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A diverse portfolio of renewable energy technologies is required to limit global warming to less than 2 ◦C. Of the possible emissions-free options, photovoltaic (PV) technologies can be most widely deployed, given the abundance of the solar resource compared. As with all power generation sources, PV adoption is predicated on the availability of technology solutions that are both inexpensive and highly efficient. One solar cell material, the metal halide perovskites (MHP), may provide the ideal combination, with > 25% efficiency devices within the first decade since their invention fabricated through simple spin coating. Despite the unprecedented rise in MHP performance, stability remains a critical challenge with the most stable devices at the 1-year benchmark compared to the >25-year lifetime of Si-based PV. Further progress concerning enduring power output will require a fundamental understanding of the impact of environmental stressors (light, temperature, bias, oxygen, and water) on the basic physical processes governing solar cell operation. Therefore, my dissertation elucidates the interplay between the ambient environment and MHP composition on both the optical and electrical behavior using in situ methods. The first part of my thesis elucidates the time-dependent optical and elec- tronic response of different MHP compositions using different in situ microscopy techniques. I capture the transient photovoltage of both Br- and I-containing per- ovskites for different photon energies using heterodyne Kelvin probe measurements. My measurements demonstrate that the voltage rise (light ON) is 104× faster than the subsequent decay (light OFF). Uniquely, the decay time for the residual voltage depends on the excitation wavelength, but only for the MAPbBr3 thin film. Next, I spatially and temporally resolve the relationship between radiative recombination and relative humidity (rH) for multi-cation films. The time-dependent photolumi- nescence (PL) indicates that the

Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis.

The ideal photovoltaic technology for global deployment must exhibit two key attributes: (i) high power-conversion efficiency, enabling a solar panel with a large power output per area and, (ii) low-cost/W, due to either being derived from earth-abundant materials and/or ease of fabrication. For the past two decades, extensive efforts have been made to boost the efficiency of some of the most promising high performance and low-cost photovoltaic materials, such as CdTe, Cu(In,Ga)Se2 (CIGS), and hybrid organic-inorganic perovskites, to achieve higher efficiency devices. However, improvement in the overall performance is still limited by the open-circuit voltage (Voc). All of the solar cell materials listed above are composed of grains and grain boundaries on the order of micro- and nanometers, respectively, and their nanoscale interfaces can cause electrical charge carriers to become trapped and recombine non-radiatively, reducing the Voc. Therefore, in this thesis, I implement high spatial resolution functional imaging techniques to resolve the local voltage variations in the thin-lm polycrystalline and hybrid perovskites materials for photovoltaic applications. First, I spectrally and spatially resolve the local photovoltage of CIGS solar cells through confocal optical microscopy to build a qualitative voltage tomography. From these photovoltage results, I discover variations in the electrical response of >20% that are also on the same length scale as the grains composing the CIGS material. Therefore, by enhancing the spatial resolution beyond the diffraction limit, the electronic properties of individual grains and the interfaces between the grains can be fully resolved. For this, I implement Kelvin probe force microscopy (KPFM), and demonstrate a universal method to directly map the Voc of any photovoltaic material with nanoscale spatial resolution. Next, we extend this ability of KPFM to rapidly image (16 sec/map) the real-time dynamics of perovskite solar cells

Metals have been widely used for harvesting and storing energy in devices such as superabsorbers and Li-ion batteries. However, incorporating metals into a wider range of energy applications is severely limited by their intrinsic optical and electrochemical properties. Therefore, in this thesis, we provide a new class of metallic materials by forming binary mixtures of Ag, Au, Cu, and Al with novel physical properties for photonics, and a comprehensive understanding of the fundamental electrochemistry in Al and Si anode all-solid-state batteries for energy storage. The first part of my thesis focuses on developing metallic alloys with a tunable optical response. We realize a new family of metallic materials by alloying Ag, Au, and Cu with on-demand dielectric functions, which can be used in superabsorbers and hot carrier devices. We design and fabricate alloyed nanostructures with engineered optical response and spatially resolve the electric field distribution at the nanoscale by utilizing near-field scanning optical microscopy, which can potentially enhance the performance of optoelectronic devices. To understand the physical origin of the optical response of the alloys, we measure the valence band spectra and calculate the band structures of Ag-Au alloys, providing direct evidence that the change in the electronic bands is responsible for its optical property. Further, we obtain a photonic device with superior performance using metallic alloys. Specifically, an Al-Cu/Si bilayer superabsorber is reached in a lithography-free manner with maximum absorption > 99%, which can be used for energy harvesting. The second part of my thesis highlights the importance of understanding the reactions and ion distribution in energy storage devices. We inspect how the Al electrode surface changes upon cycling and directly map the Li distribution in 3-dimensions within all-solid-state batteries by implementing time-of-flight secondary ion mass spectroscopy. This research indicates

Electron microscopy and spectroscopy (EMS) techniques enable (near-) surface and interfacial characterization of a variety of materials, providing insights into chemical/electrochemical and morphological information with nanoscale spatial resolution. However, the experimental realization of EMS in liquid/gaseous samples becomes problematic due to their incompatibility with high vacuum (HV) conditions. To perform EMS under elevated pressure conditions, electron transparent membranes made of thin C, SiO2 or/and Si3N4 are implemented to isolate a liquid/gas sample from HV environment. Nevertheless, even a few ten nanometer thick membrane deteriorates signal quality due to significant electron scattering. The other challenge of EMS consists in inaccessibility to probe solid state interfaces, e.g. solid-state Li-ion batteries, which makes their operando characterization problematic, limiting the analysis to ex situ and postmortem examination. The first part of my thesis focuses on developing an experimental platform for operando characterization of liquid interfaces through electron transparent membranes made of graphene (Gr)/graphene oxide (GO). The second part is dedicated to probing Li-ion transport at solid-state-battery surfaces and interfaces using ultrathin carbon anodes. I demonstrated the capability of GO to encapsulate samples with different chemical, physical, and biological properties and characterized them using EMS methods. I proposed and tested a new CVD-Gr transfer method using anthracene as a sacrificial layer. Characterization of transferred Gr revealed the advantages of our route with respect to a standard polymer based approach. A novel platform made of an array of Gr-capped liquid filled microcapsules was developed, allowing for a wide eld of view EMS. I showed the capability of conducting EMS analysis of liquid interfaces through Gr membranes using energy-dispersive X-ray spectroscopy, photoemission electron microscopy, and Auger electron spectrosco

Currently, there is a critical need for a photovoltaic design that will convert sunlight into electricity with practical efficiencies higher than 50%. Multijunction Solar Cells (MJSCs) are one of the most promising options to achieve ultra-high efficiencies. III–V compound semiconductors are generally used to fabricate MJSCs; however, limitations imposed by the lattice constants of available substrates strongly restrict which materials can be used for high-quality epitaxial growth. Herein we present an alternative design for an all lattice-matched monolithic 3-junction solar cell formed by (1.93 eV) InAlAs / (1.39 eV) InGaAsP / (0.94 eV) InGaAs, with 5.807 Å lattice constant. 1-dimensional device modeling for each individual subcell, as well as for the tandem device were performed under AM 1.5 direct illumination and concentrated sunlight. The role of concentration in each figure of merit was analyzed and Auger recombination was found to play an important role for high-injection levels. For a current match of 1.58 A/cm^2 we found that > 51% in efficiency can be achieved under 100-suns (with V_(oc)= 3.34 V). A detailed analysis of the effect of concentration on the device performance is presented.

We demonstrate high quality InAlAs epitaxial growth by metalorganic vapor phase epitaxy and wide band gap solar cell fabrication. X-ray diffraction and transmission electron microscopy were used to characterize the crystalline quality of the epitaxial InAlAs grown. InAlAs solar cells lattice-matched to InP were grown and electrically characterized under AM 1.5 global 1-sun illumination. Window layers with different composition and, therefore, band gap energies were used to compare its effect on the overall device performance. In order to improve the electrical contact at the top window (Al-rich), an InGaAs cap layer was used. The resulting first generation of InAlAs solar cells showed an efficiency higher than 14 %, open circuit voltage of V_(oc) = 1 V, J_(sc) = 19.3 mA/cm^2, and maximum external quantum efficiency of 81%.

Spectrolab has demonstrated the first lattice matched InAlAs/InGaAsP/InGaAs triple junction solar cell grown on InP substrate. X-ray diffraction characterization shows high quality solar cell materials. Preliminary 1-sun AM1.5D testing of the triple junction solar cell shows promising results with an open circuit voltage (V_(oc)) of 1.8V, a short-circuit current density (J_(sc)) of 11.0 mA/cm^2, a fill factor of 64.4%, and a 1-sun AM1.5D efficiency of 13.8%. The same cell also passes 27-suns under concentration. Improvements in layer design and crystal quality of advanced features can further raise the 1-sun and concentrated AM1.5D conversion efficiency of the InP-based triple junction cell beyond 20%.

Supermode Si/InGaAsP hybrid lasers with a varying-width Si waveguide have been fabricated and shown to be superior to those lasers with a uniform-width Si waveguide. Edge emission images demonstrate mode evolution.

Compound single junction and multijunction solar cells enable very high photovoltaic efficiencies by virtue of employing different band gap materials in seriesconnected tandem cells to access the full solar spectrum. Researchers focused on improving the electrical properties of solar cells by optimizing the material growth conditions, however relatively little work to date has been devoted to light trapping and enhanced absorption in III-V compound solar cells using back reflectors. We studied absorption enhancement in InGaAs and InGaAsP thin film solar cells by means of numerical modeling. Flat dielectric and metal back reflectors that might be introduced into the solar cell via wafer-bonding, epitaxial lift-off or deposition techniques have been shown to increase the short circuit current and the photovoltaic efficiency of solar cells.

Compound single junction and multijunction solar cells enable very high photovoltaic efficiencies by virtue of employing different band gap materials in series-connected tandem cells to access the full solar spectrum. Researchers focused on improving the electrical properties of solar cells by optimizing the material growth conditions, however relatively little work to date has been devoted to light trapping and enhanced absorption in III-V compound solar cells using back reflectors. We studied absorption enhancement in InGaAs and InGaAsP thin film solar cells by means of numerical modeling. Flat dielectric and metal back reflectors that might be introduced into the solar cell via wafer-bonding, epitaxial lift-off or deposition techniques have been shown to increase the short circuit current and the photovoltaic efficiency of solar cells.