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Artists 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 Credit K Dill, D

Researchers at the National Institute of Standards and Technology (NIST) in the US have developed a nanoresonator coating based on an optical version of a whispering gallery, which describes the phenomenon of sound waves traveling around a concave surface, such as the dome of St Paul's Cathedral in London, perhaps the most famous example of a whispering gallery. The antireflection coating comprises thousands of nanoscale glass beads that trap the light around their curved surface, enabling a solar cell to absorb about 20% more sunlight, compared with an uncoated device.

How the nanoresonator coating boosts the solar cell’s performance

In the experiments, the researchers used a laser as light source to excite individual nanoresonators in the coating. The underlying solar cell was made of gallium arsenide. “The close-packed configuration of the nanoresonator antireflection coating leads to the excitation of various optical modes, including thin-film interference effects and optical whispering gallery modes, which ultimately leads to improved optical and electronic properties of solar cells,” says Dongheon Ha, a postdoctoral researcher in NIST’s Nanoscale Imaging and Spectroscopy Group and the University of Maryland’s NanoCenter. “Due to the combined effects of thin-film interference and whispering gallery-like resonances, there is more than 20% enhancement in both absorptivity and photocurrent.”

Imaging the nanoscale photocurrent enhancement for the first time

Although calculations had suggested that the excited whispering gallery modes within such a nanoresonator coating would boost a solar cell’s ability to absorb sunlight, the effect had not been proven until the NIST team developed the necessary nanoscale measurement technologies. “For the first time,” Ha reveals, “we image nanoscale photocurrent enhancement of a solar cell induced by the excitation of whispering-gallery modes within nanoresonators using a novel near-field scanning photocurrent microscopy technique.”

Integrating the coating process into solar cell manufacturing

“The nano-resonator antireflection coating, made by the Meyer rod rolling technique, is a scalable, an inexpensive and a room-temperature process and could be compatible with large-scale roll-to-roll manufacturing,” says Ha. “Therefore, it is an excellent candidate to replace conventional thin-film-based antireflection coatings requiring expensive high-temperature vacuum deposition, and it opens a new path for developing low-cost, high-efficiency solar cells with abundant, renewable and environmentally friendly materials.” Furthermore, he points out that the suggested technique does not require direct surface processing of solar cells that is potentially harmful to the carrier collections.

Next step

The expert shares that moving forward with this research work, he and his colleagues are planning to show the tuning of excited whispering gallery resonances by selecting a different size, material or configuration of nano-resonator arrays. They also want to work on “novel hybrid configurations of nano-resonator arrays that can offer higher optoelectronic properties than those from any other conventional antireflection technologies for photovoltaics.” Moreover, the researchers will investigate the effect of assembly defects, such as voids and multilayer patches, that can be present in practical applications.

The research is described in the paper “Nanoscale imaging of photocurrent enhancement by resonator array photovoltaic coatings,” published in the journal Nanotechnology.

Written by Sandra Henderson, Research Editor, Solar Novus Today

Labels: solar cell coating,nanoresonator coating,optical whispering gallery,National Institute of Standards and Technology,NIST,Dongheon Ha

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