Rachel C. Kurchin

Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber.

Methylammonium lead halide (MAPbX3 ) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air‐Stable Bismuth Oxyiodide (BiOI)

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiOx |BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.

Segmentation and Tracking of Marine Cellular Clouds observed by Geostationary Satellites

ABSTRACT Marine stratocumulus (MSC) are shallow marine boundary layer clouds that have a significant cooling contribution to the Earth’s radiative balance. The amplitude of this cooling effect strongly depends on the properties of closed and open cells comprising MSC cloud fields. Systematic study of the underlying processes associated with cloud cell properties requires accurate and reliable cell characterization. Here we propose a method for cell segmentation of MSC clouds as observed from geostationary satellite images. The method, which is based on watershed transformation, is found to be highly efficient in segmentation of both open and closed MSC scenes. Application of the suggested methodology over a Lagrangian framework that track the clouds as they are advected by the wind and comparison of the results between pairs of consecutive images indicate that the resulted segmentation is robust and consistent. The methodology developed in this work opens the way to systematic investigation of spatiotemporal changes in MSC cloud field properties, which will improve our understanding of MSC clouds and their role in regulating Earth’s radiative budget.

Temperature- and intensity-dependent photovoltaic measurements to identify dominant recombination pathways

In novel photovoltaic absorbers, it is often difficult to assess the root causes of low open-circuit voltages, which may be due to bulk recombination or sub-optimal contacts. In the present work, we discuss the role of temperature- and illumination-dependent device electrical measurements in quantifying and distinguishing these per form ance losses — in particular, for determining bounds on interface recombination velocities, band alignment, and minority carrier lifetime. We assess the accuracy of this approach by direct comparison to photoelectron spectroscopy. Then, we demonstrate how more computationally intensive model parameter fitting approaches can draw more insights from this broad measurement space. We apply this measurement and modeling approach to high- performance III-V and thin-film chalcogenide devices.

Economically sustainable scaling of photovoltaics to meet climate targets

To meet climate goals, photovoltaics (PV) deployment will have to grow rapidly over the next fifteen years. We identify two barriers to this growth: scale-up of manufacturing capacity and the cost of PV module production. We explore several technoeconomic approaches to overcoming these barriers and identify deep reductions in the capital intensity (capex) of PV module manufacturing and large increases in module efficiency as the most promising routes to rapid deployment. Given the lag inherent in rolling out new technology, we explore an approach where growth is fueled by debt or subsidies in the short-term and technological advances in the medium term. Finally, we analyze the current capex structure of crystalline silicon PV module manufacturing to identify potential savings.