In a study of a new so-called hybrid semiconductor material, namely halide organic-inorganic perovskite (HOIP), researchers from Georgia Tech believe they have uncovered a more ‘attractive’ way to produce this light.
HOIP is a sandwich of two inorganic crystal lattice layers with an organic material between them. This organic section acts like a sheet of rubber bands, transforming the layers of crystal lattice into a wobbly, but stable surface. The material is also soft, as the HOIPs are put together with many non-covalent bonds.
The researchers discovered that these qualities result in twirling troupes of quantum particles undulating through the material, creating desirable optoelectrical properties.
"HOIPs are made using low temperatures and processed in solution," added Carlos Silva, a professor in Georgia Tech's School of Chemistry and Biochemistry. "It takes much less energy to make them, and you can make big batches."
It takes high temperatures to make most semiconductors in small quantities, and they are rigid to apply to surfaces. According to the team, HOIPs could be painted on to make LEDs, lasers or even window glass that could glow in any colour. Lighting with HOIPs may require very little energy, and solar panel makers could boost photovoltaics' efficiency and slash production costs.
Emitting light
For material to emit light, one applies energy to electrons in the material, explains the team. This results in a quantum leap from their orbits around the atoms, causing them to emit that energy as light when they hop back down to their original orbits.
Established semiconductors can trap electrons in areas of the material that strictly limit the electrons' range of motion, then apply energy to those areas to make electrons do quantum leaps in unison to emit useful light when they hop back down.
"These are quantum wells, 2D parts of the material that confine these quantum properties to create these particular light emission properties," Prof. Silva said.
An electron has a negative charge, and an orbit it vacates after having been excited by energy is a positive charge called an electron hole. The electron and the hole can gyrate around each other forming a kind of imaginary particle, or quasiparticle, called an exciton.
"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," Prof. Silva explained.
When the electron and the hole reunite it releases the binding energy to make light. But usually, excitons are very hard to maintain in a semiconductor.
"The excitonic properties in conventional semiconductors are only stable at extremely cold temperatures," Prof. Silva continued. "But in HOIPs the excitonic properties are stable at room temperature."
Excitons get freed up from their atoms and move around the material. In addition, excitons in an HOIP can whirl around other excitons, forming quasiparticles called biexcitons.
The uncommon participation of atoms of the material in these ‘dances’ with electrons, excitons, biexcitons and polarons creates repetitive nanoscale indentations in the material that are observable as wave patterns and that shift and flux with the amount of energy added to the material.
"In a ground state, these wave patterns would look a certain way, but with added energy, the excitons do things differently. That changes the wave patterns, and that's what we measure," Prof. Silva explained. "The key observation in the study is that the wave pattern varies with different types of excitons.”
The indentations also grip the excitons, slowing their mobility through the material, and all these ornate dynamics may affect the quality of light emission, the team concludes.
