Passivation of Gallium Arsenide Nanowires for Solar Cells

A strategic and diverse set of passivation methods for gallium arsenide nanowires wasstudied. Using a time-resolved photoluminescence setup at100 Kand300 K, the radiativerecombination of charge carriers was resolved on a picosecond time scale. Characterizationof these nanowire arrays on their native substrates provided reliable and nondisruptivemeasurements of thousands of nanowires simultaneously.Three promising passivation methods were explored yielding unexpected results. First,an international collaboration was created which identified poly(3-[3,7-dimethyl-octyloxy]thiophene)(P3OOT) as a novel, conjugated polymer-based passivation material. Results were incon-clusive. Second, an established hydrazine-sulfide solution treatment was compared witha low temperature, low vacuum plasma nitridation technique. The hydrazine treatmentunexpectedly demonstrated depassivation whereas the plasma method strongly exceededexpectations. These measurements of plasma treated gallium arsenide nanowires may bethe first publicly documented evidence for passivation using this procedure.Overall, the results of these experiments further demonstrate the strong need for pas-sivation of GaAs nanowires and proposes a new and relatively simple nitrogen plasmamethod which partially achieves this goal.Solar energy is a well-known pathway for reducing pollution and mitigating the climate crisis. Worldwide, millions of people have already seen the benefits of installing solar panels and generating energy locally. Despite its successes, solar energy technology is still quite new and must be developed further before it can completely surpass fossil fuels. One of the most promising ideas currently being researched combines intricate, nanosized structures with advanced, highly efficient materials. Previously, high-efficiency (but expensive) semiconductor materials like gallium arsenide could only be used in unique settings like outer space. Material rarity and production complexity made large scale implementation impractical. That era may very well be over. Cutting-edge research seems to be unlocking their potential. Over the past few decades we’ve greatly increased our ability to design structures at the atomic level. As these structures have improved, so has our understanding of them. Just a few years ago a fascinating phenomenon was discovered. Subwavelength nanowires, almost invisibly small, were shown to absorb huge amounts of light. Nearly ten times more light than their size should have allowed them to. This discovery electrified the scientific community and has stimulated enormous research. The concept has been proven; now it must be perfected. To further improve solar cell efficiency and understand this nano-sized world better, a comprehensive study of Lund University nanowires has been performed. A strategic and diverse set of surface modifications were characterized with a technique called time-resolved photoluminescence. With the help of ultra-short pulses of light, the electrical charges inside thousands of nanowires were simultaneously measured, giving detailed information about material efficiency and device performance. Several unexpected and exciting results were observed. First, a cheap and novel polymer-based surface treatment showed mixed results prompting further study. Second, an established method using a rocket fuel, hydrazine, was not found to be effective, contradicting published literature. Third, a never-before-used (and safer) nitrogen plasma alternative demonstrated preliminary success. Though the most dramatic improvements were observed at sub-freezing temperatures, significant improvements were also observed at relevant, operating temperatures and the method shows promise for future research. To the best of the author’s knowledge, the corresponding research represents the first publicly available data demonstrating the ability of nitrogen plasma to improve gallium arsenide nanowire surfaces for photovoltaics