Twirling Quantum Particles Undulating Through Hybrid Semiconductors Create Optimal Optoelectronic Properties

Quality solar panels and LED lights and monitors were the results of a revolution in the field of semiconductors. These devices have the ability to efficiently convert energy into light or vice versa.

Carlos Silva (l.) in his lab at Georgia Tech with graduate research assistant Félix Thouin examining a setup to process laser light in the visible range for the testing of quantum properties in a halide organic-inorganic perovskite. (Image credit: Georgia Tech/Rob Felt)

Currently imminent are the next-generation semiconducting materials, and in a new research, scientists have unraveled the unconventional physics behind their ability to transform photovoltaics and lighting technology one more time.

The comparison of quantum properties of these purported, emerging hybrid semiconductors with the properties of their standard antecedents is analogous to comparing the Bolshoi Ballet with jumping jacks. A group of physical chemists headed by scientists at the Georgia Institute of Technology reports that when swirling groups of quantum particles ripple through the emerging materials, they easily create highly desirable optoelectronic (light-electronic) properties.

It is practically not possible to achieve these same properties in standard semiconductors.

Similar to dancers who entice the floor to dance along with them, the particles that move through these new materials also engage the material itself in the quantum action. The scientists could evaluate patterns in the material brought about by the dancing and associate them with the quantum properties of the ensuing material and with the energy introduced into the material.

This understanding could assist engineers in productively working with the new category of semiconductors.

Unusually flexible semiconductors

The potential of the ensuing material ability to incorporate diverse, eccentric quantum particle movements, similar to the dancers, is directly associated with its strange flexibility on a molecular level, akin to the dancefloor that joins in the dances. On the contrary, the molecular structures of standard semiconductors are rigid and straight-laced, thereby leaving the dancing to quantum particles.

The category of hybrid semiconductors investigated by the scientists is known as halide organic-inorganic perovskite (HOIP), which will be described in-depth at the bottom together with the “hybrid” semiconductor designation, combining a crystal lattice—normal in semiconductors—with a layer of creatively flexing material.

Apart from their advantage of unique radiance and energy efficiency, HOIPs are also easy to synthesize and use.

Paint them on

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

Carlos Silva, Professor, School of Chemistry and Biochemistry, Georgia Tech.

Silva co-headed the research together with Ajay Ram Srimath Kandada from Georgia Tech and the Istituto Italiano di Tecnologia.

A majority of the semiconductors are produced in small quantities under high-temperature conditions, and they are rigid to be applied to surfaces; however, it is feasible to paint on HOIPs to make lasers, LEDs, or even window glass that could glow in any color from aquamarine to fuchsia. Very little energy may be needed to light with HOIPs, and makers of solar panel could boost the efficiency of the photovoltaics and decrease production costs.

The group headed by Georgia Tech included scientists from the Université de Mons in Belgium and the Istituto Italiano di Tecnologia. The outcomes of the study have been reported in the Nature Materials journal on January 14th, 2019. The study was funded by the U.S. National Science Foundation, EU Horizon 2020, the Natural Sciences and Engineering Research Council of Canada, the Fond Québécois pour la Recherche, and the Belgian Federal Science Policy Office.

Quantum jumping jacks

Semiconductors used in optoelectronic devices can either transform light into electricity or electricity into light. The scientists focused on processes related to the latter: light emission.

In a broad sense, the clue to make a material to emit light is to supply electrons in the material with energy, enabling them to take a quantum leap up from their orbits surrounding atoms and to subsequently emit that energy as light while hopping back down to the orbits that they left. Standard semiconductors have the ability to trap electrons in areas of the material where the range of motion of the electrons is strictly limited and to then apply that energy to those areas to enable electrons to do quantum leaps all at once to emit useful light while hopping back down all at once.

These are quantum wells, two-dimensional parts of the material that confine these quantum properties to create these particular light emission properties,” stated Silva.

Imaginary particle excitement

Light can be generated in a potentially more appealing way, which is a core strength of the new hybrid semiconductors.

An electron has a negative charge, and an orbit vacated by it after excitation with energy is a positive charge known as an electron hole. The hole and the electron can gyrate around each other to form a type of imaginary particle, or quasiparticle, known as 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,” stated Silva.

During reunion of the electron and the hole, the binding energy is liberated in the form of light. However, normally, it is very difficult to maintain excitons in a semiconductor.

The excitonic properties in conventional semiconductors are only stable at extremely cold temperatures. But in HOIPs the excitonic properties are very stable at room temperature.

Carlos Silva, Professor, School of Chemistry and Biochemistry, Georgia Tech.

Ornate quasiparticle twirling

Excitons that are freed up from their atoms tend to move around the material. Furthermore, excitons in an HOIP have the ability to swirl around other excitons, thereby forming quasiparticles known as biexcitons. And there is much more.

Excitons even spin around atoms in the material lattice. Quite similar to the manner in which an electron and a hole produce an exciton, this swirl of the exciton around an atomic nucleus leads to the formation of yet another quasiparticle known as a polaron. All that action can lead to transitioning of the excitons back to polarons. One can even refer to certain excitons taking on a “polaronic” subtlety.

One fact that compounds all those dynamics is that HOIPs are entirely made of positively and negatively charged ions. The opulence of these quantum dances has an overarching impact on the material itself.

Wave patterns resonate

The unusual involvement of the atoms of the material in such dances with electrons, biexcitons, excitons, and polarons results in repetitive nanoscale indentations in the material that can be observed as wave patterns and that shift and flux with the quantity 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. The key observation in the study is that the wave pattern varies with different types of excitons (exciton, biexciton, polaronic/less polaronic).

Carlos Silva, Professor, School of Chemistry and Biochemistry, Georgia Tech.

The indentations also grasp the excitons, making their mobility through the material gradual, and the light emission quality may be affected by all these ornate dynamics.

Rubber band sandwich

The material, which is a halide organic-inorganic perovskite, is a sandwich of two inorganic crystal lattice layers, where some organic material is placed between them—making HOIPs an organic-inorganic hybrid material. The quantum action takes place in the crystal lattices.

The organic layer in the middle is similar to a sheet of rubber bands that transforms the crystal lattices into a wobbly yet stable dancefloor. Moreover, HOIPs are made by many non-covalent bonds, rendering the material soft.

The crystal’s individual units take a form known as perovskite—a very even diamond shape, include a metal at the center and halogens such as iodine or chlorine at the points, hence the name “halide.” In this study, a 2D prototype with the formula (PEA)2PbI4 was used by the scientists.

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