Photokinetics transforms pure silicon from an indirect bandgap semiconductor to a direct bandgap semiconductor

Research led by the University of California, Irvine has found that the optical properties of materials can be dramatically improved by imparting new properties to light, rather than changing the material itself.

The researchers have demonstrated that by manipulating the momentum of incoming photons, they can fundamentally change the way light interacts with matter. One striking example of their discovery is that they can enhance the optical properties of pure silicon, a widely used and essential semiconductor, by an astonishing four orders of magnitude.

This breakthrough has great potential to transform solar energy conversion and optoelectronics in general. The research, featured on the cover of the September issue of ACS Nano, was conducted in collaboration with Kazan Federal University and Tel Aviv University.

“This work challenges the conventional idea that light-matter interactions are determined solely by matter,” said senior author Dmitry Fishman, adjunct professor of chemistry. “By endowing light with new properties, we can fundamentally change how light interacts with matter.”

“The result is that existing or optically ‘underappreciated’ materials can achieve functions previously thought impossible. It’s like waving a magic wand: instead of designing new materials, we enhance the properties of existing materials by simply changing the incident light.”

“This photon phenomenon arises directly from the Heisenberg uncertainty principle,” said co-author Eric Potoma, professor of chemistry. “When light is confined to scales smaller than a few nanometers, its momentum distribution spreads out. The momentum increase is so large that it exceeds the momentum of a photon in free space by a factor of 1000, and is comparable to the momentum of an electron in matter.”

Distinguished chemistry professor Ara Apkarian elaborated further, saying, “This phenomenon fundamentally changes the way light interacts with matter. Traditionally, textbooks teach about perpendicular optical transitions, where a material absorbs light and the photon changes only the energy state of the electrons.”

“But momentum-enhanced photons can change both the energy and momentum states of electrons, unlocking new transition pathways that were not previously considered. Figuratively speaking, these photons allow us to ’tilt the textbook’ because they enable diagonal transitions. This has a dramatic impact on the material’s ability to absorb and emit light.”

Fishman continues, “Silicon, for example, is the second most abundant element in the Earth’s crust and is the basis of modern electronics. Despite its widespread use, silicon’s poor absorption of light has long limited its efficiency in devices such as solar panels.”

“This is because silicon is an indirect semiconductor, which means it relies on phonons (lattice vibrations) to enable electronic transitions. The physics of light absorption in silicon is such that a photon changes the energy state of an electron, and at the same time, a phonon is needed to change the momentum state of the electron.”

“Silicon’s optical properties are inherently weak because photons, phonons and electrons are unlikely to interact at the same place and time. This poses a major challenge for optoelectronics and is even slowing progress in solar energy technology.”

“As the effects of climate change grow, the transition from fossil fuels to renewable energy is more urgent than ever. Solar energy is key to this transition, but the commercially available solar cells we rely on are inadequate,” Potoma stressed.

“Due to silicon’s poor ability to absorb light, these cells require a thick layer of pure crystalline material, about 200 micrometers, to effectively capture sunlight. This not only increases production costs but also limits efficiency due to increased carrier recombination.”

“Thin-film solar cells are widely seen as a solution to these two challenges. Alternative materials such as direct bandgap semiconductors have demonstrated thin solar cells with efficiencies greater than 20 percent, but these materials tend to degrade quickly or are expensive to manufacture, making them impractical at present.”

“Driven by the promise of silicon-based thin-film solar cells, researchers have been searching for ways to improve silicon’s light absorption for over 40 years,” Apkarian added, “but a true breakthrough has yet to be found.”

Fishman continued: “Our approach takes a fundamentally different step: by enabling diagonal transitions with momentum-enhanced photons, we effectively convert pure silicon from an indirect-bandgap semiconductor to a direct-bandgap semiconductor, without changing the material itself. This dramatically increases silicon’s ability to absorb light by several orders of magnitude.”

“This means that the thickness of the silicon layer can be reduced by the same proportion, enabling ultra-thin devices and solar cells that far exceed current technology at a fraction of the cost. Moreover, because this phenomenon does not require any material changes, this approach can be integrated into existing manufacturing techniques with little or no modifications.”

Apkarian concluded: “We are only just beginning to explore the various phenomena associated with light confinement at the nanoscale and beyond. The physics involved is rife with potential fundamental and applied discoveries, but the direct implications are already clear.”

“Converting silicon into a direct-bandgap semiconductor by increasing the momentum of photons has the potential to revolutionize energy conversion and optoelectronics.”

Co-authors of the study include Giovanny Melham, a junior specialist in chemistry at the University of California, Irvine; Kazan Federal University researchers Sergey Harintsev, Alexei Noskov and Elina Battalova; and Tel Aviv University researchers Liat Katlivas and Alexander Kotlyar.