This PDF file contains the front matter associated with SPIE Proceedings Volume 11799, including the Title Page, Copyright information, and Table of Contents.

Strong light-matter coupling generates hybrid states that inherit properties of both light and matter, effectively allowing the modification of the molecular potential energy landscape. This phenomenon opens up a plethora of options for manipulating the properties of molecules, with a broad range of applications in physics, chemistry, and materials science. In this presentation, I will discuss how the relaxation between hybrid light-matter states and molecular centered states can be used to in molecular based devices. Specifically on the possibility to efficiently harvest excitation energy in planar heterojunctions by utilizing the delocalized nature of hybrid light-matter states.

The concept of modifying molecular dynamics in strongly coupled exciton-polariton systems is an emerging topic in photonics. However, there is no consensus on the types of molecular systems whose dynamics can be modified using strong coupling. These open questions stem from persistent uncertainties concerning the lifetime and conversion dynamics of exciton-polaritons and localized excited states as well as the proper way to measure such interactions in the time-domain. Here, we provide a framework for measuring dynamical interactions between exciton-polaritons and a diverse manifold of singlet, triplet, and multiexciton states, using a model molecular spin conversion (singlet fission) system that is strongly coupled to an optical microcavity. In addition to the usual population dynamics, transient optical measurements on microcavities are sensitive to transient modifications of the exciton-polariton transition energies, exciton-photon coupling conditions, and thermal excitations of the cavity mirrors.

Vibration-cavity polaritons, which are produced by strong coupling between an optical cavity and a molecular vibration, can modify chemical reaction rates and branching ratios. However the observed effects are poorly understood. To gain insight into how these polaritons might alter molecular processes, we used ultrafast pump-probe and two-dimensional infrared spectroscopies to characterize polariton excited state dynamics. Our earlier studies on vibration-cavity polaritons with tungsten hexacarbonyl demonstrated that much of the response is due to so-called reservoir or uncoupled excited state absorption as well as polariton contraction. In recent studies, we have used 2D IR and spectrally filtered pump-probe studies on the nitroprusside anion in methanol to determine the transition frequencies and dynamics of polariton excited states allowing us to extract polariton dephasing timescales as well as incoherent polariton population which at a significantly longer timescale.

The RECTifying antENNA or rectenna device has been extensively studied for light-harvesting properties. This device utilizes the wave nature of the light through a nano-antenna and hence provides an alternative way for solar energy harvesting. In this work, we discuss nano-patch rectenna devices composed of silver nanocubes (antenna) and molecular diodes (rectifying element), which can be fabricated on a larger scale and at a lower cost than the counterpart clean room and lithographic techniques. In this work, we show the deposition of biferrocene alkane-di-thiolate molecules on template stripped gold substrates which serves three purposes: offer enhanced reproducible nano-gap, assemble patch antennas on the gold substrate and provide current rectification. This work explains both deposition of the molecular assembly through copper catalysed click reaction and characterization of the resulting SAM. We confirm the deposition and presence of both ferrocene units through cyclic voltammetry and ellipsometry. Finally, EGaIn junction measurements are performed with a plausible potential energy diagram.

Efficient energy transport is highly desirable for organic semiconductor (OSC) devices such as photovoltaics, photodetectors, and photocatalytic systems. However, photo-generated excitons in OSC films mostly occupy highly localized states over their lifetime. Energy transport is hence thought to be mainly mediated by the site-to-site hopping of localized excitons, limiting exciton diffusion coefficients to below ~10^-2 cm²/s with corresponding diffusion lengths below ~50 nm. Here, using ultrafast optical microscopy combined with non-adiabatic molecular dynamics simulations, we present evidence for a new highly-efficient energy transport regime: transient exciton delocalization, where energy exchange with vibrational modes allows excitons to temporarily re-access spatially extended states under equilibrium conditions. In films of highlyordered poly(3-hexylthiophene) nanofibers, prepared using living crystallization-driven self-assembly, we show that this enables exciton diffusion constants up to 1.1 ± 0.1 cm²/s and diffusion lengths of 300 ± 50 nm. Our results reveal the dynamic interplay between localized and delocalized exciton configurations at equilibrium conditions, calling for a re-evaluation of the basic picture of exciton dynamics. This establishes new design rules to engineer efficient energy transport in OSC films, which will enable new devices architectures not based on restrictive bulk heterojunctions.

The performance of photovoltaic and light-emitting devices that utilize singlet fission sensitization has been modest to date, despite spectroscopic measurements reporting high triplet exciton multiplication yields in the singlet fission active layer of these devices. This contrast highlights the need to characterize the factors that underpin device performance, such as triplet separation and diffusion. Here, we use ultrafast infrared spectroscopy to probe the dynamics and separation of correlated triplet pair intermediates following singlet fission in amorphous and crystalline pentacene films. The ultrafast vibrational measurements reveal that triplet-pair separation occurs on similar timescales in both types of films, despite differences in intermolecular coupling strength. Conversely, ultrafast electronic spectroscopy measurements of diffusion-controlled triplet-triplet annihilation reveal that triplet diffusion in the amorphous film is an order of magnitude lower than the crystalline analog. Together, these results suggest that sparse triplet traps limit the transport of triplet excitons in the amorphous film. Therefore, device developers should seek to identify the structural origins of these states to identify molecular structures that self-assemble in patterns that avoid triplet trap state formation.

Singlet fission (SF), the photophysical process converting an excited singlet exciton into two triplet excitons, is a promising approach to boost solar cell efficiencies. It is controlled by various parameters such as intermolecular interactions, energetics, entropy and vibronic coupling and a controlled modification of these parameters is key to a fundamental understanding. Blends of organic semiconductors present an interesting alternative to established methods of chemical functionalization and their potential for the study of SF pathways will be discussed using acene blends as example. Mixed thin films of SF chromophores and weakly interacting, high-bandgap spacer molecules allow one to study the impact of a replacement of nearest neighbors on the SF rates. While the SF rate in pentacene, for which SF is a coherent process, is unaffected by the introduction of spacer molecules into the film, we observe a significant decrease in the SF rate in tetracene, indicating incoherent SF. Mixing the two SF chromophores pentacene and tetracene with low pentacene concentrations leads to heterofission of a singlet on pentacene into two triplets on pentacene and tetracene, respectively, when selectively exciting pentacene. This heterofission process is outcompeted by pentacene homofission if the pentacene concentration exceeds 5%. Photoexcitation above the tetracene band gap additionally allows for energy transfer from tetracene to pentacene and results in complex dynamics.

Excited state dynamics play key roles in numerous condensed phase and molecular materials designed for solar energy, opto-electronics, spintronics and other applications. Controlling these far-from-equilibrium processes and steering them in desired directions require understanding of material’s response on the nanometer scale and with fine time resolution. We couple, in a unique way, real-time time-dependent density functional theory for the evolution of electrons with non-adiabatic molecular dynamics for atomic motions to model such non-equilibrium response in the time-domain and at the atomistic level. The talk will describe the basics of the simulation methodology and will discuss several recent applications, such as metal halide perovskites, metallic and semiconducting quantum dots, and transition metal dichalcogenides, among the broad variety of systems studied in our group.

2D perovskites, consisting of alternating layers of metal halide sheets and cations, tend to be more environmentally robust compared to their bulk 3D counterpart and have broad technological appeal because of their tunable mechanical, optical, and electrical properties. While these materials have promise for flexible optoelectronic applications, it is necessary to determine the impact of strain on the perovskite optical and electronic properties. Here, we discuss our work in understanding how strain modifies the carrier dynamics of 2D perovskites using time resolved spectroscopy. We compare the photoluminescence lifetime of two different 2D perovskite materials, synthesized using either phenethylammonium or butylammonium cations. Both perovskite materials exhibit about a 50% decrease in the lifetime for tensile strains <1%. The decrease in the photoluminescence lifetime, indicating a decrease in the charge carrier lifetime, is discussed in relation the materials defect states and bands

We present multi-pulse time-resolved terahertz spectroscopy study of lead halide perovskite materials. By heating carriers with infrared pulse and observed the cooling process, the carrier cooling dynamics are measured in the context of complex photoconductivity as a function of time and frequency. The result indicates non-Dude conductivity and the is discussed in conjunction with theoretical study based on polaron conductivity.

Despite a decade of research, much remains unknown about the role that grain boundaries play in determining the photovoltaic performance of perovskite solar cells. In this talk, I will describe how by combining experimental studies with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and depends on the grain area with small grains acting as recombination hot spots. We show that the distribution of grain sizes not only influences the overall performance of perovskite solar cells, but also leads to significant current exchange between small and large grains at open-circuit conditions.

Nowadays, perovskite Photovoltaic Solar Cells (PSCs) have attracted scientists’ attention due to the high- power conversion efficiency (PCE) and potentially low cost of manufacturing. However, the charge trapping and associated recombination processes negatively affect perovskite device performance. Here, we are using a new optical Pump-Push-Photocurrent (PPP) technique to study the carrier dynamic in perovskite devices. Particularly we are interested in the kinetics of charge trapping and nonradiative recombination in the device active layer. We have performed quasi-steady-state measurements of device performance under simultaneous illumination by visible and NIR laser diode. The preliminary results show that the effect of NIR push light scales linearly with NIR light intensity but depends in a more complex way from the intensity of visible light. By measuring the induced photocurrent as a function of light-modulation frequency we have performed an estimation of carrier lifetime. Moreover, the temperature-dependent current density measurements show that the effect of NIR light on the device performance does not originate from the sample heating but likely leads to the carrier detrapping.

Using transient optical spectroscopies, we study excitation recombination dynamics in manganese-doped cesium lead-halide perovskite nanocrystals. Unexpectedly, we find an increase in the intrinsic excitonic radiative recombination rate upon doping, which is typically a challenging material property to tailor. Supported by ab initio calculations, we can attribute the enhanced emission rates to increased exciton localization through lattice periodicity breaking from Mn dopants, which increases exciton effective masses and overlap of electron and hole wavefunctions and thus the oscillator strength. Our report of a fundamental strategy for improving luminescence efficiencies in perovskite nanocrystals will be valuable for maximizing efficiencies in light-emitting applications.

The electronically isolated f-orbitals of Ln3+ ions endow these species ultranarrow (atom-like) emission with a long lifetime, which are suitable for the optical generation and propagation of spin qubits, even after the coordination inside the semiconductor matrices. To overcome the low extinction coefficient of the Ln3+ ions, indirect sensitization of Ln3+ ions via energy transfer from the perovskite quantum dots (QDs) is performed through partial substitution of the perovskite matrix with Ln3+ ions. Here, we suggest a charge transfer type intermediate is involved in the energy transfer process, rather than utilizing conventional Forster or Dexter energy transfer. By comparing the static and dynamic process of the perovskite QDs doped with seven different Ln3+ species, we find that only Ln3+ species with low Ln2+ formation energy further advances the non-radiative recombination of the QDs’ delocalized charge carriers, which can potentially sensitize the Ln3+ excited states. The formation of the Ln2+ state naturally implies that energy transfer proceeds through sequential electron and hole transfer. The general mechanistic understanding of Ln3+ dopant sensitization opens the door for targeting multiple emission wavelengths by choosing the right combination of host matrices and Ln3+ species.

Functional materials combining the optoelectronic functionalities of semiconductors with control of the spin degree of freedom are highly sought after for the advancement of quantum technology devices and provide exciting avenues for polarized light-emission. Previous work towards this goal introduced small amounts of magnetic elements into crystalline semiconductor, e.g. through vacuum-based deposition, to obtain dilute magnetic semiconductors (DMS). In my talk, I will present our efforts on gaining control over spin dynamics and spin interactions through compositional and structural tuning in solution-processable hybrid perovskite semiconductors. We aim to exploit the exceptional optoelectronic properties of these hybrid perovskites, together with their tolerance in the electronic states to dopants and defects, to make advances towards high-performance DMS.

While interlayer triplet energy transfer has been studied in Ruddlesden-Popper 2D perovskites containing monovalent naphthalene cations, the photophysical properties of their Dion-Jacobson analogue have not been reported. Here we examine interlayer energy transfer in a series of mixed-halide Dion-Jacobson 2D perovskites containing divalent naphthalene cations. We find that sensitized phosphorescence in these compounds is dominated by naphthalene triplet excimer emission, but when the lead halide exciton is tuned near resonance with the triplet of naphthalene, emission from the naphthalene triplet monomer competes with triplet excimer formation. Interlayer energy transfer in these compounds is further supported by ultrafast transient absorption spectroscopy.

Recently, interactions between electronic and vibrational processes have been proposed to control various phenomena in a wide range of optoelectronic materials. Supposedly, these vibronic interactions may play the key role in physics of semiconducting materials for flexible soft photovoltaics by influencing optical, electrical, and other photovoltaic properties. Yet, their exact role in performance of real functional photovoltaic devices remains unclear, because of the current limitations of experimental and computational techniques. Here we develop a new method for studying vibronic interactions in functional optoelectronic materials based on the state-of-art combination of ultrafast spectroscopies and photocurrent detection techniques — photocurrent vibrationally promoted electronic resonance (photocurrent VIPER). The applicability of this technique is demonstrated by revealing the coupling of certain organic cation modes and inorganic lattice distortion in FaPbBr3 perovskite.

In this talk I will discuss recent collaborative work between the University of Washington, Notre Dame, and Oak Ridge National Laboratory in developing a new electron microscopy technique to perform nano-ellipsometry measurements on individual nanoparticles. Focus will be made on inverting the low-loss electron energy-loss spectrum to retrieve the complex-valued and frequency-dependent dielectric functions of a series of individual tin-doped indium oxide nanocrystals with tin doping concentration ranging from 1−10 atomic percent. This method, devoid from ensemble averaging, illustrates the potential for nanoscale electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

Semiconductor nanocrystals (NCs) have bright, narrow, and tunable emission across the visible and infrared. However, fundamental insight into NC photophysics is incomplete. At single-particle scales, NCs undergo fluorescence intermittency whereby photon emission switches between ON and OFF states with erratic kinetics. The range of timescales relevant to the excitonic lifecyle adds to the challenge of spectroscopically distinguishing between proposed mechanisms. Here, we demonstrate all-optical modulation of blinking statistics and discover that sub-bandgap light perturbs ON-times of NCs in a timescale-free manner. On the ensemble scale, NCs possess advantages when sensitizing spin-triplet states of surface-bound molecules for excitonic upconversion. In the energy transfer step, it remains unclear whether correlated transfer is outpaced by sequential electron transfer. Using tailored NC synthesis, we observe rich spectroscopic dynamics including the pervasive role of surface states.

Colloidal Semiconductor Nanocrystals offer a potential path forward to lowering the bar for access to Quantum Emitters. While demonstrations of single photons from nanocrystals have existed for two decades, intermittent periods of low light emission ("blinking") and transient emission from multiple emissive states ("spectral diffusion") have limited the usefulness of these sources. One underlying clue to the mechanism causing these two phenomena is whether or not these two effects are related. While evidence for blinking at fast timescales (~10s of us) is observable with modern electronics, the emission spectra cannot easily be captured at those timescales due to the inherent limitations of building up spectra in spectrometers. We utilize the indistinguishability of subsequent photons to determine the timescale spectral diffusion occurs and find that blinking can happen independently of spectral diffusion on the ~10us timescale, only becoming correlated at the ~1s timescale.

InP colloidal quantum dots have become widespread in luminescent colour conversion however further studies are necessary to understand the origin of the emitting state. Additionally, reports on optical gain are scarce and lack follow-up research. In this paper, we study the properties of InP/ZnSe and InP/ZnSe/ZnS quantum dots with Transient Absorption Spectroscopy. We propose a state-filling model were the band-edge bleach can be interpreted as the filling of the conduction and valence band edge states by delocalized holes and electrons. According to this interpretation, optical gain should be observed once the average exciton density is larger than 1. We explain this lower than expected threshold and the properties of the observed stimulated emission band by acknowledging that Stokes shift is the main spectral shift. The proposed exciton-phonon coupling leads to a unique mechanism where the stimulated emission band results from the counteraction between the absorption and photoluminescence bands. To fully take advantage of this mechanism, we propose that InP-based QDs with narrower emission lines and slower Auger recombination at higher pump intensities are needed.

Colloidal quantum dots, which have demonstrated its usage in optoelectronics in the visible range, are promising as infrared active materials as well. The wavelength-tunability by many variables including the nanocrystal size allows easy access to the target wavelength. Mostly, the bandgap transition is used for energy conversion. Self-doped quantum dots are the nanocrystal in which other electronic transitions are available under steady-state and ambient conditions. This talk will focus on the synthesis of the self-doped Ag2Se, HgSe QDs and their various properties including carrier dynamics studied by mid-IR spectroscopy. Additionally, several applications based on the intraband transition will be discussed.

Negative trions are exciton-electron quasiparticles that have applications in optical qubits and other quantum protocols. Colloidal cadmium selenide nanoplatelets are candidate trion materials, since trions dominate their low temperature emission spectra; however, trion formation is not understood. We observe the absence of trion absorption and a finite trion formation lifetime, requiring an alternative explanation for trion generation. Using low-temperature transient absorption (TA) and time-resolved photoluminescence, we propose that trions form through biexciton hole trapping. This contrasts the previously assumed mechanism of charged ground state absorption.

Combination of monolayer (ML) transition metal dichalcogenides (TMDC) with molecular layers with strong light matter coupling can enhance, control, and spectrally tune the absorption and emission of such heterostructures. Essential is hereby the energy level alignment at the heterointerface that governs the transfer of electronic excitations. At interfaces with a staggered type-II energy level alignment fast excited-state charge transfer has been utilized to enhance and spectrally expand the photoresponse of MoS2-based hybrid photodetectors. At interfaces with a straddling type-I energy level alignment, transfer of excitons on a sub-picosecond time scale results in an enhanced PL yield from ML-MoS2 in the heterostructure and an according overall modulation of the photo-response.

The increasing demand for efficient solar water splitting devices calls for a mechanistic understanding of trap carrier dynamics in actual working photoelectrodes. Here, we design a pump-push-photocurrent experiment to optically manipulate and detect the in-situ dynamics of trap carriers in a model photoelectrochemical cell comprising monoclinic bismuth vanadate (BiVO4) as the photoanode. We show that a near-IR (1064 nm) push pulse can be used to reactivate the pump-induced electrons that are trapped by oxygen vacancies. Meanwhile, the effect of oxygen vacancies on carrier transport is strongly affected by external bias condition. These studies enable us to better understand the role of defects in the performance of BiVO4 photoanodes, and could be used to guide the design of other promising photocatalysts.

The increasing requirement for efficient and sustainable energy generation technology demands an understanding of the mechanisms of photocurrent generation in photovoltaic systems. Device performances are largely governed by charge formation and transport dynamics as well as loss processes, which in some cases are poorly understood. Time-resolved spectroscopy is a powerful tool for detangling the origins of performance improvements and losses. We apply a combination of absorption, photoluminescence and photocurrent detection on representative photosystems, including photovoltaic materials and in vivo biofilms. By elucidating the key photophysical mechanisms in these systems, we hope to understand the pathways that determine the efficiency of photovoltaic materials and thus steer the progress of new technologies.

In the last 10 years, there has been a boom in the organic photovoltaic (OPV) community, with new devices achieving power conversion efficiencies (PCEs) of ~18%. This significant increase in device performance is due to a switch from polymer:fullerenes, such as P3HT:PCBM, to polymer:non-fullerenes, like PM6:Y6. These films are often created with the use of solvent additives which, when correctly chosen, increase the device performance. However, because many of these polymer:non-fullerene systems have only recently been developed the exact effect that these solvent additives have on the morphology, performance and electronic properties is not well understood. Here we use a combination of photoluminescence and time-resolved spectroscopy along with grazing-incidence wide-angle x-ray scattering to fully understand what is occurring at both an electronic and morphological level in a series of PM6:Y6 films with varying amounts of acetone used as a solvent additive. From these data we find that acetone changes the degree of mixing and crystal grain size, which leads to changes in polaron yield and recombination, which is reflected in device performance.

While fluorescent conjugated polymers are seeing increased use in optoelectronics, aggregation remains a roadblock due to emission quenching. It is therefore important to more fully understand the conditions that drive the emissive behavior. To properly interpret the spectral changes of P3HT from solvent poisoning, we employed a variety of single-molecule fluorescence techniques. In general, is impossible to assign spectral features in bulk emission spectra to the monomer versus aggregate or a mixture of different aggregate types without additional information. However, by measuring the difference in something like diffusion time as a function of emission wavelength, the spectral features that correspond to monomeric and aggregated chains can be assigned. Notably, these aggregates are highly emissive in the solid state though not in solution. The results suggest solvent poisoning provides a simple method of producing highly emissive aggregates from otherwise weakly emissive materials.

Triplet states are a key species that commonly form during the operation of organic electronic devices such as light emitting diodes and solar cells. Although traditionally seen as a loss mechanism in organic photovoltaics, the unique properties of triplets are increasingly being manipulated to enhance device efficiencies instead, through strategies such as singlet fission and up-conversion. Indeed, interest in triplet states has been mounting owing to their recently discovered prevalence in highly efficient non-fullerene acceptor blends. As such, it is important to elucidate triplet pathways in organic solar cells, in particular how they affect charge photogeneration and recombination. This talk will cover two blend systems with intriguing triplet / charge interactions.

The design of molecules and polymers for solution-deposited organic semiconducting materials generally considers the chemical modulation of (i) the π-conjugated backbone to modify the electronic and optical characteristics and (ii) the alkyl side chains to govern solubility. As the solid-state material forms, physical interactions among these constituents play an important, yet not well understood, role in directing the molecular-scale packing arrangements that in part determine the final material properties. In this presentation we will discuss how the dynamics of these moieties under different conditions, including the potential for conformational disorder among various points of torsion within the π-conjugated backbone, can impact aggregate formation and resulting solid-state morphology. The chemical insight developed through these investigations is beginning to refine and offer novel understanding essential to the development of next generation organic semiconducting active layers.

The question of how free charges are generated in donor:acceptor blends requires consideration that they are typically comprised of a complex phase morphology where intermixed and relatively phase-pure domains of the donor and acceptor co-exist. The local arrangement of the donor and acceptor plays thereby a decisive role in the fate of photogenerated electron-hole pairs –whether they dissociate to free charges or geminately recombine– as we demonstrate on a series of donor polymer:fullerenes binaries by combining 2D-NMR, time-resolved ultra-fast spectroscopy and detailed structural data. Our insights are important as similar considerations apply to other blends, such as semiconductor:dopant binaries that lead to highly conductive systems. We discuss how the spatial arrangement affects charge transport, and provide a tentative picture of the complex correlation of structure and electronic landscape towards the understanding of organic photovoltaics and doped, conducting plastics.

Hybrid organic electro-optic (OEO) devices consist of a layer of ordered organic chromophores confined between layers of metals or semiconductors, enabling optical fields to be tightly confined within the OEO material. The combination of tight confinement with the high electro-optic (EO) performance of state-of-the art OEO materials enables exceptional electro-optic switching performance in silicon-organic hybrid (SOH) and plasmonic-organic hybrid (POH) device architectures. Recent records in POH devices include bandwidths < 500 GHz and energy efficiency < 100 aJ/bit. However, optimization of device performance requires both understanding and improving the degree to which chromophores can be acentrically ordered near a metal or semiconductor interface. Applying bulk and/or isotropic models of OEO materials to nanophotonic device architectures often lead to overly optimistic translation of materials performance to device performance. Prior work has identified influences of high centrosymmetric order (birefringence), altered relations between acentric and centrosymmetric order (dimensionality), and surface electrostatics on chromophore ordering. We combine these models into a representation that can be used to understand the influences of these phenomena on device performance, how some prior OEO materials exhibited unusually high performance under confinement, how ordering close to surfaces may be improved, and implications for future electro-optic device design.

The spectral shape near the energy gap determines the radiative limit of the open-circuit voltage in organic photovoltaic devices. In this work, we employ ultrasensitive photocurrent measurements and detect sub-gap states with energies far below gap in a large number of different donor-acceptor blends. We provide evidence that these low-energy sub-gap states are associated with radiative mid-gap trap states, generating photocurrent via an optical release process. To account for the radiative mid-gap states, we implement a two-diode model which accurately describes both the dark current and the open-circuit voltage in organic solar cells. These findings provide important insights for our current understanding of organic photovoltaic devices.

Recent advances in organic solar cell material development based around non-fullerene electron acceptors in bulk heterojunctions have propelled power conversion efficiencies to >18%, with 20% on the horizon and 25% predicted. These efficiencies are close to traditional inorganic semiconductor photovoltaics and thus focus is now turning to manufacturability and creating a viable solar cell technology. In this presentation we report the highest efficiency to date (16% with a Fill Factor >70%) in a thick junction binary organic solar cell based upon PM6:BTP-eC9. Using a very accurate approach based upon temperature dependent ultra-sensitive EQE measurements we find that this system (and a similar one based upon PM6:Y6) have near unity charge generation yields (CGY > 99%). In this regime, we observe that a small increase in CGY of only 0.5% leads to a 2.5 times more reduction in bimolecular recombination relative to the Langevin limit enabling high efficiency thick junction solar cells.

Since the demonstration in 2018 that organic radicals can be used to make highly efficient organic light-emitting diodes there has been an explosion of interest in their capabilities and many experimental and computational studies of their performance. Here we take a theoretical view and describe the electronic structure of radicals from an algebraic perspective. By rediscovering and adapting historic investigations of organic radicals we show how many experimentally useful properties can be determined without synthesis or computation, but simply from knowledge of the molecular structure and in particular whether or not the radical is an alternant hydrocarbon. We explain these results in the context of modern organic light-emitting design in order to inform future investigations.

Strong light-matter coupling results in new eigenstates called polaritons which share properties of both light and matter and provide a useful way of modifying electronic energies. The energies of the new eigenstates depend on the concentration of molecules in the cavity. In this work, we change the concentration of Carbon Nanotubes (CNTs) in a Fabry-Perot cavity and achieve a maximum Rabi splitting of 4000 cm-1. The effects of concentration are studied with Transient Reflection (TR) spectroscopy, and we find that transfer rates between polariton states are enhanced by two orders of magnitude when their energy differences are resonant with the CNT G-band phonon energy of 1580 cm-1. The G-band phonon mode is also known to be important for exciton transfer between CNTs. The relaxation times of the system closely resemble those of CNTs outside of a cavity (~ 5 ps) which we attribute to the short lifetime of the cavity modes. The short-lived cavity modes cause the effects of the phonon mode resonances to be observed through peak intensities rather than kinetic traces. These results show that the G-band phonon mode is important for increasing excitation density in CNT polariton devices.