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Solar cells are still far from their maximum theoretical efficiency, and part of the reason is the semiconductors we use to build them don’t have ideal electrical properties.

There is now a way, however, to tweak an important electrical feature of transition metal oxides, compounds commonly used as semiconductors, to build the optimal light-absorbing material for solar cells, lasers and photoelectrochemical cells, said researchers at Northwestern University.

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In electronics, the band gap is a crucial feature of a semiconductor, measuring the amount of energy an electron needs to get before it can start conducting electricity. Its size ends up measured in electronvolts (eV) and dictates whether a material will behave as a conductor (~0 eV), a semiconductor (~1–9 eV) or an insulator (~9 or more eV).

Being able to tweak the band gap at will would be useful.

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Solar cells, for instance, produce electricity whenever a photon travels to a silicon atom and “hits” it, giving one of silicon’s electrons enough energy to jump the band gap and become conductive. Tuning the band gap would mean being able to design the ideal semiconductor that can maximize the amount of energy harvested throughout the visible spectrum. However, current methods can only change the band gap by about one eV and can only do so by modifying the material’s chemical composition, which is not ideal.

Professor James Rondinelli and colleagues at Northwestern University have found a way to tune the band gap much more effectively than before, by up to two electronvolts and without changing the material’s composition.

“There really aren’t any perfect materials to collect the sun’s light,” Rondinelli said. “So, as materials scientists, we’re trying to engineer one from the bottom up. We try to understand the structure of a material, the manner in which the atoms are arranged, and how that ‘genome’ supports a material’s properties and functionality.”

Transition metal oxides have a very well defined atomic structure organized in stacked layers of neutral and electrically charged atoms. The way in which these atoms face and interact with each other ultimately determines the mechanical and electrical properties of the material.

Using quantum mechanics, Rondinelli and team have calculated the band gap of a metal oxide can change by two electronvolts, twice as much as was previously possible, simply by reconfiguring the layout of the cations (positively charged ions) and rearranging the order of neutral and electrically charged atoms as needed.

Although this was purely a theoretical result, it is nonetheless a promising development particularly with respect to photovoltaics. For single-junction solar cells, this advance could help design materials approaching the theoretically ideal band gap of 1.34 eV, leading to significantly higher efficiencies; and for multi-junction cells, it could help optimize semiconductors in such a way that incoming light ends up captured throughout the visible spectrum, raising the energy conversion rate even further.

Besides solar cells, this advance could also apply to building better electro-optical devices like lasers and improve the rate at which sunlight can convert directly into chemical fuels for easier storage.

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