggests that the rate of back charge transfer from spin-triplet charge-transfer excitons to moleculartriplet excitons may be reduced by an order of magnitude, enabling re-dissociation of the spin-triplet charge-transfer exciton. We demonstrate NFA systems in which the formation of triplet excitons is suppressed. This work thus provides a design pathway for organic solar cells with power conversion efficiencies of 20% or more.

Vibronic Coupling and Exciton Chirality: Electronic and Structural Rearrangement between Helical to Zero Momentum Molecular Exciton States

Vibronic Coupling and Exciton Chirality: Electronic and Structural Rearrangement between Helical to Zero Momentum Molecular Exciton States

Exciton dispersion in para-quaterphenyl: Significant molecular interactions beyond Coulomb coupling

We have experimentally determined the momentum dependence of the electronic excitation spectra of para-quaterphenyl single crystals. The parallel arrangement of para-quaterphenyl molecules results in a strong Coulomb coupling of the molecular excitons. Such crystals have been considered to be a very good realization of the Frenkel exciton model, including the formation of H-type aggregates. Our data reveal an unexpected exciton dispersion of the upper Davydov component, which cannot be rationalized in terms of inter-molecular Coulomb coupling of the excitons. A significant reduction of the nearest neighbor coupling due to additional charge-transfer processes is able to provide an explanation of the data. Furthermore, the spectral onset of the excitation spectrum, which represents a heavy exciton resulting from exciton–phonon coupling, also shows a clear dispersion, which had been unknown so far. Finally, an optically forbidden excitation about 1 eV above the excitation onset is observed.