Researchers have developed a nanoscale coating for solar cells that enables the cells to absorb about 20 percent more sunlight than uncoated devices. The coating, developed by trapping light using an optical version of a whispering gallery, could open a path for developing low-cost, high-efficiency solar cells with abundant, renewable, and environmentally friendly materials. Researchers used silicon dioxide (SiO2) nanosphere arrays on a gallium arsenide (GaAs) solar cell. The setup did not require direct surface patterning. The coating consists of thousands of tiny glass beads, each one only about 100th the width of a human hair. When sunlight hits the coating, the light waves are steered around the nanoscale bead, similar to the way sound waves travel around a curved structure in an acoustic whispering gallery. Light captured by this nanoresonator coating eventually leaks out and is absorbed by the underlying GaAs solar cell. Nanoresonator coating, consisting of thousands of tiny glass beads, deposited on solar cells. The coating enhances both the absorption of sunlight and the amount of current produced by the solar cells. Courtesy of K. Dill, D. Ha, G. Holland/NIST. When researchers from the National Institute of Standards and Technology (NIST) and the University of Maryland excited individual nanoresonators in the coating, they observed about a 20 percent enhancement in both absorptivity and photocurrent. These increases were attributed to the combined effects of thin-film interference and whispering gallery-like resonances within the nanosphere arrays. To determine the effect of the resonance coupling between nanospheres, researchers performed a scanning photocurrent microscopy based on a near-field scanning optical microscopy measurement. They found a substantial local photocurrent enhancement. According to researchers, the study is the first to demonstrate the efficiency of the nanoresonator coatings using precision nanoscale measurements. Artist's representation of glass beads of slightly different diameters (denoted by different colors) in the nanoresonator coating. Each bead acts as an optical whispering gallery, or resonator, for a slightly different wavelength of sunlight. Courtesy of K. Dill, D. Ha/NIST. “Although calculations had suggested the coatings would enhance the solar cells, we could not prove this was the case until we had developed the nanoscale measurement technologies that were needed,” said NIST researcher Dongheon Ha. The team also devised a rapid, cost-efficient method of applying the nanoresonator coating. In the team’s method, droplets of the nanoresonator solution are placed on just one side of the solar cell. A wire-wound metal rod is then pulled across the cell, spreading the solution and forming a coating made of closely packed nanoresonators. “This is an inexpensive process and is compatible with mass production,” said Ha. A previous method — dipping semiconductor material in a tub of the nanoresonator solution — coated both sides of the semiconductor in a time-consuming process, even though only one side needs to be treated. The nanosphere-based antireflection coating (ARC), made by the Meyer rod rolling technique, is a scalable and a room-temperature process. It could potentially be used in place of the conventional thin-film-based ARCs requiring high-temperature vacuum deposition. The research was published in Nanotechnology (doi:10.1088/1361-6528/aaab0c).
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