Optoelectronic device engineering for energy applications

In order to effectively confront our growing climate catastrophe, we must both shift our dependence on fossil fuels to renewable resources and decrease our energy consumption. This thesis develops and optimizes two kinds of optical devices; tandem perovskite solar cells aim to increase our usage of solar energy, while dynamic windows aim to decrease our daily energy consumption in buildings. In the few short years since the inception of single-junction perovskite solar cells, their efficiencies have skyrocketed. Perovskite absorbers have at least as much to offer tandem solar cells as they do for single-junction cells due in large part to their tunable band gaps. However, we have found that modifying the perovskite band structure via halide substitution, the method that has been most effective at tuning band gaps, leads to instabilities for some compositions. Specifically, we observe a carrier-induced phase segregation that leads to trapping of carriers in a low-bandgap phase. This trapping causes lower energy emission and a corresponding decrease in open circuit voltage of solar cells made with these materials. We show that this effect, later dubbed the 'Hoke Effect', exists across many perovskite morphologies and chemistries. Recognizing that these instabilities relate to the perovskite composition, we developed a completely inorganic Cs-based mixed-halide perovskite to improve photo-stability. We found that these devices showed both improved photo-stability and improved thermal stability, and offer a promising option for top cell absorbers in tandem perovskites. But producing clean energy from the sun isn't enough; we also need to reduce day-to-day energy usage. In the United States, buildings present an obvious target for energy use reduction; they consume 72% of all electricity and produce 38% of total CO2 emissions. While conservation programs like LEED have increased buildings' energy efficiency, our increasing affection for natural daylighting and energy inefficient glass-clad facades threatens to drive those efficiency gains downward. Enter dynamic glass. Dynamic glass, capable of electronically tinting on-demand, can reduce building energy expenditure by 20%. Not only that, but it can reduce the need for overhead lighting while making sunlit spaces more comfortable. Despite its upsides, high cost, non-neutral coloring, and mediocre contrast ratios have continually plagued traditional WO3-based dynamic glass. We have developed a new breed of dynamic glass that relies on reversible metal electrodeposition (RME) to switch between clear and dark states. By developing a now-patented thiol-anchored Pt nanoparticle seed layer and optimizing electrolyte chemistry, we have been able to uniformly switch devices between opaque and transparent thousands of times with unprecedented color neutrality and contrast ratios. In addition, the devices use cheap materials and industrially compatible processing techniques. Having shown impressive device performance, we must demonstrate our path to scalability. Switching quickly and uniformly on the meter length-scale has been one of the biggest challenges for any dynamic glass technology, and ours is no exception. To this end, we have developed an ambitious new transparent electrode that aims to combine the low sheet resistance of solar modules, the cycle life of battery electrodes, and the optical quality of vision glass. Patterning a thick, highly transparent, chemically stable, UV-crosslinked polymer on indium tin oxide (ITO) has allowed us to electroplate high quality Cu mesh with nearly 90% visible light transmission, 2% haze, and sheet resistance of 0.5ohms/sq. Not only that, but our mesh construction strategy allows us to selectively protect the Cu gridlines by electroplating an inert capping layer. Critically, this capping layer has allowed us to electrochemically cycle the metal mesh in a Cu-containing electrolyte without stripping away the underlying Cu. Our highly transmissive, conductive, and durable electrodes coupled with our seed layer innovations push RME to the forefront of dynamic glass technology. With a bit more development, RME could become the basis for the next generation of commercial dynamic glass.

Keywords: Materials Science and Engineering.
Advisor: Brongersma Mark L. 
McGehee Michael
Chueh William