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Discovery Could Lead to Cheaper Solar Cell and Electronics Fabrication

Imagine printing electronics with an inkjet printer or even painting solar panels onto building walls. Such technology would cut the cost of producing electronic devices and enable new ways to use them in our everyday lives. (See photo above of graduate student Tika Kafle facing the camera while working on the time-resolved photoemission spectroscopy setup.  Courtesy of Cody Howard/University of Kansas).

Although some organic semiconductors, made out of molecules or polymers have been made suitable for such purposes, some properties of these materials pose a significant obstacle that limits their usefulness.

“In these materials, an electron is usually bound to its counterpart, a missing electron known as ‘hole,’ and can’t move freely,” said Wai-Lun Chan, associate professor of physics & astronomy at the University of Kansas. “So-called ‘free electrons,’ which wander freely in the material and conduct electricity, are rare and can’t be generated readily by light absorption. This impedes the use of these organic materials in applications like solar panels because panels built with these materials often have poor performance.”

Due to this issue, Chan said “freeing the electrons” has been a focus in developing organic semiconductors for solar cells, light sensors, and numerous other optoelectronic applications.

Now, two physics research groups at the University of Kansas, led by Chan and Hui Zhao, professor of physics & astronomy, were able to generate free electrons from organic semiconductors when combined with a single atomic layer of molybdenum disulfide (MoS2), a recently discovered 2D semiconductor.

The introduced 2D layer lets the electrons escape from “holes” and move freely. The findings were published in the Journal of American Chemical Society.

Over the last two decades, many researchers have been investigating how free charges can be produced from hybrid organic-2D interfaces.

“One of the prevailing assumptions is free electrons can be generated from the interface as long as electrons can be transferred from one material to another in a relatively short period of time — less than one-trillionth of a second,” Chan said. “However, my graduate students Tika Kafle and Bhupal Kattel and I have found the presence of the ultrafast electron transfer in itself is not sufficient to guarantee the generation of free electrons from the light absorption. That’s because the ‘holes’ can prevent the electrons from moving away from the interface. Whether the electron can be free from this binding force depends on the local energy landscape near the interface.”

Chan said the energy landscape of the electrons can be looked at like a topographic map of a mountain.

“A hiker chooses his path based on the height contour map,” he said. “Similarly, the motion of the electron at the interface between the two materials is controlled by the electron energy landscape near the interface.”

Chan and Zhao’s findings will help researchers devise general principles of how to create the “landscape” to free the electrons in such hybrid materials.

They combined two experimental tools based on ultrafast lasers; time-resolved photoemission spectroscopy in Chan’s lab and transient optical absorption in Zhao’s lab, to make the discovery. Both experimental setups are in the basement of the Integrated Science Building.

For the time-resolved photoemission spectroscopy tests, Kafle triggered the motion of electrons with an ultrashort laser pulse that only exists for 10-quadrillionths (10-14) of a second. The advantage of the short pulse is that the starting time of the electron’s journey is known.

Kafle then hit the sample again with another ultrashort laser pulse at an accurately controlled time relative to the first pulse. This second pulse provided enough energy to kick out these electrons from the sample. The researchers measured the energy of these electrons (now in a vacuum) and used the principle of energy conservation to figure out the energy of electrons before they were kicked out. This revealed the journey of these electrons since they were hit by the initial pulse.

This method resolved the energy of the excited electrons as they move across the interface after the light absorption.

Because only electrons near the sample’s front surface can be released by the second pulse, the position of the electron near the interface is also revealed with atomic precision.

KU graduate Peymon Zereshkiand and Peng Yao (a visiting student), both supervised by Zhao, also used a two-pulse technique in the transient optical absorption measurements.

The first pulse initiated the electron motion in the same way. However, for their measurements, they use the second pulse differently. Instead of kicking out electrons, it helps monitor the electrons by gauging the portion of the second laser pulse that the sample reflects.

“Because light can penetrate a longer distance, the measurement can probe electrons in the entire depth of the sample and therefore provide complementary information to the first techniques that are more’ surface sensitive,'” Zhao said. “These detailed measurements enabled us to reconstruct the trajectory of the electron and determine conditions that enable the effective generation of free electrons.”

The work from the two research teams will offer a blueprint of how to design interfaces that turn light into electrical current with high efficiency.

Both teams receive funding from the National Science Foundation; through a CAREER Award (Chan) and a Condensed Matter Physics Award (Zhao).