This PDF file contains the front matter associated with SPIE Proceedings Volume 11464, including the Title Page, Copyright information, and Table of Contents.

Transport of optical excitations in semiconducting solids plays a central role from both fundamental and technological perspectives. In systems with strong Coulomb interaction the propagation of optically injected carriers is dominated by excitons that can affect the energy landscape and the interactions with vibrational modes, with an overall strong impact on the mobility. Here, I will present recent studies of exciton diffusion in van der Waals semiconductors and hybrid two-dimensional materials, monitored via time-resolved optical microscopy. I will discuss linear and non-linear phenomena arising from efficient interactions across characteristic temperature regimes. Particular focus will be placed on the impact of interactions with vibrational modes and the influence of local fluctuations in the dielectric environment. Finally, I will discuss the interplay between free carriers and excitons and potential impact of quantum interference effects in exciton transport.

Semiconducting transition metal dichalcogenides (TMDs) continue to attract attention as components of optical devices due to remarkable refractive indices (e.g., n ≥ 4 for MoS2) from ultraviolet to near-infrared wavelengths. In recent years, TMD synthetic processes have advanced to provide sonication- and surfactant-free exfoliation methods. Such methods provide better access to high-yields of oxidatively-resistant TMDs as stable colloidal dispersions. However, inconsistent optical constants (i.e., refractive indices, n, and extinction coefficients, k) have been reported throughout the literature without clear attribution to TMD origin, exfoliation technique, or film processing procedures. Here, we offer insight toward understanding the nature of these reported discrepancies. As such, we derive broadband optical constants of redox exfoliated TMD films from 250nm – 20µm using variable angle spectroscopic ellipsometry. These data illustrate continuation of high n and low k values into the long-wavelength infrared regime. However, all the optical features (250nm – 20µm) are heavily dependent on both the selected raw material source (synthetic or natural) and accompanying post-processing conditions. These experimental optical features show significant changes in n ranging from 2.4 – 3.3. While the intrinsic lattice defect density is most likely to dominate TMD optical properties, residual species critical for exfoliation (i.e., polyoxometalates) are also suspected to contribute to variations in reported optical constants (e.g., extrinsic chemical dopant effects). Understanding such intrinsic and extrinsic optical property dependencies further expands the utility of redox exfoliated TMDs to expedite the development of next-generation optical devices.

Inhomogeneous and three-dimensional strain engineering in two dimensional materials opens up new avenues to straintronic devices for control strain sensitive photonic properties. Here we present a method to tune strain by wrinkling monolayer WSe₂ attached to a 15 nm thick ALD support layer and compressing the heterostructure on a soft substrate. The ALD film stiffens the 2D material, enabling optically resolvable micron scale wrinkling rather than nanometer scale crumpling and folding. Using photoluminescence spectroscopy, we show the wrinkling introduces periodic modulation of the bandgap by 47 meV, corresponding with strain modulation from +0.67% tensile strain at the wrinkle crest to -0.31% compressive strain at the trough. Moreover, we show that cycling the substrate strain mechanically reconfigures the magnitude and direction of wrinkling and resulting band tuning. These results pave the way towards stretchable multifuctional devices based on strained 2D materials.

Characterizing the intrinsic properties of low-dimensional transition metal dichalcogenides (TMDCs) is necessary for explaining how their novel properties arise and are modified by their local environment. Excitations in few-layer TMDCs and heterostructures are difficult to probe directly because of their low photoluminescence quantum yield. With time-resolved elastic scattering microscopy, we spatiotemporally resolve both in-plane and out-of-plane nanoscale transport in several TMDC species and architectures as a function of layer thickness and pump-induced carrier density. We directly observe interlayer exciton transport in TMDC heterostructures and find that these species diffuse an order of magnitude farther and faster than excitations do in their isolated counterparts.

In emerging photovoltaic and photocatalytic systems, correlated electron-hole excitations called excitons often serve as carriers in energy transfer processes. Structural complexities, such as reduced dimensionalities, interface compositions, and the presence of impurities, are closely coupled to exciton properties and decay processes. In this talk, I will describe a computational approach to study the excitonic phenomena in materials of complex structures, using ab initio many-body perturbation theory. I will specifically discuss many-body effects on optical and exciton phenomena in and between layered transition metal dichalcogenides, where a mixed nature of electron-hole interactions control the optical signatures and structurally-tunable selection rules. I will further present a new approach to study exciton decay processes in such functional materials from first principles, employing a rate-equation perturbative scheme to exciton-exciton and exciton-phonon interactions.

Electronics that can be stretched and feature skin-inspired functionalities are opening doors for opportunities in health and environmental monitoring, sustainability, and next-generation consumer products. Degradability is an important attribute for applications on dynamic surfaces where manual recovery would be logistically or financially unpractical. A key step to realize such electronics is the development of a stretchable and degradable transistor with electrical performance independent of large mechanical stress. Herein, we decouple the design of stretchability and transience by harmonizing polymer physics principles and molecular design in order to demonstrate for the first time a material that simultaneously possesses three disparate attributes: semiconductivity, intrinsic stretchability, and full degradability. This polymeric system represents a promising advance towards developing multifunctional materials for skin-inspired electronics.

Semicrystalline polymers are an ubiquitous class of materials incorporated into everyday life, and their functional properties are contingent on their structure over a hierarchical range of length scales. By correlating micro- and nano- structural crystalline and amorphous components of lithium triflate doped PEO thin films with fluorescent single particle trajectories, we find that crystalline fibers anisotropically constrain probe transport without altering the intrinsic diffusivity of probes. Our findings suggest controlled and periodic arrangement of crystalline fibers is a promising design principle for mass transport in semicrystalline polymers that enables the requisite mechanical stability for device applications.

Single organic molecules hold great promise for generating, manipulating, and storing single photons. Such processes form the basis for new quantum-enhanced technologies such as sensing, communications, and computation. In this talk I will focus on a particularly promising molecule – dibenzoterrylene (DBT). When DBT is introduced into a solid crystal of anthracene it is photostable and emits light between 780 and 795 nm. When cooled to cryogenic temperature, DBT is isolated from phonon-induced dephasing meaning that the photons have a lifetime-limited. I will present recent results in growing DBT-doped anthracene crystals, introducing these to nanophotonic interfaces for enhancing the collection of photons, tuning the DBT emission wavelength to coincide with rubidium atomic absorption, and finally pump-probe spectroscopy experiments to find long-lived triplet states in DBT which will be useful for building a single-molecule quantum memory.

Here, using single-material OPV device and laser system with sub-10fs time resolution, we track in time the formation of localised excitonic states. For this we employ a combination of pump-probe (PP) spectroscopy, sensitive to concentration of excited states, and pump-push-photocurrent (PPPC) technique, sensitive to the state localisation. Combining both methods to monitor charge dynamics at real operation condition allows to separate and track the evolution of strongly bound and spontaneously dissociating excited states. Our data show that PP and PPPC measurement do not follow the same trend, and the discrepancy between the states probed by PP and PPPC indicates that excitons acquire localised character within first 50fs after formation. Results may be useful for a new realisation of efficient donor-acceptor OPV design.

The integration of furan based repeat units into conjugated systems meant for optoelectronic applications has generally been limited by the photostability of the furan unit. This limitation is due to the susceptibility of furan towards reaction with singlet oxygen, which disrupts the conjugation of the system. Here, we present a family of helical, ester-functionalized polyfurans with dramatically enhanced photostability. Within this family, the emission intensity of P3HEF is essentially non-existent while a chiral branched variant, (S)-P3EHEF, is highly fluorescent. This discrepancy is due to the difference in the compactness of the helical structure of the two polymers. Interestingly, the emission wavelength of (S)-P3EHEF can be tuned through several different techniques such as deposition speed and solvation conditions. The mechanism behind the tunability was explored using fluorescence-based techniques.

Organic solar cells are undergoing a revival with the discovery of non-fullerene electron acceptors, achieving relatively high photovoltage and light harvesting yields compared to older fullerene systems. There are however scientific questions regarding their excited state dynamics which continue to be unresolved including the role of singlet and triplet excited states before and after charge generation at the electron donor-acceptor interface in the device photoactive area. The presentation will discuss our recent progress in understanding the diffusion of excited states in non-fullerene electron acceptors before charge transfer and the charge dynamics after generation, based on time-resolved laser spectroscopy experiments covering timescales from femto- to microseconds. Our analysis sheds light on the importance of both interfacial energetics and molecular packing on the excited state dynamics.

At present, white light-emitting diodes (WLEDs) are widely used in display backlights. Commercialization of WLEDs are produced by using blue LED to excite phosphors, which with different colors resulting in a color gamut is only 75 % of the NTSC standard area due to a larger full width at half maximum (FWHM) of phosphors. Because of quantum dots (QDs) have narrow FWHM that can replace traditional phosphors as promising materials for white light backlights. Although inorganic perovskite CsPbBr3 green QDs have narrow FWHM, they are limited in WLED applications due to their low stability. In order to solve the above problem, silica coating is used to passivate the surface. The results show that the emission wavelength of CsPbBr3 QDs is redshifted due to agglomeration after coating with silica. After coating, the thermal stability of the sample has 24 % of improvement. The color gamut of WLEDs obtained by mixing green QDs with red phosphors (K2SiF6:Mn4+, KSF) are 131.5 % of NTSC for as-prepared sample and 124 % of NTSC for silica-coated sample. Compared above result with LAG-based WLED, the color gamut can be increased by 40 %. The WLED stability can be enhanced 12 times after coating. These results confirm that the color gamut of WLED obtained by mixing narrow bandwidth of KSF red phosphor and CsPbBr3 green QDs show a high NTSC to 131.5 %. As QD is coated with silica, the stability of QDs and WLED can be improved very significantly. This result is beneficial to the application of the CsPbBr3 green QD in the high color gamut display.

The ability of energy carriers to move within and between atoms and molecules underlies virtually all material function. Understanding and controlling energy flow requires observing it on ultrasmall and ultrafast spatiotemporal scales, where energetic and structural roadblocks dictate the fate of energy carriers. I will describe a new optical ultrafast microscope based on stroboscopic elastic scattering that allows direct visualization of energy carrier transport in 3D with few-nm spatial precision and picosecond temporal resolution. I will demonstrate the wide applicability of the method for watching all forms of energy carriers – free charges, excitons, phonons and ions – move in materials ranging from silicon to conjugated polymers via 2D transition metal dichalcogenides and metal halide perovskites. Beyond quantifying carrier mobilities, our approach directly correlates material resistivities to local morphology, shedding light on how disorder affects transport pathways in 3D.

Scanning electron microscopy (SEM) correlated to transient absorption microscopy (TAM) is used to study heterogeneities in thin films of 3D perovskites. Statistical analysis of thousands of distinct spatial locations reveals the effects of grain boundaries and crystal size on carrier dynamics. Further, a new class of 2D perovskite materials is discovered by exploiting alloying of the organic cation. Spectroscopic and theoretical studies show that such structural deformation leads to a blue-shifted bandgap, sub-bandgap trap states with wider energetic distribution, and stronger photoluminescence quenching. These results and methods provide new insights for understanding the structure-property relationship in perovskite materials.

Hybrid inorganic-organic perovskites stand out as unique photovoltaic materials due to their exceptional optical and electronic properties. Here, we study in-plane carrier diffusion in perovskite thin films via time-resolved imaging of the photoluminescence from a diffraction-limited spot. Fitting to a 3D diffusion model allows extraction of both diffusion and recombination coefficients. Our results show two regimes of carrier diffusion, with a very rapid spreading large D = (0.968 ± 0.003) cm²s^-1 observed at short times, within a measurement window of ~1 ns, and a much smaller D = (7 ± 1) × 10^-3 cm²s^-1 in a window of 200 ns. To investigate the rapid spreading at short times, we explicitly consider the effect of photon recycling within the plane of the film. We find that photon recycling has a negligible impact on such measurements, with the exception of a small increase in the intensity of the tails of the luminescence spot.

Transient absorption spectroscopy can measure exciton dynamics and provide insight into the electronic structure of nanocrystals (NCs). This spectroscopy, however, is typically limited by the long timescales required for acquisition of transient spectra, preventing the accurate measurement of systems that are not at a structural equilibrium. The structure of NCs changes during their synthesis on a shorter timescale than that required for measurement, making it difficult to study the evolving photophysics of NCs during growth. Here, we leverage a single-shot transient absorption (SSTA) spectrometer, capable of recording transient spectra with excellent signal-to-noise in less than a minute, to measure exciton dynamics in growing NCs. The presence of internal electric fields caused by surface-trapped carriers is evident in the TA lineshape, where a distinct Stark effect is observed revealing that growing NCs are poorly passivated. This work will enable a range of future experiments to study charge carrier behavior in rapidly evolving NC systems.

Organic semi-conductors are widely used in the development and manufacturing of certain optoelectronics. However, these materials are susceptible to photodegradation in the presence of oxygen. This is due to a polymer’s populated triplet state creating singlet oxygen. In recent years, the use of plasmonic metal nanoparticles in the polymer systems or deposited on a substrate, have yielded polymer films that degrade much slower than films with the polymer alone, as long as there is good overlap with the plasmon of the metal and the emission of the polymer. Since this overlap is crucial, tunability of the plasmon is essential to “fit” various polymer systems. The research presented here provides methodology for the facile manufacturing and tuning of metal deposits for such purposes. Not only this, but through increasing the tunability of these plasmons we are able to better image various emissive pathways and species better in a polymer system deposited on film.

Strong coupling (SC) between light and matter has emerged in the last decade as a promising tool to control room-temperature photophysical processes in organic molecules. In this article, we aim to provide a pedagogical introduction to the various flavors of molecular SC involving (a) a single molecule in an optical nanocavity (e.g. a plasmonic junction), and (b) many molecules in an optical microcavity (the collective regime). Although the linear optical properties of these two systems are very similar, their chemical dynamics are drastically different from each another. We will highlight the relevant timescales and rates that can be manipulated via both flavors of SC. We will illustrate these ideas with theoretical and experimental examples from our previous work, which will help us distill the physical mechanisms that are at play in each SC case.

Quantum dynamics of the photoisomerization of a single thiacynine iodide molecule embedded in an optical microcavity was theoretically studied. The molecular model consisting of two electronic states and the reaction coordinate was coupled to a single cavity mode via the quantum Rabi Hamiltonian. We show that an electronic excitation of the molecule at cis configuration is followed by the generation of two photons in the trans configuration upon nonadiabatic isomerization. Although conditions for this phenomenon to operate in the collective strong light-matter coupling regime were found to be unfeasible for the present system, our finding suggests a new mechanism that, without ultrastrong coupling, achieves photon down-conversion by exploiting the emergent molecular dynamics arising in polaritonic architectures.

Polariton modes in organic semiconductor microcavities traditionally derive from singlet exciton states that possess no net charge or spin. This talk will explore the properties and prospects of charged polariton states that originate from cationic excitations in a heavily doped organic semiconductor. In addition to new electric and magnetic properties associated with their net charge and spin, charged polaritons are shown to be a useful platform for exploring cavity-modified photoinduced electron transfer and the mechanisms that underlie it.

Solution processed quantum dot (QD) lasers are one of the holy-grails of nanoscience. They are not yet commercialized because the lasing threshold is too high: one needs < 1 exciton per QD, which is hard to achieve due to fast non-radiative Auger recombination. The threshold can however be reduced by electronic doping of the QDs, which decreases the absorption near the band-edge, such that the stimulated emission (SE) can easily outcompete absorption. Here, we show that by electrochemically doping films of CdSe/CdS/ZnS QDs we achieve quantitative control over the gain threshold. We obtain stable and reversible doping more than two electrons per QD. We quantify the gain threshold and the charge carrier dynamics using ultrafast spectroelectrochemistry and achieve quantitative agreement between experiments and theory, including a vanishingly low gain threshold for doubly doped QDs. Over a range of wavelengths with appreciable gain coefficients, the gain thresholds reach record-low values of ~10^-5 excitons per QD. These results demonstrate an unprecedented level of control over the gain threshold in doped QD solids, paving the way for the creation of cheap, solution-processable low-threshold QD-lasers.

As the field of semiconducting quantum dots (QDs) continues to mature, the dispersity of nanocrystal sizes present in a synthesized sample is still an obstacle. Because the properties of QDs are size dependent, it is crucial to produce monodisperse QD samples to understand structure-property relations. Magic-sized clusters (MSCs) circumvent the polydispersity seen in QDs, as growth is discrete and limited to only certain sized clusters. In spite of their promise, MSCs remain poorly studied. MSCs typically exhibit broad emission with low photoluminescence quantum yields (PLQY). This presentation will describe our efforts towards CdSe MSCs with sharp, high efficiency PLQY through the growth of a passivating shell.

In recent years, research effort has been devoted to the generation of hybrid materials which change the electronic properties of one constituent by changing the optoelectronic properties of the other one. The most appealing and commonly used approach to design such novel materials relies on combining organic materials or metals with biological systems like redox-active proteins. Such hybrid systems can be used e.g. as bio-sensors, bio-fuel cells, biohybrid photoelectrochemical cells and nanosctuctured photoelectronic devices. Although experimental efforts have already resulted in the generation of a number of hybrid bio-organic materials, the main bottleneck of this technology is the formation of a stable and efficient (in terms of electronic communication) interface between the biological and the organic/metal counterparts. In particular, the efficiency of the final devices is usually very low due to two main problems related to the interfacing of such different components: charge recombination at the interface and the high possibility of losing the function of the biological component, which leads to the inactivation of the entire device. Here, we present a multiscale computational design which allows the study of complex interfaces for stable and highly efficient hybrid materials for biomimetic application, consisting of single layer graphene (SLG) as organic material/metal and small light harvesting protein complex as biological counterpart, linked together via a self-assembly monolayer (SAM), in order to create novel biomimetic materials for solar-to-fuel, bio-transistors or bioorganic electronic applications.

Hybrid metal halide perovskites have shown increasing success as active layers for photovoltaics cells, although concerns about water stability remain a hindrance to widespread commercialization. Two-dimensional, Ruddlesden-Popper phase perovskites, which incorporate larger, more hydrophobic organic cations, have been proposed as an alternative. Although they demonstrate increased water stability, they have increased exciton binding energies and thus decreased charge-carrier mobilities and charge-carrier lifetimes. To fully understand the limits that these properties impose, the interplay between exciton and free charge-carrier formation dynamics is examined.

There is an increasing interest in 2D perovskites for solar harvesting and light-emitting applications due to their superior chemical stability as compared to bulk perovskites. However, the reduced dimensionality in 2D perovskites results in excitonic excited states which dramatically modify their optoelectronic properties. While the carrier dynamics in bulk systems is increasingly well understood, a detailed understanding about the spatial dynamics of excitons in 2D perovskites is lacking. Here, we present the direct measurement of the diffusivities and diffusion lengths of excitons in 2D perovskites, revealing both the spatial and temporal exciton dynamics. We find that changing the organic spacer, cation or dimensionality of the perovskite yield dramatically different diffusivities, due to strong exciton-phonon interactions and potentially the formation of large exciton-polarons. Our results provide clear design parameters for more efficient 2D perovskite solar cells and LEDs.

2-Dimensional metal Halide Perovskites (2D-HP) are at the limelight for their potential exploitation in light-emission related applications. In particular, the most-investigated <001<-terminated 2D HP family shows dominant narrow light emission, with reduced Stokes shift, of great interest for display applications. In parallel, these systems often show additional largely Stokes-shifted emission, with reduced spectral resolution, of interest for the lighting application, e.g. development of white light-emitting diodes. Clarifying the emission mechanisms in 2D-HP and explaining the coexistence of these two contrasting emission regimes is greatly coveted, for the further exploitation of this class of semiconductors. Here, Density Functional Theory (DFT) simulations estimate total electron-phonon interaction in 2DHP in the order of few tens of meV, in agreement with ps-resolved UV-vis measurements, consistent with the reported narrow emission. On the other hand, such small coupling significantly contrasts with the assignment of broadband emission to some form of intrinsic (defect-free) self-trapped exciton. Additional DFT simulations rather assign broadband emission to extrinsic self-trapping, associated to point defects, halide interstitials in particular. This result is in line with similar findings for the parental 3D halide perovskite and highlights, on the one hand, the role of halide defects in lead-halide perovskitoids frames, and explains, on the other hand, the apparent contrasting nature of narrow and broadband emission, as due to two different emission mechanisms. Defect engineering protocol are therefore suggested, to optimize broadband emission in 2D-HP materials.

Thermally activated delayed fluorescence (TADF) has emerged as a competitive approach to provide a route towards highly energy-efficient OLED lighting and display applications. Our results show that organometallic complexes and in particular carbene metal amide materials (CMA) are an effective platform to control fundamental photophysical properties such as excited state lifetime and photoluminescence quantum yields. Comparison and key differences of the various CMA materials will be discussed to showcase the role of the metal and organic ligands, steric and electronic factors or result of their interplay. On the basis of CMA materials, we demonstrate the molecular design strategy based on twisted and tilted emitter geometries between donor and acceptor ligands to realize highly efficient TADF materials beyond the conventional co-planar approach applied in organic materials.

Room temperature phosphorescence (RTP) organic emitters have gained increasing attention in the fields of lighting, security and bioimaging. However, lack of clear mechanism and universal methods of regulating RTP property impeded the further development of RTP materials. Most importantly, the long lifetime and high quantum yield (QY) of RTP can hardly be achieved at the same time. Herein, we report a molecular design strategy to improve both lifetime and QY of RTP through intramolecular interaction. The enhancement of lifetime and QY were up to one order, respectively by introducing S···O noncovalent intramolecular interactions into the molecule. The X-ray single crystal analysis and theoretical calculations were performed together to give the solid evidence that the intramolecular interaction can not only suppress the nonradiative transition but also stabilize the triple states. This study provided a novel idea of developing high-performance RTP materials.

Thermally Activated Delayed Fluorescence (TADF) process is the new paradigm for Organic Light-Emitting Diodes. Still, a complete mechanistic understanding of TADF materials is not yet uncovered. It arises partially from the dichotomy between the need for small energy difference between the lowest singlet and triplet excited states (dEST) which have to carry a significant charge transfer (CT) character and for a significant spin-orbit coupling which requires these excited states to have different natures. In this contribution: (i) We will demonstrate that, the electronic excitations involved in the TADF process have a mixed CT-locally excited character being dynamically tuned by vibrational modes which assist upconversion and light emission. (ii) We will show, unlike conventional TADF emitters, how color purity, small dEST and high photoluminescence quantum yield in boron-centered azatriangulene-like molecule is achieved and how a negative dEST is obtained for these compounds.

Singlet exciton fission is a molecular process wherein an optically prepared singlet exciton dissociates to yield to two triplet excitons. The pair of triplets are formed on sub-picosecond timescales, leading to the assumption that the process is spin-allowed. That is, the pair of triplet excitons are in an overall singlet spin state. Recently however, we have observed a quintet triplet pair state, demonstrating that spin is not conserved. In order to completely understand this process and explain recent spectroscopic measurements, one needs to consider a time-varying spin Hamiltonian. In this presentation I will summarize recent spin-sensitive spectroscopic measurements and outline our theoretical approaches to understanding singlet fission.

Singlet fission can split a high energy singlet exciton and generate two lower energy triplet excitons. This process has shown near 200 percent triplet exciton yield. Sensitizing solar cells with singlet fission material, it can potentially increase the power conversion efficiency limit from 29 percent to 35 percent. Singlet fission in the tetracene is known to be efficient, and the energy of the triplet excitons are energetically matched to the silicon bandgap. In this work, we designed an optical measurement with an external magnetic field to determine the efficiencies of triplet exciton transfer from tetracene to silicon. Using this method, we have found that a passivation layer of 8 angstroms of hafnium oxynitride on silicon allows efficient triplet exciton transfer around 133 percent.

The ability to efficiently up-convert broadband, low-intensity infrared light to the visible would be an enabling technology for 3rd-generation photovoltaics, biological imaging, and sensitizing silicon focal plane arrays. Our approach uses PbS colloidal quantum dots to absorb infrared photons and sensitize the long-lived spin-triplet excited states of nearby semiconducting molecules, where excitations can combine via triplet fusion to create visible light emission. However, there is more to be done. For instance, energetic disorder in films of size-disperse quantum dots presently hinders transport and hampers low-intensity performance. Here, I will show that process additives can control a cluster intermediate in the synthesis of PbS quantum dots, yielding markedly narrower ensemble linewidths. Then, I will discuss why recent photophysical experiments on a novel molecular dimer suggest that the spin-statistical efficiency limit on fusion can be lifted from 25% to 66% in solution.

Silicon is an earth-abundant, inexpensive and non-toxic material. While decades of research have led to its prominence in the microelectronics industry, much less is understood about nanostructured silicon. The use of an indirect-gap material like silicon to absorb light for triplet fusion based photon upconversion proposed has no precedent and is fundamentally interesting. The benign nature of silicon will facilitate rapid adoption in biomedical applications where visible light must be delivered deep in tissue. The multi-excitonic processes investigated here may pave the way to using singlet fission to exceed the Shockley-Queisser limit in photovoltaics. In this talk, the structure property relationships governing triplet energy transfer between molecules and silicon colloids will be elucidated. Transient absorption experiments, in parallel with photon upconversion measurements using continuous wave irradiation, will reveal the mechanism of energy transfer. The relationship between the structural details and the optical properties will guide the rational design of silicon nanoparticle based light absorbers for photon upconversion.

Using time-resolved cathodoluminescence imaging, we measure the pixel-by-pixel cathodoluminescence decay of Mn2+ dopants in cesium lead chloride perovskite microplates. This measurement generates a spatially resolved map of the excited state decay dynamics of the Mn²⁺ dopants, which suggest an explanation for enhanced Mn²⁺ emission near the surface of the microplate. Near the surface, the contribution from the longer lifetime component increases, which implies that the population of excited Mn²⁺ is higher near the surface. This may arise due to the increased probability of carrier recombination at a Mn²⁺ dopant near the surface, possibly enabled by an increased concentration of traps.