Insulator pile shows solar potential


Stacks of insulating transition metal oxides could effectively convert sunlight into electricity. A team centred at the Technical University of Vienna in Austria, has calculated that layering LaVO3 on a SrTiO3 base could also deliver advantages conventional semiconductors can’t.1 ‘Using LaVO3/SrTiO3 for photovoltaics is a totally new idea,’ says Giorgio Sangiovanni, who led the team and has now moved to the University of Würzburg, Germany.

In 2002 scientists found LaAlO3/SrTiO3 interfaces can produce conductive ‘two-dimensional electron gases’ commonly found in semiconductors, despite the two oxides normally being insulators.2 Sangiovanni’s team realised this could benefit solar cells, where photons excite an electron from a semiconductor’s valence band to its conduction band. The resulting electron, and the positive hole left behind, move in opposite directions towards the cell’s anode and cathode, creating electrical current.  However, photovoltaic materials only absorb photons with more energy than the gap between bands, which can limit cells’ efficiency. At over 3eV, LaAlO3/SrTiO3’s bandgap is too large Sangiovanni says, but the 1.1eV bandgap of the related material LaVO3/SrTiO3 is ‘ideal for solar-cell applications’.

An additional benefit of layered transition metal oxides, Sangiovanni explains, is that they could also separate electrons and holes. ‘Many of these systems are polar, with nominally positively and negatively charged layers,’ he says,  creating an electric field that pushes the electrons and holes apart. And with thin enough LaVO3 the interface also behaves like a metal, which could extract solar cells’ generated electricity without needing extra electrodes.

Alternating layers should set up an electric field that helps separate electrons and holes better © American Physical Society

Meanwhile, the highest efficiency photovoltaic cells capture more solar energy by stacking semiconductor materials with progressively narrower bandgaps such as GaAsP2/GaAs/Ge. Sangiovanni’s team has calculated that transition metal oxides could do that too, for example using LaFeO3/LaVO3/SrTiO3 layers.

Jean-Marc Triscone at the University of Geneva, Switzerland, who also studies oxide interfaces, calls this study a ‘nice idea’. But he’s concerned that the electric field may be very low in the thick layers needed for solar cells. He also worries that there could be too many unwanted electron–hole recombinations in oxide materials for photovoltaic use, partly due to crystal defects. ‘Will we be able to achieve the required quality?’ he asks. ‘We should certainly try.’

Though Sangiovanni concedes that this technology is at a very early stage, his Würzburg colleagues are attempting the first physical measurements on it. ‘Now it is time for experiments,’ he says. ‘We hope the results will be as positive as we have predicted.’


Related Content

Efficient recovery justifies silver’s use in solar cells

23 January 2014 Research

news image

Photovoltaic scientists take a long term view of the large scale deployment of solar panels

Perovskite coat gives hybrid solar cells a boost

4 October 2012 Research

news image

Cheap organometal halide device overcomes problem that has dogged other low-cost solar cells

Most Read

UC Davis chemist sentenced to four years over explosion

19 November 2014 News and Analysis

news image

Postdoc sentenced over attempt to make explosive device and reckless disposal of hazardous waste

Grad student blamed for research misconduct at Utah

13 November 2014 News and Analysis

news image

Two papers have been retracted due to image falsification at the University of Utah

Most Commented

Beetle behind breath test for bank notes

17 November 2014 Research

news image

Photonic crystal inks inspired by longhorn beetle could help to fight counterfeiting

Bayer wins race to buy Merck & Co consumer care

9 May 2014 Business

news image

$14bn deal will make Aspirin inventor the number two over-the-counter healthcare company