Fifty years ago, Russian physicist Vladimir M. Fridkin created the first “dry” copy machine using a process known as xerography. Now, the foundation of his research back then may hold the key to making more efficient solar panels today.
“Vladimir is internationally renowned for his pioneering contributions to the field of electroxerography, having built the first working photocopier in the world,” says Andrew Rappe, a professor of chemistry and engineering at the University of Pennsylvania. “He then became a leader in ferroelectricity and piezoelectricity, and preeminent in understanding light interactions with ferroelectrics. It is amazing that the same person who discovered these bulk photovoltaic effects nearly 50 years ago is now helping to harness them for practical use in nanomaterials.”
Professor Fridkin has been working with Rappe and Jonathan E. Spanier, PhD, a professor of materials science, physics and electrical engineering at Drexel Univcrsity. Together they have just published a paper in journal Nature Photonics. Until they began their research, scientists believed there was a theoretical maximum efficiency of a solar panel known as the Shockley-Queisser limit.
The new research, which has been underwritten in part by U.S. Army Research Office, the Office of Naval Research, the U. S. Department of Energy, and the National Science Foundation, reveals new findings which could lead to solar panels that are 50% more efficient than the theoretical limit. If so, it could have a profound effect on the solar power industry.
The research is hard for laymen to grasp. 47 years ago, Fridkin 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 — those that carry additional energy in a photovoltaic material when excited by sunlight — before they lose their energy. Although it has received relatively little attention until recently, the so-called “bulk photovoltaic effect” might now be the key to revolutionizing our use of solar energy.
“The main result — exceeding the Shockley-Queisser limit using a small fraction of the solar spectrum — is caused by two mechanisms,” Fridkin says. “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 liberates other bound electrons in its path. Each then accelerates and triggers the release of others. This process continues successively — like setting off multiple domino cascades with a single tipped tile — amounting to a much greater current.
This second mechanism, the screening field, is an electric field 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.
“There are many exciting reports utilizing nanoscale materials or phenomena for improving solar energy conversion,” Professor Spanier says. “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 and lose their excess energy to vibrations of the crystal lattice.”
The technical explanation is a lot for non-technical people to absorb, but the upshot may be solar panels that convert significantly more sunlight to electricity than current panels are capable of. That means even more abundant solar power at even lower cost than today. Since the generation of electricity creates more carbon emissions than any other sector, including transportation, this discovery could go a long way toward slaking mankind’s thirst for fossil fuels.
