Renewable energy sources are becoming increasingly important as the world's energy demands continue to grow and climate change continues to occur. Photovoltaic power generation has great potential, but most commercial photovoltaic cells have typically been made from single-crystal or polycrystalline silicon which are expensive and energy intensive to produce. Organic semiconductors are one class of solar cell materials that have the potential for low cost due to solution processability, roll-to-roll fabrication, material tunability, and the very small amounts of material required to absorb light compared to crystalline silicon. Organic photovoltaic (OPV) efficiencies have increased rapidly in the past few years and are currently at about 11% power conversion efficiency.

To find new pathways to improve OPV performance, a deeper understanding of the relationships between morphology, electronic properties, and OPV performance is required. This work will explore these relationships, focusing on the relationship between morphology and charge transfer states. We focus on a small molecule material system with only pure donor and acceptor domains, unlike the typical bulk heterojunction composed of donor, acceptor, and mixed domains. The advantage of studying this system is a more well-defined interfacial area and simpler morphology that allows us to better characterize charge transfer states at the donor-acceptor interface.

We first review the basic principles of OPV operation, fabrication and characterization methods, and charge transfer states. We then use grazing incidence wide angle X-ray scattering to characterize the crystal structures and textures of the polymorphs of the donor and acceptor in our small molecule system, which are important for understanding their blend morphology. Next, we demonstrate a new method to control morphology in this system with the use of a thermally degradable binder polymer. Afterwards, we vary interfacial area and charge transfer state density in this system through processing and characterize it using sub-bandgap external quantum efficiency measurements, allowing us to gain valuable insights about the photophysics of this system.

Finally, many of the insights and techniques used to study this system are also useful in studying other systems of organic or solution-procesed semiconductors. We optimize the morphology and performance of OPVs made with novel low-bandgap donor-acceptor copolymers. We then find that ionic photoconductivity plays an important role in the behavior of photodetectors made with solution processed amorphous ZnO and in their interaction with organic semiconductors.

Perovskite solar cells (PSCs) have captured the focus of the photovoltaic field due to their ease of fabrication and high power conversion efficiencies of ~22%. In PSCs, the perovskite active material is sandwiched between electron and hole transport layers that serve to extract photogenerated charge carriers and to avoid direct contact between the perovskite layer and metal electrodes in order to prevent degradation. Organic semiconductors are attractive candidates for transport layers because of their low temperature processing and scalability. The major obstacle to use of organic semiconductors in PSCs is their low carrier mobility and carrier concentration, which cause them to act as a parasitic series resistance in devices. In order to overcome transport issues, doping of organic transport layers is performed to passivate trap sites and increase the concentration of free carriers.

To date, there have been few reports on understanding how the presence of small molecule dopants can influence the perovskite active layer. Small molecule dopants that have been developed to dope organic semiconductors are also candidates for charge transfer to MAPbI3. The impact of direct charge transfer from small molecules to MAPbI3 was investigated as a model system to provide a means to understand the more complex heterointerface of doped organic blends at perovskite surfaces. We studied the impact of surface doping on passivation of electron traps in MAPbI3. Using spectroscopy and photoconductivity measurements, we determined that at moderate degrees of doping the Fermi level shift will passivate this recombination center. At high degrees of doping, there is an overabundance of charge beyond what is necessary to passivate surface traps, creating an additional pathway for bimolecular recombination. In addition, excessive charge leads to chemical damage of the surface of MAPbI3, resulting in the release of mobile I- ions. Given these design principles, the impacts of doping the organic electron transport layer on solar cell performance was investigated. The power conversion efficiency of PSCs showed a rise and fall in performance with increasing doping concentration of the electron transport layer, consistent with observations from surface doping. Optimal solar cell performance was observed at moderate doping concentrations of 1 wt%. At this doping concentration, evidence of trap passivation was observed by higher open circuit voltages (VOC) and longer carrier lifetimes. Numerous studies in literature on the impact of doping the electron transport layer have identified a range of factors influencing performance, including changes to carrier mobility, resistance mismatch, energy level alignment, and morphology. This work indicates that the impacts of interfacial charge transfer on surface recombination should be added to this list of considerations.

Organic semiconductors are an exciting class of materials that have potential to produce

low-cost, printable, and flexible electronic devices. Moving to the next generation

of organic semiconductors that will result in greater efficiency requires advancements in

the areas of materials chemistry, molecular assembly, predictive modelling, and device

optimization. Here, we focus on morphology and demonstrate how it is linked to each of

these areas. Understanding the connections among chemistry, thin film microstructure,

and charge transport remains a major challenge in the field. We examined materials

systems relevant to organic solar cells, memory devices, and transistors, with a focus on

synchrotron-based X-ray techniques. For a blend of a polymer and small molecule, applicable to solar cells, control of molecular orientation in the small molecule is especially important for non-fullerene based molecules that exhibit anisotropic charge transport. In ferroelectric-semiconductor polymer blends used in organic memory, improved control over phase separation length scales is achieved by altering the chemistry of the semiconducting polymer to tune polymer-polymer interactions. Complementary simulations can facilitate characterization of organic semiconductors. First-principles predictions of X-ray absorption spectroscopy are applied to semiconducting polymers, and prove critical for understanding complex experimental data related to molecular orientation and electronic structure in general. Overall, these studies provide insights into key factors that should be considered in the development of new organic semiconductors.

Hybrid organic-inorganic perovskite (HOIP) compounds have garnered widespread research attention due to their outstanding optoelectronic properties while being solution-processable semiconductors. When used as the active material in a photovoltaic device, three-dimensionally connected HOIPs based on methylammonium lead iodide (CH₃NH₃PbI₃) and associated alloys have demonstrated power conversion efficiencies that have increased at an unprecedented rate, from an initial value of 3.8%, to present day efficiencies exceeding 25% in just a decade. This has made perovskites competitive with existing thin film solar cell technologies like CdTe and CIGS. Additionally, dispersions of nanoparticles of similar perovskite materials have shown controllable optical emission properties with narrow emission linewidths and large (~95%) photoluminescence quantum yields, making them attractive materials for light emitting applications. However, three-dimensionally connected perovskites typically suffer from poor ambient stability, impeding their feasibility in devices for long term use.

Numerous recent research efforts have shown that lower dimensional perovskites, notably two-dimensional layered structures, possess increased ambient material stability. Layered perovskite compounds correspond to structures in which the 3D perovskite structure has been sliced along a specific crystallographic direction, leading to a loss of connectivity in one dimension. Therefore, champion photovoltaic devices made with layered perovskites have only achieved efficiencies around 15% due to the loss of charge transport in one direction, presenting a fundamental tradeoff between stability and performance. Understanding the optoelectronic and structural properties of layered perovskites is therefore essential for both optimizing device efficiencies and proposing design rules for future materials discovery.

Layered perovskite compounds can be generated through two strategies -- partial anion or partial cation substitution during synthesis. This report will discuss the synthesis and characterization of three different systems -- one generated through the partial anionic substitution of iodide (I^-) anions with the pseudohalide thiocyanate (SCN^-), and two by partial cationic substitution of methylammonium (CH₃NH₃⁺) with butylammonium (C₄H₉NH₃⁺) and 2-(4-biphenyl)ethylammonium (C₆H₅C₆H₄CH₂CH₂NH₃⁺), respectively. Structural characterization with X-ray and electron diffraction reveals the difficulties in producing phase pure samples from solution processing, and the origin and impact of these impurities on electronic properties will be discussed. Transport properties were measured with contactless time-resolved microwave conductivity measurements and compared with values seen in 3D perovskites. We find that the layered perovskite systems possess similar in-plane mobilities to their three-dimensional counterparts, but with increased higher-order carrier recombination, resulting in shorter lifetimes.

The properties of organic semiconductors make them well-suited for certain applications in electronic devices or energy conversion. Because of their low inherent conductivity, organic semiconductors need to be doped to be used in many of these applications. The limited set of small molecules commonly used for organic semiconductor doping do not always have the desired combination of properties for some applications, such as high ionization energy or air stability. In addition to removing or adding charges, these small molecule dopants can significantly alter the semiconducting film structure through Coulombic interactions. These challenges suggest a need to explore new dopants and doping mechanisms and to better understand the effects of doping on the electronic structure and morphology of organic semiconductors. We used a combination of experimental methods and DFT calculations to explore new doping mechanisms based on Lewis acid-base pair chemistry. Combining UV-Vis-NIR spectroscopy, electrical transport measurements, and grazing incidence wide-angle X-ray scattering, we connected changes in the electronic structure to changes in morphology and transport in both amorphous and semicrystalline semiconductor systems. Our results suggest ways to control the electronic structure and morphology of organic semiconductors, as well as providing a starting point to further apply Lewis pair chemistry to doping.

The combination of contemporary synthetic chemistry and application-driven materials design accentuates the opportunities available at the intersection of science and engineering to advance soft, flexible, and biocompatible devices. Designing devices which will undergo deformation can be approached in two ways: (1) mitigating deformation to realize strain-tolerant devices that maintain functionality in dynamic environments, e.g., wearable devices that bend and stretch with the body, or (2) leveraging deformation as a device input or output, e.g., strain sensing or actuation. In both cases device performance can be optimized through materials development, guided by electro-mechanical modeling.Devices which leverage deformation have figures of merit that are closely linked to material mechanics. It is of interest to develop novel materials that defy conventional boundaries of mechanical behavior. Bottlebrush elastomers are an emerging class of solid materials that exhibit extremely low elastic moduli. Super-soft bottlebrush elastomer dielectrics and conductive composites have utility in enhancing the performance of flexible devices such as capacitive pressure sensors. Mitigating deformation requires quantitative modeling of strain-dependent device parameters. One particularly interesting device to model is the thin film transistor (TFT), an important building block of modern circuits. Organic TFTs can be made with materials that allow them to be deformed during electrical operation. Mechanical models of the elasticity of polymers can be applied to predict the electrical characteristics of deformable TFTs.

Organic and hybrid solar cells are complex, nanostructured devices, for which an in-depth understanding is required to improve and optimize. Both types of solar cells take advantage of the strong optical absorption of organic chromophores to collect sunlight and have reached maximum efficiencies of around 12% as of the end of 2014. They both also rely on nanostructures and interfaces to balance energy and charge transport. Despite these underlying similarities, organic solar cells and dye sensitized solar cells present different challenges to optimization. We have applied experimental and computational techniques to help explore and understand factors that affect their efficiencies and describe and predict strategies for improving them. We made a comparative study of two isostructural donor molecules and studied the exciton and charge transport in a non-fullerene acceptor molecule in bulk heterojunction organic solar cells. We then wrote a kinetic Monte Carlo simulation to study energy transfer in donor-acceptor systems and applied it to study trapping kinetics in different morphologies, energy relay dyes in dye sensitized solar cells, and energy cascades in bilayer organic solar cells. Based on these simulations, we offer design suggestions for both types of solar cells.

Phased-array metasurfaces offer a wide design space to arbitrarily shape the wavefront of light. As such, metasurfaces have been used to create various miniature and light-weight optical components such as lenses and beam steerers with applications ranging from augmented reality to space flight and LiDAR. Metasurface optical elements typically transform a well-defined incident light beam into a desired output waveform. Luminescent emission, on the other hand, is not well-defined in either space or momentum. Thus, light emitting metasurfaces offer intriguing opportunities to study fundamental light-matter interactions and further miniaturize optical components. To date, most luminescent metasurfaces have been uniform arrays of scatterers and are therefore unable to provide granular control over the wavefront of emitted light. Recent work demonstrating wavefront control of spontaneous emission using phased-array metasurfaces, on the other hand, suffer low efficiency and peculiar polarization dependencies. In this work, we develop and verify a reciprocal simulation strategy to explain the polarization disparity and improve unidirectional emission efficiency of phased-array metasurfaces. We then use these reciprocal simulations to design metasurfaces to steer light from systems where emission originates from alternate quantum mechanical processes. Additionally, we pair these reciprocal simulations with Bayesian optimization to facilitate the design of highly unidirectional photoluminescent metasurfaces capable of directive p-, s-, or combined p- and s- polarized emission at arbitrary angles. Our inverse design approach enables 54% improvement in directivity and the first-ever simultaneous directional emission of s- and p-polarized light. We expand the optimization tools developed for light emitting metasurface and apply them to quantum optics. Magneto-optical atomic traps are a critical part of modern science, but future applications in gravity mapping and space-based atomic clocks require smaller, more robust traps. We design a metasurface retroreflector to replace two bulk optical components and eventually reduce trap volume by three orders of magnitude. The metasurface is designed for polarization insensitive retroreflection of 780 nm circularly polarized light at 54.7°. The proof-of-concept device retroreflects circularly polarized 736 nm light at 50.3°. We discuss oxidation mitigation strategies for future devices and propose a corrective optic for the currently fabricated device. With a better understanding of light-matter interactions and sophisticated modeling and optimization tools, this work represents a step toward smaller, more efficient devices. Full control over the polarization and momentum of light emitted by incoherent sources will lead to lighter, more energy efficient displays whereas the miniaturization of magneto-optical traps will allow the expansion of cold-atom science beyond the laboratory.

Hybrid halide perovskites (HHPs) are being commercialized as solar cells and are being investigated for use in a wide variety of other optoelectronic devices. The most efficient solar cells rely on alloys of the cubic perovskite AMX3 structure, such as (CS.FA,MA)Pb(I,Br)3, where FA and MA stand for formamidinium and methylammonium. HHPs have incredibly versatile structure and consequently, emission color, making them attractive for light emitting diodes or lasers. In addition to the AMX3 structure, HHPs can be made as two-dimensional (2D) materials, whereby large bulky organic cations separate M-X (metal-halide) semiconducting sheets. Tuning the thickness of the M-X layer changes the color of the exciton emission, and thus much effort has been devoted to making optoelectronic devices from 2D perovskites. Generally, HHP-based devices rely on polycrystalline thin films, which poses challenges: polycrystalline thin films contain grain boundaries, exhibit film strain, and are prone to forming undesired crystalline phases.

Here, the fundamental structural, ionic and optical properties of HHP thin films are investigated. The first section is dedicated to understanding how film strains, such as those imparted by commercial fabrication procedures, may change sub-grain structure and cause degradation of the HHP. The second section examines phase stability and halide interdiffusion in mixed-halide 3D alloys. The third and fourth sections report phase-pure 2D film fabrication, and examine how strain and residual solvent can turn off certain emission features intrinsic to the 2D phase in question. These results help extend the utility of HHPs for optoelectronic devices by providing design rules for how to grow films with targeted structural and optoelectronic properties.

Semiconducting polymers and small-molecules can be readily incorporated into electronic devices such as organic photovoltaics (OPVs), thermoelectrics (OTEs), organic light emitting diodes (OLEDs), and organic thin film transistors (OTFTs). Organic materials offer the advantage of being processable from solution to form flexible and lightweight thin films. The molecular design, processing, and resulting thin film morphology of semiconducting polymers drastically affect the optical and electronic properties. Charge transport within films of semiconducting polymers relies on the nanoscale organization to ensure electronic coupling through overlap of molecular orbitals and to provide continuous transport pathways. While the angstrom-scale packing details can be studied using X-ray scattering methods, an understanding of the mesoscale, or the length scale over which smaller ordered regions connect, is much harder to achieve.

Grain boundaries play an important role in semiconducting polymer thin films where the average grain size is much smaller than the total distance which charges must traverse in order to reach the electrodes in a device. The majority of semiconducting polymers adopt a lamellar packing structure in which the conjugated backbones align in parallel π-stacks separated by the alkyl side-chains. Only two directions of transport are possible – along the conjugated backbone and in the π-stacking direction. Currently, the discussion of transport between crystallites is centered around the idea of tie-chains, or “bridging” polymer chains connecting two ordered regions. However, as molecular structures become increasingly complex with the development of new donor-acceptor copolymers, additional forms of connectivity between ordered domains should be considered.

High resolution transmission electron microscopy (HRTEM) is a powerful tool for directly imaging the crystalline grain boundaries in polymer and small-molecule thin films. Recently, structures comparable to quadrites were discovered in the semiconducting polymer, PSBTBT, where the angle of chain overlap could be predicted by the geometry of the backbone and alkyl side-chains. Such structures are hypothesized to improve the electronic connectivity and enable 3D transport. Now, it has been determined that another semiconducting polymer, PBDTTPD, forms cross-chain structures in thin films. PBDTTPD is a low band-gap donor-acceptor copolymer used in high efficiency OPVs. The effect of the alkyl side-chains on intercrystallite order is determined by examining three different derivatives of the PBDTTPD polymer with HRTEM. Additionally, the expansion and contraction of films during thermal annealing and slow cooling is monitored through in-situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. Results show that minor variations in side-chain structure drive both crystallite orientation and the formation of crossed structures. Overall, these studies suggest design principles to continue to advance the field of organic electronics.