New Photodetector Senses Light by Combining Two Distinct Inorganic Materials

A photodetector has been developed by Physicists at the University of California, Riverside. This photodetector senses light by incorporating two diverse inorganic materials and generating quantum mechanical processes capable of revolutionizing the way solar energy is collected.

Photodetectors are nearly ubiquitous, found in cell phones, remote controls, cameras, solar cells and also the panels of space shuttles. Measuring only microns across, these small devices transform light into electrons, whose subsequent movement produces an electronic signal. Increasing the efficiency of light-to-electricity conversion has been one of the main goals in photodetector construction ever since their invention.

Two atomic layers of tungsten diselenide (WSe2) were stacked by Lab Researchers on a single atomic layer of molybdenum diselenide (MoSe2). Such stacking results in properties immensely different from those of the parent layers, permitting for customized electronic engineering at the smallest possible scale.

In atoms, electrons live in states that establish their energy level. When electrons move from one state to another, they either lose or acquire energy. Electrons can move freely above a specific energy level. An electron moving into a lower energy state is capable of transferring sufficient energy in order to knock loose another electron.

UC Riverside Physicists studied that when a photon strikes the WSe2 layer, it knocks loose an electron, in order to allow it to freely conduct via the WSe2. At the junction between MoSe2 and WSe2, the electron drops down into MoSe2. The energy given off then catapults a second electron from the WSe2 into the MoSe2, where both electrons then become free to move and produce electricity.

We are seeing a new phenomenon occurring. Normally, when an electron jumps between energy states, it wastes energy. In our experiment, the waste energy instead creates another electron, doubling its efficiency. Understanding such processes, together with improved designs that push beyond the theoretical efficiency limits, will have a broad significance with regard to designing new ultra-efficient photovoltaic devices.

Nathaniel M. Gabor, Assistant Professor of Physics and Leader of the Research Team

Study results have been recently published in Nature Nanotechnology.

“The electron in WSe2 that is initially energized by the photon has an energy that is low with respect to WSe2,” said Fatemeh Barati, a Graduate Student in Gabor’s Quantum Materials Optoelectronics lab and the Co-first Author of the research paper. “With the application of a small electric field, it transfers to MoSe2, where its energy, with respect to this new material, is high.  Meaning, it can now lose energy. This energy is dissipated as kinetic energy that dislodges the additional electron from WSe2.”

In existing solar panels models, one photon can at most produce one electron. In the prototype developed by the Researchers, one photon can produce two electrons or more via a process known as electron multiplication.

The Researchers explained that electrons behave like waves in ultra-small materials. Though it is unintuitive at bigger scales, the process of producing two electrons from one photon is flawlessly allowable at very small length scales. When a material, such as MoSe2 or WSe2, gets thinned down to dimensions reaching the electron’s wavelength, the material’s properties start to change in inexplicable, mysterious and unpredictable ways.

It’s like a wave stuck between walls closing in, quantum mechanically, this changes all the scales. The combination of two different ultra small materials gives rise to an entirely new multiplication process. Two plus two equals five.

Nathaniel M. Gabor, Assistant Professor of Physics and Leader of the Research Team

“Ideally, in a solar cell we would want light coming in to turn into several electrons,” said Max Grossnickle, also a Graduate Student in Gabor’s lab and the research paper’s Co-first Author. “Our paper shows that this is possible.”

Barati highlighted that more electrons could be produced also by increasing the temperature of the device.

“We saw a doubling of electrons in our device at 340 degrees Kelvin (150 F), which is slightly above room temperature,” she said. “Few materials show this phenomenon around room temperature. As we increase this temperature, we should see more than a doubling of electrons.”

Electron multiplication in standard photocell devices usually needs applied voltages of 10-100 volts. The researchers employed only 1.2 volts, the typical voltage supplied by an AA battery, in order to observe the doubling of electrons.

“Such low voltage operation, and therefore low power consumption, may herald a revolutionary direction in photodetector and solar cell material design,” Grossnickle said.

He explained that the competence of a photovoltaic device is monitored by a simple competition: light energy is either converted into useful electronic power or waste heat.

“Ultrathin materials may tip the balance in this competition by simultaneously limiting heat generation, while increasing electronic power,” he said.

Gabor explained that the quantum mechanical phenomenon his team observed in their device is similar to what takes place when cosmic rays, getting in touch with the Earth’s atmosphere with high kinetic energy, generate an array of new particles.

He speculated that the team’s discoveries could find applications in unexpected ways.

These materials, being only an atom thick, are nearly transparent. It’s conceivable that one day we might see them included in paint or in solar cells incorporated into windows. Because these materials are flexible, we can envision their application in wearable photovoltaics, with the materials being integrated into the fabric. We could have, say, a suit that generates power – energy-harvesting technology that would be essentially invisible.

Nathaniel M. Gabor, Assistant Professor of Physics and Leader of the Research Team

Gabor, Barati and Grossnickle were joined in the study by UC Riverside’s Shanshan Su, Roger K. Lake and Vivek Aji.

The research received grants from the Air Force Office of Scientific Research, U.S. Department of Energy, a Cottrell Scholar Award and a National Science Foundation CAREER Award.