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MIT team shows how to better harness energy from sunlight

Posted: 09 Nov 2015     Print Version  Bookmark and Share

Keywords:MIT  solar cell  dielectric  photon  plasmon 

A team of researchers at MIT has figured out how to significantly boost the energy that can be harnessed from sunlight, which could lead to the development of better solar cells or light detectors. According to them, the method is based on the discovery that unexpected quantum effects enhance the number of charge carriers, known as electrons and "holes," that are knocked loose when photons of light of different wavelengths strikes a metal surface coated with a special class of oxide materials known as high-index dielectrics.

The photons generate what are known as surface plasmons, a cloud of oscillating electrons that has the same frequency as the absorbed photons. The finding is reported in the journal Physical Review Letters by authors including MIT's Nicholas Fang, an associate professor of mechanical engineering, and postdoc Dafei Jin. The researchers used a sheet of silver coated with an oxide, which converts light energy into polarisation of atoms at the interface.

"Our study reveals a surprising fact: Absorption of visible light is directly controlled by how deeply the electrons spill over the interface between the metal and the dielectric," Fang said. The strength of the effect, he added, depends directly on the dielectric constant of the material, a measure of how well it blocks the passage of electrical current and converts that energy into polarisation.

"In earlier studies," Fang stated, "this was something that was overlooked."

Previous experiments showing elevated production of electrons in such materials had been chalked up to defects in the materials. But Fang said those explanations "were not enough to explain why we observed such broadband absorption over such a thin layer" of material. But, he said, the team's experiments back the newfound quantum-based effects as an explanation for the strong interaction.

Dielectric films

Remarkable colour changes occur when different thin films of oxides (low dielectric-index SiO2 and high dielectric-index TiO2) are deposited onto the noble metals silver (Ag) and gold (Au). The colour change is due to light absorption via surface plasmons, which are strongly enhanced by the quantum spillover effect at the interface of a noble metal and a high-index dielectric. Courtesy of Qing Hu

The team found that by varying the composition and thickness of the layer of dielectric materials (such as aluminum oxide, hafnium oxide and titanium oxide) deposited on the metal surface, they could control how much energy was passed from incoming photons into generating pairs of electrons and holes in the metal, a measure of the system's efficiency in capturing light's energy. In addition, the system allowed a wide range of wavelengths, or colours, of light to be absorbed, they indicated.

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