Next-Gen Semi Materials Could Be a Boost for Photovoltaics, Lighting

Next-generation semiconducting materials have the potential to transform lighting technology and photovoltaics, suggest researchers from the Georgia Institute of Technology (Georgia Tech), in a study of the unusual physics behind hybrid semiconductors.

Laser light in the visible range is processed for use in the testing of quantum properties in materials in Carlos Silva’s lab at Georgia Tech. Courtesy of Georgia Tech/Rob Felt.

Semiconductors made with emerging materials can engage the material itself in quantum actions, according to the team. Such a material’s ability to contain diverse quantum particle movements is related to its extreme flexibility on a molecular level. Semiconductors made with traditional materials, in contrast, have rigid molecular structures.

The researchers studied the light-emission properties of a class of hybrid semiconductors called halide organic-inorganic perovskites (HOIPs). They found that the excitonic properties in HOIPs were diverse and that each excitonic resonance had a distinct degree of polaronic character.

“The positive-negative attraction in an exciton is called binding energy, and it’s a very high-energy phenomenon, which makes it great for light emitting,” said professor Carlos Silva, who co-led the study with Ajay Ram Srimath Kandada from Georgia Tech and the Istituto Italiano di Tecnologia. Unlike conventional semiconductors, the excitonic properties in HOIPs are stable at room temperature, Silva said.

This is a depiction of an HOIP, a halide organic-inorganic perovskite. The diamond shapes are perovskite, the crystal layer housing the quantum particle movement. In between is the organic layer that contributes to the overall flexibility of the HOIP, a material for this emerging generation of semiconductors. Courtesy of Georgia Tech/Silva, Thouin, Valverde-Chávez, Bargigia.

In the HOIP material’s lattice, excitons move around atoms, forming quasiparticles and creating repetitive nanoscale indentations in the material that are observable as wave patterns. Using Raman spectroscopy, the researchers measured the patterns and related them to the material’s quantum properties. The wave patterns were found to shift with the amount of energy added to the material. Also, the indentations could “grab” the excitons, slowing their mobility through the material to affect the quality of light emission.

The team concluded that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. “In a ground state, these wave patterns would look a certain way, but with added energy, the excitons do things differently,” Silva said. “That changes the wave patterns, and that’s what we measure. The key observation in the study is that the wave pattern varies with different types of excitons, for example, exciton, biexciton, polaronic, less polaronic.”

Professor Carlos Silva (l) with graduate research assistant Felix Thouin in Silva’s lab at Georgia Tech. Courtesy of Georgia Tech/Rob Felt.

The researchers said that HOIPs, which combine a crystal lattice with a layer of organic, flexible material, are easy to produce and apply, and have the potential to produce more radiant, energy-efficient light. They believe that their research into HOIPs could be relevant to a broader class of emerging hybrid semiconductor materials.

“One compelling advantage is that HOIPs are made using low temperatures and processed in solution,” Silva said. “It takes much less energy to make them, and you can make big batches.”

High temperatures are needed to make most semiconductors even in small quantities, and they are too rigid to apply to surfaces. Flexible HOIPs could be painted on surfaces to make LEDs, lasers, or even window glass that could glow in any color, said the researchers. Lighting with HOIPs could require very little energy, helping photovoltaic solar panel makers boost efficiency and reduce production costs.

The research was published in Nature Materials (https://doi.org/10.1038/s41563-018-0262-7).