This is an artist's conception of light-generated non-thermalized electrons and their collection in a ferroelectric crystal. An intense screening field results in impact ionization, producing an unexpectedly high conversion efficiency. Image: Ella Marushchenko.
This is an artist's conception of light-generated non-thermalized electrons and their collection in a ferroelectric crystal. An intense screening field results in impact ionization, producing an unexpectedly high conversion efficiency. Image: Ella Marushchenko.

A recent discovery by a team of researchers has revealed that a class of materials known as ferroelectrics could be better at converting sunlight into energy than the materials currently being used in solar arrays. The team’s research shows how this class of materials can extract power from a small portion of the sunlight spectrum with an efficiency that is above its theoretical maximum – a value known as the Shockley-Queisser limit. This finding, which could lead to more power-efficient solar cells, was inspired by a near-half-century old discovery by Russian physicist Vladimir Fridkin, a visiting professor of physics at Drexel University and one of the innovators behind the photocopier.

The team, which includes scientists from Drexel University, the Shubnikov Institute of Crystallography of the Russian Academy of Sciences, the University of Pennsylvania and the US Naval Research Laboratory recently published its findings in Nature Photonics. Their paper explains how they were able to use the ferroelectric barium titanate to convert sunlight into electrical power much more efficiently than the Shockley-Queisser limit would dictate for a material that absorbs almost no light in the visible spectrum – only ultraviolet wavelengths.

The foundation for the new findings was observed by Fridkin, who is one of the principal co-authors of the paper, some 47 years ago. This is when he discovered a physical mechanism for converting light into electrical power – one that differs from the method currently employed in solar cells. The mechanism relies on collecting ‘hot’ electrons, which contain excess energy that in conventional solar cells is lost as heat rather than converted into electricity. And though it has received relatively little attention until recently, this so-called ‘bulk photovoltaic effect’ might now be the key to revolutionizing our use of solar energy.

Up to now, solar energy conversion has been limited by solar cell design and by the electrochemical characteristics of the materials used to make solar cells.

"In a conventional solar cell – made with a semiconductor – absorption of sunlight occurs at an interface between two regions, one containing an excess of negative-charge carriers, called electrons, and the other containing an excess of positive-charge carriers, called holes," explained Alessia Polemi, a research professor in Drexel's College of Engineering and one of the co-authors of the paper.

In order to generate electron-hole pairs at the interface, which is necessary for producing an electric current, the photons in sunlight must excite the electrons so that they vacate the semiconductor’s valence band and move into the conduction band. The difference in energy levels between these two bands is referred to as the ‘band gap’.

Photons in sunlight with energies that are greater than the band gap produce excited electrons with corresponding excess energy, which is lost as heat rather than converted into electric current. This process reduces the amount of power that can be extracted from a solar cell.

"The light-induced carriers generate a voltage, and their flow constitutes a current. Practical solar cells produce power, which is the product of current and voltage," Polemi said. "This voltage, and therefore the power that can be obtained, is also limited by the band gap."

But, as Fridkin discovered in 1969 – and the team validates with this research – this limitation is not universal, which means solar cells can be improved.

When Fridkin and his colleagues at the Institute of Crystallography in Moscow observed an unusually high photovoltage while studying the ferroelectric antimony sulfide iodide – a material that does not have any junction separating the carriers – he posited that crystal symmetry could be the origin for its remarkable photovoltaic properties. He later explained how the ‘bulk photovoltaic effect’, which is very weak, transports photo-generated hot electrons in a particular direction without any collisions that could cool the electrons.

This is significant because the limit on solar power conversion specified in the Shockley-Queisser theory is based on the assumption that all of the excess energy is lost – wasted as heat. But the team's discovery shows that not all of the excess energy of hot electrons is lost, and that the energy can, in fact, be extracted as power before it is converted into heat.

"The main result – exceeding [the energy gap-specific] Shockley-Queisser [power efficiency limit] using a small fraction of the solar spectrum – is caused by two mechanisms," Fridkin said. "The first is the bulk photovoltaic effect involving hot carriers and second is the strong screening field, which leads to impact ionization and multiplication of these carriers, increasing the quantum yield."

Impact ionization, which leads to carrier multiplication, can be likened to an array of dominoes in which each domino represents a bound electron. When a photon interacts with an electron, it excites the electron, which, when subject to a strong field, accelerates and 'ionizes' or liberates other bound electrons in its path, each of which also accelerates and triggers the release of yet more electrons. Like setting off multiple domino cascades with a single tipped tile, this process can generate a much greater electric current.

The second mechanism, the screening field, is an electric field that is present in all ferroelectric materials. The nanoscale electrode used to collect the current in a solar cell enhances this screening field, which has the beneficial effect of promoting impact ionization and carrier multiplication. Following the domino analogy, the field drives the cascade effect, ensuring that it continues from one domino to the next.

"This result is very promising for high efficiency solar cells based on application of ferroelectrics having an energy gap in the higher intensity region of the solar spectrum," Fridkin said.

"Who would have expected that an electrical insulator could be used to improve solar energy conversion?" said Jonathan Spanier, a professor of materials science, physics and electrical engineering at Drexel and one of the principal authors of the study. "Barium titanate absorbs less than a tenth of the spectrum of the sun. But our device converts incident power 50% more efficiently than the theoretical limit for a conventional solar cell constructed using this material or a material of the same energy gap."

This breakthrough also builds on research conducted several years ago by Andrew Rappe, professor of chemistry and of materials science & engineering at the University of Pennsylvania, one of the principal authors, and Steve Young, also a co-author of the new report. Rappe and Young showed how bulk photovoltaic currents could be calculated, which led Spanier and his collaborators to investigate whether a higher power conversion efficiency could be attained in ferroelectrics.

"There are many exciting reports utilizing nanoscale materials or phenomena for improving solar energy conversion," Spanier said. "Professor Fridkin appreciated decades ago that the bulk photovoltaic effect enables free electrons that are generated by light and have excess energy to travel in a particular direction before they cool or 'thermalize' – and lose their excess energy to vibrations of the crystal lattice."

Rappe was also responsible for connecting Spanier to Fridkin in 2015, a collaboration that set in motion the research now detailed in the Nature Photonics paper – a validation of Fridkin's decades-old vision.

This story is adapted from material from Drexel University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.