Bending for power

Materials World magazine
,
1 Jun 2018

Deformed crystals used in solar cell semiconductors could make for more efficient photovoltaic panels. Ines Nastali reports.

Researchers at the University of Warwick, UK, have developed a way to make solar cells more efficient. As most photovoltaic cells only operate at around 20% of their power generating capability, the scientists from the Department of Physics looked to the crystals used in the cells’ semiconductors. 

They found that deforming single crystals of strontium titanate (SrTiO3), titanium dioxide (TiO2), and silicon (Si) in the semiconductors could improve efficiency. ‘Most commercial solar cells are formed of two layers creating a junction between two kinds of semiconductors, a p-type with positive charge carriers, called holes, that can be filled by electrons, which are the n-type with negative charge carriers,’ it is stated in a university press release. ‘When light is absorbed, this p-n junction of the two semiconductors sustains an internal field, splitting the photo-excited carriers in opposite directions and generating a current and voltage across the junction,’ the researchers add. 

While the junctions are necessary to harvest energy from the cell, there are natural limitations to capturing power. According to the thermodynamic Shockley-Queisser Limit, only 33.7% of the power contained in all sunlight falling on an optimised solar cell under ideal conditions can be turned into electricity.

Structural change

However, a potential solution was presented as the researchers tried to make use of the bulk photovoltaic effect, which is free from the Shockley-Queisser Limit and doesn’t need a p-n junction to work. ‘The anomalous photovoltaic effect occurs in certain semiconductors and insulators where their lack of perfect symmetry around their central point – a non-centrosymmetric structure – allows generation of voltage that is larger than the band gap of that material,’ the researchers state. 

The size of the bandgap indicates the conductivity of a material – a large bandgap means that a lot of energy is required to excite electrons and thus produce a current. ‘Unfortunately, the materials that are known to exhibit the anomalous photovoltaic effect have very low power generation efficiencies, and are never used in practical power-generation systems,’ the researchers said. 

‘Our work has introduced a new effect that is based on completely different physical principles as classical solar cells with their p-n junctions. Since the Shockley-Quesser limit is solely defined for a p-n junction, our effect is – from the fundamental physics – free of this limitation,’ Professor Marin Alexe, the leading author of the study told Materials World.

Combined forces

The Warwick team therefore tried to take the semiconductors that work well in a solar cell and manipulate them to show the non-centrosymmetric structure, which normally only ferroelectric or piezoelectric materials have, resulting in the analogous photovoltaic effect. ‘We introduce strain gradients using either an atomic force microscope or a micron-scale indentation system, creating giant photovoltaic currents from centrosymmetric single crystals of SrTiO3, TiO2, and Si. This strain-gradient-induced bulk photovoltaic effect, which we call the flexo-photovoltaic effect, functions in the absence of a p-n junction,’ the scientists report in their study, Flexo-photovoltaic effect, published in the journal Science.

Going forward

‘There are already some inquiries from companies, but it is recognised to be in a very early stage of research,’ Alexe said. ’We will need more time and manpower to understand the underlying mechanism, quantify and properly design a proof of concept device.’

One of the challenges to overcome concerns engineering the crystals for the desired use. Alexe comments, ‘It is indeed a challenge to introduce a controlled strain into the existing solar cells. There are some designs, including an array of tips indenting the top surface or even intentionally building an array of misfit dislocations. Whether these, or any other alternative design, will be commercially viable or even working, remains to be established.’

You can read the study here: bit.ly/2rJVRoK