Enhancing solar cell efficiency using nano- and micro-photonic elements

Solar energy is one of the major avenues toward clean, renewable energy future. In theory, it is supposed to be one of the cheapest energy sources around since the sun is free. However, its widespread adoption is hindered by the high cost of photovoltaic cells. Even if solar energy is free, cells that harvest this energy from the sun are expensive to develop and mass-produce, and more importantly, they are inefficient due to their low conversion rate.

The current efficiency of photovoltaic cells is limited due to the inability of a single band gap absorber to convert the energy over a broad spectrum into electric power. This means that only a small fraction of the incident light that hits the photovoltaic cell can actually be converted to electrical power.

The work of Rajesh Menon of the University of Utah and his team seeks to solve this problem by developing manufacturable nano-and microphotonics that improve the overall power-conversion efficiency of photovoltaic cells. Their approach has the potential to enable cells that can convert solar energy into electrical energy reach efficiencies of 50% or greater.

Their work focuses on extending a direct-binary search algorithm to design broadband microphotonic elements that can separate incident sunlight into spectral bands and focus these into optimized photovoltaic absorbers. Menon and his team call these optical microphotonic elements “polychromats” which have the potential to achieve optical efficiencies of greater than 90% across the solar spectrum, as compared to previous approaches using prisms, holograms, or dichroic filters. Polychromats can be incorporated into the glass covers of solar panels and can also be manufactured inexpensively via hot embossing.

Polychromats were designed using an optoelectronic model in order to maximize the total peak power density of all cells and fabricated using grayscale lithography on a glass substrate. Experiments demonstrate increases of 42% and 22% in peak power under simulated sunlight when using copper indium gallium (di)selenide solar cells and a combination of silicon and gallium arsenide solar cells, respectively. The team was also able to show that choosing an appropriate design, more compact cells can be fabricated.

Menon also developed optimization algorithms that can design nanostructured scatterers at the interfaces between the absorber and cladding layers in order to improve the coupling of broadband sunlight into resonant guided modes within the absorber. This leads to a considerable increase of the output power density in the photovoltaic cell. The optimization algorithms can be modified to account for oblique incidence angles, with simulations showing that a properly optimized cell can generate over seven times more energy over a year compared to an unoptimized cell. The fabrication of such patterned cells is still a challenge, but recent advances in aligned nanoimprint lithography have the potential for enabling the production of these cells.

One interesting direction of Menon’s research is combining light trapping and spectrum splitting with the use of nanophotonics. Aside from that, advances in high-quality thin-film gallium arsenide cells also indicate the potential of using photonics to enable higher efficiencies in photovoltaic cells in the future. Menon and his team showed that photonics can have an important role in increasing the efficiency of photovoltaic cells, leading to a decrease in their cost and hence enable their widespread adoption and use as a renewable energy source. The conditioning of sunlight using nano- and microphotonics provides unparalleled opportunities for interdisciplinary research and innovation using computation, photonics, and nanofabrication.

About the Author- This article is contributed by Martini Tech Inc., a nanotechnology company based in Tokyo, Japan and specialized in sputtering and thin-film deposition, nanoimprint lithography, nanoimprint mold and replica, photonic crystals, MEMS design and MEMS foundry services and patterned sapphire substrates ( PSS ) for LED applications.