Solar cell efficiency has made significant strides in recent times, but cells are still far from their maximum theoretical efficiency, and part of the reason is that the semiconductors we use to build them don’t have ideal electrical properties. Researchers at Northwestern University have now found a way 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.

In electronics, the band gap is a crucial feature of a semiconductor, measuring the amount of energy that an electron needs to be fed before it can start conducting electricity. Its size is 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 incredibly useful. 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," says Rondinelli. "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 that the band gap of a metal oxide can be changed 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 is 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 be converted directly into chemical fuels for easier storage.

Rondinelli and colleagues are now setting out to test their theoretical findings in the lab.

The research is described in last week’s issue of Nature Communications.