More Solar Power With Ferroelectrics

Artist's concept of optically-generated non-thermalized electrons and their collection in a ferroelectric crystal. An intense screening field results in impact ionization, enabling an unexpectedly high conversion efficiency (Source: E. Marushchenko)

Artist’s concept of optically-generated non-thermalized electrons and their collection in a ferroelectric crystal. An intense screening field results in impact ionization, enabling an unexpectedly high conversion efficiency (Source: E. Marushchenko)

Designers of solar cells may soon be setting their sights higher, as a new discovery at Drexel Uni­versity has revealed a class of materials that could be better at conver­ting sunlight into energy than those currently being used in solar arrays. The research shows how a material can be used to extract power from a small portion of the sunlight spectrum with a con­version efficiency that is above its theore­tical maximum, the Shockley-Queisser limit. This finding could lead to more power-efficient solar cells.

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 explains how they were able to use a barium titanate crystal to convert sunlight into electric power much more efficiently than the Shockley-Queisser limit would dictate for a material that absorbs almost only ultraviolet. The pheno­menon that is the foun­dation for the new findings differs from the method currently employed in solar cells. The mechanism relies on collecting hot electrons, those that carry additional energy in a photo­voltaic material when excited by sunlight, before they lose their energy. And though it has received relatively little attention until recently, the bulk photo­voltaic effect might now be the key to revolu­tionizing our use of solar energy.

“In a conven­tional solar cell absorption of sunlight occurs at an inter­face between two regions, one containing an excess of electrons, and the other containing an excess of holes,” said Alessia Polemi, a research professor in Drexel’s College of Engineering. In order to generate electron-hole pairs at the interface, which is necessary to have an electric current, the sunlight’s photons must excite the electrons to a level of energy that enables them to vacate the valence band and move into the conduction band. This means that in photo­voltaic materials, not all of the available solar spectrum can be converted into electrical power. And for sunlight photon energies that are higher than the band gap, the excited electrons will lose it excess energy as heat, rather than converting it to electric current. This process further reduces the amount of power 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 this limitation is not universal, which means solar cells can be improved. When Russian physicist Vladimir M. Fridkin and his colleagues at the Institute of Crystal­lography in Moscow observed an unusually high photo­voltage while studying the ferro­electric antimony sulfide iodide he posited that crystal symmetry could be the origin for its remarkable photo­voltaic pro­perties. He later explained how this bulk photo­voltaic effect, which is very weak, involves the transport of photo-generated hot electrons in a parti­cular direction without collisions, which cause cooling of the electrons.

This is signi­ficant because the limit on solar power conversion from the Shockley-Queisser theory is based on the assumption that all of this 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 thermalizing. “The main result exceeding the Shockley-Queisser-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 the strong field, accelerates and ‘ionizes’ or liberates other bound electrons in its path, each of which, in turn, also acce­lerates and triggers the release of others. This process continues succes­sively amounting to a much greater current. This second mechanism, the screening field, is an electric field is present in all ferroelectric materials. But with the nanoscale electrode used to collect the current in a solar cell, the field is enhanced, and this 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.

“Barium titanate absorbs less than a tenth of the spectrum of the sun. But our device converts incident power 50 percent more efficiently than the theoretical limit for a conven­tional solar cell constructed using this material or a material of the same energy gap,” said Jonathan E. Spanier, a professor of materials science, physics and electrical engi­neering at Drexel. This break­through builds on research conducted several years ago by Andrew M. Rappe and Steve M. Young of Materials Science & Engi­neering at the University of Penn­sylvania. Rappe and Young showed how bulk photovoltaic currents could be calculated, which led Spanier and colla­borators to investigate if 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. (Source: Drexel Univ.)

Reference: J. E. Spanier et al.: Power conversion efficiency exceeding the Shockley–Queisser limit in a ferroelectric insulator, Nat. Phot., online 08 August 2016; DOI: 10.1038/nphoton.2016.143

Link: Dept. of Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania, USA • Shubnikov Institute for Crystallography, Russian Academy of Sciences, Moscow, Russia

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