Strong light-matter coupling for reduced photon energy losses in organic photovoltaics

Active control of polariton-enabled long-range energy transfer

Optical control is achieved on the excited state energy transfer between spatially separated donor and acceptor molecules, both coupled to the same optical mode of a cavity. The energy transfer occurs through the formed hybrid polaritons and can be switched on and off by means of ultraviolet and visible light. The control mechanism relies on a photochromic component used as donor, whose absorption and emission properties can be varied reversibly through light irradiation, whereas in-cavity hybridization with acceptors through polariton states enables a 6-fold enhancement of acceptor/donor contribution to the emission intensity with respect to a reference multilayer. These results pave the way for synthesizing effective gating systems for the transport of energy by light, relevant for light-harvesting and light-emitting devices, and for photovoltaic cells.

Enhancement of the internal quantum efficiency in strongly coupled P3HT-C60 organic photovoltaic cells using Fabry-Perot cavities with varied cavity confinement

The short exciton diffusion length in organic semiconductors results in a strong dependence of the conversion efficiency of organic photovoltaic (OPV) cells on the morphology of the donor-acceptor bulk-heterojunction blend. Strong light-matter coupling provides a way to circumvent this dependence by combining the favorable properties of light and matter via the formation of hybrid exciton-polaritons. By strongly coupling excitons in P3HT-C60 OPV cells to Fabry-Perot optical cavity modes, exciton-polaritons are formed with increased propagation lengths. We exploit these exciton-polaritons to enhance the internal quantum efficiency of the cells, determined from the external quantum efficiency and the absorptance. Additionally, we find a consistent decrease in the Urbach energy for the strongly coupled cells, which indicates the reduction of energetic disorder due to the delocalization of exciton-polaritons in the optical cavity.

High-Brightness Blue Polariton Organic Light-Emitting Diodes

Polariton organic light-emitting diodes (POLEDs) use strong light-matter coupling as an additional degree of freedom to tailor device characteristics, thus making them ideal candidates for many applications, such as room temperature laser diodes and high-color purity displays. However, achieving efficient formation of and emission from exciton-polaritons in an electrically driven device remains challenging due to the need for strong absorption, which often induces significant nonradiative recombination. Here, we investigate a novel POLED architecture to achieve polariton formation and high-brightness light emission. We utilize the blue-fluorescent emitter material 4,4'-Bis(4-(9H-carbazol-9-yl)styryl)biphenyl (BSBCz), which exhibits strong absorption and a highly horizontal transition-dipole orientation as well as a high photoluminescence quantum efficiency, even at high doping concentrations. We achieve a peak luminance of over 20,000 cd/m2 and external quantum efficiencies of more than 2%. To the best of our knowledge, these values represent the highest reported so far for electrically driven polariton emission from an organic semiconductor emitting in the blue region of the spectrum. Our work therefore paves the way for a new generation of efficient and powerful optoelectronic devices based on POLEDs.

Controlling Exciton/Exciton Recombination in 2-D Perovskite Using Exciton-Polariton Coupling

In this paper, we demonstrate that exciton/exciton annihilation in the 2D perovskite (PEA)2PbI4 (PEPI)─a major loss mechanism in solar cells and light-emitting diodes, can be controlled through coupling of excitons with cavity polaritons. We study the excited state dynamics using time-resolved transient absorption spectroscopy and show that the system can be tuned through a strong coupling regime by varying the cavity width through the PEPI layer thickness. Remarkably, strong coupling occurs even when the cavity quality factor remains poor, providing easy optical access. We demonstrate that the observed derivative-like transient absorption spectra can be modeled using a time-dependent Rabi splitting that occurs because of transient bleaching of the excitonic states. When PEPI is strongly coupled to the cavity, the exciton/exciton annihilation rate is suppressed by 1 order of magnitude. A model that relies on the partly photonic character of polaritons explains the results as a function of detuning.

The Exciton Model for Molecular Materials: Past, Present and Future?

In assemblies of identical molecules or chromophores, electronic excitations can be described as excitons, bound electron-hole pairs that can move from site to site as a pair in a coherent manner. The understanding of excitons is crucial when trying to engineer favorable photophysical properties through structuring organic molecular matter. In recent decades, limitations of the concept of an exciton have become clear. The exciton can hybridize with phonon and photons. To clarify these issues, the exciton is discussed within the broader context of the gauge properties of the electromagnetic force.

The Mechanism and Fine-Tuning of Chiral Plexcitons in the Strong Coupling Regime

Chiral plexcitons, produced by the strong interaction between plasmonic nanocavities and chiral molecules, can provide a promising direction for controlling chiroptical responses on the nanoscale. Here, we reveal the chiral origin and electromagnetic hybridization process in chiral strongly coupled systems. The mechanism and unique advantages of chiral plexcitons for fine-tuning circular dichroism (CD) responses are demonstrated, providing a rule for controlling chiral light-matter interactions in complex chiral nanosystems. Furthermore, we experimentally demonstrate the fine-tuning of chiral plexcitons in hybrid systems consisting of plasmonic nanoparticles and chiral J-aggregates. Continuous and precise tuning of the CD resonance positions was successfully achieved in a given structure. Compared with the previous work, the CD spectral tuning accuracy has been improved by an order of magnitude, which can reach the level of 1 nm. Our findings provide a feasible strategy and theoretical basis for accurately controlling chirality in multiple dimensions.

The Rise and Current Status of Polaritonic Photochemistry and Photophysics

The interaction between molecular electronic transitions and electromagnetic fields can be enlarged to the point where distinct hybrid light-matter states, polaritons, emerge. The photonic contribution to these states results in increased complexity as well as an opening to modify the photophysics and photochemistry beyond what normally can be seen in organic molecules. It is today evident that polaritons offer opportunities for molecular photochemistry and photophysics, which has caused an ever-rising interest in the field. Focusing on the experimental landmarks, this review takes its reader from the advent of the field of polaritonic chemistry, over the split into polariton chemistry and photochemistry, to present day status within polaritonic photochemistry and photophysics. To introduce the field, the review starts with a general description of light-matter interactions, how to enhance these, and what characterizes the coupling strength. Then the photochemistry and photophysics of strongly coupled systems using Fabry-Perot and plasmonic cavities are described. This is followed by a description of room-temperature Bose-Einstein condensation/polariton lasing in polaritonic systems. The review ends with a discussion on the benefits, limitations, and future developments of strong exciton-photon coupling using organic molecules.

Identification of Design Principles for the Preparation of Colloidal Plexcitonic Materials

Colloidal plexcitonic materials (CPMs) are a class of nanosystems where molecular dyes are strongly coupled with colloidal plasmonic nanoparticles, acting as nanocavities that enhance the light field. As a result of this strong coupling, new hybrid states are formed, called plexcitons, belonging to the broader family of polaritons. With respect to other families of polaritonic materials, CPMs are cheap and easy to prepare through wet chemistry methodologies. Still, clear structure-to-properties relationships are not available, and precise rules to drive the materials' design to obtain the desired optical properties are still missing. To fill this gap, in this article, we prepared a dataset with all CPMs reported in the literature, rationalizing their design by focusing on their three main relevant components (the plasmonic nanoparticles, the molecular dyes, and the capping layers) and identifying the most used and efficient combinations. With the help of statistical analysis, we also found valuable correlations between structure, coupling regime, and optical properties. The results of this analysis are expected to be relevant for the rational design of new CPMs with controllable and predictable photophysical properties to be exploited in a vast range of technological fields.

Nonadiabatic dynamics near metal surfaces under Floquet engineering: Floquet electronic friction vs Floquet surface hopping

In the previous study Wang and Dou [J. Chem. Phys. 158, 224109 (2023)], we have derived a Floquet classical master equation (FCME) to treat nonadiabatic dynamics near metal surfaces under Floquet engineering. We have also proposed a trajectory surface hopping algorithm to solve the FCME. In this study, we map the FCME into a Floquet Fokker-Planck equation in the limit of fast Floquet driving and fast electron motion as compared to nuclear motion. The Fokker-Planck equation is then being solved using Langevin dynamics with explicit friction and random force from the nonadiabatic effects of hybridized electrons and Floquet states. We benchmark the Floquet electronic friction dynamics against Floquet quantum master equation and Floquet surface hopping. We find that Floquet driving results in a violation of the second fluctuation-dissipation theorem, which further gives rise to heating effects.

Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells

Organic solar cells benefit from non-fullerene acceptors (NFA) due to their high absorption coefficients, tunable frontier energy levels, and optical gaps, as well as their relatively high luminescence quantum efficiencies as compared to fullerenes. Those merits result in high yields of charge generation at a low or negligible energetic offset at the donor/NFA heterojunction, with efficiencies over 19% achieved for single-junction devices. Pushing this value significantly over 20% requires an increase in open-circuit voltage, which is currently still well below the thermodynamic limit. This can only be achieved by reducing non-radiative recombination, and hereby increasing the electroluminescence quantum efficiency of the photo-active layer. Here, current understanding of the origin of non-radiative decay, as well as an accurate quantification of the associated voltage losses are summarized. Promising strategies for suppressing these losses are highlighted, with focus on new material design, optimization of donor-acceptor combination, and blend morphology. This review aims at guiding researchers in their quest to find future solar harvesting donor-acceptor blends, which combine a high yield of exciton dissociation with a high yield of radiative free carrier recombination and low voltage losses, hereby closing the efficiency gap with inorganic and perovskite photovoltaics.

Developing Solution-Processed Distributed Bragg Reflectors for Microcavity Polariton Applications

Improving the performance of organic optoelectronics has been under vigorous research for decades. Recently, polaritonics has been introduced as a technology that has the potential to improve the optical, electrical, and chemical properties of materials and devices. However, polaritons have been mainly studied in optical microcavities that are made by vacuum deposition processes, which are costly, unavailable to many, and incompatible with printed optoelectronics methods. Efforts toward the fabrication of polariton microcavities with solution-processed techniques have been utterly absent. Herein, we demonstrate for the first time strong light-matter coupling and polariton photoluminescence in an organic microcavity consisting of an aluminum mirror and a distributed Bragg reflector (DBR) made by sequential dip coating of titanium hydroxide/poly(vinyl alcohol) (TiOH/PVA) and Nafion films. To fabricate and develop the solution-processed DBRs and microcavities, we automatized a dip-coating device that allowed us to produce sub-100 nm films consistently over many dip-coating cycles. Owning to the solution-based nature of our DBRs, our results pave the way to the realization of polariton optoelectronic devices beyond physical deposition methods.

Giant optomechanical spring effect in plasmonic nano- and picocavities probed by surface-enhanced Raman scattering

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for non-linear optics. Here we show the extreme confinement provided by plasmonic nano- and pico-cavities can sufficiently enhance optomechanical coupling so that intense laser illumination drastically softens the molecular bonds. This optomechanical pumping regime produces strong distortions of the Raman vibrational spectrum related to giant vibrational frequency shifts from an optical spring effect which is hundred-fold larger than in traditional cavities. The theoretical simulations accounting for the multimodal nanocavity response and near-field-induced collective phonon interactions are consistent with the experimentally-observed non-linear behavior exhibited in the Raman spectra of nanoparticle-on-mirror constructs illuminated by ultrafast laser pulses. Further, we show indications that plasmonic picocavities allow us to access the optical spring effect in single molecules with continuous illumination. Driving the collective phonon in the nanocavity paves the way to control reversible bond softening, as well as irreversible chemistry.

Trans-Reflective Structural Color Filters Assisting Multifunctional-Integrated Semitransparent Photovoltaic Window

Multifunction-integrated semitransparent organic photovoltaic cells (STOPVs), with high power generation, colorful transmittance/reflectance, excellent ultraviolet (UV) protection, and thermal insulation, are fully in line with the concept of architectural aesthetics and photoprotection characteristics for building-integrated photovoltaic-window. For the indelible rainbow color photovoltaic window, one crucial issue is to realize the integration of these photons- and photoelectric-related multifunction. Herein, dynamic transmissive and reflective structural color controllable filters, with asymmetrical metal-insulator-metal (MIM) configurations (20 nm-Ag-HATCN-30 nm-Ag) through machine learning, are deliberately designed for colorful STOPV devices. This endows the resultant integrated devices with ≈5% enhanced power conversion efficiency (PCE) than the bare-STOPVs, gifted UV (300-400 nm) blocking rates as high as 93.5, 94.1, 90.2, and 94.5%, as well as a superior infrared radiation (IR) (700-1400 nm) rejection approaching 100% for transparent purple-, blue-, green- and red-STOPV cells, respectively. Most importantly, benefiting from the photonic recycling effect beyond microcavity resonance wavelength, a reported quantum utilization efficiency (QUE) as high as 80%, is first presented for the transparent-green-STOPVs with an ultra-narrow bandgap of 1.2 eV. These asymmetrical Febry-Pérot transmissive and reflective structural color filters can also be extended to silicon- and perovskite-based optoelectric devices and make it possible to integrate additional target optical functions for multi-purpose optoelectric devices.

Low-cost machine learning prediction of excited state properties of iridium-centered phosphors

Prediction of the excited state properties of photoactive iridium complexes challenges ab initio methods such as time-dependent density functional theory (TDDFT) both from the perspective of accuracy and of computational cost, complicating high-throughput virtual screening (HTVS). We instead leverage low-cost machine learning (ML) models and experimental data for 1380 iridium complexes to perform these prediction tasks. We find the best-performing and most transferable models to be those trained on electronic structure features from low-cost density functional tight binding calculations. Using artificial neural network (ANN) models, we predict the mean emission energy of phosphorescence, the excited state lifetime, and the emission spectral integral for iridium complexes with accuracy competitive with or superseding that of TDDFT. We conduct feature importance analysis to determine that high cyclometalating ligand ionization potential correlates to high mean emission energy, while high ancillary ligand ionization potential correlates to low lifetime and low spectral integral. As a demonstration of how our ML models can be used for HTVS and the acceleration of chemical discovery, we curate a set of novel hypothetical iridium complexes and use uncertainty-controlled predictions to identify promising ligands for the design of new phosphors while retaining confidence in the quality of the ANN predictions.

Strong coupling of excitons in patterned few-layer WS2 with guided mode and bound state in the continuum

The strong coupling of excitons in few-layer transition-metal dichalcogenide (TMDC) with guided mode resonance (GMR) and bound state in the continuum (BIC) is investigated. It is shown that the strong coupling between excitons and GMR or BIC can enable a large Rabi splitting, where up to 155 meV or 162 meV Rabi splitting could be realized through changing the grating period, respectively. The physical origins behind this behavior are revealed by studying the electric field distributions at resonance. In addition, such behaviors are further theoretically verified according to the coupled-oscillator model. Moreover, the effect of the geometric dimensions on the strong coupling is also studied, which can be employed to guide real fabrication. The results will provide a new route for realization of few-layer TMDC-based light-matter interactions and may pave the way toward novel, compact, few-layer TMDC-based polaritonic devices.

Tuning the Coherent Propagation of Organic Exciton-Polaritons through Dark State Delocalization

While there have been numerous reports of long-range polariton transport at room-temperature in organic cavities, the spatiotemporal evolution of the propagation is scarcely reported, particularly in the initial coherent sub-ps regime, where photon and exciton wavefunctions are inextricably mixed. Hence the detailed process of coherent organic exciton-polariton transport and, in particular, the role of dark states has remained poorly understood. Here, femtosecond transient absorption microscopy is used to directly image coherent polariton motion in microcavities of varying quality factor. The transport is found to be well-described by a model of band-like propagation of an initially Gaussian distribution of exciton-polaritons in real space. The velocity of the polaritons reaches values of ≈ 0.65 × 10⁶ m s^-1 , substantially lower than expected from the polariton dispersion. Further, it is found that the velocity is proportional to the quality factor of the microcavity. This unexpected link between the quality-factor and polariton velocity is suggested to be a result of varying admixing between delocalized dark and polariton states.

Slip-Stacked J-Aggregate Materials for Organic Solar Cells and Photodetectors

Dye-dye interactions affect the optical and electronic properties in organic semiconductor films of light harvesting and detecting optoelectronic applications. This review elaborates how to tailor these properties of organic semiconductors for organic solar cells (OSCs) and organic photodiodes (OPDs). While these devices rely on similar materials, the demands for their optical properties are rather different, the former requiring a broad absorption spectrum spanning from the UV over visible up to the near-infrared region and the latter an ultra-narrow absorption spectrum at a specific, targeted wavelength. In order to design organic semiconductors satisfying these demands, fundamental insights on the relationship of optical properties are provided depending on molecular packing arrangement and the resultant electronic coupling thereof. Based on recent advancements in the theoretical understanding of intermolecular interactions between slip-stacked dyes, distinguishing classical J-aggregates with predominant long-range Coulomb coupling from charge transfer (CT)-mediated or -coupled J-aggregates, whose red-shifts are primarily governed by short-range orbital interactions, is suggested. Within this framework, the relationship between aggregate structure and functional properties of representative classes of dye aggregates is analyzed for the most advanced OSCs and wavelength-selective OPDs, providing important insights into the rational design of thin-film optoelectronic materials.

Evidence for a Polariton-Mediated Biexciton Transition in Single-Walled Carbon Nanotubes

Strong coupling of excitonic resonances with a cavity gives rise to exciton-polaritons which possess a modified energy landscape compared to the uncoupled emitter. However, due to the femtosecond lifetime of the so-called bright polariton states and transient changes of the cavity reflectivity under excitation, it is challenging to directly measure the polariton excited state dynamics. Here, near-infrared pump-probe spectroscopy is used to investigate the ultrafast dynamics of exciton-polaritons based on strongly coupled (6,5) single-walled carbon nanotubes in metal-clad microcavities. We present a protocol for fitting the reflectivity-associated response of the cavity using genetic algorithm-assisted transfer-matrix simulations. With this approach, we are able to identify an absorptive exciton-polariton feature in the transient transmission data. This feature appears instantaneously under resonant excitation of the upper polariton but is delayed for off-resonant excitation. The observed transition energy and detuning dependence point toward a direct upper polariton-to-biexciton transition. Our results provide direct evidence for exciton-polariton intrinsic transitions beyond the bright polariton lifetime in strongly coupled microcavities.

Not dark yet for strong light-matter coupling to accelerate singlet fission dynamics

Polaritons are unique hybrid light-matter states that offer an alternative way to manipulate chemical processes. In this work, we show that singlet fission dynamics can be accelerated under strong light-matter coupling. For superexchange-mediated singlet fission, state mixing speeds up the dynamics in cavities when the lower polariton is close in energy to the multiexcitonic state. This effect is more pronounced in non-conventional singlet fission materials in which the energy gap between the bright singlet exciton and the multiexcitonic state is large ( > 0.1 eV). In this case, the dynamics is dominated by the polaritonic modes and not by the bare-molecule-like dark states, and, additionally, the resonant enhancement due to strong coupling is robust even for energetically broad molecular states. The present results provide a new strategy to expand the range of suitable materials for efficient singlet fission by making use of strong light-matter coupling.

Ultrastrong Exciton-Photon Coupling in Broadband Solar Absorbers

The recent development of organic polaritonic solar cells, in which sunlight absorbers and photon modes of a resonator are hybridized as a result of their strong coupling, has revealed the potential this interaction offers to control and enhance the performance of these devices. In this approach, the photovoltaic cell is built in such a way that it also behaves as an optical cavity supporting spectrally well-defined resonances, which match the broad absorption bands of the dyes employed. Herein we focus on the experimental and theoretical analysis of the specific spectral and angular optical absorption characteristics of a broadband light harvester, namely a subphthalocyanine, when operating in the ultrastrong coupling regime. We discuss the implications of having a broad distribution of oscillator strengths and demonstrate that rational design of the layered structure is needed to optimize both the spectral and the angular response of the sunlight harvester dye.

Exciton-Photonics: From Fundamental Science to Applications

Semiconductors in all dimensionalities ranging from 0D quantum dots and molecules to 3D bulk crystals support bound electron-hole pair quasiparticles termed excitons. Over the past two decades, the emergence of a variety of low-dimensional semiconductors that support excitons combined with advances in nano-optics and photonics has burgeoned an advanced area of research that focuses on engineering, imaging, and modulating the coupling between excitons and photons, resulting in the formation of hybrid quasiparticles termed exciton-polaritons. This advanced area has the potential to bring about a paradigm shift in quantum optics, as well as classical optoelectronic devices. Here, we present a review on the coupling of light in excitonic semiconductors and previous investigations of the optical properties of these hybrid quasiparticles via both far-field and near-field imaging and spectroscopy techniques. Special emphasis is given to recent advances with critical evaluation of the bottlenecks that plague various materials toward practical device implementations including quantum light sources. Our review highlights a growing need for excitonic material development together with optical engineering and imaging techniques to harness the utility of excitons and their host materials for a variety of applications.

A universal Urbach rule for disordered organic semiconductors

In crystalline semiconductors, absorption onset sharpness is characterized by temperature-dependent Urbach energies. These energies quantify the static, structural disorder causing localized exponential-tail states, and dynamic disorder from electron-phonon scattering. Applicability of this exponential-tail model to disordered solids has been long debated. Nonetheless, exponential fittings are routinely applied to sub-gap absorption analysis of organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes of organic semiconductors and their blends by temperature-dependent quantum efficiency measurements. We find that sub-gap absorption due to singlet excitons is universally dominated by thermal broadening at low photon energies and the associated Urbach energy equals the thermal energy, regardless of static disorder. This is consistent with absorptions obtained from a convolution of Gaussian density of excitonic states weighted by Boltzmann-like thermally activated optical transitions. A simple model is presented that explains absorption line-shapes of disordered systems, and we also provide a strategy to determine the excitonic disorder energy. Our findings elaborate the meaning of the Urbach energy in molecular solids and relate the photo-physics to static disorder, crucial for optimizing organic solar cells for which we present a revisited radiative open-circuit voltage limit.

Barrier-free reverse-intersystem crossing in organic molecules by strong light-matter coupling

Strong light-matter coupling provides the means to challenge the traditional rules of chemistry. In particular, an energy inversion of singlet and triplet excited states would be fundamentally remarkable since it would violate the classical Hund’s rule. An organic chromophore possessing a lower singlet excited state can effectively harvest the dark triplet states, thus enabling 100% internal quantum efficiency in electrically pumped light-emitting diodes and lasers. Here we demonstrate unambiguously an inversion of singlet and triplet excited states of a prototype molecule by strong coupling to an optical cavity. The inversion not only implies that the polaritonic state lies at a lower energy, but also a direct energy pathway between the triplet and polaritonic states is opened. The intrinsic photophysics of reversed-intersystem crossing are thereby completely overturned from an endothermic process to an exothermic one. By doing so, we show that it is possible to break the limit of Hund’s rule and manipulate the energy flow in molecular systems by strong light-matter coupling. Our results will directly promote the development of organic light-emitting diodes based on reversed-intersystem crossing. Moreover, we anticipate that it provides the pathway to the creation of electrically pumped polaritonic lasers in organic systems.

Strong coupling of organic materials with optical cavities allows to manipulate the rate of energy transfer between their internal states. Here, the authors show a hybrid state of singlet character with energy lower than the triplet state, and a flow of energy from the triplet to the hybrid state.

Evolutionary optimization of light-matter coupling in open plasmonic cavities

Using a particle swarm optimization algorithm and finite-difference in time-domain simulations, we optimize the coupling strength between excitons in poly(3-hexylthiophene-2,5-diyl) (P3HT) and surface lattice resonances in open cavities defined by arrays of aluminum nanoparticles. Strong light-matter coupling and the formation of exciton-polaritons are demonstrated. Nanoparticle arrays with optimal dimensions have been fabricated and measured, validating the predictions by the numerical method. P3HT is a regioregular semiconducting polymer used as a donor material in acceptor-donor blends for organic photovoltaic applications. Our results demonstrate the efficacy of the proposed method for the optimization of light-matter coupling and its potential application for the enhanced performance of optoelectronic devices.

Polariton-assisted excitation energy channeling in organic heterojunctions

Exciton-polaritons are hybrid light-matter states resulting from strong exciton-photon coupling. The wave function of the polariton is a mixture of light and matter, enabling long-range energy transfer between spatially separated chromophores. Moreover, their delocalized nature, inherited from the photon component, has been predicted to enhance exciton transport. Here, we strongly couple an organic heterojunction consisting of energy/electron donor and acceptor materials to the same cavity mode. Using time-resolved spectroscopy and optoelectrical characterization, we show that the rate of exciton harvesting is enhanced with one order of magnitude and the rate of energy transfer in the system is increased two- to threefold in the strong coupling regime. Our results exemplify two means of efficiently channeling excitation energy to a heterojunction interface, where charge separation can occur. This study opens a new door to increase the overall efficiency of light harvesting systems using the tool of strong light-matter interactions.

Novel semi-analytical optoelectronic modeling based on homogenization theory for realistic plasmonic polymer solar cells

Numerical-based simulations of plasmonic polymer solar cells (PSCs) incorporating a disordered array of non-uniform sized plasmonic nanoparticles (NPs) impose a prohibitively long-time and complex computational demand. To surmount this limitation, we present a novel semi-analytical modeling, which dramatically reduces computational time and resource consumption and yet is acceptably accurate. For this purpose, the optical modeling of active layer-incorporated plasmonic metal NPs, which is described by a homogenization theory based on a modified Maxwell-Garnett-Mie theory, is inputted in the electrical modeling based on the coupled equations of Poisson, continuity, and drift-diffusion. Besides, our modeling considers the effects of absorption in the non-active layers, interference induced by electrodes, and scattered light escaping from the PSC. The modeling results satisfactorily reproduce a series of experimental data for photovoltaic parameters of plasmonic PSCs, demonstrating the validity of our modeling approach. According to this, we implement the semi-analytical modeling to propose a new high-efficiency plasmonic PSC based on the PM6:Y6 PSC, having the highest reported power conversion efficiency (PCE) to date. The results show that the incorporation of plasmonic NPs into PM6:Y6 active layer leads to the PCE over 18%.

Enhanced analytical and physical characterization of mixtures of random bay-position brominated boron subnaphthalocyanines enabled by establishing a partial separation method

A separation method unravelled the analytics of boron subnaphthalocyanines and enabled the investigation of the influence of bay position bromination.

Spectroscopic near-infrared photodetectors enabled by strong light-matter coupling in (6,5) single-walled carbon nanotubes

Strong light-matter coupling leads to the formation of mixed exciton-polariton states, allowing for a rigorous manipulation of the absorption and emission of excitonic materials. Here, we demonstrate the realization of this promising concept in organic photodetectors. By hybridizing the E₁₁ exciton of semiconducting (6,5) single-walled carbon nanotubes (SWNTs) with near-infrared cavity photons, we create spectrally tunable polariton states within a photodiode. In turn, we are able to red-shift the detection peak that coincides with the lower polariton band. Our photodiodes comprise a metal cavity to mediate strong coupling between light and SWNTs and utilize P3HT and PC₇₀BM as the electron donor and acceptor, respectively. The diodes are formed either via mixing of SWNTs, P3HT, and PC₇₀BM to create a bulk heterojunction or by sequential processing of layers to form flat heterojunctions. The resulting near-infrared sensors show tunable, efficient exciton harvesting in an application-relevant wavelength range between 1000 nm and 1300 nm, with optical simulations showing a possible extension beyond 1500 nm.

Fine-tuning of polariton energies in a tailored plasmon cavity and J-aggregates hybrid system

Strong coupling systems enable coherent energy exchange between a light field and material electrons in nanoscale space. Active manipulation of this phenomenon by external stimuli is crucial for the design of advanced optoelectronic devices. Two neglected points severely hinder the improvement of tuning accuracy: irreversible variation in cavity morphology and lack of control over the dielectric environment which may change during the coupling process. Here we present a chemical fine-tuning of the strong plasmon-exciton coupling process in tailored Au@Ag nanocavities. The silver shell thickness was carefully controlled to tune the plasmon resonance wavelength with an accuracy of ∼8 nm and facilitate hot spots at the edges to boost the plasmon-exciton coupling strength. Hybrid polariton states were further regulated across the zero-detuning point with a spectral accuracy of less than 1 nm via tuning the solvent refractive index, and a Rabi splitting as large as 194 meV was observed at room temperature. The fine-tuning of strong plasmon-exciton coupling by an adjacent dielectric environment provides a novel route to manipulate excitons in molecules and possesses great potential for chemical or biological sensing.

Single-Junction Organic Photovoltaic Cells with Approaching 18% Efficiency

Optimizing the molecular structures of organic photovoltaic (OPV) materials is one of the most effective methods to boost power conversion efficiencies (PCEs). For an excellent molecular system with a certain conjugated skeleton, fine tuning the alky chains is of considerable significance to fully explore its photovoltaic potential. In this work, the optimization of alkyl chains is performed on a chlorinated nonfullerene acceptor (NFA) named BTP-4Cl-BO (a Y6 derivative) and very impressive photovoltaic parameters in OPV cells are obtained. To get more ordered intermolecular packing, the n-undecyl is shortened at the edge of BTP-eC11 to n-nonyl and n-heptyl. As a result, the NFAs of BTP-eC9 and BTP-eC7 are synthesized. The BTP-eC7 shows relatively poor solubility and thus limits its application in device fabrication. Fortunately, the BTP-eC9 possesses good solubility and, at the same time, enhanced electron transport property than BTP-eC11. Significantly, due to the simultaneously enhanced short-circuit current density and fill factor, the BTP-eC9-based single-junction OPV cells record a maximum PCE of 17.8% and get a certified value of 17.3%. These results demonstrate that minimizing the alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving its photovoltaic performance.

The Cost of Converting Excitons into Free Charge Carriers in Organic Solar Cells

Efficient exciton dissociation and subsequent generation of free charge carriers at the organic donor-acceptor interface requires a number of electron-transfer processes. It is a common view that these steps result in an unavoidable energy loss in organic photovoltaic devices that is not present in other types of solar cells. The currently best performing organic solar cells with power conversion efficiencies over 16% challenge this view, and no interfacial charge-transfer states with energy significantly lower than the strongly absorbing singlet states are detected within the gap of the used donor and acceptor materials. This Perspective will discuss implications, the remaining sources of energy loss, and open questions to be solved to achieve power conversion efficiencies over 20%.

Understanding the High Performance of over 15% Efficiency in Single-Junction Bulk Heterojunction Organic Solar Cells

The highly efficient single-junction bulk-heterojunction (BHJ) PM6:Y6 system can achieve high open-circuit voltages (V_OC ) while maintaining exceptional fill-factor (FF) and short-circuit current (J_SC ) values. With a low energetic offset, the blend system is found to exhibit radiative and non-radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies reveal that the device shows moderate non-geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques are employed to understand the phase separation, long-range ordering, as well as donor:acceptor (D:A) inter- and intramolecular interactions at an atomic-level resolution. This is achieved using photo-conductive atomic force microscopy, grazing-incidence wide-angle X-ray scattering, and solid-state ¹⁹ F magic-angle-spinning NMR spectroscopy. The synergy of multifaceted characterization and device physics is used to uncover key insights, for the first time, on the structure-property relationships of this high-performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing reveals that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high J_SC and FF to be retained despite the low energetic offset.