Despite years of intense study, even today there are many novel semiconductor materials whose unique properties raise fundamental scientific questions about the relationship between structure and function. Two examples are organic semiconductors and semiconducting nanostructures. Organic semiconductors can be solution processed at room temperature, leading to thin films that are flexible, inexpensive, and more sustainable when compared to inorganic alternatives. Yet solution processing leads to complex microstructures, and the resulting polymorphism, interfaces, and defects all affect the material's behavior in complex ways. In semiconducting nanostructures, the novel behavior arises due to their nanometer-scale dimensions. Confinement and enhanced surface interactions affect the electronic structure and dynamics in sometimes unexpected ways and can produce new physics.

Chapters 1 and 2 provide the necessary background for this dissertation. Chapter 1 provides an overview of semiconductors and what makes organic semiconductors distinct. The dynamic processes that are relevant for subsequent chapters are introduced. Chapter 2 describes a home-built transient absorption (TA) microscope that was used in each project discussed here. TA is an important tool for studying a material's ultrafast excited state dynamics, and by combining TA with microscopy we have been able to investigate how those dynamics depend on the local morphology.

Chapters 3 and 4 discuss the use of TA microscopy to probe the ultrafast electronic dynamics of individual crystalline domains of organic semiconductors. In Chapter 3 we find that different domains of the material diF-TES-ADT can display different dynamics. We fit a kinetic model to the observed behaviors, quantify the amount of heterogeneity, and propose that the observations are due to polymorphism. In Chapter 4 we study singlet fission in single crystalline domains of the material TIPS-Pentacene. During singlet fission, a singlet exciton splits into two triplet excitons, initially forming a short-lived and poorly understood correlated triplet pair. By exploiting the inherent anisotropy within an individual crystalline domain we obtain unprecedented insight into nature of the correlated triplet pair. We resolve a disagreement related to its formation timescale, quantify the triplet-triplet binding energy, and measure how the charge transfer character of the correlated triplet pair perturbs its electronic structure and hence absorption spectrum.

Chapter 5 takes a step back to study the self-assembly process in the organic semiconductor rubrene. We use a variety of spatially- and temporally-resolved techniques to study the growth and morphology of spherulites, a kinetically trapped polycrystalline structure that could allow for both rapid and uniform charge transport. We use time-resolved wide-angle in-situ X-ray scattering, wide-angle X-ray microdiffraction, and atomic force microscopy to identify shear strain localized along lines of crystalline misorientation. This strain templates upon nucleation and is kinetically trapped only at higher annealing temperatures, possibly promoting the spherulite structure. Steps to extend this project are also described, using scanning transmission electron microscopy to measure the crystal structure with nanoscale resolution and TA microscopy to connect this local structure with electronic dynamics.

Chapter 6 looks at nanowires of the inorganic perovskite CsPbBr3, in which quantum confinement along two dimensions changes the excited-state dynamics. We align many nanowires into micron-scale bundles, which allows them to be studied with optical techniques while still retaining anisotropic behavior. We use TA microscopy to study the polarization-resolved ultrafast electronic dynamics of the nanowires, and find among other things that the nanowire geometry splits the degeneracy of the lowest excited states. We also use stroboscopic interferometic scattering to observed exciton diffusion within nanowire bundles, and measure the diffusivity both along and between the nanowires.

Taken together, this dissertation illustrates specific examples of the relationship between semiconductor structure and function. Small changes in processing conditions can drastically affect morphology, and small changes in morphology, whether they be defects, interfaces, or tweaks to the unit cell, can drastically affect electronic properties. Understanding these relationships is vital to incorporating novel semiconducting materials into next-generation optoelectronic devices.

Emerging semiconductors have begun to rival conventional silicon with breakthroughs in improved performance and novel and affordable processing techniques. As the applications for emerging semiconductors are explored, there is a clear need to understand the hierarchical nature of these materials to fully optimize their functionality. Consistently reproducingthe assembly and electronic functionality of organic semiconductors, for example, remains difficult given their propensity for variable molecular packing and morphology due to weak bonding interactions. As another case, metal halide perovskite semiconductors demonstrate intriguing photophysics that may change when confined in nanostructure forms. One technique alone cannot comprehensively interrogate the repercussions of changes in the molecular structure to the microstructure, morphology, and ensuing electronic functionality of a semiconductor. Only a multimodal characterization approach enables an extensive multiscale insight into the structure-function dynamics of semiconducting materials and devices.

Chapter 1 first introduces organic semiconductor properties and device applications. Common thin film processing methods and modification strategies are discussed along with the formation of crystalline microstructures in thin films. Next, a suite of characterization techniques that span multiple length and time scales are shared to inform on how one maystrategically combine individual techniques to characterize structure, morphology, and dynamics.

Chapter 2 describes a multimodal imaging study of organic semiconductor, rubrene, thin films. Scanning transmission X-ray microscopy reveals and quantifies the disappearance of orientational discontinuities in a hybrid crystalline microstructure, which is corroborated at higher spatial resolution with 4D-scanning transmission electron microscopy. In situ polarized optical microscopy during thermal annealing uncovers Arrhenius behavior for rubrene crystallization and that the finite substrate thermalization rate leads to the formation of the observed hybrid crystalline microstructure. This study emphasizes the role of temperature in organic semiconductor crystallization, and highlights the challenges and opportunities for thin film design protocols.

Chapter 3 focuses on characterizing the anisotropic behavior of inorganic lead halide perovskite (CsPbBr3) nanowires and nanowire bundles. Stroboscopic scattering interferometric microscopy shows exciton diffusion and trapping in the nanowire bundles. We use transient absorption microscopy to probe the polarization-resolved excited-state dynamics of thenanowire bundles. Coupled with polarization-resolved fluorescence, we determine that the splitting of the band-edge exciton states is driven by the shape anisotropy and a long-range exchange interaction. This study demonstrates the power of optical pump-probe microscopies to resolve charge transport and excited-state dynamics in individual structures.

This dissertation highlights the complexities of characterizing soft semiconductor thin films and nanostructures across multiple length and time scales. Despite the challenges in implementing a multimodal approach, the knowledge acquired about the semiconductors, such as defects in packing, grain boundaries, and charge carrier dynamics, is invaluable for guiding future design principles that incorporate semiconducting materials into novel technology.

There are many types of excitations that may coexist and interact after light illumination in a semiconductor. In emerging semiconductors like low-dimensional and solution-processed materials, these excitations often encounter nanoscale disorder over their lifetime, including point defects, different crystalline phases, boundaries, interfaces and dielectric disorder. In addition, distinguishing between different types of excitations is vital for teasing out the intrinsic material properties so that they may be tuned and optimized for optoelectronic device applications. Optical scattering is a sensitive probe that, when incorporated into a time-resolved pump-probe microscope, correlates sample morphology to nanoscale energy transport. The unique spectrally-dependent scattering contrast from different photoexcitations and their spatiotemporal evolution together permit the distinction between coexisting excitations. We call this approach stroboscopic scattering microscopy (stroboSCAT) and demonstrate its versatility across a range of organic, organic–inorganic, and inorganic semiconductors, enabling model-free structure–function correlations with few-nanometer precision in all spatial dimensions.

Chapter 1 provides an introduction to the two classes of materials that are explored in this dissertation with an emphasis on the 2D material archetype, molybdenum disulfide (MoS2). Next, we introduce the playing field of energy flow microscopies well-suited to study the energy transport and transduction in emerging semiconductors.

Chapter 2 explores the theoretical underpinnings and practical operation of the stroboSCAT microscope. We provide background on two approaches for understanding the optical properties of materials: scattering and frequency-dependent complex dielectric functions. We describe the photoinduced changes to the optical response and how they may be probed with transient scattering approaches in transmission or reflection. After a crash course in basic microscopy principles, the nuts and bolts of the stroboSCAT setup are described in detail, including its timing control, resolution limits, and stability.

Chapter 3 illustrates how stroboSCAT is a versatile technique that directly correlates microscopic structural motifs and macroscopic function across a broad range of materials. In TIPS-Pentacene, a molecular crystal, exciton transport is hindered by material interfaces. In a thin film halide perovskite, charge carriers find their way around resistive morphological boundaries using 3D trajectories. Finally, in silicon, oppositely signed contrast for charge carriers and heat, along with a separation in time scales, enables isolating the dynamics of each population even though they overlap.

Chapter 4 takes stroboSCAT into the 2D realm in several layered architectures and an in-depth exploration of exciton and heat transport and transduction in four-layer MoS2. We combine near- and far-from resonant stroboSCAT measurements with temperature-dependent reflectance contrast spectroscopy to isolate overlapping exciton and heat distributions. This study demonstrates the importance of access to spatially resolved information and tunable contrast for directly discerning photoinduced heat from charge without complex models or assumptions.

As summarized in Chapter 5, this dissertation illustrates the advantages of using transient light scattering to probe photoinduced changes in the optical response of a wide range of novel semiconducting materials. With high spatiotemporal resolution and sensitivity, tunable imaging contrast and direct structure-function correlation, stroboSCAT is a powerful tool for uncovering the intricacies of energy flow on relevant length- and time scales without averaging over heterogeneous energetic landscapes or misinterpreting the emergent dynamics when different excitations coexist.

This thesis focuses on the development and application of new methods for spatially and temporally resolving measurements of exciton dynamics and migration in organic semiconducting thin films, which are commonly used in organic field effect transistors (OFETs), organic light emitting diode (OLEDs), and organic photovoltaics (OPVs). Three methods of probing exciton dynamics and migration on length scales better matched to the structural heterogeneity in organic semiconductors are presented. First, the benefits of spatially resolving ultrafast dynamics are explored, by employing transient absorption microscopy on single domains of polycrystalline films of 6,13-bis-(triisopropylsilylethynyl)-pentacene (TIPS-Pn), revealing a polarization dependence that significantly aids the assignment of the excited state dynamics. A full kinetic model of population dynamics, as a function of both polarization and time, is developed and fit to the experimental data, where the polarization dependence provides a several fold increase in the number of constraints for the fitting routine. The global fitting analysis successfully reproduces the experimental data, and the observed dynamics are determined to include ultrafast thermalization of the initially hot exciton in ∼ 50 fs, followed by singlet fission in the first few picoseconds, and then internal conversion over several hundred picoseconds. The success of the kinetic model and the assignment of the dynamics are direct results of the polarization dependence, which is only revealed at the single domain level.

Second, stimulated emission depletion (STED) fluorescence microscopy, originally developed for super-resolution fluorescence imaging of isolated, robust, fluorescent dye labels in biological imaging applications, is adapted to image conjugated polymer solids using their endogenous, densely packed, non-ideal chromophores. Notably, the challenge posed by the strong two photon absorption of the so called “STED pulse”, which depletes the initially diffraction-limited excited state population to yield a sub-diffraction resolution excitation volume, in conjugated polymers is successfully mitigated through careful control of the STED pulse parameters in combination with the pile-up correction and excitation modulation. This technique is demonstrated on nanoparticles of the conjugated polymer poly(2,5-di(hexyloxy)cyanoterephthalylidene) (CN-PPV), where an imaging resolution of better than 90 nm is achieved.

Finally, a new method to measure exciton migration on its native nanometer and picosecond scales is presented, based on a further adaptation of STED microscopy, which provides ultrafast time resolution of spatial migration dynamics. This technique of time-resolved ultrafast stimulated emission depletion (TRUSTED) is achieved by adding a second STED pulse, with a controlled time delay, to define an optical quenching boundary that preferentially quenches excitation that has migrated beyond a critical radius. The theoretical and experimental sensitivity of this technique to migration processes is demonstrated, through kinetic simulations and experimental studies. The application of TRUSTED to CN-PPV thin films, in combination with a custom fitting routine, reveals the exciton migration length to be Ld = 16 ± 2 nm. Additionally, Monte Carlo simulations of incoherent exciton hopping are performed for a variety of possible spatioenergetic landscapes, revealing the migration process in CN-PPV to be approximately diffusive in nature, where the 5 ns lifetime capitalizes on the diffusive motion, resulting in the relatively long observed migration length. The simulations also reveal more generally how the energetic and spectral parameters of a material combine to determine the extent and nature of exciton migration.

The results presented here, and the future experiments enabled by this work, will reveal the importance of matching the scale of the experimental resolution to the natural scale of the process or heterogeneity of the material. The insight that stands to be gained through the continued pursuit of these research goals will elucidate the nature of the structure/function relationship in organic semiconductors, informing the rational design of the next generation of semiconducting materials for applications in displays, computing, lighting, and light harvesting.

Photosynthesis, the process by which plants and some bacteria convert sunlight into usable chemical energy, is a remarkable process that feeds nearly all of life on earth. At the smallest scales, photosynthesis is driven by molecular machinery in the form of various light-harvesting protein complexes (LHCs) contained in bacterial cells and in the chloroplasts of plant cells. At the earliest stages in photosynthesis, a photon of sunlight is absorbed by a small light-absorbing molecule (a "chromophore") and that energy is transported through a network of chromophores contained in these proteins towards specialized reaction centers where chemistry begins. This transfer process is remarkably efficient, and can result in nearly all absorbed photons reaching the reaction centers successfully. Understanding what factors of the design of these protein complexes give rise to this efficiency is thus of great interest for developing novel light-harvesting technologies. In this work, we present a series of studies focused on understanding the factors important to achieving efficient light harvesting, by spectro- and spatiotemporally resolving the few picoseconds-to-nanoseconds after photoexcitation in artificial and natural light harvesting complexes.

Chapter 1 presents a brief introduction to photosynthetic light-harvesting complexes in a variety of organisms. These complexes present a number of challenges to systematic study that kept open questions about their structure-function relationships even after over a century of study. We summarize some of these challenges and lay out our approaches to overcoming them.

Chapter 2 gives an introduction to transient absorption (TA) and transient absorption anisotropy (TAA) spectroscopy, from basic principles to practical implementation. TAA is a powerful technique for measuring energy transfer by measuring the depolarization of the transient absorption signal caused by energy transfer. We detail the implementation of a high signal-to-noise ratio TAA apparatus built in order to apply this technique to artificial light harvesting complexes.

Chapter 3 explores the role of disorder in intra-LHC energy transfer in an artificial LHC based on a circular permutant of the tobacco mosaic virus coat protein (cpTMV). This model system afford far greater control to perform systematic investigation than natural LHCs. In this study we measure the intra-complex energy transfer in these model LHCs using TAA spectroscopy and model that transport via kinetic Monte Carlo simulations. We find that fast site-to-site hopping as high as 1.6 ps^-1 is occurring in these complexes. With these simulations, we identify static disorder in orientation, site energy, and degree of coupling as key remaining factors to control to achieve long-range energy transfer in these systems. We thereby establish this system as a highly promising, bottom-up model for studying long-range energy transfer in light-harvesting protein complexes.

Chapter 4 introduces stroboscopic interferometric scattering microscopy (stroboSCAT), a label-free, time-resolved microscopy technique that can directly image energy carriers from excitons to heat in a broad range of materials. We provide an overview of the technique and detail improvements made to the technique to increase time resolution in order to study the first few picoseconds of natural light harvesting complexes. To achieve this objective we coupled an ultrafast laser source into the microscope, increasing the time resolution of the apparatus by over two orders of magnitude, to below 1 ps. We present a study of short-lived photogenerated charge carriers' migration in silicon, previously barely detectable with our lower time resolution, where we observe density-dependent diffusivity as a result of carrier-carrier scattering.

Chapter 5 presents ongoing spatially-resolved measurements of exciton migration and exciton-exciton annihilation in de-enveloped thylakoid membranes from green plants via stroboSCAT. We find that exciton-exciton annihilation dominates the observed spatial response and present a model to simultaneously fit exciton diffusion and annihilation, leveraging the spatial resolution to capture both. Finally, we provide a number of future directions and propose improvements to the apparatus to facilitate future experiments on these samples.

Taken together, this dissertation presents a set of novel approaches to studying energy transfer in LHCs that reveals the role of disorder and many-body interactions in photosynthetic light-harvesting. Both by studying novel model systems via a more well-established spectroscopic technique and by studying well-established natural photosynthetic samples via a novel microscopic technique, we unveil new findings about the important role that disorder plays in these fascinating systems.

Solution processable materials offer a variety of exciting possibilities, owing to their low-costand ease of tunability. Many next-generation electronic devices are being developed using solution processable materials, but their performance is typically hindered by the complex heterogeneous structures that result from their assembly or deposition. The characteristic length scales of this heterogeneity are typically nanometers and micrometers, and time scales of interest for material function can vary by many orders of magnitude (e.g., femtoseconds to minutes) depending on the context. In this dissertation, we employ a suite of time-resolved spectroscopies and microscopies to address material dynamics relevant for the transport of (excitation) energy and mass.

Chapter 2 describes measurements characterizing the excited state evolution of a proteinboundchromophore intended for a biomimetic photosynthetic light harvesting complex using transient absorption spectroscopy. These measurements employ a series of chromophoreprotein chemical linking groups that vary in their length and rigidity, which enables control over the chromophore-protein coupling. The findings here inform design principles for biomimetic light harvesting systems as well as the underlying photophysics potentially employed in natural photosynthetic systems for efficient excitation transport.

Chapter 3 describes correlative widefield single-particle tracking and atomic force microscopy(AFM) phase imaging to determine how the nano- and microscale semicrystalline morphology in electrolytic poly(ethylene oxide) (PEO) thin films influences the motion of small particles. The findings here suggest that polymer crystallinity, if controlled well, need not necessarily be a detriment to the transport of small particle species despite this historically being a challenge for PEO solid-state electrolytes that are investigated as next-generation battery materials. Chapter 4 gives an overview of time-resolved ultrafast STED (TRUSTED), an ultrafast trans2 formation of STED super-resolution microscopy to track exciton migration at the nanoscale in optoelectronically coupled materials. TRUSTED takes advantage of well-defined optical quenching boundaries such that exciton displacements, even over small distances, will register as a change in the number of excitons quenched. We describe the basics as well as the nuances in trying to apply this method to a series of electronically coupled materials.

Finally, Chapter 5 will discuss the ongoing investigation, at the time of this writing, ofexciton transport in Tellurium doped CdSe/CdS core-shell quantum dot superlattices. We investigate exciton transport using TRUSTED as well as time-resolved emission spectroscopy (TRES) and single-particle emission spectroscopy to characterize how the energetic disorder imposed by dopants modulates exciton transport. Steps to finish this work are detailed, which involve reconciling the time-rate of exciton energy decay measured by TRES with the exciton diffusivity measured by TRUSTED.

Taken together, this dissertation illustrates specific examples of characterizing materialstructure-function relationships across a variety of different systems. To do so, it requires using a number of different characterization tools, and, in each case, reveals the importance of matching the scale of the experimental length and/or time resolution to the native length and time scales of material structural heterogeneities and dynamics, respectively.

Super-resolution imaging has revolutionized how the structure of biological systems are observed at the nanoscale. Yet, observing dynamic processes in biology with high temporal and spatial resolution remains a significant challenge. Additionally, elucidating the nanoscale structure and dynamics in functional materials, particularly in optoelectronics, would greatly aid in the development of more efficient devices, such as solar cells and light-emitters. Unfortunately, most super-resolution microscopy platforms are designed for imaging biological samples and are incompatible with complex, functional materials. To extend super-resolution imaging to capture both biological dynamics and nanoscale material properties, we have developed cathodoluminescence imaging with low electron exposure (CILEE) and cathodoluminescence-activated imaging by resonance energy transfer (CLAIRE). Both imaging methods use cathodoluminescence (CL) microscopy to achieve nanoscale spatial resolution. The main drawback of CL microscopy is damage caused by the relatively high energy electron beam. In CILEE, the electron beam dose is significantly reduced to image samples only moderately robust to the electron beam. In CLAIRE, more fragile samples can be imaged by placing a thin scintillator film between the sample and electron beam. When excited by a focused electron beam, the scintillator film acts as a nanoscale optical excitation source, providing contrast based on interactions between luminescent dopant atoms in the sctillator and the adjacent sample in the near field. In this dissertation, the development of CILEE and CLAIRE are outlined, as well as many examples of uncovering new nanoscale phenomena with both imaging platforms.

Part I of this dissertation, which includes Chapters 2-5. focuses on using CILEE to elucidate the nanoscale structure and dynamic properties of lead halide hybrid perovskites, which are promising materials for optoelectronics. Using CILEE, we reveal a surprising degree of heterogeneity at the surface of hybrid perovskite thin films that differs greatly from the more homogeneous environment found in the bulk. Our CILEE study suggests that solar cells composed of a hybrid perovskite active layer can improve in efficiency by decreasing the heterogeneity through synthetic approaches. We also use CILEE to investigate the process by which mixed halide hybrid perovskites phase separate upon photoexcitation, a process that severely limits solar cell efficiency. Through a combination of CILEE and multiscale modeling, we find that phase separation is driven by polaronic strain in the lattice. Our results represent a new type of nanoscale phase transformation that is unique to hybrid materials. The emergence of CILEE as new approach to non-invasive super-resolution imaging has led to a greater understanding of the complex structure and dynamics in hybrid perovskite materials.

Part II of this dissertation, which includes Chapters 6-11, introduces CLAIRE as a new super-resolution imaging platform designed to image soft materials, such as organic or biological samples. In this dissertation, we describe the production of thin, free-standing scintillator films for CLAIRE and the incorporation of these scintillator films into a functional imaging device. We demonstrate that CLAIRE is capable of imaging soft materials and dynamic processes. The capability of CLAIRE to image biological samples with endogenous chromophores, such as photosynthetic membranes, is also demonstrated.

Together, CILEE and CLAIRE extend non-invasive super-resolution optical imaging to new classes of soft materials that are incompatible with current super-resolution optical imaging approaches and traditional electron microscopy. These new nanoscale imaging methods provide promising opportunities to visualize biological dynamics at high spatial and temporal resolution and to interrogate the nanoscale optical properties of functional optoelectronic materials to understand their fundamental properties, leading to higher efficiency devices.

Real-space imaging provides real-space information about the properties of a sample, which in turn provides insight into the correlation between physical properties and different regions of the sample. With repeated imaging, we can record movies that reveal spatio-temporal correlations. Many processes that are important for life and in emergent technologies occur at the nanoscale, a length scale that is typically difficult for optical microscopies to access. In particular, observing dynamics in these systems proves to be challenging, where existing microscopy techniques provide different trade-offs in spatial resolution, temporal resolution, and sample damage. In this dissertation, we use low-dose scanning electron microscopy (SEM) and cathodoluminescence (CL) microscopy to elucidate dynamics in delicate materials at the nanoscale.

Chapter 2 focuses on using time-resolved cathodoluminescence (TRCL) microscopy to determine the spatial variation in the lifetime of a Mn2+ dopant in a metal halide perovskite. We observe enhanced Mn2+ luminescence at the edges of halide perovskite microplates. Using TRCL, we reveal two luminescent decay components that we attribute to two different Mn2+ populations. While each component appears to be present both near the surface and in the bulk, the origin of the intensity variation stems from a higher proportion of the longer lifetime component near the perovskite surface. We suggest that this increased CL emission is caused by an increased probability of electron-hole recombination on the Mn2+ dopant near the perovskite surface due to an increased trap concentration there.

Chapter 3 of this dissertation focuses on developments in cathodoluminescence-activated imaging by resonant energy transfer (CLAIRE) microscopy. We discuss the production of thin, free-standing scintillator imaging chips for CLAIRE imaging and demonstrate that CLAIRE is capable of imaging dynamic processes in both soft materials and with metal nanoparticle labels. We then discuss our efforts to expand CLAIRE capabilities to other samples. We developed an aqueous encapsulation scheme using multi-layer graphene to expand the compatibility of CLAIRE imaging to samples that are otherwise incompatible with the vacuum environment of the experiment. Additionally, we report preliminary steps towards using CLAIRE imaging to study photosynthetic membranes.

Chapter 4 focuses on the use of a low-dose electron beam to both drive and record dynamics in an interfacial polycrystalline colloidal monolayer. We describe the formation of these polycrystalline monolayers at the surface of an ionic liquid droplet and the incorporation of large particle dopants in the polycrystalline lattice. We demonstrate that an electron beam perturbation drives a reduction in particle density in the center of the imaging field of view due to a combination of outward particle flow and detaching from the interface and becoming immersed into the bulk ionic liquid. We find that the rate of this reduction in particle density depends on the number of large particle dopants present in the lattice and discuss possible explanations for this dependence.

Together, these experiments demonstrate the utility of low-dose SEM and CL microscopy in capturing nanoscale dynamics in a variety of samples that are not typically robust to electron beam imaging. These methods extend nanoscale imaging to materials that are not compatible with other super-resolution imaging techniques or traditional electron microscopy, providing opportunities to explore dynamics in a wide range of other samples, from soft biological materials to next generation self assembled meta-materials.

Organic semiconductors have the potential to form the basis of the next-generation of electronic devices. Device layers formed from these molecular based materials can be solution processed at room temperature using techniques such as roll-to-roll printing, making them cheaper and more sustainable than inorganic alternatives such as silicon. Furthermore, their properties also enable the development of low-weight and flexible devices to access novel applications. Yet, solution processing also leads to the formation of complex microstructure via self-assembly that strongly impacts the function of such devices. A full understanding of the dependence of device functionality on device structure is needed to direct design of organic semiconductor based devices to drive widespread commercial adoption. The work of this thesis is focused on using time-resolved spectroscopy techniques to study the structure-function relationships of various organic semiconductors as a function of their local environment.

Chapters 2-4 focus on using transient absorption microscopy to study structure-function relationships in 6,13-Bis(triisopropylsilylethynyl)pentacene, a model organic semiconductor with applications in organic field-effect transistors and organic solar cells, as a function of the local film microstructure. Chapters 5-6 then turn to a time-resolved fluorescence study of next-generation thermally-activated delayed fluorescence organic light-emitting diodes as a function of the host matrix polarity.

Organic electronics are a fascinating field of study due to the intertwined relationship between the chaotic self-assembly of the active layers of these devices and their resulting functionality. Molecular design of the building blocks of these films affects the ensemble electronic properties of the entire film not only through the molecular excited states but also by how those molecular electronic states couple together. Efforts to correlate the impact of the local physical structure of samples on the resulting local electronic structure will inform future design decisions in the organic electronics community.