The Effect of the Defect

Iris Visoly-Fisher knew exactly what she wanted to work on when starting her Ph.D. - to follow up on a hunch that defects in a certain material used in solar cells would actually improve their performance.

But something was bugging her. She couldn’t understand why so few people were working on this topic. “It was a no-man’s-land,” she says. “It seemed clear that something was up, but there was virtually no literature on it.”

She soon found out why. The technical difficulties in examining this puzzle were daunting. To effectively tackle the problem, she would need to figure out how to zoom in on solar cell performance on the nanoscopic scale (equal to roughly one hundred-thousandth the width of a human hair).

An unexplained finding from years earlier was what had triggered this headache of a challenge. Most commercial solar cells are made of single-crystal semiconductors, such as silicon, Visoly-Fisher explains. But researchers had been looking around for alternative materials for a while, given silicon’s high manufacture costs. Then, nearly 15 years ago, a finding came up that was out of sync with all theoretical predictions. Certain solar cells made of polycrystalline (multi-grained) films were systematically outperforming their counterpart single-crystal cells.

Nobody understood why. Scientists working to improve solar cells had traditionally shied away from multi-grained films, despite their being significantly cheaper, since they contained numerous structural defects - a property believed to impair the light conversion process.

“In examining and reexamining the multi-grained films that had proven so successful, we could come up with only one key difference between them and the single-crystal films: the presence of a leading defect, known as the grain boundary defect,” says Visoly-Fisher, who performed the study under the guidance of Prof. David Cahen of the Institute’s Materials and Interfaces Department, in collaboration with Dr. Sidney Cohen of the Institute’s Chemical Research Support.

The team decided to take a closer look at these defects by studying the electrical properties of a single defect - in other words, the meeting point between two of these microscopic grains. And this is where their technical problems began.

They started out using an imaging technique based on atomic force microscopy (AFM). But given the Lilliputian scale of their study, it was impossible to determine that the results obtained were in fact due to the grain boundary defect, rather than merely being an experimental artifact.

Only after combining three different high-resolution imaging techniques, did they have an irrefutable answer: “The results were clear, and quite astounding: Contrary to earlier notions, grain boundary defects significantly enhance the efficiency of certain solar cells,” says Visoly-Fisher.

The finding, which was recently published in Advanced Materials, has to do with the basic principle underlying most solar cells. When light strikes the cell, the semiconductor within it serves as an “antenna,” absorbing the light energy, which releases electrons present in the semiconductor, allowing them to flow freely. These electrons are then harvested as an electric current for external use. As the Institute team has now shown, grain boundaries within solar cell films improve the light-to-electricity conversion because they provide a path where the freed electrons are efficiently collected and channeled on their way out.

“The grain boundaries essentially function as a freeway for electrons to exit, without traffic lights or roundabouts,” explains Cahen. “This finding offers a promising direction for improving solar cell performance while cutting production costs.”

And Visoly-Fisher adds: “There’s something immensely satisfying about solving a long-standing question in materials science by examining how the building blocks of the device work at the nano-scopic level. It’s like gaining entry to an almost imaginary world.”

Part of this study was performed in the labs of Prof. Israel Bar-Joseph of the Institute’s Condensed Matter Physics Department and Dr. Arie Ruzin of Tel Aviv University’s Department of Physical Electronics.

Sun “raycing”

On a sunny day, the sun beams approximately 1,000 watts of energy per square meter on our planet. Success in tapping this resource would transform the world’s energy industry, meeting all of our power demands for free. Solar cells are also environmentally sound, lacking any corrosive chemicals and giving off no pollutants.

More work is needed before solar technologies can become cost effective, but they are playing an increasingly important role, more than tripling their market between 1995 and 2000. Around the world, engineering students, scientists and industrialists are pushing ahead with new solar technologies - from self-sustaining solar homes to solar-powered cars. For instance, in a project launched by the U.S. Department of Energy, contractors have just put the finishing touches to a pilot $800,000 luxury home in Livermore, California, that produces its own electricity and even sells its surplus power to the local utility company. This trend has widespread parallels in Germany, where, motivated in part by the Chernobyl nuclear disaster, the government is pitching in to introduce solar-power systems to residential, public and commercial buildings.

Solar-powered car races are already a huge hit. In 2003, for example, university students from around the world participated in the World Solar Challenge across the vast Australian outback, pitting their self-designed cars, powered only by the sun, in a rigorous 3,000-km race.

Prof. Cahen’s research is supported by the Philip M. Klutznick Fund for Research; the Delores and Eugene M. Zemsky Weizmann-Johns Hopkins Research Program; the Minerva Stiftung Gesellschaft fuer die Forschung m.b.H. and the Wolfson Advanced Research Center. He is the incumbent of the Rowland Schaefer Professorial Chair in Energy Research.